TY - JOUR TI - Sensors based on piezoelectric resonators AU - Benes, E. AU - Gröschl, M. AU - Burger, W. AU - Schmid, M. T2 - Sensors and Actuators A: Physical AB - A review of sensors based on piezoelectric crystal resonators is presented. The survey focuses on the fundamental resonator modes rather than on the variety of surrounding support configurations in special sensor applications. First, the general properties of vibrating crystal sensors and their inherent superiority are described. The sensor concepts utilizing either homogeneous resonators with temperature and pressure (stress) as primary measurants or composite resonators with areal mass density and viscoelastic properties of the 'foreign' layer as primary measurands are discriminated. A comparison between bulk acoustic wave (BAW) and surface acoustic wave (SAW) resonators with respect to their primary sensitivity functions and principal capabilities for sensor applications is given and the importance of recent investigations on Lamb wave and horizontal polarized shear wave (HPLW) interdigital transducer (IDT) resonators is acknowledged. The importance of mode purity for high dynamic range sensors based on resonators and some aspects of the demand on specialized electronics are emphasized. The present state of established sensors based on primary sensitivities, e.g., quartz-crystal thermometers, pressure transducers, thin-film thickness and deposition-rate monitors, viscoelastic layer analysers (crystal/liquid composite resonators) is reviewed. A selection of the most promising recently investigated vibrating crystal sensors utilizing indirect sensitivities is described, including the wide field of analyte-selective coatings and resonator-based immunosensors or immunoassays. Finally, the potential of alternative piezoelectric materials for future sensor developments is briefly discussed. DA - 1995/05/01/ PY - 1995 DO - 10.1016/0924-4247(95)00846-2 VL - 48 IS - 1 SP - 1 EP - 21 J2 - Sens. Actuators A SN - 0924-4247 ER - TY - JOUR TI - Surface reduction state determines stabilization and incorporation of Rh on α-Fe2O3(11̅02) AU - Kraushofer, Florian AU - Resch, Nikolaus AU - Eder, Moritz AU - Rafsanjani-Abbasi, Ali AU - Tobisch, Sarah AU - Jakub, Zdenek AU - Franceschi, Giada AU - Riva, Michele AU - Meier, Matthias AU - Schmid, Michael AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - Advanced Materials Interfaces AB - Iron oxides (FeOx) are among the most common support materials utilized in single atom catalysis. The support is nominally Fe2O3, but strongly reductive treatments are usually applied to activate the as-synthesized catalyst prior to use. Here, Rh adsorption and incorporation on the (11̅02) surface of hematite (α-Fe2O3) are studied, which switches from a stoichiometric (1 × 1) termination to a reduced (2 × 1) reconstruction in reducing conditions. Rh atoms form clusters at room temperature on both surface terminations, but Rh atoms incorporate into the support lattice as isolated atoms upon annealing above 400 °C. Under mildly oxidizing conditions, the incorporation process is so strongly favored that even large Rh clusters containing hundreds of atoms dissolve into the surface. Based on a combination of low-energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) data, as well as density functional theory (DFT), it is concluded that the Rh atoms are stabilized in the immediate subsurface, rather than the surface layer. DA - 2021/// PY - 2021 DO - 10.1002/admi.202001908 VL - 8 IS - 8 SP - 2001908 J2 - Adv. Mater. Interfaces SN - 2196-7350 ER - TY - JOUR TI - Quest for a pristine unreconstructed SrTiO3(001) surface: An atomically resolved study via noncontact atomic force microscopy AU - Sokolović, Igor AU - Franceschi, Giada AU - Wang, Zhichang AU - Xu, Jian AU - Pavelec, Jiří AU - Riva, Michele AU - Schmid, Michael AU - Diebold, Ulrike AU - Setvín, Martin T2 - Physical Review B AB - The surfaces of perovskite oxides affect their functional properties, and while a bulk-truncated (1×1) termination is generally assumed, its existence and stability is controversial. Here, such a surface is created by cleaving the prototypical SrTiO3(001) in ultrahigh vacuum, and its response to thermal annealing is observed. Atomically resolved noncontact atomic force microscopy (nc-AFM) shows that intrinsic point defects on the as-cleaved surface migrate at temperatures above 200∘C. At 400∘C–500∘C, a disordered surface layer forms, albeit still with a (1×1) pattern in low-energy electron diffraction (LEED). Purely TiO2-terminated surfaces, prepared by wet-chemical treatment, are also disordered despite their (1×1) periodicity in LEED. DA - 2021/06/10/ PY - 2021 DO - 10.1103/PhysRevB.103.L241406 VL - 103 IS - 24 SP - L241406 J2 - Phys. Rev. B ST - Quest for a pristine unreconstructed ${\mathrm{SrTiO}}_{3}(001)$ surface ER - TY - JOUR TI - Ni-modified Fe3O4(001) surface as a simple model system for understanding the oxygen evolution reaction AU - Mirabella, Francesca AU - Müllner, Matthias AU - Touzalin, Thomas AU - Riva, Michele AU - Jakub, Zdenek AU - Kraushofer, Florian AU - Schmid, Michael AU - Koper, Marc T. M. AU - Parkinson, Gareth S. AU - Diebold, Ulrike T2 - Electrochimica Acta AB - Electrochemical water splitting is an environmentally friendly technology to store renewable energy in the form of chemical fuels. Among the earth-abundant first-row transition metal-based catalysts, mixed Ni-Fe oxides have shown promising performance for effective and low-cost catalysis of the oxygen evolution reaction (OER) in alkaline media, but the synergistic roles of Fe and Ni cations in the OER mechanism remain unclear. In this work, we report how addition of Ni changes the reactivity of a model iron oxide catalyst, based on Ni deposited on and incorporated in a magnetite Fe3O4(001) single crystal, using a combination of surface science techniques in ultra-high vacuum such as low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and scanning tunneling microscopy (STM), as well as atomic force microscopy (AFM) in air, and electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in alkaline media. A significant improvement in the OER activity is observed when the top surface presents an iron fraction among the cations in the range of 20-40%, which is in good agreement with what has been observed for powder catalysts. Furthermore, a decrease in the OER overpotential is observed following surface aging in electrolyte for three days. At higher Ni load, AFM shows the growth of a new phase attributed to an (oxy)-hydroxide phase which, according to CV measurements, does not seem to correlate with the surface activity towards OER. EIS suggests that the OER precursor species observed on the clean and Ni-modified surfaces are similar and Fe-centered, but form at lower overpotentials when the surface Fe:Ni ratio is optimized. We propose that the well-defined Fe3O4(001) surface can serve as a model system for understanding the OER mechanism and establishing the structure-reactivity relation on mixed Fe-Ni oxides. DA - 2021/09/01/ PY - 2021 DO - 10.1016/j.electacta.2021.138638 VL - 389 SP - 138638 J2 - Electrochim. Acta SN - 0013-4686 ER - TY - JOUR TI - Direct assessment of the acidity of individual surface hydroxyls AU - Wagner, Margareta AU - Meyer, Bernd AU - Setvin, Martin AU - Schmid, Michael AU - Diebold, Ulrike T2 - Nature AB - The state of deprotonation/protonation of surfaces has far-ranging implications in chemistry, from acid–base catalysis and the electrocatalytic and photocatalytic splitting of water, to the behaviour of minerals and biochemistry. An entity’s acidity is described by its proton affinity and its acid dissociation constant pKa (the negative logarithm of the equilibrium constant of the proton transfer reaction in solution). The acidity of individual sites is difficult to assess for solids, compared with molecules. For mineral surfaces, the acidity is estimated by semi-empirical concepts, such as bond-order valence sums, and increasingly modelled with first-principles molecular dynamics simulations. At present, such predictions cannot be tested—experimental measures, such as the point of zero charge, integrate over the whole surface or, in some cases, individual crystal facets. Here we assess the acidity of individual hydroxyl groups on In2O3(111)—a model oxide with four different types of surface oxygen atom. We probe the strength of their hydrogen bonds with the tip of a non-contact atomic force microscope and find quantitative agreement with density functional theory calculations. By relating the results to known proton affinities of gas-phase molecules, we determine the proton affinity of the different surface sites of In2O3 with atomic precision. Measurements on hydroxylated titanium dioxide and zirconium oxide extend our method to other oxides. DA - 2021/04// PY - 2021 DO - 10.1038/s41586-021-03432-3 VL - 592 IS - 7856 SP - 722 EP - 725 J2 - Nature SN - 1476-4687 ER - TY - JOUR TI - Polarons in materials AU - Franchini, Cesare AU - Reticcioli, Michele AU - Setvin, Martin AU - Diebold, Ulrike T2 - Nature Reviews Materials AB - Polarons are quasiparticles that easily form in polarizable materials due to the coupling of excess electrons or holes with ionic vibrations. These quasiparticles manifest themselves in many different ways and have a profound impact on materials properties and functionalities. Polarons have been the testing ground for the development of numerous theories, and their manifestations have been studied by many different experimental probes. This Review provides a map of the enormous amount of data and knowledge accumulated on polaron effects in materials, ranging from early studies and standard treatments to emerging experimental techniques and novel theoretical and computational approaches. DA - 2021/03/19/ PY - 2021 DO - 10.1038/s41578-021-00289-w VL - 6 SP - 560 EP - 586 J2 - Nat. Rev. Mater. SN - 2058-8437 ER - TY - JOUR TI - Resolving the structure of a well-ordered hydroxyl overlayer on In2O3(111): Nanomanipulation and theory AU - Wagner, Margareta AU - Lackner, Peter AU - Seiler, Steffen AU - Brunsch, Achim AU - Bliem, Roland AU - Gerhold, Stefan AU - Wang, Zhiming AU - Osiecki, Jacek AU - Schulte, Karina AU - Boatner, Lynn A. AU - Schmid, Michael AU - Meyer, Bernd AU - Diebold, Ulrike T2 - ACS Nano AB - Changes in chemical and physical properties resulting from water adsorption play an important role in the characterization and performance of device-relevant materials. Studies of model oxides with well-characterized surfaces can provide detailed information that is vital for a general understanding of water–oxide interactions. In this work, we study single crystals of indium oxide, the prototypical transparent contact material that is heavily used in a wide range of applications and most prominently in optoelectronic technologies. Water adsorbs dissociatively already at temperatures as low as 100 K, as confirmed by scanning tunneling microscopy (STM), photoelectron spectroscopy, and density functional theory. This dissociation takes place on lattice sites of the defect-free surface. While the In2O3(111)-(1 × 1) surface offers four types of surface oxygen atoms (12 atoms per unit cell in total), water dissociation happens exclusively at one of them together with a neighboring pair of 5-fold coordinated In atoms. These O–In groups are symmetrically arranged around the 6-fold coordinated In atoms at the surface. At room temperature, the In2O3(111) surface thus saturates at three dissociated water molecules per unit cell, leading to a well-ordered hydroxylated surface with (1 × 1) symmetry, where the three water OWH groups plus the surface OSH groups are imaged together as one bright triangle in STM. Manipulations with the STM tip by means of voltage pulses preferentially remove the H atom of one surface OSH group per triangle. The change in contrast due to strong local band bending provides insights into the internal structure of these bright triangles. The experimental results are further confirmed by quantitative simulations of the STM image corrugation. DA - 2017/11/28/ PY - 2017 DO - 10.1021/acsnano.7b06387 VL - 11 IS - 11 SP - 11531 EP - 11541 J2 - ACS Nano SN - 1936-0851 ST - Resolving the Structure of a Well-Ordered Hydroxyl Overlayer on In2O3(111) ER - TY - JOUR TI - Interplay between adsorbates and polarons: CO on rutile TiO2(110) AU - Reticcioli, Michele AU - Sokolović, Igor AU - Schmid, Michael AU - Diebold, Ulrike AU - Setvin, Martin AU - Franchini, Cesare T2 - Physical Review Letters AB - Polaron formation plays a major role in determining the structural, electrical, and chemical properties of ionic crystals. Using a combination of first-principles calculations, scanning tunneling microscopy, and atomic force microscopy, we analyze the interaction of polarons with CO molecules adsorbed on the reduced rutile TiO2(110) surface. Adsorbed CO shows attractive coupling with polarons in the surface layer, and repulsive interaction with polarons in the subsurface layer. As a result, CO adsorption depends on the reduction state of the sample. For slightly reduced surfaces, many adsorption configurations with comparable adsorption energies exist and polarons reside in the subsurface layer. At strongly reduced surfaces, two adsorption configurations dominate: either inside an oxygen vacancy, or at surface Ti5c sites, coupled with a surface polaron. Similar conclusions are predicted for TiO2(110) surfaces containing near-surface Ti interstitials. These results show that polarons are of primary importance for understanding the performance of polar semiconductors and transition metal oxides in catalysis and energy-related applications. DA - 2019/01/09/ PY - 2019 DO - 10.1103/PhysRevLett.122.016805 VL - 122 IS - 1 SP - 016805 J2 - Phys. Rev. Lett. ST - Interplay between Adsorbates and Polarons ER - TY - JOUR TI - Sexiphenyl on Cu(100): nc-AFM tip functionalization and identification AU - Wagner, Margareta AU - Setvín, Martin AU - Schmid, Michael AU - Diebold, Ulrike T2 - Surface Science T3 - Surface Structure and Dynamics – in Honor of Karl-Heinz Rieder AB - The contrast obtained in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images is determined by the tip termination and symmetry. Functionalizing the tip with a single metal atom, CO molecule or organic species has been shown to provide high spatial resolution and insights into tip-surface interactions. A topic where this concept is utilized is the adsorption of organic molecules at surfaces. With this work we aim to contribute to the growing database of organic molecules that allow assignment by intra-molecular imaging. We investigated the organic molecule para-sexiphenyl (C36H26, 6P) on Cu(100) using low-temperature STM and non-contact AFM with intra-molecular resolution. In the sub-monolayer regime we find a planar and flat adsorption with the 6P molecules rotated 10° off the〈010〉directions. In this configuration, four of six phenyl rings occupy almost equivalent sites on the surface. The 6P molecules are further investigated with CO-functionalized tips, in comparison to a single-atom metal and 6P-terminated tip. We also show that the procedure of using adsorbed CO to characterize tips introduced by Hofmann et al., Phys. Rev. B 112 (2014) 066101 is useful when the tip is terminated with an organic molecule. DA - 2018/12/01/ PY - 2018 DO - 10.1016/j.susc.2018.03.004 VL - 678 SP - 124 EP - 127 J2 - Surf. Sci. SN - 0039-6028 ST - Sexiphenyl on Cu(100) ER - TY - JOUR TI - The growth of metastable fcc Fe78Ni22 thin films on H-Si(100) substrates suitable for focused ion beam direct magnetic patterning AU - Gloss, Jonas AU - Horký, Michal AU - Křižáková, Viola AU - Flajšman, Lukáš AU - Schmid, Michael AU - Urbánek, Michal AU - Varga, Peter T2 - Applied Surface Science AB - We have studied the growth of metastable face-centered-cubic, non-magnetic Fe78Ni22 thin films on silicon substrates. These films undergo a magnetic (paramagnetic to ferromagnetic) and structural (fcc to bcc) phase transformation upon ion beam irradiation and thus can serve as a material for direct writing of magnetic nanostructures by the focused ion beam. So far, these films were prepared only on single-crystal Cu(100) substrates. We show that transformable Fe78Ni22 thin films can also be prepared on a hydrogen-terminated Si(100) with a 130-nm-thick Cu(100) buffer layer. The H-Si(100) substrates can be prepared by hydrofluoric acid etching or by annealing at 1200 °C followed by adsorption of atomic hydrogen. The Cu(100) buffer layer and Fe78Ni22 fcc metastable thin film were deposited by thermal evaporation in ultra-high vacuum. The films were consequently transformed in-situ by 4 keV Ar+ ion irradiation and ex-situ by a 30 keV Ga+ focused ion beam, and their magnetic properties were studied by magneto-optical Kerr effect magnetometry. The substitution of expensive copper single crystal substrate by standard silicon wafers dramatically expands application possibilities of metastable paramagnetic thin films for focused-ion-beam direct magnetic patterning. DA - 2019/03/01/ PY - 2019 DO - 10.1016/j.apsusc.2018.10.263 VL - 469 SP - 747 EP - 752 J2 - Appl. Surf. Sci. SN - 0169-4332 ER - TY - JOUR TI - Surface segregation and chemisorption of CO and oxygen on Pt25Ni75(111) studied by XPS and HREELS AU - Pantförder, A. AU - Skonieczny, J. AU - Janssen, E. AU - Meister, G. AU - Goldmann, A. AU - Varga, P. T2 - Surface Science AB - After sputtering and annealing to different temperatures TA the chemically disordered (111) surface of a Pt25Ni75 crystal exhibits a composition that depends strongly on TA. This has been already observed earlier by ion scattering and scanning tunneling microscopy [M. Schmid et al., Nucl. Instrum. Methods B 82 (1993) 259]. To complement this work, we have used angle-resolved X-ray photoelectron spectroscopy (XPS) to investigate the segregation behaviour a few layers below the surface. We have also used high-resolution electron energy loss spectroscopy (HREELS) to study adsorbate vibrations on differently prepared surfaces of Pt25Ni75(111) after exposure to O2 and CO at room temperature. DA - 1995/07/01/ PY - 1995 DO - 10.1016/0039-6028(95)00385-1 VL - 331-333 SP - 824 EP - 830 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Potential sputtering: desorption from insulator surfaces by impact of slow multicharged ions AU - Aumayr, F. AU - Varga, P. AU - Winter, H. P. T2 - International Journal of Mass Spectrometry AB - For certain target species, sputtering by slow multiply charged ions strongly depends on the primary ion charge state. In contrast to the usual ballistic sputtering it is not the kinetic but rather the potential energy stored in multiply charged ions that leads to this novel form of ion-induced sputtering (“potential sputtering”). A series of recent careful experiments is summarized where potential sputtering has been investigated for hyperthermal highly charged ions impact on Au, Si, GaAs, SiO2, MgO, LiF, and NaCl. Only for alkali halides (LiF, NaCl) and to some extent for SiO2 an enhancement of the total sputter yield, which is measured by a quartz crystal microbalance technique, with increasing charge state of the primary ion could be observed. All other targets showed only the common (collision induced) kinetic sputtering. With a defect mediated desorption model as known from electron stimulated desorption we can explain why potential sputtering is exclusively found for insulators with strong electron–phonon coupling. DA - 1999/09/27/ PY - 1999 DO - 10.1016/S1387-3806(99)00075-5 VL - 192 IS - 1 SP - 415 EP - 424 J2 - Int. J. Mass Spectrom. SN - 1387-3806 ST - Potential sputtering ER - TY - JOUR TI - Quenching surface states with the tip: STM scans on Fe(100) AU - Hofer, W. A. AU - Redinger, J. AU - Biedermann, A. AU - Varga, P. T2 - Surface Science AB - STM topographies and spectroscopies are compared with first principles simulations using the transfer-Hamiltonian method. We show that simulations agree with measurements in the range above 400 pm tip–sample separation, provided the tungsten tip is terminated by a (100) or (111) face. Measurements disagree with experiments for the (110) face. The approach fails to describe tunneling topographies in the range below 400 pm. It is argued that this failure is due to interactions, and it is demonstrated how quenching of surface states by the approaching tip accounts for the experimental features in this range. DA - 2001/06/20/ PY - 2001 DO - 10.1016/S0039-6028(01)00785-3 VL - 482-485 SP - 1113 EP - 1118 J2 - Surf. Sci. SN - 0039-6028 ST - Quenching surface states with the tip ER - TY - JOUR TI - Auger Electron Emission from Slow Multicharged Ions Near a Metal Surface AU - Delaunay, M. AU - Fehringer, M. AU - Geller, R. AU - Varga, P. AU - Winter, H. T2 - EPL (Europhysics Letters) AB - Electron energy spectra from multicharged ion (N 5+ , N 6+ , Ar 9+ ) impact on clean tungsten (impact energies ≥ 500 eV) revealed Auger electron emission from the projectile particles. The impact energy dependence of emission yield and kinetic-energy shift for these Auger electrons lead to a fairly detailed explanation of slow multicharged ion neutralization near metal surfaces. DA - 1987/// PY - 1987 DO - 10.1209/0295-5075/4/3/021 VL - 4 IS - 3 SP - 377 J2 - EPL SN - 0295-5075 ER - TY - JOUR TI - Neutralization and dissociative attachment in molecular ion-surface interaction: Scattering of N2+ and O2+ from Ni(111) AU - Schubert, S. AU - Neumann, J. AU - Imke, U. AU - Snowdon, K. J. AU - Varga, P. AU - Heiland, W. T2 - Surface Science AB - The influence of the symmetry of molecular states on the neutralization and dissociation of molecular ions is demonstrated experimentally for O2+ and N2+ scattered from Ni(111). DA - 1986/05/01/ PY - 1986 DO - 10.1016/0039-6028(86)90547-9 VL - 171 IS - 1 SP - L375 EP - L378 J2 - Surf. Sci. SN - 0039-6028 ST - Neutralization and dissociative attachment in molecular ion-surface interaction ER - TY - JOUR TI - Surface Depression of Series Limits in One-Electron Spectra AU - Neumann, J. AU - Schubert, S. AU - Imke, U. AU - Varga, P. AU - Heiland, W. T2 - EPL (Europhysics Letters) AB - The Balmer series of hydrogen was studied in the reflexion of hydrogen atoms from clean and cesiated nickel. We observe an enhancement of the line intensity with the decrease of the work function induced by the Cs over layer up to n = 7. For higher main quantum numbers n the series is depressed in comparison to the excitation from the clean Ni surface. DA - 1987/// PY - 1987 DO - 10.1209/0295-5075/3/7/014 VL - 3 IS - 7 SP - 859 J2 - Europhys. Lett. SN - 0295-5075 ER - TY - JOUR TI - Surface structures of ZrO2 films on Rh(111): From two layers to bulk termination AU - Lackner, Peter AU - Zou, Zhiyu AU - Mayr, Sabrina AU - Choi, Joong-Il Jake AU - Diebold, Ulrike AU - Schmid, Michael T2 - Surface Science AB - We have studied zirconia films on a Rh(111) substrate with thicknesses in the range of 2–10 monolayers (ML) using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Zirconia was deposited using a UHV-compatible sputter source, resulting in layer-by-layer growth and good uniformity of the films. For thicknesses of 2–4 ML, a layer-dependent influence of the substrate on the structure of the thin films is observed. Above this thickness, films show a (2 × 1) or a distorted (2 × 2) surface structure with respect to cubic ZrO2(111); these structures correspond to tetragonal and monoclinic zirconia, respectively. The tetragonal phase occurs for annealing temperatures of up to 730 °C; transformation to the thermodynamically stable monoclinic phase occurs after annealing at 850 °C or above. High-temperature annealing also breaks up the films and exposes the Rh(111) substrate. We argue that the tetragonal films are stabilized by the interface to the substrate and possibly oxygen deficiency, while the monoclinic films are only weakly defective and show band bending at defects and grain boundaries. This observation is in agreement with positive charge being responsible for the grain-boundary blocking effect in zirconia-based solid electrolytes. Our work introduces the tetragonal and monoclinic 5 ML-thick ZrO2 films on Rh(111) as a well-suited model system for surface-science studies on ZrO2, as they do not exhibit the charging problems of thicker films or the bulk material and show better homogeneity and stability than the previously-studied ZrO2/Pt(111) system. DA - 2019/01/01/ PY - 2019 DO - 10.1016/j.susc.2018.09.004 VL - 679 SP - 180 EP - 187 J2 - Surf. Sci. SN - 0039-6028 ST - Surface structures of ZrO2 films on Rh(111) ER - TY - JOUR TI - Adsorption of CO on the Ca3Ru2O7(001) surface AU - Mayr-Schmölzer, Wernfried AU - Halwidl, Daniel AU - Mittendorfer, Florian AU - Schmid, Michael AU - Diebold, Ulrike AU - Redinger, Josef T2 - Surface Science AB - The adsorption of CO molecules at the Ca3Ru2O7(001) surface was studied using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). Ca3Ru2O7 can be easily cleaved along the (001) plane, yielding a smooth, CaO-terminated surface. The STM shows a characteristic pattern with alternating dark and bright stripes, resulting from the tilting of the RuO6 octahedra. At 78 K, CO adsorbs at an apical surface O at the channel edge with a predicted binding energy of Eads=−0.85 eV. After annealing at room temperature, the CO forms a strong bond (Eads=−2.04 eV) with the apical O and the resulting carboxylate takes the place of the former surface O. This carboxylate can be decomposed by scanning the surface with a high sample bias voltage of +2.7 V, restoring the original surface. DA - 2019/02/01/ PY - 2019 DO - 10.1016/j.susc.2018.10.004 VL - 680 SP - 18 EP - 23 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Generation of high intensity electron beams from a duoplasmatron discharge AU - Varga, P. AU - Bruck, M. AU - Bruckmüller, R. T2 - Vacuum AB - High intensity electron beams are usually produced by thermionic cathodes. For applications where increased energy spread is tolerable the use of plasma cathodes can be advantageous. In the present paper the dependence of beam qualities on discharge parameters for a Duoplasmatron electron gun is investigated. With extraction potentials of 1–10 kV a maximum current density of 7 A cm−2 with a total current of 0.5 A can be produced. The measured microperveances are in the region of 0.5−3. DA - 1975/10// PY - 1975 DO - 10.1016/0042-207X(75)90488-1 VL - 25 IS - 9-10 SP - 421 EP - 424 J2 - Vacuum SN - 0042207X ER - TY - JOUR TI - Analysis of TiC and TiN coatings exposed to fusion plasmas AU - Taglauer, E. AU - Varga, P. AU - Ertl, K T2 - Vacuum AB - Coatings with elemental constituents of low atomic number are used for first wall surfaces and special structures in fusion plasma devices. Therefore TiC and TiN coatings on graphite and molybdenum were tested by exposing appropriate samples to the divertor plasma of the ASDEX tokamak in Garching. In parallèl experiments surface modifications under hydrogen ion bombardment were investigated. The coatings were examined by scanning electron microscopy and Auger electron spectroscopy together with Ar+ sputter depth profiling. The results show that at high heat fluxes the coatings can be partly removed from the substrate. There is also a considerable effect of preferential sputtering of C and N, which, in the case of plasma exposed samples, occurs simultaneously with deposition of metals, oxygen, sulphur and carbon contaminants from the plasma. The heat flux during neutral beam injection results in sample temperatures of about 1000°C and therefore segregation of carbon can also occur on graphite samples. DA - 1986/01/01/ PY - 1986 DO - 10.1016/0042-207X(86)90263-0 VL - 36 IS - 1 SP - 23 EP - 25 J2 - Vacuum SN - 0042-207X ER - TY - JOUR TI - Determination of cross-sections for CO desorption from Ni(111) induced by Ar-ions of very low impact energy AU - Diebold, U. AU - Varga, P. T2 - Vacuum AB - The determination of cross-sections for desorption fo CO adsorbed on a Ni(111) single crystal induced by argon ion bombardment is discussed. Experimental results are given for ions with kinetic energies ranging from 10 to 600 eV. The desorption cross-sections and the fractions of neutrals and ions desorbed from the surface were measured by means of a quadrupole mass spectrometer. In addition, the influence of the charge state of the impinging ion on the desorption cross-section has been studied. DA - 1990/// PY - 1990 DO - 10.1016/0042-207X(90)90312-M VL - 41 IS - 1-3 SP - 210 EP - 212 J2 - Vacuum SN - 0042-207X ER - TY - JOUR TI - The influence of environmental parameters on solid surface composition AU - Mezey, L. Z. AU - Giber, J. AU - Hofer, W. AU - Novacek, P. AU - Varga, P. T2 - Vacuum T3 - Surface Science Section AB - The chemical composition of the surface of a solid is determined by the following parameters: temperature, bulk chemical composition, oxygen partial pressure of the environment (or other parameters of surface chemical reactions), the time of equilibrium and the orientation of the surface. All these effects are to be described in the new MTCIP (Modern Thermodynamic Calculation of Interface Properties) theory-developed at present in a first approximation (MTCIP-1A). Varying the first three of the above mentioned parameters, experimental work has been done using Ion Scattering Spectroscopy, one of the few methods to investigate a solid's top-most atomic layer composition. Results on AuPd and on CuPd are shown and explained by theoretical calculations and considerations. DA - 1990/01/01/ PY - 1990 DO - 10.1016/S0042-207X(05)80149-6 VL - 41 IS - 1 SP - 453 EP - 456 J2 - Vacuum SN - 0042-207X ER - TY - JOUR TI - Surface composition of Cu-Pd binary alloy systems measured by low energy ion surface scattering AU - Novacek, P. AU - Liegl, A. AU - Borrell, M. AU - Hofer, W. AU - Varga, P. T2 - Vacuum T3 - Selected Proceedings of the Yugoslav-Austrian-Hungarian Fourth Joint Vacuum Conference AB - The surface composition of various copper-palladium alloy systems has been determined by means of ISS (Ion Surface Scattering). Four alloys with different bulk composition as well as the pure elements were examined. Cleaning the surface by sputtering with Ar ions results in Cu enrichment for Pd rich alloys and no or little enrichment of Pd for Cu rich alloys. Annealing the targets leads to Cu enrichment in all alloys investigated. Depth profiles of Cu35Pd65 show the altered layer several monolayers thick. DA - 1990/01/01/ PY - 1990 DO - 10.1016/0042-207X(90)90133-J VL - 40 IS - 1 SP - 113 EP - 114 J2 - Vacuum SN - 0042-207X ER - TY - JOUR TI - Modeling STM tips by single absorbed atoms on W(100) films: 5d transition metal atoms AU - Hofer, W. A. AU - Redinger, J. AU - Varga, P. T2 - Solid State Communications AB - The apex atom plays a key role in STM tunneling, in particular if subtle effects like atomic and chemical resolution on metal alloys are considered. In order to provide comprehensive data on the electronic structure of realistic tips we have calculated tungsten (100) films with single apex atoms by first principles molecular dynamics and full potential methods. Molecular dynamics using ultrasoft pseudo potentials has been used to determine the relaxation of the surface layers. The electronic structure of the relaxed film has been calculated with a first principles full potential method, which is most suitable to reproduce the subtle surface effects due to alloying. The calculations suggest that a correct result for the current and corrugation values in the perturbation approach can only be obtained by including the full electronic structure of the tip. DA - 1999/12/14/ PY - 1999 DO - 10.1016/S0038-1098(99)00489-5 VL - 113 IS - 5 SP - 245 EP - 250 J2 - Sol. State Commun. SN - 0038-1098 ST - Modeling STM tips by single absorbed atoms on W(100) films ER - TY - JOUR TI - Surface oxides on Pd(111): STM and density functional calculations AU - Klikovits, J. AU - Napetschnig, E. AU - Schmid, M. AU - Seriani, N. AU - Dubay, O. AU - Kresse, G. AU - Varga, P. T2 - Physical Review B AB - The formation of one-layer surface oxides on Pd(111) has been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). Besides the Pd5O4 structure determined previously, structural details of six different surface oxides on Pd(111) will be presented. These oxides are observed for preparation in oxygen-rich conditions, approaching the thermodynamic stability limit of the PdO bulk oxide at an oxygen chemical potential of -0.95 to -1.02 eV (570-605 K, 5x10(-4) mbar O-2). Sorted by increasing oxygen fraction in the primitive unit cell, the stoichiometry of the surface oxides is Pd5O4, Pd9O8, Pd20O18, Pd23O21, Pd19O18, Pd8O8, and Pd32O32. All structures are one-layer oxides, in which oxygen atoms form a rectangular lattice, and all structures follow the same rules of favorable alignment of the oxide layer on the Pd(111) substrate. DFT calculations were used to simulate STM images as well as to determine the stability of the surface oxide structures. Simulated and measured STM images are in excellent agreement, indicating that the structural models are correct. Since the newly found surface oxides are clearly less stable than Pd5O4, we conclude that Pd5O4 is the only thermodynamically stable phase, whereas all newly found structures are only kinetically stabilized. We also discuss possible mechanisms for the formation of these oxide structures. DA - 2007/07// PY - 2007 DO - 10.1103/PhysRevB.76.045405 VL - 76 IS - 4 SP - 045405 J2 - Phys. Rev. B SN - 1098-0121 ST - Surface oxides on Pd(111) ER - TY - JOUR TI - Pd, Co and Co-Pd clusters on the ordered alumina film on NiAl(110): Contact angle, surface structure and composition AU - Napetschnig, E. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - We have investigated the structure and morphology of Co and Pd clusters grown at room temperature on an alumina film on NiAl(110) by scanning tunneling microscopy, low energy ion scattering and Auger electron spectroscopy. We have also studied the clusters after annealing to 300 degrees C and Pd clusters deposited at 300 degrees C. Mixed Co-Pd clusters obtained by sequential deposition at room temperature were also studied. Pure Co deposited at room temperature forms a single type of clusters, most or all of them with close-packed planes parallel to the oxide surface. Their shape can be approximated by truncated spheres with a high contact angle of 115-125 degrees. These clusters are stable upon annealing up to 300 degrees C. Pd clusters deposited at room temperature grow in two different modes. At the reflection domain boundaries the clusters grow in their thermodynamically favorable shape. The clusters do not have a single crystallographic orientation and their shape can be approximated by a truncated sphere with a high contact angle of about 110 degrees, especially at very low coverages (below 0.05 ML). At the antiphase domain boundaries. the Pd clusters grow in (1 11) orientation and on some of them small (111) facets appear at their tops already at low coveraizes. For higher coverages of Pd, the majority of Pd clusters are rather flat with a large Pd(111) facet on top. The clusters' shape at the antiphase domain boundaries differs from the thermodynamically favorable one, due to kinetic limitations, especially at higher coverages. Annealing the Pd clusters to 300 degrees C leads to re-structuring of these Pd clusters. They transform into higher and more rounded clusters and a thin disordered alumina film is formed on top of the clusters. When Pd is deposited at 300 degrees C, about 16% of the Pd clusters have a steep slope and rounded tops. The rest of the Pd forms lower clusters, goes subsurface and is covered by a disordered alumina film. When Co and Pd are deposited sequentially, Pd covers the Co clusters forming a shell. The resulting mixed clusters are still truncated spheres with a lowered contact angle. For deposition in the reverse order (first Pd and then Co) we found that Co forms an alloy with Pd already at room temperature. DA - 2007/08/01/ PY - 2007 DO - 10.1016/j.susc.2007.05.047 VL - 601 IS - 15 SP - 3233 EP - 3245 J2 - Surf. Sci. SN - 0039-6028 ST - Pd, Co and Co-Pd clusters on the ordered alumina film on NiAl(110) ER - TY - JOUR TI - Nanotemplate with holes: Ultrathin alumina on Ni3Al(111) AU - Schmid, M. AU - Kresse, G. AU - Buchsbaum, A. AU - Napetschnig, E. AU - Gritschneder, S. AU - Reichling, M. AU - Varga, P. T2 - Physical Review Letters AB - We have determined the structure of the ultrathin (root 67 x root 67)R12.2 degrees aluminum oxide on Ni3Al(111) by a combination of scanning tunneling microscopy and density functional theory. In addition to other local defects, the main structural feature of the unit cell is a 0.4-nm-diameter hole reaching down to the metal substrate. Understanding the structure and metal growth on this oxide allows us to use it as a template for growing highly regular arrays of nanoparticles. DA - 2007/11/09/ PY - 2007 DO - 10.1103/PhysRevLett.99.196104 VL - 99 IS - 19 SP - 196104 J2 - Phys. Rev. Lett. SN - 0031-9007 ST - Nanotemplate with holes ER - TY - JOUR TI - Ultrathin alumina film on Cu-9at%Al(111) AU - Napetschnig, E. AU - M. Schmid AU - Varga, P. T2 - Surface Science AB - We have investigated the structure of the clean and the oxidized (111) surface of a Cu-Al alloy with 9 at% Al by scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), low energy ion scattering (LEIS) and low energy electron diffraction (LEED). Annealing of the clean crystal at 680 degrees C leads to segregation of Al to the surface. The Al concentration at the annealed surface is 23 +/- 2% and domains with a (root 3 x root 3)R30 degrees superstructure are visible, as well as small Cu(111) areas and disordered patches. Oxidation at 680 degrees C leads to the formation of a well-ordered flat alumina film with two very similar oxide structures. One oxide structure has a nearly commensurate rectangular cell rotated by 30 degrees with respect to a close-packed row of the substrate and grows in three different domains. The second structure has a commensurate cell consisting of four equivalent building blocks and has a rectangular centered symmetry. This structure is rotated by 18 degrees with respect to a close-packed row of the substrate and grows in six different domains. The rectangular building blocks of these two oxide structures have a similar thickness, the same surface termination and the same number and arrangement of the atoms as the oxide film on NiAl(110) [G. Kresse, M. Schmid, E. Napetschnig, M. Shishkin, L. Kohler, P. Varga, Science 308 (2005) 1448]. In contrast to the oxide on NiAl(110), alumina on the Cu-Al alloy crystal does not show stress-induced domain boundaries and grows in large defect-free domains. Thus, I'd deposited on this oxide nucleates not only on domain boundaries and steps but also on the unperturbed oxide, forming (111)-oriented clusters. DA - 2008/05/15/ PY - 2008 DO - 10.1016/j.susc.2008.02.040 VL - 602 IS - 10 SP - 1750 EP - 1756 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Imaging of near-field interference patterns by aperture-type SNOM – influence of illumination wavelength and polarization state AU - Dvořák, Petr AU - Édes, Zoltán AU - Kvapil, Michal AU - Šamořil, Tomáš AU - Ligmajer, Filip AU - Hrtoň, Martin AU - Kalousek, Radek AU - Křápek, Vlastimil AU - Dub, Petr AU - Spousta, Jiří AU - Varga, Peter AU - Šikola, Tomáš T2 - Optics Express AB - Scanning near-field optical microscopy (SNOM) in combination with interference structures is a powerful tool for imaging and analysis of surface plasmon polaritons (SPPs). However, the correct interpretation of SNOM images requires profound understanding of principles behind their formation. To study fundamental principles of SNOM imaging in detail, we performed spectroscopic measurements by an aperture-type SNOM setup equipped with a supercontinuum laser and a polarizer, which gave us all the degrees of freedom necessary for our investigation. The series of wavelength- and polarization-resolved measurements, together with results of numerical simulations, then allowed us to identify the role of individual near-field components in formation of SNOM images, and to show that the out-of-plane component generally dominates within a broad range of parameters explored in our study. Our results challenge the widespread notion that this component does not couple to the aperture-type SNOM probe and indicate that the issue of SNOM probe sensitivity towards the in-plane and out-of-plane near-field components – one of the most challenging tasks of near field interference SNOM measurements – is not yet fully resolved. DA - 2017/07/10/ PY - 2017 DO - 10.1364/OE.25.016560 VL - 25 IS - 14 SP - 16560 EP - 16573 J2 - Opt. Express, OE SN - 1094-4087 ER - TY - JOUR TI - Oxygen-induced surface phase transformation of Pd(111): sticking, adsorption and desorption kinetics AU - Klötzer, Bernhard AU - Hayek, Konrad AU - Konvicka, Christoph AU - Lundgren, Edvin AU - Varga, Peter T2 - Surface Science AB - Adsorption and desorption of oxygen on Pd(111) were studied by high-flux molecular beam adsorption, LEED, TDS and scanning tunnelling microscopy (STM) between 300 and 623 K sample temperature for oxygen coverages ΘO up to 1 ML. While adsorption below ΘO=0.25 is precursor mediated and proceeds without changes of the Pd substrate, it is activated for ΘO>0.25 and induces a massive change of the surface structure. STM reveals the formation of a new surface phase which consists of islands with a local oxygen coverage of 1 ML but less Pd atoms than the bulk-terminated (111) layer. Its formation and decay require activated mass transport of Pd and O atoms over mesoscopic distances. Due to island growth of this phase the oxygen sticking decreases linearly between ΘO=0.25 and 1 ML. For ΘO>0.25 ML the TPD rate maxima are shifted towards higher temperature with increasing initial coverage, indicating autocatalytic desorption kinetics. Desorption occurs preferentially from a dilute chemisorbed phase on Pd(111) terraces, with the islands of the high oxygen-density phase acting as a reservoir for O. The experimental TPD data can be well described by a simple mathematical model considering phase equilibrium during desorption. DA - 2001/06/20/ PY - 2001 DO - 10.1016/S0039-6028(01)00750-6 VL - 482-485 SP - 237 EP - 242 J2 - Surf. Sci. SN - 0039-6028 ST - Oxygen-induced surface phase transformation of Pd(111) ER - TY - JOUR TI - Adsorption kinetics and energetics of atomic hydrogen (deuterium) on oxygen and carbon covered V(100) AU - Krenn, G. AU - Eibl, C. AU - Mauritsch, W. AU - Hebenstreit, E. L. D. AU - Varga, P. AU - Winkler, A. T2 - Surface Science AB - The interaction of atomic and to some extent molecular hydrogen (deuterium) with oxygen and carbon covered V(100) surfaces has been studied. The oxygen and carbon coverage has been well characterized with respect to composition and structure by low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). Thermal desorption spectroscopy (TDS) has been used to investigate the kinetics of adsorption, absorption and desorption quantitatively. In addition, the activation barriers for the individual reaction steps during adsorption and desorption have been determined. We have focused on the advantage of using atomic hydrogen to enhance the uptake of hydrogen into the bulk of a contaminated vanadium sample. Surface, subsurface and bulk sites are occupied upon exposure to the atomic H (D) even at a surface temperature of 90K. These adsorption states exhibit desorption peaks at 150K (α-state), between 200 and 500K (β-states), and at 1000K (γ-state). The corresponding desorption energies are 0.36eV, 0.4–1.1eV, and 0.9eV, respectively. The kinetics of adsorption and absorption for molecular hydrogen depend strongly on the chemical composition of the surface, whereas the interaction of atomic hydrogen (deuterium) with V(100) is much less influenced by surface contaminants. DA - 2000/01/20/ PY - 2000 DO - 10.1016/S0039-6028(99)01080-8 VL - 445 IS - 2 SP - 343 EP - 357 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Interaction of H2, CO and O2 with a vanadium (111) surface AU - Beutl, M. AU - Lesnik, J. AU - Lundgren, E. AU - Konvicka, C. AU - Varga, P. AU - Rendulic, K. D. T2 - Surface Science AB - Adsorption of hydrogen and deuterium on V(111) is governed by dynamical steering at low molecular energies whereas at high beam energies direct adsorption is observed. Strong rotational effects and clear isotope effects in the adsorption dynamics of H2 and D2 can be seen. At elevated surface temperatures hydrogen dissolves in the bulk; the absorption coefficient is strongly dependent on surface temperature. For CO a large fraction dissociates upon adsorption. At the clean surface an intrinsic precursor facilitates adsorption and at finite coverages an extrinsic precursor leads to a sticking coefficient independent of coverage. Oxygen can adsorb up to a saturation coverage of 3.8 monolayers followed by surface oxidation. For all three gases calibration procedures for surface coverages are presented as well as quantitative values for the sticking coefficients. DA - 2000/02/20/ PY - 2000 DO - 10.1016/S0039-6028(99)01200-5 VL - 447 IS - 1 SP - 245 EP - 258 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Evidence against the "Coulomb explosion" model for desorption from insulator surfaces by slow highly charged ions AU - Aumayr, F. AU - Burgdorfer, J. AU - Hayderer, G. AU - Varga, P. AU - Winter, H. P. T2 - Physica Scripta AB - The "Coulomb explosion" model has been frequently invoked to explain that HCI induced sputtering- and secondary ion emission yields for certain (insulating) target surfaces dramatically increase with projectile charge state. Based on available experimental results and new theoretical estimates we will present evidence that "Coulomb explosion" can be ruled out as a dominant mechanism for potential sputtering of insulating surfaces, whereas all experimental features are found to be consistent with a model involving defect-mediated desorption. DA - 1999/// PY - 1999 DO - 10.1238/Physica.Topical.080a00240 VL - T80B SP - 240 EP - 242 J2 - Phys. Scr. SN - 0031-8949 ER - TY - JOUR TI - Surface composition of Pt25Ni75(111) probed by HREELS AU - Pantförder, A. AU - Skonieczny, J. AU - Janssen, E. AU - Meister, G. AU - Goldmann, A. AU - Varga, P. T2 - Surface Science T3 - Surface Reconstruction: Structure and Dynamics AB - High-resolution electron energy loss spectroscopy (HREELS) has been used to study vibrations of CO molecules chemisorbed at low coverage on a Pt25Ni75(111) sample after different surface treatments. In agreement with earlier work using other methods, we always observe that Pt is enriched in the outermost layer, with a chemical composition strongly dependent on the annealing temperature. We discuss to which extent HREELS of CO may be exploited to “titrate” the surface composition of binary alloys. DA - 1995/09/01/ PY - 1995 DO - 10.1016/0039-6028(95)00614-1 VL - 337 IS - 3 SP - 177 EP - 182 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Surface segregation of Pt10Ni90(110): experimental and theoretical results AU - Weigand, P. AU - Jelinek, B. AU - Hofer, W. AU - Varga, P. T2 - Fresenius' Journal of Analytical Chemistry AB - Results from low energy ion scattering spectroscopy of the surface composition of Pt10Ni90(110) are presented which — being performed at different scattering geometries — allow the derivation of the composition of the first and second layer. The PtNi system is an interesting example of an alternating segregation profile (reaching bulk concentration after a few layers) with an orientation-dependent change in the segregation component. Thus most theoretical descriptions fail. One of the very few successful models is a thorough thermodynamic description which will be used here in multilayer calculations. Good agreement between measurement and calculation is found, with strong Ni segregation in the topmost atomic layer and strong Pt enrichment in the second monolayer. DA - 1994/01/01/ PY - 1994 DO - 10.1007/BF00323271 VL - 349 IS - 1 SP - 199 EP - 201 J2 - Fresenius J. Anal. Chem. SN - 1432-1130 ST - Surface segregation of Pt10Ni90(110) ER - TY - JOUR TI - Pt25Ni75(100) and (110) single crystals: preferential sputtering and segregation reversal AU - Weigand, P. AU - Jelinek, B. AU - Hofer, W. AU - Varga, P. T2 - Surface Science AB - After annealing at 970 K low energy ion scattering (LEIS) reveals the topmost atomic layer of Pt25Ni75(100) to be enriched in Pt to a value of 39 at%, whereas for Pt25Ni75(110) the first monolayer is found to consist nearly exclusively of Ni atoms and the second layer shows strong Pt enrichment. LEIS measurements performed after ion bombardment at room temperature indicate the superposition of the effects of preferential sputtering of Ni atoms and segregation. Segregation is found to start at rather low annealing temperatures, whereas high annealing temperatures are necessary to allow thermodynamic equilibration by bulk diffusion. Most of the theoretical models fail to reproduce the orientation dependent change in the segregating component of the PtNi system. One of the few successful approaches is a thorough thermodynamic description which is used here to calculate the composition profile and to discuss the segregation behaviour. Good agreement between measurement and calculation is found. DA - 1994/04/20/ PY - 1994 DO - 10.1016/0039-6028(94)90429-4 VL - 307-309 SP - 416 EP - 421 J2 - Surf. Sci. SN - 0039-6028 ST - Pt25Ni75(100) and (110) single crystals ER - TY - JOUR TI - Preferential sputtering and segregation reversal: (100) and (110) surfaces of Pt25Ni75 single crystal alloys AU - Weigand, P. AU - Jelinek, B. AU - Hofer, W. AU - Varga, P. T2 - Surface Science AB - Low energy ion scattering (LEIS) and Auger electron spectroscopy (AES) are applied to determine the surface composition of Pt25Ni75(100) and Pt25Ni75(110). After annealing at 970 K the topmost atomic layer of Pt25Ni75(100) is observed to be enriched in Pt to a value of 39 at%, whereas the first monolayer of Pt25Ni75(110) is found to consist nearly exclusively of Ni atoms. The second layer of the (110) surface shows strong Pt enrichment. The bombardment with low energy ions at room temperature induces the preferential sputtering of Ni atoms. The LEIS results indicate the superposition of the effects of preferential sputtering and segregation. The dependence of the surface composition on the annealing temperature is rather similar for both Pt25Ni75 crystals under investigation though the segregating component differs. Segregation is found to occur at rather low annealing temperatures, whereas high annealing temperatures are necessary to allow thermodynamic equilibration by bulk diffusion. DA - 1994/01/10/ PY - 1994 DO - 10.1016/0039-6028(94)91310-2 VL - 301 IS - 1 SP - 306 EP - 312 J2 - Surf. Sci. SN - 0039-6028 ST - Preferential sputtering and segregation reversal ER - TY - JOUR TI - The drastic effect of oxygen on surface segregation AU - Mezey, L. Z. AU - Hofer, W. AU - Varga, P. AU - Giber, J. T2 - Surface and Interface Analysis AB - The temperature and oxygen partial pressure of the environment of a solid may produce non-oxidizing or oxidizing conditions for any of the components in the surface. Under non-oxidizing (free surface) conditions, one of the components of a binary alloy segregates. The oxidizing effect may enhance or diminish its surface content, and the other component may even segregate. This effect is investigated here using the experimental results (ISS and AES) and theoretical calculations of a new method. Agreement between theory and experiment is shown for AuPd and CuPd, and as well for S-contaminated CuFe. DA - 1990/07/01/ PY - 1990 DO - 10.1002/sia.7401601108 VL - 16 IS - 1-12 SP - 520 EP - 525 J2 - Surf. Interface Anal. SN - 1096-9918 ER - TY - JOUR TI - Investigation of Pt25Ni75(111): preferential sputtering and surface segregation AU - Weigand, P. AU - Hofer, W. AU - Varga, P. T2 - Surface Science AB - The surface composition of Pt25Ni75(111) has been determined by ion scattering spectroscopy and Auger electron spectroscopy. The surface layers are found to be enriched in Pt due to preferential sputtering. Thermal treatment induces further segregation of Pt at the topmost atomic layer. From the different sampling depths of ISS and AES composition profiles are deduced, which show a strong dependence on the temperature during thermal treatment. At an annealing temperature of 770 K the thermally induced Pt segregation is determined by the composition of the sputter induced altered layer being enriched in Pt. However, at an annealing temperature of 970 K the sputter induced altered layer has disappeared and the Pt segregation at the topmost atomic layer is determined by the bulk composition. DA - 1993/05/10/ PY - 1993 DO - 10.1016/0039-6028(93)90801-P VL - 287-288 SP - 350 EP - 354 J2 - Surf. Sci. SN - 0039-6028 ST - Investigation of Pt25Ni75(111) ER - TY - JOUR TI - Surface analysis of PtxNi1−x single crystals AU - Weigand, P. AU - Novacek, P. AU - van Husen, G. AU - Neidhart, T. AU - Varga, P. T2 - Surface Science AB - The surface composition of Pt10Ni90(100), Pt25Ni75(111) and Pt50Ni50(110) single crystal alloys has been determined by ion scattering spectroscopy (ISS) and ultraviolet photoelectron spectroscopy (UPS). Pt enrichment of the topmost layer due to preferential sputtering occurs independently of the surface orientation. Annealing the sputtered alloys results in face specific surface compositions: Pt enrichment for the (111) surface, Ni enrichment for the (110) surface and a composition close to the bulk value for the (100) surface. Exposure of the sputtered surfaces to oxygen induces segregation of Ni. Adsorbate induced segregation is correlated with segregation due to annealing. The determination of workfunctions by UPS results support the ISS results on surface composition. The experimental results on surface segregation are discussed with respect to thermodynamic calculations. DA - 1992/05/15/ PY - 1992 DO - 10.1016/0039-6028(92)91404-Y VL - 269-270 SP - 1129 EP - 1134 J2 - Surf. Sci. SN - 0039-6028 ER - TY - CHAP TI - Slow particle-induced electron emission from solid surfaces AU - Varga, P. AU - Winter, H. T2 - Particle Induced Electron Emission II A2 - Hasselkamp, Dietmar A2 - Rothard, Hermann A2 - Groeneveld, Karl-Ontjes A2 - Kemmler, Jürgen A2 - Varga, Peter A2 - Winter, Hannspeter T3 - Springer Tracts in Modern Physics CY - Berlin, Heidelberg DA - 1992/// PY - 1992 SP - 149 EP - 214 PB - Springer Berlin Heidelberg SN - 978-3-540-47447-0 ER - TY - JOUR TI - A comparison of two XPS methods for quantification of concentration profiles AU - Ebel, H. AU - Ebel, M. F. AU - Svagera, R. AU - Winklmayr, E. AU - Varga, P. T2 - Journal of Electron Spectroscopy and Related Phenomena AB - Binary AlLi alloys are known for well pronounced segregation at ambient temperatures. Literature provides a good knowledge of segregation behaviour of these alloys. An exponential concentration profile with depth can be assumed. We compare the concentration profiles obtained from SIMS measurements with results from XPS. SIMS measurements [M. Vonbank and P. Varga, Vak.-Tech., 37 (1988) 220.] performed on a specimen with 9.1 at.% Li after storage at room temperature for more than 48 h gave a surface composition of 50 at.% Li and a gradient of −20 at.% Li nm−1. We investigated the same specimen after heat treatment for 2 h 20 min at 150°C. For comparison, XPS with variable take-off angle gives 75 at.% Li and −17.8 at.% Li nm−1 assuming an exponential depth profile, 70 at.% Li and −9.9 at.% Li nm−1 with a linear response and our new imaging XPS method with a linear response gives 92 at.% Li and −15.4 at.% Li nm−1. The standard deviations of surface concentration and gradient are approximately 9 at.% Li and 8 at.% Li nm−1. The differences are due to: the statistical significance of measured data (8% relative error in case of variable take-off angle experiments and 25% for imaging experiments) especially due to weak Li1s signals; the necessity of a specimen transfer in ambient atmosphere; and uncertainties in sputter depths of about 10%. In principle the imaging method is similar to methods in which the surface is etched with a constant flux Ar ion beam and changes in the composition of the surface with time are monitored using XPS. DA - 1991/09/01/ PY - 1991 DO - 10.1016/0368-2048(91)85011-H VL - 57 IS - 1 SP - 15 EP - 32 J2 - J. Electron Spectrosc. Relat. Phenom. SN - 0368-2048 ER - TY - JOUR TI - The surface composition of binary substitutional alloys and its change caused by environmental oxygen AU - Mezey, L. Z. AU - Hofer, W. AU - Varga, P. AU - Giber, J. T2 - Surface Science AB - In view of the large practical importance of the subject, several valuable attempts were made for the theoretical description of the surface chemical composition of solids, especially during the last ten years. However, the problem is not solved so far even in the relatively most simple case of — qualitatively — just predicting, which of the components of a dilute binary substitutional alloy composed of transition metals segregates to its free surface in the thermodynamic equilibrium state. E.g. for Pt(Ni) in the polycrystalline form and for the (111) surface Ni segregation is predicted by most of the theories, while that of Pt was observed in all experiments. A new theory, the MTCIP-1A (Modern Thermodynamic Calculation of Interface Properties — First Approximation) is applied here for the description of binary substitutional alloys even in the general case of surface reactions with environmental (e.g. oxygen) atoms. It is shown, in agreement with our ISS (Ion Scattering Spectroscopy) measurements and with those of others, that for Pt(Ni) Pt segregates to the polycrystalline and the (111) free surface, while Ni to the oxidized one. Similar agreement with ISS and other experimental data is mentioned for AuPd, CuPd and CuFe. DA - 1991/07/01/ PY - 1991 DO - 10.1016/0039-6028(91)91105-7 VL - 251-252 SP - 819 EP - 824 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Eighth International Workshop on Inelastic Ion-Surface Collisions - Preface AU - Betz, G. AU - Varga, P. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms DA - 1991/06/02/ PY - 1991 DO - 10.1016/0168-583X(91)95856-9 VL - 58 IS - 3 SP - iii J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Oberflächensegregation an Au-Pd-Legierungen (Surface segregation on Au-Pd alloys) AU - Hetzendorf, G. AU - Varga, P. T2 - Fresenius' Zeitschrift für analytische Chemie AB - Measurements of the surface composition of Au-Pd binary alloys performed by Ion Scattering Spectroscopy (ISS) and X-ray Photoelectron-Spectroscopy (XPS) are presented. A set of 11 alloys with different bulk composition was used for these studies. Argon bombardment induced alternation of the surface composition due to preferential sputtering effects and surface segregation due to temperature excursions up to 500 °C have been studied. For quantitative evaluation of the ISS spectra pure metal standards and also calculated scattering cross sections were used. In general, little gold enrichment due to preferential sputtering and strong surface segregation of gold at elevated temperature was found by ISS measurements. The thickness of the altered layer is about 1 or 2 monolayers. DA - 1987/01/01/ PY - 1987 DO - 10.1007/BF00469166 VL - 329 IS - 2 SP - 332 EP - 334 J2 - Fresenius Z. Anal. Chem. SN - 1618-2650 ER - TY - JOUR TI - Determination of the surface segregation of alloys AU - Novacek, P. AU - Liegl, A. AU - Vonbank, M. AU - Borrell, M. AU - Hofer, W. AU - Varga, P. T2 - Fresenius' Zeitschrift für analytische Chemie AB - SummaryThe surface composition of CuPd- and Al-Li-alloys has been investigated by means of ISS and SIMS. Ion-induced and thermally caused segregation of Cu and Li, respectively, has been found. DA - 1989/01/01/ PY - 1989 DO - 10.1007/BF00572351 VL - 333 IS - 4 SP - 453 EP - 455 J2 - Fresenius Z. Anal. Chem. SN - 1618-2650 ER - TY - JOUR TI - The influence of environmental oxygen on the surface composition of binary substitutional alloys AU - Mezey, L. Z. AU - Hofer, W. AU - Novacek, P. AU - Varga, P. AU - Giber, J. T2 - Fresenius' Journal of Analytical Chemistry AB - SummaryThe chemical composition of free surfaces of solids (not interacting with the environmental atoms) depends on its bulk composition, on the temperature, on the time of annealing (in non-equilibrium states) and on surface orientation (in single crystals). Even in the relatively most simple case of “qualitatively” just predicting, which of the components of a dilute binary substitutional alloy composed of transitional metals segregates to its free surface, the present theories disagree with the experimental results in several cases. The problem, of course, gets even more complicated if the surface interacts with the environmental atoms (e.g. oxygen). Then environmental factors as e.g. the value of the oxygen partial pressure Po2 are as well among the variables determining the surface composition. A new theory, the MTCIP-1A (Modern Thermodynamic Calculation of Interface Properties — First Approximation) includes the description of that effect as well. Its applicability is shown here for a polycrystalline PtNi alloy with bulk content X(1)Ni=12 at % under non-oxidizing and oxidizing conditions. Our ISS (Ion Scattering Spectroscopy) results shown agree with the MTCIP-1A at both conditions (with Pt segregation in the first, while that of Ni in the second one). Some further examples of such agreement between theory and experiment are mentioned. DA - 1991/05/01/ PY - 1991 DO - 10.1007/BF00321941 VL - 341 IS - 5 SP - 383 EP - 386 J2 - Fresenius J. Anal. Chem. SN - 1432-1130 ER - TY - JOUR TI - Analysis of the surface of PtxNi1−x alloys AU - Novacek, P. AU - Taglauer, E. AU - Varga, P. T2 - Fresenius' Journal of Analytical Chemistry AB - SummaryThe surface composition of three polycrystalline PtxNi1−x alloys (metal foils) was determined by ion scattering spectroscopy (ISS) with He ions of an energy of 1 keV. The targets have been pretreated by bombarding with rare-gas ions (He, Ar and Xe), by annealing at 500°C and by exposing them to increasing amounts of oxygen (up to 20 Langmuir). The results obtained from the polycrystalline alloys are compared to results from a Pt10Ni90 silica supported catalyst. The surface composition was also measured by ISS. The catalyst was pretreated by bombarding with He ions and by exposing it to 10 L oxygen. DA - 1991/01/01/ PY - 1991 DO - 10.1007/BF00322124 VL - 341 IS - 1 SP - 136 EP - 139 J2 - Fresenius J. Anal. Chem. SN - 1432-1130 ER - TY - JOUR TI - Studies on Au-Pd alloys: Determination of the surface composition by ISS and XPS AU - Varga, P. AU - Hetzendorf, G. T2 - Surface Science AB - The surface composition of various gold-palladium alloy systems has been determined by means of ISS (Ion Surface Scattering) and XPS (X-ray Photoelectron Spectroscopy). Besides the pure elements, 8 alloys with different bulk composition were examined. After cleaning the targets by sputtering with Ar ions, Au enrichment at the surface could be detected by ISS, whereas with XPS no difference between the surface and bulk compositions could be found. DA - 1985/10/03/ PY - 1985 DO - 10.1016/0039-6028(85)90946-X VL - 162 IS - 1 SP - 544 EP - 549 J2 - Surf. Sci. SN - 0039-6028 ST - Studies on AuPd alloys ER - TY - JOUR TI - Potential emission from tungsten due to impact of slow metastable ions AU - Hofer, W. AU - Vanek, W. AU - Varga, P. AU - Winter, H. T2 - Surface Science AB - We present energy distributions of secondary electrons emitted from polycrystalline clean and gas-covered tungsten due to impact of slow singly-charged metastable noble gas ions (10–100 eV). The ion de-excitation and neutralization processes are discussed by comparing our experimental results with computed ejected-electron energy distributions, using the semi-empirical theory of Hagstrum. Since the potential energy of impinging ions strongly influences the electronic interactions, the ejected-electron energy distribution yields information on the respective initial ion states. This has also been applied for determination of metastable ion beam fractions for some standard ion sources. DA - 1983/03/02/ PY - 1983 DO - 10.1016/0039-6028(83)90764-1 VL - 126 IS - 1 SP - 605 EP - 609 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Preferential sputtering of compounds due to light ion bombardment AU - Varga, P. AU - Taglauer, E. T2 - Journal of Nuclear Materials AB - Surface composition changes of compound materials due to light ion (He, H) bombardment have been studied using Auger Electron Spectroscopy (AES) and Ion Scattering Spectroscopy (ISS). The compounds investigated are tantalum carbide and also materials which are considered for coatings of wall and limiter structures, such as TiC and TiB2. In all cases depletion of the light constituent is found. The study includes in particular the depth of the altered layer obtained from depth profiling by argon bombardment. This depth is measured for projectile energies between 300 eV and 4 keV and compared to the mean range of the projectiles. Annealing at elevated temperatures partly restores the initial surface composition. DA - 1982/11/01/ PY - 1982 DO - 10.1016/0022-3115(82)90296-3 VL - 111-112 SP - 726 EP - 731 J2 - J. Nucl. Mater. SN - 0022-3115 ER - TY - JOUR TI - Local Electronic and Chemical Structure of Oligo-acetylene Derivatives Formed Through Radical Cyclizations at a Surface AU - Riss, Alexander AU - Wickenburg, Sebastian AU - Gorman, Patrick AU - Tan, Liang Z. AU - Tsai, Hsin-Zon AU - de Oteyza, Dimas G. AU - Chen, Yen-Chia AU - Bradley, Aaron J. AU - Ugeda, Miguel M. AU - Etkin, Grisha AU - Louie, Steven G. AU - Fischer, Felix R. AU - Crommie, Michael F. T2 - Nano Letters AB - Semiconducting π-conjugated polymers have attracted significant interest for applications in light-emitting diodes, field-effect transistors, photovoltaics, and nonlinear optoelectronic devices. Central to the success of these functional organic materials is the facile tunability of their electrical, optical, and magnetic properties along with easy processability and the outstanding mechanical properties associated with polymeric structures. In this work we characterize the chemical and electronic structure of individual chains of oligo-(E)-1,1?-bi(indenylidene), a polyacetylene derivative that we have obtained through cooperative C1?C5 thermal enediyne cyclizations on Au(111) surfaces followed by a step-growth polymerization of the (E)-1,1?-bi(indenylidene) diradical intermediates. We have determined the combined structural and electronic properties of this class of oligomers by characterizing the atomically precise chemical structure of individual monomer building blocks and oligomer chains (via noncontact atomic force microscopy (nc-AFM)), as well as by imaging their localized and extended molecular orbitals (via scanning tunneling microscopy and spectroscopy (STM/STS)). Our combined structural and electronic measurements reveal that the energy associated with extended π-conjugated states in these oligomers is significantly lower than the energy of the corresponding localized monomer orbitals, consistent with theoretical predictions. DA - 2014/05/14/ PY - 2014 DO - 10.1021/nl403791q VL - 14 IS - 5 SP - 2251 EP - 2255 J2 - Nano Lett. SN - 1530-6984 ER - TY - JOUR TI - Ultrathin metal film growth on TiO2(110): an overview AU - Diebold, Ulrike AU - Pan, Jian-Mei AU - Madey, Theodore E. T2 - Surface Science AB - The interface between an oxide substrate and a metal overlayer may crucially influence the macroscopic behavior of technological devices such as sensors, catalysts, ceramics, and semiconductor chips. Hence investigations of adsorption, nucleation and growth of ultrathin metal films on metal oxide surfaces have attracted increasing interest during recent years. Experimental results on the growth of metal overlayers on a model oxide, TiO2(110), are reviewed. The emphasis is on the very initial stages of overlayer growth and on extracting general trends on metal/metal-oxide interaction by comparing results from different metal overlayers. The electronic and geometric structure, growth mode, interface formation, and thermal stability of metal films are discussed. The strength of the oxidation/reduction reaction at the interface, the wetting ability and the tendency to form a disordered layer are all related to the reactivity of the overlayer metal towards oxygen. We propose that `reactive adsorption' accounts for the observed trends. DA - 1995/07/01/ PY - 1995 DO - 10.1016/0039-6028(95)00124-7 VL - 331-333 IS - Part 2 SP - 845 EP - 854 J2 - Surf. Sci. SN - 0039-6028 ST - Ultrathin metal film growth on TiO2(110) ER - TY - JOUR TI - Ion-beam-induced magnetic transformation of CO-stabilized fcc Fe films on Cu(100) AU - Shah Zaman, Sameena AU - Oßmer, Hinnerk AU - Jonner, Jakub AU - Novotný, Zbyněk AU - Buchsbaum, Andreas AU - Schmid, Michael AU - Varga, Peter T2 - Physical Review B AB - We have grown 22-ML-thick Fe films on a Cu(100) single crystal. The films were stabilized in the face-centered-cubic (fcc) γ phase by adsorption of carbon monoxide during growth, preventing the transformation to the body-centered-cubic (bcc) α phase. A structural transformation of these films from fcc to bcc can be induced by Ar+ ion irradiation. Scanning-tunneling microscopy images show the nucleation of bcc crystallites, which grow with increasing Ar+ ion dose and eventually result in complete transformation of the film to bcc. Surface magneto-optic Kerr effect measurements confirm the transformation of the Fe film from paramagnetic (fcc) to ferromagnetic (bcc) with an in-plane easy axis. The transformation can also be observed by low-energy electron diffraction. We find only very few nucleation sites of the bcc phase and argue that nucleation of the bcc phase happens under special circumstances during resolidification of the molten iron in the thermal spike after ion impact. Intermixing with the Cu substrate impedes the transformation. We also demonstrate the transformation of films coated with Au to protect them from oxidation at ambient conditions. DA - 2010/12/01/ PY - 2010 DO - 10.1103/PhysRevB.82.235401 VL - 82 IS - 23 SP - 235401 J2 - Phys. Rev. B ER - TY - JOUR TI - Antiphase domain boundaries at the Fe3O4(001) surface AU - Parkinson, Gareth S. AU - Manz, Thomas A. AU - Novotný, Zbyněk AU - Sprunger, Phillip T. AU - Kurtz, Richard L. AU - Schmid, Michael AU - Sholl, David S. AU - Diebold, Ulrike T2 - Physical Review B AB - Antiphase domain boundaries (APDBs) in the (√2×√2)R45∘ reconstruction of the Fe3O4(001) surface were investigated using scanning tunneling microscopy (STM) and density functional theory [(DFT) + U] calculations. The equilibrium structure of the APDBs is interpreted in terms of the distorted B-layer model for the (√2×√2)R45∘ reconstruction in which a lattice distortion couples to charge order in the subsurface layers. The APDBs are observed after prolonged annealing at 700 °C, indicating that they are extremely stable. DFT + U calculations reveal that the APDB structure is linked to a disruption in the subsurface charge-order pattern, leading to an enrichment of Fe2+ cations at the APDB. Simulated STM images reproduce the appearance of the APDBs in the experimental data and reveal that they are preferential adsorption sites for hydrogen atoms. DA - 2012/05/24/ PY - 2012 DO - 10.1103/PhysRevB.85.195450 VL - 85 IS - 19 SP - 195450 J2 - Phys. Rev. B ER - TY - JOUR TI - Surface preparation of TiO2 anatase (101): Pitfalls and how to avoid them AU - Setvín, Martin AU - Daniel, Benjamin AU - Mansfeldova, Vera AU - Kavan, Ladislav AU - Scheiber, Philipp AU - Fidler, Martin AU - Schmid, Michael AU - Diebold, Ulrike T2 - Surface Science AB - TiO2 anatase is a material of high technological importance, yet studies on well-defined model surfaces are scarce. The main impediment to fundamental research of this material is the lack of high-purity single crystals of a sufficient size. Natural anatase crystals always contain impurities, while synthetic crystals are pure but usually very small. We discuss optimum surface preparation procedures that result in clean surfaces under UHV conditions and the best ways to mount small crystals on sample plates. The influence of bulk impurities on surface preparation is discussed. The most troublesome impurity in natural TiO2 crystals is Fe. Upon annealing the crystal in partial O2 pressure Fe segregates, resulting in overgrowth of iron oxide on the surface. Based on the temperature-dependence of O-induced Fe segregation, optimum sample treatment procedures are proposed. Finally, we show that the surface roughness on the anatase (101) surface increases with the number of sputter-anneal cycles. Possible reasons and ways to revert this process are described. DA - 2014/08// PY - 2014 DO - 10.1016/j.susc.2014.04.001 VL - 626 SP - 61 EP - 67 J2 - Surf. Sci. SN - 0039-6028 ST - Surface preparation of TiO2 anatase (101) ER - TY - JOUR TI - Hydrogen bonding controls the dynamics of catechol adsorbed on a TiO2(110) surface AU - Li, Shao-Chun AU - Chu, Li-Na AU - Gong, Xue-Qing AU - Diebold, Ulrike T2 - Science AB - Direct studies of how organic molecules diffuse on metal oxide surfaces can provide insights into catalysis and molecular assembly processes. We studied individual catechol molecules, C6H4(OH)2, on a rutile TiO2(110) surface with scanning tunneling microscopy. Surface hydroxyls enhanced the diffusivity of adsorbed catecholates. The capture and release of a proton caused individual molecules to switch between mobile and immobile states within a measurement period of minutes. Density functional theory calculations showed that the transfer of hydrogen from surface hydroxyls to the molecule and its interaction with surface hydroxyls substantially lowered the activation barrier for rotational motion across the surface. Hydrogen bonding can play an essential role in the initial stages of the dynamics of molecular assembly. DA - 2010/05/14/ PY - 2010 DO - 10.1126/science.1188328 VL - 328 IS - 5980 SP - 882 EP - 884 J2 - Science ER - TY - JOUR TI - Segregated carbon on Pt10Ni90(100) studied by scanning tunneling microscopy AU - Schmid, M. AU - Biedermann, A. AU - Varga, P. T2 - Surface Science AB - Scanning tunneling microscopy is used to study the arrangement of segregated carbon atoms with atomic resolution. Individual carbon atoms are visible only under special tip conditions, while they normally do not directly appear on STM topographs. Under all tip conditions, carbon atoms affect the corrugation of their metal neighbours, reducing the apparent height by 20 to 40 pm in the p(2 × 2) and 40 to 70 pm in the c(2 × 2) superstructure. Therefore the existence, structure and amount of carbon can be also derived from images without directly visible carbon atoms. A substrate lattice distortion in regions of the carbon c(2 × 2) superstructure was observed, exhibiting areas of the p4g structure known from earlier LEED studies of Ni(100). DA - 1993/09/10/ PY - 1993 DO - 10.1016/0039-6028(93)90103-Q VL - 294 IS - 3 SP - L952 EP - L958 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Subsurface islands and superstructures of Cu on Pb(111) AU - Nagl, C. AU - Platzgummer, E. AU - Schmid, M. AU - Varga, P. AU - Speller, S. AU - Heiland, W. T2 - Surface Science AB - A new growth mode, namely subsurface island growth, has been identified by means of scanning tunneling microscopy. When Cu is deposited on Pb(111), 3D-Cu islands with a thickness between 1 and 11 layers are immersed several layers deep into the Pb substrate. The Cu islands, which can be classified in four different types, are furthermore covered by a monatomic Pb film. On flat Cu islands, this Pb film shows a corrugation that is strongly dependent on the number of Cu layers beneath. Single Cu layers were found to form a new type of Cu-Pb superstructure islands. DA - 1996/05/15/ PY - 1996 DO - 10.1016/0039-6028(95)01200-1 VL - 352-354 SP - 540 EP - 545 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Segregation of impurities on Cr(100) studied by AES and STM AU - Schmid, M. AU - Pinczolits, M. AU - Hebenstreit, W. AU - Varga, P. T2 - Surface Science AB - With increasing annealing temperature, Auger electron spectroscopy (AES) of a Cr(100) single crystal shows segregation of C, N and O as the dominating segregating species, indicating competitive segregation of these elements. An STM study of N structures shows a c(2 × 2) superstructure at N coverages up to 1/2. The local N coverage can be increased by insertion of N-rich domain boundaries up to 2/3, where a c(3√2 × √2)R ± 45° structure forms, followed by a first-order phase transformation to a p(1 × 1) structure. The existence of patches of the N-rich p(1 × 1) structure at coverages below 2/3 is probably due to additional carbon impurities stabilizing this structure. The possibility of inverse corrugation on the pure Cr(100) surface is discussed. DA - 1997/04/20/ PY - 1997 DO - 10.1016/S0039-6028(96)01539-7 VL - 377-379 SP - 1023 EP - 1027 J2 - Surf. Sci. SN - 0039-6028 ST - Impurity segregation on Cr(100) ER - TY - JOUR TI - Doping and functionalization of photoactive semiconducting metal oxides AU - Di Valentin, Cristiana AU - Diebold, Ulrike AU - Selloni, Annabella T2 - Chemical Physics AB - Editorial DA - 2007/10/15/ PY - 2007 DO - 10.1016/j.chemphys.2007.09.022 VL - 339 IS - 1-3 SP - vii EP - viii J2 - Chem. Phys. SN - 0301-0104 ER - TY - CHAP TI - Desorption and secondary ion production during bombardment of CO/Ni(111) with Ne+ and Ne++ ions at very low impact energies AU - Diebold, U AU - Varga, P. T2 - Desorption Induced by Electronic Transitions Diet IV A2 - Betz, G. A2 - Varga, P. T3 - Springer Series in Surface Sciences AB - We have studied ion - induced desorption at very low impact energies. Singly and doubly charged Ne ions with impact energies E0 between 20 eV and 500 eV have been used. The desorbing species (neutrals and ions) have been analysed by means of a quadrupole mass spectrometer. The desorption cross section sigma_CO for neutral CO which is desorbed predominantly was determined for different impact energies and impact angles. When using doubly charged Ne ions instead of singly charged ones, no difference in the desorption cross section for neutral CO could be found. For impact energies below 100 eV the secondary ion yield of CO+ and Ni+ was found to be higher for bombardment with Ne++ than for Ne+. As an explanation, we propose an additional, new mechanism for secondary ion production; the particle emission is caused by momentum transfer during binary collisions, accompanied by ionization via electron capture by the primary ion from the sputtered particle. CY - Berlin DA - 1991/// PY - 1991 VL - 19 SP - 193 EP - 203 PB - Springer-Verlag SN - 0-387-52386-3 ER - TY - JOUR TI - Novel Stabilization Mechanism on Polar Surfaces: ZnO(0001)-Zn AU - Dulub, Olga AU - Diebold, Ulrike AU - Kresse, G. T2 - Physical Review Letters AB - The (1×1) terminated (0001)-Zn surface of wurtzite ZnO was investigated with scanning tunneling microscopy. The surface is characterized by the presence of nanosized islands with a size-dependent shape and triangular holes with single-height, O-terminated step edges. It is proposed that the resulting overall decrease of the surface Zn concentration stabilizes this polar surface. Ab initio calculations of test geometries predict triangularly shaped reconstructions over a wide range of oxygen and hydrogen chemical potentials. The formation of these reconstructions appears to be electrostatically driven. DA - 2003/01/08/ PY - 2003 DO - 10.1103/PhysRevLett.90.016102 VL - 90 IS - 1 SP - 016102 J2 - Phys. Rev. Lett. ST - Novel Stabilization Mechanism on Polar Surfaces ER - TY - JOUR TI - Current-controlled channel switching and magnetoresistance in an Fe3C island film supported on a Si substrate AU - Tang, Jinke AU - Dai, Jianbiao AU - Wang, Kaiying AU - Zhou, Weilie AU - Ruzycki, Nancy AU - Diebold, Ulrike T2 - Journal of Applied Physics AB - A film of magnetic Fe3C islands separated by nanochannels of graphite was prepared with pulsed laser deposition on a Si substrate with a native SiO2 surface. When the temperature is increased above 250 K the resistance suddenly drops because electron conduction switches from the film to the Si inversion layer underneath. The film shows a negative magnetoresistance. The inversion layer exhibits a large positive magnetoresistance. The transition to the low resistance channel can be reversed by applying a large measuring current, making possible current-controlled switching between two types of electron magnetotransport at room temperature. DA - 2002/05/15/ PY - 2002 DO - 10.1063/1.1447880 VL - 91 SP - 8411 EP - 8413 J2 - J. Appl. Phys. ER - TY - JOUR TI - Curve-crossing analysis for potential sputtering of insulators AU - Wirtz, L. AU - Hayderer, G. AU - Lemell, C. AU - Burgdörfer, J. AU - Hägg, L. AU - Reinhold, C.O. AU - Varga, P. AU - Winter, P. AU - Aumayr, F. T2 - Surface Science AB - We develop a theoretical model for the recently observed threshold for potential sputtering of LiF by slow singly and doubly charged ions. The threshold coincides with the potential energy to create a cold hole in the valence band of LiF by resonant neutralization. We calculate the level shift of the incident ion and the deformation of the valence band under the influence of the projectile. Resonant neutralization becomes possible for ions with recombination energies larger than 10 eV in agreement with the experimental findings. DA - 2000/04/20/ PY - 2000 DO - 10.1016/S0039-6028(00)00027-3 VL - 451 IS - 1-3 SP - 197 EP - 202 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Electronic effects in low-energy ion sputtering of LiF AU - Varga, P. AU - Diebold, U. AU - Wutte, D. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - Sputtering of a LiF(100) surface by singly (He and Ar) and doubly charged rare gas ions (Ar) with impact energies between 20 and 500 eV has been performed. The emission of Li+, Li0, F+ and F0 was measured by means of a quadrupole mass spectrometer.The yield of sputtered Li+ ions and Li0 atoms decreases with decreasing impact energy and becomes slightly higher for Ar++ compared to Ar+ at impact energies below 100 eV. The situation changes very drastically when looking at the emitted F particles. In the whole energy range investigated, the F+ and F0 yield was more than one magnitude larger for Ar++ as projectiles than for Ar+. Furthermore, for He+ bombardment a higher yield was observed than for Ar+. Therefore an electronic effect has to be assumed which causes enhancement of the sputtering of F+ if projectiles with larger potential energy are used. However, for He+ below 100 eV impact energy, a drastic decrease for the F+ yield was observe-which indicates that F+ emission cannot be due to a pure electronic effect. Our proposed model includes formation of F+ by Auger neutralization (if energetically possible) of projectiles in front of the target, and emission of the F+ ion due to momentum transfer in the collision with the already neutralized projectile. DA - 1991/06/02/ PY - 1991 DO - 10.1016/0168-583X(91)95879-I VL - 58 IS - 3-4 SP - 417 EP - 421 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Radiation-induced decomposition of PF3 on Ru(0001) AU - Tao, H.-S. AU - Diebold, U. AU - Chakarian, V. AU - Shuh, D. K. AU - Yarmoff, J. A. AU - Shinn, N. D. AU - Madey, T. E. T2 - Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films AB - Soft x-ray photoelectron spectroscopy using synchrotron radiation has been employed to study the decomposition of PF3 on Ru(0001) at 80 K induced by energetic electrons. Due to the large binding energy shifts in the P 2p core levels, the phosphorus-containing surface intermediates produced from the electron-induced decomposition of PF3 can be identified and their evolution can be followed as the fluence of electrons is changed. A stepwise decomposition of PF3 induced by energetic electrons, i.e., PF3 --> PF2 --> PF --> P, is observed. The dissociation cross section for PF3 --> PF2 induced by 550 eV electrons is measured to be on the order of ~ 1 × 10 –16 cm2. The main channels that lead to PF3 dissociation involve the excitation of F 2s as well as valence states of PF3. DA - 1995/// PY - 1995 DO - 10.1116/1.579502 VL - 13 IS - 5 SP - 2553 EP - 2557 J2 - J. Vac. Sci. Technol. A ER - TY - CHAP TI - Sputtering of metals and insulators with hyperthermal singly and doubly charged rare gas ions AU - Varga, P. AU - Diebold, U. T2 - Low Energy Ion-Surface Interactions A2 - Rabalais, J. Wayne T3 - Wiley Series in Ion Chemistry and Physics AB - The influence of the potential energy of singly and doubly charged rare gas ions (of He, Ne, Ar and Kr) on desorption, sputtering and secondary ion formation process has been studied. By using very low impact energy (10 - 500 eV), the dependence of particle emission from metals and insulators on the charge state of the primary ions can be demonstrated. The measurements have been performed using a CO covered Ni(111) surface and a LiF(100) singly crystal or LiF thin film, respectively. For sputtering of the CO covered metal surface, the positive secondary ion yields (Ni+ and CO+) are slightly higher when using doubly charged rare gas ions instead of singly charged, but only at impact energies below 200 eV. No dependence of the primary ions charge state on the sputtering yields of neutrals was observed. The enhancement in the secondary ion yield is explained by sputtering and simultaneous charge exchange in a close collision between a primary ion and a target particle. Sputtering of the LiF surface with low energetic rare gas ions has been studied as an example for an insulator surface. Extensive experiments have been performed to exclude effects caused by charging up of the surface during ion bombardment; it was found that the best method for charge neutralization was to heat the LiF samples up to 400°C. For ejection of Li+ and F- only a small influence of the charge state of the projectile has been observed at impact energies below 100 eV whereas at higher energy no effect was seen. In contrast to this it has been shown, that the sputtering of F+ strongly increases with the potential energy of the projectile. However, the F+ yield also depends on the primary energy of the projectile and a threshold energy for F+ emission is observed. This indicates that the potential energy of the incident particle alone is not sufficient for F+ emission; a certain momentum transfer is necessary for sputtering. The observed behaviour can be explained by a combination of an Auger neutralization event followed by a collisional process. F+ is formed by Auger neutralization between the lattice F- ion and the incoming projectile provided that the potential energy of the projectile exceeds at least two times the energy of the band gap of the LiF surface. The so formed F+ ion is in a weakly bound state and will be sputtered by the incoming projectile. CY - Chichester DA - 1994/04// PY - 1994 VL - 3 SP - 355 EP - 386 PB - Wiley SN - 0-471-93891-2 ER - TY - CHAP TI - Ionization of LiF by hyperthermal multiply charged ions AU - Neidhart, T. AU - Schmid, M. AU - Varga, P. T2 - Ionization of Solids by Heavy Particles A2 - Baragiola, Raul A. T3 - NATO ASI Series AB - Sputtering of a LiF thin film (0.25 µm) surface by singly- (He+, Ne+ and Ar+) and doubly-charged (Ne++ and Ar++) ions with impact energies between 10 and 500 eV has been performed. The yield of sputtered Li+ and F- ions is only slightly higher for doubly-charged ions compared to singly-charged projectiles at impact energies below 100 eV, whereas the F+ yield is substantially increased if doubly-charged projectiles are used. The experimental data are explained within the framework of a model based on calculations by Walkup and Avouris. A comparison is given with former measurements on a LiF single crystal. CY - New York DA - 1993/08/31/ PY - 1993 ET - 1 VL - B 306 SP - 447 EP - 453 PB - Plenum SN - 0-306-44489-5 ER - TY - CHAP TI - Desorption from LiF(100) by singly- and doubly-charged hyperthermal He ions AU - Neidhart, T. AU - Schmid, M AU - Varga, P. T2 - Desorption Induced by Electronic Transitions DIET V A2 - Burns, A. R. A2 - Stechel, E. B. A2 - Jennison, D. R. T3 - Springer Series in Surface Sciences AB - Sputtering of a LiF(100) surface by singly- and doubly-charged He ions with impact energies between 10 and 500 eV has been performed. The yield of sputtered Li+ and F- ions is only slightly higher for doubly-charged ions compared to singly-charged at impact energies below 100 eV, whereas the F+ yield is substantially increased if doubly-charged projectiles are used. The experimental data are explained within the framework of a model based on calculations by Walkup and Avouris. CY - Berlin DA - 1993/// PY - 1993 VL - 31 SP - 129 EP - 132 PB - Springer SN - 978-3-540-56473-7 ER - TY - JOUR TI - Sputtering of LiF(100) with low energetic Ne+ and Ne2+ ions AU - Wutte, D. AU - Diebold, U. AU - Schmid, M. AU - Varga, P. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - Sputtering of a LiF(100) surface by singly and doubly charged Ne ions with impact energies between 10 and 500 eV has been performed. The emission of Li+, Li0, F+ F0 and LiF+ was measured by means of a quadrupole mass spectrometer. The yield of sputtered Li+ ions and Li0 atoms decreases with decreasing impact energy and is slightly higher for Ne2+ compared to Ne+ at impact energies below 100 eV. The situation changes very drastically regarding the emitted F particles. In the whole energy range investigated, the F+ and F0 yield was about one order of magnitude larger if Ne2+ projectiles were used instead of Ne+. According to the experimental data we propose the following model for the emission of F+ [1,2]: (1) The F ion of the solid surface is changed to F+ by Auger neutralization of the Ne2+ into Ne+ or by double resonance transition into doubly excited states of Ne0 with subsequent autoionization into Ne+. These processes take place several Ångströms in front of the surface. (2) The initial repulsive forces between the F+ ion and the surface are changed into attractive ones within a very short time by rearrangement of the alkali halide lattice [2]. (3) The neutralized projectile sputters the F+ as in a normal sputtering event on a surface with low surface binding energy, and F+ can he emitted as long as the kinetic energy of the projectile is sufficiently high. DA - 1992/03/01/ PY - 1992 DO - 10.1016/0168-583X(92)95029-Q VL - 65 IS - 1-4 SP - 167 EP - 172 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Angle resolved electron energy loss spectroscopy on graphite AU - Diebold, U. AU - Preisinger, A. AU - Schattschneider, P. AU - Varga, P. T2 - Surface Science AB - We report on angle resolved electron energy loss spectroscopy (EELS) in reflection mode with low primary energy on a graphite single crystal. Measurements with primary electron energy of 175 eV have been performed in off-Bragg-reflex geometry in two different directions within the (0001) surface plane of the graphite single crystal. In addition, EELS measurements in specular reflection mode with different primary energies and angles of incidence were done in order to distinguish between surface and bulk plasmon losses. The energy losses and the transferred momenta of the losses have been analyzed. The results are compared with the loss functions for bulk and surface excitations calculated from the dielectric function [epsilon]([omega],q) obtained from TEELS-data (EELS in transmission mode) [Springer Tracts Mod. Phys. 54 (1970) 77]. DA - 1988/// PY - 1988 DO - 10.1016/0039-6028(88)90638-3 VL - 197 IS - 3 SP - 430 EP - 443 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - A computer-controlled system for the measurement of complete admittance spectra of piezoelectric resonators AU - Schmid, M AU - Benes, E. AU - Sedlaczek, R. T2 - Measurement Science and Technology AB - A measurement system for the characterisation of the electrical properties of piezoelectric resonators is presented. Admittance of the resonator is measured at successive frequencies controlled by an algorithm which uses small frequency steps when a resonance is found, and large steps between the resonances. During the measurement, characteristic frequencies are marked for each resonance. Further evaluation of the data is done by a least-squares fit, which yields the parameters of the equivalent circuit for each resonance in closed form. DA - 1990/// PY - 1990 DO - 10.1088/0957-0233/1/9/021 VL - 1 IS - 9 SP - 970 EP - 975 J2 - Meas. Sci. Technol. SN - 0957-0233 ER - TY - JOUR TI - H2S adsorption on chromium, chromia, and gold/chromia surfaces: Photoemission studies AU - Rodriguez, J. A. AU - Chaturvedi, S. AU - Kuhn, M. AU - van Ek, J. AU - Diebold, U. AU - Robbert, P. S. AU - Geisler, H. AU - Ventrice, Jr. T2 - The Journal of Chemical Physics AB - The reaction of H2S with chromium, chromia, and Au/chromia films grown on a Pt(111) crystal has been investigated using synchrotron-based high-resolution photoemission spectroscopy. At 300 K, H2S completely decomposes on polycrystalline chromium producing a chemisorbed layer of S that attenuates the Cr 3d valence features. No evidence was found for the formation of CrSx species. The dissociation of H2S on Cr3O4 and Cr2O3 films at room temperature produces a decrease of 0.3–0.8 eV in the work function of the surface and significant binding-energy shifts (0.2–0.6 eV) in the Cr 3p core levels and Cr 3d features in the valence region. The rate of dissociation of H2S increases following the sequence: Cr2O3H2S(5a1,2b1) bonding interactions and facilitates the decomposition of the molecule. DA - 1997/12/01/ PY - 1997 DO - 10.1063/1.475319 VL - 107 IS - 21 SP - 9146 EP - 9156 J2 - J. Chem. Phys. ST - H[sub 2]S adsorption on chromium, chromia, and gold/chromia surfaces ER - TY - JOUR TI - Water-soluble nanorods self-assembled via pristine C60 and porphyrin moieties AU - Fathalla, Maher AU - Li, Shao-Chun AU - Diebold, Ulrike AU - Alb, Alina AU - Jayawickramarajah, Janarthanan T2 - Chemical Communications AB - A novel water-soluble nanorod is discussed, which is prepared via the self-assembly of pristine C60 and a double-sided porphyrin projecting four [small beta]-cyclodextrins from each face. DA - 2009/// PY - 2009 DO - 10.1039/b908050c VL - 2009 IS - 28 SP - 4209 EP - 4211 J2 - Chem. Commun. ER - TY - JOUR TI - Decay channels for the Ti(2p1/2) core hole excitations in TiO2 observed by x-ray Raman scattering AU - Jiménez-Mier, J. AU - Diebold, U. AU - Ederer, D. AU - Callcott, T. AU - Grush, M. AU - Perera, R. T2 - Physical Review B AB - We present high-resolution x-ray fluorescence spectra, corrected for self-absorption, following resonant excitation of a Ti(2p1/2) electron in TiO2. Several transitions are studied that show complex behavior as a function of photon excitation energy, indicative of interaction in the excitation and decay channels. Three peaks are identified as transitions resulting from the excitation of a 2p1/2 electron, one of them corresponding to direct valence emission with a 3d spectator electron and the other two to participator emission following the Coster-Kronig decay of the 2p1/2 hole. Additional emission features at higher photon energies correspond to valence emission in the presence of a charge-transfer excitation that has been found in the 2p x-ray photoelectron spectroscopy spectrum, and also in the x-ray absorption spectrum. A detailed analysis of the energy dispersion versus excitation energy provides information about the dynamical processes involved. The results are interpreted in terms of the calculated band structure of the compound. DA - 2002/04// PY - 2002 DO - 10.1103/PhysRevB.65.184105 VL - 65 IS - 18 SP - 184105 J2 - Phys. Rev. B SN - 0163-1829 ER - TY - JOUR TI - One step towards bridging the materials gap: surface studies of TiO2 anatase AU - Diebold, U. AU - Ruzycki, N. AU - Herman, G. S. AU - Selloni, A. T2 - Catalysis Today AB - We present a short overview of surface studies on the main low-index surfaces of anatase, the technologically most interesting crystallographic form of titanium dioxide. Results are compared to the extensively investigated surfaces of TiO2 rutile. The anatase (1 0 1) surface is stable in a (1×1) configuration. It exhibits twofold coordinated (bridging) oxygen atoms and fivefold coordinated Ti atoms with a density comparable to the one found on rutile (1 1 0). Step edges are terminated by fourfold coordinated Ti sites. In contrast to rutile (1 1 0), anatase (1 0 1) does not show a strong tendency for losing twofold coordinated oxygen atoms upon annealing in ultrahigh vacuum. The apparent lack of point defects is also reflected in the adsorption/desorption behavior of water and methanol. The anatase (1 0 0) surface has the second-lowest surface energy and tends to from a (1×2) reconstruction. A model with (1 0 1)-oriented microfacets agrees with the observed features in atomically-resolved STM images. The (0 0 1) surface forms a (1×4) reconstruction that is well explained by an [`]ad-molecule' model predicted from density functional theory calculations. A (1×3) reconstruction was observed for the anatase (1 0 3) surface. DA - 2003/10/15/ PY - 2003 DO - 10.1016/S0920-5861(03)00378-X VL - 85 IS - 2-4 SP - 93 EP - 100 J2 - Catal. Today SN - 0920-5861 ST - One step towards bridging the materials gap ER - TY - JOUR TI - Enhanced tunneling magnetoresistance and high-spin polarization at room temperature in a polystyrene-coated Fe3O4 granular system AU - Wang, Wendong AU - Yu, Minhui AU - Batzill, Matthias AU - He, Jibao AU - Diebold, Ulrike AU - Tang, Jinke T2 - Physical Review B AB - Polystyrene-coated Fe3O4 nanoparticles through surface engineering exhibit an intergranular tunneling magnetoresistance (MR) ratio of 22.8% at room temperature and a maximum MR of 40.9% at 110 K. The drastic enhancement of the MR ratio clearly suggests that there is high degree of spin polarization even at room temperature for half metallic Fe3O4. The estimated spin polarization P is about 54% and 83% at room temperature and 110 K, respectively. DA - 2006/04// PY - 2006 DO - 10.1103/PhysRevB.73.134412 VL - 73 IS - 13 SP - 134412 J2 - Phys. Rev. B SN - 1098-0121 ER - TY - JOUR TI - Competing stabilization mechanism for the polar ZnO(0001)-Zn surface AU - Kresse, Georg AU - Dulub, Olga AU - Diebold, Ulrike T2 - Physical Review B AB - Density-functional calculations for the (0001)-Zn surface of wurtzite ZnO are reported. Different stabilization mechanisms, such as metallization of the surface layer, adsorption of OH groups or O adatoms, the formation of Zn vacancies, and large scale triangular reconstructions are considered. The calculations indicate that isolated Zn vacancies or O adatoms are unfavorable compared to triangular reconstructions. In the absence of hydrogen, these triangular features are stable under any realistic temperature and pressure. When hydrogen is present, the reconstruction is lifted, and hydroxyl groups stabilize the ideal otherwise unreconstructed surface. The transition between the unreconstructed hydroxyl covered surface and the triangular shaped features occurs abruptly; OH groups lift the reconstruction, but their adsorption is energetically unfavorable on the triangularly reconstructed surface. DA - 2003/12// PY - 2003 DO - 10.1103/PhysRevB.68.245409 VL - 68 IS - 24 SP - 245409 J2 - Phys. Rev. B SN - 0163-1829 ER - TY - JOUR TI - Ion-induced erosion of tungsten surfaces studied by a sensitive quartz-crystal-microbalance technique AU - Golczewski, A. AU - Kuzucan, A. AU - Schmid, K. AU - Roth, J. AU - Schmid, M. AU - Aumayr, F. T2 - Journal of Nuclear Materials AB - A highly sensitive quartz-crystal-microbalance (QCM) technique was used to study erosion of polycrystalline tungsten films due to impact of deuterium, carbon and argon ions, as well as retention of deuterium in these films. Polycrystalline tungsten films coated onto a SC-cut quartz crystal were bombarded by ions with impact energies from 100 eV up to a few keV and the frequency change due to mass loss (sputtering, desorption) or mass gain (implantation, adsorption) during bombardment was determined. Our setup was capable of detecting mass-changes as small as 10^-5 µg/s, which corresponds to a removal (or deposition) of only 10^-4 W monolayers/s. While our total sputtering yields for deuterium and argon projectiles compare well with the results of previous work, we derive new data on sputtering of tungsten by carbon ions. In addition we demonstrate that our setup is well suited for determining deuterium retention rates in tungsten. DA - 2009/06/15/ PY - 2009 DO - 10.1016/j.jnucmat.2009.01.279 VL - 390-391 SP - 1102 EP - 1105 J2 - J. Nucl. Mater. SN - 0022-3115 ER - TY - JOUR TI - Total sputter yield of LiF induced by hyperthermal ions measured by a quartz microbalance AU - Neidhart, T. AU - Toth, Z. AU - Hochhold, M. AU - Schmid, M. AU - Varga, P. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - The total sputter yield of LiF induced by singly charged hyperthermal He+, Ne+ and Ar+ ions with a kinetic energy of 5-500 eV is presented. The measurements have been performed with a highly sensitive quartz crystal microbalance. Results of the total sputter yield for Au show excellent agreement with the literature. The results on LiF show a large and slowly decreasing total sputter yield at low energies. This fact is interpreted as evidence for electronic processes in the sputtering of LiF. DA - 1994/05/02/ PY - 1994 DO - 10.1016/0168-583X(94)95601-4 VL - 90 IS - 1-4 SP - 496 EP - 500 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Secondary ion emission from lithium fluoride under impact of slow multicharged ions AU - Neidhart, T. AU - Pichler, F. AU - Aumayr, F. AU - Winter, H. P. AU - Schmid, M. AU - Varga, P. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - Secondary ion emission has been investigated for bombardment of polycrystalline lithium fluoride by slow multicharged Ar ions (charge state q ≤ 9, impact energy E_k ≤ 500 eV). The F- ions originate from collisional energy transfer, almost independent of the primary ion charge, whereas the F+ yield strongly increases with q. The F+ ions are produced via interatomic Auger transitions from the F- 2p valence band into projectile states, and their desorption from LiF is controlled by Coulomb interaction of F+ with Li+ and F- surface ions, and LiF lattice relaxation. At high impact energy, emission of Li+ is also mainly due to collisional energy transfer, but toward lower Ek the primary ion charge plays an increasingly important role. The present measurements demonstrate that secondary ions account for less than 0.1% of our earlier measured total sputter yields from LiF. DA - 1995/05/03/ PY - 1995 DO - 10.1016/0168-583X(95)00169-7 VL - 98 IS - 1-4 SP - 465 EP - 468 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - The accuracy of quantitative LEED in determining chemical composition profiles of substitutionally disordered alloys: a case study AU - Sporn, M. AU - Platzgummer, E. AU - Forsthuber, S. AU - Schmid, M. AU - Hofer, W. AU - Varga, P. T2 - Surface Science AB - We explore the accuracy of chemical composition profiles of substitutionally disordered alloys determined experimentally by LEED (low-energy electron diffraction) I(E) analysis. We analyse experimental I(E) spectra of pure Rh(111) for its known chemical composition by comparing them to calculations assuming a substitutionally disordered PtxRh1-x alloy surface. The layer concentrations known to be 100% Rh are reproduced with a maximum error of 8% when the Pendry R-factor (RP) is employed. This error is considerably smaller than estimated by error bars derived from the variance of RP. We argue that the same accuracy can be expected for compositional depth profiles to be determined for alloys exhibiting weak chemical order and negligible lattice distortions such as PtxRh1-x. DA - 1998/10/30/ PY - 1998 DO - 10.1016/S0039-6028(98)00596-2 VL - 416 IS - 3 SP - 423 EP - 429 J2 - Surf. Sci. SN - 0039-6028 ST - Accuracy of quantitative LEED ER - TY - JOUR TI - Trajectory-dependent neutralization of 1 keV He+ ions scattered from Pb(111) and Pb films on Cu(100) AU - Platzgummer, E. AU - Borrell, M. AU - Nagl, C. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - We observe trajectory-dependent neutralization of 1 keV He+ ions scattered from the Pb(111) surface. Varying the azimuthal angle of the ion trajectories we observe neutralization by charge densities of the next and second next neighbors of the target atom occurring at polar angles between 40 and 15°. This corresponds to ion-neighbor distances between 2.3 and 1 Å. We adjust a three-parameter neutralization model for Pb to fit the trajectory dependence of the survival probability for the well known Pb(111) surface geometry. Good agreement with experimental results is found only if a shell-like neutralization region is centered at a radius of 1.46 Å. In regard to the spatial distribution of the neutralization rate we consider the Pb 6sp electrons to be the source of the long distance neutralization that leads to the trajectory dependence. The same neutralization model is then applied for a structural analysis of ultrathin Pb films evaporated on Cu(100), where we confirm the existence of a Pb-Cu surface alloy on the c(4×4) Pb/Cu(100) and the existence of a Pb overlayer on the c(2×2) Pb/Cu(100). It turns out that the neutralization model developed for Pb atoms embedded in the (111) surface holds also for adsorbed Pb atoms. DA - 1998/09/03/ PY - 1998 DO - 10.1016/S0039-6028(98)00388-4 VL - 412-413 SP - 202 EP - 212 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Threshold for Potential Sputtering of LiF AU - Hayderer, G. AU - Schmid, M. AU - Varga, P. AU - Winter, H P. AU - Aumayr, F. AU - Wirtz, L. AU - Lemell, C. AU - Burgdörfer, J. AU - Hägg, L. AU - Reinhold, C. O. T2 - Physical Review Letters AB - We have measured total sputtering yields for impact of slow ( ≤100 eV) singly and doubly charged ions on LiF. The minimum potential energy necessary to induce potential sputtering (PS) from LiF was determined to be about 10 eV. This threshold coincides with the energy necessary to produce a cold hole in the valence band of LiF by resonant neutralization. This allows the first unambiguous identification of PS induced by cold holes. Further stepwise increase of the sputtering yield with higher projectile potential energies provides evidence for additional defect-mediated sputtering mechanisms operative in alkali halides. DA - 1999/11/08/ PY - 1999 DO - 10.1103/PhysRevLett.83.3948 VL - 83 IS - 19 SP - 3948 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Observation of a threshold in potential sputtering of LiF surfaces AU - Hayderer, G. AU - Lemell, C. AU - Wirtz, L. AU - Schmid, M. AU - Burgdörfer, J. AU - Varga, P. AU - Winter, HP. AU - Aumayr, F. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - A quartz-crystal microbalance technique is used for measuring total sputtering yields for LiF under impact of slow (20 eV, 100 eV, 500 eV and 1000 eV kinetic energy) singly and doubly charged ions. At low kinetic energies ([less-than-or-equals, slant]100 eV) potential sputtering (PS) (i.e., sputtering due to the projectiles potential energy) clearly dominates over kinetically induced sputtering. New insight into the mechanisms for PS is gained by determining the minimum potential energy necessary to induce PS. The measured potential energy threshold at around 10 eV provides evidence that PS can already be induced by the production of cold holes in the valence band of LiF via resonant neutralisation. DA - 2000/04// PY - 2000 DO - 10.1016/S0168-583X(99)01070-8 VL - 164-165 SP - 517 EP - 521 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Vibration modes of mass-loaded planoconvex quartz crystal resonators AU - Benes, Ewald AU - Schmid, Michael AU - Kravchenko, Victor T2 - Journal of the Acoustical Society of America AB - Layered piezoelectric resonators exhibit a complex spectrum of eigenfrequencies, which can be understood better if the associated vibration patterns are examined. For a closer insight into such a system, planoconvex AT-cut quartz crystals with a thick layer of copper on one side are investigated experimentally. Since presently no comprehensive three-dimensional theory of layered resonators is available, the experimental results can be compared only with one-dimensional descriptions of layered resonators or with theoretical models for unloaded contoured quartz crystals. The anharmonic spectrum of the crystals is used to find a formula by which the eigenfrequency of the corresponding infinite plate can be determined from measured frequencies of planoconvex crystals. If this correction is applied to experimental frequency spectra of composite quartz-film resonators, excellent agreement with the one-dimensional model is found. As theoretically predicted, also even harmonic overtones of heavily loaded crystals can be excited electrically. The vibration patterns are examined with an ultrasensitive laser-speckle technique. The measurements of the odd harmonic overtones agree well with the theory of energy trapping in planoconvex quartz crystals. An increase of the crystal's active area with increasing mass load is observed. The even overtones frequently exhibit no such energy trapping; in these cases, unusual vibration patterns occur. DA - 1991/// PY - 1991 DO - 10.1121/1.401940 VL - 90 IS - 2 SP - 700 EP - 706 J2 - J. Acoust. Soc. Am. ER - TY - JOUR TI - Are the surfaces of CrO2 metallic? AU - Ventrice Jr, C. A. AU - Borst, D. R. AU - Geisler, H. AU - Ek, J. van AU - Losovyj, Y. B. AU - Robbert, P. S. AU - Diebold, U. AU - Rodriguez, J. A. AU - Miao, G. X. AU - Gupta, A. T2 - Journal of Physics: Condensed Matter AB - Previous photoelectron spectroscopy studies of CrO2 have found either no density of states or a very low density of states at the Fermi level, suggesting that CrO2 is a semiconductor or a semi-metal. This is in contradiction to calculations that predict that CrO2 should be a half-metallic ferromagnet. Recently, techniques have been developed to grow high-quality epitaxial films of CrO2 on TiO2 substrates by chemical vapour deposition. We present photoelectron spectroscopy measurements of epitaxial CrO2(110)/TiO2(110) and CrO2(100)/TiO2(100) grown using a CrO3 precursor. In addition, measurements of epitaxial Cr2O3(0001)/Pt(111) films grown by thermal evaporation of Cr in an oxygen atmosphere are presented as a reference for reduced CrO2 films. The measurements of the CrO2 surfaces show no emission at the Fermi level after sputtering and annealing the surfaces in oxygen, even though our soft core photoemission data and low-energy electron diffraction measurements provide evidence that stoichiometric CrO2 is present. The consequence of this is that neither surface of CrO2 is metallic. This behaviour could result from a metal to semiconductor transition at the (110) and (100) surfaces. DA - 2007/// PY - 2007 DO - 10.1088/0953-8984/19/31/315207 VL - 19 IS - 31 SP - 315207 J2 - J. Phys.: Condens. Matter SN - 0953-8984 ER - TY - JOUR TI - Characterization of the natural barriers of intergranular tunnel junctions: Cr2O3 surface layers on CrO2 nanoparticles AU - Dai, Jianbiao AU - Tang, Jinke AU - Xu, Huiping AU - Spinu, Leonard AU - Wang, Wendong AU - Wang, Kaiying AU - Kumbhar, Amar AU - Li, Min AU - Diebold, Ulrike T2 - Applied Physics Letters AB - Cold-pressed powder compacts of CrO2 show large negative magnetoresistance (MR) due to intergranular tunneling. Powder compacts made from needle-shaped nanoparticles exhibit MR of about 28% at 5 K. Temperature dependence of the resistivity indicates that the Coulomb blockade intergranular tunneling is responsible for the conductance at low temperature. In this letter we report direct observation and characterization of the microstructure of the intergranular tunnel barriers, using transmission electron microscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). A very thin native oxide layer with a thickness of 1–3 nm on the surface of CrO2 powders has been observed. The composition and crystal structure of this surface layer has been determined to be Cr2O3 by XPS and XRD. The dense and uniform Cr2O3 surface layers play an ideal role of tunnel barriers in the CrO2 powder compacts. DA - 2000/10/30/ PY - 2000 DO - 10.1063/1.1320845 VL - 77 IS - 18 SP - 2840 EP - 2842 J2 - Appl. Phys. Lett. ST - Characterization of the natural barriers of intergranular tunnel junctions ER - TY - JOUR TI - Electron emission from polycrystalline lithium fluoride bombarded by slow multicharged ions AU - Vana, M. AU - Aumayr, F. AU - Varga, P. AU - Winter, H. P. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - Total electron yields have been determined from electron emission statistics measured for impact of H+, Nq+ (q = 1, 5, 6) and Arq+ (q = 1, 3, 6, 9) on clean, polycrystalline lithium fluoride. Ion impact energies have been varied from almost zero up to 10 × q keV. The obtained total electron yields deviate considerably from available data derived via ion- and electron current measurements for LiF single crystal targets. Our results are explained by comparison with a recent model for MCI induced potential electron emission from clean metal surfaces, which has been properly adapted, available theory for kinetic electron emission from alkalihalide surfaces, and by considering also measured secondary electron yields for LiF. Dependences of the electron emission statistics and -yields on projectile impact energy and -charge differ strongly from corresponding properties for clean metal surfaces, which can be explained from the different roles of potential- and kinetic emission and, in particular, a relatively stronger contribution from secondary electron emission induced by fast electrons from finally neutralising projectiles inside the LiF bulk. DA - 1995/06/01/ PY - 1995 DO - 10.1016/0168-583X(94)00814-0 VL - 100 IS - 2-3 SP - 284 EP - 289 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Defect mediated sputtering of amorphous LiF induced by low-energy ion bombardment AU - Seifert, N. AU - Yan, Q. AU - Barnes, A. V. AU - Tolk, N. H. AU - Neidhart, T. AU - Varga, P. AU - Husinsky, W. AU - Betz, G. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - Sputter yields of amorphous LiF caused by low-energy Ar+, Ar2+, and K+ impact have been computed using the binary collision simulation code Marlowe in conjunction with a simple, phenomenological model for defect production and diffusion. In the model used, the sputter yield is assumed to be made up by contributions due to (a) resonant or Auger neutralization (RN or AN), (b) vacancy formation due to momentum transfer, (c) inelastic energy loss channels other than RN or AN processes, and finally (d) direct sputtering of the constituents of the alkali halide solids. Our results are compared to recent investigations of the total sputter yield of Ar+ bombarded LiF films. DA - 1995/06/03/ PY - 1995 DO - 10.1016/0168-583X(95)00052-6 VL - 101 IS - 1-2 SP - 131 EP - 136 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Optimization of synthesis variables in the preparation of active sulfated zirconia catalysts AU - Cutrufello, M. AU - Diebold, U. AU - Gonzalez, R. T2 - Catalysis Letters AB - The synthesis of a series of sulfated zirconia catalysts was optimized using the isomerization of n-butane as a reaction probe. The normality of the H2SO4 solution used in the sulfation step was found to be the most important variable. A systematic change in the concentration of the H2SO4 solution showed that the optimum acid concentration was 0.25 N. When a catalyst prepared with this acid concentration was used, the conversion of n-butane at 200 °C was 35% at 5 min t-o-s. This was close to the thermodynamic equilibrium value of 56% conversion. This maximum was coincident with a catalyst with the highest specific surface area. An increase in the concentration of the H2SO4 solution above 0.25 N resulted in a decrease in both surface area and zirconia crystallinity. XPS studies showed a linear relationship between the H2SO4 solution concentration and the surface sulfur concentration. Bulk concentrations were determined by elemental analysis. The surface area increased to a maximum for a H2SO4 concentration of 0.25 N, while the concentration of bulk sulfur continued to increase when the acid concentration was progressively increased to 2.00 N. The use of a mordenite trap in the reactant stream resulted in an increase in n-butane conversion and a decrease in the rate of catalyst deactivation. XPS studies showed that the sulfur was present as sulfate species and that the oxidation state was not affected by the reaction. DA - 2005/05/01/ PY - 2005 DO - 10.1007/s10562-005-3740-x VL - 101 IS - 1 SP - 5 EP - 13 J2 - Catal. Lett. ER - TY - JOUR TI - The influence of preadsorbed K on the adsorption of PF3 on Ru(0001) studied by soft x-ray photoelectron spectroscopy AU - Chakarian, Varoujan AU - Shuh, David K. AU - Yarmoff, Jory A. AU - Tao, Hui-Shu AU - Diebold, Ulrike AU - Maschhoff, Brian L. AU - Madey, Theodore E. AU - Shinn, Neal D. T2 - The Journal of Chemical Physics AB - Soft‐x‐ray photoelectron spectroscopy (SXPS) is utilized to study the coadsorption of K and PF3 on Ru(0001) at 90 and 300 K. In the absence of K, PF3 adsorbs molecularly at both temperatures. In the presence of a fractional monolayer of K, initially PF3 completely dissociates resulting in the formation of adsorbed KF and P species. As the surface is further exposed to PF3, some of the PF3 molecules adsorb via partial dissociation, resulting in the formation of PF and PF2. This process continues until all the K has reacted. At 300 K, a fraction of the incoming PF3 molecules react with the adsorbed KF and form a species which is tentatively identified as KPF6. The data show that surface chemistry is different at the two temperatures, as some of the chemical reaction channels occurring at 300 K are blocked at 90 K. The reduced surface mobility of the incident PF3 molecules at 90 K adversely affects the probability of PF3 and KF interactions, which, in turn, causes the concentration of adsorbed PF3 relative to P to be larger at lower temperatures. DA - 1994/// PY - 1994 DO - 10.1063/1.467195 VL - 100 IS - 7 SP - 5301 J2 - J. Chem. Phys. SN - 00219606 ER - TY - JOUR TI - Metal-related gate sinking due to interfacial oxygen layer in Ir/InAlN high electron mobility transistors AU - Ostermaier, C. AU - Pozzovivo, G. AU - Basnar, B. AU - Schrenk, W. AU - Schmid, M. AU - Tóth, L. AU - Pécz, B. AU - Carlin, J.-F. AU - Gonschorek, M. AU - Grandjean, N. AU - Strasser, G. AU - Pogany, D. AU - Kuzmik, J. T2 - Applied Physics Letters AB - We report on an annealing-induced “gate sinking” effect in a 2-nm-thin In0.17Al0.83N/AlN barrier high electron mobility transistor with Ir gate. Investigations by transmission electron microscopy linked the effect to an oxygen containing interlayer between the gate metal and the InAlN layer and revealed diffusion of oxygen into iridium during annealing. Below 700 °C the diffusion is inhomogeneous and seems to occur along grain boundaries, which is consistent with the capacitance-voltage analysis. Annealing at 700 °C increased the gate capacitance over a factor 2, shifted the threshold voltage from +0.3 to +1 V and increased the transconductance from 400 to 640 mS/mm DA - 2010/// PY - 2010 DO - 10.1063/1.3458700 VL - 96 IS - 26 SP - 263515 J2 - Appl. Phys. Lett. SN - 00036951 ER - TY - JOUR TI - Surface structure and morphology of Mg-segregated epitaxial Fe3O4(001) thin films on MgO(001) AU - Anderson, J. AU - Kuhn, Markus AU - Diebold, Ulrike AU - Shaw, K. AU - Stoyanov, P. AU - Lind, D. T2 - Physical Review B AB - We have investigated the structural and compositional changes that are induced by the segregation of substrate Mg to the surface of 1μm-thick Fe3O4(001) films on MgO(001). The thin films have been grown with plasma-assisted MBE. Characterization with reflection high-energy electron diffraction, x-ray diffraction, and superconducting quantum interference device magnetometry, show slightly strained, single-crystalline Fe3O4 films. For the surface studies, we have combined low-energy electron diffraction, and scanning tunneling microscopy to study surface structure and morphology. The initial surface content was determined by x-ray photoelectron spectroscopy. The surfaces of the molecular-beam epitaxy–grown samples are flat and show a (√2×√2)R45° reconstruction with respect to the Fe3O4 surface unit cell. Onset of Mg segregation to the surface occurs around 670 K, with long, narrow extensions of terraces growing along the [110] and [11¯0] directions. Heating in an oxygen atmosphere induces a 1×4 surface reconstruction, and extremely long (≈1000 Å), wide terraces. We attribute this annealing stage to the formation of a MgFe2O4 surface phase, exhibiting highly anisotropic surface diffusion and step formation energy. DA - 1997/10// PY - 1997 DO - 10.1103/PhysRevB.56.9902 VL - 56 IS - 15 SP - 9902 EP - 9909 J2 - Phys. Rev. B SN - 0163-1829 ER - TY - JOUR TI - Wiggling its way out of surface polarity: Fe3O4(100) (A Perspectives on the article: "A combined DFT/LEED approach for complex oxide surface structure determination: Fe3O4(011)" by R. Pentcheva, W. Moritz, J. Rundgren, S. Frank, D. Schrupp, M. Scheffler) AU - Diebold, Ulrike T2 - Surface Science DA - 2008/04/01/ PY - 2008 DO - 10.1016/j.susc.2008.01.039 VL - 602 IS - 7 SP - 1297 EP - 1298 J2 - Surf. Sci. SN - 0039-6028 ST - Wiggling its way out of surface polarity ER - TY - JOUR TI - Depth profiling of the altered layer in Ta2O5 produced by sputtering with He ions AU - Varga, P. AU - Taglauer, E. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - Samples of Ta2O5 were sputtered with He+ ions of various energies until equilibrium surface concentration was reached, as measured by AES and ISS. Depth profiles of the altered layers were obtained by erosion with 1 keV Ar+ ions which have higher sputtering yields, smaller range in the target and cause comparatively small changes in the surface composition. The thickness of the altered layers produced by sputtering with He+ ions of energies between 0.7 and 3 keV could be determined and shown to be comparable with calculated mean ranges of the ions. Characteristics of the depth profiles and their dependence on ion fluence and energy are discussed. DA - 1984/03// PY - 1984 DO - 10.1016/0168-583X(84)90318-5 VL - 2 IS - 1-3 SP - 800 EP - 803 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Determination of electron-induced total sputter yield of LiF AU - Neidhart, T. AU - Sporn, M. AU - Schmid, M. AU - Varga, P. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - We present measurements of the total sputter yield for LiF at 150°C induced by electrons with an impact energy between 10 and 500 eV. A nearly linear increase with kinetic energy has been measured. The decrease of the yield with increasing electron dose for energies below 100 eV is also shown. For very high electron dose the yield even becomes zero and simultaneously a red coloration of the surface is observed. Low energy ion scattering (LEIS) measurements at such a surface showed a Li enrichment to more than 90%. To reach such a composition with electrons between 40 and 80 eV kinetic energy sputtering of about 20 monolayers LiF is necessary. DA - 1995/06/03/ PY - 1995 DO - 10.1016/0168-583X(95)00062-3 VL - 101 IS - 1-2 SP - 127 EP - 130 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - A search for surface alloy formation in faceting induced by monolayer metal films: Pd/W(111) and Ni/W(111) AU - Dong, Chengzhi AU - Zhang, Lizhong AU - Diebold, Ulrike AU - Madey, Theodore E. T2 - Surface Science AB - A monolayer of Pd on a W(111) surface will cause faceting of the surface upon annealing to T > 700 K, with the formation of {211} oriented microfacets. In order to learn whether surface alloy formation or intermixing of surface layers occurs during faceting we have employed low energy ion scattering (LEIS) , low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) to investigate the Pd/W (111) system. We find that 3.0 Å Pd deposited at room temperature almost completely covers the substrate W. In the temperature range from room temperature to ~ 1125 K (the onset of Pd desorption) there is no intermixing of Pd atoms with W atoms: Pd atoms stay on top of the planar surface as well as the faceted surface. In comparison with the Pd/W(111) system, we have also studied the Ni/W (111) system which does not exhibit faceting behavior. Within experimental uncertainty we find that there is [small tilde] 10% intermixing of Ni atoms with W atoms in the topmost surface layer, after annealing at 1175 K. DA - 1995/01/01/ PY - 1995 DO - 10.1016/0039-6028(95)90032-2 VL - 322 IS - 1-3 SP - 221 EP - 229 J2 - Surf. Sci. SN - 0039-6028 ST - A search for surface alloy formation in faceting induced by monolayer metal films ER - TY - JOUR TI - Surface segregation of silicon in platinum(111) AU - Diebold, Ulrike AU - Zhang, Lanping AU - Anderson, John F. AU - Mrozek, Pawel T2 - Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films AB - We present a study of an ultrathin surface layer of platinum silicide formed on a Pt(111) crystal as a result of surface segregation of Si and Ca trace impurities. The structure and composition of this surface compound is investigated with low energy He + ion scattering, (LEIS), x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Si surface segregation onto Pt(111) is thermally activated; annealing at temperatures between 750 and 1100 K results in the formation of a surface silicide. The highest Si coverage that can be reached is 0.4 monolayers; the Ca coverage at saturation is below 0.02 monolayers. The enthalpy of Si segregation is found to be –(105 ± 30) kJ/mole. A well-ordered ([square root of]19 × [square root of]19)R23.41° or | - 23 25| structure is observed by LEED and STM; the amount of surface area covered with this structure is proportional to the Si coverage measured with LEIS. At low annealing temperatures up to 800 K, two domains coexist with | - 23 25| and | - 32 35| orientations, but only the first one is stable at Si saturation coverage. No large relaxations of substrate interatomic distances are detected upon formation of the overlayer. DA - 1996/05// PY - 1996 DO - 10.1116/1.580318 VL - 14 SP - 1679 EP - 1683 J2 - J. Vac. Sci. Technol. A ER - TY - JOUR TI - Surface studies of gas sensing metal oxides AU - Batzill, Matthias AU - Diebold, Ulrike T2 - Physical Chemistry Chemical Physics AB - The relation of surface science studies of single crystal metal oxides to gas sensing applications is reviewed. Most metal oxide gas sensors are used to detect oxidizing or reducing gases and therefore this article focuses on surface reduction processes and the interaction of oxygen with these surfaces. The systems that are discussed are: (i) the oxygen vacancy formation on the surface of the ion conductor CeO2(111); (ii) interaction of oxygen with TiO2 (both adsorption processes and the incorporation of oxygen into the TiO2(110) lattice are discussed); (iii) the varying surface composition of SnO2(101) and its consequence for the adsorption of water; and (iv) Cu modified ZnO(0001)-Zn surfaces and its interaction with oxygen. These examples are chosen to give a comprehensive overview of surface science studies of different kinds of gas sensing materials and to illustrate the potential that surface science studies have to give fundamental insight into gas sensing phenomena. DA - 2007/// PY - 2007 DO - 10.1039/b617710g VL - 9 IS - 19 SP - 2307 EP - 2318 J2 - Phys. Chem. Chem. Phys. ER - TY - JOUR TI - Tuning surface properties of SnO2(101) by reduction AU - Batzill, Matthias AU - Katsiev, Khabibulakh AU - Burst, James M. AU - Losovyj, Yaroslav AU - Bergermayer, Wolfgang AU - Tanaka, Isao AU - Diebold, Ulrike T2 - Journal of Physics and Chemistry of Solids AB - The SnO2(1 0 1) surface can be prepared with a SnO2 or SnO composition and consequently the surface Sn-atoms are either in a Sn(II) or Sn(IV) charge state. For a Sn(II) surface, Sn-5s derived surface states are identified by resonant, angle resolved photoemission spectroscopy (ARUPS). The differences in the interface properties of the Sn(II) and Sn(IV) surfaces of SnO2(1 0 1) are reviewed on the example of benzene and water adsorption. It is found that the difference in work function of these two surfaces causes a shift of the molecular orbitals of benzene by ~1 eV with respect to the Fermi-level of the substrate. Density functional theory calculations predict dissociation of water on the stoichiometric (Sn(IV)) surface but only weak molecular adsorption on the reduced Sn(II) surface. These predictions are in agreement with ARUPS measurements that show that at 160 K no water adsorbs on the reduced surface but adsorbs dissociatively on the stoichiometric surface. A strong adsorbate induced band bending is also observed for water adsorption on the stoichiometric surface that is likely associated with the formation of surface hydroxyls. DA - 2006/// PY - 2006 DO - 10.1016/j.jpcs.2006.05.042 VL - 67 IS - 9-10 SP - 1923 EP - 1929 J2 - J. Phys. Chem. Sol. SN - 0022-3697 ER - TY - JOUR TI - Gas-phase-dependent properties of SnO2 (110), (100), and (101) single-crystal surfaces: Structure, composition, and electronic properties AU - Batzill, Matthias AU - Katsiev, Khabibulakh AU - Burst, James AU - Diebold, Ulrike AU - Chaka, Anne AU - Delley, Bernard T2 - Physical Review B AB - The dependence of the surface structure, composition, and electronic properties of three low index SnO2 surfaces on the annealing temperature in vacuum has been investigated experimentally by low energy He+ ion scattering spectroscopy (LEIS), low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and angle resolved valence band photoemission (ARUPS) using synchrotron radiation. Transitions from stoichiometric to reduced surface phases have been observed at 440–520 K, 610–660 K, and 560–660 K for the SnO2 (110), (100), and (101) surfaces, respectively. Density functional theory has been employed to assess the oxidation state and stability of different surface structures and compositions at various oxygen chemical potentials. The reduction of the SnO2 surfaces is facilitated by the dual valency of Sn, and for all three surfaces a transition from Sn(IV) to Sn(II) is observed. For the (100) and (101) surfaces, theory supports the experimental observations that the phase transitions are accomplished by removal of bridging oxygen atoms from a stoichiometric SnO2 surface, leaving a SnO surface layer with a 1×1 periodicity. For the (110) surface the lowest energy surface under reducing conditions was predicted for a model with a SnO surface layer with all bridging oxygen and every second row of in-plane oxygen atoms removed. Ab initio atomistic thermodynamic calculations predict the phase transition conditions for the (101) surface, but there are significant differences with the experimentally observed transition temperatures for the (110) and (100) surfaces. This discrepancy between experiment and thermodynamic equilibrium calculations is likely because of a dominant role of kinetic processes in the experiment. The reduction of surface Sn atoms from a Sn(IV) to a Sn(II) valence state results in filling of the Sn-5s states and, consequently, the formation of Sn derived surface states for all three investigated surfaces. The dispersion of the surface states for the reduced (101) surface was determined and found to be in good agreement with the DFT results. For the (110) surface, the 4×1 reconstruction that forms after sputter and annealing cycles was also investigated. For this surface, states that span almost the entire band gap were observed. Resonant photoemission spectroscopy identified all the surface states on the reduced SnO2 surfaces as Sn derived. DA - 2005/10// PY - 2005 DO - 10.1103/PhysRevB.72.165414 VL - 72 IS - 16 SP - 165414 J2 - Phys. Rev. B SN - 1098-0121 ST - Gas-phase-dependent properties of SnO2 (110), (100), and (101) single-crystal surfaces ER - TY - JOUR TI - Photoemission Study of Azobenzene and Aniline Adsorbed on TiO2 Anatase (101) and Rutile (110) Surfaces AU - Li, Shao-Chun AU - Losovyj, Yaroslav AU - Paliwal, Vinod Kumar AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry C AB - The electronic structure of azobenzene and aniline, adsorbed on two TiO2 surfaces, anatase (101) and rutile (110), has been studied with ultraviolet synchrotron-based photoemission spectroscopy (UPS). At saturation coverage, azobenzene and aniline exhibit very similar molecular orbitals in UPS valence band spectra. Angle-resolved UPS exhibits anisotropy of the molecular states along the polar and azimuthal direction, as is expected for highly oriented superstructures. For a low coverage of azobenzene adsorbed on anatase, photon irradiation results in the conversion of the flat-lying molecule into two upright phenyl imide species. An irradiation-induced trans–cis isomer conversion is proposed to facilitate the azobenzene cleavage. These results confirm that the N═N double bond of azobenzene is cleaved by TiO2 in the full-coverage regime and that the resulting intermediate is bonded to the substrate, in agreement with a previous scanning tunneling microscopy study and a proposed reaction scheme for azobenzene ↔ aniline conversion at TiO2 surfaces [Li, S.-C. ; Diebold, U. J. Am. Chem. Soc. 2010, 132, 64]. DA - 2011/05/26/ PY - 2011 DO - 10.1021/jp202029a VL - 115 IS - 20 SP - 10173 EP - 10179 J2 - J. Phys. Chem. C ER - TY - JOUR TI - Interaction of Molecular Oxygen with the Vacuum-Annealed TiO2(110) Surface: Molecular and Dissociative Channels AU - Henderson, Michael A. AU - Epling, William S. AU - Perkins, Craig L. AU - Peden, Charles H. F. AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry B AB - We have examined the interaction of molecular oxygen with the TiO2(110) surface using temperature-programmed desorption (TPD), isotopic labeling studies, sticking probability measurements, and electron energy loss spectroscopy (ELS). Molecular oxygen does not adsorb on the TiO2(110) surface in the temperature range between 100 and 300 K unless surface oxygen vacancy sites are present. These vacancy defects are generated by annealing the crystal at 850 K, and can be quantified reliably using water TPD. Adsorption of O2 at 120 K on a TiO2(110) surface with 8% oxygen vacancies (about 4*10^13 sites/cm2) occurs with an initial sticking probability of 0.5-0.6 that diminishes as the surface is saturated. The saturation coverage at 120 K, as estimated by TPD uptake measurements, is approximately three times the surface vacancy population. Coverage-dependent TPD shows little or no O2 desorption below a coverage of 4*10^13 molecules/cm2 (the vacancy population), presumably due to dissociative filling of the vacancy sites in a 1:1 ratio. Above a coverage of 4*10^13 molecules/cm2, a first-order O2 TPD peak appears at 410 K. Oxygen molecules in this peak do not scramble oxygen atoms with either the surface or with other coadsorbed oxygen molecules. Sequential exposures of 16O2 and 18O2 at 120 K indicate that each adsorbed O2 molecule, irrespective of its adsorption sequence, has equivalent probabilities with respect to its neighbors to follow the two channels (molecular and dissociative), suggesting that O2 adsorption is not only precursor-mediated, as the sticking probability measurements indicate, but that all O2 molecules reside in this precursor state at 120 K. This precursor state may be associated with a weak 145 K O2 TPD state observed at high O2 exposures. ELS measurements suggest charge transfer from the surface to the O2 molecule based on disappearance of the vacancy loss feature at 0.8 eV, and the appearance of a 2.8 eV loss that can be assigned to an adsorbed O2- species based on comparisons with TiO2 inorganic complexes in the literature. Utilizing results from recent spin-polarized DFT calculations in the literature, we propose a model where three O2 molecules are bound in the vicinity of each vacancy site at 120 K. For adsorption temperatures above 150 K, the dissociation channel completely dominates and the surface adsorbs oxygen in a 1:1 ratio with each vacancy site. ELS measurements indicate that the vacancies are filled, and the remaining oxygen adatom, which is apparent in TPD, is transparent in ELS. On the basis of the variety of oxygen adsorption states observed in this study, further work is needed in order to determine which oxygen-related species play important roles in chemical and photochemical oxidation processes on TiO2 surfaces. DA - 1999/06/01/ PY - 1999 DO - 10.1021/jp990655q VL - 103 IS - 25 SP - 5328 EP - 5337 J2 - J. Phys. Chem. B ST - Interaction of Molecular Oxygen with the Vacuum-Annealed TiO2(110) Surface ER - TY - JOUR TI - Photocatalysts: Closing the gap AU - Diebold, Ulrike T2 - Nature Chemistry AB - Photocatalysts such as titanium dioxide that use sunlight to split water and produce hydrogen would be a clean and sustainable solution to many problems, but their efficiency is currently too low to be widely used. Two approaches to engineer the surface properties of titanium dioxide offer hope that its efficiency can be increased. DA - 2011/04// PY - 2011 DO - 10.1038/nchem.1019 VL - 3 IS - 4 SP - 271 EP - 272 J2 - Nat. Chem. SN - 1755-4330 ST - Photocatalysts ER - TY - JOUR TI - Adsorbate dependent neutralization of ions near a surface AU - Hofer, W. AU - Varga, P. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - We present energy distributions of secondary electrons emitted from polycrystalline clean and oxygen covered tungsten due to impact of slow singly and doubly charged noble gas ions (15 eV). To discuss the influence of surface contamination on the various ion neutralization and de-excitation processes and the resulting measured ejected-electron energy distributions, our experimental results have been compared with electron spectra as computed with the semi-empirical theory of Hagstrum. With oxygen adsorbed at the tungsten surface a drastic change of the electron density of states distribution occurs, which has been included in our calculations by modifying the state density distribution with increasing target contamination as found from UPS measurements. DA - 1984/03// PY - 1984 DO - 10.1016/0168-583X(84)90228-3 VL - 2 IS - 1-3 SP - 391 EP - 395 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Influence of subsurface, charged impurities on the adsorption of chlorine at TiO2(110) AU - Batzill, Matthias AU - Hebenstreit, Eleonore L. D. AU - Hebenstreit, Wilhelm AU - Diebold, Ulrike T2 - Chemical Physics Letters AB - Adsorption of chlorine on TiO2(1 1 0) surfaces modified by subsurface impurity atoms was studied by scanning tunneling microscopy. Adsorption was found to be suppressed in the vicinity of positively charged subsurface donor atoms. The reduced adsorption is explained by an increased electron affinity close to the impurity atoms that renders these sites energetically unfavorable for acceptor-like adsorbates. DA - 2003/01/06/ PY - 2003 DO - 10.1016/S0009-2614(02)01635-4 VL - 367 IS - 3-4 SP - 319 EP - 323 J2 - Chem. Phys. Lett. SN - 0009-2614 ER - TY - JOUR TI - TiO2 by XPS AU - Diebold, Ulrike AU - Madey, T. E. T2 - Surface Science Spectra AB - The surfaces of titanium oxide belong to the most-studied oxide systems in the surface science literature. This is in part because TiO2 surfaces and interfaces play a major role in several technological applications, e.g., as promoters in catalysis, as photocatalysts, and as gas sensors. TiO2 is a reducible oxide, i.e., several phases with different stoichiometries exist. Because Ti is highly reactive towards oxygen, titanium oxides are formed readily when Ti is exposed to an atmosphere containing water or oxygen. The oxidation behavior of the metal is of interest for the properties of protective coatings. Although accurate knowledge of the XPS binding energies of different oxidation states is necessary for XPS investigations of titanium oxides, a recent review of the 16 literature data of the binding energy of Ti 2p3/2 from Ti4 + showed wide scatter of the reported values with a mean of 458.7 eV and a standard deviation of 1.3 eV [J. Mayer, E. Garfunkel, T. E. Madey, and U. Diebold, J. Electron Spectrosc. Relat. Phenom. 73, 1 (1995)]. TiO2 is easy to handle experimentally. Although it has a bulk band gap of 3 eV, no charging problems occur during surface spectroscopies after single-crystalline samples are reduced by heating in UHV (1000 K, 45 min). This treatment causes loss of bulk oxygen and results in n-type doping. A stochiometric TiO2 surface can reproducibly be prepared through sputtering and annealing in oxygen (2 × 10 – 4 Pa, 900 K). Our XPS core level spectra are measured from a bulk-reduced titanium dioxide (rutile) (110) surface using a VSW hemispherical analyzer. The binding energy of Ti 2p3/2 is determined as 459.3 eV, and the binding energy of O 1s as 530.4 eV. DA - 1996/07// PY - 1996 DO - 10.1116/1.1247794 VL - 4 IS - 3 SP - 227 EP - 231 J2 - Surf. Sci. Spectra ER - TY - JOUR TI - Adsorption and thermal stability of Mn on TiO2(110): 2p X-ray absorption spectroscopy and soft X-ray photoemission AU - Diebold, Ulrike AU - Shinn, Neal D. T2 - Surface Science AB - X-ray absorption and photoelectron spectroscopies are used to study the adsorption and reaction of Mn films deposited on TiO2(110) at 25°C and after annealing to ~650°C. Fractional monolayer coverages of Mn at 25°C produce a reactive, disordered interface consisting of reduced Ti cations and oxidized Mn overlayer atoms. Metallic Mn is found only for thicker layers at 25°C. Annealing Mn films to ~650°C leads to several changes that are largely independent of initial overlayer coverage: Metallic Mn thermally desorbs leaving only Mn2+ ions; interfacial Ti cations are largely re-oxidized to the Ti4+ state; and the local order is increased at the interface. The formation of a crystalline ternary surface oxide, MnTiOx, is proposed to account for these chemical and structural changes. DA - 1995/12/01/ PY - 1995 DO - 10.1016/0039-6028(95)00780-6 VL - 343 IS - 1-2 SP - 53 EP - 60 J2 - Surf. Sci. SN - 0039-6028 ST - Adsorption and thermal stability of Mn on TiO2(110) ER - TY - JOUR TI - Titanium and reduced titania overlayers on titanium dioxide(110) AU - Mayer, J. T. AU - Diebold, U. AU - Madey, T. E. AU - Garfunkel, E. T2 - Journal of Electron Spectroscopy and Related Phenomena AB - The adsorption of titanium on titanium dioxide TiO2(110) has been studied by X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS). The XPS data for Ti overlayers are interpreted using peak fitting based on experimental standard spectra. 4 Å of Ti deposited at 150 K reacts with the substrate to produce [approximate] 12 Å of intermediate oxidation state Ti. Adsorption of neutral metal begins on top of this interface oxide film, but 20 Å of deposited Ti are needed to cover the oxide completely. LEIS data indicate a tendency for clustering of Ti on top of the interface oxide. Ar+ sputtering of stoichiometric TiO2 leads to preferential loss of O from the near surface region. This reduction of the clean, annealed oxide surface by Ar+ ion bombardment starts immediately and does not reach a steady state until 3 × 1017 ions cm-2, at which point the reduced overlayer is 17 Å thick. DA - 1995/05/31/ PY - 1995 DO - 10.1016/0368-2048(94)02258-5 VL - 73 IS - 1 SP - 1 EP - 11 J2 - J. Electron Spectrosc. Relat. Phenom. SN - 0368-2048 ER - TY - JOUR TI - Electronic structure of ultrathin Fe films on TiO2(110) studied with soft-x-ray photoelectron spectroscopy and resonant photoemission AU - Diebold, Ulrike AU - Tao, Hui-Shu AU - Shinn, Neal D. AU - Madey, Theodore E. T2 - Physical Review B AB - We report on soft-x-ray photoelectron spectroscopy (SXPS) of a TiO2(110) surface during deposition of Fe in the monolayer regime. At low fractional monolayer coverages, the adsorbed Fe atoms are oxidized and Ti cations at the interface become reduced due to Fe adsorption. SXPS from shallow core levels and valence bands show that Fe starts to exhibit metallic character at a coverage of approximately 0.7 equivalent monolayers. Two well separated defect states appear in the band gap of TiO2 at iron coverages well below one monolayer. We use resonant photoemission to obtain information on the partial density of states, and we assign these defect states as being Fe-derived and Ti-derived states, located at the Fe and Ti sites, respectively. We suggest that a position change of oxygen is involved in the bonding of Fe on the TiO2(110) surface. DA - 1994/11/15/ PY - 1994 DO - 10.1103/PhysRevB.50.14474 VL - 50 IS - 19 SP - 14474 J2 - Phys. Rev. B ER - TY - JOUR TI - Supression of electron-induced positive ion emission by a molecular overlayer: Ion-molecule charge exchange at a surface AU - Diebold, Ulrike AU - Madey, Theodore E. T2 - Physical Review Letters AB - We report the suppression of electron-stimulated desorption of positive ions (O+ and F+) from a TiO2 (110) surface caused by adsorption of a fractional monolayer of molecular NH3. A linear decrease of O+ and F+ emission with NH3 coverage is observed. This system allows us to distinguish between neutralization of desorbing ions via interaction with the substrate or with adsorbed molecules. We propose a novel charge exchange mechanism where electron transfer from the occupied orbitals of the adsorbed molecules to the desorbing ions causes the decrease in detected ion yield. Charge transfer cross sections of 2.8(±0.5) and 2.7(±1) (×10-15) cm2 have been determined for O+ and F+ ions, respectively. DA - 1994/02/14/ PY - 1994 DO - 10.1103/PhysRevLett.72.1116 VL - 72 IS - 7 SP - 1116 EP - 1119 J2 - Phys. Rev. Lett. ST - Supression of electron-induced positive ion emission by a molecular overlayer ER - TY - JOUR TI - Structural study of ultrathin metal films on TiO2 using LEED, ARXPS and MEED AU - Pan, Jian-Mei AU - Maschhoff, Brian L. AU - Diebold, Ulrike AU - Madey, Theodore E. T2 - Surface Science AB - A structural study of ultrathin Cr, Fe and Cu metal films on stoichiometric and Ar+ sputtered TiO2(110) surfaces has been carried out using low energy electron diffraction (LEED), angle-resolved XPS (ARXPS) and two-dimensional medium energy backscattered electron diffraction (MEED). Although LEED results indicate only weak long range order for all three metal films (Cr, Fe and Cu) on TiO2(110), clear evidence for a locally ordered structure can be observed using ARXPS and MEED. Both Cr and Fe films grow with bcc(100) structures on stoichiometric TiO2(110) yielding pronounced forward scattering features in ARXPS and MEED data. Cu overlayers grow with fcc(111) structures exhibiting two equivalent domain orientations on TiO2(110) that yield less pronounced features in ARXPS. However, MEED measurements of Cu films with different electron surface incidence angles show clear fcc(111) patterns for different domains. Both bcc(100) and fcc(111) MEED patterns are simulated by single scattering cluster (SSC) calculations, and the results are qualitatively consistent with experimental data. A bcc(100) short range ordering is also observed for ultrathin Cr and Fe films on long range disordered Ar+ sputtered TiO2(110) surfaces. DA - 1993/07/10/ PY - 1993 DO - 10.1016/0039-6028(93)90455-S VL - 291 IS - 3 SP - 381 EP - 394 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Interaction of water, oxygen, and hydrogen with TiO2(110) surfaces having different defect densities AU - Pan, J.-M. AU - Maschhoff, B. L. AU - Diebold, U. AU - Madey, T. E. T2 - Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films AB - We have studied the stoichiometric (annealed in oxygen), the slightly oxygen-deficient (annealed in vacuum), and the highly defective (sputtered with Ar+) TiO2(110) surfaces and their reactivities to molecular oxygen, molecular water, and 50-eV hydrogen ions using x-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LEIS). The use of isotopically labeled 18O enables us to distinguish adsorbed oxygen from lattice oxygen, and the concentration of surface oxygen vacancies is titrated by 18O2 adsorption. LEIS (1-keV He+) is used to analyze the chemical composition of the outermost surface layer before and after 18O2 and H218O exposure. Water adsorbs on both stoichiometric and slightly O-deficient surfaces [with oxygen vacancies ~0 and 0.08 monolayer (ML), respectively] at room temperature. There is little or no dependence of saturation water coverage (lower limit of ~0.07 ML for both surfaces) on the concentration of surface oxygen vacancies. On the highly defective surfaces, the saturation water coverage increases to a lower limit of 0.15 ML and the saturation 18O coverage increases to 0.4 ML. The interaction of hydrogen with the stoichiometric surface creates defect states that can be observed by XPS and by subsequent adsorption of 18O. DA - 1992/07// PY - 1992 DO - 10.1116/1.577986 VL - 10 IS - 4 SP - 2470 EP - 2476 J2 - J. Vac. Sci. Technol. A ER - TY - JOUR TI - Adsorption and electron stimulated desorption of NH3/TiO2(110) AU - Diebold, U. AU - Madey, T. E. T2 - Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films AB - Adsorption and electron stimulated processes of NH3 on rutile TiO2(110) have been studied by means of x-ray photoemission spectroscopy (XPS) and low-energy ion scattering (LEIS). Defects, mostly surface oxygen vacancies, can be produced easily on TiO2 surfaces by thermal treatment or by rare gas ion sputtering. We have used three differently prepared TiO2 surfaces to study the influence of the substrate defect structure on the interaction with NH3: a stoichiometric surface, a thermally treated (slightly oxygen deficient), and a sputtered (highly oxygen-deficient) surface. The adsorption behavior of NH3 on all these surfaces is quite similar. NH3 adsorbs molecularly, with a saturation coverage of ~0.16–0.19 monolayers, the latter for a highly oxygen-deficient surface. The electron stimulated desorption (ESD) behavior is quite different: In the limit of a stoichiometric surface, electron irradiation induces the desorption of NH3 molecules, with a desorption cross section of ~ 1 × 10–16 cm2. When the highly oxygen-deficient TiO2 surface is used as substrate, besides desorption, electron stimulated dissociation of NH3 takes place, with atomic nitrogen being the final product of the dissociation process. This atomic nitrogen is adsorbed in subsurface sites, as confirmed by angle-resolved XPS. Under certain conditions, an electron stimulated formation of surface nitride is observed. These measurements provide direct evidence for the role of surface defects in electron stimulated reaction pathways. DA - 1992/07// PY - 1992 DO - 10.1116/1.577939 VL - 10 IS - 4 SP - 2327 EP - 2335 J2 - J. Vac. Sci. Technol. A ER - TY - JOUR TI - Understanding Metal Oxide Surfaces at the Atomic Scale: STM Investigations of Bulk-defect Dependent Surface Processes AU - Diebold, Ulrike T2 - Proceedings of the Materials Research Society AB - Surface defects are important in oxide surface chemistry, because they change not only the surface geometric structure, but also affect the local electronic structure. Scanning Tunneling Microscopy (STM) images with atomic-scale resolution, in combination with area-averaging surface spectroscopies, is an ideal tool to study local surface defects and their relationship to surface reactivity. We report STM results on TiO2(110) surfaces which show the surprising influence of bulk defects on surface properties. The reduced crystals used in this and other surface science studies contain Ti interstitials and oxygen vacancies. Re-oxidation at elevated temperatures results in the growth of additional TiO2 layers with Ti coming from the bulk of the crystal and O from the gas phase. This often result in partially incomplete surface structures with many undercoordinated atoms. The desorption behavior of elemental S, dosed at room temperature, depends on the reduction state of the sample. This is explained by a mechanism where desorption from a weakly-bound precursor state competes with the availability of new adsorption sites in the form of oxygen vacancies which migrate from the bulk to the surface. DA - 2001/// PY - 2001 VL - 654 SP - AA5.1 J2 - Mat. Res. Soc. Symp. Proc. ER - TY - JOUR TI - Structure of an ultrathin TiOx film, formed by the strong metal support interaction (SMSI), on Pt nanocrystals on TiO2(110) AU - Jennison, D. R. AU - Dulub, O. AU - Hebenstreit, W. AU - Diebold, U. T2 - Surface Science AB - We use first-principles density functional theory to study ultrathin TiOx films on Pt(111). The preferred interface with Pt has Ti with O as an overlayer. However, this ordering, preferred over Ti/O/Pt by 2.9 eV/Ti–O unit, produces >10% stress. This explains a complex structure, seen using STM, of TiOx bilayers encapsulating Pt(111) nanofacets: the energetics of stress relief is about ten times that of differences in the various possible O/Ti/Pt(111)-layer site occupations, thus favoring dislocation formation. A structure is found that is stable. It consists of a series of linear misfit dislocations at the relatively weak Ti/Pt interface that are 6/7 Ti/Pt rows wide. In addition, strong interactions at the O/Ti interface and O-layer strain also cause the Ti/Pt interface to abruptly change from hcp- to fcc-site Ti, producing linear “canyons” (Ti/Pt dislocation cores occur between these stripes). Furthermore, alternating hcp- and fcc-site triangles, each with ten O-atoms, are separated by bridging O in an abrupt O/Ti misfit dislocation, thus producing a zigzag pattern. The above dislocations release strain along both the x- and y-directions in the surface plane. However, we have been unable to find a stable structure if the zigzag ends consist of O, but stability is found if they consist of Ti. Finally, reverse bias STM images indicate the ends might indeed different than the line portions of the zigzag features. DA - 2001/10/10/ PY - 2001 DO - 10.1016/S0039-6028(01)01460-1 VL - 492 IS - 1-2 SP - L677 EP - L687 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Tuning the chemical functionality of a gas sensitive material: Water adsorption on SnO2(101) AU - Batzill, Matthias AU - Bergermayer, Wolfgang AU - Tanaka, Isao AU - Diebold, Ulrike T2 - Surface Science AB - Photoemission and density functional theory studies show that water adsorbs dissociatively on the SnO2(1 0 1) surface in the presence of terminating oxygen atoms and molecularly if these surface oxygen atoms are removed. The different chemical surface responses of these two bulk terminations of SnO2 also change the water induced band bending and consequently the conductivity of the gas sensing material. DA - 2006/02/15/ PY - 2006 DO - 10.1016/j.susc.2005.11.034 VL - 600 IS - 4 SP - 29 EP - 32 J2 - Surf. Sci. SN - 0039-6028 ST - Tuning the chemical functionality of a gas sensitive material ER - TY - JOUR TI - Surface studies of nitrogen implanted TiO2 AU - Batzill, Matthias AU - Morales, Erie H. AU - Diebold, Ulrike T2 - Chemical Physics AB - Rutile TiO2(1 1 0) single crystals have been doped by nitrogen-ion implantation. The change in the valence band and in the core level peak shapes are characterized by photoemission spectroscopy. Surface morphologies are characterized by scanning tunneling microscopy. N-dopants are observed to be in a 3- charge state and to substitute for O-anions in the TiO2 lattice for N-concentrations up to ~5% of the anions. The higher valency of the N-dopants compared to the host O-anions is proposed to be compensated by the formation of O-vacancies and/or Ti-interstitials. Two chemically shifted components arise in the Ti-2p core level upon N-doping. These components, shifted by 0.9 eV and 2.1 eV, are assigned to Ti-bound to N-ligands and possibly due to O-vacancies in the lattice. The Ti-3d band gap state observed in UPS is initially suppressed upon room temperature N-implantation and recovers a similar intensity as for undoped TiO2 samples upon annealing. This indicates that electrons left behind upon creation of O-vacancies are filling the N-2p level rather than Ti-3d states. The filled N-2p state is found at the top of the TiO2 valence band and is believed to be responsible for the band gap narrowing of N-doped TiO2 that shifts the photoactivity of TiO2 into the visible spectrum. DA - 2007/10/15/ PY - 2007 DO - 10.1016/j.chemphys.2007.07.037 VL - 339 IS - 1-3 SP - 36 EP - 43 J2 - Chem. Phys. SN - 0301-0104 ER - TY - JOUR TI - Oxygen adsorption on Cu∕ZnO(0001)-Zn AU - Lazcano, Paola AU - Batzill, Matthias AU - Diebold, Ulrike AU - Häberle, Patricio T2 - Physical Review B AB - We hereby describe the modifications induced in a Cu thin film induced by different oxidation processes. By using plasma assisted deposition, we have adsorbed oxygen onto the Cu∕ZnO(0001)-Zn surface. Cu was deposited on the sputtered-annealed ZnO substrate at room temperature, which was later exposed to oxygen. Using x-ray photoelectron spectroscopy, we verified the effect of the surface treatment on the electronic structure. Our findings are consistent with a partially oxidized Cu layer, with the CuO located at the interface between ZnO and the adsorbed Cu islands. Further Cu deposition induces the formation of Cu2O as judged by the evolution of the spectra. Annealing the sample up to 750 °C in UHV induces further reduction of the oxide, metallic Cu is recovered on the top layer, with evidence of Cu desorption into the vacuum or incorporation into the substrate. DA - 2008/01// PY - 2008 DO - 10.1103/PhysRevB.77.035435 VL - 77 IS - 3 SP - 035435 J2 - Phys. Rev. B SN - 1098-0121 ER - TY - JOUR TI - Decomposition of catechol and carbonaceous residues on TiO2(110): A model system for cleaning of extreme ultraviolet lithography optics AU - Jacobson, Peter AU - Li, Shao-Chun AU - Wang, Chuandao AU - Diebold, Ulrike T2 - Journal of Vacuum Science & Technology B AB - High energy photons used to expose photoresists in extreme ultraviolet lithography (92 eV, 13.5 nm) photoexcite electrons from Mo/Si multilayer mirror surfaces. Photoemitted electrons participate in the formation of carbonaceous residues on the mirror surface significantly affecting the mirror reflectivity. We explore mitigation strategies utilizing TiO2(110) as a model for the capping layer. Two carbon containing surfaces are examined; an ordered catechol monolayer and a carbonaceous layer. Excimer laser sources (XeF and KrF) coupled with oxidizing gas backgrounds (NO and O2) are shown to be effective for the photocatalytic removal of carbon. Utilizing x-ray photoemission spectroscopy and scanning tunneling microscopy carbon removal is shown to proceed through oxidation of the overlayer. DA - 2008/11// PY - 2008 DO - 10.1116/1.3002566 VL - 26 IS - 6 SP - 2236 EP - 2240 J2 - J. Vac. Sci. Technol. B ST - Decomposition of catechol and carbonaceous residues on TiO[sub 2](110) ER - TY - JOUR TI - Direction-dependent intermolecular interactions: catechol on TiO2(110)-1 x 1 AU - Li, Shao-Chun AU - Diebold, Ulrike T2 - Proceedings of the SPIE AB - The adsorption of a submonolayer of catechol (C6H6O2) on the rutile TiO2(110)-1×1 surface has been investigated by Scanning Tunneling Microscopy (STM). The catechol molecules are preferentially adsorbed on the surface 5-fold coordinated Ti4+ sites, and occupy two neighboring lattice Ti sites. No preference for adsorption at surface step edges is observed at room temperature. A statistical analysis of intermolecular distances demonstrates that the interaction between the molecules strongly depends on the surface crystallographic direction: catechol molecules exhibit attractive interaction along [1-1 0], while they repel each other along the [001] direction. The attractive interaction is proposed to be caused by the coupling of pi bonding electrons and the repulsive interaction is possibly mediated by substrate. DA - 2009/// PY - 2009 DO - doi:10.1117/12.828204 VL - 7396 SP - 73960P EP - 7 J2 - Proc. SPIE ST - Direction-dependent intermolecular interactions ER - TY - JOUR TI - The relationship between bulk and surface properties of rutile TiO2(110) AU - Diebold, Ulrike AU - Li, Min AU - Dulub, Olga AU - Hebenstreit, Eleonore L. D. AU - Hebenstreit, Wilhelm T2 - Surface Review and Letters AB - We report scanning tunneling microscopy and complementary spectroscopic measurements on TiO2(110) surfaces. We show data on (i) a surface restructuring process that results from annealing in oxygen; (ii) Pt clusters, grown at room temperature and encapsulated upon high temperature annealing; and (iii) adsorption of sulfur. In each case, heavily reduced, dark crystals show a very different behavior than more stoichiometric, light blue ones. DA - 2000/// PY - 2000 DO - 10.1142/S0218625X0000052X VL - 7 IS - 5-6 SP - 613 EP - 617 J2 - Surf. Rev. Lett. SN - 0218-625X ER - TY - JOUR TI - Evidence for oxygen adatoms on TiO2(110) resulting from O2 dissociation at vacancy sites AU - Epling, William S. AU - Peden, Charles H. F. AU - Henderson, Michael A. AU - Diebold, Ulrike T2 - Surface Science AB - Annealing TiO2(110) in vacuum at high temperature (above about 800 K) generates oxygen vacancy sites that are associated with reduced surface cations. Numerous studies have shown that these sites can be oxidized by exposure to molecular oxygen, but the mechanism and temperature dependence of this oxidation process are not well understood. We present results that suggest low temperature (<600 K) O2 exposure oxidizes oxygen vacancies but also leaves oxygen-containing species on the surface that we propose are oxygen adatoms. The presence of these oxygen adatoms is evident in the temperature-programmed desorption (TPD) spectrum of water. Oxidizing the vacuum annealed surface at 700 K produces a fully oxidized TiO2(110) surface that gives a single monolayer TPD state for water at 270 K. Exposing the vacuum annealed surface to O2 at temperatures between 90 and 600 K followed by water adsorption at 90 K results in a new water TPD state 25 K higher in temperature. Similar results were obtained using ammonia instead of water. Isotopic labeling experiments, in which the vacuum annealed surface was dosed with at 135 K followed by at 135 K, indicate that the new water TPD state results from recombinative desorption, whereas no such effect is observed for the surface exposed to at 700 K. The effect on water is also absent in TPD if the low temperature O2 treated surface is heated to 600 K prior to water adsorption at 90 K, suggesting that the oxygen adatoms desorb from the surface or diffuse into the bulk. We propose that at low temperatures, O2 dissociates at oxygen vacancies filling each defect site with one O atom and depositing a second O adatom at a five-coordinate Ti4+ site or that O2 interacts with surface hydroxyl groups resulting in O2 dissociation and the presence of the O adatom. The new dissociative water chemistry results from the interaction of water molecules with these oxygen adatoms. After high temperature (>600 K) O2 exposure, no dissociative water chemistry is observed, suggesting that these oxygen adatoms are not present on the surface. The presence of surface O adatoms may explain inconsistencies in the literature regarding the reactivity of water, and potentially other species, on TiO2(110). These results also detail the importance of sample preparation techniques on the chemistry which can occur at a solid surface. DA - 1998/09/03/ PY - 1998 DO - 10.1016/S0039-6028(98)00446-4 VL - 412-413 SP - 333 EP - 343 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Variations of the local electronic surface properties of TiO2(110) induced by intrinsic and extrinsic defects AU - Batzill, Matthias AU - Katsiev, Khabibulakh AU - Gaspar, Daniel J. AU - Diebold, Ulrike T2 - Physical Review B AB - Variation of the local electronic structure at rutile TiO2(110) surfaces was studied by scanning tunneling spectroscopy (STS). Structural surface features such as step edges, (1×2) reconstructed strands, and their terminations were correlated to changes in tunneling spectra. In particular, band-gap states, associated with a reduced surface, showed characteristic variations. In addition, electronic variations due to extrinsic defects are discussed. Nanometer wide protrusions in constant current scanning tunneling microscopy images were identified in STS as local electronic alterations. These features are interpreted to be due to local band bending induced by individual, charged impurity atoms. DA - 2002/12/04/ PY - 2002 DO - 10.1103/PhysRevB.66.235401 VL - 66 IS - 23 SP - 235401 J2 - Phys. Rev. B ER - TY - JOUR TI - Steps on anatase TiO2(101) AU - Gong, Xue-Qing AU - Selloni, Annabella AU - Batzill, Matthias AU - Diebold, Ulrike T2 - Nature Materials AB - Surface defects strongly influence the surface chemistry of metal oxides, and a detailed picture of defect structures may help to understand reactivity and overall materials performance in many applications. We report first-principles calculations of step edges, the most common intrinsic defects on surfaces (and probably the predominant ones on nanoparticles). We have determined the structure, energetics, and chemistry of step edges on the (101) surface of TiO2 anatase, an important photocatalytic material. Scanning tunnelling microscopy measurements of step-edge configurations and the contrast in atomically resolved images agree remarkably well with the theoretical predictions. Step-edge formation energies as well as the adsorption energies of water scale with the surface energy of the step facet, a trend that is expected to generally hold for metal oxide surfaces. Depending on the terrace/step configuration, this can lead to a situation where a step is less reactive than the flat terrace. DA - 2006/// PY - 2006 DO - 10.1038/nmat1695 VL - 5 IS - 8 SP - 665 EP - 670 J2 - Nat. Mater. SN - 1476-1122 ER - TY - JOUR TI - The Influence of the Bulk Reduction State on the Surface Structure and Morphology of Rutile TiO2(110) Single Crystals AU - Li, Min AU - Hebenstreit, Wilhelm AU - Diebold, Ulrike AU - Tyryshkin, Alexei M. AU - Bowman, Michael K. AU - Dunham, Glen G. AU - Henderson, Michael A. T2 - The Journal of Physical Chemistry B AB - We have investigated the relationship between different types and amounts of bulk defects and the surface morphology of TiO2(110) single crystals prepared by annealing in ultrahigh vacuum and in oxygen. Rutile TiO2(110) specimens were cut from the same crystal and were heated in a furnace to different temperatures which resulted in different states of reduction (colors of the crystals). After characterization of the bulk defects with electron paramagnetic resonance (EPR), the specimens were studied with scanning tunneling microscopy (STM), low-energy He+ ion scattering (LEIS), and work function measurements. EPR reveals that darker rutile crystals exhibit higher concentrations of extended Ti3+ related bulk defects such as crystallographic shear planes (CSP), with a decrease in substitutional and interstitial defects as compared to lighter crystals. Surface structures with (1 2) features are preferably formed upon UHV annealing on these darker crystals. LEIS measurements show that all of the crystals' (110) surfaces are reoxidized upon annealing in 18O2 (573 K, 1 10-6 mbar, 10 min) and that the 18O surface content is proportional to the bulk reduction state. UVvisible adsorption spectra and resistivity measurements also scale with the reduction states of crystals. Only the (1 1) structure is observed on the surface of slightly reduced crystals. Annealing in oxygen induces additional metastable structures, i.e., TiO2 clusters on blue crystals and rosette networks on dark blue crystals. DA - 2000/05/01/ PY - 2000 DO - 10.1021/jp9943272 VL - 104 IS - 20 SP - 4944 EP - 4950 J2 - J. Phys. Chem. B ER - TY - JOUR TI - The surface and materials science of tin oxide AU - Batzill, Matthias AU - Diebold, Ulrike T2 - Progress in Surface Science AB - The study of tin oxide is motivated by its applications as a solid state gas sensor material, oxidation catalyst, and transparent conductor. This review describes the physical and chemical properties that make tin oxide a suitable material for these purposes. The emphasis is on surface science studies of single crystal surfaces, but selected studies on powder and polycrystalline films are also incorporated in order to provide connecting points between surface science studies with the broader field of materials science of tin oxide. The key for understanding many aspects of SnO2 surface properties is the dual valency of Sn. The dual valency facilitates a reversible transformation of the surface composition from stoichiometric surfaces with Sn4+ surface cations into a reduced surface with Sn2+ surface cations depending on the oxygen chemical potential of the system. Reduction of the surface modifies the surface electronic structure by formation of Sn 5s derived surface states that lie deep within the band gap and also cause a lowering of the work function. The gas sensing mechanism appears, however, only to be indirectly influenced by the surface composition of SnO2. Critical for triggering a gas response are not the lattice oxygen concentration but chemisorbed (or ionosorbed) oxygen and other molecules with a net electric charge. Band bending induced by charged molecules cause the increase or decrease in surface conductivity responsible for the gas response signal. In most applications tin oxide is modified by additives to either increase the charge carrier concentration by donor atoms, or to increase the gas sensitivity or the catalytic activity by metal additives. Some of the basic concepts by which additives modify the gas sensing and catalytic properties of SnO2 are discussed and the few surface science studies of doped SnO2 are reviewed. Epitaxial SnO2 films may facilitate the surface science studies of doped films in the future. To this end film growth on titania, alumina, and Pt(111) is reviewed. Thin films on alumina also make promising test systems for probing gas sensing behavior. Molecular adsorption and reaction studies on SnO2 surfaces have been hampered by the challenges of preparing well-characterized surfaces. Nevertheless some experimental and theoretical studies have been performed and are reviewed. Of particular interest in these studies was the influence of the surface composition on its chemical properties. Finally, the variety of recently synthesized tin oxide nanoscopic materials is summarized. DA - 2005/// PY - 2005 DO - 10.1016/j.progsurf.2005.09.002 VL - 79 IS - 2-4 SP - 47 EP - 154 J2 - Prog. Surf. Sci. SN - 0079-6816 ER - TY - CHAP TI - Local structural study of ultrathin metal films on TiO2(110) using ARXPS and MEED AU - Pan, J.-M. AU - Maschhoff, B. L. AU - Diebold, U. AU - Madey, T. E. T2 - The Structure of Surfaces IV: Proceedings of the 4th International Conference on the Structure of Surfaces Shanghai, China August 16-19, 1993 A2 - Tong, S. Y. A2 - Xie, Xide A2 - Hsieh, Hsi-Te A2 - Hove, M. A. Van DA - 1994/// PY - 1994 SP - 328 PB - World Scientific SN - 981-02-1551-7 ER - TY - CHAP TI - Intrinsic Point Defects on TiO2(110) Surface and Their Reaction With Oxygen: A Scanning-Tunneling-Microscopy Study AU - Kuhn, K. AU - Anderson, J. F. AU - Lehman, J. AU - Mahmoud, T. AU - Diebold, U. T2 - Epitaxial Oxide Thin Films III A2 - Foster, C. A2 - Speck, J.S. A2 - Schlom, D. A2 - Eom, C.-B. A2 - Hawley, M.E. T3 - Mater. Res. Soc. Symp. Proc. AB - The interaction of molecular oxygen, at room temperature, with a reduced TiO2(110) surface has been studied in situ by scanning tunneling microscopy (STM). Oxygen vacancies (point defects) were created on a clean TiO2(110) surface by annealing in ultra-high vacuum and successfully imaged on the atomic scale. These point defect sites were stable under ultrahigh vacuum conditions. During exposure to molecular oxygen, new point defects appear at different locations on the surface although their overall number is reduced. A mechanism for this dynamic healing process is proposed. CY - Warrendale, PA DA - 1997/// PY - 1997 VL - 474 SP - 359 SN - 1-55899-378-9 ER - TY - JOUR TI - Tuning the oxide/organic interface: Benzene on SnO[sub 2](101) AU - Batzill, Matthias AU - Katsiev, Khabibulakh AU - Diebold, Ulrike T2 - Applied Physics Letters AB - Two different SnO2(101) bulk terminations have been prepared in order to demonstrate the impact of the oxide surface composition on the interface properties between SnO2(101) and an organic film. The change in work function causes a rigid shift of the molecular orbitals of the condensed organic film by 1 eV with respect to the valence band of SnO2. This change in the band alignment between an organic film and an oxide electrode material allows tuning of the barriers for charge transfer across this interface in molecular electronics applications. DA - 2004/12/06/ PY - 2004 DO - 10.1063/1.1831565 VL - 85 IS - 23 SP - 5766 EP - 5768 J2 - Appl. Phys. Lett. ST - Tuning the oxide/organic interface ER - TY - JOUR TI - A LEED study of NO superstructures on the Pd(111) surface AU - Kostelník, Petr AU - Šikola, Tomas AU - Varga, Peter AU - Schmid, Michael T2 - Journal of Physics: Condensed Matter AB - We have examined two adsorption structures of NO on the Pd(111) surface and the transformation between them. Low-energy electron diffraction (LEED) I(V) curves of the Pd(111)-p(2 x 2)-NO and Pd(111)-c(4 x 2)-NO surface structures were acquired and analyzed using tensor LEED. Our structural models confirm a previous study by scanning tunneling microscopy and DFT (Hansen et al 2002 Surf. Sci. 496 1). In the c(4 x 2)-NO structure, which forms at an NO coverage of 0.5 monolayers (ML), the NO molecules occupy fcc and hcp hollow sites and are almost upright with only slight tilting, possibly related to NO-NO repulsion. In the p(2 x 2)-NO structure (0.75 ML), with two NO molecules in hollow sites and one in an on-top site, we find strong tilting of the on-top molecule. Upon heating, thermal desorption of NO leads to a transition from the p(2 x 2) to the c(4 x 2) structure, which leads to splitting of the diffraction spots and/or streaky spots. The transition is discussed in terms of domain walls. DA - 2009/// PY - 2009 DO - 10.1088/0953-8984/21/13/134005 VL - 21 IS - 13 SP - 134005 J2 - J. Phys.: Condens. Matter SN - 0953-8984 ER - TY - JOUR TI - Partial Dissociation of Water Leads to Stable Superstructures on the Surface of Zinc Oxide AU - Meyer, Bernd AU - Marx, Dominik AU - Dulub, Olga AU - Diebold, Ulrike AU - Kunat, Martin AU - Langenberg, Deler AU - Wöll, Christof T2 - Angewandte Chemie International Edition AB - Half-dissociated: Experimental and computational findings conclude that water forms a highly ordered superstructure on defect-free surfaces of zinc oxide, in which every second water molecule is dissociated (see picture). The results are of general relevance for heterogeneous catalysis. DA - 2004/// PY - 2004 DO - 10.1002/anie.200461696 VL - 43 IS - 48 SP - 6641 EP - 6645 J2 - Angew. Chem. Int. Ed. ER - TY - CHAP TI - Surface structure and morphology of Mg-segregated, epitaxial Fe3O4 thin films on MgO(001) AU - Anderson, J. F. AU - Kuhn, K. AU - Diebold, U. AU - Shaw, K. T2 - Epitaxial Oxide Thin Films III A2 - Foster, C. A2 - Speck, J.S. A2 - Schlom, D. A2 - Eom, C.-B. A2 - Hawley, M.E. T3 - Mater. Res. Soc. Symp. Proc. AB - Fe3O4 films on MgO(001). The thin films have been grown with plasma-assisted MBE, and characterization with RHEED (reflection high-energy electron diffraction), x-ray diffraction (XRD), and Superconducting Quantum Interference Device (SQUID) magnetometry show slightly strained, singlecrystalline Fe3O4 films. For the surface studies, we have combined Low-Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM). Initial and final surface characterization employed X-ray Photoelectron Spectroscopy (XPS) and Ion Scattering Spectroscopy (ISS) respectively. The surfaces of the MBE-grown samples are flat and show a (√2×√2)R45° reconstruction with respect to the Fe3O4 surface unit cell. We observe the onset of Mg segregation to the surface at around 700 K, with long, narrow extensions of terraces being observed growing along the [110] and [1i0] directions. Upon prolonged heating at 800 K, massive Mg segregation to the surface is observed. Heating in an oxygen atmosphere induces a 1x4 surface reconstruction, and results in extremely long (; 1000 A), wide terraces. CY - Warrendale, PA DA - 1997/// PY - 1997 VL - 474 SP - 265 SN - 1-55899-378-9 ER - TY - JOUR TI - Decomposition of P(CH3)3 on Ru(0001): comparison with PH3 and PCl3 AU - Tao, H.-S. AU - Diebold, U. AU - Shinn, N.D. AU - Madey, T.E. T2 - Surface Science AB - The decomposition of P(CH3)3 adsorbed on Ru(0001) at 80 K is studied by soft X-ray photoelectron spectroscopy using synchrotron radiation. Using the chemical shifts in the P 2p core levels, we are able to identify various phosphorus-containing surface reaction products and follow their reactions on Ru(0001). It is found that P(CH3)3 undergoes a step-wise demethylation on Ru(0001), P(CH3)3 --> P(CH3)2 --> P(CH3) --> P, which is complete around ~450 K. These results are compared with the decomposition of isostructural PH3 and PCl3 on Ru(0001). The decomposition of PH3 involves a stable intermediate, labeled as PHx, and follows a reaction of: PH3 --> PHx --> P, which is complete around ~190 K. The conversion of chemisorbed phosphorus to ruthenium phosphide is observed and is complete around ~700 K on Ru(0001). PCl3 also follows a step-wise decomposition reaction, PCl3 --> PCl2 --> PCl --> P, which is complete around ~300 K. The energetics of the adsorption and the step-wise decomposition reactions of PH3, PCl3 and P(CH3)3 are estimated using the bond order conservation Morse potential (BOCMP) method. The energetics calculated using the BOCMP method agree qualitatively with the experimental data. DA - 1997/04/01/ PY - 1997 DO - 10.1016/S0039-6028(96)01280-0 VL - 375 IS - 2-3 SP - 257 EP - 267 J2 - Surf. Sci. SN - 0039-6028 ST - Decomposition of P(CH3)3 on Ru(0001) ER - TY - JOUR TI - Reduced thickness of contamination layers determined from C 1s- and CKVV-lines AU - Ebel, Maria F. AU - Schmid, M. AU - Ebel, H. AU - Vogel, A. T2 - Journal of Electron Spectroscopy and Related Phenomena DA - 1984/// PY - 1984 DO - 10.1016/0368-2048(84)80076-6 VL - 34 IS - 3 SP - 313 EP - 316 J2 - J. Electron Spectrosc. Relat. Phenom. SN - 0368-2048 ER - TY - JOUR TI - A quartz-crystal-microbalance technique to investigate ion-induced erosion of fusion relevant surfaces AU - Golczewski, A. AU - Dobes, K. AU - Wachter, G. AU - Schmid, M. AU - Aumayr, F. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - We describe a highly sensitive quartz-crystal-microbalance technique capable of determining erosion as well as implantation and retention rates for fusion relevant surfaces under ion bombardment. Total sputtering yields obtained with this technique for Ar ion impact on polycrystalline gold and tungsten surfaces are presented. The results compare well with existing experimental data as well as theoretical predictions and thus demonstrate the feasibility of the developed technique. Our setup is capable of detecting mass-changes as small as 10-5 [mu]g/s, which corresponds to a removal of only 10-4 W monolayers/s. DA - 2009/02// PY - 2009 DO - 10.1016/j.nimb.2008.10.088 VL - 267 IS - 4 SP - 695 EP - 699 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Surface reconstruction of Fe3O4(001) AU - Stanka, B. AU - Hebenstreit, W. AU - Diebold, U. AU - Chambers, S. A. T2 - Surface Science AB - We have investigated the surface termination, structure, morphology and composition of Fe3O4(001) using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), low-energy He+-ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). The samples consisted of not, vert, similar5000 Å thick epitaxial films of Fe3O4(001) grown by oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) on MgO(001) substrates. The (√2 × √2)R45° surface reconstruction that is present on the as-grown surface is recovered by heating the sample in oxygen (10−6–10−7 mbar) at temperatures between 420 and 770 K after a through-air transfer from the MBE chamber. STM results are best interpreted by assuming an autocompensated B-layer termination, which consists of a layer of octahedrally coordinated iron and tetrahedrally coordinated oxygen, along with one oxygen vacancy per unit cell. Evidence for a vacancy-induced lateral relaxation of the adjacent octahedral iron ions is presented. Further annealing in ultrahigh vacuum causes a transformation to either a (1×n) or a The (√2 × √2)R45° structure. These surfaces can be reproducibly transformed back to the The (√2 × √2)R45° reconstruction by annealing in oxygen. Interestingly, at no time do we observe the other autocompensated termination, which consists of one-half of a monolayer of tetrahedrally coordinated Fe(III), despite its observation on the as-grown surface. Thus, it appears that the surface termination is critically dependent on the method of surface preparation. DA - 2000/03/01/ PY - 2000 DO - 10.1016/S0039-6028(99)01182-6 VL - 448 IS - 1 SP - 49 EP - 63 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Vanadium surface oxides on Pd(111): A structural analysis AU - Klein, C. AU - Kresse, G. AU - Surnev, S. AU - Netzer, F. P. AU - Schmid, M. AU - Varga, P. T2 - Physical Review B AB - Scanning tunneling microscopy studies of vanadium oxides grown on Pd(111) show interesting structures especially in the low-coverage region. Evaporation of V in an oxygen background at elevated sample temperature (250 °C) results in the formation of a nonperiodic honeycomb-like structure growing from the steps, which starts to transform into an ordered phase at a vanadium coverage of ≈0.2 ML (monolayer). At 0.31 ML the entire surface is covered by this well-ordered open (4×4) structure. Annealing this structure in H2 atmosphere transforms the phase into a V2O3 surface oxide with (2×2) periodicity, whose optimal coverage is reached at 0.5 ML vanadium. Models for both ordered structures have been suggested before on the basis of ab initio density-functional theory (DFT) calculations and molecular-dynamics simulations and these models are now unambiguously confirmed by quantitative low-energy electron-diffraction (LEED) analyses. In the (4×4) phase, the V atoms are surrounded by four oxygen atoms in an unusual tetrahedral coordination leading to a V5O14 stoichiometry. This tetrahedral coordination allows the oxide to adopt open loosely packed two-dimensional (2D) and 1D structures, which are stabilized by the surface-oxide interface energy. Furthermore, it is shown that state of the art DFT calculations can indeed predict complex structures exactly as well as that modern quantitative LEED is capable of dealing with very large unit cells. DA - 2003/12/30/ PY - 2003 DO - 10.1103/PhysRevB.68.235416 VL - 68 IS - 23 SP - 235416 J2 - Phys. Rev. B ST - Vanadium surface oxides on Pd(111) ER - TY - JOUR TI - Coexistence of fcc- and bcc-like crystal structures in ultrathin Fe films grown on Cu(111) AU - Biedermann, A. AU - Rupp, W. AU - Schmid, M. AU - Varga, P. T2 - Physical Review B AB - We report on bcc-like phases in ultrathin Fe films grown by thermal deposition on Cu(111) previously thought to consist exclusively of fcc phases distinguished only by their magnetic order. Our scanning tunneling microscopy and spectroscopy data together with published x-ray photoelectron diffraction results [M. T. Kief and W. F. Egelhoff, Jr., Phys. Rev. B 47, 10785 (1993)] provide us with sufficient detail to deduce the film structure. Two growth regimes are considered: (1) films with 1–2 monolayer average thickness grown near 200 K, which nucleate as bcc-like bilayer islands: Larger islands show bcc-like fringes coexisiting with an fcc center domain; i.e., the bcc-like phase is stable only within a certain distance to a step edge. The presence of a bcc-like bilayer phase provides a straightforward explanation for the ferromagnetism previously observed in these films. In addition we find that the bcc-like phase can be promoted by H adsorption at 80 K. The bcc domains form "displacement vortex" structures to simultaneously minimize film stress and interface energy. (2) In films grown at room temperature, between pseudomorphic fcc areas, we observe a more ideal but still strained bcc phase in regions with a local thickness of at least 4 monolayers. Also in this growth regime, the fcc-bcc transformation is facilitated by step edges, which are abundant due to the imperfect layer-by-layer growth. DA - 2006/04/15/ PY - 2006 DO - 10.1103/PhysRevB.73.165418 VL - 73 IS - 16 SP - 165418 EP - 16 J2 - Phys. Rev. B ER - TY - JOUR TI - Ion-beam induced fcc-bcc transition in ultrathin Fe films for ferromagnetic patterning AU - Rupp, W. AU - Biedermann, A. AU - Kamenik, B. AU - Ritter, R. AU - Klein, Ch. AU - Platzgummer, E. AU - Schmid, M. AU - Varga, P. T2 - Applied Physics Letters AB - Ar+ ion irradiation is used to induce a structural change from fcc to bcc in a 1.5 nm thick Fe film epitaxially grown on a Cu(100) crystal. Scanning tunneling microscopy and low-energy electron diffraction show the nucleation of bcc nanocrystals, which grow with increasing ion dose. As a consequence of the structural change, the irradiated iron film becomes strongly ferromagnetic at room temperature. We present a model for the process of the transformation and demonstrate writing a magnetic pattern at the 100 nm scale by ion-beam projection lithography. DA - 2008/// PY - 2008 DO - 10.1063/1.2969795 VL - 93 IS - 6 SP - 063102 EP - 3 J2 - Appl. Phys. Lett. ER - TY - JOUR TI - Two-dimensional oxide on Pd(111) AU - Lundgren, E. AU - Kresse, G. AU - Klein, C. AU - Borg, M. AU - Andersen, J. N. AU - De Santis, M. AU - Gauthier, Y. AU - Konvicka, C. AU - Schmid, M. AU - Varga, P. T2 - Physical Review Letters AB - The oxidation of Pd(111) leads to an incommensurate surface oxide, which was studied by the use of scanning tunneling microscopy, surface x-ray diffraction, high resolution core level spectroscopy, and density functional calculations. A combination of these methods reveals a two-dimensional structure having no resemblance to bulk oxides of Pd. Our study also demonstrates how the atomic arrangement of a nontrivial incommensurate surface can be solved by molecular dynamics in a case where experimental techniques alone give no solution. DA - 2002/06/03/ PY - 2002 DO - 10.1103/PhysRevLett.88.246103 VL - 88 IS - 24 SP - 246103 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Surface structure and composition of Pt50Rh50(110): room temperature analysis of the (1 × 3) missing-row reconstruction AU - Koller, R. AU - Gauthier, Y. AU - Klein, C. AU - De Santis, M. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - The room temperature structure and composition of the clean Pt50Rh50(110) surface is investigated by low energy ion scattering (LEIS), scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and grazing incidence X-ray diffraction (GIXRD). While Pt25Rh75(1 1 0) reconstructs with a (1 × 2) missing-row structure, Pt50Rh50(110) exhibits a (1 × 3) structure in analogy with Pt80Fe20(110) and Pt90Co10(110). Three missing rows lead to the formation of (111) facets in which all atomic sites are enriched with platinum. Similarly all sites directly underneath are enriched with Rh leading to oscillation of the Pt concentrations similar to that of (111) surfaces. These composition changes are accompanied by a marked inwards relaxation of the top row (-10%) and large buckling in layers 3-5. Additionally the atomic positions of the facets are shifted laterally towards the valleys formed by the missing rows. Consistent pictures are derived from LEED, GIXRD, STM and LEIS concerning the composition while some discrepancy--similar to that recorded for pure Pt--are found concerning the interlayer distances between LEED and X-rays. DA - 2003/05/01/ PY - 2003 DO - 10.1016/S0039-6028(03)00381-9 VL - 530 IS - 3 SP - 121 EP - 135 J2 - Surf. Sci. SN - 0039-6028 ST - Surface structure and composition of Pt50Rh50(110) ER - TY - JOUR TI - Structure of Ag(111)-p(4 x 4)-O: No Silver Oxide AU - Schmid, M. AU - Reicho, A. AU - Stierle, A. AU - Costina, I. AU - Klikovits, J. AU - Kostelník, P. AU - Dubay, O. AU - Kresse, G. AU - Gustafson, J. AU - Lundgren, E. AU - Andersen, J. N. AU - Dosch, H. AU - Varga, P. T2 - Physical Review Letters AB - The structure of the oxygen-induced p(4×4) reconstruction of Ag(111) is determined by a combination of scanning tunneling microscopy, surface x-ray diffraction, core level spectroscopy, and density functional theory. We demonstrate that all previous models of this surface structure are incorrect and propose a new model which is able to explain all our experimental findings but has no resemblance to bulk silver oxide. We also shed some light on the limitations of current density functional theories and the potential role of van der Waals interactions in the stabilization of oxygen-induced surface reconstructions of noble metals. DA - 2006/04/14/ PY - 2006 DO - 10.1103/PhysRevLett.96.146102 VL - 96 IS - 14 SP - 146102 EP - 4 J2 - Phys. Rev. Lett. ST - Structure of Ag(111)-p(4 x 4)-O ER - TY - JOUR TI - Strain-induced local surface chemical ordering observed by STM AU - Ritz, G. AU - Schmid, M. AU - Biedermann, A. AU - Varga, P. T2 - Physical Review B AB - PtNi alloys are known to exhibit a tendency towards chemical ordering, which also effects surface segregation. Scanning tunneling microscopy results obtained in the strain field of dislocations on PtxNi1-x(110) surfaces show a (2×1) superstructure of alternating Pt and Ni atoms in some regions close to the dislocation core. In other regions, the apparent height of all surface atoms is equal, in agreement with low energy ion scattering results yielding a surface concentration of almost 100% Ni. This indicates that the strain present in the vicinity of dislocations influences both the surface composition and chemical order. The experimental results are compared to simulation calculations of chemical ordering and segregation, using embedded atom method potentials and linear elasticity theory. The simulations indicate that the (2×1) superstructure is due to an L10 ordered phase in regions where the tetragonal distortion of the L10 phase with respect to the cubic substrate can alleviate stress. It is argued that this dislocation-induced ordering can immobilize dislocations. DA - 1996/06// PY - 1996 DO - 10.1103/PhysRevB.53.16019 VL - 53 IS - 23 SP - 16019 EP - 16026 J2 - Phys. Rev. B SN - 0163-1829 ER - TY - JOUR TI - Surface effects on Pt-Ni single crystals calculated with the embedded-atom method AU - Stadler, H. AU - Hofer, W. AU - Schmid, M. AU - Varga, P. T2 - Physical Review B AB - Recent scanning-tunneling-microscopy (STM) experiments on Pt25Ni75(111) and Pt10Ni90(110) surfaces showing lattice mismatch dislocations near the surface and surface ordering phenomena have been verified in simulation calculations using the embedded-atom method (EAM). It was found that the EAM can be successfully applied for the calculation of ordering effects and crystal defects such as dislocations using energy minimization routines and Monte Carlo calculations. By comparing experimental data and EAM results, it was possible to determine the depth of the dislocations and therefore the number of layers with Pt enrichment on the surface. For the (111) face exhibiting surface ordering phenomena, the calculated short-range order is in agreement with STM data where the two species could be clearly distinguished. DA - 1993/10/15/ PY - 1993 DO - 10.1103/PhysRevB.48.11352 VL - 48 IS - 15 SP - 11352 J2 - Phys. Rev. B ER - TY - JOUR TI - Direct observation of surface chemical order by scanning tunneling microscopy AU - Schmid, M. AU - Stadler, H. AU - Varga, P. T2 - Physical Review Letters AB - We present the first scanning tunneling microscopy (STM) study which allows clear discrimination of two chemical species in a metal alloy. Special tunneling conditions, which we attribute to an adsorbate at the STM tip, cause a difference in corrugation between Pt and Ni atoms of 0.3 Å. The STM data reveal chemical short-range order at the surface, which is in agreement with embedded atom simulations and can be understood as small domains of an L10 ordered phase. DA - 1993/03/08/ PY - 1993 DO - 10.1103/PhysRevLett.70.1441 VL - 70 IS - 10 SP - 1441 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Inverse corrugation and corrugation enhancement of Pb superstructures on Cu(111) and (110) AU - Nagl, C. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - Contrary to intuitive expectation, a monatomic hexagonal close-packed Pb film on Cu(111) shows an "inverse" corrugation, i.e. the Pb atoms in on-top adsorption sites appear lowest in scanning tunneling microscopy (STM) images, as well as in effective medium theory (EMT) simulation. On small subsurface Cu islands on a Pb(111) substrate, which are also covered by a monatomic Pb film, the corrugation of this film was found to be strongly dependent on the thickness of the Cu island (the thinner the island, the larger the corrugation). EMT simulations reproduce this enhanced corrugation qualitatively and thus further confirm the formation of these subsurface Cu islands. On a p(8 × 1) superstructure of Pb/Cu(110), similarly, the lowest coordinated Pb atoms also show the lowest apparent height in STM images. DA - 1996/12/20/ PY - 1996 DO - 10.1016/S0039-6028(96)00907-7 VL - 369 IS - 1-3 SP - 159 EP - 168 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Pt(100) quasihexagonal reconstruction: A comparison between scanning tunneling microscopy data and effective medium theory simulation calculations AU - Ritz, G. AU - Schmid, M. AU - Varga, P. AU - Borg, A. AU - Rønning, M. T2 - Physical Review B AB - The interpretation of scanning tunneling microscopy (STM) data is usually limited to first-layer effects, but with increasing resolution of the STM images deeper-layer effects may also become visible in the top-layer corrugations. We have investigated the clean Pt(100) surface, which is known to be pseudohexagonally reconstructed and for which there is some evidence for a second-layer reconstruction. The big unit cell makes it difficult to investigate the deeper layers by traditional methods like low-energy-electron diffraction (LEED). We have, therefore, used a “fingerprint” technique to compare highly resolved STM data of the clean Pt(100) surface to effective-medium-theory simulation calculations in order to determine the geometric structure of the second atomic layer. We were able to show that STM can be sensitive to deeper layer effects and that excellent agreement could only be achieved for an unreconstructed second layer. The simulation results also agree well with the corrugations determined by LEED whereas the maximum corrugation amplitude is higher than previously derived from helium-diffraction measurements. DA - 1997/10/15/ PY - 1997 DO - 10.1103/PhysRevB.56.10518 VL - 56 IS - 16 SP - 10518 J2 - Phys. Rev. B ST - Pt(100) quasihexagonal reconstruction ER - TY - JOUR TI - Adatom interlayer diffusion on Pt(111): an embedded atom method study AU - Leonardelli, G. AU - Lundgren, E. AU - Schmid, M. T2 - Surface Science AB - We use embedded atom method (EAM) potentials to calculate the Schwoebel barriers for a large number of hopping and exchange processes of Pt and Ni adatoms descending steps of the Pt(111) surface. The barriers we find for hopping processes are too high to play any role in homo- and heteroepitaxy, but we find very low and even negative Schwoebel barriers for exchange processes at concave corners and kinks. On straight steps we find the process taking place on B-steps rather than on A-steps, with very similar Schwoebel barriers for Ni and Pt adatoms. We also find a strong dependence of the Schwoebel barrier on the lateral relaxation of step edges as caused by surface stress. For vicinal surfaces with high step density this effect causes an increase of the Schwoebel barrier if the width of the (111)-terraces is reduced. DA - 2001/09/01/ PY - 2001 DO - 10.1016/S0039-6028(01)01212-2 VL - 490 IS - 1-2 SP - 29 EP - 42 J2 - Surf. Sci. SN - 0039-6028 ST - Adatom interlayer diffusion on Pt(111) ER - TY - JOUR TI - A misfit structure in the Co/Pt(111) system studied by scanning tunnelling microscopy and embedded atom method calculations AU - Lundgren, E. AU - Leonardelli, G. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - We have performed scanning tunneling microscopy (STM) measurements of thin films of Co and CoPt alloys on Pt(111). In contrast to the interpretation of an earlier low energy electron diffraction investigation by Tsay and Shern [J.S. Tsay, C.S. Shern, Surf. Sci. 396 (1998) 313] we find that the structure observed upon annealing a Pt(111) sample with a Co film of 2 monolayer thickness does not consist of a rotated Co film but rather of a CoPt alloy film with hexagonal areas of fcc stacking, single and double tacking faults, delimited by misfit dislocations (Shockley partial dislocations). Comparison of STM images with embedded atom method (EAM) calculations confirms our model. The structure at the Pt-PtCo interface in our model is similar to that proposed by Henzler [M. Henzler, Surf. Sci. 419 (1999) 321]. DA - 2002/02/10/ PY - 2002 DO - 10.1016/S0039-6028(01)01754-X VL - 498 IS - 3 SP - 257 EP - 265 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Growth and decay of the Pd(111)-Pd5O4 surface oxide: Pressure-dependent kinetics and structural aspects AU - Gabasch, Harald AU - Unterberger, Werner AU - Hayek, Konrad AU - Klötzer, Bernhard AU - Kresse, Georg AU - Klein, Christof AU - Schmid, Michael AU - Varga, Peter T2 - Surface Science AB - Growth and decomposition of the Pd5O4 surface oxide on Pd(111) were studied at sample temperatures between 573 and 683 K and O2 gas pressures between 10-7 and 6 × 10-5 mbar, by means of an effusive O2 beam from a capillary array doser, scanning tunnelling microscopy (STM) and thermal desorption spectrometry (TDS). Exposures beyond the p(2 × 2)O adlayer (saturation coverage 0.25) at 683 K (near thermodynamic equilibrium with respect to Pd5O4 surface oxide formation) lead to incorporation of additional oxygen into the surface. To initiate the incorporation, a critical pressure beyond the thermodynamic stability limit of the surface oxide is required. This thermodynamic stability limit is near 8.9 × 10-6 mbar at 683 K, in good agreement with calculations by density functional theory. A controlled kinetic study was feasible by generating nuclei by only a short O2 pressure pulse and then following further growth kinetics in the lower (10-6 mbar) pressure range. Growth of the surface oxide layer at a lower temperature (573 K) studied by STM is characterized by a high degree of heterogeneity. Among various metastable local structures, a seam of disordered oxide formed at the step edges is a common structural feature characteristic of initial oxide growth. Further oxide nucleation appears to be favoured along the interface between the p(2 × 2)O structure and these disordered seams. Among the intermediate phases one specifically stable phase was detected both during growth and decomposition of the Pd5O4 layer. It is hexagonal with a distance of about 0.62 nm between the protrusions. Its well-ordered form is a (√67 x √67)R12.1° superstructure. Isothermal decay of the Pd5O4 oxide layer at 693 K involves at first a rearrangement into the (√67 x √67)R12.1° structure, indicating its high-temperature stability. This structure can break up into small clusters of uniform size and leaves a free metal surface area covered by a p(2 × 2)O adlayer. The rate of desorption increases autocatalytically with increasing phase boundary metal-oxide. We propose that at close-to-equilibrium conditions (693 K) surface oxide growth and decay occur via this intermediate structure. DA - 2006/01/01/ PY - 2006 DO - 10.1016/j.susc.2005.09.052 VL - 600 IS - 1 SP - 205 EP - 218 J2 - Surf. Sci. SN - 0039-6028 ST - Growth and decay of the Pd(1 1 1)-Pd5O4 surface oxide ER - TY - JOUR TI - Characterization of Ca impurity segregation on the TiO2(110) surface AU - Zhang, L. P. AU - Li, M. AU - Diebold, U. T2 - Surface Science AB - TiO2(110) single crystals with trace amounts of Ca impurities were investigated by ISS, XPS, LEED, and STM. After initial cleaning and with increasing annealing temperature, ISS and XPS clearly show the segregation of Ca to the surface of fresh TiO2(110) crystals. As revealed by LEED and STM, a [6 0; 3 1] structure is formed as an overlayer, with row-like features along the substrate [001] direction. In line with the Ca 2p3/2 peak position of 347.4 eV, we propose the formation of a CaTiO3-like compound, oriented with its (110) face parallel to the substrate. DA - 1998/09/03/ PY - 1998 DO - 10.1016/S0039-6028(98)00432-4 VL - 412-413 SP - 242 EP - 251 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Oxygen-induced restructuring of the rutile TiO2(110)(1×1) surface AU - Li, M AU - Hebenstreit, W AU - Diebold, U T2 - Surface Science AB - We report scanning tunneling microscopy (STM) results of bulk-reduced rutile TiO2 single crystals. TiO2(110) surfaces, prepared by sputtering and annealing at 850 K in UHV, exhibit a (1×1) surface termination and flat, several-hundred-angstrom-wide terraces. After exposure to oxygen at elevated temperatures (onset ~470 K), the surfaces are covered with small (typically tens of angstroms wide) terraces with monoatomic step height and the same (1×1) structure. On top and in between these terraces appear patches of an irregular network consisting of interconnected rosettes (width ~7 Å) with pseudohexagonal symmetry. The positions of atoms within the network are consistent with an incomplete TiO2(110) layer. This is a kinetically limited, metastable phase that easily transforms into the regular (1×1) structure upon further annealing. Oxygen-induced surface segregation of interstitial Ti atoms from the reduced bulk is invoked for this "restructuring" of the initially flat TiO2(110)(1×1) surface. DA - 1998/09/11/ PY - 1998 DO - 10.1016/S0039-6028(98)00549-4 VL - 414 IS - 1-2 SP - L951 EP - L956 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Oxygen-induced restructuring of the TiO2(110) surface: a comprehensive study AU - Li, Min AU - Hebenstreit, Wilhelm AU - Gross, Leo AU - Diebold, Ulrike AU - Henderson, M. A. AU - Jennison, D. R. AU - Schultz, P. A. AU - Sears, M. P. T2 - Surface Science AB - We report a comprehensive experimental and theoretical study of the effect of oxidizing a TiO2(110) surface at moderate temperatures. The surfaces are investigated with scanning tunneling microscopy (STM), low-energy He+ ion scattering (LEIS) and static secondary ion mass spectroscopy (SSIMS). Flat (1×1)-terminated TiO2(110) surfaces are obtained by sputtering and annealing in UHV at 880 K. These surfaces are exposed to oxygen gas at elevated temperatures in the range 470–830 K. Formation of irregular networks of pseudo-hexagonal rosettes (6.5 Å×6 Å) and small (1Image 0] oriented (1×1) islands along with {001}-oriented strands is induced at temperatures from 470 to 660 K. After annealing above 830 K, only regular (1×1) terraces and white strands are observed. The composition of these oxygen-induced phases is quantified using 18O2 gas in combination with LEIS and SSIMS measurements. The dependence of the restructuring process on annealing time, annealing temperature, and sample history is systematically investigated. Exposure to H218O and air in the same temperature regime fails to induce the restructuring. UHV annealing of restructured, oxygen-enriched TiO2(110) surface smooths the surfaces and converts the rosette networks into strands and finally into the regular (1×1) terraces. This is reported in an accompanying paper [M. Li, W. Hebenstreit, U. Diebold, Phys. Rev. B (1999), submitted]. The rosette model is supported by first-principles density functional calculations which show a stable structure results, accompanied by significant relaxations from bulk-truncated positions. A mechanism for the dynamic processes of the formation of rosettes and (1×1) islands is presented and the importance of these results for the surface chemistry of TiO2(110) surfaces is discussed. DA - 1999/08/20/ PY - 1999 DO - 10.1016/S0039-6028(99)00720-7 VL - 437 IS - 1-2 SP - 173 EP - 190 J2 - Surf. Sci. SN - 0039-6028 ST - Oxygen-induced restructuring of the TiO2(110) surface ER - TY - JOUR TI - Oxygen-induced restructuring of rutile TiO2(110): formation mechanism, atomic models, and influence on surface chemistry AU - Li, Min AU - Hebenstreit, Wilhelm AU - Diebold, Ulrike AU - Henderson, Michael A. AU - Jennison, Dwight R. T2 - Faraday Discussions AB - The rutile TiO2(110) (1x1) surface is considered the prototypical 'well-defined' system in the surface science of metal oxides. Its popularity results partly from two experimental advantages: (i) bulk-reduced single crystals do not exhibit charging, and (ii) stoichiometric surfaces, as judged by electron spectroscopies, can be prepared reproducibly by sputtering and annealing in oxygen. We present results that show that this commonly applied preparation procedure may result in a surface structure that is by far more complex than generally anticipated. Flat, (1x1)-terminated surfaces are obtained by sputtering and annealing in ultrahigh vacuum. When re-annealed in oxygen at moderate temperatures (470-660 K), irregular networks of partially connected, pseudohexagonal rosettes (6.5x6 Å wide), one-unit cell wide strands, and small (≈tens of Å) (1x1) islands appear. This new surface phase is formed through reaction of oxygen gas with interstitial Ti from the reduced bulk. Because it consists of an incomplete, kinetically limited (1x1) layer, this phenomenon has been termed 'restructuring'. We report a combined experimental and theoretical study that systematically explores this restructuring process. The influence of several parameters (annealing time, temperature, pressure, sample history, gas) on the surface morphology is investigated using STM. The surface coverage of the added phase as well as the kinetics of the restructuring process are quantified by LEIS and SSIMS measurements in combination with annealing in 18O-enriched gas. Atomic models of the essential structural elements are presented and are shown to be stable with first-principles density functional calculations. The effect of oxygen-induced restructuring on surface chemistry and its importance for TiO2 and other bulk-reduced oxide materials is briefly discussed. DA - 1999/// PY - 1999 DO - 10.1039/a903598b VL - 114 SP - 245 EP - 258 J2 - Faraday Discuss. ST - Oxygen-induced restructuring of rutile TiO2(110) ER - TY - JOUR TI - Morphology change of oxygen-restructured TiO2(110) surfaces by UHV annealing: Formation of a low-temperature (1×2) structure AU - Li, Min AU - Hebenstreit, Wilhelm AU - Diebold, Ulrike T2 - Physical Review B AB - When reduced TiO2(110) single crystals are oxidized at moderate temperatures (470–660 K), the surfaces restructure. Interstitial Ti atoms from the bulk diffuse to the surface where they react with gaseous oxygen and form new, added TiO2 layers. These are characterized by three structural elements: small (1×1) islands, irregular networks of pseudohexagonal rosettes, and [001]-oriented strands. The strands exhibit the same structural characteristics as the (1×2) surface reconstruction, which forms upon annealing at higher temperatures. Atomic-resolution scanning tunneling microscopy images of the strands are consistent with the added-Ti2O3-row model. UHV annealing of oxygen-restructured surfaces smooths the surfaces and converts the rosette networks into strands and finally into regular (1×1) terraces. The composition of these oxygen-induced phases is quantified using 18O2 gas in combination with low-energy He+ ion scattering measurements. Dynamic processes for the conversion from rosette networks into (1×2) strands and ultimately into (1×1) terraces are discussed. DA - 2000/02// PY - 2000 DO - 10.1103/PhysRevB.61.4926 VL - 61 IS - 7 SP - 4926 EP - 4933 J2 - Phys. Rev. B SN - 0163-1829 ST - Morphology change of oxygen-restructured TiO2(110) surfaces by UHV annealing ER - TY - JOUR TI - Adsorption of sulfur on TiO2(110) studied with STM, LEED and XPS: temperature-dependent change of adsorption site combined with O-S exchange AU - Hebenstreit, E. L. D. AU - Hebenstreit, W. AU - Diebold, U. T2 - Surface Science AB - We have studied the adsorption of elemental sulfur on TiO2(110) for exposure at room temperature and at 300°C. Depending on the substrate temperature we found different adsorption sites with scanning tunneling microscopy (STM). At room temperature sulfur adsorbs on the titanium rows with a high mobility along the [001] direction. At 300°C sulfur adsorbs at the position of the bridging oxygen rows and forms short chains along the [1Image 0] direction at low coverages. High coverages result in the formation of a (3×1) superstructure. X-ray photoelectron spectroscopy (XPS) shows a 2 eV shift of the sulfur 2p peak to lower binding energy concurrently with the change of adsorption site from the titanium to the oxygen rows. For the (3×1) structure a distinct Ti3+ shoulder appears at the Ti 2p3/2 peak. Based on these measurements we present a model where sulfur replaces every third bridging oxygen atom of the substrate and the other bridging oxygens are completely removed. DA - 2000/08/01/ PY - 2000 DO - 10.1016/S0039-6028(00)00538-0 VL - 461 IS - 1-3 SP - 87 EP - 97 J2 - Surf. Sci. SN - 0039-6028 ST - Adsorption of sulfur on TiO2(110) studied with STM, LEED and XPS ER - TY - JOUR TI - Scanning tunneling microscopy investigation of the TiO2 anatase (101) surface AU - Hebenstreit, Wilhelm AU - Ruzycki, Nancy AU - Herman, Gregory AU - Gao, Yufei AU - Diebold, Ulrike T2 - Physical Review B AB - We report the first scanning tunneling microscopy (STM) study of single-crystalline anatase. Atomically resolved images of the (101) surface are consistent with a bulk-truncated (1×1) termination. Step edges run predominantly in the [010], [-111], and [-1-11] directions. The surface is stable with very few point defects. Fourfold-coordinated Ti atoms at step edges are preferred adsorption sites and allow the identification of tunneling sites in STM. DA - 2000/12// PY - 2000 DO - 10.1103/PhysRevB.62.R16334 VL - 62 IS - 24 SP - R16334 EP - R16336 J2 - Phys. Rev. B SN - 0163-1829 ER - TY - JOUR TI - Structures of sulfur on TiO2(110) determined by scanning tunneling microscopy, X-ray photoelectron spectroscopy and low-energy electron diffraction AU - Hebenstreit, E. L. D. AU - Hebenstreit, W. AU - Diebold, U. T2 - Surface Science AB - The temperature dependent adsorption of sulfur on TiO2(1 1 0) has been studied with X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). Sulfur adsorbs dissociatively at room temperature and binds to fivefold coordinated Ti atoms. Upon heating to ~120°C, 80% of the sulfur desorbs and the S 2p peak position changes from 164.3±0.1 to 162.5±0.1 eV. This peak shift corresponds to a change of the adsorption site to the position of the bridging oxygen atoms of TiO2(1 1 0). Further heating causes little change in S coverage and XPS binding energies, up to a temperature of ~430°C where most of the S desorbs and the S 2p peak shifts back to higher binding energy. Sulfur adsorption at 150°C, 200°C, and 300°C leads to a rich variety of structures and adsorption sites as observed with LEED and STM. At low coverages, sulfur occupies the position of the bridging oxygen atoms. At 200°C these S atoms arrange in a (3×1) superstructure. For adsorption between 300°C and 400°C a (3×3) and (4×1) LEED pattern is observed for intermediate and saturation coverage, respectively. Adsorption at elevated temperature reduces the substrate as indicated by a strong Ti3+ shoulder in the XPS Ti 2p3/2 peak, with up to 15.6% of the total peak area for the (4×1) structure. STM of different coverages adsorbed at 400°C indicates structural features consisting of two single S atoms placed next to each other along the [0 0 1] direction at the position of the in-plane oxygen atoms. The (3×3) and the (4×1) structure are formed by different arrangements of these S pairs. DA - 2001/01/01/ PY - 2001 DO - 10.1016/S0039-6028(00)00849-9 VL - 470 IS - 3 SP - 347 EP - 360 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Sulfur on TiO2(110) studied with resonant photoemission AU - Hebenstreit, E. AU - Hebenstreit, W. AU - Geisler, H. AU - Thornburg, S. AU - Ventrice, C. AU - Hite, D. AU - Sprunger, P. AU - Diebold, U. T2 - Physical Review B AB - Adsorption of sulfur on TiO2(110) at room temperature (RT) and 350°C has been studied with ultraviolet photoelectron spectroscopy. A TiO2(110) (1×1) surface with a small amount of oxygen vacancies was prepared by sputtering and annealing in ultrahigh vacuum. Oxygen vacancies induce a defect state that pins the Fermi level just below the conduction-band minimum. Sulfur adsorption at room temperature leads to the disappearance of this vacancy-related band-gap state, indicating that the surface oxygen vacancies are filled by sulfur. Sulfur-induced valence-band features are identified at binding energies of 3.4 and 8 eV. Adsorption of S at 350°C forms a (4×1) superstructure at high coverages [≈0.9 monolayer (ML)] that is visible with low-energy electron diffraction. In a previously proposed model for this superstructure, sulfur replaces half of the in-plane oxygen atoms and all the bridging oxygen atoms are removed. In agreement with this model, the oxygen 2s peak is decreased significantly and the defect state is increased. Two additional valence features are observed: one at 2.7 eV and one at 3.9 eV. Due to those features the band gap vanishes. In resonant photoemission, these features show a similar, but weaker, resonance profile than the vacancy-related defect state. Hybridized Ti-derived states extend across the whole valence-band region. Generally, a higher resonant photon energy is found for valence-band states with lower binding energies, indicating mainly 3p-4s transitions in the upper valence band. Adsorption of sulfur reduces the strength of the resonances. DA - 2001/08// PY - 2001 DO - 10.1103/PhysRevB.64.115418 VL - 64 IS - 11 SP - 115418 J2 - Phys. Rev. B SN - 0163-1829 ER - TY - JOUR TI - The adsorption of chlorine on TiO2(110) studied with scanning tunneling microscopy and photoemission spectroscopy AU - Hebenstreit, E. L. D. AU - Hebenstreit, W. AU - Geisler, H. AU - Ventrice, C. A. AU - Hite, D. A. AU - Sprunger, P. T. AU - Diebold, U. T2 - Surface Science AB - The adsorption of chlorine, dosed from an electrochemical cell, on the (1 1 0)-surface of a bulk-reduced TiO2 crystal was studied with scanning tunneling microscopy, X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy (UPS). At room temperature chlorine adsorbs on the fivefold coordinated Ti atoms and on the vacancies in the bridging oxygen rows. Chlorine exposure at 200 and 300 °C leads to adsorption at the position of bridging oxygen atoms. In addition, not, vert, similar15 Å wide protrusions are formed that contain several chlorine atoms. No long-range ordering was found in the adsorbed layer. UPS shows that adsorption of Cl on the hot surface leads to an increase in the Ti 3d-related defect state. This indicates a replacement of bridging oxygen atoms by chlorine. Interestingly, the rate of the site exchange depends on the bulk reduction state of the TiO2 crystal. After dosing Cl at 200 °C, negligible adsorption was found on a slightly reduced, light blue crystal. The data are compared and contrasted with similar experimental results on the S/TiO2(110) system. DA - 2002/05// PY - 2002 DO - 10.1016/S0039-6028(02)01385-7 VL - 505 SP - 336 EP - 348 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Ab initio and experimental studies of chlorine adsorption on the rutile TiO2 (110) surface AU - Vogtenhuber, Doris AU - Podloucky, Raimund AU - Redinger, Josef AU - Hebenstreit, Eleonore AU - Hebenstreit, Wilhelm AU - Diebold, Ulrike T2 - Physical Review B AB - We report a comprehensive study on the adsorption of Cl on the clean rutile TiO2 (110) (1×1) surface. STM and photoemission spectroscopy results are compared to ab initio results. At room temperature, Cl adsorbs dissociatively and binds to the fivefold coordinated Ti atoms in an on-top configuration. The calculations predict energetically more favorable adsorption on a reduced surface, where bridging oxygen atoms are missing in a (1×4) geometry. Experimental photoemission data indicate a quenching of the oxygen-vacancy-related defect state upon chlorine adsorption at room temperature, in agreement with the theoretical results. Chlorine atoms appear as bright, extended spots in experimental empty-states STM images. Calculations of density-of-states contours indicate that Ti states underneath the Cl atoms are predominantly responsible for the tunneling current. When chlorine is dosed onto a hot surface, a replacement of bridging oxygen atoms is observed in addition to features of unidentified structure and stoichiometry. The experimental results on the change in work function and shift of Cl core levels agree semiquantitatively with calculations of different test geometries. DA - 2002/03// PY - 2002 DO - 10.1103/PhysRevB.65.125411 VL - 65 IS - 12 SP - 125411 J2 - Phys. Rev. B SN - 0163-1829 ER - TY - JOUR TI - Surface Structure of TiO2(011)-(2×1) AU - Beck, T. AU - Klust, Andreas AU - Batzill, Matthias AU - Diebold, Ulrike AU - Di Valentin, Cristiana AU - Selloni, Annabella T2 - Physical Review Letters AB - A combined experimental and first principles study of the (2×1)-reconstructed rutile TiO2(011) surface is presented. Our results provide evidence that the surface structure is described by a model that includes onefold coordinated (titanyl) oxygen atoms giving rise to double bonded Ti=O species. These species should play a special role in the enhanced photocatalytic activity of the TiO2(011) surface. DA - 2004/07// PY - 2004 DO - 10.1103/PhysRevLett.93.036104 VL - 93 IS - 3 SP - 036104 J2 - Phys. Rev. Lett. SN - 0031-9007 ER - TY - JOUR TI - Dispersed Au atoms, supported on TiO2(110) AU - Diebold, Ulrike T2 - Surface Science DA - 2005/03/10/ PY - 2005 DO - 10.1016/j.susc.2005.01.029 VL - 578 IS - 1-3 SP - 1 EP - 3 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Adsorption of Water on Reconstructed Rutile TiO2(011)-(2×1): Ti=O Double Bonds and Surface Reactivity AU - Di Valentin, Cristiana AU - Tilocca, Antonio AU - Selloni, Annabella AU - Beck, T. J. AU - Klust, Andreas AU - Batzill, Matthias AU - Losovyj, Yaroslav AU - Diebold, Ulrike T2 - Journal of the American Chemical Society AB - Recent combined experimental and theoretical studies (Beck et al., Phys. Rev. Lett. 2004, 93, 036104) have provided evidence for Ti=O double-bonded titanyl groups on the reconstructed rutile TiO2(011)-(2x1) surface. The adsorption of water on the same surface is now investigated to further probe the properties of these groups, as well as to confirm their existence. Ultraviolet photoemission experiments show that water is adsorbed in molecular form at a sample temperature of 110 K. At the same time, the presence of a 3sigma state in the photoemission spectra and work function measurements indicate a significant amount of hydroxyls within the first monolayer of water. At room temperature, scanning tunneling microscopy (STM) suggests that dissociated water is present, and about 30% of the surface active sites are hydroxylated. These findings are well explained by total energy density functional theory calculations and CarParrinello molecular dynamics simulations for water adsorption on the titanyl model of TiO2(011)-(21). The theoretical results show that a mixed molecular/dissociative layer is the most stable configuration in the monolayer regime at low temperatures, while complete dissociation takes place at 250 K. The arrangement of the protonated mono-coordinated oxygens in the mixed molecular/dissociated layer is consistent with the observed short-range order of the hydroxyls in the STM images. DA - 2005/07/01/ PY - 2005 DO - 10.1021/ja0511624 VL - 127 IS - 27 SP - 9895 EP - 9903 J2 - J. Am. Chem. Soc. ST - Adsorption of Water on Reconstructed Rutile TiO2(011)-(2×1) ER - TY - JOUR TI - Structure, defects, and impurities at the rutile TiO2(011)-(2 × 1) surface: A scanning tunneling microscopy study AU - Dulub, Olga AU - Valentin, Cristiana Di AU - Selloni, Annabella AU - Diebold, Ulrike T2 - Surface Science AB - The titanium dioxide rutile (0 1 1) (equivalent to (1 0 1)) surface reconstructs to a stable (2 × 1) structure upon sputtering and annealing in ultrahigh vacuum. A previously proposed model (T.J. Beck, A. Klust, M. Batzill, U. Diebold, C. Di Valentin, A. Selloni, Phys. Rev. Lett. 93 (2004) 036104/1) containing onefold coordinated oxygen atoms (titanyl groups, TiO) is supported by Scanning Tunneling Microscopy (STM) measurements. These TiO sites are imaged bright in empty-states STM. A few percent of these terminal oxygen atoms are missing at vacuum-annealed surfaces of bulk-reduced samples. These O vacancies are imaged as dark spots. Their number density depends on the reduction state of the bulk. Double vacancies are the most commonly observed defect configuration; single vacancies and vacancies involving several O atoms are present as well. Formation of oxygen vacancies can be suppressed by annealing a sputtered surface first in vacuum and then in oxygen; annealing a sputtered surface in oxygen results in surface restructuring and a (3 × 1) phase. Anti-phase domain boundaries in the (2 × 1) structure are active adsorption sites. Segregation of calcium impurities from the bulk results in an ordered overlayer that exhibits domains with a centered (2 × 1) periodicity in STM. DA - 2006/10/01/ PY - 2006 DO - 10.1016/j.susc.2006.06.042 VL - 600 IS - 19 SP - 4407 EP - 4417 J2 - Surf. Sci. SN - 0039-6028 ST - Structure, defects, and impurities at the rutile TiO2(0 1 1)-(2 × 1) surface ER - TY - JOUR TI - Scanning tunneling microscopy of binary-alloy surfaces: is chemical contrast a consequence of alloying? AU - Hofer, W. A. AU - Ritz, G. AU - Hebenstreit, W. AU - Schmid, M. AU - Varga, P. AU - Redinger, J. AU - Podloucky, R. T2 - Surface Science AB - Recent STM studies achieved chemical resolution on PtRh and PtNi alloy surfaces. By a first-principles method employing the Tersoff-Hamann model, we have simulated STM scans on PtRh and PtNi(100) surfaces by calculating the apparent heights of individual surface atoms. The difference in apparent heights between Pt and Rh atoms is caused by changes in the density of states due to alloying. The simulations for the PtNi(100) surface, however, yield apparent heights of Pt and Ni atoms below atomic resolution, indicating that in the experiment, tip-sample interactions are responsible for chemical and atomic resolution. DA - 1998/05/15/ PY - 1998 DO - 10.1016/S0039-6028(98)00140-X VL - 405 IS - 2-3 SP - L514 EP - L519 J2 - Surf. Sci. SN - 0039-6028 ST - Scanning tunneling microscopy of binary-alloy surfaces ER - TY - JOUR TI - Oxygen-induced vacancy formation on a metal surface AU - Schmid, M. AU - Leonardelli, G. AU - Sporn, M. AU - Platzgummer, E. AU - Hebenstreit, W. AU - Pinczolits, M. AU - Varga, P. T2 - Physical Review Letters AB - Using scanning tunneling microscopy, low-energy ion scattering, and quantitative low-energy electron diffraction, we find about 17% metal vacancies on the oxygen-covered Cr(100) surface. The oxygen atoms occupy all the hollow sites of the first layer, including those neighboring a Cr vacancy. We argue that the vacancy formation is energetically favored and not caused by stress but by electronic effects. DA - 1999/01/11/ PY - 1999 DO - 10.1103/PhysRevLett.82.355 VL - 82 IS - 2 SP - 355 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Atomic resolution by STM on ultra-thin films of alkali halides: experiment and local density calculations AU - Hebenstreit, W. AU - Redinger, J. AU - Horozova, Z. AU - Schmid, M. AU - Podloucky, R. AU - Varga, P. T2 - Surface Science AB - Atomically resolved scanning tunneling microscopy (STM) of ultra-thin NaCl films on Al(111) and Al(100) demonstrates that only one atomic species of NaCl is imaged as a protrusion. By comparison of the constant current STM images with ab-initio calculations of the local density of states (LDOS) by means of the full-potential linearized augmented plane wave (FLAPW) method, the protrusions could be attributed to the anion Cl-. The calculations show that a higher density of occupied states at the Cl sites than for the Na sites around the Fermi level causes the STM contrast between Cl and Na. With increasing number of NaCl layers the density of states in the bandgap is reduced and the apparent height of additional NaCl layers decreases. The maximum film thickness allowing successful imaging by STM was found to be three layers. DA - 1999/04/01/ PY - 1999 DO - 10.1016/S0039-6028(99)00095-3 VL - 424 IS - 2-3 SP - L321 EP - L328 J2 - Surf. Sci. SN - 0039-6028 ST - Atomic resolution by STM on ultra-thin films of alkali halides ER - TY - JOUR TI - Surface and subsurface alloy formation of vanadium on Pd(111) AU - Konvicka, Ch. AU - Jeanvoine, Y. AU - Lundgren, E. AU - Kresse, G. AU - Schmid, M. AU - Hafner, J. AU - Varga, P. T2 - Surface Science AB - We have studied the submonolayer growth of vanadium on the Pd(111) surface at different substrate temperatures. By using LEIS (Low energy ion spectroscopy), AES (Auger electron spectroscopy), STM (Scanning tunneling microscopy), XPD (X-ray photoelectron diffraction) and ab-initio local-density-functional calculations we find that V atoms deposited at room temperature substitute surface Pd atoms. In addition, islands are formed on the surface, which consist mostly of the substituted Pd atoms. At higher temperatures, V diffuses into subsurface layers and at a temperature of 300 °C only a small amount of V is observed in the top-most layer. By using STM a (√3 x √3)R30° superstructure is observed to be formed and XPD measurements demonstrate that this structure is due to V atoms incorporated in the second layer. This finding is confirmed by ab-initio calculations. Further, a model for the (√3 x √3)R30° structure based on the experiments and the ab-initio calculations is given. DA - 2000/09/10/ PY - 2000 DO - 10.1016/S0039-6028(00)00643-9 VL - 463 IS - 3 SP - 199 EP - 210 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Direct imaging of catalytically important processes in the oxidation of CO over RuO2(110) AU - Over, H. AU - Seitsonen, A. P. AU - Lundgren, E. AU - Schmid, M. AU - Varga, P. T2 - Journal of the American Chemical Society AB - Ruthenium dioxide (RuO2) reveals unique and promising redox properties, making RuO2 a potential candidate for a versatile oxidation catalyst. Recently Zhang and Kisch reported, for instance, that RuO2 is a robust and efficient catalyst for room temperature (RT) oxidation of CO by humid air; recall that typical metal oxides do not tolerate humidity. In this contribution we present scanning tunneling microscopy (STM) data which directly image the catalytically important processes occurring on the RuO2(110) surface after exposing the pristine surface to CO and O2. These data are complemented by density functional theory (DFT) calculations. The following processes are governing the catalytic activity of RuO2 on atomic scale. The reactants from the gas phase encounter strongly binding adsorption sites on the RuO2(110) surface in the form of the under-coordinated Ru atoms. For instance, CO adsorbs on the stoichiometric RuO2(110) surface by 1.2 eV (over the 1f-cus-Ru), while on the reduced RuO2(110) surface the CO binding energies are 1.61 eV and 1.85 eV for adsorption over 1f-cus-Ru and 2f-cus-Ru atoms, respectively. The RuO2 surface provides an active oxygen species to react with CO, i.e. the under-coordinated (bridging) lattice oxygen atoms Obr. The recombination of adsorbed CO with Obr creates vacancies, which are identified with STM. At RT, oxygen molecules from the gas phase can efficiently dissociate on RuO2(110) via the molecular precursor state. This leads to weakly held O atoms, which are grouped in pairs as imaged with STM. It is argued that the weakly held oxygen plays an important role in replenishing the consumed (bridging) lattice oxygen atoms on the (partially) reduced RuO2(110) surface. DA - 2001/11/01/ PY - 2001 DO - 10.1021/ja016408t VL - 123 IS - 47 SP - 11807 EP - 11808 J2 - J. Am. Chem. Soc. ER - TY - JOUR TI - Oxygen adsorption on Al(111): low transient mobility AU - Schmid, M. AU - Leonardelli, G. AU - Tscheließnig, R. AU - Biedermann, A. AU - Varga, P. T2 - Surface Science AB - Adsorption of oxygen on Al(111) is studied by scanning tunneling microscopy at 80 and 300 K. After adsorption at 130-195 K, STM images taken at 80 K show pairs of oxygen adatoms with interatomic distances mainly between one and three Al interatomic spacings. This clearly shows that dissociation of the oxygen molecules results in a rather low transient mobility of the two oxygen atoms, a fact which is in contrast to previous work [Phys. Rev. Lett. 68 (1992) 624]. We also find evidence for oxygen atoms in a second metastable adsorption site at these temperatures. At room temperature, we find groups of two or more oxygen atoms in adjacent fcc hollow sites, but no single oxygen atoms. We therefore explain the room-temperature results by part of the oxygen pairs remaining or becoming nearest neighbors, whereas others separate by diffusion and their oxygen atoms attach to other pairs or groups, forming the larger groups found. The pairs and larger groups are stable due to an attractive interaction of oxygen atoms in adjacent fcc hollow sites. DA - 2001/05/10/ PY - 2001 DO - 10.1016/S0039-6028(01)00967-0 VL - 478 IS - 3 SP - L355 EP - L362 J2 - Surf. Sci. SN - 0039-6028 ST - Oxygen adsorption on Al(111) ER - TY - JOUR TI - The structure of the oxygen induced (1×5) reconstruction of V(100) AU - Koller, R. AU - Bergermayer, W. AU - Kresse, G. AU - Hebenstreit, E. L. D. AU - Konvicka, C. AU - Schmid, M. AU - Podloucky, R. AU - Varga, P. T2 - Surface Science AB - The adsorption of 1 Langmuir oxygen on the V(100) surface at 200 °C leads to a (1x5) reconstruction. The structure of this surface was determined by scanning tunneling microscopy (STM), low energy electron diffraction (quantitative LEED) and density functional theory calculations. The STM images show dark lines every fifth vanadium lattice constant. Between these (1x5) lines, oxygen resides in four-fold coordinated hollow sites with a coverage of ~70%. From the LEED analysis (Pendry R-factor = 0.17) it was found that in the dark lines oxygen adsorbs in bridge sites, in agreement with the ab initio calculations. The driving force behind this reconstruction is surface stress induced by the vanadium-oxygen bonds. DA - 2001/05/30/ PY - 2001 DO - 10.1016/S0039-6028(01)00978-5 VL - 480 IS - 1-2 SP - 11 EP - 24 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Experimental and simulated STM images of stoichiometric and partially reduced RuO2(110) surfaces including adsorbates AU - Over, H. AU - Seitsonen, A. P. AU - Lundgren, E. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - We present experimental and DFT-simulated STM images of ultrathin RuO2(110) films on Ru(0001), including adsorbates such as oxygen and CO. We are able to identify the under-coordinated O atoms on the RuO2(110) surface with STM, i.e. the bridging O atoms and the on-top O atoms. The partial reduction of the RuO2(110) surface by CO exposure at room temperature leads to a surface where part of the bridging O atoms have been removed and some of the vacancies are occupied by bridging CO. When dosing 10 L of CO at room temperature, all the RuO2(110) surface becomes mildly reduced in that all bridging O atoms are replaced by bridging CO molecules. Annealing the surface to 600 K produces holes on the terraces of such a mildly reduced RuO2(110) surface. These pits are not generated by the recombination of lattice O with CO, but rather these pits are assigned to a complex temperature-induced rearrangement of surface atoms in the topmost RuO2 double layer of RuO2(110). With this process the bridging O atoms are again populated and surplus Ru atoms agglomerate in small islands at the rims of the holes. DA - 2002/08// PY - 2002 DO - 10.1016/S0039-6028(02)01853-8 VL - 515 IS - 1 SP - 143 EP - 156 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - The structure of the oxygen-induced c(6×2) reconstruction of V(110) AU - Koller, R. AU - Bergermayer, W. AU - Kresse, G. AU - Konvicka, C. AU - Schmid, M. AU - Redinger, J. AU - Podloucky, R. AU - Varga, P. T2 - Surface Science AB - The adsorption of 2.4 Langmuir oxygen on V(110) induces a c(6×2) reconstruction with an oxygen coverage of 0.5 ML. Its structure was determined using STM, quantitative LEED and ab initio density functional calculations in combination with molecular dynamics. Driven by the strong vanadium-oxygen bonding, the vanadium atoms at the surface are significantly rearranged compared to their bulk positions. The reconstructed geometry offers threefold and fourfold coordinated hollow sites, which are partially occupied by oxygen. The large set of structural data derived from LEED I-V analysis (R_Pe=0.11) and ab initio calculations, as well as the experimental and simulated STM images agree well. Additionally the structure of clean V(110) was determined. DA - 2002/06/20/ PY - 2002 DO - 10.1016/S0039-6028(02)01722-3 VL - 512 IS - 1-2 SP - 16 EP - 28 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Interaction of oxygen with palladium deposited on a thin alumina film AU - Shaikhutdinov, Sh. AU - Heemeier, M. AU - Hoffmann, J. AU - Meusel, I. AU - Richter, B. AU - Bäumer, M. AU - Kuhlenbeck, H. AU - Libuda, J. AU - Freund, H. -J. AU - Oldman, R. AU - Jackson, S. D. AU - Konvicka, C. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - The interaction of oxygen with Pd particles, vapor deposited onto a thin alumina film grown on a NiAl(110) substrate, was studied by STM, AES, LEED, XPS, TPD and molecular beam techniques. The results show that O2 exposure at 400-500 K strongly influences the oxide support. We suggest that the oxygen atoms formed by dissociation on the Pd surface can diffuse through the alumina film and react with the NiAl substrate underneath the Pd particles, thus increasing the thickness of the oxide film. The surface oxygen inhibits hydrogen adsorption, and readily reacts with CO at 300-500 K. For large and crystalline Pd particles, the system exhibits adsorption-desorption properties which are very similar to those of the Pd(111) single crystal surface. The molecular beam and TPD experiments reveal that, at low coverage, CO adsorbs slightly stronger on the smaller Pd particles, with an adsorption energy difference of ca. 5-7 kJmol^-1 for 1 and 3-5 nm Pd particles studied. DA - 2002/04/01/ PY - 2002 DO - 10.1016/S0039-6028(01)01850-7 VL - 501 IS - 3 SP - 270 EP - 281 J2 - Surf. Sci. SN - 0039-6028 ST - Interaction of O with Pd/alumina ER - TY - JOUR TI - Superstructures of carbon on V(100) AU - Bergermayer, W. AU - Koller, R. AU - Konvicka, C. AU - Schmid, M. AU - Kresse, G. AU - Redinger, J. AU - Varga, P. AU - Podloucky, R. T2 - Surface Science AB - Carbon adsorption on V(100) was studied by both experimental methods and density functional theory. At low carbon coverages of theta_C = 0.18 ML and oxygen below the experimental detection limit, measured scanning tunneling microscopy (STM) images show both areas of local c(2x2) structure and <010> oriented rows of C atoms. At higher coverages of ThetaC = 0.41 ML, mainly <010> oriented C rows with some local p(1x2) patterns are formed. The observed c(2x2) pattern is attributed to the presence of oxygen, since a mixture of carbon and oxygen favours the c(2x2) superstructure according to both the STM and the ab initio results. The calculations show that for theta_C = 0.50 ML the p(1x2) structure is more stable than c(2x2) by 0.13 eV per adsorbed atom. From the ab initio results it is predicted that p(1x2) changes into c(2x2) at a mixed coverage of about theta_C ≈ 0.37 ML and theta_O ≈ 0.13 ML. The geometry of the c(2x2) structure was determined using quantitative LEED showing good agreement with the ab initio data. Also the simulated STM images agree well with the experimental STM data. DA - 2002/01/20/ PY - 2002 DO - 10.1016/S0039-6028(01)01659-4 VL - 497 IS - 1-3 SP - 294 EP - 304 J2 - Surf. Sci. SN - 0039-6028 ST - Carbon on V(100) ER - TY - JOUR TI - Stabilizing single metal adatoms at room temperature: Pd on C- and O-covered V(100) AU - Konvicka, C. AU - Hammerschmid, A. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - The initial stages of Pd thin film growth on clean and C- and O-precovered V(100) surfaces at room temperature (RT) and 200 °C have been studied by means of scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). In the presence of C and O, the adsorption of Pd atoms in the clean four-fold hollow sites of the V surface is strongly preferred. Upon deposition of Pd in the submonolayer range, it was possible to stabilize single Pd adatoms and small clusters at RT. Annealing such a surface at 200 °C leads to the formation of rectangular Pd islands (size ≈40×40 Å2) and to the compression of the initial C and O adlayer in the areas between the Pd. For higher Pd coverages we found that oxygen acts as an anti-surfactant, shifting the onset of second layer growth dramatically. The reason for the change of the growth mode of Pd on V(100) from Stranski–Krastanov to a Volmer–Weber-type growth in the presence of oxygen can be found in a higher free energy of the film/substrate interface compared to the clean surface. DA - 2002/01/10/ PY - 2002 DO - 10.1016/S0039-6028(01)01605-3 VL - 496 IS - 3 SP - 209 EP - 220 J2 - Surf. Sci. SN - 0039-6028 ST - Single metal adatoms at room temperature ER - TY - JOUR TI - Complex surface reconstructions solved by ab initio molecular dynamics AU - Kresse, G. AU - Bergermayer, W. AU - Podloucky, R. AU - Lundgren, E. AU - Koller, R. AU - Schmid, M. AU - Varga, P. T2 - Applied Physics A: Materials Science & Processing AB - Abstract. Complex surface reconstructions and surface oxides, in particular, often exhibit complicated atomic arrangements, which are difficult to resolve with traditional experimental methods, such as low energy electron diffraction (LEED), surface X-ray diffraction (SXRD) or scanning tunnelling microscopy (STM) alone. Therefore, ab initio density functional calculations are used as a supplement to the experimental techniques, but even then the structural determination usually relies on a simple trial and error procedure, in which conceivable models are first constructed and then tested for their stability in ab initio calculations. An exhaustive search of the configuration space is usually difficult and requires a significant human effort. Solutions to this problem, such as simulated annealing, have long been known, but are usually considered to be too time-consuming in combination with first principles methods. In this work, we show that ab initio density functional codes are now sufficiently fast to perform extensive finite temperature molecular dynamics. The merits of this approach are exemplified for two cases, for a complex two-dimensional surface oxide on Pd(111), and for the oxygen induced c(6×2) reconstruction of V(110). DA - 2003/03/15/ PY - 2003 DO - 10.1007/s00339-002-2007-2 VL - 76 IS - 5 SP - 701 EP - 710 J2 - Appl. Phys. A ER - TY - JOUR TI - Structure of the cobalt-filled missing-row reconstruction of Pt(110) AU - Klein, C. AU - Koller, R. AU - Lundgren, E. AU - Máca, F. AU - Redinger, J. AU - Schmid, M. AU - Varga, P. T2 - Physical Review B AB - The atomic structure of 0.5 monolayer (ML) Co deposited on Pt(110) was investigated by quantitative low-energy electron diffraction and ab initio density functional theory calculations, showing a pronounced inward relaxation and a filling of the missing-row sites of the Pt(110) substrate by Co atoms. Up to this Co coverage no significant intermixing of Pt atoms with Co atoms was observed by scanning tunneling microscopy, resulting in an alternating arrangement of pure Co and Pt rows. DA - 2004/10/18/ PY - 2004 DO - 10.1103/PhysRevB.70.153403 VL - 70 IS - 15 SP - 153403 J2 - Phys. Rev. B ER - TY - JOUR TI - Structure of a thin oxide film on Rh(100) AU - Gustafson, J. AU - Mikkelsen, A AU - Borg, M. AU - Andersen, J. N. AU - Lundgren, E. AU - Klein, C. AU - Hofer, W. AU - Schmid, M. AU - Varga, P. AU - Köhler, L. AU - Kresse, G. AU - Kasper, N. AU - Stierle, A. AU - Dosch, H. T2 - Physical Review B AB - The initial oxidation of Rh(100) has been studied using high resolution core level spectroscopy, low energy electron diffraction, surface x-ray diffraction, scanning tunneling microscopy, and density functional theory. We report a structural study of an oxygen induced structure displaying a c(8×2) periodicity at an oxygen pressure above 10−5 mbar and using a sample temperature of 700 K . Our experimental and theoretical data demonstrate that this structure is due to the formation of a thin surface oxide with a hexagonal trilayer O-Rh-O structure. DA - 2005/03/31/ PY - 2005 DO - 10.1103/PhysRevB.71.115442 VL - 71 IS - 11 SP - 115442 J2 - Phys. Rev. B ER - TY - JOUR TI - Chemical ordering and composition fluctuations at the (001) surface of the Fe64Ni36 Invar alloy AU - Ondráček, M. AU - Máca, F. AU - Kudrnovský, J. AU - Redinger, J. AU - Biedermann, A. AU - Fritscher, C. AU - Schmid, M. AU - Varga, P. T2 - Physical Review B AB - We report on a study of (001) oriented fcc Fe-Ni alloy surfaces which combines first-principles calculations and low-temperature scanning tunneling microscopy (STM) experiments. Density functional theory calculations show that Fe-Ni alloy surfaces are buckled with the Fe atoms slightly shifted outwards and the Ni atoms inwards. This is consistent with the observation that the atoms in the surface layer can be chemically distinguished in the STM image: brighter spots (corrugation maxima with increased apparent height) indicate iron atoms, darker ones nickel atoms. This chemical contrast reveals a c(2×2) chemical order (50% Fe) with frequent Fe-rich defects on the Fe64Ni36(001) surface. The calculations also indicate that subsurface composition fluctuations may additionally modulate the apparent height of the surface atoms. The STM images show that this effect is pronounced compared to the surfaces of other disordered alloys, which suggests that some chemical order and corresponding concentration fluctuations exist also in the subsurface layers of Invar alloy. In addition, detailed electronic structure calculations allow us to identify the nature of a distinct peak below the Fermi level observed in the tunneling spectra. This peak corresponds to a surface resonance band which is particularly pronounced in iron-rich surface regions and provides a second type of chemical contrast with less spatial resolution but one that is essentially independent of the subsurface composition. DA - 2006/12/15/ PY - 2006 DO - 10.1103/PhysRevB.74.235437 VL - 74 IS - 23 SP - 235437 EP - 7 J2 - Phys. Rev. B ER - TY - JOUR TI - Oxygen-deficient line defects in an ultrathin aluminum oxide film AU - Schmid, M. AU - Shishkin, M. AU - Kresse, G. AU - Napetschnig, E. AU - Varga, P. AU - Kulawik, M. AU - Nilius, N. AU - Rust, H.-P. AU - Freund, H.-J. T2 - Physical Review Letters AB - A model for the straight antiphase domain boundary of the ultrathin aluminum oxide film on the NiAl(110) substrate is derived from scanning tunneling microscopy measurements and density-functional theory calculations. Although the local bonding environment of the perfect film is maintained, the structure is oxygen deficient and possesses a favorable adsorption site. The domain boundary exhibits a downwards band bending and three characteristic unoccupied electronic states, in excellent agreement with scanning tunneling spectroscopy measurements. DA - 2006/07/28/ PY - 2006 DO - 10.1103/PhysRevLett.97.046101 VL - 97 IS - 4 SP - 046101 EP - 4 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Oxygen-induced step bunching and faceting of Rh(553): Experiment and ab initio calculations AU - Gustafson, J. AU - Resta, A. AU - Mikkelsen, A. AU - Westerström, R. AU - Andersen, J. N. AU - Lundgren, E. AU - Weissenrieder, J. AU - Schmid, M. AU - Varga, P. AU - Kasper, N. AU - Torrelles, X. AU - Ferrer, S. AU - Mittendorfer, F. AU - Kresse, G. T2 - Physical Review B AB - Using a combined experimental and theoretical approach, we show that the initial oxidation of a Rh(553) surface, a surface vicinal to (111), undergoes step bunching when exposed to oxygen, forming lower-index facets. At a pressure of about 10–6 mbar and a temperature of 380 °C this leads to (331) facets with one-dimensional oxide chains along the steps, coexisting with (111) facets. Further increase of the pressure and temperature results in (111) facets only, covered by an O-Rh-O surface oxide. Our density functional theory calculations provide an atomistic understanding of the observed behavior. DA - 2006/07/15/ PY - 2006 DO - 10.1103/PhysRevB.74.035401 VL - 74 IS - 3 SP - 035401 J2 - Phys. Rev. B ST - Oxygen-induced step bunching and faceting of Rh(553) ER - TY - JOUR TI - Combined STM, LEED and DFT study of Ag(100) exposed to oxygen near atmospheric pressures AU - Costina, I. AU - Schmid, M. AU - Schiechl, H. AU - Gajdos, M. AU - Stierle, A. AU - Kumaragurubaran, S. AU - Hafner, J. AU - Dosch, H. AU - Varga, P. T2 - Surface Science AB - We have investigated the interaction of molecular oxygen with the Ag(100) surface in a temperature range from 130 K to 470 K and an oxygen partial pressure ranging up to 10 mbar by scanning tunneling microscopy, low electron energy diffraction, Auger electron spectroscopy and ab initio density functional calculations. We find that at 130 K, following oxygen exposures of 6000 Langmuirs O2, the individual oxygen atoms are randomly distributed on the surface. When the sample is exposed to 10 mbar O2 at room temperature, small, p(2 × 2) reconstructed patches are formed on the surface. After oxidation at [approximate]470 K and 10 mbar O2 pressure the surface undergoes a c(4 × 6) reconstruction coexisting with a (6 × 6) superstructure. By ab initio thermodynamic calculations it is shown that the c(4 × 6) reconstruction is an oxygen adsorption induced superstructure which is thermodynamically stable for an intermediate range of oxygen chemical potential. DA - 2006/02/01/ PY - 2006 DO - 10.1016/j.susc.2005.11.020 VL - 600 IS - 3 SP - 617 EP - 624 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Structure and catalytic reactivity of Rh oxides AU - Gustafson, J. AU - Westerström, R. AU - Resta, A. AU - Mikkelsen, A. AU - Andersen, J.N. AU - Balmes, O. AU - Torrelles, X. AU - Schmid, M. AU - Varga, P. AU - Hammer, B. AU - Kresse, G. AU - Baddeley, C.J. AU - Lundgren, E. T2 - Catalysis Today AB - Using a combination of experimental and theoretical techniques, we show that a thin RhO2 surface oxide film forms prior to the bulk Rh2O3 corundum oxide on all close-packed single crystal Rh surfaces. Based on previous reports, we argue that the RhO2 surface oxide also forms on vicinal Rh surfaces as well as on Rh nanoparticles. The detailed structure of this film was previously determined using UHV based techniques and density functional theory. In the present paper, we also examine the structure of the bulk Rh2O3 corundum oxide using surface X-ray diffraction. Being armed with this structural information, we have explored the CO oxidation reaction over Rh(111), Rh(100) and Pt25Rh75(100) at realistic pressures using in situ surface X-ray diffraction and online mass spectrometry. In all three cases we find that an increase of the CO2 production coincides with the formation of the thin RhO2 surface oxide film. In the case of Pt25Rh75(100), our measurements demonstrate that the formation of bulk Rh2O3 corundum oxide poisons the reaction, and argue that this is also valid for all other Rh surfaces. Our study implies that the CO oxidation reaction over Rh surfaces at realistic conditions is insensitive to the exact Rh substrate orientation, but is rather governed by the formation of a specific surface oxide phase. DA - 2009/07/30/ PY - 2009 DO - 10.1016/j.cattod.2008.11.011 VL - 145 IS - 3-4 SP - 227 EP - 235 J2 - Catal. Today SN - 0920-5861 ER - TY - JOUR TI - Spatial self-organization of a nanoscale structure on the Pt(111) surface AU - Zhang, L. AU - van Ek, J. AU - Diebold, U. T2 - Physical Review B AB - A self-organized nanostructure has been observed on the Cr/Pt(111) system. Cr overlayers with various coverages were deposited on a Pt(111) surface. Upon annealing to different temperatures, metastable surface alloys characteristic for a miscible metallic system have been observed. For a limited coverage range from 1.5 to 3.0 ML and an annealing temperature of 800 K, a spatial self-organization takes place on the bimetallic surface, forming a highly ordered hexagonal superstructure. The scanning tunnel microscope contrast of the superstructure is sensitive to the bias voltage, resulting in different patterns: either a mosaic structure and/or a network consisting of one-atom-wide lines. Relative to the Pt(111) surface, the superstructure has a (√39×√39)R16.1° unit cell with a dimension of 17.3 Å. We propose that the network consists of Pt dislocation lines, and is caused by a stress-induced reconstruction of the top Pt(111) layer. The mosaic pattern is related to a two-dimensional array of Cr clusters containing only ten atoms. DA - 1999/02// PY - 1999 DO - 10.1103/PhysRevB.59.5837 VL - 59 IS - 8 SP - 5837 EP - 5846 J2 - Phys. Rev. B SN - 0163-1829 ER - TY - JOUR TI - STM study of Cu growth on the ZnO(10-10) surface AU - Dulub, Olga AU - Boatner, Lynn A. AU - Diebold, Ulrike T2 - Surface Science AB - The room temperature growth of Cu islands on the non-polar (10-10) surface of zinc oxide was studied using scanning tunneling microscopy. Images of the clean (10-10) surface prepared by sputtering and annealing at 550–700 °C exhibit flat terraces with a high density of steps, mostly running along [0001] and [1-210] directions. For Cu coverages of 0.025–1 ML deposited on flat, freshly annealed surfaces, preferential nucleation occurs at the step edges oriented perpendicular to the [1-210] atomic row direction. Exclusively three-dimensional (3D) islands have been observed at all coverages. For comparison, the same Cu coverages were deposited onto the surface contaminated by adsorption from the residual gas in the ultrahigh vacuum chamber. Both 2D and 3D islands, randomly distributed across the terraces, were observed reproducibly on such slightly contaminated surfaces. The density of the islands, as well as their average diameter and height, increases with increasing Cu coverage for both surface preparations. DA - 2002/04/20/ PY - 2002 DO - 10.1016/S0039-6028(02)01107-X VL - 504 SP - 271 EP - 281 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Surface morphologies of SnO2(110) AU - Batzill, Matthias AU - Katsiev, Khabibulakh AU - Diebold, Ulrike T2 - Surface Science AB - Scanning tunneling microscopy (STM), ion scattering spectroscopy (ISS), and low energy electron diffraction (LEED) was used to investigate the surface morphology of SnO2(1 1 0) for different preparation conditions. Annealing in 10-3 mbar oxygen results in a 1 × 1 diffraction pattern. Such surfaces exhibit terraces separated predominantly by straight step edges along low index crystallographic directions. The terraces exhibit a high density of defects. Annealing to 810 K results in the loss of surface oxygen but the surface retains a 1 × 1 periodicity. Steps of less than monolayer-height, however, indicate that a significant reordering of the surface atoms occurs already at this temperature. Annealing to higher temperature or preparation of the surface by sputtering and vacuum annealing always results in a superstructure in the diffraction pattern and a low [O]/[Sn] ratio in ISS. For annealing temperatures between 920 and 1050 K co-existence of c(2 × 2) and 4 × 1 reconstructed domains is observed. In this regime small adislands are always present at the surface; extended terraces were imaged by STM with a 4 × 1 periodicity. This implies that the c(2 × 2)-structure is associated with adislands at the surface. Annealing to 1100 K resulted in the formation of a 4 × 1 surface only. This surface exhibits terraces with meandering step edges and antiphase domain boundaries of the 4 × 1 surface structure. A new model for this reconstruction is proposed including Sn atoms occupying interstitial surface sites. Annealing to 1180 K results in the fragmentation of the 4 × 1 structure and the surface looses its long-range order. This causes a 1 × 1 LEED pattern originating from the underlying substrate. Surface undulations with sub-interlayer step heights are explained by the frequent presence of stacking faults and other bulk defects that are also accompanied by variations in the electronic structure due to a locally altered Sn/O stoichiometry. DA - 2003/04/10/ PY - 2003 DO - 10.1016/S0039-6028(03)00357-1 VL - 529 IS - 3 SP - 295 EP - 311 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - STM Study of Copper Growth on ZnO(0001)−Zn and ZnO(0001̄)−O Surfaces AU - Koplitz, Lynn Vogel AU - Dulub, Olga AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry B AB - The room-temperature growth of Cu on the polar (0001)-Zn and (0001̄)-O surfaces of zinc oxide has been studied with scanning tunneling microscopy (STM). Copper grows on the (0001)-Zn surface as three-dimensional clusters even at low coverages (0.050.25 monolayers (ML)); two-dimensional (2D) islands are only observed at very low coverages (0.0010.05 ML). The average size of the 3D clusters increases with coverage, and their density increases slowly. Surface roughness and sputter damage change the growth mode to more 2D-like. The Cu clusters are well-separated and exhibit a well-defined hexagonal shape. Equilibrium crystal shape analysis yields an apparent work of adhesion of 3.4 +- 0.1 J/m2 for the largest clusters. On the (0001̄)-O surface, formation of two-dimensional Cu clusters was observed at coverages of less than 0.1 ML. DA - 2003/// PY - 2003 DO - 10.1021/jp0352175 VL - 107 IS - 38 SP - 10583 EP - 10590 J2 - J. Phys. Chem. B ER - TY - JOUR TI - Atomic-scale properties of low-index ZnO surfaces AU - Diebold, Ulrike AU - Koplitz, Lynn Vogel AU - Dulub, Olga T2 - Applied Surface Science AB - Zinc oxide (ZnO) is an important material in heterogeneous catalysis and has recently attracted interest as a wide band-gap semiconductor for electro-optical devices. The surfaces and interfaces of ZnO are critical for understanding the mechanistics of surface chemical reactions and for the fabrication of high quality hetero- and homoepitaxial films with long-term stability. The surfaces of the main low-index planes, i.e., surfaces with (0 0 0 1), (000-1), (10-10) orientations are characterized with high-resolution scanning tunneling microscopy and compared to first-principles total-energy calculations. DA - 2004/10/15/ PY - 2004 DO - 10.1016/j.apsusc.2004.06.040 VL - 237 IS - 1-4 SP - 336 EP - 342 J2 - Appl. Surf. Sci. SN - 0169-4332 ER - TY - JOUR TI - Characterization of individual SnO2 nanobelts with STM AU - Katsiev, Khabibulakh AU - Kolmakov, Andrei AU - Fang, Minghu AU - Diebold, Ulrike T2 - Surface Science AB - The surface morphology of tin oxide nanobelts (NB) was studied with scanning tunneling microscopy (STM), dry-deposited on TiO2 substrates. XPS shows carbon contamination on as-grown, air-exposed SnO2 nanobelts, which was removed by oxygen plasma cleaning. The thermal stability of the NBs was studied with SEM and critical temperatures, where structural changes occur in UHV, O2 atmosphere, and air, were determined. Atomically resolved STM images show a SnO2(1 0 1)-(1 × 1) structure on the top NB surface. DA - 2008/07/15/ PY - 2008 DO - 10.1016/j.susc.2008.05.042 VL - 602 IS - 14 SP - L112 EP - L114 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Surface structure of Sn-doped In2O3 (111) thin films by STM AU - Morales, Erie H. AU - He, Yunbin AU - Vinnichenko, Mykola AU - Delley, Bernard AU - Diebold, Ulrike T2 - New Journal of Physics AB - High-quality Sn-doped In2O3 (ITO) films were grown epitaxially on yttria stabilized zirconia (111) with oxygen-plasma assisted molecular beam epitaxy (MBE). The 12 nm thick films, containing 2-6% Sn, are fully oxidized. Angle-resolved x-ray photoelectron spectroscopy (ARXPS) confirms that the Sn dopant substitutes In atoms in the bixbyite lattice. From XPS peak shape analysis and spectroscopic ellipsometry measurements it is estimated that, in a film with 6 at.% Sn, ≈1/3 of the Sn atoms are electrically active. Reflection high energy electron diffraction (RHEED) shows a flat surface morphology and scanning tunneling microscopy (STM) shows terraces several hundred nanometers in width. The terraces consist of 10 nm wide orientational domains, which are attributed to the initial nucleation of the film. Low energy electron diffraction (LEED) and STM results show a bulk-terminated (1 x 1) surface, which is supported by first-principles density functional theory (DFT) calculations. Atomically resolved STM images are consistent with Tersoff-Hamann calculations that show that surface In atoms are imaged bright or dark, depending on the configuration of their O neighbors. The coordination of surface atoms on the In2O3(111)-1x1 surface is analyzed in terms of their possible role in surface chemical reactions. DA - 2008/// PY - 2008 DO - 10.1088/1367-2630/10/12/125030 VL - 10 IS - 12 SP - 125030 J2 - New J. Phys. SN - 1367-2630 ER - TY - JOUR TI - Analysis of vibration-isolating systems for scanning tunneling microscopes AU - Schmid, M. AU - Varga, P. T2 - Ultramicroscopy AB - For high-resolution and nanotechnology devices, such as the STM, the effective isolation of environmental vibrations plays a key role. Different types of vibration-reducing systems are analyzed by means of mechanical four-pole theory. A comparison of one- and two-stage spring-suspended systems with magnetic eddy-current damping shows that one-stage systems suffer from poor isolation at high frequencies, which can be improved by additional elastomer elements. Contrary to common belief, two-stage spring systems with eddy-current damping for the upper stage only are superior to similar systems with eddy-current damping of both stages. Plate stacks with elastomer elements yield good amplitude reduction at high frequencies, while vibrations in the 10-100 Hz range may be even enhanced. Several stacks with different arrangements of masses and spring constants are compared, and it is shown that stacks should have only few (2 or 3) plates. For optimum performance it is necessary to combine the plate stack inside the vacuum chamber with soft (pneumatic or spring) suspension of the whole chamber. DA - 1992/07// PY - 1992 DO - 10.1016/0304-3991(92)90493-4 VL - 42-44 IS - Part 2 SP - 1610 EP - 1615 J2 - Ultramicroscopy SN - 0304-3991 ER - TY - JOUR TI - p(n x 1) superstructures of Pb on Cu(110) AU - Nagl, C. AU - Pinczolits, M. AU - Schmid, M. AU - Varga, P. AU - Robinson, I. K. T2 - Physical Review B AB - The structures of the p(n×1) superstructures (n=4, 5, and 9) of Pb on Cu(110) in the coverage range between FTHETA=0.75 and 0.8 are revealed by atomically resolved scanning tunneling microscopy. All three superstructures are formed by substitution of every nth row of Cu atoms (n=4, 5, and 9) in the [001] direction by Pb atoms. The Pb atoms in between are lined up in the [1¯10] direction. The p(4×1) structure appears in two different modifications: one with substitutional rows of Pb atoms and one with a simple overlayer structure without substituted rows of Cu atoms. Alternating succession of these two modifications results in p(12×1) domains. It is further shown that the p(9×1) structure is not a succession of p(4×1) and p(5×1) but a superstructure on its own. The p(5×1) structure proposed here agrees with previous x-ray-diffraction data at least as well as the quasihexagonal model proposed earlier. We have, in addition, identified the nature of the phase that has been described incommensurate obtained by desorption of Pb upon annealing above 600 K. DA - 1995/12/15/ PY - 1995 DO - 10.1103/PhysRevB.52.16796 VL - 52 IS - 23 SP - 16796 J2 - Phys. Rev. B ER - TY - JOUR TI - Direct Observation of a New Growth Mode: Subsurface Island Growth of Cu on Pb(111) AU - Nagl, C. AU - Platzgummer, E. AU - Schmid, M. AU - Varga, P. AU - Speller, S. AU - Heiland, W. T2 - Physical Review Letters AB - Atomically resolved scanning tunneling microscopy on Cu/Pb(111) reveals a new growth mode, contrary to the Volmer-Weber mode expected from the significantly lower surface energy of Pb. (111)-oriented Cu islands with a thickness of 3–11 layers are immersed in the Pb substrate and covered by a single close-packed Pb layer. This subsurface growth mode occurring at room temperature can be explained by simple thermodynamic considerations. DA - 1995/10/16/ PY - 1995 DO - 10.1103/PhysRevLett.75.2976 VL - 75 IS - 16 SP - 2976 J2 - Phys. Rev. Lett. ST - Direct Observation of a New Growth Mode ER - TY - JOUR TI - STM study of the (111) and (100) surfaces of PdAg AU - Wouda, P. T. AU - Schmid, M. AU - Nieuwenhuys, B. E. AU - Varga, P. T2 - Surface Science AB - The (111) and (100) surfaces of the Pd67Ag33 alloy have been imaged with atomic resolution by scanning tunneling microscopy. On the (111) surface, it was found that Pd atoms appear in the images about 25 pm higher than the Ag atoms. The surface concentration of palladium was determined as a function of annealing temperature (720-920 K) and was found to vary between 5 and 11%. Analysis of the relative positions of the Pd atoms showed a tendency towards formation of isolated palladium sites. On the (100) surface, the palladium concentration in the first layer is extremely low and the system has to be forced into a non-equilibrium state to find palladium atoms in the first monolayer. Chemical contrast here amounts to a 60 pm apparent height difference. DA - 1998/11/20/ PY - 1998 DO - 10.1016/S0039-6028(98)00673-6 VL - 417 IS - 2-3 SP - 292 EP - 300 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Unreconstructed Au(100) monolayers on a Au3Pd(100) single-crystal surface AU - Aschoff, M. AU - Speller, S. AU - Kuntze, J. AU - Heiland, W. AU - Platzgummer, E. AU - Schmid, M. AU - Varga, P. AU - Baretzky, B. T2 - Surface Science AB - The Au3Pd(100) single-crystal surface was studied with ion scattering methods, low-energy electron diffraction (LEED) and scanning tunneling microscopy. The crystal is covered at room temperature with a pure, (100)-ordered gold layer. Palladium is found in the second layer only. The lattice constant of the gold surface as evaluated by ion scattering and a tensor low-energy electron diffraction (TLEED) analysis is equal to the bulk lattice constant of 4.017 Å as evaluated by X-ray analysis. The surface lattice constant of the gold layer on the alloy surface is 0.08 Å smaller than that of bulk gold. DA - 1998/10/09/ PY - 1998 DO - 10.1016/S0039-6028(98)00564-0 VL - 415 IS - 3 SP - L1051 EP - L1054 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - High temperature growth of Pt on the Rh(111) surface AU - Duisberg, M. AU - Dräger, M. AU - Wandelt, K. AU - Gruber, E. L. D. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - The epitaxial growth of Pt on the Rh(111) surface at 700 K was studied with AES, UPS, ISS and STM. From AES and ISS measurements a 2D growth mode is concluded at this substrate temperature. The morphology of the surface is studied by photoemission spectra of adsorbed Xe (PAX) and STM. A disperse distribution of the Pt atoms is suggested by PAX and is consistent with an incorporation of these atoms into the first substrate layer. Atomically and chemically resolved STM measurements confirm these conclusions. The interaction of CO with the surface alloy is investigated by UPS. The CO-induced features in UP spectra show significant differences in the peak positions and shape between the clean substrate and the surface precovered with different amounts of Pt. The CO induced emissions are, thus, used for a quantitative titration of Pt on the Rh surface. DA - 1999/08/02/ PY - 1999 DO - 10.1016/S0039-6028(99)00037-0 VL - 433-435 SP - 554 EP - 558 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Pt25Rh75(111), (110), and (100) studied by scanning tunnelling microscopy with chemical contrast AU - Hebenstreit, E. L. D. AU - Hebenstreit, W. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - Scanning tunneling microscopy images with chemical contrast allowed the direct determination of the composition and short-range order behaviour of the clean Pt25Rh75(111), (110), and (100) alloy surfaces. All measurements were performed at room temperature and showed a strong platinum enrichment depending on the preparation temperature. In the top layers of both Pt25Rh75(111) and Pt25Rh75(110)-(1x2) we find a preference for unlike nearest neighbors. Pt25Rh75(100) exhibits after a preparation temperature of 900°C a preference for clustering whereas after a preparation temperature of 600°C, comparable to Pt25Rh75(111) and (110) ordering tendencies appear. Additional investigations of the chemical identity of atoms surrounding hollow positions show mostly small deviations from a random distribution. However, the number of hollow sites surrounded by Rh atoms only can be significantly affected by the short range order. Pt25Rh75(110) exhibits a (1x2) missing-row reconstruction after annealing above 700°C. After the first annealing of the sputtered surface it is accompanied by mesoscopic long range "waves" with a height of approximately 2 nm and a wavelength up to 200 nm depending on the preparation temperature. DA - 1999/11/01/ PY - 1999 DO - 10.1016/S0039-6028(99)00880-8 VL - 441 IS - 2-3 SP - 441 EP - 453 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Chemical discrimination on atomic level by STM AU - Varga, P. AU - Schmid, M. T2 - Applied Surface Science AB - Chemical information with spatial atomic resolution on multicomponent surfaces (especially alloys) can be achieved by STM (scanning tunnelling microscopy) with constant current imaging. Therefore STM can not only be used for determination of the crystallographic structure of single crystal surfaces but also for finding the chemical composition of bimetallic surfaces. This possibility makes the STM a unique instrument to find out the local chemical structure of multicomponent surfaces on an atomic scale. This ability can be used for studying in great detail segregation processes on metal surfaces. Examples of chemical discrimination between different metals on low index single crystal surfaces of bulk alloys we have seen so far are PtNi, PtRh, PtCo, PtAu and AgPd. For surfaces where the identification of the alloy constituents in the STM images is ambiguous (e.g., because of unknown or equal concentrations), it will be shown how ab initio calculations of the electron density of states using the FLAPW (full potential linearized augmented plane waves) method can help to interpret constant current STM topographs just by following the simple theory of Tersoff-Hamann. On the other hand, tip changes (e.g., adsorbates) can strongly influence the chemical contrast on constant current STM images. DA - 1999/03// PY - 1999 DO - 10.1016/S0169-4332(98)00514-5 VL - 141 IS - 3-4 SP - 287 EP - 293 J2 - Appl. Surf. Sci. SN - 0169-4332 ER - TY - JOUR TI - Temperature-dependent segregation on Pt25Rh75(111) and (100) AU - Platzgummer, E. AU - Sporn, M. AU - Koller, R. AU - Forsthuber, S. AU - Schmid, M. AU - Hofer, W. AU - Varga, P. T2 - Surface Science AB - Surface segregation is studied on Pt25Rh75(111) and Pt25Rh75(100) by LEED intensity analysis and LEIS. Although both equilibrated surfaces are strongly Pt-enriched (up to 80 at.%), we find an interesting difference in the segregation behavior when annealing the sputtered surfaces. The Pt concentration grows continuously on Pt25Rh75(111) until 1000°C, whereas it reaches a maximum enrichment around 500°C on Pt25Rh75(100) and decreases thereafter. This contrasting behavior results solely from the kinetic limitation in the low-temperature regime, and is not due to energetic reasons. From temperature-dependent composition profiles we determine the segregation kinetics as well as the annealing temperature necessary for thermodynamic equilibration. We find that an equilibrium is acquired on the Pt25Rh75(100) surface by the interchange of Pt and Rh atoms within the near-surface layers, and on the Pt25Rh75(111) surface by a diffusion of Pt atoms from bulk to the near-surface region. The latter leads to an overall Pt enrichment of several layers, and is only observed after annealing at 1100°C. The presence of carbon contamination on the Pt25Rh75(100) surface causes a significant reduction of the Pt segregation. There is excellent agreement between the top-layer concentrations derived by LEIS and quantitative LEED. DA - 1999/01/04/ PY - 1999 DO - 10.1016/S0039-6028(98)00800-0 VL - 419 IS - 2-3 SP - 236 EP - 248 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Temperature-dependent segregation and (1×2) missing-row reconstruction of Pt25Rh75(110) AU - Platzgummer, E. AU - Sporn, M. AU - Koller, R. AU - Schmid, M. AU - Hofer, W. AU - Varga, P. T2 - Surface Science AB - The surface structure and composition of the clean Pt25Rh75(110) surface is investigated by low energy electron diffraction (LEED) and low energy ion scattering (LEIS). For the equilibrated Pt25Rh75(110) surface we observe a (1x2) missing-row reconstruction in analogy to the pure Pt(110) surface, and a significant Pt enrichment of the topmost atomic layer (up to 80 at.% Pt). As the same strong surface enrichment in Pt was found in a previous study on the (100) and (111) surface of the same bulk composition, this means that in contrast to Pt-Ni and Pt-Co alloys, for Pt25Rh75 alloys the segregation behavior is not influenced extensively by the surface orientation. In addition to the structure analysis by LEED we performed LEIS experiments to determine the temperature induced changes of the surface composition and structure. Since the Pt segregation is less pronounced at elevated temperature, the surface reveals a temperature induced deconstruction of the (1x2) structure around 750°C, resulting in an fcc(110) (1x1) surface at high temperature. Temperature dependent measurements further show a hysteresis-like behavior of the top-layer composition, which is attributed to an enhanced Pt segregation on the (1x2) reconstructed surface. DA - 1999/03/01/ PY - 1999 DO - 10.1016/S0039-6028(98)00924-8 VL - 423 IS - 1 SP - 134 EP - 143 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Adsorbate migration on PdAg(111) AU - Wouda, P. T. AU - Schmid, M. AU - Nieuwenhuys, B. E. AU - Varga, P. T2 - Surface Science AB - Scanning Tunneling Microscopy was used to study the adsorption of oxygen on the PdAg(111) surface. Oxygen atoms appear as dark holes of about 40 pm depth, whereas palladium atoms appear as bright spots. By comparing consecutive STM images, oxygen was found to occupy only palladium sites: the oxygen was found to travel exclusively to and from the palladium atoms. The surface concentration of oxygen was low under all the experimental conditions used, as indicated by STM as well as Auger electron spectroscopy. DA - 1999/03/01/ PY - 1999 DO - 10.1016/S0039-6028(98)00937-6 VL - 423 IS - 1 SP - L229 EP - L235 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - An atomic-scale study of the Co induced dendrite formation on Pt(111) AU - Lundgren, E. AU - Stanka, B. AU - Koprolin, W. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - We have studied the initial growth of Co on the Pt(111) surface in a temperature range of 300-400 K with scanning tunneling microscopy, low energy ion scattering and Auger electron spectroscopy. In agreement with previous work by Grütter and Dürig, when depositing 0.1 ML Co on Pt(111) at 400 K, the formation of large dendrites is observed. These dendrites are formed in conjunction with a Co induced local reconstruction of the Pt(111) surface, resulting in dislocations. The dendrites, however, show no evidence for any dislocations below their surface, the local reconstruction is observed to be lifted by the formation of the dendrites. LEIS data suggest that the dendrites consist mainly of Pt, implying, together with the STM data, that Co is incorporated underneath the Pt. A model for this process is proposed. DA - 1999/03/10/ PY - 1999 DO - 10.1016/S0039-6028(98)00931-5 VL - 423 IS - 2-3 SP - 357 EP - 363 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Segregation and chemical ordering in the surface layers of Pt25Co75(111): a LEED/STM study AU - Gauthier, Y. AU - Baudoing-Savois, R. AU - Bugnard, J. M. AU - Hebenstreit, W. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - Segregation and chemical ordering on Pt25Co75(111) are studied by quantitative low energy electron diffraction (LEED) analysis and scanning tunnelling microscopy (STM). Although LEED patterns do not show any sign of superstructure, LEED calculations undoubtedly point to a surface which contains about the same amount of both species and reveal significant short range chemical ordering (down to the third layer). Pt and Co surface sites are locally arranged with a (1x2) unit cell, in the manner of the ordered tetragonal L10 phase. More direct evidence is given by STM images which exhibit parallel Pt and Co monoatomic chains a few lattice constants long and an apparent height difference of about 0.2 Å for Pt and Co sites. LEED shows that the Pt sublattice in the top layer actually resides 0.1 Å above the Co one. Otherwise the surface is bulklike, with weak relaxations of interlayer distances. The use of a chemically ordered model for the LEED analysis, in which sublattice occupancies are optimised, results in a remarkable improvement of the fit with experiment as compared to a totally random distribution; however, most interestingly, it does not modify the average layer concentration profile versus depth (55, 5 and 35 at% Pt respectively for the three outermost layers). The distortions needed for the tetragonal L1_0 phase with respect to the fcc L1_2 phase explains why chemical order does not extend over larger domains. Finally, both techniques yield complementary pictures and quite consistent results as to the top layer content and chemical order. DA - 2000/11/01/ PY - 2000 DO - 10.1016/S0039-6028(00)00751-2 VL - 466 IS - 1-3 SP - 155 EP - 166 J2 - Surf. Sci. SN - 0039-6028 ST - Segregation and chemical ordering in Pt25Co75(111) ER - TY - JOUR TI - Temperature-dependent segregation reversal and (1×3) missing-row structure of Pt90Co10(110) AU - Platzgummer, E. AU - Sporn, M. AU - Koller, R. AU - Schmid, M. AU - Hofer, W. AU - Varga, P. T2 - Surface Science AB - The surface structure and composition of the clean Pt90Co10(110) surface is investigated by low energy electron diffraction (LEED) and low energy ion scattering (LEIS). Through LEED I-V analysis we find a (1x3) missing-row reconstruction on the equilibrated Pt90Co10(110) surface - comparable with the pure Pt(110) (1x3) surface - in which all atomic positions in the topmost layer and in the (111) oriented micro facets are Pt-enriched. Due to the fact that the unreconstructed Pt25Co75(110) surface is known to exhibit an almost pure Co top layer the Pt segregation reported in this study is undoubtedly connected to the existence of the missing-row reconstruction. The proposed structural influence on the composition is confirmed by LEIS experiments performed on the hot Pt90Co10(110) surface, in which simultaneously temperature induced changes of the surface composition and qualitative changes in the surface structure are monitored. The measured low energy ion spectra do not only reproduce the calculated first-layer composition of the LEED analysis, they also show a less pronounced Pt segregation at temperatures around 750°C, and eventually a reversed Pt segregation above 750°C, i.e. Co enrichment of the Pt90Co10(110) surface with respect to the bulk concentration. We find a clear correlation between the thermal deconstruction and the surface composition. The striking segregation reversal during temperature variation is attributed to the high excess value of the mixing enthalpy, which implies a structure-dominated segregation behavior. DA - 2000/05/10/ PY - 2000 DO - 10.1016/S0039-6028(00)00351-4 VL - 453 IS - 1-3 SP - 214 EP - 224 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - STM and STS of bulk electron scattering by subsurface objects AU - Schmid, M. AU - Crampin, S. AU - Varga, P. T2 - Journal of Electron Spectroscopy and Related Phenomena AB - Results obtained on aluminium and copper surfaces are used to demonstrate the ability of scanning tunnelling microscopy (STM) and spectroscopy (STS) to detect subsurface structures through their influence on the electronic structure. Subsurface Ar bubbles in Al lead to a quantum well bounded by the outer surface and the top of the bubbles. Using z(V) spectroscopy, where the STM feedback loop keeps the current constant while ramping the voltage, it is possible to detect the energy steps between the quantum well states; combined with a one-dimensional model employing a realistic potential for the electrons, this allows an estimate of the thickness of the quantum well, i.e., the depth of the bubbles. Simulated STM images calculated with a three-dimensional scattering theory reproduce many details of the interference pattern, and confirm the size and geometry of the bubbles. Interference patterns attributed to subsurface scatterers have been also detected on Cu(111) and Cu(100). We propose that the patterns observed on Cu(111) are due to focusing of electron waves in certain crystallographic directions, whereas those on Cu(100) are unexplained up to now. DA - 2000/08// PY - 2000 DO - 10.1016/S0368-2048(00)00108-0 VL - 109 IS - 1-2 SP - 71 EP - 84 J2 - J. Electron Spectrosc. Relat. Phenom. SN - 0368-2048 ER - TY - JOUR TI - Segregation and ordering at Fe1-xAlx(100) surfaces - a model case for binary alloys AU - Blum, V. AU - Hammer, L. AU - Meier, W. AU - Heinz, K. AU - Schmid, M. AU - Lundgren, E. AU - Varga, P. T2 - Surface Science AB - The geometrical structure, chemical order and composition of (100) oriented surfaces of the binary alloy system Fe_1-xAl_x were investigated in the Fe-rich regime (x = 0.03, 0.15, and 0.30) using quantitative low-energy electron diffraction. Low-energy He+ ion scattering and scanning tunneling microscopy were additionally employed to characterize the x = 0.15 sample. The equilibrium structures developing with increasing bulk Al content can be consistently explained by the interplay between Al surface segregation and ordering processes which are controlled by atomic interactions similar to those in the bulk. These interactions divide the process of Al segregation to the very surface into two steps whereby Al atoms occupy sites of two different sublattices of c(2x2) periodicity with different probability. While one sublattice is already completely filled at low bulk Al concentration, the other sublattice fills only gradually with increasing bulk Al content. The local order in deeper layers is consistent with the bulk phase diagram. DA - 2001/03/01/ PY - 2001 DO - 10.1016/S0039-6028(00)00987-0 VL - 474 IS - 1-3 SP - 81 EP - 97 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Exchange processes in interlayer diffusion – kinks, corners and the growth mode AU - Schmid, M. AU - Lundgren, E. AU - Leonardelli, G. AU - Hammerschmid, A. AU - Stanka, B. AU - Varga, P. T2 - Applied Physics A: Materials Science & Processing AB - Abstract. Interlayer diffusion, i.e. mass transport between different terraces, is known to be an essential process for obtaining layer-by-layer growth, avoiding formation of three-dimensional (3D) islands when growing thin films. We present experimental results for the growth of cobalt on Pt(111), which demonstrate the importance of kinks and corners for interlayer diffusion. We show that Co grows two-dimensionally as long as strain caused by the Pt-Co interface keeps the step edges rough, with a high kink density, and then transforms to 3D growth with straight steps. The results for growth with adsorbed carbon monoxide show that CO acts as a surfactant, causing two-dimensional growth unless heterogeneous nucleation occurs. Again, this process is related to roughening of the steps, being a new mechanism for the action of a surfactant. A scanning tunneling microscopy study at the atomic scale confirms the fact that step descent happens only at kinks and (concave) corners, and in conjunction with simulations allows us to identify some of the relevant atomic-exchange processes. We finally argue that the dependence of the growth mode on the step morphology, together with straightening of the steps by step–step interaction, can lead to an instability of the growth mode. DA - 2001/04/15/ PY - 2001 DO - 10.1007/s003390100753 VL - 72 IS - 4 SP - 405 EP - 412 J2 - Appl. Phys. A ER - TY - JOUR TI - Nucleation of bcc Iron in Ultrathin fcc Films AU - Biedermann, Albert AU - Schmid, Michael AU - Varga, Peter T2 - Physical Review Letters AB - Needle-shaped bcc nucleation centers in fcc films of Fe on Cu(100) are observed by scanning tunneling microscopy. They form virtually without mass transfer and nearly under conservation of volume, which causes a large strain within the nascent bcc grain. The corresponding strain energy almost equals the gain in structural energy, rendering the bcc nucleation very sensitive to any effect influencing this subtle balance. We suggest that modifying the film by straining, alloying, or surface adsorption may inhibit the bcc nucleation and lead to thick metastable fcc films. DA - 2001/01/15/ PY - 2001 DO - 10.1103/PhysRevLett.86.464 VL - 86 IS - 3 SP - 464 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Scanning tunneling spectroscopy on clean and contaminated V(001) AU - Bischoff, M. M. J. AU - Konvicka, C. AU - Quinn, A. J. AU - Schmid, M. AU - Redinger, J. AU - Podloucky, R. AU - Varga, P. AU - van Kempen, H. T2 - Surface Science AB - Scanning tunneling spectroscopy (STS) measurements on clean V(001), carbon-covered V(001) and the oxygen-induced V(001) (1x5) reconstruction are reported. The clean V(001) surface shows a strong surface state 0.03 eV below the Fermi level. Isolated impurities shift the surface state 0.05 eV upwards in energy and broaden the peak observed in dI/dV. No significant influence of monoatomic steps on the surface state could be observed. For tunneling resistances down to about 1 MOhm the surface state is unaffected by the tip of the STM. A surface state is detected around +0.75 eV in small c(2x2) patches which are observed at higher carbon (and oxygen) coverages. The oxygen induced (1x5) reconstruction of V(001) shows a peak at similar energy (+0.63 eV) in the areas with O and C atoms in fourfold hollow sites and a peak around +0.91 eV above the rows of bridge-site oxygen. Ab-initio band structure calculations confirm the existence of a surface state of dz2 symmetry with an energy close to that observed experimentally on clean V(001). This agreement provides strong evidence that the V(001) surface is not magnetic (at least at room temperature) as predicted by the calculations. We also compare the experimentally observed peak shifts on the carbon and oxygen covered surfaces with calculational results for carbon-covered geometries. DA - 2002/07// PY - 2002 DO - 10.1016/S0039-6028(02)01783-1 VL - 513 IS - 1 SP - 9 EP - 25 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Geometry of the valence transition induced surface reconstruction of Sm(0001) AU - Lundgren, E. AU - Andersen, J. N. AU - Nyholm, R. AU - Torrelles, X. AU - Rius, J. AU - Delin, A. AU - Grechnev, A. AU - Eriksson, O. AU - Konvicka, C. AU - Schmid, M. AU - Varga, P. T2 - Physical Review Letters AB - We present a structural determination of the surface reconstruction of the Sm(0001) surface using surface x-ray diffraction, scanning tunneling microscopy, and ab initio calculations. The reconstruction is associated with a large (22%) expansion of the atomic radius for the top monolayer surface Sm atoms. The mechanism driving the surface reconstruction in Sm is unique among all elements and is connected to the strong correlations of the 4f electrons in Sm and the intermediate valence observed in certain Sm compounds. The atoms constituting the top monolayer of Sm(0001) have vastly different chemical properties compared to the layer underneath and behave as if they were an adsorbate of a different chemical species. DA - 2002/03/18/ PY - 2002 DO - 10.1103/PhysRevLett.88.136102 VL - 88 IS - 13 SP - 136102 J2 - Phys. Rev. Lett. ER - TY - CHAP TI - Segregation and surface chemical ordering-an experimental view on the atomic scale AU - Schmid, M. AU - Varga, P. T2 - Surface Alloys and Alloy Surfaces A2 - Woodruff, D.P. T3 - The Chemical Physics of Solid Surfaces AB - Scanning tunneling microscopy with atomic resolution and chemical contrast offers unique possibilities in studying segregation and chemical ordering of alloy surfaces. Chemical contrast in STM can have three different reasons, (a) true topographic effects, (b) different density of states of the alloy constituents, and (c) tip-sample interaction depending on the chemical identity of the atom imaged. The composition and chemical order on surfaces is determined by an interplay of ordering and segregation. If the chemical ordering is weak, segregation is mainly determined by the differences in surface energies. On the surfaces of alloys with a strong tendency towards ordering, segregation and ordering can either compete, e.g., in cases where preserving the bulk chemical order requires the surface to assume the bulk composition, or ordering can enhance segregation, e.g. in cases where bulk ordering allows a pure-metal termination. In cases where only short-range chemical ordering occurs at the surface, the trends in surface chemical ordering were found to correspond well to the ordering tendency observed in the bulk. We also show that the study of surface composition and chemical order is essential for understanding adsorption on alloy surface. Even weak ordering can lead to significant changes in the availability of some adsorption sites. We could also obtain STM images of an alloy surface with chemical contrast and images of adsorbates in the same surface area, revealing the chemical structure of adsorption sites. We could thereby demonstrate the ligand effect, i.e., the dependence of adsorption strength on the atoms neighbouring an adsorption site. DA - 2002/// PY - 2002 VL - Volume 10 SP - 118 EP - 151 PB - Elsevier SN - 1571-0785 ER - TY - JOUR TI - Two-dimensional alloy of immiscible metals: Single and binary monolayer films of Pb and Sn on Rh(111) AU - Yuhara, J. AU - Schmid, M. AU - Varga, P. T2 - Physical Review B AB - The single and binary metal films of Pb and Sn on Rh(111) have been studied at room temperature by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. Both Pb and Sn are mobile at low coverage and form commensurate overlayers of (4×4)-Pb and c(2×4)-Sn, respectively. From atomically resolved STM images, the atomic arrangements of (4×4)-Pb and c(2×4)-Sn have been identified to be hexagonal and rectangular structures, respectively. With increasing coverage, both commensurate phases change into incommensurate phases followed by island formation (Stranski-Krastanov growth). This shows that Rh(111) is static and inert enough to support two-dimensional (2D) phases of Pb and/or Sn without alloying at room temperature. The Pb-Sn bimetallic film on Rh(111) forms an ordered 2D alloy of PbSn3 with an incommensurate structure close to (sqrt[7]×sqrt[7]), contrary to the immiscibility of Pb and Sn in the bulk. From atomically resolved STM images, the atomic arrangement of this (sqrt[7]×sqrt[7])-(Pb,Sn) structure has been determined. DA - 2003/05/12/ PY - 2003 DO - 10.1103/PhysRevB.67.195407 VL - 67 IS - 19 SP - 195407 J2 - Phys. Rev. B ST - Two-dimensional alloy of immiscible metals ER - TY - JOUR TI - Reconstruction of the clean and H covered "magnetic live surface layer" of Fe films grown on Cu(100) AU - Biedermann, Albert AU - Tscheliessnig, Rupert AU - Klein, Christof AU - Schmid, Michael AU - Varga, Peter T2 - Surface Science AB - The surface of 6-7 monolayer thick fcc Fe films grown at room temperature on a Cu(100) substrate is characterized by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The STM images show a p4g(2 × 2) structure at 5 and 80 K, but not at 300 K. LEED, however, indicates an expansion of the interlayer distance and lateral distortions of similar magnitude both at 150 K and at 300 K. No evidence for a significant change of the surface structure is detected by a LEED spot profile analysis between 150 and 300 K. We attribute the apparent absence of the reconstruction in the STM images at 300 K to surface dynamics caused by domain boundary motion. The particular surface structure with bond angles and distances similar to bcc Fe suggests a driving force of the reconstruction which is similar to that operative in the fcc-to-bcc transition of bulk Fe. Dosing less than 5 L H2 decorates the p4g(2 × 2) surface reconstruction, while higher hydrogen doses transform the surface reconstruction to p(2 × 1). DA - 2004/08/10/ PY - 2004 DO - 10.1016/j.susc.2004.06.150 VL - 563 IS - 1-3 SP - 110 EP - 126 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Growth of Ce on Rh(111) AU - Napetschnig, E. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - We have studied the growth of cerium films on Rh(111) using STM (scanning tunneling microscopy), LEED (low energy electron diffraction), XPS (X-ray photoelectron spectroscopy) and AES (Auger electron spectroscopy). Measurements of the Ce films after room temperature deposition showed that Ce is initially forming nanoclusters in the low coverage regime. These clusters consist of 12 Ce atoms and have the shape of pinwheels. At a coverage of 0.25 ML (monolayer, ML) an adatom layer with a (2 × 2) superstructure is observed. Above 0.4 ML, Rh is diffusing through pinholes into the film, forming an unstructured mixed layer. Annealing at 250 °C leads to the formation of ordered Ce-Rh compounds based on the bulk compound CeRh3. At a coverage of 0.1 ML, small ordered (2 × 2) surface alloy domains are observed. The exchanged Rh atoms form additional alloy islands situated on the pure Rh(1 1 1) surface, showing the same (2 × 2) superstructure as the surface alloy. At a coverage of 0.25 ML, the surface is completely covered by the surface alloy and alloy islands. The (2 × 2) structure is equivalent to a (111)-plane of CeRh3, contracted by 6%. Annealing a 1 ML thick Ce layer leads to a flat surface consisting of different rotational domains of CeRh3(100). The Rh needed for alloy formation comes from 50 Å deep pits in the substrate. Finally we show that LEIS (low energy ion scattering) is not suitable for the characterization of Ce and CeRh films due to strong effects of neutralization. DA - 2004/05/01/ PY - 2004 DO - 10.1016/j.susc.2004.03.006 VL - 556 IS - 1 SP - 1 EP - 10 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Local atomic structure of ultra-thin Fe films grown on Cu(100) AU - Biedermann, A. AU - Tscheliessnig, R. AU - Schmid, M. AU - Varga, P. T2 - Applied Physics A: Materials Science & Processing AB - Ultra-thin epitaxial Fe films grown by thermal deposition on Cu(100) are analyzed by scanning tunneling microscopy. Evidence is presented that the morphological characteristics and magnetic properties are a direct consequence of FCC-to-BCC transitions reminiscent of those occurring in bulk Fe. In contrast to the assumption of a ferromagnetic FCC phase in previous models of the Fe/Cu(100) system, we observe a tightly twinned and strained BCC-like phase termed nanomartensite in films below 5 ML thickness, which encompasses almost the entire film volume of 3 ML films. In addition, the surface of 7–8 ML films reconstructs by forming non-close-packed structures with BCC-like bond angles. The formation of these BCC-like phases is the reason for the expansion of the interlayer spacing observed in these films and correlates perfectly with their ferromagnetic ordering. DA - 2004/03/01/ PY - 2004 DO - 10.1007/s00339-003-2435-7 VL - 78 IS - 6 SP - 807 EP - 816 J2 - Appl. Phys. A ER - TY - JOUR TI - Growth of ultrathin Fe films on Cu(111) by pulsed laser deposition AU - Schiechl, H. AU - Rauchbauer, G. AU - Biedermann, A. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - Ultrathin Fe films have been grown on Cu(111) by pulsed laser deposition (PLD) and thermal deposition (TD) and analyzed by scanning tunneling microscopy (STM) and low-energy ion scattering (LEIS). PLD was performed using nanosecond pulses of a Nd:YAG laser providing three different wavelengths. Compared to the widely investigated Fe films grown by thermal deposition on Cu(111), which exhibit bilayer and multi-layer (3D) island growth at low coverage, PLD-grown films show enhanced layer-by-layer growth when using sufficiently high laser fluence. However, by increasing the laser spot size on the Fe target and adjusting the laser power to achieve an unchanged deposition rate, resulting in a lower laser fluence, we observe bilayer growth reminiscent of the TD films. Using STM and LEIS, we observe an increasing number of Fe atoms implanted into the Cu substrate with increasing laser fluence and consequently mixing of Fe and Cu in the layer-by-layer films even at a preparation temperature of 200 K. We therefore suggest that the reason for layer-by-layer growth in this system is not the high instantaneous deposition rate of PLD, but the implantation of Fe atoms due to their higher kinetic energy at higher fluences. DA - 2005/12/01/ PY - 2005 DO - 10.1016/j.susc.2005.07.016 VL - 594 IS - 1-3 SP - 120 EP - 131 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Thin films of Co on Pt(111): Strain relaxation and growth AU - Lundgren, E. AU - Stanka, B. AU - Schmid, M. AU - Varga, P. T2 - Physical Review B AB - The growth, structure and morphology of thin Co layers with a thickness ranging from 1 to 15 monolayers deposited at room temperature on Pt(111) have been studied by the use of scanning tunneling microscopy. We demonstrate that the first Co layer grows preferably in the Pt fcc lattice sites, with a high density of defects due to the lattice mismatch. The second Co layer is found to exhibit a moiré structure, with the Co in-plane lattice distance close to that of bulk Co. The growth of thin Co films is observed to be mostly in terms of flat layers (two dimensional) up to a Co coverage of about 3.5 ML. At higher coverages, we find that the Co grows in (three dimensional) islands and we show that the growth is characterized by a mainly twinned fcc-like stacking. We argue that the reason for the two dimensional growth mode at lower Co coverages is due to the strained interface between the Co overlayers and the Pt(111) surface, resulting in a large number of kinks and corners which facilitate interlayer diffusion. For higher coverage such sites become less common, due to the decreasing influence of the strained interface, resulting in no interlayer diffusion leading to a three dimensional growth mode. The implications by these observations on the magnetic properties of the Co/Pt(111) interface system are discussed. DA - 2000/07/15/ PY - 2000 DO - 10.1103/PhysRevB.62.2843 VL - 62 IS - 4 SP - 2843 J2 - Phys. Rev. B ST - Thin films of Co on Pt(111) ER - TY - JOUR TI - Time-of-flight spectroscopy of the energy distribution of laser-ablated atoms and ions AU - Buchsbaum, A. AU - Rauchbauer, G. AU - Varga, P. AU - Schmid, M. T2 - Review of Scientific Instruments AB - The growth of ultrathin films, deposited by laser ablation, crucially depends on the energy of the ablated species. Therefore, a time-of-flight (TOF) spectrometer has been constructed and measurements have been carried out in order to determine the energy distribution of laser-ablated Fe and Pt atoms and ions in the plasma created by nanosecond pulses of a frequency-doubled neodymium doped yttrium aluminum garnet laser. The experiments have been performed in ultrahigh vacuum at relatively low laser power. For measuring the spectra of the neutrals, a cross-beam electron source for postionization and electric as well as magnetic fields for repelling the ions are employed. Nevertheless, measurements of neutral particles are restricted to low plasma densities due to electrostatic shielding within the plasma, leading to an inefficient deflection of charged particles by electrostatic and magnetic fields. Test measurements have been performed by utilizing the TOF spectrometer as a pressure gauge and also by chopping the electron beam, running the TOF spectrometer as a residual gas mass spectrometer. The spectra of the laser-ablated plasmas have shown plasma conditions with a Debye length of approximately 10^−4 m, densities of 10^15–10^16 m^−3 and ion energies up to 150 eV. Neutral spectra have shown an unexpectedly low fraction of neutrals (10^−3–10^−4) and hyperthermal energies up to several 10 eV, possibly contributed by recombination of ions and electrons in the plasma. Even though gas spectra had demonstrated the expected sensitivity of the TOF spectrometer for low-energy neutrals, no thermally evaporated neutral atoms could be found. DA - 2008/04/04/ PY - 2008 DO - 10.1063/1.2901607 VL - 79 IS - 4 SP - 043301 J2 - Rev. Sci. Instrum. ER - TY - JOUR TI - Sputtering of Au and Al2O3 surfaces by slow highly charged ions AU - Hayderer, G. AU - Cernusca, S. AU - Hoffmann, V. AU - Niemann, D. AU - Stolterfoht, N. AU - Schmid, M. AU - Varga, P. AU - Winter, HP. AU - Aumayr, F. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - A quartz crystal microbalance technique was used for measuring total sputtering yields for polycrystalline Au and Al2O3 under impact of slow (100-1500 eV) multiply charged Ar and Xe ions. Up to the highest charge states investigated (Xe25+), the sputter yield for the Au target remains independent on the projectile charge state and can be well described by kinetic sputtering only. For Al2O3, on the contrary, a dramatic increase in total sputtering yield with increasing projectile charge state was found, showing that in this case potential sputtering (PS), i.e., sputtering due to the potential energy of the projectile, clearly dominates over kinetically induced sputtering. Results can be explained within the defect-mediated desorption model of PS. DA - 2001/08// PY - 2001 DO - 10.1016/S0168-583X(01)00668-1 VL - 182 IS - 1-4 SP - 143 EP - 147 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Sputter yields of insulators bombarded with hyperthermal multiply charged ions AU - Varga, P. AU - Neidhart, T. AU - Sporn, M. AU - Libiseller, G. AU - Schmid, M. AU - Aumayr, F. AU - Winter, H. P. T2 - Physica Scripta AB - The total sputter yield for Au, Si, GaAs, SiO2, MgO, LiF and NaCl bombarded with hyperthermal highly charged Ar^q+ ions (q = 1-9) has been measured. Only for alkali halides (LiF, NaCl) and to some extent for SiO2 potential sputtering (enhancement of the sputter yield with increasing charge state of the primary ion) has been observed. All other targets showed normal collision induced sputtering. From that result it is obvious that the mechanisms for sputtering can not be explained by the Coulomb explosion model, because in this model insulators like MgO and semiconductors like Si and GaAs should also show charge state dependence of the sputtering yield. Alkali halides and SiO2 are materials which are known for strong electron phonon coupling where electronic excitations in the valence band are localized by formation of self trapped excitons (STE) and/or self trapped holes (STH). During bombardment with highly charged ions the neutralization process in front of, at and below the surface causes the formation of STE and/or STH. Therefore the potential sputtering can be explained as a defect mediated sputtering process which is well known in electron stimulated desorption (ESD) where the decay of STH and/or STE into different colour centers leads at the end to the desorption of neutralized anions. The also created neutral cations are either evaporated (as it is the case for the alkali halides) or have to be removed by momentum transfer by the impinging projectiles. Therefore it is very likely that in the case of SiO2 for very low impact energy mainly only oxygen is enhanced sputtered, the surface is enriched in Si and the potential sputtering effect decreases with increasing ion dose. DA - 1997/// PY - 1997 DO - 10.1088/0031-8949/1997/T73/100 VL - T73 SP - 307 EP - 310 J2 - Phys. Scripta SN - 1402-4896 ER - TY - JOUR TI - Defects and Pd growth on the reduced SnO2(100) surface AU - Katsiev, Khabibulakh AU - Batzill, Matthias AU - Boatner, Lynn A. AU - Diebold, Ulrike T2 - Surface Science AB - Scanning tunneling microscopy experiments on a clean, reduced SnO2(1 0 0)-(1 × 1) surface reveal surface defects with zero-, one-, and two-dimensions. Point defects consist of missing SnO/SnO2 units. Line defects are probably crystallographic shear planes that extend to the surface and manifest themselves as rows of atoms, shifted half a unit cell along the [0 1 0] direction. Their ends act as preferential nucleation sites for the formation of Pd clusters upon vapor deposition. Areas of a more reduced surface phase, still with a (1 × 1) structure and a half-unit cell deep, form at [0 0 1]-oriented step edges. DA - 2008/05/01/ PY - 2008 DO - 10.1016/j.susc.2008.03.003 VL - 602 IS - 9 SP - 1699 EP - 1704 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Growth, structure and thermal properties of chromium oxide films on Pt(111) AU - Zhang, Lanping AU - Kuhn, Markus AU - Diebold, Ulrike T2 - Surface Science AB - We have prepared chromium oxide films on Pt(111) with thicknesses ranging from less than a monolayer to more than eight monolayers. The films were grown by Cr vapor deposition under an oxygen background pressure of 2 × 10-6 mbar and a sample temperature of 600 K. The growth, structure and thermal properties of the resulting overlayers have been studied using a combination of ISS, XPS, LEED and STM. During the initial stage of growth, the film nucleates at Pt step edges, and grows in a step-flow mode. For the first two monolayers, a well-ordered p(2 × 2) structure is observed and attributed to the epitaxial growth of metastable Cr3O4. At higher coverage, a ([radical sign]3 × [radical sign]3)R30° structure appears, due to the formation of the stable Cr2O3 phase. After heating in vacuum to about 700 K, the p(2 × 2) film begins to decompose and agglomerate, and finally becomes a ([radical sign]3 × [radical sign]3)R30° overlayer. The latter is stable up to the highest temperature used in these studies (1000 K), although at this temperature it also shows dewetting and further clustering. DA - 1997/03/20/ PY - 1997 DO - 10.1016/S0039-6028(96)01256-3 VL - 375 IS - 1 SP - 1 EP - 12 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Epitaxial growth of ultrathin films of chromium and its oxides on Pt(111) AU - Zhang, Lanping AU - Kuhn, Markus AU - Diebold, Ulrike T2 - Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films AB - The growth of vapor-deposited Cr and chromium oxide on Pt(111) has been studied by ion scattering spectroscopy, x-ray photoelectron spectroscopy, low energy electron diffraction, and scanning tunneling microscopy. Both metal Cr film and its oxide nucleate at Pt(111) step edges, and grow in a step-flow mode. The metal Cr film is pseudomorphic to the Pt(111) substrate within the first two monolayers. At higher coverages, the chromium overlayer adopts a bcc(110) surface structure and forms three-dimensional elongated islands oriented along the three close-packed directions of the Pt(111) surface. For the chromium oxide, a well ordered p(2 × 2) structure is observed within the first two monolayers and attributed to the epitaxial growth of metastable Cr3O4. At higher coverages, a ([square root of]3 × [square root of]3)R30° structure appears, due to the formation of the stable Cr2O3 phase. DA - 1997/05// PY - 1997 DO - 10.1116/1.580635 VL - 15 SP - 1576 EP - 1580 J2 - J. Vac. Sci. Technol. A ER - TY - JOUR TI - Magnesium outdiffusion through magnetite films grown on magnesium oxide (001) (abstract) AU - Shaw, K. A. AU - Lochner, E. AU - Lind, D. M. AU - Anderson, J. F. AU - Kuhn, M. AU - Diebold, U. T2 - Journal of Applied Physics AB - Scanning tunneling microscopy (STM) studies of 1 µm thick films of single crystalline Fe3O4 grown on MgO(001) indicate that repeated annealing of the sample in UHV causes Mg diffusion through the Fe3O4 film. The onset of this effect was clearly seen by STM at room temperature for samples raised above 400–430 °C. It appears that the annealing process causes the migration of Mg from the substrate entirely through the Fe3O4 lattice, and that the migration tends to fill the surface layer first, with lower layers filling as anneal time is increased. Upon detection of this effect, several complementary sample analysis techniques were employed to determine the extent of the changes observed. X-ray diffraction studies indicate shifts in the lattice constant from the cubic constant of magnetite, Fe3O4, (8.396 Å), which is already strained in thin-film growth on a substrate, further toward the cubic lattice constant of magnesioferrite, MgFe2O4, (8.375 Å) in order to accommodate the Mg that has migrated to the surface. Superconducting quantum interference device magnetometry studies reveal a significant change in the magnetic behavior of the film and large decreases in the saturation moment, remanence, and coercive field. The Verwey transition is greatly altered in these films after the annealing sequence. X-ray photoelectron spectroscopy studies of the films confirm the presence of magnesium in the uppermost layers of the film, and indicate a concentration gradient, with the highest concentrations of magnesium in the surface layer. X-ray fluorescence in scanning electron microscopy qualitatively indicate the presence of magnesium throughout the film, consistent with migration of the magnesium from the substrate. These results are compared with those on an unannealed Fe3O4 film of the same thickness and growth parameters, which shows no magnesium migration into the film during growth up to substrate temperatures of 300 °C. DA - 1997/04/15/ PY - 1997 DO - 10.1063/1.365161 VL - 81 IS - 8 SP - 5176 J2 - J. Appl. Phys. ER - TY - JOUR TI - Novel electronic and magnetic properties of ultrathin chromium oxide films grown on Pt(111) AU - Robbert, P. S. AU - Geisler, H. AU - Ventrice, Jr. AU - van Ek, J. AU - Chaturvedi, S. AU - Rodriguez, J. A. AU - Kuhn, M. AU - Diebold, U. T2 - Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films AB - The growth of epitaxial metal–oxide films on lattice-mismatched metal substrates often results in the formation of unique overlayer structures. In particular, epitaxial chromium oxide films grown on Pt(111) exhibit a p(2 × 2) symmetry through the first two monolayers of growth which is followed by a (√3 × √3)R30° phase that is attributed to the growth of a Cr2O3(0001) overlayer. Ultraviolet photoelectron spectroscopy measurements have been performed on the CrOx/Pt(111) system. The electronic structures of CrO2, Cr2O3, and Cr3O4 were calculated using the linear muffin-tin orbital method in the atomic sphere approximation. Comparison of the photoemission valence band spectra with the calculated density of states indicates that the CrOx initially grows in a cubic spinel Cr3O4 structure. Beyond ~ 0.2 monolayers, the metallic behavior of the CrOx overlayer begins a transformation to an insulating state. The measured valence emission for the p(2 × 2) phase beyond ~ 0.2 monolayers is more consistent with either a gamma-Cr2O3(111) overlayer or possibly a reconstructed Cr2O3(0001) overlayer. DA - 1998/05// PY - 1998 DO - 10.1116/1.581283 VL - 16 SP - 990 EP - 995 J2 - J. Vac. Sci. Technol. A ER - TY - JOUR TI - Epitaxial growth and properties of ferromagnetic co-doped TiO2 anatase AU - Chambers, S. A. AU - Thevuthasan, S. AU - Farrow, R. F. C. AU - Marks, R. F. AU - Thiele, J. U. AU - Folks, L. AU - Samant, M. G. AU - Kellock, A. J. AU - Ruzycki, N. AU - Ederer, D. L. AU - Diebold, U. T2 - Applied Physics Letters AB - We have used oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) to grow CoxTi1–xO2 anatase on SrTiO3(001) for x = ~0.01–0.10, and have measured the structural, compositional, and magnetic properties of the resulting films. Whether epitaxial or polycrystalline, these CoxTi1–xO2 films are ferromagnetic semiconductors at and above room temperature. However, the magnetic and structural properties depend critically on the Co distribution, which varies widely with growth conditions. Co is substitutional in the anatase lattice and in the +2 formal oxidation state in ferromagnetic CoxTi1–xO2. The magnetic properties of OPA-MBE grown material are significantly better than those of analogous pulsed laser deposition-grown material. DA - 2001/11/19/ PY - 2001 DO - 10.1063/1.1420434 VL - 79 IS - 21 SP - 3467 EP - 3469 J2 - Appl. Phys. Lett. ER - TY - JOUR TI - Metastable surface oxide on CoGa(100): Structure and stability AU - Vlad, A. AU - Stierle, A. AU - Marsman, M. AU - Kresse, G. AU - Costina, I. AU - Dosch, H. AU - Schmid, M. AU - Varga, P. T2 - Physical Review B AB - We investigated the structure and formation of a surface oxide and bulk β-Ga2O3 on CoGa(100) from ultrahigh vacuum to 1 bar oxygen pressure in a temperature range from 300 to 1040 K. We combined in situ surface x-ray diffraction with scanning tunneling microscopy, atomic force microscopy, and density-functional theory calculations. We find that the two-dimensional epitaxial surface oxide layer exhibits a p2mm symmetry with an additional mirror plane as compared to the bulk oxide. The surface oxide layer is found to form under metastable conditions at an oxygen chemical potential ∼1.6 eV above the stability limit for bulk β-Ga2O3. The formation of the bulk oxide is kinetically hindered by the presence of the oxygen-terminated surface oxide, which most likely hampers dissociative oxygen chemisorption. We observe that below 620 K, the surface oxide is surprisingly stable at 1 bar oxygen pressure. Substrate faceting accompanies the bulk oxide formation at temperatures higher than 1020 K. DA - 2010/03/02/ PY - 2010 DO - 10.1103/PhysRevB.81.115402 VL - 81 IS - 11 SP - 115402 J2 - Phys. Rev. B ST - Metastable surface oxide on CoGa(100) ER - TY - JOUR TI - Highly ordered Pd, Fe, and Co clusters on alumina on Ni3Al(111) AU - Buchsbaum, Andreas AU - De Santis, Maurizio AU - Tolentino, Helio C. N. AU - Schmid, Michael AU - Varga, Peter T2 - Physical Review B AB - Template-mediated growth of metals has attracted much interest due to the remarkable magnetic and catalytic properties of clusters in the nanometer range and provides the opportunity to grow clusters with narrow size distributions. We have grown well-ordered Fe and Co clusters on the ultrathin aluminum oxide on Ni3Al(111), a template with a 4.1 nm lattice. The structure of the ≈0.5-nm-thick oxide film exhibits holes reaching down to the metal substrate at the corners of the (√67×√67)R12.2° unit cell. Pd atoms trapped in these corner holes create metallic nucleation sites where Fe as well as Co clusters can nucleate and form a well-ordered hexagonal arrangement on the oxide nanomesh. We have studied these Fe and Co clusters and applied different methods such as scanning tunneling microscopy and surface x-ray diffraction to determine the morphology and crystallography of the clusters. For Fe, we found cluster growth in either bcc[110] or bcc[100] direction, depending on the deposition temperature and for Co we found close-packed planes on top of the clusters and random stacking of close-packed planes. Pd clusters grow with fcc(111) orientation. DA - 2010/03/11/ PY - 2010 DO - 10.1103/PhysRevB.81.115420 VL - 81 IS - 11 SP - 115420 J2 - Phys. Rev. B ER - TY - JOUR TI - Mismatch dislocations caused by preferential sputtering of a platinum-nickel alloy surface AU - Schmid, M. AU - Biedermann, A. AU - Stadler, H. AU - Slama, C. AU - Varga, P. T2 - Applied Physics A: Materials Science & Processing AB - Preferential sputtering and recoil mixing of a Pt25Ni75(111) single crystal surface leads to platinum enrichment in the upper monolayers, thereby increasing the lattice constant in these layers. This results in subsurface lattice mismatch dislocations, which have been studied by scanning tunneling microscopy. While the subsurface dislocations are only visible as shallow ditches in STM topographs, the Burgers vectors of the dislocation system can be determined by means of atomically resolved images of dislocations reaching the surface. A comparison with simulations of lattice relaxation using embedded-atom potentials shows good agreement with STM data and further allows the determination of the thickness of the Pt enrichment. We have estimated the Pt concentration in these layers from the dislocation density and studied the annealing behaviour of the surface. DA - 1992/11/01/ PY - 1992 DO - 10.1007/BF00348334 VL - 55 IS - 5 SP - 468 EP - 475 J2 - Appl. Phys. A ER - TY - JOUR TI - Embedded-atom method calculations applied to surface segregation of Pt-Ni single crystals AU - Stadler, H. AU - Hofer, W. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - The embedded-atom method, a model for the calculation of various crystal and alloy properties, has also been applied to the study of surface segregation phenomena. We employ this formalism for calculating surface segregation on PtxNi1-x single-crystal low-index faces which are known to show an orientation-dependent segregation behaviour. Pt enrichment in the topmost layer and alternating segregation profiles for the (100) and (111) surfaces are found to be in accordance with experimental data. The results for the (110) surface, showing a different behaviour, are discussed. In addition, surface relaxation is calculated and found to correspond with experimental results. DA - 1993/05/10/ PY - 1993 DO - 10.1016/0039-6028(93)90804-S VL - 287-288 IS - Part 1 SP - 366 EP - 370 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Preparation of a pristine TiO2 anatase (101) surface by cleaving AU - Dulub, Olga AU - Diebold, Ulrike T2 - Journal of Physics: Condensed Matter AB - A natural TiO2 anatase crystal, cut to exhibit its (010) surface, was cleaved by breaking off one of its corners. The resulting sample exhibited a small, flat area ca. 2 mm2 in size with a (101) orientation as confirmed by LEED. The evolution of the surface morphology was monitored with UHV-STM. After one sputtering/annealing cycle the surface is characterized by periodic ridges that run parallel to the [010] direction. The ridges are ~ 3 nm high and 10–15 nm wide and have a spacing of 30 nm. Interestingly, [1-1-1]/11-1] -oriented step edges are not observed, despite them having the lowest formation energy. The ridges flatten with repeated sputter/annealing cycles. After a total of three cycles a flat surface is achieved, which exhibits trapezoidal terraces that are typical for anatase (101). The importance of preparing such a pristine surface for understanding the surface structure and chemistry of TiO2 anatase is discussed. DA - 2010/03// PY - 2010 DO - 10.1088/0953-8984/22/8/084014 VL - 22 IS - 8 SP - 084014 J2 - J. Phys.: Condens. Matter SN - 0953-8984 ER - TY - JOUR TI - The 2 × 1 reconstruction of the rutile TiO2(011) surface: A combined density functional theory, X-ray diffraction, and scanning tunneling microscopy study AU - Gong, Xue-Qing AU - Khorshidi, Navid AU - Stierle, Andreas AU - Vonk, Vedran AU - Ellinger, Claus AU - Dosch, Helmut AU - Cheng, Hongzhi AU - Selloni, Annabella AU - He, Yunbin AU - Dulub, Olga AU - Diebold, Ulrike T2 - Surface Science AB - An extensive search for possible structural models of the (2 × 1)-reconstructed rutile TiO2(0 1 1) surface was carried out by means of density functional theory (DFT) calculations. A number of models were identified that have much lower surface energies than the previously-proposed 'titanyl' and 'microfaceting' models. These new structures were tested with surface X-ray diffraction (SXRD) and voltage-dependent STM measurements. The model that is (by far) energetically most stable shows also the best agreement with SXRD data. Calculated STM images agree with the experimental ones for appropriate tunneling conditions. In contrast to previously-proposed models, this structure is not of missing-row type; because of its similarity to the fully optimized brookite TiO2(001) surface, we call it the 'brookite (001)-like' model. The new surface structure exhibits two different types of undercoordinated oxygen and titanium atoms, and is, in its stoichiometric form, predicted to be rather inert towards the adsorption of probe molecules. DA - 2009/01/01/ PY - 2009 DO - 10.1016/j.susc.2008.10.034 VL - 603 IS - 1 SP - 138 EP - 144 J2 - Surf. Sci. SN - 0039-6028 ST - The 2 × 1 reconstruction of the rutile TiO2(0 1 1) surface ER - TY - JOUR TI - Bulk-defect dependent adsorption on a metal oxide surface: S/TiO2(110) AU - Hebenstreit, E. L. D. AU - Hebenstreit, W. AU - Geisler, H. AU - Ventrice Jr., C. A. AU - Sprunger, P. T. AU - Diebold, U. T2 - Surface Science AB - The adsorption of molecular sulfur on TiO2(110)(1×1) has been studied with scanning tunneling microscopy and photoelectron spectroscopy. At room temperature S binds dissociatively to 5-fold coordinated Ti atoms and oxygen vacancies. At elevated temperatures (120–440°C) sulfur replaces surface oxygen atoms. Evidence was found that the reduction state of TiO2 crystals strongly affects the surface coverage of S at elevated temperatures. The rate of the O–S site exchange is kinetically limited by the arrival of diffusing bulk defects at the surface. DA - 2001/07/10/ PY - 2001 DO - 10.1016/S0039-6028(01)01067-6 VL - 486 IS - 3 SP - L467 EP - L474 J2 - Surf. Sci. SN - 0039-6028 ST - Bulk-defect dependent adsorption on a metal oxide surface ER - TY - JOUR TI - Ultrathin reactive metal films on TiO2(110): growth, interfacial interaction and electronic structure of chromium films AU - Pan, Jian-Mei AU - Diebold, Ulrike AU - Zhang, Lizhong AU - Madey, Theodore E. T2 - Surface Science AB - A study of ultrathin Cr films on TiO2(110) surfaces is reported. The combination of low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS) enables us to study quantitatively the growth of reactive metal films on metal oxides, and the chemical interactions at the interface. Ultrathin Cr films grow initially in a quasi-two-dimensional fashion at room temperature. Changes in oxidation states of both Ti and Cr during film formation suggest that strong chemical interactions at the interface are of great importance in the growth of reactive metal films. Cr "wets" the surface more effectively at 300 K than ultrathin films of less reactive metals, Fe and Cu. This suggests that a relation exists between the metal reactivity with oxygen and the wetting of metal films on oxides: The more reactive the metal towards oxygen, the better is the wetting ability. The chemical interaction is accompanied by charge transfer at the interface, causing a reduction of the surface work function. The interfacial interaction also causes the interfacial Cr layer to behave differently from the top metallic Cr atoms upon thermal annealing. The top layers of metallic Cr show a strong clustering tendency at 500°C, while the interfacial Cr seems to have less surface mobility and slow bulk diffusion. DA - 1993/10/01/ PY - 1993 DO - 10.1016/0039-6028(93)90288-U VL - 295 IS - 3 SP - 411 EP - 426 J2 - Surf. Sci. SN - 0039-6028 ST - Ultrathin reactive metal films on TiO2(110) ER - TY - JOUR TI - Growth mode of ultrathin copper overlayers on TiO2(110) AU - Diebold, U. AU - Pan, J.-M. AU - Madey, T. E. T2 - Physical Review B AB - Copper overlayers with thicknesses up to several tens of angstroms have been vapor deposited at various substrate temperatures onto rutile TiO2(110) surfaces that have different defect concentrations. The metal films have been studied by means of He+ low-energy ion scattering, x-ray photoelectron spectroscopy, and LEED (low-energy electron diffraction). Our measurements clearly show that Volmer-Weber growth (formation of three-dimensional crystallites) occurs even at sample temperatures as low as 160 K. Defects created by sputtering the substrate prior to Cu deposition do not influence the subsequent growth of the Cu films. The clusters are oriented with their (111) orientation parallel to the surface as confirmed by LEED. During low-temperature annealing, a coarsening of the crystallites takes place. The size of the clusters and the coverage have been modeled using simple assumptions about their shape. An average thickness of approximately 10 Å is needed to cover half of the sample with Cu. The growth mode can be attributed to a very weak interaction between Cu and the substrate. DA - 1993/02/15/ PY - 1993 DO - 10.1103/PhysRevB.47.3868 VL - 47 IS - 7 SP - 3868 J2 - Phys. Rev. B ER - TY - JOUR TI - Evidence for the Tunneling Site on Transition-Metal Oxides: TiO2(110) AU - Diebold, Ulrike AU - Anderson, J. AU - Ng, Kwok-On AU - Vanderbilt, David T2 - Physical Review Letters AB - We present atomically resolved scanning tunneling microscopy (STM) images from TiO2(110) surfaces. After annealing nearly perfect stoichiometric 1×1 surfaces to elevated temperatures in ultrahigh vacuum, randomly distributed oxygen vacancies are observed. The apparent shape of these defects provides strong evidence that the STM is imaging undercoordinated Ti atoms, as do first-principles pseudopotential calculations of the electronic states accessible to tunneling. We thus resolve a controversy as to whether STM imaging on this surface is dominated by geometric-structure or electronic effects. DA - 1996/08// PY - 1996 DO - 10.1103/PhysRevLett.77.1322 VL - 77 IS - 7 SP - 1322 EP - 1325 J2 - Phys. Rev. Lett. SN - 0031-9007 ST - Evidence for the Tunneling Site on Transition-Metal Oxides ER - TY - JOUR TI - Intrinsic defects on a TiO2(110)(1×1) surface and their reaction with oxygen: a scanning tunneling microscopy study AU - Diebold, Ulrike AU - Lehman, Jeremiah AU - Mahmoud, Talib AU - Kuhn, Markus AU - Leonardelli, Georg AU - Hebenstreit, Wilhelm AU - Schmid, Michael AU - Varga, Peter T2 - Surface Science AB - We report a scanning tunneling microscopy (STM) study of the rutile TiO2(110) surface. The surface was prepared by sputtering and annealing in an ultrahigh vacuum (UHV). After annealing to 1100 K in UHV, a (1×1) surface with a terrace width of ~100 Å is obtained. The terraces are separated by monoatomic step edges running predominantly parallel to <001> and type directions. Approximately half of the <001>-type steps have a kinked appearance that is attributed to a (4×1)-reconstructed step edge. Atomic models for autocompensated step edges are presented. Oxygen vacancies (point defects) in the bridging oxygen rows are created by the high-temperature anneal in UHV. In STM images, these oxygen vacancies appear as bright features centered on dark rows. Their density is 7±3% per surface unit cell and is reduced upon exposure to molecular oxygen at room temperature. Dark features on bright rows are also seen; these are not affected by molecular oxygen and are tentatively assigned to subsurface defects. Hydroxyl groups from spurious water in the oxygen gas stream are observed to adsorb dissociatively at step edges and on the in-plane Ti rows on the terraces. The appearance of the surface oxygen vacancies depends on the state of the STM tip; asymmetric tips skew the appearance of the point defects and may even render images where they are invisible. Tip changes occur frequently, especially when the surface has been exposed to oxygen, and may lead to images that are hard to interpret. The "normal" tip state where the vacancies appear as bright spots connecting bright rows can be regained reproducibly by scanning with a high (up to +10 V) tip voltage; the tip is then possibly covered with substrate material. The oxygen vacancies show strong interactions with the STM tip, i.e. tip-induced oxidation and mobility. These interactions depend strongly on the state of the tip, and are enhanced by the presence of oxygen in the ambient. A model for the tip-induced oxidation is presented where oxygen atoms hop between the tip and sample to explain these effects. DA - 1998/08/11/ PY - 1998 DO - 10.1016/S0039-6028(98)00356-2 VL - 411 IS - 1-2 SP - 137 EP - 153 J2 - Surf. Sci. SN - 0039-6028 ST - Intrinsic defects on a TiO2(110)(1×1) surface and their reaction with oxygen ER - TY - JOUR TI - Study of silica supported PtxNi1-x catalysts by ion scattering spectroscopy AU - Borrell, Martine AU - Jentys, Andreas AU - Varga, Peter T2 - Microchimica Acta AB - As supported PtxNi1-x catalysts are used for hydrogenation reactions, it seemed necessary to assess the surface composition of the reduced samples. To approach the usual reduction conditions we applied in situ reduction in a reaction chamber (1 mbar H2 up to 500 °C) placed in ultra high vacuum recipient (UHV: 10-9 to 2.10-10mbar). All ion scattering spectroscopy measurements were performed in UHV. Charging of the samples was avoided by electron bombardment (5 eV). The variation of the surface composition was determined after subsequent sputtering, thermal treatment at 500 °C and during oxygen adsorption. A comparison with previous results of surface compositions of binary alloys (polycrystalline foils [1, 2] and single crystals PtxNi1-x [3]) is given. DA - 1997/03/01/ PY - 1997 DO - 10.1007/BF01246216 VL - 125 IS - 1 SP - 389 EP - 393 J2 - Microchim. Acta ER - TY - JOUR TI - Hochauflösende Rastertunnelmikroskopie unterscheidet Atome AU - Varga, Peter AU - Schmid, Michael AU - Redinger, Josef T2 - Physik in unserer Zeit AB - Rastertunnelmikroskopie mit atomarer Auflösung ermöglicht die chemische Analyse an Oberflächen von metallischen Legierungen und ultradünnen Filmen auf atomarer Skala und trägt damit zum besseren Verständnis von katalytischen Prozessen bei. DA - 2000/// PY - 2000 DO - 10.1002/1521-3943(200009)31:5<215::AID-PIUZ215>3.0.CO;2-0 VL - 31 IS - 5 SP - 215 EP - 221 J2 - Physik in unserer Zeit ER - TY - JOUR TI - The structure of the polar Sn-doped indium oxide (001) surface AU - Morales, Erie H. AU - Diebold, Ulrike T2 - Applied Physics Letters AB - Epitaxial Sn-doped In2O3 (ITO) thin films were grown using oxygen plasma-assisted molecular beam epitaxy (MBE) on (001) oriented Yttria Stabilized Zirconia. Low-energy-electron-diffraction shows that ITO(001) surface is oxygen terminated and has a c(1×1)-structure with p4g symmetry. Atomically-resolved Scanning Tunneling Microscopy suggests that surface oxygen atoms undergo dimerization; possible adsorption sites are identified. The density of surface oxygen depends on the Sn concentration and it is suggested that both, dimerization and doping stabilize the polar ITO(001) surface. DA - 2009/// PY - 2009 DO - 10.1063/1.3275716 VL - 95 IS - 25 SP - 253105 J2 - Appl. Phys. Lett. SN - 00036951 ER - TY - JOUR TI - On the role of kinks and strain in heteroepitaxial growth: An STM study AU - Lundgren, E. AU - Schmid, M. AU - Leonardelli, G. AU - Hammerschmid, A. AU - Stanka, B. AU - Varga, P. T2 - Surface Review and Letters AB - Interlayer diffusion of Co over steps of vacancy islands on the Pt(111) surface as studied by scanning tunneling microscopy is presented. It is demonstrated that Co atoms descend Pt steps by an exchange diffusion process at the step edge with the Pt atoms. Further, the exchange diffusion process is observed to occur at the corners (kinks) of the vacancy islands. The importance of kinks concerning whether the growth mode of a heteropitaxial film is two-dimensional or three-dimensional is demonstrated for the case of thin Co films on Pt(111). We argue that the strain in the Co film is to a large extent responsible for the kink formation. DA - 2000/// PY - 2000 DO - 10.1142/S0218625X00000750 VL - 7 IS - 5-6 SP - 673 EP - 677 J2 - Surf. Rev. Lett. ER - TY - JOUR TI - Thermal Stability of Ultrathin Cr Films on Pt(111) AU - Zhang, Lanping AU - Kuhn, Markus AU - Diebold, Ulrike AU - Rodriguez, Jose A. T2 - The Journal of Physical Chemistry B AB - The thermal stability of ultrathin chromium films on Pt(111) has been studied using low-energy He+ ion scattering spectroscopy, X-ray photoelectron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy. At room temperature, Cr grows on Pt(111) with a modified StranskiKrastanov growth mode (almost complete wetting of the first two layers with subsequent three-dimensional island growth). Upon annealing, chromium diffuses into the Pt lattice. This causes a smoothening of the surface features. A flat Pt(111) surface, devoid of Cr, is regained after prolonged annealing above 770 K. At lower temperatures, metastable CrPt surface alloys are formed. The apparent composition and stability of these surface alloys are dependent upon the amount of Cr deposited initially. Both the Cr adlayer and Pt surface atoms were found to be perturbed at the interface. Experimental and theoretical results indicate that a redistribution of charge between Pt and Cr occurs upon alloy formation with the greatest perturbation experienced by the Pt atoms. DA - 1997/06/01/ PY - 1997 DO - 10.1021/jp9627786 VL - 101 IS - 23 SP - 4588 EP - 4596 J2 - J. Phys. Chem. B ER - TY - JOUR TI - Segregated Si on Fe96.5Si3.5(110): Domain-wall structures in a two-dimensional alloy AU - Biedermann, A. AU - Schmid, M. AU - Varga, P. T2 - Physical Review B AB - The surface structures formed by segregated silicon on a bcc Fe96.5Si3.5(110) surface have been studied by scanning tunneling microscopy (STM). Additionally Auger-electron spectroscopy and low-energy ion-scattering spectroscopy have been used to obtain the chemical and structural information necessary to decide between possible configurations suggested by the STM images. Near the expected saturation coverage a substitutional c(1×3)Si superstructure weakly disordered by 〈110〉 aligned domain boundaries was observed. Their arrangement is equivalent to the incommensurate domain-wall structures of chemisorption systems like H/Fe(110), which allow a continuous increase of the surface coverage by successive insertion of equidistantly arranged heavy domain walls. Our observations suggest a possible increase of the silicon coverage above the 33.3% of the perfect c(1×3) structure at least up to 40% corresponding to a c(1×5)Si commensurate domain-wall structure. Correlation analysis of the disordered phase at 15% coverage, obtained by short annealing, allows a qualitative determination of the interaction forces among individual silicon atoms on the surface, yielding attraction of third-nearest neighbors and repulsion of nearest neighbors. DA - 1994/12/15/ PY - 1994 DO - 10.1103/PhysRevB.50.17518 VL - 50 IS - 23 SP - 17518 J2 - Phys. Rev. B ST - Segregated Si on Fe96.5Si3.5(110) ER - TY - JOUR TI - An STM study of the step structure of Pb(110) and Pb(111) AU - Speller, S. AU - Heiland, W. AU - Biedermann, A. AU - Platzgummer, E. AU - Nagl, C. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - The (110) and (111) surfaces of lead are investigated by scanning tunneling microscopy. They are atomically resolved with corrugations of 0.1 Å and 0.6 Å respectively. The STM images allow conclusions about the motion of the atoms at step edges and step step interaction. At room temperature the steps on both surfaces are below the roughening transition. The influence of impurities and tip surface interaction on the step fluctuation is discussed. DA - 1995/07/01/ PY - 1995 DO - 10.1016/0039-6028(95)00257-X VL - 331-333 IS - Part 2 SP - 1056 EP - 1061 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Chemically resolved STM on a PtRh(100) surface AU - Wouda, P. T. AU - Nieuwenhuys, B. E. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - Scanning tunnelling microscopy on PtRh(100) (molar bulk composition 1 : 1) has revealed the possibility of direct determination of the surface of this system. During measurements at low tunnelling resistance (<500 kΩ), the Pt and Rh atoms appear with a clearly observable apparent height difference of more than 20 pm. No long range ordering has been found. Variation of the sample preparation method and comparison of STM and Auger electron spectroscopy measurements led to the conclusions that there is a preferential surface segregation of platinum, that rhodium atoms are the ones with the highest apparent height, and that there is limited tendency of clustering on the surface. Furthermore, it was found that platinum atoms preferentially populate the step edges on this crystal surface. DA - 1996/07// PY - 1996 DO - 10.1016/0039-6028(96)00022-2 VL - 359 IS - 1-3 SP - 17 EP - 22 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Scanning tunneling microscopy studies of niobium carbide (100) and (110) surfaces AU - Tsong, R. M. AU - Schmid, M. AU - Nagl, C. AU - Varga, P. AU - Davis, R. F. AU - Tsong, I. S. T. T2 - Surface Science AB - Scanning tunneling microscopy (STM) studies were conducted on the surfaces of NbC0.75(100) and NbC0.865(110) single crystals after in situ cleaning treatments of sputtering and annealing cycles. STM images show atom-resolved structures of both surfaces. On NbC0.75(100), localized areas of a square (1 × 1) structure were observed, together with regions of hexagonal structure, indicative of a coexisting surface phase, possibly that of Nb4C3-x. On the NbC0.865(110) surface, a ridge-and-valley grating structure consisting of both (4 × 1) and (5 × 1) geometries was observed over large areas. The nanometer-scale faceting phenomenon may be common to the (110) surfaces of most transition-metal carbides. DA - 1996/10/10/ PY - 1996 DO - 10.1016/0039-6028(96)00804-7 VL - 366 IS - 1 SP - 85 EP - 92 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Surface structure of the missing-row reconstruction of VC0.8(110): a scanning tunneling microscopy analysis AU - Gauthier, Y. AU - Schmid, M. AU - Hebenstreit, W. AU - Varga, P. T2 - Surface Science AB - Scanning tunnelling microscopy (STM) was used to study the (110) surface of a VC0.8 sample. The surface shows a missing-row reconstruction, i.e., a grating structure with ridges and valleys oriented along the [001] direction and (100) and (010) facets. We did not find unreconstructed (110) terraces. The regular spacing of the ridges corresponds to a periodicity of (3 × 1) or (4 × 1), depending on preparation, presumably related to different concentrations of carbon vacancies. In the STM images, we can also observe apparent pairing of atoms in the rows, leading to the larger c(6 × 2) and (4 × 2) superstructure cells, which also show up in LEED. We attribute these additional periodicities to ordering of carbon vacancies in the surface rows. DA - 2003/12/20/ PY - 2003 DO - 10.1016/j.susc.2003.10.016 VL - 547 IS - 3 SP - 394 EP - 402 J2 - Surf. Sci. SN - 0039-6028 ST - Surface structure of the missing-row reconstruction of VC0.8(1 1 0) ER - TY - JOUR TI - Atomic structure of an Al-Co-Ni decagonal quasicrystalline surface AU - Yuhara, J. AU - Klikovits, J. AU - Schmid, M. AU - Varga, P. AU - Yokoyama, Y. AU - Shishido, T. AU - Soda, K. T2 - Physical Review B AB - We have analyzed the structure and composition of the first layer of an Al72Co16Ni12 tenfold surface by means of scanning tunneling microscopy (STM), ion scattering spectroscopy (ISS), and Auger electron spectroscopy (AES). High-resolution STM images reveal local structures that have decagonal symmetry in addition to the usual pentagonal symmetry of the surface. This quasicrystal surface resembles a random tiling instead of an ideal quasiperiodic tiling. After annealing at 1100 K , the total surface atomic density found by ISS is (9±1)×10^14 cm−2 . The surface densities of Al and TM (transition metal, i.e., Co and Ni) are determined as (8±1)×10^14 cm−2 and (1.0±0.2)×10^14 cm−2 , respectively from ISS, indicating a similar density of Al and much lower density of the TM atoms in the surface layer than in a truncated bulk. The Al surface atomic density agrees well with the number of corrugation maxima in the STM images. A model of the arrangement of the Al atoms in the top layer is presented. Scanning tunneling spectroscopy (STS) is performed to study the local electronic structure. The STS spectrum at the corrugation maxima is similar to that at the corrugation minima. A few ≈0.12 nm high protrusions in the STM images are attributed to local oxide clusters due to their STS spectra different from the corrugation maxima and through in situ STM observations during exposure to O2 gas at 2×10^−6 Pa at RT. DA - 2004/07/07/ PY - 2004 DO - 10.1103/PhysRevB.70.024203 VL - 70 IS - 2 SP - 024203 J2 - Phys. Rev. B ER - TY - JOUR TI - Low-energy ion impact desorption cross sections of carbon monoxide from Ni(111) AU - Diebold, U. AU - Möller, W. AU - Varga, P. T2 - Surface Science AB - We report on the determination of cross sections for desorption ([sigma]D) of CO/Ni(111) induced by rare-gas ion (He, Ne, Ar and Kr) bombardment at very low impact energy (10-500 eV). When bombarding the initially fully covered surface, mainly neutral CO was desorbed as was measured by the means of a quadrupole mass spectrometer. The desorption cross section was determined from the exponential decay of the signal of desorbed CO. The experiment was simulated with the dynamic Monte Carlo code TRIDYN [W. Möller, W. Eckstein and J.P. Biersack, Comput. Phys. Commun. 51 (1988) 355], and a very good agreement with [sigma]D obtained in the experiment was found. By comparing the calculated values of sputtering yields with [sigma]D, the role of recoil implantation and dissociation is discussed. DA - 1991/05/03/ PY - 1991 DO - 10.1016/0039-6028(91)90068-4 VL - 248 IS - 1-2 SP - 147 EP - 157 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Influence of the primary ion charge state on secondary ion production: bombardment of CO/Ni(111) with Ne+, Ne2+, Kr+ and Kr2+ at low impact energies AU - Diebold, U. AU - Varga, P. T2 - Surface Science Letters AB - We report on the secondary ion emission from Ni(111) covered with one monolayer of CO. The ion emission was induced by bombardment with singly and doubly charged Ne and Kr ions (impact energy 20-500 eV). At low impact energies (< 200 eV), Ne2+ or Kr2+ caused a significantly higher Ni+ secondary ion yield than Ne+ and Kr+ respectively. A mechanism for this charge-dependent secondary ion formation at low impact energy is discussed. DA - 1991/01/01/ PY - 1991 DO - 10.1016/0167-2584(91)91044-W VL - 241 IS - 1-2 SP - L6 EP - L10 J2 - Surf. Sci. SN - 0167-2584 ST - Influence of the primary ion charge state on secondary ion production ER - TY - JOUR TI - Growth and structure of ultrathin Cr films on Pt(111) AU - Zhang, Lanping AU - Kuhn, Markus AU - Diebold, Ulrike T2 - Surface Science AB - The growth and structure of ultrathin chromium films on Pt(111) have been studied using low-energy He+ ion scattering spectroscopy, X-ray photoelectron spectroscopy, low energy electron diffraction and scanning tunneling microscopy. Upon initial deposition at room temperature, the Cr atoms migrate along the Pt terraces and populate the step edges. Further growth results in two-dimensional islands of chromium developing from the step edges and across the Pt terraces. The film is pseudomorphic to the Pt(111) substrate. At coverages above 3 ML, chromium forms three-dimensional elongated islands oriented along the three closepacked directions of the Pt(111) surface. The chromium overlayer adopts a bcc (110) surface structure with the close-packed overlayer directions, <111>, aligned with the close-packed Pt(111) directions, <110>. The presence of the Cr overlayer was found to electronically perturb the Pt surface atoms leading to a redistribution of charge between Pt and Cr. DA - 1997/02/01/ PY - 1997 DO - 10.1016/S0039-6028(96)01001-1 VL - 371 IS - 2-3 SP - 223 EP - 234 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Lattice mismatch dislocations in a preferentially sputtered alloy studied by scanning tunneling microscopy AU - Schmid, M. AU - Biedermann, A. AU - Stadler, H. AU - Varga, P. T2 - Physical Review Letters AB - Scanning tunneling microscopy (STM) on a sputtered and annealed Pt25Ni75(111) single crystal reveals a network of subsurface lattice mismatch dislocations caused by platinum enrichment due to preferential sputtering and recoil mixing. Atomically resolved STM topographs are compared with simulations of these dislocations using embedded atom potentials. This allows one to estimate the depth of the dislocations, and thus the thickness of Pt enrichment, which is three monolayers on the 500 eV Xe+ sputtered and five monolayers on the Ar+ sputtered surface, compatible with the depth of radiation damage. DA - 1992/// PY - 1992 DO - 10.1103/PhysRevLett.69.925 VL - 69 IS - 6 SP - 925 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Surface stress, surface elasticity, and the size effect in surface segregation AU - Schmid, M. AU - Hofer, W. AU - Varga, P. AU - Stoltze, P. AU - Jacobsen, K. W. AU - Nørskov, J. K. T2 - Physical Review B AB - Surface stress and surface elasticity of low-index fcc surfaces have been studied using effective-medium theory potentials. In addition to total-energy calculations giving stress components and elastic data for the surface as a whole, the use of artificial atoms with modified size allows us to probe the stress and elasticity of individual layers. This method of artificial atoms provides a direct way to study the contribution of atomic size to segregation in alloys as well as the driving force of reconstructions driven by surface stress. As an example, we give a qualitative explanation of the face-dependent segregation of Pt-Ni alloys. We also compare results of these atomic-scale calculations with continuum elasticity. DA - 1995/04/15/ PY - 1995 DO - 10.1103/PhysRevB.51.10937 VL - 51 IS - 16 SP - 10937 J2 - Phys. Rev. B ER - TY - JOUR TI - Imaging Cluster Surfaces with Atomic Resolution: The Strong Metal-Support Interaction State of Pt Supported on TiO2(110) AU - Dulub, Olga AU - Hebenstreit, Wilhelm AU - Diebold, Ulrike T2 - Physical Review Letters AB - Nanosized platinum clusters were grown on a TiO2(110) surface and annealed in ultrahigh vacuum at high temperatures. This leads to the so-called strong metal-support interaction (SMSI) state, characterized by a complete encapsulation of the clusters with a reduced titanium oxide layer. We present atomically resolved scanning tunneling microscopy measurements of the cluster surfaces and an atomic model of the SMSI state. The ability to resolve the cluster surface geometry with atomistic detail may help to identify the active sites responsible for the SMSI. DA - 2000/04// PY - 2000 DO - 10.1103/PhysRevLett.84.3646 VL - 84 IS - 16 SP - 3646 EP - 3649 J2 - Phys. Rev. Lett. SN - 0031-9007 ST - Imaging Cluster Surfaces with Atomic Resolution ER - TY - JOUR TI - Local ordering and electronic signatures of submonolayer water on anatase TiO2(101) AU - He, Yunbin AU - Tilocca, Antonio AU - Dulub, Olga AU - Selloni, Annabella AU - Diebold, Ulrike T2 - Nature Materials AB - The interaction of water with metal oxide surfaces is of fundamental importance to various fields of science, ranging from geophysics to catalysis and biochemistry. In particular, the discovery that TiO2 photocatalyses the dissociation of water has triggered broad interest and intensive studies of water adsorption on TiO2 over decades. So far, these studies have mostly focused on the (110) surface of the most stable polymorph of TiO2, rutile, whereas it is the metastable anatase form that is generally considered photocatalytically more efficient. The present combined experimental (scanning tunnelling microscopy) and theoretical (density functional theory and first-principles molecular dynamics) study gives atomic-scale insights into the adsorption of water on anatase (101), the most frequently exposed surface of this TiO2 polymorph. Water adsorbs as an intact monomer with a computed binding energy of 730 meV. The charge rearrangement at the molecule–anatase interface affects the adsorption of further water molecules, resulting in short-range repulsive and attractive interactions along the [010] and [11-1]/[1-1-1] directions, respectively, and a locally ordered (2×2) superstructure of molecular water. DA - 2009/07// PY - 2009 DO - 10.1038/nmat2466 VL - 8 IS - 7 SP - 585 EP - 589 J2 - Nat. Mater. SN - 1476-1122 ER - TY - JOUR TI - Highly ordered nanoscale surface alloy formed through Cr-induced Pt(111) reconstruction AU - Zhang, Lanping AU - van Ek, J. AU - Diebold, Ulrike T2 - Physical Review B AB - We present scanning tunneling microscopy (STM) results of an ordered Cr/Pt surface alloy formed upon annealing 1.5–3 ML Cr/Pt(111) to 800 K. This alloy exhibits a nanoscale pattern with remarkable features: (i) it appears only along surface step edges; (ii) it forms a highly symmetric hexagonal network of one-atom-wide dislocation lines, with a unit-cell dimension of 17.3 Å; and (iii) the contrast in STM images is sensitive to the STM bias voltage, reflecting a strong local variation of the electronic structure due to the presence of a regular array of two-dimensional Cr clusters containing ten atoms. DA - 1998/02/15/ PY - 1998 DO - 10.1103/PhysRevB.57.R4285 VL - 57 IS - 8 SP - R4285 EP - R4288 J2 - Phys. Rev. B ER - TY - JOUR TI - STM study of the geometric and electronic structure of ZnO(0001)-Zn, (000-1)-O, (10-10), and (11-20) surfaces AU - Dulub, Olga AU - Boatner, Lynn A AU - Diebold, Ulrike T2 - Surface Science AB - The geometric and electronic structure of clean (0001), (000-1), (11-20), and (10-10) faces of ZnO single crystals have been studied with scanning tunneling microscopy (STM) and spectroscopy (STS), low-energy electron diffraction (LEED), and low-energy He+ ion scattering spectroscopy (LEIS). All surfaces exhibit a (1×1) termination but distinctly different terrace and step structures. On the zinc-terminated (0001)-Zn surface, the terraces are covered with triangular islands and pits of different sizes, rotated by 180° with respect to those in the neighboring terraces. Single-layer steps with a height of ≈2.7 Å are observed. Vicinal surfaces of (0001)-Zn consist of terraces separated by alternating straight and saw-tooth-shaped steps. On the oxygen-terminated (000-1)-O surface, flat hexagonal terraces are separated by predominantly ≈5.3 Å high-double-layer steps. The terraces are wide (≈500 Å) and smooth with no added islands and holes. They are not covered with a saturation coverage of hydrogen. Near-atomic-resolution images of the prism (10-10) surface show flat, rectangular terraces separated by single-layer steps (≈3 Å) running perpendicular to the <0001> and <1-210> directions. A high density of terraces with atomic rows running preferentially along the <0001> directions was observed on the as-grown (11-20) surface. This surface is the least stable and tends to form long grooves that are ≈250 Å wide and ≈50 Å deep along the <1-100> directions. STS measurements show semiconductor-like behavior of all the surfaces, but a slightly different I–V characteristic of the (000-1)-O face. Based on these results, structural models for the different surfaces are proposed and related to the stability and reactivity of ZnO surfaces. DA - 2002/// PY - 2002 DO - 10.1016/S0039-6028(02)02211-2 VL - 519 IS - 3 SP - 201 EP - 217 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Growth of Copper on Single Crystalline ZnO: Surface Study of a Model Catalyst AU - Dulub, Olga AU - Batzill, Matthias AU - Diebold, Ulrike T2 - Topics in Catalysis AB - Copper, vapor-deposited on the polar, Zn-terminated ZnO(0001) surface is investigated in view of its suitability as model system for the technologically important Cu/ZnO catalyst. The structure and electronic properties of Cu clusters on ZnO(0001)–Zn have been studied with scanning tunneling microscopy (STM), low energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), and low-energy He+ ion scattering (LEIS). At room temperature copper grows as two-dimensional (2D) clusters only at very low coverages of 0.001–0.05 equivalent monolayers (ML). At coverages greater than 0.01 ML, 3D clusters start to develop. This is contrasted to Cu growth on the oxygen-terminated ZnO(0001bar) surface, where a strong adhesion between Cu and the ZnO substrate results in an initial wetting of the surface by Cu. On ZnO(0001)–Zn, surface roughness and sputter damage change the growth mode to more 2D-like. Annealing in UHV results in well-separated, hexagonal clusters rotationally aligned with the substrate. Annealing of 2–5 ML Cu deposits on the ZnO(0001)–Zn surface in 10−6 mbar O2 results in the formation of a (√3 × √3)R30° superstructure with respect to the ZnO lattice. This superstructure likely contains Cu+ sites. The suitability of the different surface morphologies to probe specific sites that are thought to be active for catalytic processes is discussed. DA - 2005/// PY - 2005 DO - 10.1007/s11244-005-7863-5 VL - 36 IS - 1 SP - 65 EP - 76 J2 - Top. Catal. ST - Growth of Copper on Single Crystalline ZnO ER - TY - JOUR TI - Semiconductor-half metal transition at the Fe3O4(001) surface upon hydrogen adsorption AU - Parkinson, Gareth S. AU - Mulakaluri, Narasimham AU - Losovyj, Yaroslav AU - Jacobson, Peter AU - Pentcheva, Rossitza AU - Diebold, Ulrike T2 - Physical Review B AB - The adsorption of H on the magnetite (001) surface was studied with photoemission spectroscopies, scanning tunneling microscopy, and density-functional theory. At saturation coverage the insulating (√2×√2)R45° reconstruction is lifted and the surface undergoes a semiconductor–half metal transition. This transition involves subtle changes in the local geometric structure linked to an enrichment of Fe2+ cations at the surface. The ability to manipulate the electronic properties by surface engineering has important implications for magnetite-based spintronic devices. DA - 2010/// PY - 2010 DO - 10.1103/PhysRevB.82.125413 VL - 82 IS - 12 SP - 125413 J2 - Phys. Rev. B ER - TY - JOUR TI - Deconvolution of STM images using entropy as a regularization functional AU - Böhmig, S. AU - Schmid, M. AU - Störi, H. T2 - Fresenius' Journal of Analytical Chemistry AB - The Maximum Entropy approach is applied to restore and sharpen scanning tunneling microscopy (STM) images with atomic resolution. Based on the STM theory of Tersoff and Hamann the process of data acquisition can be approximated by the convolution of a localised atomic density of states (i.e. narrow spots in the reconstruction) of the sample and a Gaussian resolution function which limits the resolution. In STM practice a good and robust estimation of the atomic core positions is necessary for different reasons, such as to be able to calculate the characteristics of the atomic lattice or to study non-periodicities. DA - 1995/10/25/ PY - 1995 DO - 10.1007/s0021653530439 VL - 353 IS - 3 SP - 439 EP - 442 J2 - Fresenius J. Anal. Chem. ER - TY - JOUR TI - Surface composition of Pt10Ni90(110) AU - Weigand, P. AU - Jelinek, B. AU - Hofer, W. AU - Varga, P. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - Low energy ion scattering spectroscopy results are presented which allow to derive the first and second layer's composition of the Pt10Ni90(110) single crystal. After annealing at 970 K the topmost layer is found to be nearly pure Ni, whereas the second layer shows a strong enrichment in Pt. After bombarding the surface with ions a similar depth profile is preserved with a slight Ni enrichment in the first monolayer. Thermal treatment of the sputtered surface induces site changes at low temperatures between the first and second layer and at high temperatures equilibration between surface and bulk. Only very few theoretical models successfully describe the oscillating segregation profile and the orientation dependent segregating component of the PtNi system in the equilibrium state. A thorough thermodynamic description (previously applied in a monolayer approximation) will be used here in multilayer calculations to study the influence of the different effects on the composition profile. Pt and Ni differ negligibly in the surface free energies giving way to a competition between an ordering effect and a size effect. Good agreement between measurement and calculation is found. DA - 1994/03/02/ PY - 1994 DO - 10.1016/0168-583X(94)95857-2 VL - 85 IS - 1-4 SP - 424 EP - 428 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Surface composition of Pt25Ni75(111) investigated by ISS and STM AU - Weigand, P. AU - Nagl, C. AU - Schmid, M. AU - Varga, P. T2 - Fresenius' Journal of Analytical Chemistry AB - Ion scattering spectroscopy results show Pt enrichment in the topmost atomic layers of a Pt25Ni75(111) single crystal due to preferential sputtering. The increased lattice constant caused thereby leads to subsurface lattice mismatch dislocations, which have been studied by scanning tunneling microscopy. Agreement is found for the Pt concentration estimated from the density of dislocations and the results obtained by ISS. Thermal annealing induces further segregation of Pt in the topmost atomic layer. The composition of the subsurface layers has been studied and a strong dependence on the annealing temperature has been found. The observed Pt enrichment in the first monolayer for the thermodynamic equilibrium state agrees well with a thermodynamic theory. DA - 1993/01/01/ PY - 1993 DO - 10.1007/BF00321431 VL - 346 IS - 1 SP - 281 EP - 283 J2 - Fresenius J. Anal. Chem. ER - TY - JOUR TI - Preferential sputtering of Pt-Ni alloy single crystals studied by scanning tunneling microscopy AU - Schmid, M. AU - Biedermann, A. AU - Slama, C. AU - Stadier, H. AU - Weigand, P. AU - Varga, P. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - Due to its composition, the altered layer of preferentially sputtered alloys has a lattice constant different from that of the bulk. This lattice mismatch can lead to the formation of dislocations or reconstructions, which have been studied on different crystallographic faces. While a subsurface dislocation network exists on the (111) plane, parallel dislocations are found below the (110) surface. The (100) surface exhibits a shifted-row reconstruction, which is tentatively attributed to the stress induced by lattice mismatch between the bulk and the Pt-enriched surface. The annealing process of the sputtered Pt25Ni75(111) surface is studied in detail by evaluation of the mismatch dislocations and low energy ion scattering data. DA - 1993/07/02/ PY - 1993 DO - 10.1016/0168-583X(93)96028-B VL - 82 IS - 2 SP - 259 EP - 268 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Chemical analysis of PtxNi1−x alloy single crystal surfaces by scanning tunnelling microscopy AU - Biedermann, A. AU - Schmid, M. AU - Varga, P. T2 - Fresenius' Journal of Analytical Chemistry AB - Two STM investigations are presented, in which irregular tip conditions enable direct access to chemical and structural information of a surface on an atomic scale, otherwise invisible for the STM. They allow a study of surface ordering of a Pt25Ni75(111) crystal by chemical contrast between the alloy components, and a study of carbon superstructures on a Pt10Ni90(100) surface by simultaneous imaging of substrate lattice and carbon atoms. All these images were obtained at very low tunnelling resistances and thus at small tip-sample distances. A chemical interaction between the probably adsorbate covered tip and the sample is proposed to explain these images. DA - 1994/01/01/ PY - 1994 DO - 10.1007/BF00323272 VL - 349 IS - 1 SP - 201 EP - 203 J2 - Fresenius J. Anal. Chem. ER - TY - JOUR TI - Domain wall structures in an ordered Si/Fe(110) surface alloy AU - Biedermann, A. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - During the annealing process of an Fe96.5Si3.5(100)/(110) bicrystal, silicon and impurity carbon segregate to the surface. The structures formed by the segregands on the (110) surface have been studied by STM (geometry) and AES (chemical information). Silicon substitutes iron surface atoms and forms a two-dimensional alloy, whereas carbon occupies hollow sites in the first monolayer, leading to a distortion of the substrate lattice. The structures are based on a c(1 × 3)Si [theta] = 1/3 ordered surface alloy. Additional silicon as well as the co-segregating impurity carbon are inserted into this structure by formation of domain walls. If the density of these nearly straight and parallel domain walls becomes high enough, commensurate domain wall structures with c(1 × n) supercells can be observed. DA - 1995/07/01/ PY - 1995 DO - 10.1016/0039-6028(95)00339-8 VL - 331-333 IS - Part 1 SP - 787 EP - 793 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Competitive segregation of Si and P on Fe96.5Si3.5 (100) and (110) AU - Biedermann, A. AU - Schmid, M. AU - Reichl, B. AU - Varga, P. T2 - Fresenius' Journal of Analytical Chemistry AB - Annealing an Fe96.5Si3.5 (100)/(110) bicrystal, containing 90 ppm P, leads immediately to a strong segregation of silicon. The Si atoms, however, desegregate subsequently and are displaced by P, whose segregation enthalpy is larger than that of silicon. The corresponding surface structures formed on both faces have been studied using complementary methods: Scanning tunneling microscopy (STM) to obtain atomically resolved geometrical information and Auger electron spectroscopy (AES) for the determination of the surface composition. Si substitutes surface Fe atoms on both faces and forms ordered surface alloys, whereas P occupies hollow sites on the surface. Si and P form c(2 × 2) superstructures on the (100) surface, whereby each segregated phosphorus atom blocks in the average one silicon segregation site. The (110) surface, on the other hand, is characterized by a c(1 × 3) Si superstructure. Due to the anisotropy of this surface the P/Si exchange proceeds by the formation of silicon coverage decreasing domain boundaries within the silicon structure, which are simultaneously occupied by P atoms. Furthermore the comparison of the AES and STM derived phosphorus coverages indicates a P multilayer segregation on the (110) surface. DA - 1995/10/25/ PY - 1995 DO - 10.1007/s0021653530259 VL - 353 IS - 3 SP - 259 EP - 262 J2 - Fresenius J. Anal. Chem. ER - TY - JOUR TI - Probing the surface phase diagram of Fe3O4(001) towards the Fe-rich limit: Evidence for progressive reduction of the surface AU - Novotny, Zbynek AU - Mulakaluri, Narasimham AU - Edes, Zoltan AU - Schmid, Michael AU - Pentcheva, Rossitza AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - Physical Review B AB - Reduced terminations of the Fe3O4(001) surface were studied using scanning tunneling microscopy, x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Fe atoms, deposited onto the thermodynamically stable, distorted B-layer termination at room temperature (RT), occupy one of two available tetrahedrally coordinated sites per (√2×√2)R45° unit cell. Further RT deposition results in Fe clusters. With mild annealing, a second Fe adatom per unit cell is accommodated, though not in the second tetrahedral site. Rather both Fe atoms reside in octahedral coordinated sites, leading to a “Fe-dimer” termination. At four additional Fe atoms per unit cell, all surface octahedral sites are occupied, resulting in a FeO(001)-like phase. The observed configurations are consistent with the calculated surface phase diagram. Both XPS and DFT+U results indicate a progressive reduction of surface iron from Fe3+ to Fe2+ upon Fe deposition. The antiferromagnetic FeO layer on top of ferromagnetic Fe3O4(001) suggests possible exchange bias in this system. DA - 2013/05/08/ PY - 2013 DO - 10.1103/PhysRevB.87.195410 VL - 87 IS - 19 SP - 195410 J2 - Phys. Rev. B ST - Probing the surface phase diagram of Fe_{3}O_{4}(001) towards the Fe-rich limit ER - TY - JOUR TI - Tailoring the photocatalytic reaction rate of a nanostructured TiO2 matrix using additional gas phase oxygen AU - Uddin, Mohammed Jasim AU - Alam, Md Mohibul AU - Islam, Md Akhtarul AU - Snigda, Sharmin Rahman AU - Das, Sreejon AU - Rahman, Mohammed Mastabur AU - Uddin, Md Nizam AU - Morris, Cindy A. AU - Gonzalez, Richard D. AU - Diebold, Ulrike AU - Dickens, Tarik J. AU - Okoli, Okenwa I. T2 - International Nano Letters AB - Nanostructured TiO2 was synthesized by the sol-gel method. The titania was supported on nanoporous poly(styrene-co-divinylbenzene) (PS). The samples were characterized by several techniques (scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and ultraviolet-visible spectroscopy). Three types of TiO2 samples were prepared using various temperatures and were studied for the photocatalytic degradation of methylene blue. The photocatalytic efficiency of TiO2 was observed to increase by activating the TiO2 surface using nanoporous PS. The photocatalytic performance of the synthesized samples showed a higher performance using molecular O2, which was purged through the reactor. DA - 2013/03/21/ PY - 2013 DO - 10.1186/2228-5326-3-16 VL - 3 IS - 1 SP - 16 J2 - Int. Nano Lett. SN - 2228-5326 ER - TY - JOUR TI - Interaction of slow multicharged ions with solid surfaces AU - Arnau, A. AU - Aumayr, F. AU - Echenique, P.M. AU - Grether, M. AU - Heiland, W. AU - Limburg, J. AU - Morgenstern, R. AU - Roncin, P. AU - Schippers, S. AU - Schuch, R. AU - Stolterfoht, N. AU - Varga, P. AU - Zouros, T.J.M. AU - Winter, H.P. T2 - Surface Science Reports AB - The present report deals with the main aspects of the interaction of slow (impact velocity typically below 1 a.u.) multicharged ions (MCI) with atomically clean solid surfaces of metals, semiconductors and insulators. It is based to a large extent on the results obtained by the authors and their affiliates within the Human Capital and Mobility Network of the European Union on “Interaction of Slow Highly Charged Ions with Solid Surfaces”, which has been carried out during the last three years. After briefly reviewing the pertinent historical developments, the experimental and theoretical techniques applied nowadays in the field of MCI-surface interaction studies are explained in detail, discussing especially the transient formation and relaxation of “hollow atoms” formed in such collisions. Further on, the status of the field is exemplified by numerous results from recent studies on MCI-induced emission of slow and fast electrons (yields and energy distributions), projectile soft X-ray spectroscopy, charge-changing and energy loss of scattered and surface-channelled projectiles, MCI-induced sputtering and secondary ion emission, and coincidence measurements involving different signatures from the above processes. The presented theoretical and experimental work has greatly contributed to an improved understanding of the strongly inter-related electronic transitions taking place for MCI above, at and below a solid surface. DA - 1997/// PY - 1997 DO - 10.1016/S0167-5729(97)00002-2 VL - 27 IS - 4–6 SP - 113 EP - 239 J2 - Surf. Sci. Rep. SN - 0167-5729 ER - TY - JOUR TI - Interface-confined mixing and buried partial dislocations for Ag bilayer on Pt(111) AU - Aït-Mansour, Kamel AU - Brune, Harald AU - Passerone, Daniele AU - Schmid, Michael AU - Xiao, Wende AU - Ruffieux, Pascal AU - Buchsbaum, Andreas AU - Varga, Peter AU - Fasel, Roman AU - Gröning, Oliver T2 - Physical Review B AB - The trigonal strain-relief pattern formed by an Ag bilayer on Pt(111) is a prominent example for dislocation networks and their use as nanotemplates. However, its atomic structure has not been solved. Combining scanning tunneling microscopy, low-energy ion scattering, and x-ray photoelectron diffraction, we demonstrate that, unexpectedly, about 22% of the atoms exchange across the Ag/Pt interface, and that the partial dislocations defining the trigonal network are buried in the Pt interface layer. We present an embedded-atom-method simulation identifying the lowest energy structure compatible with all experimental findings. DA - 2012/08/03/ PY - 2012 DO - 10.1103/PhysRevB.86.085404 VL - 86 IS - 8 SP - 085404 J2 - Phys. Rev. B ER - TY - JOUR TI - Imaging physical phenomena with local probes: From electrons to photons AU - Bonnell, Dawn A. AU - Basov, D. N. AU - Bode, Matthias AU - Diebold, Ulrike AU - Kalinin, Sergei V. AU - Madhavan, Vidya AU - Novotny, Lukas AU - Salmeron, Miquel AU - Schwarz, Udo D. AU - Weiss, Paul S. T2 - Reviews of Modern Physics AB - The invention of scanning tunneling and atomic force probes revolutionized our understanding of surfaces by providing real-space information about the geometric and electronic structure of surfaces at atomic spatial resolution. However, the junction of a nanometer-sized probe tip and a surface contains much more information than is intrinsic to conventional tunneling and atomic force measurements. This review summarizes recent advances that push the limits of the probing function at nanometer-scale spatial resolution in the context of important scientific problems. Issues such as molecular interface contact, superconductivity, electron spin, plasmon field focusing, surface diffusion, bond vibration, and phase transformations are highlighted as examples in which local probes elucidate complex function. The major classes of local probes are considered, including those of electromagnetic properties, electron correlations, surface structure and chemistry, optical interactions, and electromechanical coupling. DA - 2012/09/26/ PY - 2012 DO - 10.1103/RevModPhys.84.1343 VL - 84 IS - 3 SP - 1343 EP - 1381 J2 - Rev. Mod. Phys. ST - Imaging physical phenomena with local probes ER - TY - CHAP TI - Tailoring the Interface Properties of Magnetite for Spintronics AU - Parkinson, Gareth S. AU - Diebold, Ulrike AU - Tang, Jinke AU - Malkinski, Leszek T2 - Advanced Magnetic Materials A2 - Malkinski, Leszek CY - Rijeka DA - 2012/05/24/ PY - 2012 SP - 61 EP - 88 PB - InTech SN - 978-953-51-0637-1 ER - TY - JOUR TI - Bulk and surface characterization of In2O3(001) single crystals AU - Hagleitner, Daniel R. AU - Menhart, Manfred AU - Jacobson, Peter AU - Blomberg, Sara AU - Schulte, Karina AU - Lundgren, Edvin AU - Kubicek, Markus AU - Fleig, Jürgen AU - Kubel, Frank AU - Puls, Christoph AU - Limbeck, Andreas AU - Hutter, Herbert AU - Boatner, Lynn A. AU - Schmid, Michael AU - Diebold, Ulrike T2 - Physical Review B AB - A comprehensive bulk and surface investigation of high-quality In2O3(001) single crystals is reported. The transparent-yellow, cube-shaped single crystals were grown using the flux method. Inductively coupled plasma mass spectrometry (ICP-MS) reveals small residues of Pb, Mg, and Pt in the crystals. Four-point-probe measurements show a resistivity of 2.0 ± 0.5 × 10^5 Ω cm, which translates into a carrier concentration of ≈10^12 cm−3. The results from x-ray diffraction (XRD) measurements revise the lattice constant to 10.1150(5) Å from the previously accepted value of 10.117 Å. Scanning tunneling microscopy (STM) images of a reduced (sputtered/annealed) and oxidized (exposure to atomic oxygen at 300 °C) surface show a step height of 5 Å, which indicates a preference for one type of surface termination. The surfaces stay flat without any evidence for macroscopic faceting under any of these preparation conditions. A combination of low-energy ion scattering (LEIS) and atomically resolved STM indicates an indium-terminated surface with small islands of 2.5 Å height under reducing conditions, with a surface structure corresponding to a strongly distorted indium lattice. Scanning tunneling spectroscopy (STS) reveals a pronounced surface state at the Fermi level (EF). Photoelectron spectroscopy (PES) shows additional, deep-lying band gap states, which can be removed by exposure of the surface to atomic oxygen. Oxidation also results in a shoulder at the O 1s core level at a higher binding energy, possibly indicative of a surface peroxide species. A downward band bending of 0.4 eV is observed for the reduced surface, while the band bending of the oxidized surface is of the order of 0.1 eV or less. DA - 2012/03/28/ PY - 2012 DO - 10.1103/PhysRevB.85.115441 VL - 85 IS - 11 SP - 115441 J2 - Phys. Rev. B ER - TY - JOUR TI - Segregation and reconstructions of PtxNi1-x(100) AU - Hebenstreit, W. AU - Ritz, G. AU - Schmid, M. AU - Biedermann, A. AU - Varga, P. T2 - Surface Science AB - It is known that on (100) surfaces of PtxNi1 - x single crystals Pt segregates. With increasing Pt concentration in the surface the transition from unreconstructed Ni(100) to the pseudo hexagonal Pt(100) reconstruction occurs via a shifted row reconstruction and several pseudo hexagonal (n × 1) superstructures (n = 7, 12 and 19) consisting of similar (7 × 1) and (5 × 1) subcells. This was revealed by atomically resolved scanning tunnelling microscopy (STM). From low energy ion scattering measurements it becomes clear that the formation of the pseudo hexagonal structure leads to strong amplification of Pt segregation. Chemically resolved STM on the atomic scale shows that Pt prefers the highly coordinated four-fold hollow sites in the pseudo hexagonal structures and Ni is pushed into nearly on-top or bridge sites. Therefore the strong tendency of Pt to increase its coordination is proposed as the driving force of the reconstructions. Corrugations and chemical ordering measured by STM within the pseudo hexagonal reconstructions are confirmed by simulations based on embedded atom method potentials. DA - 1997/10/23/ PY - 1997 DO - 10.1016/S0039-6028(97)00392-0 VL - 388 IS - 1-3 SP - 150 EP - 161 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - The Rh(100)-(3 × 1)-2O structure AU - Gustafson, J. AU - Lundgren, E. AU - Mikkelsen, A. AU - Borg, M. AU - Klikovits, J. AU - Schmid, M. AU - Varga, P. AU - Andersen, J. N. T2 - Journal of Physics: Condensed Matter AB - The O adsorption on Rh(100) has been studied using high resolution core level spectroscopy, low energy electron diffraction and scanning tunnelling microscopy. In addition to the well known (2 × 2), (2 × 2)-pg and c(8 × 2) structures at coverages of 0.25, 0.5 and 1.75 ML respectively, an intermediate (3 × 1) structure with a coverage of 2/3 ML is identified. DA - 2012/06/06/ PY - 2012 DO - 10.1088/0953-8984/24/22/225006 VL - 24 IS - 22 SP - 225006 J2 - J. Phys.: Condens. Matter SN - 0953-8984, 1361-648X ER - TY - JOUR TI - Interaction of oxygen with PtRh(100) studied with STM AU - Wouda, P. T. AU - Schmid, M. AU - Hebenstreit, W. AU - Varga, P. T2 - Surface Science AB - The adsorption of oxygen at 500°C on a Pt50Rh50(100) single crystal surface was studied using UHV-STM and Auger electron spectroscopy. Images were taken of the p(3 × 1) phase; of a mixed phase with p(2 × 2), c(2 × 2) and (3 × 3) units; and of rhodium oxide patches. Possible models for these structures involving surface reconstruction are presented. Exposure of the p(3 × 1)O/PtRh(100) to H2 at room temperature led to the conversion to the p(1 × 1) substrate structure. The ordering and composition of this substrate structure after reduction is discussed. DA - 1997/10/23/ PY - 1997 DO - 10.1016/S0039-6028(97)00375-0 VL - 388 IS - 1-3 SP - 63 EP - 70 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Surface alloying and superstructures of Pb on Cu(100) AU - Nagl, C. AU - Platzgummer, E. AU - Haller, O. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - The growth of submonolayer and monolayer Pb films on Cu(100) has been investigated by STM. In the submonolayer region a disordered surface alloy is found in spite of the immiscibility of bulk Cu and Pb. At a coverage of theta = 3/8 a c(4 × 4) superstructure is observed. The atomic arrangement of the c(4 × 4) superstructure unit cell could be revealed; it is formed by linear chains of Pb atoms in two rotational domains. Increasing the coverage to theta = 0.5, a c(2 × 2) structure can be observed. The transition to the c(5√2 × √2)R45° superstructure of the dense overlayer proceeds via the insertion of antiphase domain boundary. The model for the c(5√2 × √2)R45° superstructure presented in literature is confirmed. DA - 1995/07/01/ PY - 1995 DO - 10.1016/0039-6028(95)00387-8 VL - 331-333 IS - Part 1 SP - 831 EP - 837 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - The nitrogen-induced herringbone reconstruction of Cr(110) AU - Schmid, M. AU - Pinczolits, M. AU - Hebenstreit, W. AU - Varga, P. T2 - Surface Science AB - Segregation of nitrogen causes herringbone-like (3 × n) surface reconstructions, which were studied by atomically resolved scanning tunneling microscopy (STM). The main building blocks of the reconstruction are domains of (√6 × √6)R ± 35° cells, separated by domain boundaries running in the [110] direction. Since the density of Cr atoms in the surface is reduced by the reconstruction, the driving force of the reconstruction is believed to be compressive stress caused by the incorporation of N atoms into the surface. DA - 1997/11/06/ PY - 1997 DO - 10.1016/S0039-6028(97)00474-3 VL - 389 IS - 1-3 SP - L1140 EP - L1146 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Chemical ordering and reconstruction of Pt25Co75(100): an LEED/STM study AU - Gauthier, Y. AU - Dolle, P. AU - Baudoing-Savois, R. AU - Hebenstreit, W. AU - Platzgummer, E. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - The surface of a disordered Pt25Co75(100) alloy has been investigated using quantitative LEED, AES and UHV-STM at room temperature. Atomic-resolution images reveal that it reconstructs with close-packed rows shifted by half the interatomic distance, from hollow to bridge sites. The density of shifted rows increases with the surface Pt concentration, leading to (1 × 5), (1 × 6) and (1 × 7) patterns. Segregation and chemical ordering lead to the formation of c(2 × 2) domains between the shifted rows. Chemical resolution was achieved with STM: the apparent height of the Pt atoms in the STM topographs is about 0.1-0.4 Å above that of Co, whereas LEED shows that Pt atoms are geometrically ~0.04 Å higher. The composition was determined down to the fourth layer. An oscillatory segregation profile is observed, with Pt-rich layers ( = 62.6% Pt, = 53.5%) and Pt-depleted layers ( = 6.9%, = 2.7%). Chemical ordering is present in the third layer and the four-layer surface slab stabilises with a structure and a composition quite similar to that of the L1_2 PtCo3 phase. As regards the composition and ordering of the top layer, there is a remarkable agreement between chemically resolved STM analysis and LEED analysis. DA - 1998/01/20/ PY - 1998 DO - 10.1016/S0039-6028(97)00665-1 VL - 396 IS - 1-3 SP - 137 EP - 155 J2 - Surf. Sci. SN - 0039-6028 ST - Chemical ordering and reconstruction of Pt25Co75(100) ER - TY - JOUR TI - Anti-corrugation and nitrogen c(2 × 2) on Cr(100): STM on atomic scale and quantitative LEED AU - Sporn, M. AU - Platzgummer, E. AU - Pinczolits, M. AU - Hebenstreit, W. AU - Schmid, M. AU - Hofer, W. AU - Varga, P. T2 - Surface Science AB - We present a LEED I-V analysis of c(2 × 2)-N/Cr(100). We found nitrogen residing in fourfold hollow sites and exclude adsorption models in which nitrogne adsorbs on a metal site (on-top, substitutional or second-layer interstitial). We achieved a Pendry R-factor of 0.16 for the best-fit structure. Nitrogen resides at a vertical distance of 0.36 Å above the first chromium layer. The interlayer spacing between the first and the second chromium layer is expanded to 1.55 Å (7.5% with respect to the bulk value of 1.44 Å). The interlayer spacing between the second and the third layer is contracted to 1.41 Å. The second chromium layer is buckled (0.13 Å). The second-layer chromium atom beneath a nitrogen atom resides deeper in the bulk. The nitrogen bond length to the four first-layer chromium atoms amounts to 2.07 Å, the bond length to the second-layer chromium atom amounts to 1.97 Å. The nitrogen position in c(2 × 2)-N determined by LEED is used to identify hollow sites in scanning tunnelling microscopy images. We found that hollow sites in p(1 × 1)-Cr(100) are imaged as hillocks and chromium atoms as depressions. This is anti-corrugation of clean Cr(100). Anti-corrugation seems to be related to a surface state of clean Cr(100) and is lifted in p(1 × 1)-N/Cr(100) at a (local) nitrogen coverage of 1 monolayer. DA - 1998/01/20/ PY - 1998 DO - 10.1016/S0039-6028(97)00660-2 VL - 396 IS - 1-3 SP - 78 EP - 86 J2 - Surf. Sci. SN - 0039-6028 ST - Anti-corrugation and nitrogen c(2 × 2) on Cr(100) ER - TY - JOUR TI - Submonolayer growth of Pb on Cu(111): surface alloying and de-alloying AU - Nagl, C. AU - Haller, O. AU - Platzgummer, E. AU - Schmid, M. AU - Varga, P. T2 - Surface Science AB - In spite of the immiscibility of Pb in bulk Cu, atomically resolved scanning tunneling microscopy reveals surface alloy formation of Pb deposited on Cu(111), even at 300 K. Due to kinetic limitations at room temperature, the incorporation of Pb is restricted to advance from step edges, while after annealing to 470 K or higher, embedded Pb atoms are found to be randomly distributed over terraces. At low tunneling voltages, standing waves of surface-state electrons scattered by embedded Pb atoms could be observed. The maximum packing density of the surface alloy is about 40% (= 0.4 ML) of a close-packed Pb overlayer. Thus, deposition above 0.4 ML and subsequent annealing results in hexagonal close-packed Pb regions, whereas on the non-annealed surface hexagonal close-packed Pb islands are already found at 0.2 ML. Eventually, at 1 ML the surface alloy is entirely replaced by a Pb overlayer. DA - 1994/12/20/ PY - 1994 DO - 10.1016/0039-6028(94)90189-9 VL - 321 IS - 3 SP - 237 EP - 248 J2 - Surf. Sci. SN - 0039-6028 ST - Submonolayer growth of Pb on Cu(111) ER - TY - JOUR TI - Growth of ultrathin cobalt oxide films on Pt(111) AU - De Santis, Maurizio AU - Buchsbaum, Andreas AU - Varga, Peter AU - Schmid, Michael T2 - Physical Review B AB - Cobalt surface oxides were grown on Pt(111) by depositing Co and dosing with molecular oxygen at temperatures ranging between 300 and 740 K. Oxidation of 1 monolayer (ML) Co results in a two-dimensional (2D) moiré structure, observed using both low-energy electron diffraction and scanning tunneling microscopy and interpreted as a polar (oxygen terminated) CoO(111) atomic bilayer. It is expanded by 2.7 ± 0.6% in the surface plane with respect to bulk CoO. An almost-flawless moiré pattern is obtained after a final step of annealing at 740 K in oxygen. Insufficient oxidation leads to defects in the moiré pattern, consisting of triangular dislocation loops of different sizes; the smaller ones occupy half of the moiré cell. Low-temperature annealing (450 K) can be used to create a zigzag phase, which is mainly observed in 1-ML-thick areas after several cycles of Co deposition (1 ML each) and oxidation at 10−7 mbar. The CoO films obtained by deposition/oxidation cycles exhibit Stranski-Krastanov growth; the structure of the 2D layer between the islands depends on the thermal treatment. It exhibits the moiré pattern after annealing at 740 K, whereas the zigzag phase was observed after low-temperature annealing. The second monolayer consists of a moiré pattern different from that of the first layer, presumably a wurtzite-like structure. Above the third layer, we observe only small three-dimensional islands, which exhibit a bandgap. We have also studied oxidation of surface alloys obtained by depositing Co and annealing. On these surfaces, we found a quasi-(3 × 3) reconstruction. Structure models are presented for all phases observed, and we argue that some of the moirélike structures might be useful as templates for metal cluster growth. DA - 2011/// PY - 2011 DO - 10.1103/PhysRevB.84.125430 VL - 84 IS - 12 SP - 125430 J2 - Phys. Rev. B ER - TY - JOUR TI - Complex magnetic phase in submonolayer Fe stripes on Pt(997) AU - Honolka, J. AU - Lee, T. Y. AU - Kuhnke, K. AU - Repetto, D. AU - Sessi, V. AU - Wahl, P. AU - Buchsbaum, A. AU - Varga, P. AU - Gardonio, S. AU - Carbone, C. AU - Krishnakumar, S. R. AU - Gambardella, P. AU - Komelj, M. AU - Singer, R. AU - Fähnle, M. AU - Fauth, K. AU - Schütz, G. AU - Enders, A. AU - Kern, K. T2 - Physical Review B AB - Correlations between magnetism and morphology of iron nanostructures of monatomic height on Pt(997) substrates are studied using x-ray magnetic circular dichroism as well as scanning tunneling microscopy and helium scattering. A drastic collapse of the average magnetization by more than a factor of 4 is observed when increasing the iron coverage from 0.1 to 0.2 ML. This effect goes along with a softening of the magnetic anisotropy energy and a gradual reorientation of the magnetic easy axis from in plane to out of plane. The experimental findings together with electronic density-functional calculations suggest the formation of a complex magnetic phase in corrugated rim regions of Fe islands, leading to both ferromagnetic and antiferromagnetic exchange couplings of Fe moments depending on their various local bonding configurations. DA - 2009/03/27/ PY - 2009 DO - 10.1103/PhysRevB.79.104430 VL - 79 IS - 10 SP - 104430 J2 - Phys. Rev. B ER - TY - JOUR TI - In-situ magnetic nano-patterning of Fe films grown on Cu(100) AU - Shah Zaman, Sameena AU - Dvořák, Petr AU - Ritter, Robert AU - Buchsbaum, Andreas AU - Stickler, Daniel AU - Oepen, Hans Peter AU - Schmid, Michael AU - Varga, Peter T2 - Journal of Applied Physics AB - Metastable paramagnetic face-centered cubic (fcc) Fe films grown on a Cu(100) single crystal at room temperature can be transformed to the ferromagnetic body-centered cubic (bcc) structure by ion irradiation. We have employed this technique to write small ferromagnetic patches by Ar+ irradiation through a gold coated SiN mask with regularly arranged 80-nm diameter holes, which was placed on top of the as-prepared fcc Fe films. Nanopatterning was performed on both 8-monolayer (ML) Fe films grown in ultrahigh vacuum as well as 22-ML films stabilized by dosing carbon monoxide during growth. The structural transformation of these nano-patterned films was investigated using scanning tunneling microscopy. In both 8 and 22-ML fcc Fe films, the bcc needles are found to protrude laterally out of the irradiated part of the sample, limiting the resolution of the technique to a few 10 nm. The magnetic transformation was confirmed by magnetic force microscopy. DA - 2011/07/27/ PY - 2011 DO - 10.1063/1.3609078 VL - 110 IS - 2 SP - 024309 J2 - J. Appl. Phys. SN - 10897550 ER - TY - JOUR TI - Oxide surfaces: Surface science goes inorganic AU - Diebold, Ulrike T2 - Nature Materials DA - 2010/03// PY - 2010 DO - 10.1038/nmat2708 VL - 9 IS - 3 SP - 185 EP - 187 J2 - Nat. Mater. SN - 1476-1122 ST - Oxide surfaces ER - TY - JOUR TI - Anisotropic two-dimensional electron gas at SrTiO3(110) AU - Wang, Zhiming AU - Zhong, Zhicheng AU - Hao, Xianfeng AU - Gerhold, Stefan AU - Stöger, Bernhard AU - Schmid, Michael AU - Sánchez-Barriga, Jaime AU - Varykhalov, Andrei AU - Franchini, Cesare AU - Held, Karsten AU - Diebold, Ulrike T2 - Proceedings of the National Academy of Sciences AB - Two-dimensional electron gases (2DEGs) at oxide heterostructures are attracting considerable attention, as these might one day substitute conventional semiconductors at least for some functionalities. Here we present a minimal setup for such a 2DEG––the SrTiO3(110)-(4 × 1) surface, natively terminated with one monolayer of tetrahedrally coordinated titania. Oxygen vacancies induced by synchrotron radiation migrate underneath this overlayer; this leads to a confining potential and electron doping such that a 2DEG develops. Our angle-resolved photoemission spectroscopy and theoretical results show that confinement along (110) is strikingly different from the (001) crystal orientation. In particular, the quantized subbands show a surprising “semiheavy” band, in contrast with the analog in the bulk, and a high electronic anisotropy. This anisotropy and even the effective mass of the (110) 2DEG is tunable by doping, offering a high flexibility to engineer the properties of this system. DA - 2014/03/03/ PY - 2014 DO - 10.1073/pnas.1318304111 VL - 111 IS - 11 SP - 3933 EP - 3937 J2 - Proc. Natl. Acad. Sci. USA SN - 0027-8424, 1091-6490 ER - TY - JOUR TI - Real-space imaging of the Verwey transition at the (100) surface of magnetite AU - de la Figuera, Juan AU - Novotny, Zbynek AU - Setvin, Martin AU - Liu, Tijiang AU - Mao, Zhiqiang AU - Chen, Gong AU - N’Diaye, Alpha T. AU - Schmid, Michael AU - Diebold, Ulrike AU - Schmid, Andreas K. AU - Parkinson, Gareth S. T2 - Physical Review B AB - Effects of the Verwey transition on the (100) surface of magnetite were studied using scanning tunneling microscopy and spin polarized low-energy electron microscopy. On cooling through the transition temperature TV, the initially flat surface undergoes a rooflike distortion with a periodicity of ∼0.5 μm due to ferroelastic twinning within monoclinic domains of the low-temperature monoclinic structure. The monoclinic c axis orients in the surface plane, along the [001]c directions. At the atomic scale, the charge-ordered (√2×√2)R45∘ reconstruction of the (100) surface is unperturbed by the bulk transition, and is continuous over the twin boundaries. Time resolved low-energy electron microscopy movies reveal the structural transition to be first order at the surface, indicating that the bulk transition is not an extension of the Verwey-like (√2×√2)R45∘ reconstruction. Although conceptually similar, the charge-ordered phases of the (100) surface and sub-TV bulk of magnetite are unrelated phenomena. DA - 2013/10/29/ PY - 2013 DO - 10.1103/PhysRevB.88.161410 VL - 88 IS - 16 SP - 161410(R) J2 - Phys. Rev. B ER - TY - CHAP TI - The structure of TiO2 surfaces AU - Diebold, Ulrike T2 - Oxide Surfaces A2 - Woodruff, D.P. T3 - The Chemical Physics of Solid Surfaces AB - Unraveling the relationship between the atomic surface structure and other physical and chemical properties is probably one of the most important achievements of surface science. Because of the mixed ionic and covalent bonding in metal-oxide systems, the surface structure has an even stronger influence on local surface chemistry as compared to metals or elemental semiconductors. This chapter describes the bulk structure of TiO2 crystals and their stable crystal planes. Because bulk nonstoichiometries influence the surface properties of TiO2 in a variety of ways, the chapter discusses the bulk defects. The bulk-truncated 1 × 1 surface is known with a very high accuracy from experimental and theoretical studies. Surface defects are categorized as step edges, oxygen vacancies, line defects (closely related to the 1 × 2 reconstruction), common impurities, and the manifestation of crystallographic shear planes on surfaces. The long-standing argument of the structure of the 1 × 2 phase is discussed in the chapter. Scanning tunneling microscopy (STM) and, more recently, atomic force microscopy (AFM) studies have revealed the complexity of the seemingly simple RuO2 (110) surface; hence, the chapter presents the way to best prepare this surface. The two other low-index planes RuO2 (100) and (001) are also described in the chapter. DA - 2001/// PY - 2001 VL - 9 SP - 443 EP - 484 PB - Elsevier SN - 978-0-444-50745-7 ER - TY - JOUR TI - Stoichiometry-driven switching between surface reconstructions on SrTiO3(001) AU - Gerhold, Stefan AU - Wang, Zhiming AU - Schmid, Michael AU - Diebold, Ulrike T2 - Surface Science AB - Controlling the surface structure on the atomic scale is a major difficulty for most transition metal oxides; this is especially true for the ternary perovskites. The influence of surface stoichiometry on the atomic structure of the SrTiO3(001) surface was examined with scanning tunneling microscopy, low-energy electron diffraction, low-energy He+ ion scattering (LEIS), and X-ray photoelectron spectroscopy (XPS). Vapor deposition of 0.8 monolayer (ML) strontium and 0.3 ML titanium, with subsequent annealing to 850 °C in 4 × 10^-6 mbar O2, reversibly switches the surface between c(4 × 2) and (2 × 2) reconstructions, respectively. The combination of LEIS and XPS shows a different stoichiometry that is confined to the top layer. Geometric models for these reconstructions need to take into account these different surface compositions. DA - 2014/03// PY - 2014 DO - 10.1016/j.susc.2013.10.015 VL - 621 SP - L1 EP - L4 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Surface composition of PtxNi1-x single crystal alloys AU - Weigand, P. AU - Novacek, P. AU - van Husen, G. AU - Neidhart, T. AU - Mezey, L. Z. AU - Hofer, W. AU - Varga, P. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - We present results of the surface composition of the topmost layer of three low index planes of PtxNi1-x single crystal alloys: Pt10Ni90(100), Pt25Ni75(111) and Pt50N50(110). The alloys were investigated by ion scattering spectroscopy (ISS). We observed Pt enrichment of the surface layers caused by preferential sputtering. After exposing the surfaces to oxygen, adsorbate induced segregation of Ni was measured. Annealing the sputtered alloys results in a site specific change in surface composition. We found Pt enrichment for the (111) surface and Ni enrichment for the (110) surface. The surface of Pt10Ni90(100) showed a Pt concentration close to the bulk composition. Work functions were determined by ultraviolet photoelectron spectroscopy (UPS). The UPS results support the ISS results of the surface composition. The results of thermodynamic calculations are discussed with respect to the experimental results on surface segregation. DA - 1992/02/02/ PY - 1992 DO - 10.1016/0168-583X(92)95444-V VL - 64 IS - 1-4 SP - 93 EP - 97 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - CHAP TI - Structure and kinetics of electron beam damage in a chemisorbed monolayer: PF3 on Ru(0001) AU - Madey, T. E. AU - Tao, H. S. AU - Nair, L. AU - Diebold, U. AU - Shivaprasad, S. M. AU - Johnson, A. L. AU - Poradzsiz, A. AU - Shinn, N. D. AU - Yarmoff, J. A. AU - Chakarian, V. AU - Shuh, D. T2 - Desorption Induced by Electronic Transitions DIET V A2 - Burns, A. R. A2 - Stechel, E. B. A2 - Jennison, D. R. T3 - Springer Series in Surface Sciences CY - Berlin DA - 1993/// PY - 1993 VL - 31 SP - 182 EP - 188 PB - Springer SN - 978-3-540-56473-7 ER - TY - JOUR TI - Ultrathin metal films on TiO2(110): metal overlayer spreading and surface reactivity AU - Diebold, U. AU - Pan, J.-M. AU - Madey, T. E. T2 - Surface Science AB - We have studied the growth of ultrathin (< 50 Å) Cu overlayers on stoichiometric TiO2(110) surfaces. Using low energy ion scattering, the growth mode of Cu was determined to be Volmer-Weber type (three-dimensional islands) at room temperature; evidence for cluster growth is observed even at 160 K. A comparison with Fe and Cr metal films shows that the spreading of the films correlates with the heat of formation of the metal oxide of the overlayer. Fe and Cr cause a reduction of the Ti4+ surface cations at the metal/metal oxide interface as observed with X-ray photoelectron spectroscopy. No reduction of the TiO2 cations takes place upon contact with a Cu overlayer. DA - 1993/05/10/ PY - 1993 DO - 10.1016/0039-6028(93)91095-7 VL - 287-288 IS - Part 2 SP - 896 EP - 900 J2 - Surf. Sci. SN - 0039-6028 ST - Ultrathin metal films on TiO2(110) ER - TY - JOUR TI - Surface chemistry of PH3, PF3 and PCl3 on Ru(0001) AU - Tao, H.-S. AU - Diebold, U. AU - Shinn, N. D. AU - Madey, T. E. T2 - Surface Science AB - The adsorption, desorption and decomposition of PH3, PF3 and PCl3 on Ru(0001) have been studied by soft X-ray photoelectron spectroscopy (SXPS) using synchrotron radiation. Due to large chemical shifts in the P 2p core levels, different phosphorus containing surface species can be identified. We find that PF3 adsorbs molecularly on Ru(0001) at 80 and 300 K. At 80 K, PH3 saturates the surface with one layer of atomic hydrogen, elemental phosphorus, subhydride (i.e., PHx (0 < x < 3)) and PH3, with a total phosphorus coverage of 0.4 ML. At 300 K, PH3 decomposes into atomic hydrogen and elemental phosphorus with a phosphorus coverage of 0.8 ML. At 80 K, PCl3 adsorbs dissociatively into atomic chlorine, elemental phosphorus, PCl and possibly PCl2 and PCl3 in the first monolayer. Formation of multilayers of PCl3 is observed at 80 K. At 300 K, PCl3 adsorbs dissociatively as atomic chlorine and elemental phosphorus with a saturation phosphorus coverage of 0.1 ML. The variation in total phosphorus uptake at 300 K from PX3 (X = H, FandCl) adsorption is a result of competition between site blocking by dissociation fragments and displacement reactions. Annealing surfaces with adsorbed phosphorus to 1000 K results in formation of RuzP ( z = 1 or 2), which is manifested by the chemical shifts in the P2p core level, as well as the P LVV Auger transition. The recombination of adsorbed phosphorus and adsorbed X (x = H, F and Cl) from decomposition is also observed, but is a minor reaction channel on the surface. Thermochemical data are used to analyze the different stabilities of PX3 at 300 K, namely, PF3 adsorbs molecularly and PH3 and PCl3 dissociate completely. First, we compare the heat of molecular adsorption and the heat of dissociative adsorption of PX3 on Ru(0001), using an enthalpy approach, and find results consistent with experimental observations. Second, we compare the total bond energy difference between molecular adsorption and complete dissociation of PX3 on Ru(0001). In particular, we apply Shustorovich's bond-order conservation-Morse potential (BOC-MP) method to estimate the heat of adsorption for PH3 and PCl3 and the bond energies of the relaxed P-X bonds of the adsorbed PX3 on the surface. The bond strength difference among the relaxed P-X bonds (i.e., the relaxed P-F bond () is much stronger than either the relaxed P-H bond () or the relaxed P-Cl bond ()) suggests that PF3 is more stable than PH3 and PCl3 on Ru(0001) at 300 K. These values are used to evaluate the total bond energy differences between molecular adsorption and complete dissociation for each of the PX3, and the results agree with the experimental trends. DA - 1994/06/10/ PY - 1994 DO - 10.1016/0039-6028(94)90725-0 VL - 312 IS - 3 SP - 323 EP - 344 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Enhancement of STM images and estimation of atomic positions based on maximum entropy deconvolution AU - Böhmig, S. D. AU - Schmid, M. AU - Störi, H. T2 - Surface Science AB - A new method for restoration and sharpening of scanning tunneling microscopy (STM) data is presented. According to the STM theory of Tersoff and Hamann it is assumed that the response of the STM can be approximated by the convolution of a localized atomic density of states of the sample and a Gaussian, which limits the resolution. Therefore, one must find the solution of an inverse problem, which is done by minimizing the mean square deviation between the measured and the reconstructed image using entropy as a regularization functional. This nonlinear method is shown to be superior to linear filters such as the Wiener filter in that the solution carries as minimal information as is necessary to fit the data and is not constrained to low frequencies. On metals, where atomically resolved STM images show mainly geometrical information, the centers of mass of the resulting peaks are taken as the atomic positions which are compared to those estimated visually from the STM images. The method has been applied to both the periodic Cu(111) surface and to the nonperiodic shifted row reconstruction of Pt10Ni90(100). DA - 1994/06/20/ PY - 1994 DO - 10.1016/0039-6028(94)91152-5 VL - 313 IS - 1-2 SP - 6 EP - 16 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - The shifted-row reconstruction of PtxNi1-x(100) AU - Schmid, M. AU - Biedermann, A. AU - Böhmig, S. D. AU - Weigand, P. AU - Varga, P. T2 - Surface Science AB - Scanning tunneling microscopy on Pt10Ni90(100) and Pt25Ni75(100) single crystals reveals close-packed rows of atoms, which are shifted by <110> along the direction of the rows into a bridge position and slightly outward of the surface. Maximum entropy deconvolution of atomically resolved STM data shows that all atoms between the shifted rows are close to the unreconstructed positions. The density of the shifted rows increases with increasing Pt surface concentration up to a maximum value of each 5th row shifted. The reconstruction shows little dependence on the carbon contamination of the surface, but it is lifted by a full c(2 × 2) coverage of carbon monoxide, which can be imaged simultaneously with the substrate, indicating an on-top position of CO. The driving force of the shifted-row reconstruction is discussed. DA - 1994/10/20/ PY - 1994 DO - 10.1016/0039-6028(94)90103-1 VL - 318 IS - 3 SP - 289 EP - 298 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - An STM study of growth and alloying of Cr on Ru(0001) and CO adsorption on the alloy AU - Engelhardt, M. P. AU - Schmid, M. AU - Biedermann, A. AU - Denecke, R. AU - Steinrück, H.-P. AU - Varga, P. T2 - Surface Science AB - We have investigated the growth of Cr on Ru(0001) using scanning tunneling microscopy (STM). In the submonolayer regime (theta_Cr = 0.25 ML) monatomically high islands are formed at 300 K, accompanied by step decoration. The island density decreases upon heating to 700 K. Small amounts of Cr are incorporated in the Ru substrate at this temperature. For higher Cr coverages (2 ML), three-dimensional growth is observed at 300 K. Annealing this Cr layer to 500 and 700 K leads to the formation of Cr islands (4-5 layers high) with an elongated shape and bcc(110) structure with a pseudomorphic Cr monolayer covering the remaining substrate. As shown by atomically resolved images, the Cr bcc(110) islands have Kurdjumov-Sachs orientation with respect to the Ru(0001) substrate. After annealing at 1000 K, the formation of a hexagonal, chemically disordered CrRu alloy is observed, in agreement with a previous study [Engelhardt et al., Surf. Sci. 512 (2002) 107]. The room-temperature STM investigation of CO adsorption on the CrRu alloy with approx. 34% Cr in the first layer shows that adsorbed molecular CO resides only on Ru atoms, not on Cr, mostly in an on-top geometry. The CO occupation of Ru sites increases slightly with the number of neighbouring Cr atoms, indicating a weak ligand effect. We also find indications that the few threefold hollow sites surrounded by three Cr atoms become occupied by atomic C or O, due to dissociation of a small fraction of the adsorbed CO molecules; thus, we cannot unambiguously determine whether molecular CO can bind to these threefold sites at room temperature. DA - 2005/03/10/ PY - 2005 DO - 10.1016/j.susc.2005.01.022 VL - 578 IS - 1-3 SP - 124 EP - 135 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - The surface oxide as a source of oxygen on Rh(111) AU - Lundgren, E. AU - Gustafson, J. AU - Resta, A. AU - Weissenrieder, J. AU - Mikkelsen, A. AU - Andersen, J. N. AU - Köhler, L. AU - Kresse, G. AU - Klikovits, J. AU - Biederman, A. AU - Schmid, M. AU - Varga, P. T2 - Journal of Electron Spectroscopy and Related Phenomena AB - The reduction of a thin surface oxide on the Rh(111) surface by CO is studied in situ by photoemission spectroscopy, scanning tunneling microscopy, and density functional theory. CO molecules are found not to adsorb on the surface oxide at a sample temperature of 100[thin space]K, in contrast to on the clean and chemisorbed oxygen covered surface. Despite this behavior, the surface oxide may still be reduced by CO, albeit in a significantly different fashion as compared to the reduction of a phase containing only chemisorbed on surface oxygen. The experimental observations combined with theoretical considerations concerning the stability of the surface oxide, result in a model of the reduction process at these pressures suggesting that the surface oxide behaves as a source of oxygen for the CO-oxidation reaction. DA - 2005/06// PY - 2005 DO - 10.1016/j.elspec.2005.01.004 VL - 144-147 SP - 367 EP - 372 J2 - J Electron Spectrosc. Relat. Phenom. SN - 0368-2048 ER - TY - JOUR TI - Mixed dissociated/molecular monolayer of water on the TiO2(011)-(2 × 1) surface AU - Beck, T. J. AU - Klust, Andreas AU - Batzill, Matthias AU - Diebold, Ulrike AU - Di Valentin, Cristiana AU - Tilocca, Antonio AU - Selloni, Annabella T2 - Surface Science AB - A combined theoretical and experimental approach is used to study water on the TiO2(0 1 1)-(2 × 1) surface. Based on simple proximity arguments dissociative adsorption is expected. Density functional theory and photoemission spectroscopy show that, at low temperatures, a mixed molecular/dissociated water monolayer is stabilized by a H-bonding network. Scanning tunneling microscopy and molecular dynamics simulations provide evidence of a dissociated layer with a preferential non-uniform arrangement of the adsorbates at room temperature. DA - 2005/10/20/ PY - 2005 DO - 10.1016/j.susc.2005.06.021 VL - 591 IS - 1-3 SP - L267 EP - L272 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Stressing Pd atoms: Initial oxidation of the Pd(110) surface AU - Westerström, R. AU - Weststrate, C. J. AU - Resta, A. AU - Mikkelsen, A. AU - Schnadt, J. AU - Andersen, J. N. AU - Lundgren, E. AU - Schmid, M. AU - Seriani, N. AU - Harl, J. AU - Mittendorfer, F. AU - Kresse, G. T2 - Surface Science AB - We have investigated the oxygen induced structures of the Pd(110) surface in the pressure range of 10-5-10-3 mbar of oxygen, at a sample temperature of around 300 °C. These structures, denoted as "(7x√3)" and "(9x√3)", are studied in detail by the use of a combination of low-energy electron diffraction, scanning tunneling microscopy, high-resolution core level spectroscopy, and ab-initio simulations. Based on our data a model is proposed for these structures containing segments of Pd atoms in the [1 -1 0] direction, in which the Pd rows are decorated by O atoms in a zig-zag pattern. The segments are periodically separated by displaced Pd atoms. Density functional theory calculations show that the displacements reduce the oxygen induced stress significantly, as compared to a structure with no displacements. The calculations also suggest that the new structures are stabilized by domain formation. DA - 2008/// PY - 2008 DO - 10.1016/j.susc.2008.05.033 VL - 602 IS - 14 SP - 2440 EP - 2447 J2 - Surf. Sci. SN - 0039-6028 ST - Stressing Pd atoms ER - TY - JOUR TI - Ultra-thin Fe films grown on Cu by pulsed laser deposition: Intermixing and bcc-like structures AU - Rauchbauer, G. AU - Buchsbaum, A. AU - Schiechl, H. AU - Varga, P. AU - Schmid, M. AU - Biedermann, A. T2 - Surface Science AB - Pulsed laser deposition (PLD) with nanosecond pulses has been used for growing ultrathin Fe films on Cu(100) and Cu(111) single crystal surfaces. We have studied the morphology as well as the crystallographic structure of these films by scanning tunneling microscopy, and we compare the films with thermally deposited (TD) films. For Fe/Cu(100), bcc-like (nanomartensitic) structures are found in roughly the same thickness range for PLD and TD films but occupy a lower fraction of the films when deposited by PLD. The situation is different for thin Fe/Cu(111) films, where PLD films exhibit a higher bcc-like fraction, especially in islands of two monolayers thickness. Similar to TD films, we also observe surface reconstructions with bcc-like bond angles for the otherwise fee Fe/Cu(100) films in the thickness range above 5 ML. For both Fe/Cu(111) and Fe/Cu(100), we find a stronger intermixing between substrate and film compared to films grown by thermal deposition. Even in the seventh monolayer of Fe/Cu(100), approximately 10% Cu have been measured. We argue that the compositional heterogeneity is the reason for the absence of long-range order in the bcc-like phases, hiding them from diffraction techniques. We also discuss the results in the context of the magnetic properties of these films described in the literature. (c) 2008 Elsevier B.V. All rights reserved. DA - 2008/04/15/ PY - 2008 DO - 10.1016/j.susc.2008.02.024 VL - 602 IS - 8 SP - 1589 EP - 1598 J2 - Surf. Sci. SN - 0039-6028 ST - Ultra-thin Fe films grown on Cu by pulsed laser deposition ER - TY - JOUR TI - An in vitro controlled release study of valproic acid encapsulated in a titania ceramic matrix AU - Uddin, M. J. AU - Mondal, D. AU - Morris, C. A. AU - Lopez, T. AU - Diebold, U. AU - Gonzalez, R. D. T2 - Applied Surface Science AB - Despite the therapeutic efficacy of valproic acid towards numerous diseases, its poor bioavailability and systemic side effects pose significant barriers to long term treatment. In order to take advantage of controlled release implants of valproic acid, the drug was encapsulated into titania ceramic matrices via a sol-gel process. The integrity and structure of valproic acid-containing matrices were characterized through the use of FESEM, TEM, and BET analyses. In vitro controlled release studies and kinetic analyses were performed under ambient conditions (25 °C, atmospheric pressure) and controlled release behaviors were studied using a GC-MS method. Results showed first order dependence in the rate of valproic acid release as a function of drug concentrations in the titania ceramic device. A marked dependence on the surface area and pore size distribution with drug loading was also observed. This research opens new possibilities for the design of novel time-delayed controlled release systems for valproic acid encapsulates. DA - 2011/07/01/ PY - 2011 DO - 16/j.apsusc.2011.03.079 VL - 257 IS - 18 SP - 7920 EP - 7927 J2 - Appl. Surf. Sci. SN - 0169-4332 ER - TY - JOUR TI - Growth and structure of an ultrathin tin oxide film on Rh(111) AU - Yuhara, J. AU - Tajima, D. AU - Matsui, T. AU - Tatsumi, K. AU - Muto, S. AU - Schmid, M. AU - Varga, P. T2 - Journal of Applied Physics AB - The oxidation of submonolayer tin films on a Rh(111) surface by O2 gas was studied using low energy electron diffraction, Auger electron spectroscopy, x-ray photoemission spectroscopy(XPS), and scanning tunneling microscopy. A uniform tin oxide monolayer film formed at oxidation temperatures around 500 °C and a partial pressure of 2 × 10 ^-7 mbar O2 . The tin oxide film had (2 × 2) periodicity on the Rh(111) surface, and the resulting tin coverage was determined to be 0.5 ML. Using XPS, the compositional ratio O/Sn was determined to be 3/2. XPS spectra showed a single component for the Sn and O peaks, indicating a uniform bonding environment. Finally, ab initio density-functional theory total energy calculations and molecular dynamics simulations were performed using the projector augmented wave method to determine the detailed structure of the tin oxide thin film. DA - 2011/01/20/ PY - 2011 DO - 10.1063/1.3537871 VL - 109 IS - 2 SP - 024903 J2 - J. Appl. Phys. SN - 0021-8979, 1089-7550 ER - TY - JOUR TI - Segregation phenomena on Pt_x Ni_1−x low-index single crystal surfaces studied by STM AU - Varga, P. AU - Schmid, M. AU - Hofer, W. T2 - Surface Review and Letters AB - Surface segregation changes the composition of alloy surfaces. It influences both the geometrical and the chemical structure of the surface. In this paper segregation phenomena are shown for low index single crystal surfaces of different PtNi alloys which can be seen only by scanning tunneling microscopy (STM). STM experiments performed with atomic resolution revealed the existence of subsurface dislocation networks. A closer study of the conditions of their existence allowed us to understand the effects of preferential sputtering and annealing on the segregation behavior (i.e. building up a rather stable altered layer and its disappearance only at elevated temperatures). In addition, local chemical ordering in small domains and shifted row reconstructions with a large and varying periodicity (i.e. phenomena that are hardly seen by other methods like e.g. LEED) have been observed. DA - 1996/10// PY - 1996 DO - 10.1142/S0218625X9600276X VL - 03 IS - 05-06 SP - 1831 EP - 1845 J2 - Surf. Rev. Lett. SN - 0218-625X, 1793-6667 ER - TY - JOUR TI - Atomic-scale structure and catalytic reactivity of the RuO2(110) surface AU - Over, H. AU - Kim, Y. D. AU - Seitsonen, A. P. AU - Wendt, S. AU - Lundgren, E. AU - Schmid, M. AU - Varga, P. AU - Morgante, A. AU - Ertl, G. T2 - Science AB - Exposure of a Ru(0001) surface to large doses of O2 at elevated temperatures leads to the growth of an epitaxial layer of RuO2 with (110) surface orientation whose structure was analyzed quantitatively by combination of low energy electron diffraction, scanning tunneling microscopy and density functional calculations. The surface exposes essentially bridging O atoms and Ru atoms not capped by oxygen. The latter play the role of coordinatively unsaturated sites (cus) - a hypothesis introduced long ago to account for the catalytic activity of oxide surfaces - onto which CO may become chemisorbed and from where it may react with neighboring lattice-O to CO2. The distortion of the surface lattice thereby caused is restored by uptake of oxygen from the gas phase, i.e., the oxide surface itself is actively participating in the catalytic reaction. In this way a general mechanism originally proposed by Mars and van Krevelen could be verified. DA - 2000/02/25/ PY - 2000 DO - 10.1126/science.287.5457.1474 VL - 287 IS - 5457 SP - 1474 EP - 1476 J2 - Science ER - TY - JOUR TI - Carbon monoxide-induced adatom sintering in a Pd–Fe3O4 model catalyst AU - Parkinson, Gareth S. AU - Novotny, Zbynek AU - Argentero, Giacomo AU - Schmid, Michael AU - Pavelec, Jiří AU - Kosak, Rukan AU - Blaha, Peter AU - Diebold, Ulrike T2 - Nature Materials AB - The coarsening of catalytically active metal clusters is often accelerated by the presence of gases, but the role played by gas molecules is difficult to ascertain and varies from system to system. We use scanning tunnelling microscopy to follow the CO-induced coalescence of Pd adatoms supported on the Fe3O4(001) surface at room temperature, and find Pd-carbonyl species to be responsible for mobility in this system. Once these reach a critical density, clusters nucleate; subsequent coarsening occurs through cluster diffusion and coalescence. Whereas CO induces the mobility in the Pd/Fe3O4 system, surface hydroxyls have the opposite effect. Pd atoms transported to surface OH groups are no longer susceptible to carbonyl formation and remain isolated. Following the evolution from well-dispersed metal adatoms into clusters, atom-by-atom, allows identification of the key processes that underlie gas-induced mass transport. DA - 2013/08// PY - 2013 DO - 10.1038/nmat3667 VL - 12 IS - 8 SP - 724 EP - 728 J2 - Nat. Mater. SN - 1476-1122 ER - TY - JOUR TI - Pt3Zr(0001): A substrate for growing well-ordered ultrathin zirconia films by oxidation AU - Antlanger, Moritz AU - Mayr-Schmölzer, Wernfried AU - Pavelec, Jiří AU - Mittendorfer, Florian AU - Redinger, Josef AU - Varga, Peter AU - Diebold, Ulrike AU - Schmid, Michael T2 - Physical Review B AB - We have studied the surface of pure and oxidized Pt3Zr(0001) by scanning tunneling microscopy (STM), Auger electron microscopy, and density functional theory (DFT). The well-annealed alloy surface shows perfect long-range chemical order. Occasional domain boundaries are probably caused by nonstoichiometry. Pt3Zr exhibits ABAC stacking along [0001]; only the A-terminated surfaces are seen by STM, in agreement with DFT results showing a lower surface energy for the A termination. DFT further predicts a stronger inward relaxation of the surface Zr than for Pt, in spite of the larger atomic size of Zr. A closed ZrO2 film is obtained by oxidation in 10^-7 mbar O2 at 400 °C and post-annealing at ≈800 ∘C. The oxide consists of an O-Zr-O trilayer, equivalent to a (111) trilayer of the fluorite structure of cubic ZrO2, but contracted laterally. The oxide forms a (√19×√19)R23∘ superstructure. The first monolayer of the substrate consists of Pt and contracts, similar to the metastable reconstruction of pure Pt(111). DFT calculations show that the oxide trilayer binds rather weakly to the substrate. In spite of the O-terminated oxide, bonding to the substrate mainly occurs via the Zr atoms in the oxide, which strongly buckle down toward the Pt substrate atoms if near a Pt position. According to DFT, the oxide has a band gap; STM indicates that the conduction band minimum lies ≈2.3 eV above E_F. DA - 2012/07/30/ PY - 2012 DO - 10.1103/PhysRevB.86.035451 VL - 86 IS - 3 SP - 035451 J2 - Phys. Rev. B ST - ZrO2/Pt3Zr(0001) ER - TY - JOUR TI - Potential-driven molecular tiling of a charged polycyclic aromatic compound AU - Cui, Kang AU - Ivasenko, Oleksandr AU - Mali, Kunal S. AU - Wu, Dongqing AU - Feng, Xinliang AU - Müllen, Klaus AU - De Feyter, Steven AU - Mertens, Stijn F. L. T2 - Chemical Communications AB - Using in situ electrochemical scanning tunnelling microscopy (EC-STM), we demonstrate fully reversible tuning of molecular tiling between self-assembled structures with supramolecular motifs containing 2, 3, 4, 6 or 7 tectons. The structures can be explained by electrocompression of the cationic adlayer at the solid–liquid interface. DA - 2014/07/31/ PY - 2014 DO - 10.1039/C4CC04189E VL - 50 IS - 72 SP - 10376 J2 - Chem. Commun. SN - 1359-7345, 1364-548X ER - TY - JOUR TI - A metastable Fe(A) termination at the Fe3O4(001) surface AU - Parkinson, Gareth S. AU - Novotný, Zbynek AU - Jacobson, Peter AU - Schmid, Michael AU - Diebold, Ulrike T2 - Surface Science AB - A metastable Fe(A) terminated Fe3O4(001) surface was prepared by tailoring the surface preparation conditions. STM, LEIS and LEED are utilized to demonstrate that annealing the Ar+ sputtered surface to 350 °C produces an Fe(A) terminated surface with a (√2 × √2)R45° superstructure. Within the superstructure both single Fe atoms and Fe dimer species are observed. The surface is reoxidized upon annealing to higher temperatures, eventually leading to the recovery of the energetically favorable Jahn-Teller distorted surface at 700 °C. The ability to reproducibly prepare the Fe(A) termination in this simple manner will allow investigations into the structure-function relationship for this important technological material. DA - 2011/08// PY - 2011 DO - 10.1016/j.susc.2011.05.018 VL - 605 IS - 15-16 SP - L42 EP - L45 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Subsurface cation vacancy stabilization of the magnetite (001) surface AU - Bliem, R. AU - McDermott, E. AU - Ferstl, P. AU - Setvin, M. AU - Gamba, O. AU - Pavelec, J. AU - Schneider, M. A. AU - Schmid, M. AU - Diebold, U. AU - Blaha, P. AU - Hammer, L. AU - Parkinson, G. S. T2 - Science AB - Iron oxides play an increasingly prominent role in heterogeneous catalysis, hydrogen production, spintronics, and drug delivery. The surface or material interface can be performance-limiting in these applications, so it is vital to determine accurate atomic-scale structures for iron oxides and understand why they form. Using a combination of quantitative low-energy electron diffraction, scanning tunneling microscopy, and density functional theory calculations, we show that an ordered array of subsurface iron vacancies and interstitials underlies the well-known (√2 x √2)R45° reconstruction of Fe3O4(001). This hitherto unobserved stabilization mechanism occurs because the iron oxides prefer to redistribute cations in the lattice in response to oxidizing or reducing environments. Many other metal oxides also achieve stoichiometry variation in this way, so such surface structures are likely commonplace. DA - 2014/12/05/ PY - 2014 DO - 10.1126/science.1260556 VL - 346 IS - 6214 SP - 1215 EP - 1218 J2 - Science SN - 0036-8075, 1095-9203 ER - TY - JOUR TI - Point defects at cleaved Srn+1RunO3n+1(001) surfaces AU - Stöger, Bernhard AU - Hieckel, Marcel AU - Mittendorfer, Florian AU - Wang, Zhiming AU - Schmid, Michael AU - Parkinson, Gareth S. AU - Fobes, David AU - Peng, Jin AU - Ortmann, John E. AU - Limbeck, Andreas AU - Mao, Zhiqiang AU - Redinger, Josef AU - Diebold, Ulrike T2 - Physical Review B AB - The (001) surfaces of cleaved Sr3Ru2O7 and Sr2RuO4 samples were investigated using low-temperature scanning tunneling microscopy and density functional theory calculations. Intrinsic defects are not created during cleaving. This experimental observation is consistent with calculations, where the formation energy for a Sr and O vacancy, 4.19 eV and 3.81 eV, respectively, is significantly larger than that required to cleave the crystal, 1.11 eV/(1 × 1) unit cell. Surface oxygen vacancies can be created through electron bombardment, however, and their appearance is shown to vary strongly with the imaging conditions. Point defects observed on as-cleaved surfaces result from bulk impurities and adsorption from the residual gas. DA - 2014/10/30/ PY - 2014 DO - 10.1103/PhysRevB.90.165438 VL - 90 IS - 16 SP - 165438 J2 - Phys. Rev. B ER - TY - JOUR TI - Partielle Dissoziation von Wasser führt zu stabilen Überstrukturen auf der Oberfläche von Zinkoxid AU - Meyer, Bernd AU - Marx, Dominik AU - Dulub, Olga AU - Diebold, Ulrike AU - Kunat, Martin AU - Langenberg, Deler AU - Wöll, Christof T2 - Angewandte Chemie AB - Zur Hälfte dissoziiert: Experimentelle und rechnerische Befunde lassen den Schluss zu, dass Wasser auf defektfreien Oberflächen von Zinkoxid eine weitreichend geordnete Überstruktur bildet, in der jedes zweite Wassermolekül dissoziiert ist. Die Ergebnisse sind von allgemeiner Relevanz für die heterogene Katalyse. DA - 2004/12/10/ PY - 2004 DO - 10.1002/ange.200461696 VL - 116 IS - 48 SP - 6809 EP - 6814 J2 - Angew. Chem. SN - 1521-3757 ER - TY - JOUR TI - Surface oxygen chemistry of a gas-sensing material: SnO2(101) AU - Batzill, M. AU - Chaka, A. M. AU - Diebold, U. T2 - EPL (Europhysics Letters) AB - Experimental techniques and density-functional theory have been employed to identify the surface composition and structure of SnO2(101). The stoichiometric Sn^4+O2^2- surface is only stable at high oxygen chemical potential. For lower oxidizing potential of the gas phase a Sn^2+O^2- bulk termination is favored. These two surfaces convert into each other without reconstruction by occupying and vacating bridging oxygen sites. This variability of the surface composition is possible because of the dual valency of Sn and may be one of the fundamental mechanisms responsible for the performance of this material in gas-sensing devices. DA - 2004/// PY - 2004 DO - 10.1209/epl/i2003-10044-0 VL - 65 IS - 1 SP - 61 EP - 67 J2 - Europhys. Lett. SN - 0295-5075 ST - Surface oxygen chemistry of a gas-sensing material ER - TY - JOUR TI - Progress in monitoring thin film thickness by use of quartz crystals AU - Benes, E. AU - Schmid, M. AU - Thorn, G. T2 - Thin Solid Films AB - The sensor function of high-accuracy quartz crystal thin film thickness monitors is determined by the properties of the unloaded crystal and the acoustic impedance zF of the film material. This material constant zF is often not precisely known, but it can be determined by the introduced two-frequency or Auto-Z-Match® technique which uses the frequencies of two different quasiharmonic modes of the crystal. In comparison with standard crystal thickness measurement techniques as well as with atomic absorption spectrometry and the conventional microbalance, the superiority of the Auto-Z-Match® technique is demonstrated for the evaporation materials copper, titanium, aluminium and lead. As the two-frequency technique allows the determination of the effective acoustic impedance of the film and thus thickness, determinations with higher accuracy are possible in the case of poorly known or unknown z-values. Other contributions to the practical performance and accuracy of quartz crystal thin film thickness monitors, such as the influence of crystal temperature, thin films stresses and the relaxation phenomenon, as well as the occurrence of frequency jumps, are discussed on the basis of the most recent research results. DA - 1989/07// PY - 1989 DO - 10.1016/0040-6090(89)90907-3 VL - 174 IS - Part 1 SP - 307 EP - 314 J2 - Thin Solid Films SN - 0040-6090 ER - TY - JOUR TI - Pure and cobalt-doped SnO2(101) films grown by molecular beam epitaxy on Al2O3 AU - Batzill, Matthias AU - Burst, James M. AU - Diebold, Ulrike T2 - Thin Solid Films AB - Pure and Co-doped epitaxial SnO2 films grown by oxygen plasma assisted molecular beam epitaxy on r-cut [alpha]-alumina substrates were investigated by electron diffraction, X-ray photoelectron spectroscopy (XPS), and X-ray photoelectron diffraction (XPD). On hot alumina substrates (~ 800 °C) only a submonolayer amount of Sn adsorbs, indicating a strong adhesion of the first monolayer of tin on the alumina surface. SnO2 films grown at ~ 400-600 °C substrate temperature exhibit a SnO2(101)[010]||Al2O3(-1012)[12-10] epitaxial relationship. Subtle differences in the XPD data of SnO2 films compared to measurements on SnO2(101) single crystal surfaces are consistent with the presence of a high density of stoichiometric antiphase domain boundaries in the film. These planar defects are introduced in the SnO2 film to compensate for the more than 10% lattice mismatch between the SnO2 films and the alumina substrate along the SnO2[-101] direction. CoxSn1-xO2 films with a Co-cation concentration of 5-15% were also grown. XPS indicates that Co is in a 2+ oxidation state and XPD shows that tin is replaced substitutionally by Co. DA - 2005/07/22/ PY - 2005 DO - 10.1016/j.tsf.2005.02.016 VL - 484 IS - 1-2 SP - 132 EP - 139 J2 - Thin Solid Films SN - 0040-6090 ER - TY - JOUR TI - Identification of adsorbed molecules via STM tip manipulation: CO, H2O, and O2 on TiO2 anatase (101) AU - Setvin, Martin AU - Daniel, Benjamin AU - Aschauer, Ulrich AU - Hou, Weiyi AU - Li, Ye-Fei AU - Schmid, Michael AU - Selloni, Annabella AU - Diebold, Ulrike T2 - Physical Chemistry Chemical Physics AB - While Scanning Tunneling Microscopy (STM) has evolved as an ideal tool to study surface chemistry at the atomic scale, the identification of adsorbed species is often not straightforward. This paper describes a way to reliably identify H2O, CO and O2 on the TiO2 anatase (101) surface with STM. These molecules are of a key importance in the surface chemistry of this and many other (photo-) catalytic materials. They exhibit a wide variety of contrasts in STM images, depending on the tip condition. With clean, metallic tips the molecules appear very similar, i.e., as bright, dimer-like features located in the proximity of surface Ti5c atoms. However, each species exhibits a specific response to the electric field applied by the STM tip. It is shown that this tip–adsorbate interaction can be used to reliably ascertain the identity of such species. The tip–adsorbate interactions, together with comparison of experimental and calculated STM images, are used to analyse and revisit the assignments of molecular adsorbed species reported in recent studies. DA - 2014/09/17/ PY - 2014 DO - 10.1039/C4CP03212H VL - 16 IS - 39 SP - 21524 EP - 21530 J2 - Phys. Chem. Chem. Phys. SN - 1463-9084 ST - Identification of adsorbed molecules via STM tip manipulation ER - TY - JOUR TI - Vacancy clusters at domain boundaries and band bending at the SrTiO3(110) surface AU - Wang, Zhiming AU - Hao, Xianfeng AU - Gerhold, Stefan AU - Schmid, Michael AU - Franchini, Cesare AU - Diebold, Ulrike T2 - Physical Review B AB - Antiphase domain boundaries (APDBs) in the (n×1)(n=4,5) reconstructions of the SrTiO3(110) surface were studied with scanning tunneling microscopy, x-ray photoemission spectroscopy, and density functional theory (DFT) calculations. Two types of APDBs form on each reconstruction; they consist of TixOy vacancy clusters with a specific stoichiometry. The presence of these clusters is controlled by the oxygen pressure during annealing. The structural models of the vacancy clusters are resolved with DFT, which also shows that their relative stability depends on the chemical potential of oxygen. The surface band bending can be tuned by controlling the vacancy clusters at the domain boundaries. DA - 2014/07/23/ PY - 2014 DO - 10.1103/PhysRevB.90.035436 VL - 90 IS - 3 SP - 035436 J2 - Phys. Rev. B ER - TY - JOUR TI - Hybrid exchange density functional study of vicinal anatase TiO2 surfaces AU - Sanches, F. F. AU - Mallia, G. AU - Liborio, L. AU - Diebold, U. AU - Harrison, N. M. T2 - Physical Review B AB - The observation of photocatalytic water splitting on the surface of anatase TiO2 crystals has stimulated many investigations of the underlying processes. Nevertheless, a molecular-level understanding of the reaction is not available. This requires knowledge of the crystal facets present, the atomistic structure of the surfaces, and thus the reaction sites involved. In this paper we establish the atomistic structure of two surfaces, vicinal to the low-energy (101) surface. We compute the relative stability and electronic properties of the (514) and (516) surfaces and compare these to the low-index (101), (001), and (100) surfaces. The (516) surface is remarkably stable, and is predicted to contribute significantly to the surface area of a crystallite in equilibrium. We simulate constant current scanning tunneling microscopy images and, by comparing with those measured, we conclude that a surface previously observed in a miscut single crystal is the (516) surface described here. The computed stability of this surface indicates that it will be present in TiO2 nanostructures and the relative positions of its band edges suggests that it will play a significant role in the water-TiO2 reactions in solar water splitting. DA - 2014/06/23/ PY - 2014 DO - 10.1103/PhysRevB.89.245309 VL - 89 IS - 24 SP - 245309 J2 - Phys. Rev. B ER - TY - JOUR TI - Imaging and Tuning Molecular Levels at the Surface of a Gated Graphene Device AU - Riss, Alexander AU - Wickenburg, Sebastian AU - Tan, Liang Z. AU - Tsai, Hsin-Zon AU - Kim, Youngkyou AU - Lu, Jiong AU - Bradley, Aaron J. AU - Ugeda, Miguel M. AU - Meaker, Kacey L. AU - Watanabe, Kenji AU - Taniguchi, Takashi AU - Zettl, Alex AU - Fischer, Felix R. AU - Louie, Steven G. AU - Crommie, Michael F. T2 - ACS Nano AB - Gate-controlled tuning of the charge carrier density in graphene devices provides new opportunities to control the behavior of molecular adsorbates. We have used scanning tunneling microscopy (STM) and spectroscopy (STS) to show how the vibronic electronic levels of 1,3,5-tris(2,2-dicyanovinyl)benzene molecules adsorbed onto a graphene/BN/SiO2 device can be tuned via application of a backgate voltage. The molecules are observed to electronically decouple from the graphene layer, giving rise to well-resolved vibronic states in dI/dV spectroscopy at the single-molecule level. Density functional theory (DFT) and many-body spectral function calculations show that these states arise from molecular orbitals coupled strongly to carbon–hydrogen rocking modes. Application of a back-gate voltage allows switching between different electronic states of the molecules for fixed sample bias. DA - 2014/04/18/ PY - 2014 DO - 10.1021/nn501459v VL - 8 IS - 6 SP - 5395 EP - 5401 J2 - ACS Nano SN - 1936-0851 ER - TY - JOUR TI - Characterizing solid state gas responses using surface charging in photoemission: water adsorption on SnO2(101) AU - Batzill, Matthias AU - Diebold, Ulrike T2 - Journal of Physics: Condensed Matter AB - A novel experimental approach for studying the gas response mechanism of semiconducting gas sensor materials is demonstrated using the example of water adsorption on SnO2(101). In this approach, valence band photoemission as a chemical probe is combined with photocurrent induced surface charging as a basis for contactless sample conductivity measurement. DA - 2006/// PY - 2006 DO - 10.1088/0953-8984/18/8/L03 VL - 18 IS - 8 SP - L129 EP - L134 J2 - J. Phys.: Condens. Matter SN - 0953-8984 ST - Characterizing solid state gas responses using surface charging in photoemission ER - TY - JOUR TI - Adjusting island density and morphology of the SrTiO3(110)-(4 × 1) surface: Pulsed laser deposition combined with scanning tunneling microscopy AU - Gerhold, Stefan AU - Riva, Michele AU - Yildiz, Bilge AU - Schmid, Michael AU - Diebold, Ulrike T2 - Surface Science AB - The first stages of homoepitaxial growth of the (4 × 1) reconstructed surface of SrTiO3(110) are probed by a combination of pulsed laser deposition (PLD) with in-situ reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). Considerations of interfacing high-pressure PLD growth with ultra-high-vacuum surface characterization methods are discussed, and the experimental setup and procedures are described in detail. The relation between RHEED intensity oscillations and ideal layer-by-layer growth is confirmed by analysis of STM images acquired after deposition of sub-monolayer amounts of SrTiO3. For a quantitative agreement between RHEED and STM results one has to take into account two interfaces: the steps at the circumference of islands, as well as the borders between two different reconstruction phases on the islands themselves. Analysis of STM images acquired after one single laser shot reveals an exponential decrease of the island density with increasing substrate temperature. This behavior is also directly visible from the temperature dependence of the relaxation times of the RHEED intensity. Moreover, the aspect ratio of islands changes considerably with temperature. The growth mode depends on the laser pulse repetition rate, and can be tuned from predominantly layer-by-layer to the step-flow growth regime. DA - 2016/09// PY - 2016 DO - 10.1016/j.susc.2016.03.010 VL - 651 SP - 76 EP - 83 J2 - Surf. Sci. SN - 0039-6028 ST - Adjusting island density and morphology of the SrTiO3(110)-(4 × 1) surface ER - TY - JOUR TI - Fe3O4(110)–(1 × 3) revisited: Periodic (111) nanofacets AU - Parkinson, Gareth S. AU - Lackner, Peter AU - Gamba, Oscar AU - Maaß, Sebastian AU - Gerhold, Stefan AU - Riva, Michele AU - Bliem, Roland AU - Diebold, Ulrike AU - Schmid, Michael T2 - Surface Science AB - The structure of the Fe3O4(110)–(1 × 3) surface was studied with scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and reflection high-energy electron diffraction (RHEED). The so-called one-dimensional reconstruction is characterized by bright rows that extend hundreds of nanometers in the [ 1 ¯ 10] direction and have a periodicity of 2.52 nm in [001] in STM. It is concluded that this reconstruction is the result of a periodic faceting to expose {111}-type planes with a lower surface energy. DA - 2016/07// PY - 2016 DO - 10.1016/j.susc.2016.02.020 VL - 649 SP - 120 EP - 123 J2 - Surf. Sci. SN - 0039-6028 ST - Fe3O4(110)–(1 × 3) revisited ER - TY - JOUR TI - Adsorption of water at the SrO surface of ruthenates AU - Halwidl, Daniel AU - Stöger, Bernhard AU - Mayr-Schmölzer, Wernfried AU - Pavelec, Jiri AU - Fobes, David AU - Peng, Jin AU - Mao, Zhiqiang AU - Parkinson, Gareth S. AU - Schmid, Michael AU - Mittendorfer, Florian AU - Redinger, Josef AU - Diebold, Ulrike T2 - Nature Materials AB - Although perovskite oxides hold promise in applications ranging from solid oxide fuel cells to catalysts, their surface chemistry is poorly understood at the molecular level. Here we follow the formation of the first monolayer of water at the (001) surfaces of Srn+1RunO3n+1 (n = 1, 2) using low-temperature scanning tunnelling microscopy, X-ray photoelectron spectroscopy, and density functional theory. These layered perovskites cleave between neighbouring SrO planes, yielding almost ideal, rocksalt-like surfaces. An adsorbed monomer dissociates and forms a pair of hydroxide ions. The OH stemming from the original molecule stays trapped at Sr–Sr bridge positions, circling the surface OH with a measured activation energy of 187 ± 10 meV. At higher coverage, dimers of dissociated water assemble into one-dimensional chains and form a percolating network where water adsorbs molecularly in the gaps. Our work shows the limitations of applying surface chemistry concepts derived for binary rocksalt oxides to perovskites. DA - 2016/// PY - 2016 DO - 10.1038/nmat4512 VL - 15 SP - 450 EP - 455 J2 - Nat. Mater. SN - 1476-1122 ER - TY - JOUR TI - A multitechnique study of CO adsorption on the TiO2 anatase (101) surface AU - Setvin, Martin AU - Buchholz, Maria AU - Hou, Weiyi AU - Zhang, Cui AU - Stöger, Bernhard AU - Hulva, Jan AU - Simschitz, Thomas AU - Shi, Xiao AU - Pavelec, Jiri AU - Parkinson, Gareth S. AU - Xu, Mingchun AU - Wang, Yuemin AU - Schmid, Michael AU - Wöll, Christof AU - Selloni, Annabella AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry C AB - The adsorption of carbon monoxide on the anatase TiO2 (101) surface was studied with infrared reflection absorption spectroscopy (IRRAS), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density functional theory (DFT). The IRRAS data reveal only one CO band at ≈2181 cm^-1 for both stoichiometric and reduced TiO2(101) surfaces. From TPD, an adsorption energy of 0.37 ± 0.03 eV is estimated for the isolated molecule, which shifts to slightly smaller values at higher coverages. Combining STM imaging and controlled annealing of the sample confirms the adsorption energies estimated from TPD and the slight repulsive intermolecular interaction. CO molecules desorb from electron-rich, extrinsic donor defect sites at somewhat higher temperatures. Confronting the experimental results with DFT calculations indicates that the anatase (101) surface does not contain any significant concentration of subsurface oxygen vacancies in the near-surface region. Comparison with CO adsorption on the rutile TiO2(110) surface shows that the tendency for excess electron localization in anatase is much weaker than in rutile. DA - 2015/09/10/ PY - 2015 DO - 10.1021/acs.jpcc.5b07999 VL - 119 IS - 36 SP - 21044 EP - 21052 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - An atomic-scale view of CO and H2 oxidation on a Pt/Fe3O4 model catalyst AU - Bliem, Roland AU - van der Hoeven, Jessi AU - Zavodny, Adam AU - Gamba, Oscar AU - Pavelec, Jiri AU - de Jongh, Petra E. AU - Schmid, Michael AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - Angewandte Chemie International Edition AB - Metal–support interactions are frequently invoked to explain the enhanced catalytic activity of metal nanoparticles dispersed over reducible metal oxide supports, yet the atomic-scale mechanisms are rarely known. In this report, scanning tunneling microscopy was used to study a Pt1-6/Fe3O4 model catalyst exposed to CO, H2, O2, and mixtures thereof at 550 K. CO extracts lattice oxygen atoms at the cluster perimeter to form CO2, creating large holes in the metal oxide surface. H2 and O2 dissociate on the metal clusters and spill over onto the support. The former creates surface hydroxy groups, which react with the support, ultimately leading to the desorption of water, while oxygen atoms react with Fe from the bulk to create new Fe3O4(001) islands. The presence of the Pt is crucial because it catalyzes reactions that already occur on the bare iron oxide surface, but only at higher temperatures. DA - 2015/11/16/ PY - 2015 DO - 10.1002/anie.201507368 VL - 54 IS - 47 SP - 13999 EP - 14002 J2 - Angew. Chem. Int. Ed. SN - 1521-3773 ER - TY - JOUR TI - Adsorption of formic acid on the Fe3O4(001) surface AU - Gamba, Oscar AU - Noei, Heshmat AU - Pavelec, Jiří AU - Bliem, Roland AU - Schmid, Michael AU - Diebold, Ulrike AU - Stierle, Andreas AU - Parkinson, Gareth S. T2 - The Journal of Physical Chemistry C AB - The adsorption of formic acid (HCOOH) on the Fe3O4(001) surface was studied using X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy (IRRAS), low-energy electron diffraction, and scanning tunneling microscopy (STM). At room temperature, HCOOH dissociates to form formate (HCOO–) and hydroxyl groups, facilitated by the close proximity of undercoordinated Fe3+/O2– cation/anion pairs at the Fe3O4(001) surface. Bidentate formate species are observed in IRRAS, and their position on Fe–Fe bridge sites can be inferred from STM data. At 70 K, HCOOH is adsorbed both dissociatively and molecularly, suggesting two active sites for dissociation. Our study also demonstrates that IRRAS is possible on Fe3O4 single crystals with good sensitivity but that unusual peak shapes occur because the substrate is midway between a perfect conductor and a perfect dielectric. DA - 2015/09/03/ PY - 2015 DO - 10.1021/acs.jpcc.5b05560 VL - 119 IS - 35 SP - 20459 EP - 20465 J2 - J. Phys. Chem. C SN - 1932-7447, 1932-7455 ER - TY - JOUR TI - Nickel-oxide-modified SrTiO3 (110)-(4 × 1) surfaces and their interaction with water AU - Gerhold, Stefan AU - Riva, Michele AU - Wang, Zhiming AU - Bliem, Roland AU - Wagner, Margareta AU - Osiecki, Jacek AU - Schulte, Karina AU - Schmid, Michael AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry C AB - Nickel oxide (NiO), deposited onto the strontium titanate (SrTiO3) (110)-(4 × 1) surface, was studied using photoemission spectroscopy (PES), X-ray absorption near edge structure (XANES), and low-energy He+ ion scattering (LEIS), as well as scanning tunneling microscopy (STM). The main motivation for studying this system comes from the prominent role it plays in photocatalysis. The (4 × 1) reconstructed SrTiO3(110) surface was previously found to be remarkably inert toward water adsorption under ultrahigh-vacuum conditions. Nickel oxide grows on this surface as patches without any apparent ordered structure. PES and LEIS reveal an upward band bending, a reduction of the band gap, and reactivity toward water adsorption upon deposition of NiO. Spectroscopic results are discussed with respect to the enhanced reactivity toward water of the NiO-loaded surface. DA - 2015/09/03/ PY - 2015 DO - 10.1021/acs.jpcc.5b06144 VL - 119 IS - 35 SP - 20481 EP - 20487 J2 - J. Phys. Chem. C SN - 1932-7447, 1932-7455 ER - TY - JOUR TI - Growth of an ultrathin zirconia film on Pt3Zr examined by high-resolution x-ray photoelectron spectroscopy, temperature-programmed desorption, scanning tunneling microscopy, and density functional theory AU - Li, Hao AU - Choi, Joong-Il Jake AU - Mayr-Schmölzer, Wernfried AU - Weilach, Christian AU - Rameshan, Christoph AU - Mittendorfer, Florian AU - Redinger, Josef AU - Schmid, Michael AU - Rupprechter, Günther T2 - The Journal of Physical Chemistry C AB - Ultrathin (~3 Å) zirconium oxide films were grown on a single-crystalline Pt3Zr(0001) substrate by oxidation in 1 x 10^-7 mbar of O2 at 673 K, followed by annealing at temperatures up to 1023 K. The ZrO2 films are intended to serve as model supports for reforming catalysts and fuel cell anodes. The atomic and electronic structure and composition of the ZrO2 films were determined by synchrotron-based high-resolution X-ray photoelectron spectroscopy (HR-XPS) (including depth profiling), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. Oxidation mainly leads to ultrathin trilayer (O-Zr-O) films on the alloy; only a small area fraction (10-15%) is covered by ZrO2 clusters (thickness ~0.5-10 nm). The amount of clusters decreases with increasing annealing temperature. Temperature-programmed desorption (TPD) of CO was utilized to confirm complete coverage of the Pt3Zr substrate by ZrO2, that is, formation of a closed oxide overlayer. Experiments and DFT calculations show that the core level shifts of Zr in the trilayer ZrO2 films are between those of metallic Zr and thick (bulklike) ZrO2. Therefore, the assignment of such XPS core level shifts to substoichiometric ZrOx is not necessarily correct, because these XPS signals may equally well arise from ultrathin ZrO2 films or metal/ZrO2 interfaces. Furthermore, our results indicate that the common approach of calculating core level shifts by DFT including final-state effects should be taken with care for thicker insulating films, clusters, and bulk insulators. DA - 2015/// PY - 2015 DO - 10.1021/jp5100846 VL - 119 IS - 5 SP - 2462 EP - 2470 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - Molecular ordering at the interface between liquid water and rutile TiO2(110) AU - Serrano, Giulia AU - Bonanni, Beatrice AU - Di Giovannantonio, Marco AU - Kosmala, Tomasz AU - Schmid, Michael AU - Diebold, Ulrike AU - Di Carlo, Aldo AU - Cheng, Jun AU - VandeVondele, Joost AU - Wandelt, Klaus AU - Goletti, Claudio T2 - Advanced Materials Interfaces AB - The pivotal importance of TiO2 as a technological material involves most applications in an aqueous environment, but the single-crystal TiO2/bulk-water interfaces are almost completely unexplored, since up to date solid/liquid interfaces are more difficult to access than surfaces in ultrahigh vacuum (UHV). Only a few techniques (as scanning probe microscopy) offer the opportunity to explore these systems under realistic conditions. The rutile TiO2(110) surface immersed in high-purity water is studied by in situ scanning tunneling microscopy. The large-scale surface morphology as obtained after preparation under UHV conditions remains unchanged upon prolonged exposure to bulk water. Moreover, in contrast to UHV, atomically resolved images show a twofold periodicity along the [001] direction, indicative of an ordered structure resulting from the hydration layer. This is consistent with density-functional theory based molecular dynamics simulations where neighboring interfacial molecules of the first water layer in contact with the bulk liquid form dimers. By contrast, this dimerization is not observed for a single adsorbed water monolayer, i.e., in the absence of bulk water. DA - 2015/11/01/ PY - 2015 DO - 10.1002/admi.201500246 VL - 2 IS - 17 SP - 1500246 J2 - Adv. Mater. Interfaces SN - 2196-7350 ER - TY - JOUR TI - Characterization of collective ground states in single-layer NbSe2 AU - Ugeda, Miguel M. AU - Bradley, Aaron J. AU - Zhang, Yi AU - Onishi, Seita AU - Chen, Yi AU - Ruan, Wei AU - Ojeda-Aristizabal, Claudia AU - Ryu, Hyejin AU - Edmonds, Mark T. AU - Tsai, Hsin-Zon AU - Riss, Alexander AU - Mo, Sung-Kwan AU - Lee, Dunghai AU - Zettl, Alex AU - Hussain, Zahid AU - Shen, Zhi-Xun AU - Crommie, Michael F. T2 - Nature Physics AB - Layered transition metal dichalcogenides are ideal systems for exploring the effects of dimensionality on correlated electronic phases such as charge density wave (CDW) order and superconductivity. In bulk NbSe2 a CDW sets in at TCDW = 33 K and superconductivity sets in at Tc = 7.2 K. Below Tc these electronic states coexist but their microscopic formation mechanisms remain controversial. Here we present an electronic characterization study of a single two-dimensional (2D) layer of NbSe2 by means of low-temperature scanning tunnelling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and electrical transport measurements. We demonstrate that 3 × 3 CDW order in NbSe2 remains intact in two dimensions. Superconductivity also still remains in the 2D limit, but its onset temperature is depressed to 1.9 K. Our STS measurements at 5 K reveal a CDW gap of Δ = 4 meV at the Fermi energy, which is accessible by means of STS owing to the removal of bands crossing the Fermi level for a single layer. Our observations are consistent with the simplified (compared to bulk) electronic structure of single-layer NbSe2, thus providing insight into CDW formation and superconductivity in this model strongly correlated system. DA - 2016/01// PY - 2016 DO - 10.1038/nphys3527 VL - 12 IS - 1 SP - 92 EP - 97 J2 - Nat. Phys. SN - 1745-2473 ER - TY - JOUR TI - Dual role of CO in the stability of subnano Pt clusters at the Fe3O4(001) surface AU - Bliem, Roland AU - Hoeven, Jessi E. S. van der AU - Hulva, Jan AU - Pavelec, Jiri AU - Gamba, Oscar AU - Jongh, Petra E. de AU - Schmid, Michael AU - Blaha, Peter AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - Proceedings of the National Academy of Sciences AB - Interactions between catalytically active metal particles and reactant gases depend strongly on the particle size, particularly in the subnanometer regime where the addition of just one atom can induce substantial changes in stability, morphology, and reactivity. Here, time-lapse scanning tunneling microscopy (STM) and density functional theory (DFT)-based calculations are used to study how CO exposure affects the stability of Pt adatoms and subnano clusters at the Fe3O4(001) surface, a model CO oxidation catalyst. The results reveal that CO plays a dual role: first, it induces mobility among otherwise stable Pt adatoms through the formation of Pt carbonyls (Pt1–CO), leading to agglomeration into subnano clusters. Second, the presence of the CO stabilizes the smallest clusters against decay at room temperature, significantly modifying the growth kinetics. At elevated temperatures, CO desorption results in a partial redispersion and recovery of the Pt adatom phase. DA - 2016/08/09/ PY - 2016 DO - 10.1073/pnas.1605649113 VL - 113 IS - 32 SP - 8921 EP - 8926 J2 - Proc. Natl. Acad. Sci. USA SN - 0027-8424, 1091-6490 ER - TY - JOUR TI - Composition and local atomic arrangement of decagonal Al-Co-Cu quasicrystal surfaces AU - Zenkyu, R. AU - Yuhara, J. AU - Matsui, T. AU - Shah Zaman, S. AU - Schmid, M. AU - Varga, P. T2 - Physical Review B AB - We investigated the composition of decagonal Al-Co-Cu surface by Auger electron spectroscopy (AES) and low-energy ion scattering (LEIS). The surface composition after annealing was Al richer and Co poorer compared to that after sputtering or bulk composition. Two types of the characteristic clusters were observed by scanning tunneling microscopy (STM) and no bias voltage dependence of the image was observed. On the other hand, scanning tunneling spectroscopy revealed a subtle difference of local density of states in unoccupied states between different sites. Structural optimization using ab initio calculations based on density functional theory (DFT) was performed on several compositional models, which are based on the W-(AlCoNi) bulk model. The surface structures of two types of the characteristic clusters were determined by comparison of the STM image and the simulated image of the structures obtained by DFT. The topmost layer was composed of Al and Cu atoms, and the compositional ratio was consistent with the AES and LEIS results. DA - 2012/09/17/ PY - 2012 DO - 10.1103/PhysRevB.86.115422 VL - 86 IS - 11 SP - 115422 J2 - Phys. Rev. B ER - TY - JOUR TI - Ion-beam-induced magnetic and structural phase transformation of Ni-stabilized face-centered-cubic Fe films on Cu(100) AU - Gloss, Jonas AU - Shah Zaman, Sameena AU - Jonner, Jakub AU - Novotny, Zbynek AU - Schmid, Michael AU - Varga, Peter AU - Urbánek, Michal T2 - Applied Physics Letters AB - Metastable face-centered cubic (fcc) Fe/Cu(100) thin films are good candidates for ion-beam magnetic patterning due to their magnetic transformation upon ion-beam irradiation. However, pure fcc Fe films undergo spontaneous transformation when their thickness exceeds 10 ML. This limit can be extended to approximately 22 ML by deposition of Fe at increased CO background pressures. We show that much thicker films can be grown by alloying with Ni for stabilizing the fcc γ phase. The amount of Ni necessary to stabilize nonmagnetic, transformable fcc Fe films in dependence on the residual background pressure during the deposition is determined and a phase diagram revealing the transformable region is presented. DA - 2013/12/26/ PY - 2013 DO - 10.1063/1.4856775 VL - 103 IS - 26 SP - 262405 J2 - Appl. Phys. Lett. SN - 0003-6951, 1077-3118 ER - TY - JOUR TI - Following the reduction of oxygen on TiO2 anatase (101) step by step AU - Setvin, Martin AU - Aschauer, Ulrich AU - Hulva, Jan AU - Simschitz, Thomas AU - Daniel, Benjamin AU - Schmid, Michael AU - Selloni, Annabella AU - Diebold, Ulrike T2 - Journal of the American Chemical Society AB - We have investigated the reaction between O2 and H2O, coadsorbed on the (101) surface of a reduced TiO2 anatase single crystal by scanning tunneling microscopy, density functional theory, temperature-programmed desorption, and X-ray photoelectron spectroscopy. While water adsorbs molecularly on the anatase (101) surface, the reaction with O2 results in water dissociation and formation of terminal OH groups. We show that these terminal OHs are the final and stable reaction product on reduced anatase. We identify OOH as a metastable intermediate in the reaction. The water dissociation reaction runs as long as the surface can transfer enough electrons to the adsorbed species; the energy balance and activation barriers for the individual reaction steps are discussed, depending on the number of electrons available. Our results indicate that the presence of donor dopants can significantly reduce activation barriers for oxygen reduction on anatase. DA - 2016/08/03/ PY - 2016 DO - 10.1021/jacs.6b04004 VL - 138 IS - 30 SP - 9565 EP - 9571 J2 - J. Am. Chem. Soc. SN - 0002-7863 ER - TY - JOUR TI - Tailoring the nature and strength of electron-phonon interactions in the SrTiO3(001) 2D electron liquid AU - Wang, Z. AU - McKeown Walker, S. AU - Tamai, A. AU - Wang, Y. AU - Ristic, Z. AU - Bruno, F. Y. AU - de la Torre, A. AU - Riccò, S. AU - Plumb, N. C. AU - Shi, M. AU - Hlawenka, P. AU - Sánchez-Barriga, J. AU - Varykhalov, A. AU - Kim, T. K. AU - Hoesch, M. AU - King, P. D. C. AU - Meevasana, W. AU - Diebold, U. AU - Mesot, J. AU - Moritz, B. AU - Devereaux, T. P. AU - Radovic, M. AU - Baumberger, F. T2 - Nature Materials AB - Surfaces and interfaces offer new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides. Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO3(001) surface to reveal a remarkably complex evolution of electron–phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO3/SrTiO3 interface, our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron–phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO3-based 2DELs. DA - 2016/08// PY - 2016 DO - 10.1038/nmat4623 VL - 15 IS - 8 SP - 835 EP - 839 J2 - Nat. Mater. SN - 1476-1122 ER - TY - JOUR TI - Iron oxide surfaces AU - Parkinson, Gareth S. T2 - Surface Science Reports AB - The current status of knowledge regarding the surfaces of the iron oxides, magnetite (Fe3O4), maghemite (γ-Fe2O3), haematite (α-Fe2O3), and wüstite (Fe1−xO) is reviewed. The paper starts with a summary of applications where iron oxide surfaces play a major role, including corrosion, catalysis, spintronics, magnetic nanoparticles (MNPs), biomedicine, photoelectrochemical water splitting and groundwater remediation. The bulk structure and properties are then briefly presented; each compound is based on a close-packed anion lattice, with a different distribution and oxidation state of the Fe cations in interstitial sites. The bulk defect chemistry is dominated by cation vacancies and interstitials (not oxygen vacancies) and this provides the context to understand iron oxide surfaces, which represent the front line in reduction and oxidation processes. Fe diffuses in and out from the bulk in response to the O2 chemical potential, forming sometimes complex intermediate phases at the surface. For example, α-Fe2O3 adopts Fe3O4-like surfaces in reducing conditions, and Fe3O4 adopts Fe1−xO-like structures in further reducing conditions still. It is argued that known bulk defect structures are an excellent starting point in building models for iron oxide surfaces. The atomic-scale structure of the low-index surfaces of iron oxides is the major focus of this review. Fe3O4 is the most studied iron oxide in surface science, primarily because its stability range corresponds nicely to the ultra-high vacuum environment. It is also an electrical conductor, which makes it straightforward to study with the most commonly used surface science methods such as photoemission spectroscopies (XPS, UPS) and scanning tunneling microscopy (STM). The impact of the surfaces on the measurement of bulk properties such as magnetism, the Verwey transition and the (predicted) half-metallicity is discussed. The best understood iron oxide surface at present is probably Fe3O4(100); the structure is known with a high degree of precision and the major defects and properties are well characterised. A major factor in this is that a termination at the Feoct–O plane can be reproducibly prepared by a variety of methods, as long as the surface is annealed in 10−7−10−5 mbar O2 in the final stage of preparation. Such straightforward preparation of a monophase termination is generally not the case for iron oxide surfaces. All available evidence suggests the oft-studied (√2×√2)R45° reconstruction results from a rearrangement of the cation lattice in the outermost unit cell in which two octahedral cations are replaced by one tetrahedral interstitial, a motif conceptually similar to well-known Koch–Cohen defects in Fe1−xO. The cation deficiency results in Fe11O16 stoichiometry, which is in line with the chemical potential in ultra-high vacuum (UHV), which is close to the border between the Fe3O4 and Fe2O3 phases. The Fe3O4(111) surface is also much studied, but two different surface terminations exist close in energy and can coexist, which makes sample preparation and data interpretation somewhat tricky. Both the Fe3O4(100) and Fe3O4(111) surfaces exhibit Fe-rich terminations as the sample selvedge becomes reduced. The Fe3O4(110) surface forms a one-dimensional (1×3) reconstruction linked to nanofaceting, which exposes the more stable Fe3O4(111) surface. α-Fe2O3(0001) is the most studied haematite surface, but difficulties preparing stoichiometric surfaces under UHV conditions have hampered a definitive determination of the structure. There is evidence for at least three terminations: a bulk-like termination at the oxygen plane, a termination with half of the cation layer, and a termination with ferryl groups. When the surface is reduced the so-called “bi-phase” structure is formed, which eventually transforms to a Fe3O4(111)-like termination. The structure of the bi-phase surface is controversial; a largely accepted model of coexisting Fe1−xO and α-Fe2O3(0001) islands was recently challenged and a new structure based on a thin film of Fe3O4(111) on α-Fe2O3(0001) was proposed. The merits of the competing models are discussed. The α-Fe2O3(1 1 ¯ 02) “R-cut” surface is recommended as an excellent prospect for future study given its apparent ease of preparation and its prevalence in nanomaterial. In the latter sections the literature regarding adsorption on iron oxides is reviewed. First, the adsorption of molecules (H2, H2O, CO, CO2, O2, HCOOH, CH3OH, CCl4, CH3I, C6H6, SO2, H2S, ethylbenzene, styrene, and Alq3) is discussed, and an attempt is made to relate this information to the reactions in which iron oxides are utilized as a catalyst (water–gas shift, Fischer–Tropsch, dehydrogenation of ethylbenzene to styrene) or catalyst supports (CO oxidation). The known interactions of iron oxide surfaces with metals are described, and it is shown that the behaviour is determined by whether the metal forms a stable ternary phase with the iron oxide. Those that do not, (e.g. Au, Pt, Ag, Pd) prefer to form three-dimensional particles, while the remainder (Ni, Co, Mn, Cr, V, Cu, Ti, Zr, Sn, Li, K, Na, Ca, Rb, Cs, Mg, Ca) incorporate within the oxide lattice. The incorporation temperature scales with the heat of formation of the most stable metal oxide. A particular effort is made to underline the mechanisms responsible for the extraordinary thermal stability of isolated metal adatoms on Fe3O4 surfaces, and the potential application of this model system to understand single atom catalysis and sub-nano cluster catalysis is discussed. The review ends with a brief summary, and a perspective is offered including exciting lines of future research. DA - 2016/03// PY - 2016 DO - 10.1016/j.surfrep.2016.02.001 VL - 71 IS - 1 SP - 272 EP - 365 J2 - Surf. Sci. Rep. SN - 0167-5729 ER - TY - JOUR TI - Experimental Investigation of the Interaction of Water and Methanol with Anatase−TiO2(101) AU - Herman, G. S. AU - Dohnalek, Z. AU - Ruzycki, N. AU - Diebold, U. T2 - The Journal of Physical Chemistry B AB - The interaction of water and methanol with well-defined (1 x 1) terminated surfaces of anatase-TiO2(101) were investigated with temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). For water, three desorption states were observed in the TPD spectra at 160, 190, and 250 K. The three desorption peaks were assigned to multilayer water, water adsorbed to 2-fold-coordinated O, and water adsorbed to 5-fold-coordinated Ti, respectively. The TPD spectra for methanol were more complicated. For methanol, five desorption peaks were observed in the TPD spectra at 135, 170, 260, 410, and 610 K. The five desorption peaks were assigned to multilayer methanol, methanol adsorbed to 2-fold-coordinated O, methanol adsorbed to 5-fold-coordinated Ti, methoxy adsorbed to 5-fold-coordinated Ti, and methoxy adsorbed to Ti at step edges, respectively. The XPS results indicated that the adsorbed water and methanol were predominantly bound to the surface in a molecular state, with no evidence for dissociation. Furthermore, the O 1s core-level binding energies for water and methanol were found to shift to an 0.75 eV lower binding energy for coverages before multilayer desorption is observed in the TPD spectra. The O 1s core-level binding-energy shift appears to be linear in this region and corresponds to water and methanol bonding to Ti cation and O anion sites on the surface. The C 1s core-level binding energy for methanol was found to remain approximately constant in the same coverage regime. DA - 2003/03/01/ PY - 2003 DO - 10.1021/jp0275544 VL - 107 IS - 12 SP - 2788 EP - 2795 J2 - J. Phys. Chem. B ER - TY - JOUR TI - Dynamics of the TiO2(110) surface and step: Onset of defects in the ordered structure AU - Kuyanov, Igor AU - Lacks, Daniel AU - Diebold, Ulrike T2 - Physical Review B AB - Molecular dynamics simulations are carried out on planar and stepped TiO2 (110) surfaces. The simulations focus on the development of defects in the initially ordered structure as the temperature increases. For the planar surface, bridging oxygen atoms are found to leave their equilibrium positions and subsequently roam one-dimensional lanes formed by adjacent rows of other bridging oxygen atoms, until they become stuck on a previously fivefold-coordinated titanium atom. For the stepped surface, the first defects to occur correspond to titanium-oxygen bonds breaking near the step edge. DA - 2003/12// PY - 2003 DO - 10.1103/PhysRevB.68.233404 VL - 68 IS - 23 SP - 233404 J2 - Phys. Rev. B SN - 0163-1829 ST - Dynamics of the TiO2 (110) surface and step ER - TY - JOUR TI - The locus of sulfate sites on sulfated zirconia AU - Marcus, Rachel AU - Diebold, Ulrike AU - Gonzalez, Richard D. T2 - Catalysis Letters AB - The surface of sulfated zirconia was probed using X-ray photoelectron spectroscopy. It was observed that the entire inventory of sulfur could be completely removed by sputtering the surface using an argon beam. Calibration using a TiO2(110) standard resulted in a surface concentration of 2.85 sulfur atoms/nm2. This is in reasonable agreement with a value of 4.15 sulfur atoms/nm2 based on sulfur analysis on the assumption that all of the sulfur was located at the surface. These results suggest that most, if not all, of the sulfur is near or at the surface. When charging was taken into account, we observed that the oxidation state of sulfur did not change following catalyst deactivation during the isomerization of n-butane. We also determined that the entire inventory of sulfur was present as SO42-. These results reinforce previous studies suggesting that catalyst deactivation occurs as the result of carbon deposition and not a change in the oxidation state of sulfur. DA - 2003/03/01/ PY - 2003 DO - 10.1023/A:1022680421726 VL - 86 IS - 4 SP - 151 EP - 156 J2 - Catal. Lett. ER - TY - JOUR TI - Scanning tunneling microscopy study of the anatase (100) surface AU - Ruzycki, Nancy AU - Herman, Gregory S. AU - Boatner, Lynn A. AU - Diebold, Ulrike T2 - Surface Science AB - Scanning tunneling microscopy (STM) has been used to investigate the structure of the TiO2 anatase (100) surface. Natural single crystals of anatase were employed; and after several cycles of sputtering and annealing at T=450 °C, the TiO2(100) surface was free of impurities, and reconstructed to a (1×n) termination. No evidence for point defects was found in the atomic resolution STM images. The STM results were accounted for on the basis of a surface structure model in which (101)-oriented microfacets run along the anatase [010] direction. DA - 2003/04/01/ PY - 2003 DO - 10.1016/S0039-6028(03)00117-1 VL - 529 IS - 1-2 SP - L239 EP - L244 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Switching stiction and adhesion of a liquid on a solid AU - Mertens, Stijn F. L. AU - Hemmi, Adrian AU - Muff, Stefan AU - Gröning, Oliver AU - De Feyter, Steven AU - Osterwalder, Jürg AU - Greber, Thomas T2 - Nature AB - When a gecko moves on a ceiling it makes use of adhesion and stiction. Stiction—static friction—is experienced on microscopic and macroscopic scales and is related to adhesion and sliding friction. Although important for most locomotive processes, the concepts of adhesion, stiction and sliding friction are often only empirically correlated. A more detailed understanding of these concepts will, for example, help to improve the design of increasingly smaller devices such as micro- and nanoelectromechanical switches. Here we show how stiction and adhesion are related for a liquid drop on a hexagonal boron nitride monolayer on rhodium, by measuring dynamic contact angles in two distinct states of the solid–liquid interface: a corrugated state in the absence of hydrogen intercalation and an intercalation-induced flat state. Stiction and adhesion can be reversibly switched by applying different electrochemical potentials to the sample, causing atomic hydrogen to be intercalated or not. We ascribe the change in adhesion to a change in lateral electric field of in-plane two-nanometre dipole rings, because it cannot be explained by the change in surface roughness known from the Wenzel model. Although the change in adhesion can be calculated for the system we study, it is not yet possible to determine the stiction at such a solid–liquid interface using ab initio methods. The inorganic hybrid of hexagonal boron nitride and rhodium is very stable and represents a new class of switchable surfaces with the potential for application in the study of adhesion, friction and lubrication. DA - 2016/06/30/ PY - 2016 DO - 10.1038/nature18275 VL - 534 IS - 7609 SP - 676 EP - 679 J2 - Nature SN - 0028-0836 ER - TY - JOUR TI - Imaging single-molecule reaction intermediates stabilized by surface dissipation and entropy AU - Riss, Alexander AU - Paz, Alejandro Pérez AU - Wickenburg, Sebastian AU - Tsai, Hsin-Zon AU - De Oteyza, Dimas G. AU - Bradley, Aaron J. AU - Ugeda, Miguel M. AU - Gorman, Patrick AU - Jung, Han Sae AU - Crommie, Michael F. AU - Rubio, Angel AU - Fischer, Felix R. T2 - Nature Chemistry AB - Chemical transformations at the interface between solid/liquid or solid/gaseous phases of matter lie at the heart of key industrial-scale manufacturing processes. A comprehensive study of the molecular energetics and conformational dynamics that underlie these transformations is often limited to ensemble-averaging analytical techniques. Here we report the detailed investigation of a surface-catalysed cross-coupling and sequential cyclization cascade of 1,2-bis(2-ethynyl phenyl)ethyne on Ag(100). Using non-contact atomic force microscopy, we imaged the single-bond-resolved chemical structure of transient metastable intermediates. Theoretical simulations indicate that the kinetic stabilization of experimentally observable intermediates is determined not only by the potential-energy landscape, but also by selective energy dissipation to the substrate and entropic changes associated with key transformations along the reaction pathway. The microscopic insights gained here pave the way for the rational design and control of complex organic reactions at the surface of heterogeneous catalysts. DA - 2016/07// PY - 2016 DO - 10.1038/nchem.2506 VL - 8 SP - 678 EP - 683 J2 - Nat. Chem. SN - 1755-4330 ER - TY - JOUR TI - Magnetism of FePt surface alloys AU - Honolka, J. AU - Lee, T. Y. AU - Kuhnke, K. AU - Enders, A. AU - Skomski, R. AU - Bornemann, S. AU - Mankovsky, S. AU - Minar, J. AU - Staunton, J. AU - Ebert, H. AU - Hessler, M. AU - Fauth, K. AU - Schutz, G. AU - Buchsbaum, A. AU - Schmid, M. AU - Varga, P. AU - Kern, K. T2 - Physical Review Letters AB - The complex correlation of structure and magnetism in highly coercive monoatomic FePt surface alloys is studied using scanning tunneling microscopy, X-ray magnetic circular dichroism and ab-initio theory. Depending on the specific lateral atomic coordination of Fe either hard magnetic properties comparable to that of bulk FePt or complex non-collinear magnetism due to Dzyaloshinski-Moriya interactions are observed. Our calculations confirm the subtle dependence of the magnetic anisotropy and spin alignment on the local coordination and suggest that 3D stacking of Fe and Pt layers in bulk L1_0 magnets is not essential to achieve high anisotropy values. DA - 2009/02/13/ PY - 2009 DO - 10.1103/PhysRevLett.102.067207 VL - 102 IS - 6 SP - 067207 EP - 4 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - High island densities in pulsed laser deposition: Causes and implications AU - Schmid, M. AU - Lenauer, C. AU - Buchsbaum, A. AU - Wimmer, F. AU - Rauchbauer, G. AU - Scheiber, P. AU - Betz, G. AU - Varga, P. T2 - Physical Review Letters AB - By studying metal growth on Pt(111), we determine the reasons for the high island densities observed in pulsed laser deposition (PLD) compared to conventional thermal deposition. For homoepitaxy by PLD with moderate energies (≲100 eV) of the deposited ions, high island densities are caused by the high instantaneous flux of arriving particles. Additional nuclei are formed at high ion energies (≳200 eV) by adatoms created by the impinging ions. For heteroepitaxy, the island density is also increased by intermixing (deposited material implanted in the surface), creating an inhomogeneous potential energy surface for diffusing atoms. We discuss implications for layer-by-layer growth and sputter deposition. DA - 2009/08/10/ PY - 2009 DO - 10.1103/PhysRevLett.103.076101 VL - 103 IS - 7 SP - 076101 J2 - Phys. Rev. Lett. ST - High Island Densities in Pulsed Laser Deposition ER - TY - JOUR TI - Observation and destruction of an elusive adsorbate with STM: O2/TiO2(110) AU - Scheiber, Philipp AU - Riss, Alexander AU - Schmid, Michael AU - Varga, Peter AU - Diebold, Ulrike T2 - Physical Review Letters AB - When a slightly defective rutile TiO2(110) surface is exposed to O2 at elevated temperatures, the molecule dissociates at defects, filling O vacancies (VO) and creating O adatoms (Oad) on Ti5c rows. The adsorption of molecular O2 at low temperatures has remained controversial. Low-temperature scanning tunneling microscopy of O2, dosed on TiO2(110) at a sample temperature of ≈100 K and imaged at 17 K, shows a molecular precursor at VO as a faint change in contrast. The adsorbed O2 easily dissociates during the STM measurements, and the formation of Oad’s at both sides of the original VO is observed. DA - 2010/11/15/ PY - 2010 DO - 10.1103/PhysRevLett.105.216101 VL - 105 IS - 21 SP - 216101 J2 - Phys. Rev. Lett. ST - Observation and Destruction of an Elusive Adsorbate with STM ER - TY - JOUR TI - Room temperature water splitting at the surface of magnetite AU - Parkinson, Gareth S. AU - Novotný, Zbyněk AU - Jacobson, Peter AU - Schmid, Michael AU - Diebold, Ulrike T2 - Journal of the American Chemical Society AB - An array of surface science measurements has revealed novel water adsorption behavior at the Fe3O4(001) surface. Following room temperature exposure to water, a low coverage of hydrogen atoms is observed, with no associated water hydroxyl group. Mild annealing of the hydrogenated surface leads to desorption of water via abstraction of surface oxygen atoms, leading to a reduction of the surface. These results point to an irreversible splitting of the water molecule. The observed phenomena are discussed in the context of recent DFT calculations ( Mulakaluri, N. ; Pentcheva, R. ; Scheffler, M. J. Phys. Chem. C 2010, 114, 11148 ), which show that the Jahn–Teller distorted surface isolates adsorbed H in a geometry that could kinetically hinder recombinative desorption. In contrast, the adsorption geometry facilitates interaction between water hydroxyl species, which are concluded to leave the surface following a reactive desorption process, possibly via the creation of O2. DA - 2011/// PY - 2011 DO - 10.1021/ja203432e VL - 133 IS - 32 SP - 12650 EP - 12655 J2 - J. Am. Chem. Soc. ER - TY - JOUR TI - Unusual cluster shapes and directional bonding of an fcc metal: Pt/Pt(111) AU - Schmid, Michael AU - Garhofer, Andreas AU - Redinger, Josef AU - Wimmer, Florian AU - Scheiber, Philipp AU - Varga, Peter T2 - Physical Review Letters AB - Small clusters of Pt adatoms grown on Pt(111) exhibit a preference for the formation of linear chains, which cannot be explained by simple diffusion-limited aggregation. Density functional theory calculations show that short chains are energetically favorable to more compact configurations due to strong directional bonding by dz2-like orbitals, explaining the stability of the chains. The formation of the chains is governed by substrate distortions, leading to funneling towards the chain ends. DA - 2011/06/29/ PY - 2011 DO - 10.1103/PhysRevLett.107.016102 VL - 107 IS - 1 SP - 016102 J2 - Phys. Rev. Lett. ST - Unusual Cluster Shapes and Directional Bonding of an fcc Metal ER - TY - JOUR TI - Nickel carbide as a source of grain rotation in epitaxial graphene AU - Jacobson, Peter AU - Stöger, Bernhard AU - Garhofer, Andreas AU - Parkinson, Gareth S. AU - Schmid, Michael AU - Caudillo, Roman AU - Mittendorfer, Florian AU - Redinger, Josef AU - Diebold, Ulrike T2 - ACS Nano AB - Graphene has a close lattice match to the Ni(111) surface, resulting in a preference for 1 × 1 configurations. We have investigated graphene grown by chemical vapor deposition (CVD) on the nickel carbide (Ni2C) reconstruction of Ni(111) with scanning tunneling microscopy (STM). The presence of excess carbon, in the form of Ni2C, prevents graphene from adopting the preferred 1 × 1 configuration and leads to grain rotation. STM measurements show that residual Ni2C domains are present under rotated graphene. Nickel vacancy islands are observed at the periphery of rotated grains and indicate Ni2C dissolution after graphene growth. Density functional theory (DFT) calculations predict a very weak (van der Waals type) interaction of graphene with the underlying Ni2C, which should facilitate a phase separation of the carbide into metal-supported graphene. These results demonstrate that surface phases such as Ni2C can play a major role in the quality of epitaxial graphene. DA - 2012/04/24/ PY - 2012 DO - 10.1021/nn300625y VL - 6 IS - 4 SP - 3564 EP - 3572 J2 - ACS Nano SN - 1936-0851, 1936-086X ER - TY - JOUR TI - Ordered array of single adatoms with remarkable thermal stability: Au/Fe3O4(001) AU - Novotný, Zbyněk AU - Argentero, Giacomo AU - Wang, Zhiming AU - Schmid, Michael AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - Physical Review Letters AB - Gold deposited on the Fe3O4(001) surface at room temperature was studied using scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS). This surface forms a (√2×√2)R45° reconstruction, where pairs of Fe and neighboring O ions are slightly displaced laterally producing undulating rows with “narrow” and “wide” hollow sites. At low coverages, single Au adatoms adsorb exclusively at the narrow sites, with no significant sintering up to annealing temperatures of 400 °C. We propose the strong site preference to be related to charge and orbital ordering within the first subsurface layer of Fe3O4(001)-(√2×√2)R45°. Because of its high thermal stability, this could prove an ideal model system for probing the chemical reactivity of single atomic species. DA - 2012/05/24/ PY - 2012 DO - 10.1103/PhysRevLett.108.216103 VL - 108 IS - 21 SP - 216103 J2 - Phys. Rev. Lett. ST - Ordered Array of Single Adatoms with Remarkable Thermal Stability ER - TY - JOUR TI - (Sub)surface mobility of oxygen vacancies at the TiO2 anatase (101) surface AU - Scheiber, Philipp AU - Fidler, Martin AU - Dulub, Olga AU - Schmid, Michael AU - Diebold, Ulrike AU - Hou, Weiyi AU - Aschauer, Ulrich AU - Selloni, Annabella T2 - Physical Review Letters AB - Anatase is a metastable polymorph of TiO2. In contrast to the more widely studied TiO2 rutile, O vacancies (VO’s) are not stable at the anatase (101) surface. Low-temperature STM shows that surface VO’s, created by electron bombardment at 105 K, start migrating to subsurface sites at temperatures ≥200 K. After an initial decrease of the VO density, a temperature-dependent dynamic equilibrium is established where VO’s move to subsurface sites and back again, as seen in time-lapse STM images. We estimate that activation energies for subsurface migration lie between 0.6 and 1.2 eV; in comparison, density functional theory calculations predict a barrier of ca. 0.75 eV. The wide scatter of the experimental values might be attributed to inhomogeneously distributed subsurface defects in the reduced sample. DA - 2012/09/28/ PY - 2012 DO - 10.1103/PhysRevLett.109.136103 VL - 109 IS - 13 SP - 136103 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Reaction of O2 with subsurface oxygen vacancies on TiO2 anatase (101) AU - Setvín, Martin AU - Aschauer, Ulrich AU - Scheiber, Philipp AU - Li, Ye-Fei AU - Hou, Weiyi AU - Schmid, Michael AU - Selloni, Annabella AU - Diebold, Ulrike T2 - Science AB - Oxygen (O2) adsorbed on metal oxides is important in catalytic oxidation reactions, chemical sensing, and photocatalysis. Strong adsorption requires transfer of negative charge from oxygen vacancies (VOs) or dopants, for example. With scanning tunneling microscopy, we observed, transformed, and, in conjunction with theory, identified the nature of O2 molecules on the (101) surface of anatase (titanium oxide, TiO2) doped with niobium. VOs reside exclusively in the bulk, but we pull them to the surface with a strongly negatively charged scanning tunneling microscope tip. O2 adsorbed as superoxo (O2–) at fivefold-coordinated Ti sites was transformed to peroxo (O22–) and, via reaction with a VO, placed into an anion surface lattice site as an (O2)O species. This so-called bridging dimer also formed when O2 directly reacted with VOs at or below the surface. DA - 2013/08/30/ PY - 2013 DO - 10.1126/science.1239879 VL - 341 IS - 6149 SP - 988 EP - 991 J2 - Science SN - 0036-8075, 1095-9203 ER - TY - JOUR TI - The role of defects in the local reaction kinetics of CO oxidation on low-index Pd surfaces AU - Vogel, D. AU - Spiel, C. AU - Schmid, M. AU - Stöger-Pollach, M. AU - Schlögl, R. AU - Suchorski, Y. AU - Rupprechter, G. T2 - The Journal of Physical Chemistry C AB - The role of artificially created defects and steps in the local reaction kinetics of CO oxidation on the individual domains of a polycrystalline Pd foil was studied by photoemission electron microscopy (PEEM), mass spectroscopy (MS), and scanning tunneling microscopy (STM). The defects and steps were created by STM-controlled Ar+ sputtering and the novel PEEM-based approach allowed the simultaneous determination of local kinetic phase transitions on differently oriented ?m-sized grains of a polycrystalline sample. The independent (single-crystal-like) reaction behavior of the individual Pd(hkl) domains in the 10?5 mbar pressure range changes upon Ar+ sputtering to a correlated reaction behavior, and the reaction fronts propagate unhindered across the grain boundaries. The defect-rich surface shows also a significantly higher CO tolerance as reflected by the shift of both the global (MS-measured) and the local (PEEM-measured) kinetic diagrams toward higher CO pressure. DA - 2013/06/13/ PY - 2013 DO - 10.1021/jp312510d VL - 117 IS - 23 SP - 12054 EP - 12060 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - Water adsorption at the tetrahedral titania surface layer of SrTiO3(110)-(4 × 1) AU - Wang, Zhiming AU - Hao, Xianfeng AU - Gerhold, Stefan AU - Novotny, Zbynek AU - Franchini, Cesare AU - McDermott, Eamon AU - Schulte, Karina AU - Schmid, Michael AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry C AB - The interaction of water with oxide surfaces is of great interest for both fundamental science and applications. We present a combined theoretical (density functional theory (DFT)) and experimental (scanning tunneling microscopy (STM) and photoemission spectroscopy (PES)) study of water interaction with the two-dimensional titania overlayer that terminates the SrTiO3(110)-(4 x 1) surface and consists of TiO4 tetrahedra. STM and core-level and valence band PES show that H2O neither adsorbs nor dissociates on the stoichiometric surface at room temperature, whereas it does dissociate at oxygen vacancies. This is in agreement with DFT calculations, which show that the energy barriers for water dissociation on the stoichiometric and reduced surfaces are 1.7 and 0.9 eV, respectively. We propose that water weakly adsorbs on two-dimensional, tetrahedrally coordinated overlayers. DA - 2013/12/12/ PY - 2013 DO - 10.1021/jp407889h VL - 117 IS - 49 SP - 26060 EP - 26069 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - Metal adatoms and clusters on ultrathin zirconia films AU - Choi, Joong Il Jake AU - Mayr-Schmölzer, Wernfried AU - Valenti, Ilaria AU - Luches, Paola AU - Mittendorfer, Florian AU - Redinger, Josef AU - Diebold, Ulrike AU - Schmid, Michael T2 - The Journal of Physical Chemistry C AB - Nucleation and growth of transition metals on zirconia has been studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Since STM requires electrical conductivity, ultrathin ZrO2 films grown by oxidation of Pt3Zr(0001) and Pd3Zr(0001) were used as model systems. DFT studies were performed for single metal adatoms on supported ZrO2 films as well as the (1̅11) surface of monoclinic ZrO2. STM shows decreasing cluster size, indicative of increasing metal–oxide interaction, in the sequence Ag < Pd ≈ Au < Ni ≈ Fe. Ag and Pd nucleate mostly at steps and domain boundaries of ZrO2/Pt3Zr(0001) and form three-dimensional clusters. Deposition of low coverages of Ni and Fe at room temperature leads to a high density of few-atom clusters on the oxide terraces. Weak bonding of Ag to the oxide is demonstrated by removing Ag clusters with the STM tip. DFT calculations for single adatoms show that the metal–oxide interaction strength increases in the sequence Ag < Au < Pd < Ni on monoclinic ZrO2, and Ag ≈ Au < Pd < Ni on the supported ultrathin ZrO2 film. With the exception of Au, metal nucleation and growth on ultrathin zirconia films follow the usual rules: More reactive (more electropositive) metals result in a higher cluster density and wet the surface more strongly than more noble metals. These bind mainly to the oxygen anions of the oxide. Au is an exception because it can bind strongly to the Zr cations. Au diffusion may be impeded by changing its charge state between −1 and +1. We discuss differences between the supported ultrathin zirconia films and the surfaces of bulk ZrO2, such as the possibility of charge transfer to the substrate of the films. Due to their large in-plane lattice constant and the variety of adsorption sites, ZrO2{111} surfaces are more reactive than many other oxygen-terminated oxide surfaces. DA - 2016/05/12/ PY - 2016 DO - 10.1021/acs.jpcc.6b03061 VL - 120 IS - 18 SP - 9920 EP - 9932 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - Direct view at excess electrons in TiO2 rutile and anatase AU - Setvin, Martin AU - Franchini, Cesare AU - Hao, Xianfeng AU - Schmid, Michael AU - Janotti, Anderson AU - Kaltak, Merzuk AU - Van de Walle, Chris G. AU - Kresse, Georg AU - Diebold, Ulrike T2 - Physical Review Letters AB - A combination of scanning tunneling microscopy and spectroscopy and density functional theory is used to characterize excess electrons in TiO2 rutile and anatase, two prototypical materials with identical chemical composition but different crystal lattices. In rutile, excess electrons can localize at any lattice Ti atom, forming a small polaron, which can easily hop to neighboring sites. In contrast, electrons in anatase prefer a free-carrier state, and can only be trapped near oxygen vacancies or form shallow donor states bound to Nb dopants. The present study conclusively explains the differences between the two polymorphs and indicates that even small structural variations in the crystal lattice can lead to a very different behavior. DA - 2014/08/18/ PY - 2014 DO - 10.1103/PhysRevLett.113.086402 VL - 113 IS - 8 SP - 086402 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Charge trapping at the step edges of TiO2 anatase (101) AU - Setvin, Martin AU - Hao, Xianfeng AU - Daniel, Benjamin AU - Pavelec, Jiri AU - Novotny, Zbynek AU - Parkinson, Gareth S. AU - Schmid, Michael AU - Kresse, Georg AU - Franchini, Cesare AU - Diebold, Ulrike T2 - Angewandte Chemie International Edition AB - A combination of photoemission, atomic force, and scanning tunneling microscopy/spectroscopy measurements shows that excess electrons in the TiO2 anatase (101) surface are trapped at step edges. Consequently, steps act as preferred adsorption sites for O2. In density functional theory calculations electrons localize at clean step edges, this tendency is enhanced by O vacancies and hydroxylation. The results show the importance of defects for the wide-ranging applications of titania. DA - 2014/// PY - 2014 DO - 10.1002/anie.201309796 VL - 53 IS - 18 SP - 4714 EP - 4716 J2 - Angew. Chem. Int. Ed. SN - 1521-3773 ER - TY - JOUR TI - High chemical activity of a perovskite surface: reaction of CO with Sr3Ru2O7 AU - Stöger, Bernhard AU - Hieckel, Marcel AU - Mittendorfer, Florian AU - Wang, Zhiming AU - Fobes, David AU - Peng, Jin AU - Mao, Zhiqiang AU - Schmid, Michael AU - Redinger, Josef AU - Diebold, Ulrike T2 - Physical Review Letters AB - Adsorption of CO at the Sr3Ru2O7(001) surface was studied with low-temperature scanning tunneling microscopy (STM) and density functional theory. In situ cleaved single crystals terminate in an almost perfect SrO surface. At 78 K, CO first populates impurities and then adsorbs above the apical surface O with a binding energy Eads=−0.7 eV. Above 100 K, this physisorbed CO replaces the surface O, forming a bent CO2 with the C end bound to the Ru underneath. The resulting metal carboxylate (Ru-COO) can be desorbed by STM manipulation. A low activation (0.2 eV) and high binding (−2.2 eV) energy confirm a strong reaction between CO and regular surface sites of Sr3Ru2O7; likely, this reaction causes the “UHV aging effect” reported for this and other perovskite oxides. DA - 2014/09/10/ PY - 2014 DO - 10.1103/PhysRevLett.113.116101 VL - 113 IS - 11 SP - 116101 J2 - Phys. Rev. Lett. ST - High Chemical Activity of a Perovskite Surface ER - TY - JOUR TI - Stabilizing single Ni adatoms on a two-dimensional porous titania overlayer at the SrTiO3(110) surface AU - Wang, Zhiming AU - Hao, Xianfeng AU - Gerhold, Stefan AU - Mares, Petr AU - Wagner, Margareta AU - Bliem, Roland AU - Schulte, Karina AU - Schmid, Michael AU - Franchini, Cesare AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry C AB - Nickel vapor-deposited on the SrTiO3(110) surface was studied using scanning tunneling microscopy, photoemission spectroscopy (PES), and density functional theory calculations. This surface forms a (4 ? 1) reconstruction, composed of a 2-D titania structure with periodic six- and ten-membered nanopores. Anchored at these nanopores, Ni single adatoms are stabilized at room temperature. PES measurements show that the Ni adatoms create an in-gap state located at 1.9 eV below the conduction band minimum and induce an upward band bending. Both experimental and theoretical results suggest that Ni adatoms are positively charged. Our study produces well-dispersed single-adatom arrays on a well-characterized oxide support, providing a model system to investigate single-adatom catalytic and magnetic properties. DA - 2014/08/28/ PY - 2014 DO - 10.1021/jp506234r VL - 118 IS - 34 SP - 19904 EP - 19909 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - Reducing the In2O3(111) surface results in ordered indium adatoms AU - Wagner, Margareta AU - Seiler, Steffen AU - Meyer, Bernd AU - Boatner, Lynn A. AU - Schmid, Michael AU - Diebold, Ulrike T2 - Advanced Materials Interfaces AB - When the In2O3 surface is subjected to reducing conditions it responds with the formation of an ordered array of isolated In adatoms. The facile back-and-forth between the oxidized and reduced surface should allow tuning surface properties in a judicious and clear-cut manner. On a more fundamental level, the ordered adatom structure is an entirely novel way of any oxide surface to respond to reducing conditions. DA - 2014/11/01/ PY - 2014 DO - 10.1002/admi.201400289 VL - 1 IS - 8 SP - 1400289 J2 - Adv. Mater. Interfaces SN - 2196-7350 ER - TY - JOUR TI - Interplay between steps and oxygen vacancies on curved TiO2(110) AU - Miccio, Luis A. AU - Setvin, Martin AU - Müller, Moritz AU - Abadía, Mikel AU - Piquero, Ignacio AU - Lobo-Checa, Jorge AU - Schiller, Frederik AU - Rogero, Celia AU - Schmid, Michael AU - Sánchez-Portal, Daniel AU - Diebold, Ulrike AU - Ortega, J. Enrique T2 - Nano Letters AB - A vicinal rutile TiO2(110) crystal with a smooth variation of atomic steps parallel to the [1?10] direction was analyzed locally with STM and ARPES. The step edge morphology changes across the samples, from [1?11] zigzag faceting to straight [1?10] steps. A step-bunching phase is attributed to an optimal (110) terrace width, where all bridge-bonded O atom vacancies (Obr vacs) vanish. The [1?10] steps terminate with a pair of 2-fold coordinated O atoms, which give rise to bright, triangular protrusions (St) in STM. The intensity of the Ti 3d-derived gap state correlates with the sum of Obr vacs plus St protrusions at steps, suggesting that both Obr vacs and steps contribute a similar effective charge to sample doping. The binding energy of the gap state shifts when going from the flat (110) surface toward densely stepped planes, pointing to differences in the Ti3+ polaron near steps and at terraces. DA - 2016/03/09/ PY - 2016 DO - 10.1021/acs.nanolett.5b05286 VL - 16 IS - 3 SP - 2017 EP - 2022 J2 - Nano Lett. SN - 1530-6984 ER - TY - JOUR TI - Transition from reconstruction toward thin film on the (110) surface of strontium titanate AU - Wang, Z. AU - Loon, A. AU - Subramanian, A. AU - Gerhold, S. AU - McDermott, E. AU - Enterkin, J. A. AU - Hieckel, M. AU - Russell, B. C. AU - Green, R. J. AU - Moewes, A. AU - Guo, J. AU - Blaha, P. AU - Castell, M. R. AU - Diebold, U. AU - Marks, L. D. T2 - Nano Letters AB - The surfaces of metal oxides often are reconstructed with a geometry and composition that is considerably different from a simple termination of the bulk. Such structures can also be viewed as ultrathin films, epitaxed on a substrate. Here, the reconstructions of the SrTiO3 (110) surface are studied combining scanning tunneling microscopy (STM), transmission electron diffraction, and X-ray absorption spectroscopy (XAS), and analyzed with density functional theory calculations. Whereas SrTiO3 (110) invariably terminates with an overlayer of titania, with increasing density its structure switches from n ? 1 to 2 ? n. At the same time the coordination of the Ti atoms changes from a network of corner-sharing tetrahedra to a double layer of edge-shared octahedra with bridging units of octahedrally coordinated strontium. This transition from the n ? 1 to 2 ? n reconstructions is a transition from a pseudomorphically stabilized tetrahedral network toward an octahedral titania thin film with stress-relief from octahedral strontia units at the surface. DA - 2016/04/13/ PY - 2016 DO - 10.1021/acs.nanolett.5b05211 VL - 16 IS - 4 SP - 2407 EP - 2412 J2 - Nano Lett. SN - 1530-6984 ER - TY - JOUR TI - NO adsorption and diffusion on hydroxylated rutile TiO2(110) AU - Yu, Yan-Yan AU - Diebold, Ulrike AU - Gong, Xue-Qing T2 - Physical Chemistry Chemical Physics AB - We report a computational study of NO adsorption and diffusion on the hydroxylated rutile TiO2(110) surface performed with density functional theory (DFT) calculations corrected by on-site Coulomb corrections and long-range dispersion interactions. NO prefers to adsorb with its N-end down at surface Ti5c sites. The excess electron that is located at a subsurface site for the hydroxylated surface localizes in the 2π* orbital of the adsorbed NO. A novel ‘roll-over’ diffusion scheme is proposed that involves three neighboring Ti5c atoms and one surface hydroxyl, with an O-end down NO at the middle Ti5c as the intermediate state. During the migration, NO can also form bridging species between two Ti5c atoms. The calculated scanning tunneling microscopy (STM) features with the “bright-dark-bright” configuration corresponding to diffusing NO at different positions are consistent with the experimental STM results. DA - 2015/10/07/ PY - 2015 DO - 10.1039/C5CP04584C VL - 17 IS - 40 SP - 26594 EP - 26598 J2 - Phys. Chem. Chem. Phys. SN - 1463-9084 ER - TY - JOUR TI - Adsorption and incorporation of transition metals at the magnetite Fe3O4(001) surface AU - Bliem, Roland AU - Pavelec, Jiri AU - Gamba, Oscar AU - McDermott, Eamon AU - Wang, Zhiming AU - Gerhold, Stefan AU - Wagner, Margareta AU - Osiecki, Jacek AU - Schulte, Karina AU - Schmid, Michael AU - Blaha, Peter AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - Physical Review B AB - The adsorption of Ni, Co, Mn, Ti, and Zr at the (√2×√2)R45°-reconstructed Fe3O4(001) surface was studied by scanning tunneling microscopy, x-ray and ultraviolet photoelectron spectroscopy, low-energy electron diffraction (LEED), and density functional theory (DFT). Following deposition at room temperature, metals are either adsorbed as isolated adatoms or fill the subsurface cation vacancy sites responsible for the (√2×√2)R45° reconstruction. Both configurations coexist, but the ratio of adatoms to incorporated atoms depends on the metal; Ni prefers the adatom configuration, Co and Mn form adatoms and incorporated atoms in similar numbers, and Ti and Zr are almost fully incorporated. With mild annealing, all adatoms transition to the incorporated cation configuration. At high coverage, the (√2×√2)R45° reconstruction is lifted because all subsurface cation vacancies become occupied with metal atoms, and a (1×1) LEED pattern is observed. DFT+U calculations for the extreme cases, Ni and Ti, confirm the energetic preference for incorporation, with calculated oxidation states in good agreement with photoemission experiments. Because the site preference is analogous to bulk ferrite (XFe2O4) compounds, similar behavior is likely to be typical for elements forming a solid solution with Fe3O4. DA - 2015/08/26/ PY - 2015 DO - 10.1103/PhysRevB.92.075440 VL - 92 IS - 7 SP - 075440 J2 - Phys. Rev. B ER - TY - CHAP TI - The growth and structure of ultrathin metal films on TiO2(110) AU - Madey, T. E. AU - Diebold, U. AU - Pan, J.-M. T2 - Adsorption on Ordered Surfaces on Ionic Solids and Thin Films A2 - Umbach, E. A2 - Freund, H.-J. T3 - Springer Series in Surface Sciences AB - We have studied the growth kinetics and structure of 3d transition metals (Cr, Fe, Cu) evaporated onto TiO2(110), a model oxide substrate. Using Low Energy Ion Scattering (LEIS) and X-ray Photoelectron Spectroscopy (XPS), we find that Cu grows in the Volmer-Weber (3-D island) mode. The mobility of Cu atoms is very high, and 3-dimensional clusters are formed even at 160 K for Cu thicknesses < 30 A. In contrast, the Cr and Fe films completely cover the Ti~ substrate for thicknesses of a few ML, indicating that Cr and Fe "wet" the surface better than Cu. The ability of the metal film to cover (wet) the oxide substrate correlates directly with the reactivity of the metal toward oxygen. There is no XPS evidence for a strong chemical interaction between the Cu film and the oxide substrate. However, for the Cr and Fe films, we observe a strong interaction between the films and the oxide substrate, i.e., oxidation of the metal overlayers and reduction of the substrate, at submonolayer thickness. A bcc(lOO) structure is observed for Cr and Fe films with thickness ~ 10 A, on both stoichiometric and sputtered substrates using Angle-Resolved XPS (ARXPS), Medium Energy backscattered Electron Diffraction (MEED) and LEED. The overlayer structure orientation is related to the substrate local structure. CY - Berlin DA - 1993/// PY - 1993 VL - 33 SP - 147 EP - 155 PB - Springer SN - 978-3-540-56473-7 ER - TY - CHAP TI - Electron stimulated desorption (ESD) of ammonia on TiO2(110): The influence of substrate defect structure AU - Diebold, U. AU - Madey, T. E. T2 - Desorption Induced by Electronic Transitions DIET V A2 - Burns, A. R. A2 - Stechel, E. B. A2 - Jennison, D. R. T3 - Springer Series in Surface Sciences AB - Electron stimulated processes for NH3 adsorbed on rutile TiO2(11O) have been studied by means of XPS and mass-resolved ESDIAD (electron stimulated desorption angular ion distribution). We have used three differently prepared TiO2 surfaces to study the influence of the substrate defect structure on the interaction with NH3: a stoichiometric nearly perfect surface, a high-temperature annealed slightly oxygen deficient surface and a sputtered highly oxygen deficient surface. In the limit of a stoichiometric surface, electron irradiation induces the desorption of NH3 molecules. When the highly oxygen deficient TiO2 surface is used as substrate, both desorption and electron stimulated dissociation of NH3 take place with atomic nitrogen the final product of the dissociation process. These measurements provide direct evidence for the role of surface defects of an oxide substrate in electron stimulated reaction pathways. CY - Berlin DA - 1993/// PY - 1993 VL - 31 SP - 284 EP - 288 PB - Springer SN - 978-3-540-56473-7 ER - TY - CHAP TI - Specimen Treatment: Preparation of Metal Compound Materials (Mainly Oxides) AU - Diebold, Ulrike T2 - Specimen handling, preparation, and treatments in surface characterization A2 - Czanderna, Alvin Warren A2 - Powell, Cedric John A2 - Madey, Theodore E. T3 - Methods of Surface Characterization DA - 1998/// PY - 1998 VL - 4 SP - 145 EP - 172 PB - Springer SN - 0-306-45887-X 978-0-306-45887-3 ER - TY - JOUR TI - Aggregation and electronically induced migration of oxygen vacancies in TiO2 anatase AU - Setvin, Martin AU - Schmid, Michael AU - Diebold, Ulrike T2 - Physical Review B AB - The influence of the electric field and electric current on the behavior of oxygen vacancies (VOs) in TiO2 anatase was investigated with scanning tunneling microscopy (STM). At the anatase (101) surface VOs are not stable; they migrate into the bulk at temperatures above 200 K. Scanning a clean anatase (101) surface at a sample bias greater than ≈+4.3 V results in surface VOs in the scanned area, suggesting that subsurface VOs migrate back to the surface. To test this hypothesis, surface VOs were first created through bombardment with energetic electrons. The sample was then mildly annealed, which caused the VOs to move to the subsurface region, where they formed vacancy clusters. These VO clusters have various, distinct shapes. Scanning VO clusters with a high STM bias reproducibly converts them back into groupings of surface VO, with a configuration that is characteristic for each type of cluster. The dependence of the subsurface-to-surface VO migration on the applied STM bias voltage, tunneling current, and sample temperature was investigated systematically. The results point towards a key role of energetic, “hot” electrons in this process. The findings are closely related to the memristive behavior of oxides and oxygen diffusion in solid-oxide membranes. DA - 2015/05/06/ PY - 2015 DO - 10.1103/PhysRevB.91.195403 VL - 91 IS - 19 SP - 195403 J2 - Phys. Rev. B ER - TY - JOUR TI - In situ scanning tunneling microscopy study of Ca-modified rutile TiO2(110) in bulk water AU - Serrano, Giulia AU - Bonanni, Beatrice AU - Kosmala, Tomasz AU - Di Giovannantonio, Marco AU - Diebold, Ulrike AU - Wandelt, Klaus AU - Goletti, Claudio T2 - Beilstein Journal of Nanotechnology AB - Despite the rising technological interest in the use of calcium-modified TiO2 surfaces in biomedical implants, the Ca/TiO2 interface has not been studied in an aqueous environment. This investigation is the first report on the use of in situ scanning tunneling microscopy (STM) to study calcium-modified rutile TiO2(110) surfaces immersed in high purity water. The TiO2 surface was prepared under ultrahigh vacuum (UHV) with repeated sputtering/annealing cycles. Low energy electron diffraction (LEED) analysis shows a pattern typical for the surface segregation of calcium, which is present as an impurity on the TiO2 bulk. In situ STM images of the surface in bulk water exhibit one-dimensional rows of segregated calcium regularly aligned with the [001] crystal direction. The in situ-characterized morphology and structure of this Ca-modified TiO2 surface are discussed and compared with UHV-STM results from the literature. Prolonged immersion (two days) in the liquid leads to degradation of the overlayer, resulting in a disordered surface. X-ray photoelectron spectroscopy, performed after immersion in water, confirms the presence of calcium. DA - 2015/02/12/ PY - 2015 DO - 10.3762/bjnano.6.44 VL - 6 SP - 438 EP - 443 J2 - Beilstein J. Nanotechnol. SN - 2190-4286 ER - TY - JOUR TI - Coexistence of trapped and free excess electrons in SrTiO3 AU - Hao, Xianfeng AU - Wang, Zhiming AU - Schmid, Michael AU - Diebold, Ulrike AU - Franchini, Cesare T2 - Physical Review B AB - The question whether excess electrons in SrTiO3 form free or trapped carriers is a crucial aspect for the electronic properties of this important material. This fundamental ambiguity prevents a consistent interpretation of the puzzling experimental situation, where results support one or the other scenario depending on the type of experiment that is conducted. Using density functional theory with an on-site Coulomb interaction U, we show that excess electrons form small polarons if the density of electronic carriers is higher than ≈10^20 cm^-3. Below this value, the electrons stay delocalized or become large polarons. For oxygen-deficient SrTiO3, small polarons confined to Ti3+ sites are immobile at low temperature but can be thermally activated into a conductive state, which explains the metal-insulator transition observed experimentally. DA - 2015/02/12/ PY - 2015 DO - 10.1103/PhysRevB.91.085204 VL - 91 IS - 8 SP - 085204 J2 - Phys. Rev. B ER - TY - JOUR TI - Electron transfer between anatase TiO2 and an O2 molecule directly observed by atomic force microscopy AU - Setvin, Martin AU - Hulva, Jan AU - Parkinson, Gareth S. AU - Schmid, Michael AU - Diebold, Ulrike T2 - Proceedings of the National Academy of Sciences AB - Activation of molecular oxygen is a key step in converting fuels into energy, but there is precious little experimental insight into how the process proceeds at the atomic scale. Here, we show that a combined atomic force microscopy/scanning tunneling microscopy (AFM/STM) experiment can both distinguish neutral O2 molecules in the triplet state from negatively charged (O2)− radicals and charge and discharge the molecules at will. By measuring the chemical forces above the different species adsorbed on an anatase TiO2 surface, we show that the tip-generated (O2)− radicals are identical to those created when (i) an O2 molecule accepts an electron from a near-surface dopant or (ii) when a photo-generated electron is transferred following irradiation of the anatase sample with UV light. Kelvin probe spectroscopy measurements indicate that electron transfer between the TiO2 and the adsorbed molecules is governed by competition between electron affinity of the physisorbed (triplet) O2 and band bending induced by the (O2)− radicals. Temperature–programmed desorption and X-ray photoelectron spectroscopy data provide information about thermal stability of the species, and confirm the chemical identification inferred from AFM/STM. DA - 2017/03/28/ PY - 2017 DO - 10.1073/pnas.1618723114 VL - 114 IS - 13 SP - E2556 EP - E2562 J2 - Proc. Natl. Acad. Sci. USA SN - 0027-8424, 1091-6490 ER - TY - JOUR TI - Physical-chemical stability of fluorinated III-N surfaces: Towards the understanding of the (0001) AlxGa1-xN surface donor modification by fluorination AU - Reiner, Maria AU - Schellander, Josef AU - Denifl, Günter AU - Stadtmueller, Michael AU - Schmid, Michael AU - Frischmuth, Tobias AU - Schmid, Ulrich AU - Pietschnig, Rudolf AU - Ostermaier, Clemens T2 - Journal of Applied Physics AB - Gallium nitride based high electron mobility transistors are widely known for their operational instabilities regarding interface defects to the dielectric. In this paper, we discuss a III-N surface treatment that results in an electrically more defined interface and hence a narrower distribution of electrically present interface states compared to the original, untreated interface. This surface modification is caused by a remote plasma fluorination of the III-N surface. We show that it is a very distinctive surface processing which cannot be reproduced by other plasma techniques or ion implantation. Applying physical and chemical analyses, the fluorination is found to have a remarkable stability towards temperatures up to 700 °C and is also stable in air for up to 180 h. However, an aqueous clean allows the surface to return to its original state. Even though the exact physical origin of the responsible surface donor cannot be inferred, we suggest that fluorine itself might not directly represent the new surface donor but that it rather activates the III-N surface prior to the dielectric deposition or even substitutes and hence reduces the concentration of surface hydroxides. DA - 2017/06/09/ PY - 2017 DO - 10.1063/1.4985345 VL - 121 IS - 22 SP - 225704 J2 - J. Appl. Phys. SN - 0021-8979 ST - Physical-chemical stability of fluorinated III-N surfaces ER - TY - JOUR TI - Atomic structure and stability of magnetite Fe3O4(001): An X-ray view AU - Arndt, Björn AU - Bliem, Roland AU - Gamba, Oscar AU - van der Hoeven, Jessi E. S. AU - Noei, Heshmat AU - Diebold, Ulrike AU - Parkinson, Gareth S. AU - Stierle, Andreas T2 - Surface Science AB - The structure of the Fe3O4(001) surface was studied using surface X-ray diffraction in both ultra-high vacuum, and higher-pressure environments relevant to water–gas shift catalysis. The experimental X-ray structure factors from the 2 x 2 R 4 5 ∘ reconstructed surface are found to be in excellent agreement with the recently proposed subsurface cation vacancy (SCV) model for this surface (Science 346 (2014), 1215). Further refinement of the structure results in small displacements of the iron atoms in the first three double layers compared to structural parameters deduced from LEED I–V experiments and DFT calculations. An alternative, previously proposed structure, based on a distorted bulk truncation (DBT), is conclusively ruled out. The lifting of the 2 × 2 R 4 5 ∘ reconstruction upon exposure to water vapor in the mbar pressure regime was studied at different temperatures under flow conditions, and a roughening of the surface was observed. Addition of CO flow did not further change the roughness perpendicular to the surface but decreased the lateral correlations. DA - 2016/11// PY - 2016 DO - 10.1016/j.susc.2016.06.002 VL - 653 SP - 76 EP - 81 J2 - Surf. Sci. SN - 0039-6028 ST - Atomic structure and stability of magnetite Fe3O4(001) ER - TY - JOUR TI - Co on Fe3O4(001): Towards precise control of surface properties AU - Gargallo-Caballero, Raquel AU - Martín-García, Laura AU - Quesada, Adrián AU - Granados-Miralles, Cecilia AU - Foerster, Michael AU - Aballe, Lucía AU - Bliem, Roland AU - Parkinson, Gareth S. AU - Blaha, Peter AU - Marco, José F. AU - de la Figuera, Juan T2 - The Journal of Chemical Physics AB - A novel approach to incorporate cobalt atoms into a magnetite single crystal is demonstrated by a combination of x-ray spectro-microscopy, low-energy electron diffraction, and density-functional theory calculations. Co is deposited at room temperature on the reconstructed magnetite (001) surface filling first the subsurface octahedral vacancies and then occupying adatom sites on the surface. Progressive annealing treatments at temperatures up to 733 K diffuse the Co atoms into deeper crystal positions, mainly into octahedral ones with a marked inversion level. The oxidation state, coordination, and magnetic moments of the cobalt atoms are followed from their adsorption to their final incorporation into the bulk, mostly as octahedral Co2+. This precise control of the near-surface Co atoms location opens up the way to accurately tune the surface physical and magnetic properties of mixed spinel oxides. DA - 2016/03/02/ PY - 2016 DO - 10.1063/1.4942662 VL - 144 IS - 9 SP - 094704 J2 - J. Chem. Phys. SN - 0021-9606 ST - Co on Fe3O4(001) ER - TY - JOUR TI - Surface point defects on bulk oxides: atomically-resolved scanning probe microscopy AU - Setvín, Martin AU - Wagner, Margareta AU - Schmid, Michael AU - Parkinson, Gareth S. AU - Diebold, Ulrike T2 - Chemical Society Reviews AB - Metal oxides are abundant in nature and they are some of the most versatile materials for applications ranging from catalysis to novel electronics. The physical and chemical properties of metal oxides are dramatically influenced, and can be judiciously tailored, by defects. Small changes in stoichiometry introduce so-called intrinsic defects, e.g., atomic vacancies and/or interstitials. This review gives an overview of using Scanning Probe Microscopy (SPM), in particular Scanning Tunneling Microscopy (STM), to study the changes in the local geometric and electronic structure related to these intrinsic point defects at the surfaces of metal oxides. Three prototypical systems are discussed: titanium dioxide (TiO2), iron oxides (Fe3O4), and, as an example for a post-transition-metal oxide, indium oxide (In2O3). Each of these three materials prefers a different type of surface point defect: oxygen vacancies, cation vacancies, and cation adatoms, respectively. The different modes of STM imaging and the promising capabilities of non-contact Atomic Force Microscopy (nc-AFM) techniques are discussed, as well as the capability of STM to manipulate single point defects. DA - 2017/04/03/ PY - 2017 DO - 10.1039/C7CS00076F VL - 46 IS - 7 SP - 1772 EP - 1784 J2 - Chem. Soc. Rev. SN - 1460-4744 ST - Surface point defects on bulk oxides ER - TY - JOUR TI - Well-ordered In adatoms at the In2O3(111) surface created by Fe deposition AU - Wagner, Margareta AU - Lackner, Peter AU - Seiler, Steffen AU - Gerhold, Stefan AU - Osiecki, Jacek AU - Schulte, Karina AU - Boatner, Lynn A. AU - Schmid, Michael AU - Meyer, Bernd AU - Diebold, Ulrike T2 - Physical Review Letters AB - Metal deposition on oxide surfaces usually results in adatoms, clusters, or islands of the deposited material, where defects in the surface often act as nucleation centers. Here an alternate configuration is reported. After the vapor deposition of Fe on the In2O3(111) surface at room temperature, ordered adatoms are observed with scanning tunneling microscopy. These are identical to the In adatoms that form when the sample is reduced by heating in ultrahigh vacuum. Density functional theory calculations confirm that Fe interchanges with In in the topmost layer, pushing the excess In atoms to the surface where they arrange as a well-ordered adatom array. DA - 2016/11/11/ PY - 2016 DO - 10.1103/PhysRevLett.117.206101 VL - 117 IS - 20 SP - 206101 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Oxygen-stabilized Rh adatoms: 0D oxides on a vicinal surface AU - Mittendorfer, Florian AU - Franz, Thomas AU - Klikovits, Jan AU - Schmid, Michael AU - Merte, Lindsay R. AU - Shah Zaman, Sameena AU - Varga, Peter AU - Westerström, Rasmus AU - Resta, Andrea AU - Andersen, Jesper N. AU - Gustafson, Johan AU - Lundgren, Edvin T2 - The Journal of Physical Chemistry Letters AB - We have investigated the initial oxidation of the Rh(113) and Rh(223) vicinal surfaces by STM and ab initio simulations. Upon adsorption of small amounts of oxygen, the surface morphology is completely altered. Surprisingly, oxygen-stabilized Rh adatoms can be observed on the (113) facets, with oxide-like electronic properties. We present models of these "0D oxide" phases and discuss reasons for their stability. We have investigated the initial oxidation of the Rh(113) and Rh(223) vicinal surfaces by STM and ab initio simulations. Upon adsorption of small amounts of oxygen, the surface morphology is completely altered. Surprisingly, oxygen-stabilized Rh adatoms can be observed on the (113) facets, with oxide-like electronic properties. We present models of these "0D oxide" phases and discuss reasons for their stability. DA - 2011/10/19/ PY - 2011 DO - 10.1021/jz2011308 VL - 2 IS - 21 SP - 2747 EP - 2751 J2 - J. Phys. Chem. Lett. SN - 1948-7185 ST - Oxygen-Stabilized Rh Adatoms ER - TY - JOUR TI - Strain-induced defect superstructure on the SrTiO3(110) surface AU - Wang, Zhiming AU - Li, Fengmiao AU - Meng, Sheng AU - Zhang, Jiandi AU - Plummer, E. W. AU - Diebold, Ulrike AU - Guo, Jiandong T2 - Physical Review Letters AB - We report on a combined scanning tunneling microscopy and density functional theory calculation study of the SrTiO3(110)-(4×1) surface. It is found that antiphase domains are formed along the [11̅ 0]-oriented stripes on the surface. The domain boundaries are decorated by defect pairs consisting of Ti2O3 vacancies and Sr adatoms, which relieve the residual stress. The formation energy of and interactions between vacancies result in a defect superstructure. It is suggested that the density and distributions of defects can be tuned by strain engineering, providing a flexible platform for the designed growth of complex oxide materials. DA - 2013/08/02/ PY - 2013 DO - 10.1103/PhysRevLett.111.056101 VL - 111 IS - 5 SP - 056101 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Area-selective passivation of sp2 carbon surfaces by supramolecular self-assembly AU - Li, Zhi AU - Gorp, Hans Van AU - Walke, Peter AU - Phan, Thanh Hai AU - Fujita, Yasuhiko AU - Greenwood, John AU - Ivasenko, Oleksandr AU - Tahara, Kazukuni AU - Tobe, Yoshito AU - Uji-i, Hiroshi AU - Mertens, Stijn F. L. AU - Feyter, Steven De T2 - Nanoscale AB - Altering the chemical reactivity of graphene can offer new opportunities for various applications. Here, we report that monolayers of densely packed n-pentacontane significantly reduce the covalent grafting of aryl radicals to graphitic surfaces. The effect is highly local in nature and on fully covered substrates grafting can occur only at monolayer imperfections such as interdomain borders and vacancy defects. Grafting partially covered substrates primarily results in the covalent modification of uncoated areas. DA - 2017/04/20/ PY - 2017 DO - 10.1039/C7NR00022G VL - 9 IS - 16 SP - 5188 EP - 5193 J2 - Nanoscale SN - 2040-3372 ER - TY - JOUR TI - Nanoconfined self-assembly on a grafted graphitic surface under electrochemical control AU - Huynh, Thi Mien Trung AU - Phan, Thanh Hai AU - Ivasenko, Oleksandr AU - Mertens, Stijn F. L. AU - Feyter, Steven De T2 - Nanoscale AB - Highly oriented pyrolytic graphite (HOPG) can be covalently grafted with aryl radicals generated via the electrochemical reduction of 3,5-bis-tert-butyl-diazonium cations (3,5-TBD). The structure of the grafted layer and its stability under electrochemical conditions were assessed with electrochemical scanning tunneling microscopy (EC-STM) and cyclic voltammetry (CV). Stable within a wide (>2.5 V) electrochemical window, the grafted species can be locally removed using EC-STM-tip nanolithography. Using dibenzyl viologen as an example, we show that the generated nanocorrals of bare graphitic surface can be used to study nucleation and growth of self-assembled structures under conditions of nanoconfinement and electrochemical potential control. DA - 2016/12/22/ PY - 2016 DO - 10.1039/C6NR07519C VL - 9 IS - 1 SP - 362 EP - 368 J2 - Nanoscale SN - 2040-3372 ER - TY - JOUR TI - Cluster nucleation and growth from a highly supersaturated adatom phase: silver on magnetite AU - Bliem, Roland AU - Kosak, Rukan AU - Perneczky, Lukas AU - Novotny, Zbynek AU - Gamba, Oscar AU - Fobes, David AU - Mao, Zhiqiang AU - Schmid, Michael AU - Blaha, Peter AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - ACS Nano AB - The atomic-scale mechanisms underlying the growth of Ag on the (√2x√2)R45°-Fe3O4(001) surface were studied using scanning tunneling microscopy and density functional theory based calculations. For coverages up to 0.5 ML, Ag adatoms populate the surface exclusively; agglomeration into nanoparticles occurs only with the lifting of the reconstruction at 720 K. Above 0.5 ML, Ag clusters nucleate spontaneously and grow at the expense of the surrounding material with mild annealing. This unusual behavior results from a kinetic barrier associated with the (√2x√2)R45° reconstruction, which prevents adatoms from transitioning to the thermodynamically favorable 3D phase. The barrier is identified as the large separation between stable adsorption sites, which prevents homogeneous cluster nucleation and the instability of the Ag dimer against decay to two adatoms. Since the system is dominated by kinetics as long as the (√2x√2)R45° reconstruction exists, the growth is not well described by the traditional growth modes. It can be understood, however, as the result of supersaturation within an adsorption template system. DA - 2014/07/22/ PY - 2014 DO - 10.1021/nn502895s VL - 8 IS - 7 SP - 7531 EP - 7537 J2 - ACS Nano SN - 1936-0851 ST - Cluster Nucleation and Growth from a Highly Supersaturated Adatom Phase ER - TY - JOUR TI - Construction and evaluation of an ultrahigh-vacuum-compatible sputter deposition source AU - Lackner, Peter AU - Choi, Joong Il Jake AU - Diebold, Ulrike AU - Schmid, Michael T2 - Review of Scientific Instruments AB - A sputter deposition source for the use in ultrahigh vacuum (UHV) is described, and some properties of the source are analyzed. The operating principle is based on the design developed by Mayr et al. [Rev. Sci. Instrum. 84, 094103 (2013)], where electrons emitted from a filament ionize argon gas and the Ar+ ions are accelerated to the target. In contrast to the original design, two grids are used to direct a large fraction of the Ar+ ions to the target, and the source has a housing cooled by liquid nitrogen to reduce contaminations. The source has been used for the deposition of zirconium, a material that is difficult to evaporate in standard UHV evaporators. At an Ar pressure of 9×10^−6 mbar in the UHV chamber and moderate emission current, a highly reproducible deposition rate of ≈1 ML in 250 s was achieved at the substrate (at a distance of ≈50 mm from the target). Higher deposition rates are easily possible. X-ray photoelectron spectroscopy shows a high purity of the deposited films. Depending on the grid voltages, the substrate gets mildly sputtered by Ar+ ions; in addition, the substrate is also reached by electrons from the negatively biased sputter target. DA - 2017/10/01/ PY - 2017 DO - 10.1063/1.4998700 VL - 88 IS - 10 SP - 103904 J2 - Rev. Sci. Instrum. SN - 0034-6748 ER - TY - JOUR TI - Synthesis, characterization, and computation of catalysts at the Center for Atomic-Level Catalyst Design AU - Spivey, James J. AU - Krishna, Katla Sai AU - Kumar, Challa S.S.R. AU - Dooley, Kerry M. AU - Flake, John C. AU - Haber, Louis H. AU - Xu, Ye AU - Janik, Michael J. AU - Sinnott, Susan B. AU - Cheng, Yu-Ting AU - Liang, Tao AU - Sholl, David S. AU - Manz, Thomas A. AU - Diebold, Ulrike AU - Parkinson, Gareth S. AU - Bruce, David A. AU - de Jongh, Petra T2 - The Journal of Physical Chemistry C AB - Energy Frontier Research Centers have been developed by the Department of Energy to accelerate research synergism among experimental and theoretical scientists in catalysis. The overall goal is to advance tools of synthesis, characterization, and computation of solid catalysts to design and predict catalytic properties at the atomic level. The Center for Atomic-Level Catalyst Design (CALC-D) has the goal of significantly advancing: (a) the tools of materials synthesis, allowing catalysts identified by computation to be prepared with atomic-level precision, (b) characterization methods such as advanced spectroscopy to understand surface structures of the working catalyst unambiguously, and (c) the ability of computational catalysis to accurately model reactions at working conditions. DA - 2014/09/04/ PY - 2014 DO - 10.1021/jp502556u VL - 118 IS - 35 SP - 20043 EP - 20069 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - Ordered hydroxyls on Ca3Ru2O7(001) AU - Halwidl, Daniel AU - Mayr-Schmölzer, Wernfried AU - Fobes, David AU - Peng, Jin AU - Mao, Zhiqiang AU - Schmid, Michael AU - Mittendorfer, Florian AU - Redinger, Josef AU - Diebold, Ulrike T2 - Nature Communications AB - As complex ternary perovskite-type oxides are increasingly used in solid oxide fuel cells, electrolysis and catalysis, it is desirable to obtain a better understanding of their surface chemical properties. Here we report a pronounced ordering of hydroxyls on the cleaved (001) surface of the Ruddlesden-Popper perovskite Ca3Ru2O7 upon water adsorption at 105 K and subsequent annealing to room temperature. Density functional theory calculations predict the dissociative adsorption of a single water molecule (E ads = 1.64 eV), forming an (OH)ads group adsorbed in a Ca-Ca bridge site, with an H transferred to a neighboring surface oxygen atom, Osurf. Scanning tunneling microscopy images show a pronounced ordering of the hydroxyls with (2 × 1), c(2 × 6), (1 × 3), and (1 × 1) periodicity. The present work demonstrates the importance of octahedral rotation and tilt in perovskites, for influencing surface reactivity, which here induces the ordering of the observed OH overlayers. As ternary perovskite-type oxides are increasingly used in fuel cells and catalysis, greater understanding of their surface chemical properties is required. Here the authors report a pronounced ordering of hydroxyls on the cleaved (001) surface of Ca3Ru2O7 perovskite induced by O-octahedral rotation and tilt. DA - 2017/06/20/ PY - 2017 DO - 10.1038/s41467-017-00066-w VL - 8 IS - 1 SP - 23 J2 - Nat. Commun. SN - 2041-1723 ER - TY - JOUR TI - Trapping nitric oxide by surface hydroxyls on rutile TiO2(110) AU - Li, Shao-Chun AU - Jacobson, Peter AU - Zhao, Shu-Lei AU - Gong, Xue-Qing AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry C AB - Hydroxyls are omnipresent on oxide surfaces under ambient conditions. While they unambiguously play an important role in many catalytic processes, it is not well-understood how these species influence surface chemistry at atomic scale. We investigated the adsorption of nitric oxide (NO) on a hydroxylated rutile TiO2(110) surface with scanning tunneling microscopy (STM), X-ray/ultraviolet photoemission spectroscopy (XPS/UPS), and density functional theory (DFT) calculations. At room temperature adsorption of NO is only possible in the vicinity of a surface hydroxyl, and leads to a change of the local electronic structure. DFT calculations confirm that the surface hydroxyl-induced excess charge is transferred to the NO adsorbate, which results in an electrostatic stabilization of the adsorbate and, consequently, a significantly stronger bonding. Hydroxyls are omnipresent on oxide surfaces under ambient conditions. While they unambiguously play an important role in many catalytic processes, it is not well-understood how these species influence surface chemistry at atomic scale. We investigated the adsorption of nitric oxide (NO) on a hydroxylated rutile TiO2(110) surface with scanning tunneling microscopy (STM), X-ray/ultraviolet photoemission spectroscopy (XPS/UPS), and density functional theory (DFT) calculations. At room temperature adsorption of NO is only possible in the vicinity of a surface hydroxyl, and leads to a change of the local electronic structure. DFT calculations confirm that the surface hydroxyl-induced excess charge is transferred to the NO adsorbate, which results in an electrostatic stabilization of the adsorbate and, consequently, a significantly stronger bonding. DA - 2012/// PY - 2012 DO - 10.1021/jp209290a VL - 116 IS - 2 SP - 1887 EP - 1891 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - Disorder and defect healing in graphene on Ni(111) AU - Jacobson, Peter AU - Stöger, Bernhard AU - Garhofer, Andreas AU - Parkinson, Gareth S. AU - Schmid, Michael AU - Caudillo, Roman AU - Mittendorfer, Florian AU - Redinger, Josef AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry Letters AB - The structural evolution of graphene on Ni(111) is investigated as a function of growth temperature by scanning tunneling microscopy (STM). Low temperature (400–500 °C) growth results in a continuous but highly defective film with small ordered graphene domains and disordered domains composed of Stone-Wales (SW)-like defects. As the growth temperature is increased, the disordered domains shrink leaving small clusters of defects alongside epitaxially matched graphene. Density functional theory (DFT) calculations indicate the crucial role of the metallic support for the healing of SW defects, as the interaction with the substrate leads to a stabilization of the reaction intermediate. This work highlights the effect of the graphene-substrate interaction on the temperature dependence of the defect concentration in epitaxial graphene on Ni(111). DA - 2012/// PY - 2012 DO - 10.1021/jz2015007 VL - 3 SP - 136 EP - 139 J2 - J. Phys. Chem. Lett. SN - 1948-7185 ER - TY - JOUR TI - Auger neutralization of multiply charged noble gas ions at a tungsten surface AU - Varga, P. AU - Hofer, W. AU - Winter, H. T2 - Surface Science AB - Neutralization processes for slow doubly and triply charged noble gas ions impinging on clean polycrystalline tungsten are studied by observation of the resulting ejected electron energy distributions. A sequence of Auger de-excitation and Auger neutralization processes is demonstrated to occur and therefore strong evidence for participation of one-electron like transitions is presented. In the special case of Ne2+ impact this may yield straightforward information on the density of states of the uppermost surface layer. DA - 1982/05/02/ PY - 1982 DO - 10.1016/0039-6028(82)90494-0 VL - 117 IS - 1 SP - 142 EP - 153 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Measurements of impurity fluxes in the ASDEX plasma boundary AU - Taglauer, E. AU - Scherzer, B. M. U. AU - Varga, P. AU - Behrisch, R. AU - Chen, Cheng Kai AU - Asdex-Team T2 - Journal of Nuclear Materials AB - Impurity fluxes parallel to the magnetic field in the plasma boundary of ASDEX were measured with a collector probe. The probe allows simultaneous measurements of the radial distribution and can be rotated in order to obtain time resolution. The amount of impurities deposited on the graphite collector stripes is subsequently analysed by Rutherford backscattering. The predominant species detected are oxygen and stainless steel components, other impurities such as Ti, Si, Mo, and Au are also found. For typical divertor discharges the average Fe flux near the outer wall is about 1.3 × 1014 Fe-atoms/cm2· s with an e-folding length of 1.1 cm. The oxygen flux is about one order of magnitude higher. Time-resolved measurements show a maximum deposition rate at the beginning of the discharge. Impurity fluxes for discharges with various parameters including limiter and divertor operation are discussed. DA - 1982/11/01/ PY - 1982 DO - 10.1016/0022-3115(82)90197-0 VL - 111-112 SP - 142 EP - 146 J2 - J. Nucl. Mater. SN - 0022-3115 ER - TY - JOUR TI - Time-resolved measurements of hydrogen and deuterium fluxes in the ASDEX plasma boundary AU - Roth, J. AU - Varga, P. AU - Martinelli, A. P. AU - Scherzer, B. M. U. AU - Chen, Cheng-Kai AU - Wampler, W. R. AU - Taglauer, E. AU - ASDEX-team T2 - Journal of Nuclear Materials AB - Hydrogen and deuterium fluxes parallel to the toroidal magnetic field were measured in the plasma boundary of ASDEX using graphite collector probes. Time resolution of the order of 100 ms was obtained by rotating the cylindrical probes behind an aperture during the discharge. The trapped amount of hydrogen was determined by subsequent thermal desorption, in the analysis of deuterium the D(3He, p)4He nuclear reaction was used. Both methods yield quantitative results. Measurements were done for limiter and divertor discharges in the range of 4 to 20 cm outside the limiter or separatrix. The time distributions show a maximum flux at the beginning and at the end of the discharge. The relatively lower flux during the plateau phase of the discharge is in the range of 1015 to 2 × 1017 cm−2/s, depending on the radial probe position, the maximum values are higher by a factor of 5 to 50. During neutral hydrogen injection an additional maximum can be observed. The radial 1/e-decay length is about 0.9 cm in front and 0.4 cm behind the fixed limiter. The results are compared with independent measurements in ASDEX and other plasma machines. DA - 1982/11/01/ PY - 1982 DO - 10.1016/0022-3115(82)90194-5 VL - 111-112 SP - 123 EP - 129 J2 - J. Nucl. Mater. SN - 0022-3115 ER - TY - JOUR TI - A new approach to interface segregation: Surface dangling and interatomic bond effects in binary alloys AU - Mezey, Lörinc Zoltán AU - Giber, Janos AU - Hofer, Wilhelm AU - Varga, Peter T2 - Surface Science AB - First a new theory and method of calculation, describing also surface interatomic bonds with environmental atoms is outlined. This theory is termed the MTCIP-1A (modern thermodynamic calculation of interface properties-first approximation). The method permits the calculation of the X1 thermodynamic equilibrium composition (given by the mole fraction of a selected component, e.g. the solute component) of the topmost atomic sublayer of a binary solid. The value of X1 is shown to be determined by the following factors: the “mixing”, the “non-bonding”, the “bonding” and the “real mixture” factors. The first three are described here more deeply. The numerical calculations are shown in detail for an AuPd alloy with a bulk Au content of 25 at%, with (100) surface orientation, with a temperature T = 773.2 K and under an environmental oxygen partial pressure PO2 = 5 × 10−10Pa. Further results are outlined (in appendix 6) for T = 293.2 K and PO2 = 5 × 10−5Pa (much stronger oxidizing conditions). The X1Au value at% in the first case (Au segregation) while it is practically 0 at% in the second case (Pd segregation). Results on this alloy, obtained by ISS (ion scattering spectroscopy) are used for comparison. The initial Au content of the surface (after sputtering) was 29 at%. After 15 min. of equilibration in the first case 75 at% was measured (Au segregation). In the second case just 26 at% was obtained (Pd segregation compared with the initial composition). The calculated X1Au value is close to the experimental one at 773.2 K. At T = 293.2 K the agreement seems to be only qualitative. This is due to the slow approach to equilibrium (from 29 to about 0 at% of Au) by diffusion at such a low temperature and to the relatively small time of equilibration. Several other cases giving good agreement between theory and experimental results are found in other works mentioned here. DA - 1990/08/01/ PY - 1990 DO - 10.1016/0039-6028(90)90677-Z VL - 234 IS - 1 SP - 197 EP - 209 J2 - Surf. Sci. SN - 0039-6028 ST - A new approach to interface segregation ER - TY - JOUR TI - Analyses of the adsorption of oxygen and hydrogen on PtxNi1−x alloys using ion scattering spectroscopy AU - Novacek, P. AU - Varga, P. T2 - Surface Science AB - The ISS-signals of Pt and Ni were used to detect the surface composition of three polycrystalline PtxNi1−x alloys after adsorbing various amounts of oxygen and hydrogen. Oxygen was adsorbed in the range from 0–20 langmuir at room temperature on Pt9Ni91, Pt34Ni66, Pt88Ni12, pure NI and pure Pt. Below 2 L the adsorption process was found to be quite the same on the pure metals as well as on the respective metals in the alloys. At exposures above 2 L there are distinct differences between the reaction of the alloys and the pure metals. They are interpreted as an enrichment of Ni atoms in the topmost layer due to oxygen induced segregation. The adsorption of hydrogen at T = 80 K does not show any significant differences in the adsorption process between the pure metals and the alloys, which implies that there is no hydrogen induced segregation. DA - 1991/05/03/ PY - 1991 DO - 10.1016/0039-6028(91)90071-Y VL - 248 IS - 1 SP - 183 EP - 192 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Suface segregation in Al_xLi_1-x alloys AU - Vonbank, M. AU - Varga, P. T2 - Vakuum-Technik DA - 1988/12// PY - 1988 VL - 37 IS - 7-8 SP - 220 EP - 225 J2 - Vakuum-Technik SN - 0042-2266 ER - TY - JOUR TI - Neutralization of multiply charged ions at a surface AU - Varga, P. T2 - Applied Physics A AB - We report on emission processes induced by particle-solid interaction involving ions with a large potential (i.e., high ion charge state) and low kinetic energy. After an introduction into existing neutralization models for ion scattering at a metal surface a detailed discussion on the electron emission processes is presented.The number of electrons emitted per incident ion is shown to be proportional to the potential energy only within a restricted parameter field involving charge state and ion velocity. The kinetic energy distribution of emitted electrons is dominated by low-energetic electrons (≦30 eV), while inner shell holes of the projectile ion can initiate high-energetic characteristic Auger electrons. The presence of inner shell holes is also of importance for the charge state of highly charged ions being scattered at surfaces whereas normally the charge state distribution of scattered ions depends on the impact parameter only.The influence of the primary ion charge state on the sputtering yield of insulating surfaces is seen for the charge state of sputtered particles, whereas the total sputtering yield seems to be insensitive. This question is still subject to controversy, however.Photon emission dependent on the charge state of the impinging ion has been observed up to now only for extremely highly charged ions as hydrogenlike Ar or Kr. DA - 1987/09/01/ PY - 1987 DO - 10.1007/BF00617889 VL - 44 IS - 1 SP - 31 EP - 41 J2 - Appl. Phys. A SN - 1432-0630 ER - TY - JOUR TI - Potential emission for multicharged ion impact on clean tungsten above the kinetic emission threshold AU - Fehringer, M. AU - Delaunay, M. AU - Geller, R. AU - Varga, P. AU - Winter, H. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - Apparent electron emission yields for impact of Nq+ (q = 4-6) and Neq+ (q = 4-7) on polycrystalline tungsten at impact velocities of 0.5 - 3 × 105 ms-1 are presented. For [nu] >[nu]t [approximate] 105 ms-1 with [nu]t being threshold velocity for kinetic electron emission the data have been corrected for the latter contribution, which permits investigation of potential emission toward higher impact velocities. DA - 1987/04/02/ PY - 1987 DO - 10.1016/0168-583X(87)90455-1 VL - 23 IS - 1-2 SP - 245 EP - 247 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - JOUR TI - Electron emission from a metal surface bombarded by slow highly charged ions AU - Delaunay, M. AU - Fehringer, M. AU - Geller, R. AU - Hitz, D. AU - Varga, P. AU - Winter, H. T2 - Physical Review B AB - We have measured total electron-emission yields γ for clean tungsten bombarded with various multiply charged ions: Zq+ up to N6+, Ne7+, Ar12+, and Kr11+. At low impact energy (≤10 eV/amu) a linear dependence of γ versus ion potential energy could be observed up to the respectively highest q, for which a pronounced decrease appeared superimposed on the gradual decrease of all γ toward higher impact energy. Our results are discussed within a multiple-step ion-neutralization model. DA - 1987/03/15/ PY - 1987 DO - 10.1103/PhysRevB.35.4232 VL - 35 IS - 9 SP - 4232 EP - 4235 J2 - Phys. Rev. B ER - TY - JOUR TI - Preferential sputtering and surface segregation in Au-Pd alloys AU - Hetzendorf, G. AU - Varga, P. T2 - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms AB - The influence of sputtering and heating on the surface composition of Au-Pd alloys has been measured with ISS (ion scattering spectroscopy) and compared with results of XPS measurements (X-ray photoelectron spectroscopy). After sputter cleaning and heating up to 500°C, the composition of polycrystalline alloy targets with different compositions was investigated. Pure metal standards were used for quantitative evaluation of the spectra. Gold enrichment due to preferential sputtering and strong gold segregation was found by ISS measurements; the XPS results do not show the same amount of enrichment. The differences in the information depth for ISS and XPS is responsible for the contradicting results. DA - 1987/// PY - 1987 DO - 10.1016/S0168-583X(86)80076-3 VL - 18 IS - 4-6 SP - 501 EP - 503 J2 - Nucl. Instrum. Methods B SN - 0168-583X ER - TY - CHAP TI - Ion neutralization of multiply charged paricles near a surface AU - P. Varga AU - W. Hofer AU - H. Winter T2 - Scanning Electron Microscopy CY - Chicago DA - 1982/// PY - 1982 VL - Part 3 SP - 967 EP - 971 PB - Scanning Microscopy Int. ER - TY - JOUR TI - Apparent cross sections for metastable ion production by electron impact AU - Varga, P. AU - Hofer, W. AU - Winter, H. T2 - Journal of Physics B: Atomic and Molecular Physics AB - Fractions of long-lived excited species in noble-gas ion beams which were produced by various methods have been determined by virtue of slow ion impact induced electron emission from clean polycrystalline tungsten. Investigations with ion beams produced by single electron impact on atoms permitted the determination of cross sections for the production of singly charged metastable ions of the noble gases Ar, Kr and Xe. The cross sections obtained increase steeply from threshold up to their maximum values of about 10 -17 cm 2 within 1.5 time threshold energy. Processes responsible for production of these metastable ions are discussed and the method applied is compared with other techniques for metastable ion detection. DA - 1981/// PY - 1981 DO - 10.1088/0022-3700/14/8/019 VL - 14 IS - 8 SP - 1341 J2 - J. Phys. B: At. Mol. Phys. SN - 0022-3700 ER - TY - JOUR TI - Apparent cross sections for electron impact production of metastable noble gas ions AU - Hofer, W. AU - Varga, P. AU - Winter, H. T2 - Acta Physica Academiae Scientiarum Hungaricae DA - 1980/08/01/ PY - 1980 DO - 10.1007/BF03158768 VL - 49 IS - 1 SP - 313 EP - 313 J2 - Acta Physica SN - 0001-6705 ER - TY - JOUR TI - Determination of metastable fractions in noble-gas-ion beams AU - Varga, P. AU - Winter, H. T2 - Physical Review A AB - Yields and kinetic energy distributions of secondary electrons emitted by potential emission due to impact of slow singly or multiply charged noble-gas ions on a clean polycrystalline tungsten target have been determined. An empirical relation for the secondary-electron yield of ground-state ions is applied to obtain the secondary-electron yield of corresponding metastably excited ions. By this technique metastable fractions of noble-gas-ion beams could be evaluated. Results for singly and doubly charged ions of Ar, Kr, and Xe, are presented. DA - 1978/12/01/ PY - 1978 DO - 10.1103/PhysRevA.18.2453 VL - 18 IS - 6 SP - 2453 EP - 2458 J2 - Phys. Rev. A ER - TY - JOUR TI - Real-time observation of self-limiting SiO2/Si decomposition catalysed by gold silicide droplets AU - Bábor, Petr AU - Duda, Radek AU - Polčák, Josef AU - Průša, Stanislav AU - Potoček, Michal AU - Varga, Peter AU - Čechal, Jan AU - Šikola, Tomáš T2 - RSC Advances AB - The thermal decomposition of thin SiO2 layers on silicon substrates draws significant attention due to its high technological importance in the semiconductor industry and in all relevant fields where silicon is employed as a substrate or part of an active device. Understanding of the underlying processes on silicon surfaces is therefore of fundamental importance. Here we show that the presence of gold silicide (AuSi) catalytically enhances the decomposition of SiO2 layers on a Si substrate, which proceeds via void nucleation under the positions of Au nanoparticles and subsequent lateral growth of the void. Our real-time secondary electron microscopy data reveal that the presence of a AuSi droplet within the void enhances the reaction rate due to an increased pre-exponential factor of the rate limiting step (i.e., SiO desorption at temperatures beyond 700 °C). While the SiO2 is decomposed the silicon surface in the open voids is covered by an Au monolayer. Consequently, as the void grows, the AuSi droplet is depleted of gold and the reaction rate enhancement is terminated when the supply of gold stops. Hence, the size of the pits is determined by the initial size of the Au nanoparticle. Our work thus provides insight into Au-enhanced SiO2 decomposition and its self-limiting nature offers a way for the preparation of nanoscale features with nanometer precision. DA - 2015/11/24/ PY - 2015 DO - 10.1039/C5RA19472E VL - 5 IS - 123 SP - 101726 EP - 101731 J2 - RSC Adv. SN - 2046-2069 ER - TY - JOUR TI - Water adsorption at zirconia: from the ZrO2(111)/Pt3Zr(0001) model system to powder samples AU - Lackner, Peter AU - Hulva, Jan AU - Köck, Eva-Maria AU - Mayr-Schmölzer, Wernfried AU - Choi, Joong Il J. AU - Penner, Simon AU - Diebold, Ulrike AU - Mittendorfer, Florian AU - Redinger, Josef AU - Klötzer, Bernhard AU - Parkinson, Gareth S. AU - Schmid, Michael T2 - Journal of Materials Chemistry A AB - We present a comprehensive study of water adsorption and desorption on an ultrathin trilayer zirconia film using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), as well as scanning tunneling microscopy (STM) at different temperatures. The saturation coverage is one H2O per surface Zr atom, with about 12% dissociation. The monolayer TPD peak (180 K, desorption barrier 0.57 ± 0.04 eV) has a tail towards higher temperatures, caused by recombinative desorption from defect sites with dissociated water. STM shows that the defects with the strongest H2O adsorption are found above subsurface dislocations. Additional defect sites are created by multiple water adsorption/desorption cycles; these water-induced changes were also probed by CO2 TPD. Nevertheless, the defect density is much smaller than in previous studies of H2O/ZrO2. To validate our model system, transmission Fourier-transform infrared absorption spectroscopy (FTIR) studies at near-ambient pressures were carried out on monoclinic zirconia powder, showing comparable adsorption energies as TPD on the ultrathin film. The results are also compared with density functional theory (DFT) calculations, which suggest that sites with strong H2O adsorption contain twofold-coordinated oxygen. DA - 2018/09/18/ PY - 2018 DO - 10.1039/C8TA04137G VL - 6 IS - 36 SP - 17587 EP - 17601 J2 - J. Mater. Chem. A SN - 2050-7496 ST - Water adsorption at zirconia ER - TY - JOUR TI - Influence of surface atomic structure demonstrated on oxygen incorporation mechanism at a model perovskite oxide AU - Riva, Michele AU - Kubicek, Markus AU - Hao, Xianfeng AU - Franceschi, Giada AU - Gerhold, Stefan AU - Schmid, Michael AU - Hutter, Herbert AU - Fleig, Juergen AU - Franchini, Cesare AU - Yildiz, Bilge AU - Diebold, Ulrike T2 - Nature Communications AB - Perovskite oxide surfaces catalyze oxygen exchange reactions that are crucial for fuel cells, electrolyzers, and thermochemical fuel synthesis. Here, by bridging the gap between surface analysis with atomic resolution and oxygen exchange kinetics measurements, we demonstrate how the exact surface atomic structure can determine the reactivity for oxygen exchange reactions on a model perovskite oxide. Two precisely controlled surface reconstructions with (4 × 1) and (2 × 5) symmetry on 0.5 wt.% Nb-doped SrTiO3(110) were subjected to isotopically labeled oxygen exchange at 450 °C. The oxygen incorporation rate is three times higher on the (4 × 1) surface phase compared to the (2 × 5). Common models of surface reactivity based on the availability of oxygen vacancies or on the ease of electron transfer cannot account for this difference. We propose a structure-driven oxygen exchange mechanism, relying on the flexibility of the surface coordination polyhedra that transform upon dissociation of oxygen molecules. DA - 2018/09/13/ PY - 2018 DO - 10.1038/s41467-018-05685-5 VL - 9 IS - 1 SP - 3710 J2 - Nat. Commun. SN - 2041-1723 ER - TY - JOUR TI - High-affinity adsorption leads to molecularly ordered interfaces on TiO2 in air and solution AU - Balajka, Jan AU - Hines, Melissa A. AU - DeBenedetti, William J. I. AU - Komora, Mojmir AU - Pavelec, Jiri AU - Schmid, Michael AU - Diebold, Ulrike T2 - Science AB - Researchers around the world have observed the formation of molecularly ordered structures of unknown origin on the surface of titanium dioxide (TiO2) photocatalysts exposed to air and solution. Using a combination of atomic-scale microscopy and spectroscopy, we show that TiO2 selectively adsorbs atmospheric carboxylic acids that are typically present in parts-per-billion concentrations while effectively repelling other adsorbates, such as alcohols, that are present in much higher concentrations. The high affinity of the surface for carboxylic acids is attributed to their bidentate binding. These self-assembled monolayers have the unusual property of being both hydrophobic and highly water-soluble, which may contribute to the self-cleaning properties of TiO2. This finding is relevant to TiO2 photocatalysis, because the self-assembled carboxylate monolayers block the undercoordinated surface cation sites typically implicated in photocatalysis. Self-assembled monolayers that form on titanium dioxide surfaces in air are attributed to atmospheric organic acids. DA - 2018/08/24/ PY - 2018 DO - 10.1126/science.aat6752 VL - 361 IS - 6404 SP - 786 EP - 789 J2 - Science SN - 0036-8075, 1095-9203 ER - TY - JOUR TI - Apparatus for dosing liquid water in ultrahigh vacuum AU - Balajka, Jan AU - Pavelec, Jiri AU - Komora, Mojmir AU - Schmid, Michael AU - Diebold, Ulrike T2 - Review of Scientific Instruments AB - The structure of the solid-liquid interface often defines the function and performance of materials in applications. To study this interface at the atomic scale, we extended an ultrahigh vacuum (UHV) surface-science chamber with an apparatus that allows bringing a surface in contact with ultrapure liquid water without exposure to air. In this process, a sample, typically a single crystal prepared and characterized in UHV, is transferred into a separate, small chamber. This chamber already contains a volume of ultrapure water ice. The ice is at cryogenic temperature, which reduces its vapor pressure to the UHV range. Upon warming, the ice melts and forms a liquid droplet, which is deposited on the sample. In test experiments, a rutile TiO2(110) single crystal exposed to liquid water showed unprecedented surface purity, as established by X-ray photoelectron spectroscopy and scanning tunneling microscopy. These results enabled us to separate the effect of pure water from the effect of low-level impurities present in the air. Other possible uses of the setup are discussed. DA - 2018/08/01/ PY - 2018 DO - 10.1063/1.5046846 VL - 89 IS - 8 SP - 083906 J2 - Rev. Sci. Instrum. SN - 0034-6748 ER - TY - JOUR TI - Prototypical organic–oxide interface: Intramolecular resolution of sexiphenyl on In2O3(111) AU - Wagner, Margareta AU - Hofinger, Jakob AU - Setvín, Martin AU - Boatner, Lynn A. AU - Schmid, Michael AU - Diebold, Ulrike T2 - ACS Applied Materials & Interfaces AB - The performance of an organic semiconductor device is critically determined by the geometric alignment, orientation, and ordering of the organic molecules. Although an organic multilayer eventually adopts the crystal structure of the organic material, the alignment and configuration at the interface with the substrate/electrode material are essential for charge injection into the organic layer. This work focuses on the prototypical organic semiconductor para-sexiphenyl (6P) adsorbed on In2O3(111), the thermodynamically most stable surface of the material that the most common transparent conducting oxide, indium tin oxide, is based on. The onset of nucleation and formation of the first monolayer are followed with atomically resolved scanning tunneling microscopy and noncontact atomic force microscopy (nc-AFM). Annealing to 200 °C provides sufficient thermal energy for the molecules to orient themselves along the high-symmetry directions of the surface, leading to a single adsorption site. The AFM data suggests an essentially planar adsorption geometry. With increasing coverage, the 6P molecules first form a loose network with a poor long-range order. Eventually, the molecules reorient into an ordered monolayer. This first monolayer has a densely packed, well-ordered (2 × 1) structure with one 6P per In2O3(111) substrate unit cell, that is, a molecular density of 5.64 × 1013 cm–2. DA - 2018/03/28/ PY - 2018 DO - 10.1021/acsami.8b02177 VL - 10 IS - 16 SP - 14175 EP - 14182 J2 - ACS Appl. Mater. Interfaces ST - Prototypical Organic–Oxide Interface AN - world ER - TY - JOUR TI - Surface oxides on close-packed surfaces of late transition metals AU - Lundgren, Edvin AU - Mikkelsen, Anders AU - Andersen, Jesper N. AU - Kresse, Georg AU - Schmid, Michael AU - Varga, Peter T2 - Journal of Physics: Condensed Matter AB - In recent years, the formation of thin, well-ordered but complex surface oxides on late transition metals has been discovered. The driving force for this line of research has been the strong incentive to increase the partial pressure of oxygen from ultra-high vacuum to conditions more relevant for heterogeneous catalysis. Here we review the present status of the research field. Compared to oxygen adatom superstructures, the structure of the surface oxides has proven to be extremely complex, and the investigations have therefore relied on a combination of several experimental and theoretical techniques. The approach to solving the structures formed on close-packed surfaces of Pd and Rh is presented in some detail. Focusing on the structures found, we show that the surface oxides share some general properties with the corresponding bulk oxides. Nevertheless, of all surface oxide structures known today, only the two-dimensional surface oxides on Pd(100) and Pt(110) have the same lattice as the bulk oxides (PdO and PtO, respectively). In addition to two-dimensional oxides, including the O-Rh-O trilayers found on Rh, one-dimensional oxides were observed at ridges or steps of open surfaces such as (110) or vicinal surfaces. Finally, we briefly report on a few studies of the reactivity of surface oxides with well-known structure. DA - 2006/// PY - 2006 VL - 18 IS - 30 SP - R481 EP - R499 J2 - J. Phys.: Condens. Matter SN - 0953-8984 ER - TY - JOUR TI - The Pd(100)-(√5 x √5)R27°-O surface oxide: A LEED, DFT and STM study AU - Kostelník, Petr AU - Seriani, Nicola AU - Kresse, Georg AU - Mikkelsen, Anders AU - Lundgren, Edvin AU - Blum, Volker AU - Šikola, Tomás AU - Varga, Peter AU - Schmid, Michael T2 - Surface Science AB - Using low energy electron diffraction (LEED), density functional theory (DFT) and scanning tunneling microscopy (STM), we have re-analyzed the Pd(100)-(√5 x √5)R27°-O surface oxide structure consisting, in the most recent model, of a strained PdO(101) layer on top of the Pd(100) surface [M. Todorova et al., Surf. Sci. 541 (2003) 101]. Both, DFT simulations using the Vienna Ab initio Simulation Package (VASP) and tensor LEED I(V) analysis of newly acquired LEED experimental data, show that the PdO(101) model is essentially correct. However, compared to the previous study, there is a horizontal shift of the PdO(101) layer with respect to the Pd(100) substrate. The atomic coordinates derived by DFT and LEED (RP = 0.162) are in excellent agreement with each other. We also present STM images with atomic resolution showing domain boundaries on the surface oxide and discuss the bonding geometry between the surface oxide and the substrate. DA - 2007/03/15/ PY - 2007 DO - 10.1016/j.susc.2007.01.026 VL - 601 IS - 6 SP - 1574 EP - 1581 J2 - Surf. Sci. SN - 0039-6028 ST - Pd(100) surface oxide ER - TY - JOUR TI - Oxidation of Pd(553): From ultrahigh vacuum to atmospheric pressure AU - Westerström, R. AU - Gustafson, J. AU - Resta, A. AU - Mikkelsen, A. AU - Andersen, J. N. AU - Lundgren, E. AU - Seriani, N. AU - Mittendorfer, F. AU - Schmid, M. AU - Klikovits, J. AU - Varga, P. AU - Ackermann, M. D. AU - Frenken, J. W. M. AU - Kasper, N. AU - Stierle, A. T2 - Physical Review B AB - The oxidation of a vicinal Pd(553) surface has been studied from ultrahigh vacuum (UHV) to atmospheric oxygen pressures at elevated sample temperatures. The investigation combines traditional electron based UHV techniques such as high resolution core level spectroscopy, low-energy electron diffraction, scanning tunneling microscopy with in situ surface x-ray diffraction, and ab initio simulations. In this way, we show that the O atoms preferentially adsorb at the step edges at oxygen pressures below 10(-6) mbar and that the (553) surface is preserved. In the pressure range between 10(-6) and 1 mbar and at a sample temperature of 300-400 degrees C, a surface oxide forms and rearranges the (553) surface facets and forming (332) facets. Most of the surface oxide can be described as a PdO(101) plane, similar to what has been found previously on other Pd surfaces. However, in the present case, the surface oxide is reconstructed along the step edges, and the stability of this structure is discussed. In addition, the (√6 x √6) Pd5O4 surface oxide can be observed on (111) terraces larger than those of the (332) terraces. Increasing the O pressure above 1 mbar results in the disappearance of the (332) facets and the formation of PdO bulk oxide. DA - 2007/10/15/ PY - 2007 DO - 10.1103/PhysRevB.76.155410 VL - 76 IS - 15 SP - 155410 J2 - Phys. Rev. B ST - Oxidation of Pd(553) ER - TY - JOUR TI - Water agglomerates on Fe3O4(001) AU - Meier, Matthias AU - Hulva, Jan AU - Jakub, Zdeněk AU - Pavelec, Jiří AU - Setvin, Martin AU - Bliem, Roland AU - Schmid, Michael AU - Diebold, Ulrike AU - Franchini, Cesare AU - Parkinson, Gareth S. T2 - Proceedings of the National Academy of Sciences AB - Determining the structure of water adsorbed on solid surfaces is a notoriously difficult task and pushes the limits of experimental and theoretical techniques. Here, we follow the evolution of water agglomerates on Fe3O4(001); a complex mineral surface relevant in both modern technology and the natural environment. Strong OH–H2O bonds drive the formation of partially dissociated water dimers at low coverage, but a surface reconstruction restricts the density of such species to one per unit cell. The dimers act as an anchor for further water molecules as the coverage increases, leading first to partially dissociated water trimers, and then to a ring-like, hydrogen-bonded network that covers the entire surface. Unraveling this complexity requires the concerted application of several state-of-the-art methods. Quantitative temperature-programmed desorption (TPD) reveals the coverage of stable structures, monochromatic X-ray photoelectron spectroscopy (XPS) shows the extent of partial dissociation, and noncontact atomic force microscopy (AFM) using a CO-functionalized tip provides a direct view of the agglomerate structure. Together, these data provide a stringent test of the minimum-energy configurations determined via a van der Waals density functional theory (DFT)-based genetic search. DA - 2018/06/19/ PY - 2018 DO - 10.1073/pnas.1801661115 VL - 115 IS - 25 SP - E5642 EP - E5650 J2 - Proc. Natl. Acad. Sci. USA SN - 0027-8424, 1091-6490 ER - TY - JOUR TI - Formation and dynamics of small polarons on the rutile TiO2(110) surface AU - Reticcioli, Michele AU - Setvin, Martin AU - Schmid, Michael AU - Diebold, Ulrike AU - Franchini, Cesare T2 - Physical Review B AB - Charge trapping and the formation of polarons is a pervasive phenomenon in transition-metal oxide compounds, in particular at the surface, affecting fundamental physical properties and functionalities of the hosting materials. Here we investigate via first-principles techniques the formation and dynamics of small polarons on the reduced surface of titanium dioxide, an archetypal system for polarons, for a wide range of oxygen vacancy concentrations. We report how the essential features of polarons can be adequately accounted for in terms of a few quantities: the local structural and chemical environment, the attractive interaction between negatively charged polarons and positively charged oxygen vacancies, and the spin-dependent polaron-polaron Coulomb repulsion. We combined molecular-dynamics simulations on realistic samples derived from experimental observations with simplified static models, aiming to disentangle the various variables at play. We find that depending on the specific trapping site, different types of polarons can be formed, with distinct orbital symmetries and a different degree of localization and structural distortion. The energetically most stable polaron site is the subsurface Ti atom below the undercoordinated surface Ti atom, due to a small energy cost to distort the lattice and a suitable electrostatic potential. Polaron-polaron repulsion and polaron-vacancy attraction determine the spatial distribution of polarons as well as the energy of the polaronic in-gap state. In the range of experimentally reachable oxygen vacancy concentrations, the calculated data are in excellent agreement with observations, thus validating the overall interpretation. DA - 2018/07/20/ PY - 2018 DO - 10.1103/PhysRevB.98.045306 VL - 98 IS - 4 SP - 045306 J2 - Phys. Rev. B ER - TY - JOUR TI - Pushing the detection of cation nonstoichiometry to the limit AU - Riva, Michele AU - Franceschi, Giada AU - Lu, Qiyang AU - Schmid, Michael AU - Yildiz, Bilge AU - Diebold, Ulrike T2 - Physical Review Materials AB - Nanoscale complex-oxide thin films prepared by well-established growth techniques, such as pulsed-laser deposition or molecular-beam epitaxy, often exhibit compositions that deviate from the ideal stoichiometry. Even small variations in composition can lead to substantial changes in the technologically relevant electronic, magnetic, and optical properties of these materials. To assess the reasons behind this variability, and ultimately to allow tuning the properties of oxide films with precise control of the deposition parameters, high-resolution detection of the nonstoichiometry introduced during growth is needed. The resolution of current techniques, such as x-ray diffraction, fluorescence, or spectroscopy, is limited to estimating composition differences in the percent level, which is often insufficient for electronic-device quality. We develop an unconventional approach based on scanning tunneling microscopy for enabling the determination of cation imbalance introduced in thin films with exceptionally small detection limit. We take advantage of the well-controlled surface reconstructions on SrTiO3(110), and use the established relation between those reconstructions and the surface composition to assess the cation excess deposited in pulsed-laser grown SrTiO3(110) films. We demonstrate that a <0.1% change in cation nonstoichiometry is detectable by our approach. Furthermore, we show that, for thin films that accommodate all the nonstoichiometry at the surface, this method has no fundamental detection limit. DA - 2019/04/25/ PY - 2019 DO - 10.1103/PhysRevMaterials.3.043802 VL - 3 IS - 4 SP - 043802 J2 - Phys. Rev. Materials ER - TY - JOUR TI - A multi-technique study of CO2 adsorption on Fe3O4 magnetite AU - Pavelec, Jiri AU - Hulva, Jan AU - Halwidl, Daniel AU - Bliem, Roland AU - Gamba, Oscar AU - Jakub, Zdenek AU - Brunbauer, Florian AU - Schmid, Michael AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - The Journal of Chemical Physics AB - The adsorption of CO2 on the Fe3O4(001)-(√2 ×√2)R45° surface was studied experimentally using temperature programmed desorption (TPD), photoelectron spectroscopies (UPS and XPS), and scanning tunneling microscopy. CO2 binds most strongly at defects related to Fe2+, including antiphase domain boundaries in the surface reconstruction and above incorporated Fe interstitials. At higher coverages,CO2 adsorbs at fivefold-coordinated Fe3+ sites with a binding energy of 0.4 eV. Above a coverage of 4 molecules per (√2 × √2)R45° unit cell, further adsorption results in a compression of the first monolayer up to a density approaching that of a CO2 ice layer. Surprisingly, desorption of the second monolayer occurs at a lower temperature (≈84 K) than CO2 multilayers (≈88 K), suggestive of a metastable phase or diffusion-limited island growth. The paper also discusses design considerations for a vacuum system optimized to study the surface chemistry of metal oxide single crystals, including the calibration and characterisation of a molecular beam source for quantitative TPD measurements. DA - 2017/01/03/ PY - 2017 DO - 10.1063/1.4973241 VL - 146 IS - 1 SP - 014701 J2 - J. Chem. Phys. SN - 0021-9606 ER - TY - JOUR TI - Research update: Focused ion beam direct writing of magnetic patterns with controlled structural and magnetic properties AU - Urbánek, Michal AU - Flajšman, Lukáš AU - Křižáková, Viola AU - Gloss, Jonáš AU - Horký, Michal AU - Schmid, Michael AU - Varga, Peter T2 - APL Materials AB - Focused ion beam irradiation of metastable Fe78Ni22 thin films grown on Cu(100) substrates is used to create ferromagnetic, body-centered cubic patterns embedded into paramagnetic, face-centered-cubic surrounding. The structural and magnetic phase transformation can be controlled by varying parameters of the transforming gallium ion beam. The focused ion beam parameters such as the ion dose, number of scans, and scanning direction can be used not only to control a degree of transformation but also to change the otherwise four-fold in-plane magnetic anisotropy into the uniaxial anisotropy along a specific crystallographic direction. This change is associated with a preferred growth of specific crystallographic domains. The possibility to create magnetic patterns with continuous magnetization transitions and at the same time to create patterns with periodical changes in magnetic anisotropy makes this system an ideal candidate for rapid prototyping of a large variety of nanostructured samples. Namely, spin-wave waveguides and magnonic crystals can be easily combined into complex devices in a single fabrication step. DA - 2018/06/01/ PY - 2018 DO - 10.1063/1.5029367 VL - 6 IS - 6 SP - 060701 J2 - APL Mater. ST - Research Update ER - TY - JOUR TI - Epitaxially Grown Fe3O4 Thin Films: An XPS Study AU - Anderson, J. F. AU - Kuhn, Markus AU - Diebold, Ulrike T2 - Surface Science Spectra AB - Magnetite is a technologically important material with interesting magnetic and electronic properties. The applications of this oxide include, but are not limited to, magnetic storage media, as a catalyst in the plastics industry, electronic circuit applications, and may have important future applications in the micro-electronics industry as well. Since the transition between different iron oxide phases can be easily achieved, characterization of well-defined materials with XPS is quite important. We report XPS measurements of a 1 µm thick Fe3O4 thin film on a MgO(001) substrate. The film was grown using plasma-assisted MBE, and was characterized in situ with RHEED which indicated a well ordered surface. A companion sample was characterized ex situ with XRD and SQUID which showed it to be single-crystalline, stoichiometric Fe3O3. The sample was transported under argon atmosphere and introduced into the XPS chamber under flowing nitrogen. The sample was then cleaned by annealing in a 2 × 10 – 6 oxygen at 620 K until XPS indicated no carbon present. The x-ray source is a non-monochromated Al Kalpha and Mg K alpha , and the resulting spectra were analyized using a cylindrical sectional analyzer (CSA) with entrance lens (Omicron Vakuumphysic Ges.m.b.H). Survey scans, Fe 2p, and O 1s transitions are presented. DA - 1996/07// PY - 1996 DO - 10.1116/1.1247796 VL - 4 IS - 3 SP - 266 EP - 272 J2 - Surf. Sci. Spectra ST - Epitaxially Grown Fe3O4 Thin Films ER - TY - JOUR TI - Incipient ferroelectricity: A route towards bulk-terminated SrTiO3 AU - Sokolović, Igor AU - Schmid, Michael AU - Diebold, Ulrike AU - Setvin, Martin T2 - Physical Review Materials AB - An investigation of bulk-terminated (001) surfaces of SrTiO3, a prototypical cubic perovskite, was made possible with a novel cleaving procedure. Controlled application of strain on a SrTiO3 single-crystal results in a flat cleavage with μm-size domains of SrO and TiO2. Distribution of these two terminations is dictated by ferroelectric domains induced by strain during the cleavage process. Atomically resolved scanning tunneling microscopy/atomic force microscopy measurements reveal the presence of point defects in a well-defined concentration of (14±2)%; Sr vacancies form at the SrO termination and complementary Sr adatoms appear at the TiO2 termination. These intrinsic defects stabilize the surface by balancing the interplay between ferroelectricity, surface polarity, and surface charge. DA - 2019/03/14/ PY - 2019 DO - 10.1103/PhysRevMaterials.3.034407 VL - 3 IS - 3 SP - 034407 J2 - Phys. Rev. Materials ST - Incipient ferroelectricity ER - TY - JOUR TI - Observation of the dynamical change in a water monolayer adsorbed on a ZnO surface AU - Dulub, Olga AU - Meyer, Bernd AU - Diebold, Ulrike T2 - Physical Review Letters AB - A combined scanning tunneling microscopy and density-functional theory (DFT) study shows a rich structure of water monolayers adsorbed on ZnO(1 0 -1 0) at room temperature. Most of the water is in a lowest-energy configuration where every second molecule is dissociated. It coexists with an energetically almost degenerate configuration consisting of a fully molecular water monolayer. Parts of the layer continuously switch back and forth between these two states. DFT calculations reveal that water molecules repeatedly associate and dissociate in this sustained dynamical process. DA - 2005/09// PY - 2005 DO - 10.1103/PhysRevLett.95.136101 VL - 95 IS - 13 SP - 136101 J2 - Phys. Rev. Lett. SN - 0031-9007 ER - TY - JOUR TI - Structure of the ultrathin aluminum oxide film on NiAl(110) AU - Kresse, Georg AU - Schmid, Michael AU - Napetschnig, Evelyn AU - Shishkin, Maxim AU - Köhler, Lukas AU - Varga, Peter T2 - Science AB - The well-ordered aluminum oxide film formed by oxidation of the NiAl(110) surface is the most intensely studied metal surface oxide, but its structure was previously unknown. We determined the structure by extensive ab initio modeling and scanning tunneling microscopy experiments. Because the topmost aluminum atoms are pyramidally and tetrahedrally coordinated, the surface is different from all Al2O3 bulk phases. The film is a wide-gap insulator, although the overall stoichiometry of the film is not Al2O3 but Al10O13. We propose that the same building blocks can be found on the surfaces of bulk oxides, such as the reduced corundum (0001) surface. DA - 2005/06/03/ PY - 2005 DO - 10.1126/science.1107783 VL - 308 IS - 5727 SP - 1440 EP - 1442 J2 - Science ER - TY - JOUR TI - One-dimensional PtO2 at Pt steps: Formation and reaction with CO AU - Wang, J. G. AU - Li, W. X. AU - Borg, M. AU - Gustafson, J. AU - Mikkelsen, A. AU - Pedersen, T. M. AU - Lundgren, E. AU - Weissenrieder, J. AU - Klikovits, J. AU - Schmid, M. AU - Hammer, B. AU - Andersen, J. N. T2 - Physical Review Letters AB - Using core-level spectroscopy and density functional theory we show that a one-dimensional (1D) PtO2 oxide structure forms at the steps of the Pt(332) surface after O2 exposure. The 1D oxide is found to be stable in an oxygen pressure range, where bulk oxides are only metastable, and is therefore argued to be a precursor to the Pt oxidation. As an example of the consequences of such a precursor exclusively present at the steps, we investigate the reaction of CO with oxygen covered Pt(332). Albeit more strongly bound, the oxidic oxygen is found to react more easily with CO than oxygen chemisorbed on the Pt terraces. DA - 2005/12/15/ PY - 2005 DO - 10.1103/PhysRevLett.95.256102 VL - 95 IS - 25 SP - 256102 J2 - Phys. Rev. Lett. ST - One-Dimensional PtO2 at Pt Steps ER - TY - JOUR TI - Influence of nitrogen doping on the defect formation and surface properties of TiO2 Rutile and anatase AU - Batzill, Matthias AU - Morales, Erie AU - Diebold, Ulrike T2 - Physical Review Letters AB - Nitrogen doping-induced changes in the electronic properties, defect formation, and surface structure of TiO2 rutile(110) and anatase(101) single crystals were investigated. No band gap narrowing is observed, but N doping induces localized N 2p states within the band gap just above the valence band. N is present in a N(III) valence state, which facilitates the formation of oxygen vacancies and Ti 3d band gap states at elevated temperatures. The increased O vacancy formation triggers the 1×2 reconstruction of the rutile (110) surface. This thermal instability may degrade the catalyst during applications. DA - 2006/01// PY - 2006 DO - 10.1103/PhysRevLett.96.026103 VL - 96 IS - 2 SP - 026103 J2 - Phys. Rev. Lett. SN - 0031-9007 ER - TY - JOUR TI - Kinetics of the reduction of the Rh(111) surface oxide: Linking spectroscopy and atomic-scale information AU - Klikovits, J. AU - Schmid, M. AU - Gustafson, J. AU - Mikkelsen, A. AU - Resta, A. AU - Lundgren, E. AU - Andersen, J. N. AU - Varga, P. T2 - The Journal of Physical Chemistry B AB - The reduction of the surface oxide on Rh(111) by H2 was observed in situ by scanning tunneling microscopy (STM) and high-resolution core level spectroscopy (HRCLS). At room temperature, H2 does not adsorb on the oxide, only in reduced areas. Reduction starts in very few sites, almost exclusively in stepped areas. One can also initiate the reduction process by deliberately creating defects with the STM tip allowing us to examine the reduction kinetics in detail. Depending on the size of the reduced area and the hydrogen pressure, two growth regimes were found. At low H2 pressures or small reduced areas, the reduction rate is limited by hydrogen adsorption on the reduced area. For large reduced areas, the reduction rate is limited by the processes at the border of the reduced area. Since a near-random distribution of the reduction nuclei was found and the reduction process at defects starts at a random time, one can use JohnsonMehlAvramiKolmogoroff (JMAK) theory to describe the process of reduction. The microscopic data from STM agree well with spatially averaged data from HRCLS measurements. DA - 2006/05/01/ PY - 2006 DO - 10.1021/jp0611875 VL - 110 IS - 20 SP - 9966 EP - 9975 J2 - J. Phys. Chem. B ST - Kinetics of the Reduction of the Rh(111) Surface Oxide ER - TY - JOUR TI - Unusual process of water formation on RuO2(110) by hydrogen exposure at room temperature AU - Knapp, M. AU - Crihan, D. AU - Seitsonen, A. P. AU - Resta, A. AU - Lundgren, E. AU - Andersen, J. N. AU - Schmid, M. AU - Varga, P. AU - Over, H. T2 - The Journal of Physical Chemistry B AB - The reduction mechanism of the RuO2(110) surface by molecular hydrogen exposure is unraveled to an unprecedented level by a combination of temperature programmed reaction, scanning tunneling microscopy, high-resolution core level shift spectroscopy, and density functional theory calculations. We demonstrate that even at room temperature hydrogen exposure to the RuO2(110) surface leads to the formation of water. In a two-step process, hydrogen saturates first the bridging oxygen atoms to form (ObrH) species and subsequently part of these ObrH groups move to the undercoordinated Ru atoms where they form adsorbed water. This latter process is driven by thermodynamics leaving vacancies in the bridging O rows. DA - 2006/07/01/ PY - 2006 DO - 10.1021/jp0626622 VL - 110 IS - 29 SP - 14007 EP - 14010 J2 - J. Phys. Chem. B ER - TY - JOUR TI - Electron-induced oxygen desorption from the TiO2(011)-2x1 surface leads to self-organized vacancies AU - Dulub, Olga AU - Batzill, Matthias AU - Solovev, Sergey AU - Loginova, Elena AU - Alchagirov, Alim AU - Madey, Theodore E. AU - Diebold, Ulrike T2 - Science AB - When low-energy electrons strike a titanium dioxide surface, they may cause the desorption of surface oxygen. Oxygen vacancies that result from irradiating a TiO2(011)-2x1 surface with electrons with an energy of 300 electron volts were analyzed by scanning tunneling microscopy. The cross section for desorbing oxygen from the pristine surface was found to be 9 ({+/-}6) x 10-17 square centimeters, which means that the initial electronic excitation was converted into atomic motion with a probability near unity. Once an O vacancy had formed, the desorption cross sections for its nearest and next-nearest oxygen neighbors were reduced by factors of 100 and 10, respectively. This site-specific desorption probability resulted in one-dimensional arrays of oxygen vacancies. DA - 2007/08/24/ PY - 2007 DO - 10.1126/science.1144787 VL - 317 IS - 5841 SP - 1052 EP - 1056 J2 - Science ER - TY - JOUR TI - Growth of one-dimensional Pd nanowires on the terraces of a reduced SnO2(101) surface AU - Katsiev, Khabibulakh AU - Batzill, Matthias AU - Diebold, Ulrike AU - Urban, Alexander AU - Meyer, Bernd T2 - Physical Review Letters AB - Palladium, vapor-deposited at room temperature on a reduced SnO2(101) surface, forms one-dimensional islands, one atomic layer high, 5 Å wide, and up to 350 Å long. Scanning tunneling microscopy shows that neighboring islands do not merge. First-principles calculations reveal the atomistic processes that lead to this, for metal oxide substrates unusual, overlayer growth. Formation of 1D islands is mediated by a large anisotropy in surface diffusion, strong Pd-Sn interaction, and the lack of stable binding sites at the sides of the Pd islands. Nucleation is defect mediated, and the initial nucleation site determines the width of the resulting nanocluster. DA - 2007/05// PY - 2007 DO - 10.1103/PhysRevLett.98.186102 VL - 98 IS - 18 SP - 186102 J2 - Phys. Rev. Lett. SN - 0031-9007 ER - TY - JOUR TI - Surface and interface properties of metal-organic chemical vapor deposition grown a-plane Mg_x Zn_1– x O (0 ≤ x ≤ 0.3) films AU - Saraf, Gaurav AU - Zhong, Jian AU - Dulub, Olga AU - Diebold, Ulrike AU - Siegrist, Theo AU - Lu, Yicheng T2 - Journal of Electronic Materials AB - The a-plane Mg_x Zn_1−x O (0 ≤ x ≤ 0.3) films were grown on r-plane (0 1 -1 2) sapphire substrates using metal-organic chemical vapor deposition (MOCVD). Growth was done at temperatures from 450 °C to 500 °C, with a typical growth rate of ∼500 nm/h. Field emission scanning electron microscopy (FESEM) images show that the films are smooth and dense. X-ray diffraction (XRD) scans confirm good crystallinity of the films. The interface of Mg_x Zn_1−x O films with r-sapphire was found to be semicoherent as characterized by high-resolution transmission electron microscopy (HRTEM). The Mg_x Zn_1−x O surfaces were characterized using scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). Low-energy electron diffraction (LEED) shows well-ordered and single-crystalline surfaces. The films have a characteristic wavelike surface morphology with needle-shaped domains running predominantly along the crystallographic c-direction. Photoluminescence (PL) measurements show a strong near-band-edge emission without observable deep level emission, indicating a low defect concentration. In-plane optical anisotropic transmission was observed by polarized transmission measurements. DA - 2007/04/01/ PY - 2007 DO - 10.1007/s11664-006-0052-x VL - 36 IS - 4 SP - 446 EP - 451 J2 - J. Electron. Mater. ER - TY - JOUR TI - Fabrication of a well-ordered nanohole array stable at room temperature AU - Aït-Mansour, K. AU - Buchsbaum, A. AU - Ruffieux, P. AU - Schmid, M. AU - Gröning, P. AU - Varga, P. AU - Fasel, R. AU - Gröning, O. T2 - Nano Letters AB - We report on the fabrication of a new type of nanotemplate surface consisting of a hexagonally well-ordered array of one monolayer deep holes with a tunable size of about 4 nm2 and a fixed spacing of 7 nm. The nanohole array fabrication is based on the strain-relief trigonal network formed in the 2 monolayer Ag on Pt(111) system. Removing about 0.1 ML of the Ag top layer of this surface structure, for example, by He- or Ar-ion sputtering, leads to the formation of nanoholes at specific domains of the trigonal network, which are stable at room temperature. DA - 2008/07/01/ PY - 2008 DO - 10.1021/nl8013378 VL - 8 IS - 7 SP - 2035 EP - 2040 J2 - Nano Lett. ER - TY - JOUR TI - Small Au and Pt clusters at the anatase TiO2(101) surface: behavior at Terraces, steps, and surface oxygen vacancies AU - Gong, Xue-Qing AU - Selloni, Annabella AU - Dulub, Olga AU - Jacobson, Peter AU - Diebold, Ulrike T2 - Journal of the American Chemical Society AB - The adsorption properties of Au and Pt metal nanoclusters on TiO2 anatase (101) were calculated using density functional theory. Structures and energetics of adsorbed Au and Pt monomers, dimers, and trimers at clean anatase TiO2(101) terraces and two major step edges, as well as O-vacancies, were systematically determined. The theoretical predictions were tested by vapor-depositing small coverages of Au and Pt on anatase (101) and investigating the resulting clusters with Scanning Tunneling Microscopy. On the clean surface, Au shows a strong tendency to form large clusters that nucleate on step edges. A preference for adsorption at type D-(112) steps is observed, which is probably a result of kinetic effects. For Pt, clusters as small as monomers are observed on the terraces, in agreement with the predicted large binding energy of 2.2 eV. Step edges play a less important role than in the case of Au. Oxygen vacancies, produced by electron irradiation, dramatically influence the growth of Au, while the nucleation behavior of Pt was found to be less affected. DA - 2008/01/01/ PY - 2008 DO - 10.1021/ja0773148 VL - 130 IS - 1 SP - 370 EP - 381 J2 - J. Am. Chem. Soc. ST - Small Au and Pt Clusters at the Anatase TiO2(101) Surface ER - TY - JOUR TI - Step-orientation-dependent oxidation: From 1D to 2D oxides AU - Klikovits, J. AU - Schmid, M. AU - Merte, LR. AU - Varga, P. AU - Westerström, R. AU - Resta, A AU - Andersen, JN AU - Gustafson, J AU - Mikkelsen, A AU - Lundgren, E AU - Mittendorfer, F AU - Kresse, G T2 - Physical Review Letters AB - Using scanning tunneling microscopy and density functional theory, we have studied the initial oxidation of Rh(111) surfaces with two types of straight steps, having {100} and {111} microfacets. The one-dimensional (1D) oxide initially formed at the steps acts as a barrier impeding formation of the 2D oxide on the (111) terrace behind it. We demonstrate that the details of the structure of the 1D oxide govern the rate of 2D oxidation and discuss implications for oxidation of nanoparticles. DA - 2008/12/31/ PY - 2008 DO - 10.1103/PhysRevLett.101.266104 VL - 101 IS - 26 SP - 266104 J2 - Phys. Rev. Lett. SN - 0031-9007 ST - Step-Orientation-Dependent Oxidation ER - TY - JOUR TI - Scanning tunneling microscopy study of a vicinal anatase TiO2 surface AU - Li, Shao-Chun AU - Dulub, Olga AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry C AB - Using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED), the structure of the anatase TiO2 (5 -1 4) surface, ∼10° vicinal to the lowest-energy (101) plane, has been studied. The surface was found to facet into a structure composed of ridges with a uniform width of five lattice units. On the basis of atomically resolved STM and electron counting rules, it is proposed that the sides of the ridges are parallel to (1 -1 0) and (112) planes. These sides might be reconstructed to stabilize the microfaceted structure. Vapor-deposited gold shows pronounced clustering between the ridges, indicating a one-dimensional template effect of the vicinal surface, which supports denser and more uniformly sized Au clusters, as compared to the flat (101) surface. DA - 2008/10/23/ PY - 2008 DO - 10.1021/jp806383c VL - 112 IS - 42 SP - 16166 EP - 16170 J2 - J. Phys. Chem. C ER - TY - JOUR TI - Evidence for the predominance of subsurface defects on reduced anatase TiO2(101) AU - He, Yunbin AU - Dulub, Olga AU - Cheng, Hongzhi AU - Selloni, Annabella AU - Diebold, Ulrike T2 - Physical Review Letters AB - Scanning tunneling microscopy (STM) images taken on a freshly cleaved anatase TiO2(101) sample show an almost perfect surface with very few subsurface impurities and adsorbates. Surface oxygen vacancies are not typically present but can be induced by electron bombardment. In contrast, a reduced anatase (101) crystal shows isolated as well as ordered intrinsic subsurface defects in STM, consistent with density functional theory (DFT) calculations which predict that O vacancies (VO's) at subsurface and bulk sites are significantly more stable than on the surface. DA - 2009/03/13/ PY - 2009 DO - 10.1103/PhysRevLett.102.106105 VL - 102 IS - 10 SP - 106105 EP - 4 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Nucleation and growth of 1D water clusters on rutile TiO2(011)-2×1 AU - He, Yunbin AU - Li, Wei-Kun AU - Gong, Xue-Qing AU - Dulub, Olga AU - Selloni, Annabella AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry C AB - We present a combined experimental and theoretical study of the adsorption of water on the rutile TiO2(011)-2×1 surface, whose “brookite (001)-like” reconstruction has been recently elucidated. By using scanning tunneling microscopy and density functional theory calculations, we provide evidence that water adsorbs weakly on the stoichiometric surface, while hydroxyls resulting from water dissociation at surface O vacancies act as nucleation centers for the growth of H-bonded water clusters that are confined in one dimension. DA - 2009/06/18/ PY - 2009 DO - 10.1021/jp903017x VL - 113 IS - 24 SP - 10329 EP - 10332 J2 - J. Phys. Chem. C ER - TY - JOUR TI - Correlation between bonding geometry and band gap states at organic−inorganic interfaces: catechol on rutile TiO2(110) AU - Li, Shao-Chun AU - Wang, Jian-guo AU - Jacobson, Peter AU - Gong, X.-Q. AU - Selloni, Annabella AU - Diebold, Ulrike T2 - Journal of the American Chemical Society AB - Adsorbate-induced band gap states in semiconductors are of particular interest due to the potential of increased light absorption and photoreactivity. A combined theoretical and experimental (STM, photoemission) study of the molecular-scale factors involved in the formation of gap states in TiO2 is presented. Using the organic catechol on rutile TiO2(110) as a model system, it is found that the bonding geometry strongly affects the molecular electronic structure. At saturation catechol forms an ordered 4 × 1 overlayer. This structure is attributed to catechol adsorbed on rows of surface Ti atoms with the molecular plane tilted from the surface normal in an alternating fashion. In the computed lowest-energy structure, one of the two terminal OH groups at each catechol dissociates and the O binds to a surface Ti atom in a monodentate configuration, whereas the other OH group forms an H-bond to the next catechol neighbor. Through proton exchange with the surface, this structure can easily transform into one where both OH groups dissociate and the catechol is bound to two surface Ti in a bidentate configuration. Only bidendate catechol introduces states in the band gap of TiO2. DA - 2009/01/28/ PY - 2009 DO - 10.1021/ja803595u VL - 131 IS - 3 SP - 980 EP - 984 J2 - J. Am. Chem. Soc. ST - Correlation between Bonding Geometry and Band Gap States at Organic−Inorganic Interfaces ER - TY - JOUR TI - Reactivity of Fe0 atoms, clusters, and nanoparticles with CCl4 multilayers on FeO(111) AU - Parkinson, Gareth S. AU - Dohnálek, Zdenek AU - Smith, R. Scott AU - Kay, Bruce D. T2 - The Journal of Physical Chemistry C AB - The interaction of Fe0 atoms and clusters with CCl4 multilayers was investigated using a novel “atom dropping” method at 30 K over a FeO(111) thin film. Temperature programmed desorption experiments over a range of Fe0 and CCl4 coverages demonstrate a rich surface chemistry with several reaction products (C2Cl4, C2Cl6, OCCl2, CO, FeCl2, and FeCl3) observed. X-ray photoelectron spectroscopy data show that the initial reactive interaction occurs spontaneously at 30 K, with the experimentally observed reaction products formed at higher temperature, in agreement with the results of theoretical calculations. The formation of OCCl2 and CO is concluded to occur through abstraction of O atoms from the generally inert FeO(111) substrate. The buffer layer assisted growth technique is used to show that the reactivity, and interestingly the reaction products, are determined by the size of Fe0 nanoparticles which interact with CCl4. DA - 2009/02/05/ PY - 2009 DO - 10.1021/jp8076062 VL - 113 IS - 5 SP - 1818 EP - 1829 J2 - J. Phys. Chem. C ER - TY - JOUR TI - Influence of subsurface defects on the surface reactivity of TiO2: Water on anatase (101) AU - Aschauer, Ulrich AU - He, Yunbin AU - Cheng, Hongzhi AU - Li, Shao-Chun AU - Diebold, Ulrike AU - Selloni, Annabella T2 - The Journal of Physical Chemistry C AB - The adsorption of water on a reduced TiO2 anatase (101) surface is investigated with scanning tunneling microscopy and density functional theory calculations. The presence of subsurface defects, which are prevalent on reduced anatase (101), leads to a higher desorption temperature of adsorbed water, indicating an enhanced binding due to the defects. Theoretical calculations of water adsorption on anatase (101) surfaces containing subsurface oxygen vacancies or titanium interstitials show a strong selectivity for water binding to sites in the vicinity of the subsurface defects. Moreover, the water adsorption energy at these sites is considerably higher than that on the stoichiometric surface, thus giving an explanation for the experimental observations. The calculations also predict facile water dissociation at these sites, confirming the important role of defects in the surface chemistry of TiO2. DA - 2010/01/21/ PY - 2010 DO - 10.1021/jp910492b VL - 114 IS - 2 SP - 1278 EP - 1284 J2 - J. Phys. Chem. C ST - Influence of Subsurface Defects on the Surface Reactivity of TiO2 ER - TY - JOUR TI - Reactivity of TiO2 rutile and anatase surfaces toward nitroaromatics AU - Li, Shao-Chun AU - Diebold, Ulrike T2 - Journal of the American Chemical Society AB - The Au−TiO2 system is a promising catalyst for the synthesis of nitro-aromatic compounds. The adsorption of azobenzene (C6H5N═NH5C6) and aniline (C6H5NH2) on two single-crystalline TiO2 surfaces, anatase (101) and rutile (110), has been investigated with scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and X-ray photoemission spectroscopy (XPS). While azobenzene adsorbs as an intact molecule at low coverages, ordered overlayers of phenyl imide (C6H5N) form at saturation coverage, indicating that TiO2 surfaces cleave the N═N bond even without the presence of Au. The same superstructures, p(1 × 2) on anatase and c(2 × 2) on rutile, form upon adsorption of aniline, suggesting the formation of the same, or a very similar, reaction intermediate. These results suggest that the main role of the supported Au in catalytic aniline ↔ azobenzene conversion is the activation of O2/H2 for de/hydrogenation reactions. DA - 2010/01/13/ PY - 2010 DO - 10.1021/ja907865t VL - 132 IS - 1 SP - 64 EP - 66 J2 - J. Am. Chem. Soc. ER - TY - JOUR TI - Adsorption-site-dependent electronic structure of catechol on the anatase TiO2(101) surface AU - Li, Shao-Chun AU - Losovyj, Yaroslav AU - Diebold, Ulrike T2 - Langmuir AB - The adsorption of catechol (1,2-benzendiol) on the anatase TiO2(101) surface was studied with synchrotron-based ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). Catechol adsorbs with a unity sticking coefficient and the phenyl ring intact. STM reveals preferred nucleation at step edges and subsurface point defects, followed by 1D growth and the formation of a 2 × 1 superstructure at full coverage. A gap state of ∼1 eV above the valence band maximum is observed for dosages in excess of ∼0.4 Langmuir, but such a state is absent for lower coverages. The formation of the band gap states thus correlates with the adsorption at regular lattice sites and the onset of self-assembled superstructures. DA - 2011/07/19/ PY - 2011 DO - 10.1021/la201553k VL - 27 IS - 14 SP - 8600 EP - 8604 J2 - Langmuir ER - TY - JOUR TI - Growth and organization of an organic molecular monolayer on TiO2: Catechol on anatase (101) AU - Liu, Li-Min AU - Li, Shao-Chun AU - Cheng, Hongzhi AU - Diebold, Ulrike AU - Selloni, Annabella T2 - Journal of the American Chemical Society AB - Anatase TiO2 is a widely used photocatalytic material, and catechol (1,2-benzendiol) is a model organic sensitizer for dye-sensitized solar cells. The growth and the organization of a catecholate monolayer on the anatase (101) surface were investigated with scanning tunneling microscopy and density functional theory calculations. Isolated molecules adsorb preferentially at steps. On anatase terraces, monodentate (‘D1’) and bidentate (‘D2’) conformations are both present in the dilute limit, and frequent interconversions can take place between these two species. A D1 catechol is mobile at room temperature and can explore the most favorable surface adsorption sites, whereas D2 is essentially immobile. When a D1 molecule arrives in proximity of another adsorbed catechol in an adjacent row, it is energetically convenient for them to pair up in nearest-neighbor positions taking a D2–D2 or D2–D1 configuration. This intermolecular interaction, which is largely substrate mediated, causes the formation of one-dimensional catecholate islands that can change in shape but are stable to break-up. The change between D1 and D2 conformations drives both the dynamics and the energetics of this model system and is possibly of importance in the functionalization of dye-sensitized solar cells. DA - 2011/05/25/ PY - 2011 DO - 10.1021/ja200001r VL - 133 IS - 20 SP - 7816 EP - 7823 J2 - J. Am. Chem. Soc. ST - Growth and Organization of an Organic Molecular Monolayer on TiO2 ER - TY - JOUR TI - Evidence for s–d hybridization in Au38 clusters AU - Losovyj, Yaroslav B. AU - Li, Shao-Chun AU - Lozova, Natalia AU - Katsiev, Khabibulakh AU - Stellwagen, Daniel AU - Diebold, Ulrike AU - Kong, Lingmei AU - Kumar, Challa S. S. R. T2 - The Journal of Physical Chemistry C AB - Resonant enhancement is seen in some gold clusters at the gold 4f7/2 and 4f5/2 threshold, indicative of an f to d transition. The existence of such resonance is generally not observed in ultrasmall Au nanoclusters because this resonant transition is forbidden in atomic gold, as gold represents a filled 5d106s1 system. Here we report, for the first time, the presence of a strong resonant enhancement in photoemission at the 4f7/2 threshold and a weaker resonant enhancement at the 4f5/2 threshold, in the open (undressed) Au38 cluster system, indicating not only an f to d Coster-Kronig resonance but also s-d hybridization, much like what is observed in gold films. This f to d Coster-Kronig photoemission resonance is not observed in the "undressed" thiol-terminated gold clusters characteristic of far more localized orbitals. These results indicate that the unusual catalytic properties of ultrasmall gold nanoclusters such as Au38 are not a result of localized orbitals, which is the case in a molecular system. DA - 2012/// PY - 2012 DO - 10.1021/jp3010508 VL - 116 IS - 9 SP - 5857 EP - 5861 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - Control and near-field detection of surface plasmon interference patterns AU - Dvořák, Petr AU - Neuman, Tomáš AU - Břínek, Lukáš AU - Šamořil, Tomáš AU - Kalousek, Radek AU - Dub, Petr AU - Varga, Peter AU - Šikola, Tomáš T2 - Nano Letters AB - The tailoring of electromagnetic near-field properties is the central task in the field of nanophotonics. In addition to 2D optics for optical nanocircuits, confined and enhanced electric fields are utilized in detection and sensing, photovoltaics, spatially localized spectroscopy (nanoimaging), as well as in nanolithography and nanomanipulation. For practical purposes, it is necessary to develop easy-to-use methods for controlling the electromagnetic near-field distribution. By imaging optical near-fields using a scanning near-field optical microscope, we demonstrate that surface plasmon polaritons propagating from slits along the metal–dielectric interface form tunable interference patterns. We present a simple way how to control the resulting interference patterns both by variation of the angle between two slits and, for a fixed slit geometry, by a proper combination of laser beam polarization and inhomogeneous far-field illumination of the structure. Thus the modulation period of interference patterns has become adjustable and new variable patterns consisting of stripelike and dotlike motifs have been achieved, respectively. DA - 2013/06/12/ PY - 2013 DO - 10.1021/nl400644r VL - 13 IS - 6 SP - 2558 EP - 2563 J2 - Nano Lett. SN - 1530-6984 ER - TY - JOUR TI - Direct imaging of covalent bond structure in single-molecule chemical reactions AU - Oteyza, Dimas G. de AU - Gorman, Patrick AU - Chen, Yen-Chia AU - Wickenburg, Sebastian AU - Riss, Alexander AU - Mowbray, Duncan J. AU - Etkin, Grisha AU - Pedramrazi, Zahra AU - Tsai, Hsin-Zon AU - Rubio, Angel AU - Crommie, Michael F. AU - Fischer, Felix R. T2 - Science AB - Observing the intricate chemical transformation of an individual molecule as it undergoes a complex reaction is a long-standing challenge in molecular imaging. Advances in scanning probe microscopy now provide the tools to visualize not only the frontier orbitals of chemical reaction partners and products, but their internal covalent bond configurations as well. We used noncontact atomic force microscopy to investigate reaction-induced changes in the detailed internal bond structure of individual oligo-(phenylene-1,2-ethynylenes) on a (100) oriented silver surface as they underwent a series of cyclization processes. Our images reveal the complex surface reaction mechanisms underlying thermally induced cyclization cascades of enediynes. Calculations using ab initio density functional theory provide additional support for the proposed reaction pathways. Watching Organic Reactions Single-molecule studies can overcome the difficulty of inferring the various outcomes of reactions in ensemble measurements. De Oteyza et al. (p. 1434, published online 30 May; see the Perspective by Giessibl) used a variation of noncontact atomic force microscopy in which the imaging tip was derivatized with a single CO molecule to obtain subnanometer-resolution images of conjugated organic molecules undergoing reaction on a silver surface. Different thermally induced cyclization reactions of oligo- (phenylene-1,2-ethynylenes) were observed. DA - 2013/06/21/ PY - 2013 DO - 10.1126/science.1238187 VL - 340 IS - 6139 SP - 1434 EP - 1437 J2 - Science SN - 0036-8075, 1095-9203 ER - TY - JOUR TI - Squeezing, then stacking: From breathing pores to three-dimensional ionic self-assembly under electrochemical control AU - Cui, Kang AU - Mali, Kunal S. AU - Ivasenko, Oleksandr AU - Wu, Dongqing AU - Feng, Xinliang AU - Walter, Michael AU - Müllen, Klaus AU - De Feyter, Steven AU - Mertens, Stijn F. L. T2 - Angewandte Chemie International Edition AB - We demonstrate the spontaneous and reversible transition between the two- and three-dimensional self-assembly of a supramolecular system at the solid–liquid interface under electrochemical conditions, using in situ scanning tunneling microscopy. By tuning the interfacial potential, we can selectively organize our target molecules in an open porous pattern, fill these pores to form an auto-host–guest structure, or stack the building blocks in a stratified bilayer. Using a simple electrostatic model, we rationalize which charge density is required to enable bilayer formation, and conversely, which molecular size/charge ratio is necessary in the design of new building blocks. Our results may lead to a new class of electrochemically controlled dynamic host–guest systems, artificial receptors, and smart materials. DA - 2014/11/17/ PY - 2014 DO - 10.1002/anie.201406246 VL - 53 IS - 47 SP - 12951 EP - 12954 J2 - Angew. Chem. Int. Ed. SN - 14337851 ST - Squeezing, Then Stacking ER - TY - JOUR TI - Wetting, adhesion and stiction of 2D materials AU - Mertens, Stijn F. L. T2 - ECS Transactions AB - The wetting properties of surfaces, including the adhesion and stiction (static friction) of liquid drops, are critical for processing materials and post-production problems such as fouling. For the novel but very active research field of 2D materials, the interaction of liquids with one- or few-atom-thick matter and its support poses new questions in our understanding of wetting. This paper presents a brief overview of recent research in this area, highlighting differences with materials of other dimensionalities, and remaining issues. DA - 2017/08/15/ PY - 2017 DO - 10.1149/08002.0023ecst VL - 80 IS - 2 SP - 23 EP - 27 J2 - ECS Trans. SN - 1938-6737, 1938-5862 ER - TY - JOUR TI - Real-time observation of collector droplet oscillations during growth of straight nanowires AU - Kolíbal, Miroslav AU - Vystavěl, Tomáš AU - Varga, Peter AU - Šikola, Tomáš T2 - Nano Letters AB - A liquid droplet sitting on top of a pillar is crucially important for semiconductor nanowire growth via a vapor–liquid–solid (VLS) mechanism. For the growth of long and straight nanowires, it has been assumed so far that the droplet is pinned to the nanowire top and any instability in the droplet position leads to nanowire kinking. Here, using real-time in situ scanning electron microscopy during germanium nanowire growth, we show that the increase or decrease in the droplet wetting angle and subsequent droplet unpinning from the growth interface may also result in the growth of straight nanowires. Because our argumentation is based on terms and parameters common for VLS-grown nanowires, such as the geometry of the droplet and the growth interface, these conclusions are likely to be relevant to other nanowire systems. DA - 2014/04/09/ PY - 2014 DO - 10.1021/nl404159x VL - 14 IS - 4 SP - 1756 EP - 1761 J2 - Nano Lett. SN - 1530-6984 ER - TY - JOUR TI - Guided assembly of gold colloidal nanoparticles on silicon substrates prepatterned by charged particle beams AU - Kolíbal, Miroslav AU - Konečný, Martin AU - Ligmajer, Filip AU - Škoda, David AU - Vystavěl, Tomáš AU - Zlámal, Jakub AU - Varga, Peter AU - Šikola, Tomáš T2 - ACS Nano AB - Colloidal gold nanoparticles represent technological building blocks which are easy to fabricate while keeping full control of their shape and dimensions. Here, we report on a simple two-step maskless process to assemble gold nanoparticles from a water colloidal solution at specific sites of a silicon surface. First, the silicon substrate covered by native oxide is exposed to a charged particle beam (ions or electrons) and then immersed in a HF-modified solution of colloidal nanoparticles. The irradiation of the native oxide layer by a low-fluence charged particle beam causes changes in the type of surface-terminating groups, while the large fluences induce even more profound modification of surface composition. Hence, by a proper selection of the initial substrate termination, solution pH, and beam fluence, either positive or negative deposition of the colloidal nanoparticles can be achieved. DA - 2012/11/27/ PY - 2012 DO - 10.1021/nn3038226 VL - 6 IS - 11 SP - 10098 EP - 10106 J2 - ACS Nano SN - 1936-0851 ER - TY - JOUR TI - Probing the role of interlayer coupling and Coulomb interactions on electronic structure in few-layer MoSe2 nanostructures AU - Bradley, Aaron J. AU - M. Ugeda, Miguel AU - da Jornada, Felipe H. AU - Qiu, Diana Y. AU - Ruan, Wei AU - Zhang, Yi AU - Wickenburg, Sebastian AU - Riss, Alexander AU - Lu, Jiong AU - Mo, Sung-Kwan AU - Hussain, Zahid AU - Shen, Zhi-Xun AU - Louie, Steven G. AU - Crommie, Michael F. T2 - Nano Letters AB - Despite the weak nature of interlayer forces in transition metal dichalcogenide (TMD) materials, their properties are highly dependent on the number of layers in the few-layer two-dimensional (2D) limit. Here, we present a combined scanning tunneling microscopy/spectroscopy and GW theoretical study of the electronic structure of high quality single- and few-layer MoSe2 grown on bilayer graphene. We find that the electronic (quasiparticle) bandgap, a fundamental parameter for transport and optical phenomena, decreases by nearly one electronvolt when going from one layer to three due to interlayer coupling and screening effects. Our results paint a clear picture of the evolution of the electronic wave function hybridization in the valleys of both the valence and conduction bands as the number of layers is changed. This demonstrates the importance of layer number and electron–electron interactions on van der Waals heterostructures and helps to clarify how their electronic properties might be tuned in future 2D nanodevices. DA - 2015/04/08/ PY - 2015 DO - 10.1021/acs.nanolett.5b00160 VL - 15 IS - 4 SP - 2594 EP - 2599 J2 - Nano Lett. SN - 1530-6984, 1530-6992 ER - TY - CHAP TI - Adsorption on metal oxide surfaces AU - Parkinson, Gareth S. AU - Diebold, Ulrike T2 - Surface and Interface Science A2 - Wandelt, Klaus AB - In this chapter we describe how the ionic bonding present in metal oxide materials has important consequences for the adsorption of molecules at their surfaces. The chapter begins with a brief discussion of metal oxide surface structure, and explains why polarity compensation ensures that both charged cations and anions are present at many surfaces. Molecular and dissociative adsorption are described in terms of acid-base reactions, using simple concepts borrowed from solution chemistry. The importance of defects such as steps and surface oxygen vacancies are described, first in terms of increased binding strength due to the higher coordinative unsaturation of lattice atoms at these sites, and second as a source of electrons that can be transferred to adsorbates. The chapter culminates with a perspective that highlights the need for further studies to develop an understanding of hydroxylated and doped metal oxide surfaces, as well as multi-valent and ternary compounds. DA - 2015/// PY - 2015 VL - 6 SP - 793 EP - 817 PB - Wiley-VCH Verlag GmbH & Co. KGaA SN - 978-3-527-41158-0 ER - TY - JOUR TI - Reversible anion-driven switching of an organic 2D crystal at a solid–liquid interface AU - Cui, Kang AU - Mali, Kunal S. AU - Wu, Dongqing AU - Feng, Xinliang AU - Müllen, Klaus AU - Walter, Michael AU - De Feyter, Steven AU - Mertens, Stijn F. L. T2 - Small AB - Ionic self-assembly of charged molecular building blocks relies on the interplay between long-range electrostatic forces and short-range, often cooperative, supramolecular interactions, yet has been seldom studied in two dimensions at the solid–liquid interface. Here, we demonstrate anion-driven switching of two-dimensional (2D) crystal structure at the Au(111)/octanoic acid interface. Using scanning tunneling microscopy (STM), three organic salts with identical polyaromatic cation (PQPC6+) but different anions (perchlorate, anthraquinonedisulfonate, benzenesulfonate) are shown to form distinct, highly ordered self-assembled structures. Reversible switching of the supramolecular arrangement is demonstrated by in situ exchange of the anion on the pre-formed adlayer, by changing the concentration ratio between the incoming and outgoing anion. Density functional theory (DFT) calculations reveal that perchlorate is highly mobile in the adlayer, and corroborate why this anion is only resolved transiently in STM. Surprisingly, the templating effect of the anion persists even where it does not become part of the adlayer 2D fabric, which we ascribe to differences in stabilization of cation conformations by the anion. Our results provide important insight into the structuring of mixed anion–cation adlayers. This is essential in the design of tectons for ionic self-assembled superstructures and biomimetic adaptive materials and valuable also to understand adsorbate–adsorbate interactions in heterogeneous catalysis. DA - 2017/12/13/ PY - 2017 DO - 10.1002/smll.201702379 VL - 13 IS - 46 SP - 1702379 J2 - Small SN - 1613-6829 ER - TY - JOUR TI - Unravelling single atom catalysis: The surface science approach AU - Parkinson, Gareth S. T2 - Chinese Journal of Catalysis AB - This perspective discusses how studies of idealised model systems can shed light on the fundamental mechanisms of single-atom catalysis. DA - 2017/09// PY - 2017 DO - 10.1016/S1872-2067(17)62878-X VL - 38 IS - 9 SP - 1454 EP - 1459 J2 - Chinese J. Catal. SN - 18722067 ST - Unravelling single atom catalysis ER - TY - JOUR TI - Zirconium-palladium interactions during dry reforming of methane AU - Köpfle, Norbert AU - Mayr, Lukas AU - Lackner, Peter AU - Schmid, Michael AU - Schmidmair, Daniela AU - Götsch, Thomas AU - Penner, Simon AU - Kloetzer, Bernhard T2 - ECS Transactions AB - Catalytic investigations on chemical-vapor-deposition (CVD)-prepared Pd/Zr^0 /ZrO_x H_y inverse model catalysts and Pd/Zr intermetallic compound system in dry reforming of methane (DRM) are presented. DRM, which produces syngas, is an economically favourable way to operate an SOFC by reusing the already heated CO2 exhaust. The catalytic investigations of the Pd/Zr system yield important information for the design of novel electrode materials or external reforming catalysts. From a catalytic perspective, the initially bimetallic Pd-Zr pre-catalyst shows a distinct activity for dry reforming of methane. This activity can be ascribed to synergistic bifunctional cooperation of palladium and zirconium. Moreover, the investigations clearly demonstrate that metallic Zr is crucial to observe any activity. Therefore, different bulk and surface sensitive methods are used to follow the evolution of structural and redox changes of Zr. Studies of single-crystalline Pd(111) show that Zr^0 is located exclusively in subsurface layers after annealing in vacuum and prior to reaction. DA - 2017/// PY - 2017 DO - 10.1149/07801.2419ecst VL - 78 IS - 1 SP - 2419 EP - 2430 J2 - ECS Trans. SN - 2151-2041, 2151-2043 ER - TY - JOUR TI - A full monolayer of superoxide: oxygen activation on the unmodified Ca3Ru2O7(001) surface AU - Halwidl, Daniel AU - Mayr-Schmölzer, Wernfried AU - Setvin, Martin AU - Fobes, David AU - Peng, Jin AU - Mao, Zhiqiang AU - Schmid, Michael AU - Mittendorfer, Florian AU - Redinger, Josef AU - Diebold, Ulrike T2 - Journal of Materials Chemistry A AB - Activating the O2 molecule is at the heart of a variety of technological applications, most prominently in energy conversion schemes including solid oxide fuel cells, electrolysis, and catalysis. Perovskite oxides, both traditionally-used and novel formulations, are the prime candidates in established and emerging energy devices. This work shows that the as-cleaved and unmodified CaO-terminated (001) surface of Ca3Ru2O7, a Ruddlesden–Popper perovskite, supports a full monolayer of superoxide ions, O2−, when exposed to molecular O2. The electrons for activating the molecule are transferred from the subsurface RuO2 layer. Theoretical calculations using both, density functional theory (DFT) and more accurate methods (RPA), predict the adsorption of O2− with Eads = 0.72 eV and provide a thorough analysis of the charge transfer. Non-contact atomic force microscopy (nc-AFM) and scanning tunnelling microscopy (STM) are used to resolve single molecules and confirm the predicted adsorption structures. Local contact potential difference (LCPD) and X-ray photoelectron spectroscopy (XPS) measurements on the full monolayer of O2− confirm the negative charge state of the molecules. The present study reports the rare case of an oxide surface without dopants, defects, or low-coordinated sites readily activating molecular O2. DA - 2018/04/03/ PY - 2018 DO - 10.1039/C8TA00265G VL - 6 IS - 14 SP - 5703 EP - 5713 J2 - J. Mater. Chem. A SN - 2050-7496 ST - A full monolayer of superoxide ER - TY - JOUR TI - Metal Tungstates at the Ultimate Two-Dimensional Limit: Fabrication of a CuWO4 Nanophase AU - Denk, Martin AU - Kuhness, David AU - Wagner, Margareta AU - Surnev, Svetlozar AU - Negreiros, Fabio R. AU - Sementa, Luca AU - Barcaro, Giovanni AU - Vobornik, Ivana AU - Fortunelli, Alessandro AU - Netzer, Falko P. T2 - ACS Nano AB - Metal tungstates (with general formula MWO4) are functional materials with a high potential for a diverse set of applications ranging from low-dimensional magnetism to chemical sensing and photoelectrocatalytic water oxidation. For high level applications, nanoscale control of film growth is necessary, as well as a deeper understanding and characterization of materials properties at reduced dimensionality. We succeeded in fabricating and characterizing a two-dimensional (2-D) copper tungstate (CuWO4). For the first time, the atomic structure of an ultrathin ternary oxide is fully unveiled. It corresponds to a CuWO4 monolayer arranged in three sublayers with stacking O–W–O/Cu from the interface. The resulting bidimensional structure forms a robust framework with localized regions of anisotropic flexibility. Electronically it displays a reduced band gap and increased density of states close to the Fermi level with respect to the bulk compound. These unique features open a way for new applications in the field of photo- and electrocatalysis, while the proposed synthesis method represents a radically new and general approach toward the fabrication of 2-D ternary oxides. DA - 2014/04/22/ PY - 2014 DO - 10.1021/nn500867y VL - 8 IS - 4 SP - 3947 EP - 3954 J2 - ACS Nano SN - 1936-0851 ST - Metal Tungstates at the Ultimate Two-Dimensional Limit ER - TY - JOUR TI - Oxide surface science AU - Diebold, Ulrike AU - Li, Shao-Chun AU - Schmid, Michael T2 - Annual Review of Physical Chemistry AB - Most metals are oxidized under ambient conditions, and metal oxides show interesting and technologically promising properties. This has motivated much recent research on oxide surfaces. The combination of scanning tunneling microscopy with first-principles density functional theory–based computational techniques provides an atomic-scale view of the properties of metal-oxide materials. Surface polarity is a key concept for predicting the stability of oxide surfaces and is discussed using ZnO as an example. This review also highlights the role of surface defects for surface reactivity, and their interplay with defects in the bulk, for the case of TiO2. Ultrathin metal-oxide films, grown either through reactive evaporation on metal single crystals or through oxidation of metal alloys (such as Al2O3/NiAl), have gained popularity as supports for planar model catalysts. The surface oxides that form upon oxidation on Pt-group metals (e.g., Ru, Rh, Pd, and Pt) are considered as model systems for CO oxidation. DA - 2010/03// PY - 2010 DO - 10.1146/annurev.physchem.012809.103254 VL - 61 IS - 1 SP - 129 EP - 148 J2 - Annu. Rev. Phys. Chem. SN - 0066-426X ER - TY - JOUR TI - Polarity compensation mechanisms on the perovskite surface KTaO3(001) AU - Setvin, Martin AU - Reticcioli, Michele AU - Poelzleitner, Flora AU - Hulva, Jan AU - Schmid, Michael AU - Boatner, Lynn A. AU - Franchini, Cesare AU - Diebold, Ulrike T2 - Science AB - Compensating a polar surface An ionic crystal surface can be electrostatically unstable, and the surface must reconstruct in some way to avoid this “polar catastrophe.” Setvin et al. used scanning probe microscopies and density functional theory to study the changes in the polar surface of the perovskite KTaO3. They observed several structural reconstructions as the surface cleaved in vacuum was heated to higher temperatures. These ranged from surface distortions to the formation of oxygen vacancies to the development of KO and TaO2 stripes. Hydroxylation after exposure to water vapor also stabilized the surface. Science, this issue p. 572 The stacking of alternating charged planes in ionic crystals creates a diverging electrostatic energy—a “polar catastrophe”—that must be compensated at the surface. We used scanning probe microscopies and density functional theory to study compensation mechanisms at the perovskite potassium tantalate (KTaO3) (001) surface as increasing degrees of freedom were enabled. The as-cleaved surface in vacuum is frozen in place but immediately responds with an insulator-to-metal transition and possibly ferroelectric lattice distortions. Annealing in vacuum allows the formation of isolated oxygen vacancies, followed by a complete rearrangement of the top layers into an ordered pattern of KO and TaO2 stripes. The optimal solution is found after exposure to water vapor through the formation of a hydroxylated overlayer with ideal geometry and charge. An ionic material can alleviate the energetic instability of its polar surface in several different ways. An ionic material can alleviate the energetic instability of its polar surface in several different ways. DA - 2018/02/02/ PY - 2018 DO - 10.1126/science.aar2287 VL - 359 IS - 6375 SP - 572 EP - 575 J2 - Science SN - 0036-8075, 1095-9203 ER - TY - JOUR TI - Surface structure of TiO2 rutile (011) exposed to liquid water AU - Balajka, Jan AU - Aschauer, Ulrich AU - Mertens, Stijn F. L. AU - Selloni, Annabella AU - Schmid, Michael AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry C AB - The rutile TiO2(011) surface exhibits a (2 × 1) reconstruction when prepared by standard techniques in ultrahigh vacuum (UHV). Here we report that a restructuring occurs upon exposing the surface to liquid water at room temperature. The experiment was performed in a dedicated UHV system, equipped for direct and clean transfer of samples between UHV and liquid environment. After exposure to liquid water, an overlayer with a (2 × 1) symmetry was observed containing two dissociated water molecules per unit cell. The two OH groups yield an apparent “c(2 × 1)” symmetry in scanning tunneling microscopy (STM) images. On the basis of STM analysis and density functional theory (DFT) calculations, this overlayer is attributed to dissociated water on top of the unreconstructed (1 × 1) surface. Investigation of possible adsorption structures and analysis of the domain boundaries in this structure provide strong evidence that the original (2 × 1) reconstruction is lifted. Unlike the (2 × 1) reconstruction, the (1 × 1) surface has an appropriate density and symmetry of adsorption sites. The possibility of contaminant-induced restructuring was excluded based on X-ray photoelectron spectroscopy (XPS) and low-energy He+ ion scattering (LEIS) measurements. DA - 2017/11/30/ PY - 2017 DO - 10.1021/acs.jpcc.7b09674 VL - 121 IS - 47 SP - 26424 EP - 26431 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - The role of surface defects in the adsorption of methanol on Fe3O4(001) AU - Gamba, Oscar AU - Hulva, Jan AU - Pavelec, Jiri AU - Bliem, Roland AU - Schmid, Michael AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - Topics in Catalysis AB - The adsorption of methanol (CH3OH) at the Fe3O4(001)−(√2 × √2)R45° surface was studied using X-ray photoelectron spectroscopy, scanning tunneling microscopy, and temperature-programmed desorption (TPD). CH3OH adsorbs exclusively at surface defect sites at room temperature to form hydroxyl groups and methoxy (CH3O) species. Active sites are identified as step edges, iron adatoms, antiphase domain boundaries in the (√2 × √2)R45° reconstruction, and above Fe atoms incorporated in the subsurface. In TPD, recombinative desorption is observed around 300 K, and a disproportionation reaction to form methanol and formaldehyde occurs at 470 K. DA - 2017/05/01/ PY - 2017 DO - 10.1007/s11244-016-0713-9 VL - 60 IS - 6-7 SP - 420 EP - 430 J2 - Top Catal SN - 1022-5528, 1572-9028 ER - TY - JOUR TI - Self-limiting adsorption of WO3 oligomers on oxide substrates in solution AU - Müllner, Matthias AU - Balajka, Jan AU - Schmid, Michael AU - Diebold, Ulrike AU - Mertens, Stijn F. L. T2 - The Journal of Physical Chemistry C AB - Electrochemical surface science of oxides is an emerging field with expected high impact in developing, for instance, rationally designed catalysts. The aim in such catalysts is to replace noble metals by earth-abundant elements, yet without sacrificing activity. Gaining an atomic-level understanding of such systems hinges on the use of experimental surface characterization techniques such as scanning tunneling microscopy (STM), in which tungsten tips have been the most widely used probes, both in vacuum and under electrochemical conditions. Here, we present an in situ STM study with atomic resolution that shows how tungsten(VI) oxide, spontaneously generated at a W STM tip, forms 1D adsorbates on oxide substrates. By comparing the behavior of rutile TiO2(110) and magnetite Fe3O4(001) in aqueous solution, we hypothesize that, below the point of zero charge of the oxide substrate, electrostatics causes water-soluble WO3 to efficiently adsorb and form linear chains in a self-limiting manner up to submonolayer coverage. The 1D oligomers can be manipulated and nanopatterned in situ with a scanning probe tip. As WO3 spontaneously forms under all conditions of potential and pH at the tungsten–aqueous solution interface, this phenomenon also identifies an important caveat regarding the usability of tungsten tips in electrochemical surface science of oxides and other highly adsorptive materials. DA - 2017/09/14/ PY - 2017 DO - 10.1021/acs.jpcc.7b04076 VL - 121 IS - 36 SP - 19743 EP - 19750 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - Formaldehyde adsorption on the anatase TiO2(101) surface: Experimental and theoretical investigation AU - Setvin, Martin AU - Hulva, Jan AU - Wang, Honghong AU - Simschitz, Thomas AU - Schmid, Michael AU - Parkinson, Gareth S. AU - Di Valentin, Cristiana AU - Selloni, Annabella AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry C AB - Formaldehyde (CH2O) adsorption on the anatase TiO2(101) surface was studied with a combination of experimental and theoretical methods. Scanning tunneling microscopy, noncontact atomic force microscopy, temperature-programmed desorption, and X-ray photoelectron spectroscopy were employed on the experimental side. Density functional theory was used to calculate formaldehyde adsorption configurations and energy barriers for transitions between them. At low coverages (<0.25 monolayer), CH2O binds via its oxygen atom to the surface 5-coordinated Ti atoms Ti5c (monodentate configuration). At higher coverages, many adsorption configurations with comparable adsorption energies coexist, including a bidentate configuration and paraformaldehyde chains. The adsorption energies of all possible adsorption configurations lie in the range from 0.6 to 0.8 eV. Upon annealing, all formaldehyde molecules desorb below room temperature; no other reaction products were detected. DA - 2017/04/27/ PY - 2017 DO - 10.1021/acs.jpcc.7b01434 VL - 121 IS - 16 SP - 8914 EP - 8922 J2 - J. Phys. Chem. C SN - 1932-7447 ST - Formaldehyde Adsorption on the Anatase TiO2(101) Surface ER - TY - JOUR TI - Methanol on anatase TiO2 (101): mechanistic insights into photocatalysis AU - Setvin, Martin AU - Shi, Xiao AU - Hulva, Jan AU - Simschitz, Thomas AU - Parkinson, Gareth S. AU - Schmid, Michael AU - Di Valentin, Cristiana AU - Selloni, Annabella AU - Diebold, Ulrike T2 - ACS Catalysis AB - The photoactivity of methanol adsorbed on the anatase TiO2 (101) surface was studied by a combination of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations. Isolated methanol molecules adsorbed at the anatase (101) surface show a negligible photoactivity. Two ways of methanol activation were found. First, methoxy groups formed by reaction of methanol with coadsorbed O2 molecules or terminal OH groups are photoactive, and they turn into formaldehyde upon UV illumination. The methoxy species show an unusual C 1s core-level shift of 1.4 eV compared to methanol; their chemical assignment was verified by DFT calculations with inclusion of final-state effects. The second way of methanol activation opens at methanol coverages above 0.5 monolayer (ML), and methyl formate is produced in this reaction pathway. The adsorption of methanol in the coverage regime from 0 to 2 ML is described in detail; it is key for understanding the photocatalytic behavior at high coverages. There, a hydrogen-bonding network is established in the adsorbed methanol layer, and consequently, methanol dissociation becomes energetically more favorable. DFT calculations show that dissociation of the methanol molecule is always the key requirement for hole transfer from the substrate to the adsorbed methanol. We show that the hydrogen-bonding network established in the methanol layer dramatically changes the kinetics of proton transfer during the photoreaction. DA - 2017/10/06/ PY - 2017 DO - 10.1021/acscatal.7b02003 VL - 7 IS - 10 SP - 7081 EP - 7091 J2 - ACS Catal. ST - Methanol on Anatase TiO2 (101) ER - TY - JOUR TI - Atomic-scale structure of the hematite α-Fe2O3(11̅02) “r-cut” surface AU - Kraushofer, Florian AU - Jakub, Zdenek AU - Bichler, Magdalena AU - Hulva, Jan AU - Drmota, Peter AU - Weinold, Michael AU - Schmid, Michael AU - Setvin, Martin AU - Diebold, Ulrike AU - Blaha, Peter AU - Parkinson, Gareth S. T2 - The Journal of Physical Chemistry C AB - The α-Fe2O3(11̅02) surface (also known as the hematite r-cut or (012) surface) was studied using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), and ab initio density functional theory (DFT)+U calculations. Two surface structures are stable under ultrahigh vacuum (UHV) conditions; a stoichiometric (1 × 1) surface can be prepared by annealing at 450 °C in ≈10–6 mbar O2, and a reduced (2 × 1) reconstruction is formed by UHV annealing at 540 °C. The (1 × 1) surface is close to an ideal bulk termination, and the undercoordinated surface Fe atoms reduce the surface bandgap by ≈0.2 eV with respect to the bulk. The work function is measured to be 5.7 ± 0.2 eV, and the VBM is located 1.5 ± 0.1 eV below EF. The images obtained from the (2 × 1) reconstruction cannot be reconciled with previously proposed models, and a new “alternating trench” structure is proposed based on an ordered removal of lattice oxygen atoms. DFT+U calculations show that this surface is favored in reducing conditions and that 4-fold-coordinated Fe2+ cations at the surface introduce gap states approximately 1 eV below EF. The work function on the (2 × 1) termination is 5.4 ± 0.2 eV. DA - 2018/01/25/ PY - 2018 DO - 10.1021/acs.jpcc.7b10515 VL - 122 IS - 3 SP - 1657 EP - 1669 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - Perspective: A controversial benchmark system for water-oxide interfaces: H2O/TiO2(110) AU - Diebold, Ulrike T2 - The Journal of Chemical Physics AB - The interaction of water with the single-crystalline rutile TiO2(110) surface has been the object of intense investigations with both experimental and computational methods. Not only is TiO2(110) widely considered the prototypical oxide surface, its interaction with water is also important in many applications where this material is used. At first, experimental measurements were hampered by the fact that preparation recipes for well-controlled surfaces had yet to be developed, but clear experimental evidence that water dissociation at defects including oxygen vacancies and steps emerged. For a perfect TiO2(110) surface, however, an intense debate has evolved whether or not water adsorbs as an intact molecule or if it dissociates by donating a proton to a so-called bridge-bonded surface oxygen atom. Computational studies agree that the energy difference between these two states is very small and thus depends sensitively on the computational setup and on the approximations used in density functional theory (DFT). While a recent molecular beam/STM experiment [Z.-T. Wang et al., Proc. Natl. Acad. Sci. U. S. A. 114(8), 1801–1805 (2017)] gives conclusive evidence for a slight preference (0.035 eV) for molecular water and a small activation energy of (0.36 eV) for dissociation, understanding the interface between liquid water and TiO2(110) arises as the next controversial frontier. DA - 2017/07/25/ PY - 2017 DO - 10.1063/1.4996116 VL - 147 IS - 4 SP - 040901 J2 - J. Chem. Phys. SN - 0021-9606 ST - Perspective ER - TY - JOUR TI - Revealing the Buried Metal–Organic Interface: Restructuring of the First Layer by van der Waals Forces AU - Wagner, Margareta AU - Berkebile, Stephen AU - Netzer, Falko P. AU - Ramsey, Michael G. T2 - ACS Nano AB - With the use of molecular manipulation in a cryogenic scanning tunneling microscope, the structure and rearrangement of sexiphenyl molecules at the buried interface of the organic film with the Cu(110) substrate surface have been revealed. It is shown that a reconstruction of the first monolayer of flat lying molecules occurs due to the van der Waals pressure from subsequent layers. In this rearrangement, additional sexiphenyl molecules are forced into the established complete monolayer and adopt an edge-on configuration. Incorporation of second layer molecules into the first layer is also demonstrated by purposely pushing sexiphenyl molecules with the STM tip. The results indicate that even chemisorbed organic layers at interfaces can be significantly influenced by external stress from van der Waals forces of subsequent layers. DA - 2015/12/22/ PY - 2015 DO - 10.1021/acsnano.5b05013 VL - 9 IS - 12 SP - 12070 EP - 12078 J2 - ACS Nano SN - 1936-0851 ST - Revealing the Buried Metal–Organic Interface ER - TY - JOUR TI - Copper underpotential deposition on boron nitride nanomesh AU - Mertens, Stijn F. L. T2 - Electrochimica Acta AB - The boron nitride nanomesh is a corrugated monolayer of hexagonal boron nitride (h-BN) on Rh(111), which so far has been studied mostly under ultrahigh vacuum conditions. Here, we investigate how copper underpotential deposition (upd) can be used to quantify defects in the boron nitride monolayer and to assess the potential window of the nanomesh, which is important to explore its functionality under ambient and electrochemical conditions. In dilute sulfuric acid, the potential window of h-BN/Rh(111) is close to 1volt, i.e. larger than that of the Rh substrate, and is limited by molecular hydrogen evolution on the negative and by oxidative removal on the positive side. From copper upd on pristine h-BN/Rh(111) wafer samples, we estimate a collective defect fraction on the order of 0.08–0.7% of the geometric area, which may arise from line and point defects in the h-BN layer that are created during its chemical vapour deposition. Overpotential deposition (opd) is demonstrated to have significant consequences on the defect area. We hypothesise that this non-innocent Cu electrodeposition involves intercalation originating at initial defects, causing irreversible delamination of the h-BN layer; this effect may be used for 2D material nanoengineering. On the relevant timescale, upd itself does not alter the defect area on repeated cycling; therefore, metal upd may find use as a general tool to determine the collective defect area in hybrids between 2D materials and various substrate metals. DA - 2017/08/20/ PY - 2017 DO - 10.1016/j.electacta.2017.06.082 VL - 246 IS - Supplement C SP - 730 EP - 736 J2 - Electrochim. Acta SN - 0013-4686 ER - TY - JOUR TI - Motional capacitance of layered piezoelectric thickness-mode resonators AU - Schmid, M. AU - Benes, E. AU - Burger, W. AU - Kravchenko, V. T2 - IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control AB - The Butterworth-Van Dyke equivalent circuit for description of the electrical behavior of piezoelectric bulk resonators is considered. The motional capacitance, C1, in the circuit characterizes the strength of piezoelectric excitability of a vibration mode. For layered one-dimensional (1-D) structures this parameter can be calculated from the admittance given by the transfer matrix description of H. Nowotny and E. Benes (1987). Introducing the equivalent area of a vibration mode, the calculation is generalized for the three-dimensional (3-D) case of thickness-mode vibration amplitudes varying only slowly in the lateral directions. Detailed formulae are given for the case of singly rotated quartz crystals or ultrasonic transducers with additional layers on one or two sides. Good agreement of the calculated C1 with experimental data is shown for mass-loaded planoconvex AT-cut quartz crystals. DA - 1991/05// PY - 1991 DO - 10.1109/58.79604 VL - 38 IS - 3 SP - 199 EP - 206 J2 - IEEE Trans. Ultrason. Ferroelectr. Freq. Control SN - 0885-3010 ER - TY - CONF TI - Investigation of quartz crystal thickness shear and twist modes using a new noninterferometric laser speckle measurement method AU - Hertl, S. AU - Benes, E. AU - Wimmer, L. AU - Schmid, M. T2 - 39th Annual Symposium on Frequency Control. 1985 AB - The measurement of the mechanical vibration amplitudes of quartz crystals has been carried out by a variety of different methods. Interferometric methods firstly were used to investigate out-of-plane vibrations. Enhanced interferometric setups also allow the measurement of in-plane vibrations. Holographic techniques present an instantaneous picture of the whole object, whereas the interferometric methods supply the vibration amplitude distribution point after point. A very powerful technique to study amplitude distributios of quartz crystals is the use of X-ray diffraction. Using this method, many quartz-crystal vibration modes have been investigated so far. The measurement setup, however, is complicated and the resolution of the graphic output is rather poor. Looking for a simple mechanical setup that is insensitive to environmental disturbances, we developed a new method which allows the measurement of in-plane vibration amplitudes of quartz crystals. The detection range is about one to some hundred nanometers. Due to the use of a microcomputer that controls the whole measurement the graphic output can easily be adapted to the user's needs. C3 - 39th Annual Symposium on Frequency Control. 1985 DA - 1985/// PY - 1985 DO - 10.1109/FREQ.1985.200895 SP - 535 EP - 543 ER - TY - JOUR TI - Layered piezoelectric resonators with an arbitrary number of electrodes (general one-dimensional treatment) AU - Nowotny, Helmut AU - Benes, Ewald AU - Schmid, Michael T2 - Journal of the Acoustical Society of America AB - A general transfer matrix description for one-dimensional layered structures consisting of piezoelectric and nonpiezoelectric anisotropic layers of arbitrary number is used to calculate the electrical admittance matrix for such resonators with N electrodes. The calculation is done in detail for linearly stacked resonators with two free surfaces as well as for ring resonators with a closed acoustical path. Experimental and theoretical results are given and compared for a ring resonator with two piezoelectric layers excited by four electrodes. Such a configuration can be used to generate unidirectional resonant waves. DA - 1991/// PY - 1991 DO - 10.1121/1.401915 VL - 90 IS - 3 SP - 1238 EP - 1245 J2 - J. Acoust. Soc. Am. ER - TY - CONF TI - The role of localized rotational imbalance in drive level dependence phenomena AU - EerNisse, E.P. AU - Benes, E. AU - Schmid, M. T2 - Frequency Control Symposium and PDA Exhibition, 2002. IEEE International AB - The motivation for this study comes from mode shape measurements made of a thickness shear quartz plate with a splatter accidentally attached during Cu deposition. The results show distinct flexural motions extending out to the blank edges in narrow bands. These flexural motions are not present for a clean blank. Finite element analysis calculations are performed for the motion of a contoured quartz thickness shear blank with a small amount of mass attached to the top surface. A 2-dimensional model of the blank is used to facilitate calculation. The added mass causes a localized perturbation of rotational motion that excites a small flexural component extending to the blank edge. This flexural component is similar to a nearby overtone flexural resonant mode. The magnitude of the induced flexural motion varies strongly as the location of the particle moves a few microns from a node to an antinode of the flexural component. This would explain why high level drive can cure a crystal exhibiting drive level dependence (DLD) without the particles being removed and cause others with particles originally not exhibiting DLD to become DLD. It also explains why some blanks with particle contamination function well without showing DLD. C3 - Frequency Control Symposium and PDA Exhibition, 2002. IEEE International DA - 2002/// PY - 2002 DO - 10.1109/FREQ.2002.1075847 SP - 2 EP - 7 PB - IEEE ER - TY - JOUR TI - Bulk terminated NaCl(111) on aluminum: A polar surface of an ionic crystal? AU - Hebenstreit, W. AU - Schmid, M. AU - Redinger, J. AU - Podloucky, R. AU - Varga, P. T2 - Physical Review Letters AB - Atomically resolved scanning tunneling microscopy reveals the existence of triangular (111) bulk terminated NaCl islands. The islands can be grown by subsequent adsorption of Na and Cl2 on Al(111) and Al(100) or by conversion of stoichiometric NaCl(100) islands to NaCl(111) via additional Na adsorption. The NaCl(111) islands have Na-Cl-Na sandwich structure. Ab initio calculations of the electronic structure of these islands show that each of the Na atoms carries half a positive elementary charge, leaving the islands neutral and explaining the existence of an otherwise unstable surface. DA - 2000/12/18/ PY - 2000 DO - 10.1103/PhysRevLett.85.5376 VL - 85 IS - 25 SP - 5376 J2 - Phys. Rev. Lett. ST - Bulk Terminated NaCl(111) on Aluminum ER - TY - JOUR TI - Interlayer diffusion of adatoms: A scanning-tunneling microscopy study AU - Lundgren, E. AU - Stanka, B. AU - Leonardelli, G. AU - Schmid, M. AU - Varga, P. T2 - Physical Review Letters AB - Low coverages of Co are deposited at room temperature on a Pt(111) surface with vacancy islands providing a high density of steps. Scanning-tunneling microscopy images with chemical contrast show that Co atoms do not descend Pt steps by diffusing over the step, but descend from the upper terrace to the lower by an exchange diffusion process with the Pt atoms at the step edge. The Co atoms descend a Pt step edge by this process neither at straight A nor at B steps, but rather at the corners or kinks of the vacancy islands. DA - 1999/06/21/ PY - 1999 DO - 10.1103/PhysRevLett.82.5068 VL - 82 IS - 25 SP - 5068 J2 - Phys. Rev. Lett. ST - Interlayer Diffusion of Adatoms ER - TY - JOUR TI - High transient mobility of chlorine on TiO2(110): Evidence for "cannon-ball" trajectories of hot adsorbates AU - Diebold, Ulrike AU - Hebenstreit, Wilhelm AU - Leonardelli, Georg AU - Schmid, Michael AU - Varga, Peter T2 - Physical Review Letters AB - Scanning tunneling microscopy was used to study the initial stages of Cl2 adsorption on TiO2(110). Cl atoms adsorb on the rows of fivefold coordinated surface Ti atoms, and mostly form well separated pairs (average distance 26 Å, atoms can be two or three rows apart). Abstractive adsorption results in 10% single Cl adatoms. We propose that Cl2 dissociates in an approximately upright position. The outer Cl atom is emitted along the bond axis and can surmount the substrate bridging oxygen rows in a “cannon-ball”-like trajectory. Channeling along the Ti rows leads to large average Cl-Cl distances. DA - 1998/07/13/ PY - 1998 DO - 10.1103/PhysRevLett.81.405 VL - 81 IS - 2 SP - 405 J2 - Phys. Rev. Lett. ST - High Transient Mobility of Chlorine on TiO2(110) ER - TY - JOUR TI - Potential sputtering of clean SiO2 by slow highly charged ions AU - Sporn, M. AU - Libiseller, G. AU - Neidhart, T. AU - Schmid, M. AU - Aumayr, F. AU - Winter, HP. AU - Varga, P. AU - Grether, M. AU - Niemann, D. AU - Stolterfoht, N. T2 - Physical Review Letters AB - The recently discovered phenomenon of potential sputtering, i.e., the efficient removal of neutral and ionized target particles from certain insulator surfaces due to the potential rather than the kinetic energy of impinging slow highly charged ions, has now also been observed for stoichiometric SiO2 surfaces. Using a sensitive quartz crystal microbalance technique, total sputter yields induced by Ar^q+ (q≤14) and Xe^q+ (q≤27) ions have been determined for LiF and SiO2 surfaces. The primary mechanisms for potential sputtering (defect mediated sputtering) and its considerable practical relevance for highly charged ion-induced surface modification of insulators are discussed. DA - 1997/// PY - 1997 DO - 10.1103/PhysRevLett.79.945 VL - 79 IS - 5 SP - 945 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Quantum wells and electron interference phenomena in Al due to subsurface noble gas bubbles AU - Schmid, M. AU - Hebenstreit, W. AU - Varga, P. AU - Crampin, S. T2 - Physical Review Letters AB - Scanning tunneling microscopy on Ar ion bombarded and annealed aluminum surfaces shows electron interference between the surface and subsurface bubbles of implanted gas. The depth of the bubbles as determined from the energy dependence of the standing waves indicates a minimum around 6–7 layers on Al(111). The appearance and energy dependence of the interference pattern is in good agreement with scattering theory based on free electrons, and indicates the bubbles have a shape given by the Wulff construction. DA - 1996/03/25/ PY - 1996 DO - 10.1103/PhysRevLett.76.2298 VL - 76 IS - 13 SP - 2298 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Scanning tunneling spectroscopy of one-dimensional surface states on a metal surface AU - Biedermann, A. AU - Genser, O. AU - Hebenstreit, W. AU - Schmid, M. AU - Redinger, J. AU - Podloucky, R. AU - Varga, P. T2 - Physical Review Letters AB - Scanning tunneling spectroscopy permits real-space observation of one-dimensional electronic states on a Fe(100) surface alloyed with Si. These states are localized along chains of Fe atoms in domain boundaries of the Fe(100) c(2×2)Si surface alloy, as confirmed by first-principles spin-polarized calculations. The calculated charge densities illustrate the d-like orbital character of the one-dimensional state and show its relationship to a two-dimensional state existing on the pure Fe(100) surface. DA - 1996/05/27/ PY - 1996 DO - 10.1103/PhysRevLett.76.4179 VL - 76 IS - 22 SP - 4179 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Potential sputtering of lithium fluoride by slow multicharged ions AU - Neidhart, T. AU - Pichler, F. AU - Aumayr, F. AU - Winter, HP. AU - Schmid, M. AU - Varga, P. T2 - Physical Review Letters AB - Thin polycrystalline LiF films have been bombarded by slow (≤1 keV) multicharged Arq+ ions (q≤9), in order to study the resulting total sputter yields by means of a quartz crystal microbalance. More than 99% of sputtered particles are neutral and show yields, at given impact energy, in proportion to the potential energy of projectile ions. The respective “potential sputtering” process already takes place far below 100 eV impact energy. It can be related to defect production in LiF following electron capture by the multicharged ions, and removes about one LiF molecule per 100 eV of projectile potential energy. DA - 1995/06/26/ PY - 1995 DO - 10.1103/PhysRevLett.74.5280 VL - 74 IS - 26 SP - 5280 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Slowing down adatom diffusion by an adsorbate: Co on Pt(111) with and without preadsorbed CO AU - Kalousek, R. AU - Schmid, M. AU - Hammerschmid, A. AU - Lundgren, E. AU - Varga, P. T2 - Physical Review B AB - Submonolayer deposition of cobalt on Pt(111) was studied by scanning tunneling microscopy at room temperature. It was found that deposition on a surface with adsorbed carbon monoxide (saturation coverage) leads to an island density almost ten times as high as that resulting from deposition on the clean platinum surface. Based on nucleation theory, we explain this fact as the result of a reduction of the diffusion coefficient of the Co adatoms in the presence of CO by more than two orders of magnitude. This effect is attributed to the displacement and/or rearrangement of the CO molecules necessary when Co adatoms diffuse over the “crowded” surface. DA - 2003/12/05/ PY - 2003 DO - 10.1103/PhysRevB.68.233401 VL - 68 IS - 23 SP - 233401 J2 - Phys. Rev. B ST - Slowing down adatom diffusion by an adsorbate ER - TY - JOUR TI - Using photoelectron spectroscopy to observe oxygen spillover to zirconia AU - Lackner, Peter AU - Zou, Zhiyu AU - Mayr, Sabrina AU - Diebold, Ulrike AU - Schmid, Michael T2 - Physical Chemistry Chemical Physics AB - X-ray photoelectron spectroscopy (XPS) of five-monolayer-thick ZrO2 films reveals a core level binding energy difference of up to 1.8 eV between the tetragonal and monoclinic phase. This difference is explained by positively charged oxygen vacancies in the tetragonal films, which are slightly reduced. Due to the large band gap of zirconia (≈5–6 eV), these charges shift the electron levels, leading to higher binding energies of reduced tetragonal films w.r.t. fully oxidized monoclinic films. These core level shifts have the opposite direction than what is usually encountered for reduced transition metal oxides. The vacancies can be filled via oxygen spillover from a catalyst that enables O2 dissociation. This can be either a metal deposited on the film, or, if the film has holes, the metallic (in our case, Rh) substrate. Our study also confirms that tetragonal ZrO2 is stabilized via oxygen vacancies and shows that the XPS binding energy difference between O 1s and Zr 3d solely depends on the crystallographic phase. DA - 2019/08/15/ PY - 2019 DO - 10.1039/C9CP03322J VL - 21 IS - 32 SP - 17613 EP - 17620 J2 - Phys. Chem. Chem. Phys. SN - 1463-9084 ER - TY - JOUR TI - Crystallographic structure of ultrathin Fe films on Cu(100) AU - Biedermann, Albert AU - Tscheließnig, Rupert AU - Schmid, Michael AU - Varga, Peter T2 - Physical Review Letters AB - We report the observation of bcc-like crystal structures in 2–4 monolayer (ML) Fe films grown on fcc Cu(100) using scanning tunneling microscopy. The local bcc structure provides a straightforward explanation for their frequently reported outstanding magnetic properties, i.e., ferromagnetic ordering in all layers with a Curie temperature above 300 K. The nonpseudomorphic structure, which becomes pseudomorphic above 4 ML film thickness, is unexpected in terms of conventional rules of thin film growth and stresses the importance of finite thickness effects in ferromagnetic ultrathin films. DA - 2001/// PY - 2001 DO - 10.1103/PhysRevLett.87.086103 VL - 87 IS - 8 SP - 086103 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Influence of impurities on localized transition metal surface states: scanning tunneling spectroscopy on V(001) AU - Bischoff, M. M. J. AU - Konvicka, C. AU - Quinn, A. J. AU - Schmid, M. AU - Redinger, J. AU - Podloucky, R. AU - Varga, P. AU - van Kempen, H. T2 - Physical Review Letters AB - The first scanning tunneling spectroscopy measurements on V(001) are reported. A strong surface state is detected which is very sensitive to the presence of segregated carbon impurities. The surface state energy shifts from 0.03 eV below the Fermi level at clean areas towards higher energies (up to ∼0.2 eV) at contaminated areas. Because of the negative dispersion of this state, the upward shift cannot be described in a simple confinement picture. Rather, the surface state energy is governed by vanadium surface s- d interactions which are altered by carbon coverage. DA - 2001/03/12/ PY - 2001 DO - 10.1103/PhysRevLett.86.2396 VL - 86 IS - 11 SP - 2396 J2 - Phys. Rev. Lett. ST - Influence of Impurities on Localized Transition Metal Surface States ER - TY - JOUR TI - Adsorption sites and ligand effect for CO on an alloy surface: A direct view AU - Gauthier, Y. AU - Schmid, M. AU - Padovani, S. AU - Lundgren, E. AU - Buš, V. AU - Kresse, G. AU - Redinger, J. AU - Varga, P. T2 - Physical Review Letters AB - CO adsorption on a PtCo(111) surface was studied by scanning tunneling microscopy. Comparison of images with chemical contrast of Pt and Co and images showing the CO molecules indicates that CO resides exclusively on top of Pt sites and never on Co. CO bonding is highly sensitive to the chemical environment. The probability to find CO on a Pt atom increases drastically with the number of its Co nearest neighbors. Ab initio calculations show that this ligand effect is due to different positions of the center of the Pt d band. DA - 2001/06/28/ PY - 2001 DO - 10.1103/PhysRevLett.87.036103 VL - 87 IS - 3 SP - 036103 J2 - Phys. Rev. Lett. ST - Adsorption Sites and Ligand Effect for CO on an Alloy Surface ER - TY - JOUR TI - Kinetically assisted potential sputtering of insulators by highly charged ions AU - Hayderer, G. AU - Cernusca, S. AU - Schmid, M. AU - Varga, P. AU - Winter, HP. AU - Aumayr, F. AU - Niemann, D. AU - Hoffmann, V. AU - Stolterfoht, N. AU - Lemell, C. AU - Wirtz, L. AU - Burgdörfer, J. T2 - Physical Review Letters AB - A new form of potential sputtering has been found for impact of slow ( ≤1500 eV) multiply charged Xe ions (charge states up to q = 25) on MgOx. In contrast to alkali-halide or SiO2 surfaces this mechanism requires the simultaneous presence of electronic excitation of the target material and of a kinetically formed collision cascade within the target in order to initiate the sputtering process. This kinetically assisted potential sputtering mechanism has been identified to be present for other insulating surfaces as well. DA - 2001/04/16/ PY - 2001 DO - 10.1103/PhysRevLett.86.3530 VL - 86 IS - 16 SP - 3530 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - STM studies of HCI-induced surface damage on highly oriented pyrolytic graphite AU - Hayderer, G. AU - Cernusca, S. AU - Schmid, M. AU - Varga, P. AU - Winter, HP AU - Aumayr, F. T2 - Physica Scripta AB - Scanning tunneling microscopy (STM) with atomic scale resolution has been applied to study surface defects in highly oriented pyrolytic graphite (HOPG) which have been produced by impact of 150 eV singly and multiply charged Ar ions (charge state up to 9+). The most prominent surface defects are protrusions. Their area density is in good agreement with the applied ion dose, implying that about every single ion impact causes one protrusion. A (√3 x √3) R 30° surface reconstruction, as characteristic for interstitial defects in HOPG, is observed in the vicinity of most defects. As the most remarkable result we find that the measured size of the hillocks (mean diameter and height) increases with projectile charge state. DA - 2001/// PY - 2001 DO - 10.1238/Physica.Topical.092a00156 VL - T92 SP - 156 EP - 157 J2 - Phys. Scripta SN - 1402-4896 ER - TY - JOUR TI - Ultrathin films of Co on Pt(111): an STM view AU - Varga, P. AU - Lundgren, E. AU - Redinger, J. AU - Schmid, M. T2 - physica status solidi (a) AB - The growth, structure and morphology of ultrathin Co layers with a thickness up to 15 layers deposited at room temperature on Pt(111) have been studied by using scanning tunnelling microscopy (STM) with atomic resolution and chemical discrimination between Co and Pt. This chemical contrast has been confirmed by simulations with an FLAPW (Full Potential Linearized Augmented Plane Waves) ab-initio computer code based on density functional theory. By the help of this contrast between Pt and Co atoms in STM constant current images it is shown that in the early stages of submonolayer growth Co is incorporated into the Pt surface, thereby forming dislocation lines. We were also able to demonstrate that Co atoms descend from the upper terrace to the lower one by an exchange diffusion process with the Pt atoms at the step edges. It is shown that this interlayer diffusion does not take place at straight steps, but rather at corners or kinks. The first completed Co monolayer (ML) is almost pseudomorphic (Co in the Pt fcc lattice sites) with a high density of defects due to the lattice mismatch. The second layer exhibits a moiré structure, with the Co in-plane lattice distance close to that of bulk Co. The step edges which are very rough at a coverage of two monolayers become smoother with increasing Co deposition. The growth mode is two-dimensional (layer-by-layer) around two to three monolayers and changes afterward into three-dimensional growth (island growth). We observe that the change of the step edge morphology is also correlated to this change from 2D to 3D growth mode. The reason for the 2D growth at the beginning is attributed to the strained interface between the Co overlayer and the Pt(111) surface which hinders the formation of straight steps. Therefore, many kinks and corners are formed, increasing the probability for interlayer diffusion by the above mentioned exchange process. With increasing number of layers the strain decreases, steps become smoother, interlayer diffusion decreases and therefore island growth develops. Up to the highest coverage (15 ML) studied the growth is characterised by a mainly twinned fcc-like stacking. Only a small amount of hcp stacking has been observed. Further experiments showed that preadsorption of carbon monoxide acts as a surfactant which extends the layer-by-layer growth up to higher Co coverages. DA - 2001/// PY - 2001 DO - 10.1002/1521-396X(200109)187:1<97::AID-PSSA97>3.0.CO;2-A VL - 187 IS - 1 SP - 97 EP - 112 J2 - Physica Stat. Sol. ST - Ultrathin Films of Co on Pt(111) ER - TY - JOUR TI - When scanning tunneling microscopy gets the wrong adsorption site: H on Rh(100) AU - Klein, C. AU - Eichler, A. AU - Hebenstreit, E. L. D. AU - Pauer, G. AU - Koller, R. AU - Winkler, A. AU - Schmid, M. AU - Varga, P. T2 - Physical Review Letters AB - At low tunneling resistance, scanning tunneling microscopy (STM) images of a Rh(100) surface with adsorbed hydrogen reproducibly show protrusions in all bridge sites of the surface, leading to a naive interpretation of all bridge sites being occupied with H atoms. Using quantitative low-energy electron diffraction and temperature programmed desorption we find a much lower H coverage, with most H atoms in fourfold hollow sites. Density functional theory calculations show that the STM result is due to the influence of the tip, attracting the mobile H atoms into bridge sites. This demonstrates that STM images of highly mobile adsorbates can be strongly misleading and underlines the importance of additional analysis techniques. DA - 2003/04/29/ PY - 2003 DO - 10.1103/PhysRevLett.90.176101 VL - 90 IS - 17 SP - 176101 J2 - Phys. Rev. Lett. ST - When Scanning Tunneling Microscopy Gets the Wrong Adsorption Site ER - TY - JOUR TI - Self-limited growth of a thin oxide layer on Rh(111) AU - Gustafson, J. AU - Mikkelsen, A. AU - Borg, M. AU - Lundgren, E. AU - Köhler, L. AU - Kresse, G. AU - Schmid, M. AU - Varga, P. AU - Yuhara, J. AU - Torrelles, X. AU - Quirós, C. AU - Andersen, J. N. T2 - Physical Review Letters AB - The oxidation of the Rh(111) surface at oxygen pressures from 10-10 mbar to 0.5 bar and temperatures between 300 and 900 K has been studied on the atomic scale using a multimethod approach of experimental and theoretical techniques. Oxidation starts at the steps, resulting in a trilayer O-Rh-O surface oxide which, although not thermodynamically stable, prevents further oxidation at intermediate pressures. A thick corundum like Rh2O3 bulk oxide is formed only at significantly higher pressures and temperatures. DA - 2004/03/26/ PY - 2004 DO - 10.1103/PhysRevLett.92.126102 VL - 92 IS - 12 SP - 126102 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Visualization of atomic processes on ruthenium dioxide using scanning tunneling microscopy AU - Over, H. AU - Knapp, M. AU - Lundgren, E. AU - Seitsonen, A. P. AU - Schmid, M. AU - Varga, P. T2 - ChemPhysChem AB - The visualization of surface reactions on the atomic scale provides direct insight into the microscopic reaction steps taking place in a catalytic reaction at a (model) catalyst's surface. Employing the technique of scanning tunneling microscopy (STM), we investigated the CO oxidation reaction over the RuO2(110) and RuO2(100) surfaces. For both surfaces the protruding bridging O atoms are imaged in STM as bright features. The reaction mechanism is identical on both orientations of RuO2. CO molecules adsorb on the undercoordinated surface Ru atoms from where they recombine with undercoordinated O atoms to form CO2 at the oxide surface. In contrast to the RuO2(110) surface, the RuO2(100) surface stabilizes also a catalytically inactive c(2×2) surface phase onto which CO is not able to adsorb above 100 K. We argue that this inactive RuO2(100)-c(2×2) phase may play an important role in the deactivation of RuO2 catalysts in the electrochemical Cl2 evolution and other heterogeneous reactions. DA - 2004/// PY - 2004 DO - 10.1002/cphc.200300833 VL - 5 IS - 2 SP - 167 EP - 174 J2 - ChemPhysChem ER - TY - JOUR TI - Understanding the structural deactivation of ruthenium catalysts on an atomic scale under both oxidizing and reducing conditions AU - Aßmann, Jens AU - Crihan, Daniela AU - Knapp, Marcus AU - Lundgren, Edvin AU - Löffler, Elke AU - Muhler, Martin AU - Narkhede, Vijay AU - Over, Herbert AU - Schmid, Michael AU - Seitsonen, Ari P. AU - Varga, Peter T2 - Angewandte Chemie International Edition AB - No Abstract The surface-science approach coupled with industrial catalyst research offers a synergistic strategy to improve the performance of industrial catalysts. The poorly understood microscopic processes that determine the structural deactivation of ruthenium-based catalysts during CO oxidation have been elucidated. Based on these results measures are proposed to improve the performance of ruthenium catalysts. DA - 2005/// PY - 2005 DO - 10.1002/anie.200461805 VL - 44 IS - 6 SP - 917 EP - 920 J2 - Angew. Chem. Int. Ed. ER - TY - JOUR TI - Probing the geometry of copper and silver adatoms on magnetite: quantitative experiment versus theory AU - Meier, Matthias AU - Jakub, Zdeněk AU - Balajka, Jan AU - Hulva, Jan AU - Bliem, Roland AU - Thakur, Pardeep K. AU - Lee, Tien-Lin AU - Franchini, Cesare AU - Schmid, Michael AU - Diebold, Ulrike AU - Allegretti, Francesco AU - Duncan, David A. AU - Parkinson, Gareth S. T2 - Nanoscale AB - Accurately modelling the structure of a catalyst is a fundamental prerequisite for correctly predicting reaction pathways, but a lack of clear experimental benchmarks makes it difficult to determine the optimal theoretical approach. Here, we utilize the normal incidence X-ray standing wave (NIXSW) technique to precisely determine the three dimensional geometry of Ag1 and Cu1 adatoms on Fe3O4(001). Both adatoms occupy bulk-continuation cation sites, but with a markedly different height above the surface (0.43 ± 0.03 Å (Cu1) and 0.96 ± 0.03 Å (Ag1)). HSE-based calculations accurately predict the experimental geometry, but the more common PBE + U and PBEsol + U approaches perform poorly. DA - 2018/// PY - 2018 DO - 10.1039/C7NR07319D VL - 10 IS - 5 SP - 2226 EP - 2230 J2 - Nanoscale SN - 2040-3364, 2040-3372 ST - Probing the geometry of copper and silver adatoms on magnetite ER - TY - JOUR TI - Stability and catalytic performance of reconstructed Fe3O4(001) and Fe3O4(110) surfaces during oxygen evolution reaction AU - Müllner, Matthias AU - Riva, Michele AU - Kraushofer, Florian AU - Schmid, Michael AU - Parkinson, Gareth S. AU - Mertens, Stijn F. L. AU - Diebold, Ulrike T2 - The Journal of Physical Chemistry C AB - Earth-abundant oxides are promising candidates as effective and low-cost catalysts for the oxygen evolution reaction (OER) in alkaline media, which remains one of the bottlenecks in electrolysis and artificial photosynthesis. A fundamental understanding of the atomic-scale reaction mechanism during OER could drive further progress, but a stable model system has yet to be provided. Here we show that Fe3O4 single crystal surfaces, prepared in ultrahigh vacuum (UHV) are stable in alkaline electrolytein the range pH 7–14 and under OER conditions in 1 M NaOH. Fe3O4(001) and Fe3O4(110) surfaces were studied with X-ray photoelectron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy in UHV, and atomic force microscopy in air. Fe3O4(110) is found to be more reactive for oxidative water splitting than (001)-oriented magnetite samples. Magnetite is electrically conductive, and the structure and properties of its major facets are well understood in UHV. With these newly obtained results, we propose magnetite (Fe3O4) as a promising model system for further mechanistic studies of electrochemical reactions in alkaline media and under highly oxidizing conditions. DA - 2019/04/04/ PY - 2019 DO - 10.1021/acs.jpcc.8b08733 VL - 123 IS - 13 SP - 8304 EP - 8311 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - High-coverage oxygen structures on Rh(111): adsorbate repulsion and site preference is not enough AU - Köhler, L. AU - Kresse, G. AU - Schmid, M. AU - Lundgren, E. AU - Gustafson, J. AU - Mikkelsen, A. AU - Borg, M. AU - Yuhara, J. AU - Andersen, J. N. AU - Marsman, M. AU - Varga, P. T2 - Physical Review Letters AB - A new O induced structure on Rh(111) displaying a (2sqrt[3]×2sqrt[3])R30° periodicity with an oxygen coverage of 2/3 has been studied by high resolution core level spectroscopy, scanning tunneling microscopy, and density functional theory. Although O favors fcc hollow sites in all other known phases, it occupies both fcc and hcp sites in this structure, which cannot be explained by pairwise adsorbate repulsion only. Both the (2sqrt[3]×2sqrt[3])R30° and (2×2)-3O structures also exemplify that density-of-states contrast can lead to oxygen adatoms appearing as protrusions in scanning tunneling microscopy images. DA - 2004/12/21/ PY - 2004 DO - 10.1103/PhysRevLett.93.266103 VL - 93 IS - 26 SP - 266103 J2 - Phys. Rev. Lett. ST - High-Coverage Oxygen Structures on Rh(111) ER - TY - JOUR TI - Adsorption of CO on the Fe3O4(001) surface AU - Hulva, Jan AU - Jakub, Zdeněk AU - Novotny, Zbynek AU - Johansson, Niclas AU - Knudsen, Jan AU - Schnadt, Joachim AU - Schmid, Michael AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - The Journal of Physical Chemistry B AB - The interaction of CO with the Fe3O4(001)-(√2 × √2)R45° surface was studied using temperature-programmed desorption (TPD), scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS), the latter both under ultrahigh vacuum (UHV) conditions and in CO pressures up to 1 mbar. In general, the CO–Fe3O4 interaction is found to be weak. The strongest adsorption occurs at surface defects, leading to small TPD peaks at 115, 130, and 190 K. Desorption from the regular surface occurs in two distinct regimes. For coverages up to two CO molecules per (√2 × √2)R45° unit cell, the desorption maximum shows a large shift with increasing coverage, from initially 105 to 70 K. For coverages between 2 and 4 molecules per (√2 × √2)R45° unit cell, a much sharper desorption feature emerges at ∼65 K. Thermodynamic analysis of the TPD data suggests a phase transition from a dilute 2D gas into an ordered overlayer with CO molecules bound to surface Fe3+ sites. XPS data acquired at 45 K in UHV are consistent with physisorption. Some carbon-containing species are observed in the near-ambient-pressure XPS experiments at room temperature but are attributed to contamination and/or reaction with CO with water from the residual gas. No evidence was found for surface reduction or carburization by CO molecules. DA - 2018/01/18/ PY - 2018 DO - 10.1021/acs.jpcb.7b06349 VL - 122 IS - 2 SP - 721 EP - 729 J2 - J. Phys. Chem. B SN - 1520-6106 ER - TY - CHAP TI - Adsorption and self-organization of organic molecules under electrochemical control AU - Mertens, Stijn F. L. T2 - Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry A2 - Wandelt, Klaus AB - Adsorption and self-organization of organic molecules on solid substrates have been studied extensively in vacuum and at solid–liquid interfaces. Under electrochemical conditions, however, an often exceptional level of control over adsorption and self-organization can be achieved. The source of this control lies in modulating the strength of the interactions between adsorbates, substrate, and electrolyte through the electric field at the electrode–electrolyte interface and has both thermodynamic and kinetic origins and implications. This article briefly introduces the most important substrates for studying adsorption and self-organization under electrochemical control and the main techniques used. We then discuss the principles responsible for switching and tuning supramolecular structures under electrochemical control, by analyzing representative examples. DA - 2018/// PY - 2018 VL - 4 IS - 7 SP - 13 EP - 23 PB - Elsevier SN - 978-0-12-809894-3 ER - TY - JOUR TI - Interfacial supramolecular electrochemistry AU - Cui, Kang AU - Dorner, Iris AU - Mertens, Stijn F. L. T2 - Current Opinion in Electrochemistry AB - Supramolecular chemistry at solid–liquid interfaces is guided by the interactions between the molecular building blocks, the solid substrate and the liquid phase. At an electrochemical interface (i.e., at the interface between an electronic and ionic conductor), the substrate potential allows modulating many of these interactions, resulting in a high level of control over the supramolecular structures that are formed and their reactivity. In this paper, we review key principles and recent work in this area, and discuss how a standard scanning tunneling microscope setup allows to scale down interfacial supramolecular electrochemistry to the few-molecule level. DA - 2018/03/01/ PY - 2018 DO - 10.1016/j.coelec.2018.06.002 VL - 8 SP - 156 EP - 163 J2 - Curr. Opin. Electrochem. SN - 2451-9103 ER - TY - JOUR TI - A highly sensitive quartz-crystal microbalance for sputtering investigations in slow ion-surface collisions AU - Hayderer, G. AU - Schmid, M. AU - Varga, P. AU - Winter, HP. AU - Aumayr, F. T2 - Review of Scientific Instruments AB - A quartz-crystal microbalance technique for measuring total sputter yields in ion-surface collisions is described. The electronic circuit to drive the quartz crystal ensures low noise and high frequency stability. By measuring the total sputter yields for impact of singly charged ions on LiF target films a sensitivity limit of 0.5% of a monolayer per minute could be achieved. DA - 1999/// PY - 1999 DO - 10.1063/1.1149979 VL - 70 IS - 9 SP - 3696 EP - 3700 J2 - Rev. Sci. Instrum. ER - TY - JOUR TI - Metastable ion beam fractions measured for different ion sources AU - Hofer, W. AU - Vanek, W. AU - Varga, P. AU - Winter, H. T2 - Review of Scientific Instruments AB - Metastable fractions of singly charged ion beams extracted from different ion sources have been determined by measurement of secondary electron spectra which result from slow ion‐induced potential emission from a well‐defined solid surface. Data are presented for Ar+, Kr+, Xe+ beams produced by a NIER‐type electron impact source, a ‘‘COLUTRON’’‐type low‐pressure arc plasma source, and a ‘‘DUOPLASMATRON’’‐type magnetically compressed arc plasma source, respectively. The measured metastable fractions are accurate within 25%, and their dependence on various ion source parameters can be well explained with a simple model of metastable ion production and loss kinetics. DA - 1983/02/01/ PY - 1983 DO - 10.1063/1.1137361 VL - 54 IS - 2 SP - 150 EP - 157 J2 - Rev. Sci. Instrum. SN - 0034-6748 ER - TY - JOUR TI - Structure and properties of TiO2 surfaces: a brief review AU - Diebold, U. T2 - Applied Physics A: Materials Science & Processing AB - Titanium oxides are used in a wide variety of technological applications where surface properties play a role. TiO2 surfaces, especially the (110) face of rutile, have become prototypical model systems in the surface science of metal oxides. Reduced TiO2 single crystals are easy to work with experimentally, and their surfaces have been characterized with virtually all surface-science techniques. Recently, TiO2 has also been used to refine computational ab initio approaches and to calculate properties of adsorption systems. Scanning tunneling microscopy (STM) studies have shown that the surface structure of TiO2(110) is more complex than originally anticipated. The reduction state of the sample, i.e. the number and type of bulk defects, as well as the surface treatment (annealing in vacuum vs. annealing in oxygen), can give rise to different structures, such as two different (1×2) reconstructions, a ‘rosette’ overlayer, and crystallographic shear planes. Single point defects can be identified with STM and influence the surface chemistry in a variety of ways; the adsorption of water is discussed as one example. The growth of a large number of different metal overlayers has been studied on TiO2(110). Some of these studies have been instrumental in furthering the understanding of the ‘strong metal support interaction’ between group-VIII metals and TiO2, as well as low-temperature oxidation reactions on TiO2-supported nanoscopic gold clusters. The growth morphology, interfacial oxidation/reduction reaction, thermal stability, and geometric structure of ultra-thin metal overlayers follow general trends where the most critical parameter is the reactivity of the overlayer metal towards oxygen. It has been shown recently that the technologically more relevant TiO2 anatase phase can also be made accessible to surface investigations. DA - 2003/03/15/ PY - 2003 DO - 10.1007/s00339-002-2004-5 VL - 76 IS - 5 SP - 681 EP - 687 J2 - Appl. Phys. A ST - Structure and properties of TiO2 surfaces ER - TY - JOUR TI - The surface science of titanium dioxide AU - Diebold, Ulrike T2 - Surface Science Reports AB - Titanium dioxide is the most investigated single-crystalline system in the surface science of metal oxides, and the literature on rutile (1 1 0), (1 0 0), (0 0 1), and anatase surfaces is reviewed. This paper starts with a summary of the wide variety of technical fields where TiO2 is of importance. The bulk structure and bulk defects (as far as relevant to the surface properties) are briefly reviewed. Rules to predict stable oxide surfaces are exemplified on rutile (1 1 0). The surface structure of rutile (1 1 0) is discussed in some detail. Theoretically predicted and experimentally determined relaxations of surface geometries are compared, and defects (step edge orientations, point and line defects, impurities, surface manifestations of crystallographic shear planes--CSPs) are discussed, as well as the image contrast in scanning tunneling microscopy (STM). The controversy about the correct model for the (1×2) reconstruction appears to be settled. Different surface preparation methods, such as reoxidation of reduced crystals, can cause a drastic effect on surface geometries and morphology, and recommendations for preparing different TiO2(1 1 0) surfaces are given. The structure of the TiO2(1 0 0)-(1×1) surface is discussed and the proposed models for the (1×3) reconstruction are critically reviewed. Very recent results on anatase (1 0 0) and (1 0 1) surfaces are included. The electronic structure of stoichiometric TiO2 surfaces is now well understood. Surface defects can be detected with a variety of surface spectroscopies. The vibrational structure is dominated by strong Fuchs-Kliewer phonons, and high-resolution electron energy loss spectra often need to be deconvoluted in order to render useful information about adsorbed molecules. The growth of metals (Li, Na, K, Cs, Ca, Al, Ti, V, Nb, Cr, Mo, Mn, Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au) as well as some metal oxides on TiO2 is reviewed. The tendency to [`]wet' the overlayer, the growth morphology, the epitaxial relationship, and the strength of the interfacial oxidation/reduction reaction all follow clear trends across the periodic table, with the reactivity of the overlayer metal towards oxygen being the most decisive factor. Alkali atoms form ordered superstructures at low coverages. Recent progress in understanding the surface structure of metals in the [`]strong-metal support interaction' (SMSI) state is summarized. Literature is reviewed on the adsorption and reaction of a wide variety of inorganic molecules (H2, O2, H2O, CO, CO2, N2, NH3, NOx, sulfur- and halogen-containing molecules, rare gases) as well as organic molecules (carboxylic acids, alcohols, aldehydes and ketones, alkynes, pyridine and its derivates, silanes, methyl halides). The application of TiO2-based systems in photo-active devices is discussed, and the results on UHV-based photocatalytic studies are summarized. The review ends with a brief conclusion and outlook of TiO2-based surface science for the future. DA - 2003/01// PY - 2003 DO - 10.1016/S0167-5729(02)00100-0 VL - 48 IS - 5-8 SP - 53 EP - 229 J2 - Surf. Sci. Rep. SN - 0167-5729 ER - TY - JOUR TI - Substoichiometric ultrathin zirconia films cause strong metal–support interaction AU - Lackner, Peter AU - Choi, Joong Il Jake AU - Diebold, Ulrike AU - Schmid, Michael T2 - Journal of Materials Chemistry A AB - The strong metal–support interaction (SMSI) leads to substantial changes of the properties of an oxide-supported catalyst after annealing under reducing conditions. The common explanation is the formation of heavily reduced, ultrathin oxide films covering metal particles. This is typically encountered for reducible oxides such as TiO2 or Fe3O4. Zirconia (ZrO2) is a catalyst support that is difficult to reduce and therefore no obvious candidate for the SMSI effect. In this work, we use inverse model systems with Rh(111), Pt(111), and Ru(0001) as supports. Contrary to expectations, we show that SMSI is encountered for zirconia. Upon annealing in ultra-high vacuum, oxygen-deficient ultrathin zirconia films (≈ZrO1.5) form on all three substrates. However, Zr remains in its preferred charge state of 4+, as electrons are transferred to the underlying metal. At high temperatures, the stability of the ultrathin zirconia films depends on whether alloying of Zr and the substrate metal occurs. The SMSI effect is reversible; the ultrathin suboxide films can be removed by annealing in oxygen. DA - 2019/11/21/ PY - 2019 DO - 10.1039/C9TA08438J VL - 7 IS - 43 SP - 24837 EP - 24846 J2 - J. Mater. Chem. A SN - 2050-7496 ER - TY - JOUR TI - Few-monolayer yttria-doped zirconia films: Segregation and phase stabilization AU - Lackner, Peter AU - Brandt, Amy J. AU - Diebold, Ulrike AU - Schmid, Michael T2 - The Journal of Chemical Physics AB - For most applications, zirconia (ZrO2) is doped with yttria. Doping leads to the stabilization of the tetragonal or cubic phase and increased oxygen ion conductivity. Most previous surface studies of yttria-doped zirconia were plagued by impurities, however. We have studied doping of pure, 5-monolayer ZrO2 films on Rh(111) by x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). STM and LEED show that the tetragonal phase is stabilized by unexpectedly low dopant concentrations, 0.5 mol % Y2O3, even when the films are essentially fully oxidized (as evidenced by XPS core level shifts). XPS also shows Y segregation to the surface with an estimated segregation enthalpy of −23 ± 4 kJ/mol. DA - 2020/02/11/ PY - 2020 DO - 10.1063/1.5140266 VL - 152 IS - 6 SP - 064709 J2 - J. Chem. Phys. SN - 0021-9606 ST - Few-monolayer yttria-doped zirconia films ER - TY - JOUR TI - Zero-field propagation of spin waves in waveguides prepared by focused ion beam direct writing AU - Flajšman, Lukáš AU - Wagner, Kai AU - Vaňatka, Marek AU - Gloss, Jonáš AU - Křižáková, Viola AU - Schmid, Michael AU - Schultheiss, Helmut AU - Urbánek, Michal T2 - Physical Review B AB - Metastable face-centered-cubic Fe78Ni22 thin films are excellent candidates for focused ion beam direct writing of magnonic structures due to their favorable magnetic properties after ion-beam-induced transformation. The focused ion beam transforms the originally nonmagnetic fcc phase into the ferromagnetic bcc phase with additional control over the direction of uniaxial magnetic in-plane anisotropy and saturation magnetization. Local magnetic anisotropy direction control eliminates the need for external magnetic fields, paving the way towards complex magnonic circuits with waveguides pointing in different directions. In the present study, we show that the magnetocrystalline anisotropy in transformed areas is strong enough to stabilize the magnetization in the direction perpendicular to the long axis of narrow waveguides. Therefore, it is possible to propagate spin waves in these waveguides in the favorable Damon-Eshbach geometry without the presence of any external magnetic field. Phase-resolved microfocused Brillouin light scattering yields the dispersion relation of these waveguides in zero as well as in nonzero external magnetic fields. DA - 2020/01/24/ PY - 2020 DO - 10.1103/PhysRevB.101.014436 VL - 101 IS - 1 SP - 014436 J2 - Phys. Rev. B ER - TY - JOUR TI - Epitaxial growth of complex oxide films: Role of surface reconstructions AU - Riva, Michele AU - Franceschi, Giada AU - Schmid, Michael AU - Diebold, Ulrike T2 - Physical Review Research AB - Achieving atomically flat and stoichiometric films of complex multicomponent oxides is crucial to integrating these materials in both established and emerging technologies. While pulsed laser deposition (PLD) can in principle produce these high-quality films, growth experiments often result in unsatisfactory morphologies with rough surfaces and nonstoichiometric compositions. To understand the cause, the growth needs to be followed at an atomic level from its early stages as a function of the growth conditions. By combining PLD with atomically resolved scanning tunneling microscopy, as well as surface spectroscopic and diffraction techniques, we address the origin of surface roughening in SrTiO3(110) homoepitaxial films and pinpoint optimal growth conditions. We highlight the importance of surface reconstructions at all stages of growth: The different sticking on coexisting surface structures is responsible for the roughening of SrTiO3(110) films and affects their stoichiometry. DA - 2019/10/29/ PY - 2019 DO - 10.1103/PhysRevResearch.1.033059 VL - 1 IS - 3 SP - 033059 J2 - Phys. Rev. Research ST - Epitaxial growth of complex oxide films ER - TY - JOUR TI - Growth of In2O3(111) thin films with optimized surfaces AU - Franceschi, Giada AU - Wagner, Margareta AU - Hofinger, Jakob AU - Krajňák, Tomáš AU - Schmid, Michael AU - Diebold, Ulrike AU - Riva, Michele T2 - Physical Review Materials AB - Indium oxide is widely employed in applications ranging from optoelectronics and gas sensing to catalysis, as well as in thin-film heterostructures. To probe the fundamentals of phenomena at the heart of In2O3-based devices that are tied to the intrinsic surface and interface properties of the material, well-defined single-crystalline In2O3 surfaces are needed. We report on how to grow atomically flat In2O3(111) thin films on yttria-stabilized zirconia substrates by pulsed laser deposition. The films are largely relaxed and reproduce the atomic-scale details of the surfaces of single crystals, except for line defects originating from the antiphase domain boundaries that form because of the one-on-four lattice match between the surface unit cells of In2O3(111) and of the substrate. While optimizing the growth conditions, we observe that the morphology of the films is ruled by the oxygen chemical potential, which determines the nature and diffusivity of adspecies during growth. DA - 2019/10/10/ PY - 2019 DO - 10.1103/PhysRevMaterials.3.103403 VL - 3 IS - 10 SP - 103403 J2 - Phys. Rev. Materials ER - TY - JOUR TI - Self-limited growth of an oxyhydroxide phase at the Fe3O4(001) surface in liquid and ambient pressure water AU - Kraushofer, Florian AU - Mirabella, Francesca AU - Xu, Jian AU - Pavelec, Jiří AU - Balajka, Jan AU - Müllner, Matthias AU - Resch, Nikolaus AU - Jakub, Zdeněk AU - Hulva, Jan AU - Meier, Matthias AU - Schmid, Michael AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - The Journal of Chemical Physics AB - Atomic-scale investigations of metal oxide surfaces exposed to aqueous environments are vital to understand degradation phenomena (e.g., dissolution and corrosion) as well as the performance of these materials in applications. Here, we utilize a new experimental setup for the ultrahigh vacuum-compatible dosing of liquids to explore the stability of the Fe3O4(001)-(√2 × √2)R45° surface following exposure to liquid and ambient pressure water. X-ray photoelectron spectroscopy and low-energy electron diffraction data show that extensive hydroxylation causes the surface to revert to a bulklike (1 × 1) termination. However, scanning tunneling microscopy (STM) images reveal a more complex situation, with the slow growth of an oxyhydroxide phase, which ultimately saturates at approximately 40% coverage. We conclude that the new material contains OH groups from dissociated water coordinated to Fe cations extracted from subsurface layers and that the surface passivates once the surface oxygen lattice is saturated with H because no further dissociation can take place. The resemblance of the STM images to those acquired in previous electrochemical STM studies leads us to believe that a similar structure exists at the solid-electrolyte interface during immersion at pH 7. DA - 2019/10/16/ PY - 2019 DO - 10.1063/1.5116652 VL - 151 IS - 15 SP - 154702 J2 - J. Chem. Phys. SN - 0021-9606 ER - TY - JOUR TI - Polaron-driven surface reconstructions AU - Reticcioli, Michele AU - Setvin, Martin AU - Hao, Xianfeng AU - Flauger, Peter AU - Kresse, Georg AU - Schmid, Michael AU - Diebold, Ulrike AU - Franchini, Cesare T2 - Physical Review X AB - Geometric and electronic surface reconstructions determine the physical and chemical properties of surfaces and, consequently, their functionality in applications. The reconstruction of a surface minimizes its surface free energy in otherwise thermodynamically unstable situations, typically caused by dangling bonds, lattice stress, or a divergent surface potential, and it is achieved by a cooperative modification of the atomic and electronic structure. Here, we combined first-principles calculations and surface techniques (scanning tunneling microscopy, non-contact atomic force microscopy, scanning tunneling spectroscopy) to report that the repulsion between negatively charged polaronic quasiparticles, formed by the interaction between excess electrons and the lattice phonon field, plays a key role in surface reconstructions. As a paradigmatic example, we explain the (1×1) to (1×2) transition in rutile TiO2(110). DA - 2017/09/25/ PY - 2017 DO - 10.1103/PhysRevX.7.031053 VL - 7 IS - 3 SP - 031053 J2 - Phys. Rev. X ER - TY - JOUR TI - Unraveling CO adsorption on model single-atom catalysts AU - Hulva, Jan AU - Meier, Matthias AU - Bliem, Roland AU - Jakub, Zdenek AU - Kraushofer, Florian AU - Schmid, Michael AU - Diebold, Ulrike AU - Franchini, Cesare AU - Parkinson, Gareth S. T2 - Science AB - Understanding how the local environment of a “single-atom” catalyst affects stability and reactivity remains a challenge. We present an in-depth study of copper1, silver1, gold1, nickel1, palladium1, platinum1, rhodium1, and iridium1 species on Fe3O4(001), a model support in which all metals occupy the same twofold-coordinated adsorption site upon deposition at room temperature. Surface science techniques revealed that CO adsorption strength at single metal sites differs from the respective metal surfaces and supported clusters. Charge transfer into the support modifies the d-states of the metal atom and the strength of the metal–CO bond. These effects could strengthen the bond (as for Ag1–CO) or weaken it (as for Ni1–CO), but CO-induced structural distortions reduce adsorption energies from those expected on the basis of electronic structure alone. The extent of the relaxations depends on the local geometry and could be predicted by analogy to coordination chemistry. DA - 2021/01/22/ PY - 2021 DO - 10.1126/science.abe5757 VL - 371 IS - 6527 SP - 375 EP - 379 J2 - Science SN - 0036-8075, 1095-9203 ER - TY - JOUR TI - Electrochemical stability of the reconstructed Fe3O4(001) surface AU - Grumelli, Doris AU - Wiegmann, Tim AU - Barja, Sara AU - Reikowski, Finn AU - Maroun, Fouad AU - Allongue, Philippe AU - Balajka, Jan AU - Parkinson, Gareth S. AU - Diebold, Ulrike AU - Kern, Klaus AU - Magnussen, Olaf M. T2 - Angewandte Chemie International Edition AB - Establishing the atomic-scale structure of metal-oxide surfaces during electrochemical reactions is a key step to modeling this important class of electrocatalysts. Here, we demonstrate that the characteristic (√2×√2)R45° surface reconstruction formed on (001)-oriented magnetite single crystals is maintained after immersion in 0.1 M NaOH at 0.20 V vs. Ag/AgCl and we investigate its dependence on the electrode potential. We follow the evolution of the surface using in situ and operando surface X-ray diffraction from the onset of hydrogen evolution, to potentials deep in the oxygen evolution reaction (OER) regime. The reconstruction remains stable for hours between −0.20 and 0.60 V and, surprisingly, is still present at anodic current densities of up to 10 mA cm−2 and strongly affects the OER kinetics. We attribute this to a stabilization of the Fe3O4 bulk by the reconstructed surface. At more negative potentials, a gradual and largely irreversible lifting of the reconstruction is observed due to the onset of oxide reduction. DA - 2020/// PY - 2020 DO - https://doi.org/10.1002/anie.202008785 VL - 59 IS - 49 SP - 21904 EP - 21908 J2 - Angew. Chem. Int. Ed. SN - 1521-3773 ER - TY - JOUR TI - Adsorption and reaction of methanol on Fe3O4(001) AU - Marcinkowski, Matthew D. AU - Adamsen, Kræn C. AU - Doudin, Nassar AU - Sharp, Marcus A. AU - Smith, R. Scott AU - Wang, Yang AU - Wendt, Stefan AU - Lauritsen, Jeppe V. AU - Parkinson, Gareth S. AU - Kay, Bruce D. AU - Dohnálek, Zdenek T2 - The Journal of Chemical Physics AB - The interaction of methanol with iron oxide surfaces is of interest due to its potential in hydrogen storage and from a fundamental perspective as a chemical probe of reactivity. We present here a study examining the adsorption and reaction of methanol on magnetite Fe3O4(001) at cryogenic temperatures using a combination of temperature programmed desorption, x-ray photoelectron spectroscopy, and scanning tunneling microscopy. The methanol desorption profile from Fe3O4(001) is complex, exhibiting peaks at 140 K, 173 K, 230 K, and 268 K, corresponding to the desorption of intact methanol, as well as peaks at 341 K and 495 K due to the reaction of methoxy intermediates. The saturation of a monolayer of methanol corresponds to ∼5 molecules/unit cell (u.c.), which is slightly higher than the number of surface octahedral iron atoms of 4/u.c. We probe the kinetics and thermodynamics of the desorption of molecular methanol using inversion analysis. The deconvolution of the complex desorption profile into individual peaks allows for calculations of both the desorption energy and the prefactor of each feature. The initial 0.7 methanol/u.c. reacts to form methoxy and hydroxy intermediates at 180 K, which remain on the surface above room temperature after intact methanol has desorbed. The methoxy species react via one of two channels, a recombination reaction with surface hydroxyls to form additional methanol at ∼350 K and a disproportionation reaction to form methanol and formaldehyde at ∼500 K. Only 20% of the methoxy species undergo the disproportionation reaction, with most of them reacting via the 350 K pathway. DA - 2020/02/10/ PY - 2020 DO - 10.1063/1.5139418 VL - 152 IS - 6 SP - 064703 J2 - J. Chem. Phys. SN - 0021-9606 ER - TY - JOUR TI - A high temperature dual-mode quartz crystal microbalance technique for erosion and thermal desorption spectroscopy measurements AU - Stadlmayr, Reinhard AU - Szabo, Paul Stefan AU - Biber, Herbert AU - Koslowski, Hans Rudolf AU - Kadletz, Elisabeth AU - Cupak, Christian AU - Wilhelm, Richard Arthur AU - Schmid, Michael AU - Linsmeier, Christian AU - Aumayr, Friedrich T2 - Review of Scientific Instruments AB - An improved quartz crystal microbalance measurement method is described, which allows us to determine erosion, implantation, and release rates of thin films, during changing temperatures and up to 700 K. A quasi-simultaneous excitation of two eigenmodes of the quartz resonator is able to compensate for frequency drifts due to temperature changes. The necessary electronics, the controlling behavior, and the dual-mode temperature compensation are described. With this improved technique, quantitative in situ temperature-programmed desorption measurements are possible and the quartz crystal microbalance can be used for quantification of thermal desorption spectroscopy measurements with a quadrupole mass spectrometer. This is demonstrated by a study of the retention and release behavior of hydrogen isotopes in fusion-relevant materials. We find that more than 90% of the deuterium implanted into a thin film of beryllium is released during a subsequent temperature ramp up to 500 K. DA - 2020/12/01/ PY - 2020 DO - 10.1063/5.0012028 VL - 91 IS - 12 SP - 125104 J2 - Rev. Sci. Instrum. SN - 0034-6748 ER - TY - JOUR TI - IrO2 surface complexions identified through machine learning and surface investigations AU - Timmermann, Jakob AU - Kraushofer, Florian AU - Resch, Nikolaus AU - Li, Peigang AU - Wang, Yu AU - Mao, Zhiqiang AU - Riva, Michele AU - Lee, Yonghyuk AU - Staacke, Carsten AU - Schmid, Michael AU - Scheurer, Christoph AU - Parkinson, Gareth S. AU - Diebold, Ulrike AU - Reuter, Karsten T2 - Physical Review Letters AB - A Gaussian approximation potential was trained using density-functional theory data to enable a global geometry optimization of low-index rutile IrO2 facets through simulated annealing. Ab initio thermodynamics identifies (101) and (111) (1×1) terminations competitive with (110) in reducing environments. Experiments on single crystals find that (101) facets dominate and exhibit the theoretically predicted (1×1) periodicity and x-ray photoelectron spectroscopy core-level shifts. The obtained structures are analogous to the complexions discussed in the context of ceramic battery materials. DA - 2020/11/10/ PY - 2020 DO - 10.1103/PhysRevLett.125.206101 VL - 125 IS - 20 SP - 206101 J2 - Phys. Rev. Lett. ER - TY - JOUR TI - Atomically resolved surface phases of La0.8Sr0.2MnO3(110) thin films AU - Franceschi, Giada AU - Schmid, Michael AU - Diebold, Ulrike AU - Riva, Michele T2 - Journal of Materials Chemistry A AB - The atomic-scale properties of lanthanum–strontium manganite (La1−xSrxMnO3−δ, LSMO) surfaces are of high interest because of the roles of the material as a prototypical complex oxide, in the fabrication of spintronic devices and in catalytic applications. This work combines pulsed laser deposition (PLD) with atomically resolved scanning tunneling microscopy (STM) and surface analysis techniques (low-energy electron diffraction – LEED, X-ray photoelectron spectroscopy – XPS, and low-energy He+ ion scattering – LEIS) to assess the atomic properties of La0.8Sr0.2MnO3(110) surfaces and their dependence on the surface composition. Epitaxial films with 130 nm thickness were grown on Nb-doped SrTiO3(110) and their near-surface stoichiometry was adjusted by depositing La and Mn in sub-monolayer amounts, quantified with a movable quartz-crystal microbalance. The resulting surfaces were equilibrated at 700 °C under 0.2 mbar O2, i.e., under conditions that bridge the gap between ultra-high vacuum and the operating conditions of high-temperature solid-oxide fuel cells, where LSMO is used as the cathode. The atomic details of various composition-related surface phases have been unveiled. The phases are characterized by distinct structural and electronic properties and vary in their ability to accommodate deposited cations. DA - 2020/11/10/ PY - 2020 DO - 10.1039/D0TA07032G VL - 8 IS - 43 SP - 22947 EP - 22961 J2 - J. Mater. Chem. A SN - 2050-7496 ER - TY - JOUR TI - Electron emission from polycrystalline lithium fluoride induced by slow multicharged ions AU - Vana, M. AU - Aumayr, F. AU - Varga, P. AU - Winter, H. P. T2 - EPL (Europhysics Letters) AB - Total electron yields have been determined from electron emission statistics measured for impact of H + , N q + ( q = 1, 5, 6) and Ar q + ( q = 1, 3, 6, 9) on clean, polycrystalline lithium fluoride, by varying the impact energy from almost zero up to (10 × q ) keV. Dependences of the electron emission statistics and yields on projectile charge and impact energy deviate strongly from the corresponding properties for clean metal surfaces, because of grossly different contributions of potential and kinetic emission and, in particular, a more efficient secondary electron emission if fast electrons are ejected from neutralising projectiles inside the LiF bulk. DA - 1995/// PY - 1995 DO - 10.1209/0295-5075/29/1/010 VL - 29 IS - 1 SP - 55 J2 - Europhys. Lett. SN - 0295-5075 ER - TY - JOUR TI - Atomic-scale studies of Fe3O4(001) and TiO2(110) surfaces following immersion in CO2-acidified water AU - Mirabella, Francesca AU - Balajka, Jan AU - Pavelec, Jiri AU - Göbel, Markus AU - Kraushofer, Florian AU - Schmid, Michael AU - Parkinson, Gareth S. AU - Diebold, Ulrike T2 - ChemPhysChem AB - Abstract Difficulties associated with the integration of liquids into a UHV environment make surface-science style studies of mineral dissolution particularly challenging. Recently, we developed a novel experimental setup for the UHV-compatible dosing of ultrapure liquid water and studied its interaction with TiO2 and Fe3O4 surfaces. Herein, we describe a simple approach to vary the pH through the partial pressure of CO2 (p_CO2 ) in the surrounding vacuum chamber and use this to study how these surfaces react to an acidic solution. The TiO2(110) surface is unaffected by the acidic solution, except for a small amount of carbonaceous contamination. The Fe3O4(001)-(√2 × √2)R45° surface begins to dissolve at a pH 4.0–3.9 ( p_CO2 = 0.8–1 bar) and, although it is significantly roughened, the atomic-scale structure of the Fe3O4(001) surface layer remains visible in scanning tunneling microscopy (STM) images. X-ray photoelectron spectroscopy (XPS) reveals that the surface is chemically reduced and contains a significant accumulation of bicarbonate (HCO3-) species. These observations are consistent with Fe(II) being extracted by bicarbonate ions, leading to dissolved iron bicarbonate complexes (Fe(HCO3)2), which precipitate onto the surface when the water evaporates. DA - 2020/08/18/ PY - 2020 DO - 10.1002/cphc.202000471 VL - 21 IS - 16 SP - 1788 EP - 1796 J2 - ChemPhysChem SN - 1439-4235 ER - TY - JOUR TI - Carbide-modified Pd on ZrO2 as active phase for CO2-reforming of methane—A model phase boundary approach AU - Köpfle, Norbert AU - Ploner, Kevin AU - Lackner, Peter AU - Götsch, Thomas AU - Thurner, Christoph AU - Carbonio, Emilia AU - Hävecker, Michael AU - Knop-Gericke, Axel AU - Schlicker, Lukas AU - Doran, Andrew AU - Kober, Delf AU - Gurlo, Aleksander AU - Willinger, Marc AU - Penner, Simon AU - Schmid, Michael AU - Klötzer, Bernhard T2 - Catalysts AB - Starting from subsurface Zr0-doped "inverse" Pd and bulk-intermetallic Pd0Zr0 model catalyst precursors, we investigated the dry reforming reaction of methane (DRM) using synchrotron-based near ambient pressure in-situ X-ray photoelectron spectroscopy (NAP-XPS), in-situ X-ray diffraction and catalytic testing in an ultrahigh-vacuum-compatible recirculating batch reactor cell. Both intermetallic precursors develop a Pd0–ZrO2 phase boundary under realistic DRM conditions, whereby the oxidative segregation of ZrO2 from bulk intermetallic PdxZry leads to a highly active composite layer of carbide-modified Pd0 metal nanoparticles in contact with tetragonal ZrO2. This active state exhibits reaction rates exceeding those of a conventional supported Pd–ZrO2 reference catalyst and its high activity is unambiguously linked to the fast conversion of the highly reactive carbidic/dissolved C-species inside Pd0 toward CO at the Pd/ZrO2 phase boundary, which serves the role of providing efficient CO2 activation sites. In contrast, the near-surface intermetallic precursor decomposes toward ZrO2 islands at the surface of a quasi-infinite Pd0 metal bulk. Strongly delayed Pd carbide accumulation and thus carbon resegregation under reaction conditions leads to a much less active interfacial ZrO2–Pd0 state. DA - 2020/09// PY - 2020 DO - 10.3390/catal10091000 VL - 10 IS - 9 SP - 1000 J2 - Catalysts ER - TY - JOUR TI - Atomic force and scanning tunneling microscopy of ordered ionic liquid wetting layers from 110 K up to room temperature AU - Meusel, Manuel AU - Lexow, Matthias AU - Gezmis, Afra AU - Schötz, Simon AU - Wagner, Margareta AU - Bayer, Andreas AU - Maier, Florian AU - Steinrück, Hans-Peter T2 - ACS Nano AB - Ionic liquids (ILs) are used as ultrathin films in many applications. We studied the nanoscale arrangement within the first layer of 1,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C1C1Im] [Tf2N]) on Au(111) between 400 and 110 K in ultrahigh vacuum by scanning tunneling and noncontact atomic force microscopy with molecular resolution. Compared to earlier studies on similar ILs, a different behavior is observed, which we attribute to the small size and symmetrical shape of the cation: (a) In both AFM and STM only the anions are imaged; (b) only long-range-ordered but no amorphous phases are observed; (c) the hexagonal room-temperature phase melts 30–50 K above the IL’s bulk melting point; (d) at 110 K, striped and hexagonal superstructures with two and three ion pairs per unit cell, respectively, are found. AFM turned out to be more stable at higher temperature, while STM revealed more details at low temperature. DA - 2020/07/28/ PY - 2020 DO - 10.1021/acsnano.0c03841 VL - 14 IS - 7 SP - 9000 EP - 9010 J2 - ACS Nano SN - 1936-0851 ER - TY - JOUR TI - Oxide chemistry and catalysis AU - Baker, L. Robert AU - Diebold, Ulrike AU - Park, Jeong Young AU - Selloni, Annabella T2 - The Journal of Chemical Physics AB - Editorial of a JCP special issue focused on oxide chemistry and catalysis DA - 2020/08/03/ PY - 2020 DO - 10.1063/5.0021819 VL - 153 IS - 5 SP - 050401 J2 - J. Chem. Phys. SN - 0021-9606 ER - TY - JOUR TI - A quantitative LEED analysis of the oxygen-induced p(3×1) reconstruction of Pt25Rh75(100) AU - Sporn, M. AU - Platzgummer, E. AU - Gruber, E. L. D. AU - Schmid, M. AU - Hofer, W. AU - Varga, P. T2 - Surface Science AB - Pt25Rh75(100) forms a p(3×1) reconstruction at saturation coverage of oxygen (23 L O2, 600°C). A previous STM study on O/Pt50Rh50(100) suggests that every third row of the first substrate layer is shifted by half a lattice constant ("shifted rows"). We present a LEED I(E) analysis of Pt25Rh75(100) confirming the shifted-row model and find that oxygen resides in threefold-coordinated sites on both sides of the shifted rows. The adsorbate occupies those of the threefold-coordinated sites that are directly separated by the metal atom in the shifted row. Further I(E) calculations exclude the alternative threefold-coordinated adsorption site beside the shifted row as well as the fourfold-coordinated site symmetrically in between the shifted rows. We achieve a Pendry R-factor of 0.14 for the best-fit structure. Oxygen has equal bond lengths to its three metal neighbours, amounting to 1.95 Å. The first substrate layer relaxes outward by 8.8% of the bulk value to 2.08 Å, but we do not observe any significant relaxations of deeper layer spacings. The shifted rows pop out of the surface by 0.38 Å. After determination of the oxygen adsorption site with LEED, we examine local adsorption structures on Pt25Rh75(100) at low oxygen coverage with STM. We resolve the shifted rows in real-space, and for special tip conditions, we find maxima of apparent height at in-plane positions that coincide with the oxygen position as established by quantitative LEED. We determine chemical-composition depth-profiles by quantitative LEED for three surface preparations occurring during sample preparation. While the first substrate layer of clean and annealed Pt25Rh75(100) is enriched in Pt (76%) as compared to the bulk value (25%), that in p(3×1)-O/Pt25Rh75(100) is enriched in Rh (90%). Oxygen adsorption at a moderate temperature (600°C) and formation of the p(3×1) structure reverse segregation on Pt25Rh75(100). Finally, oxygen can be removed at room temperature by exposure of the surface to hydrogen. This lifts the reconstruction but keeps the Rh enrichment of the first substrate layer. DA - 1998/10/30/ PY - 1998 DO - 10.1016/S0039-6028(98)00574-3 VL - 416 IS - 3 SP - 384 EP - 395 J2 - Surf. Sci. SN - 0039-6028 ER - TY - JOUR TI - Propagation of spin waves through a Néel domain wall AU - Wojewoda, O. AU - Hula, T. AU - Flajšman, L. AU - Vaňatka, M. AU - Gloss, J. AU - Holobrádek, J. AU - Staňo, M. AU - Stienen, S. AU - Körber, L. AU - Schultheiss, K. AU - Schmid, M. AU - Schultheiss, H. AU - Urbánek, M. T2 - Applied Physics Letters AB - Spin waves have the potential to be used as a next-generation platform for data transfer and processing as they can reach wavelengths in the nanometer range and frequencies in the terahertz range. To realize a spin-wave device, it is essential to be able to manipulate the amplitude as well as the phase of spin waves. Several theoretical and recent experimental works have also shown that the spin-wave phase can be manipulated by the transmission through a domain wall (DW). Here, we study propagation of spin waves through a DW by means of micro-focused Brillouin light scattering microscopy (μBLS). The 2D spin-wave intensity maps reveal that spin-wave transmission through a Néel DW is influenced by a topologically enforced circular Bloch line in the DW center and that the propagation regime depends on the spin-wave frequency. In the first regime, two spin-wave beams propagating around the circular Bloch line are formed, whereas in the second regime, spin waves propagate in a single central beam through the circular Bloch line. Phase-resolved μBLS measurements reveal a phase shift upon transmission through the domain wall for both regimes. Micromagnetic modeling of the transmitted spin waves unveils a distortion of their phase fronts, which needs to be taken into account when interpreting the measurements and designing potential devices. Moreover, we show that, by means of micromagnetic simulations, an external magnetic field can be used to move the circular Bloch line within the DW and to manipulate spin-wave propagation. DA - 2020/07/13/ PY - 2020 DO - 10.1063/5.0013692 VL - 117 IS - 2 SP - 022405 J2 - Appl. Phys. Lett. SN - 0003-6951 ER - TY - JOUR TI - Resolving the adsorption of molecular O2 on the rutile TiO2(110) surface by noncontact atomic force microscopy AU - Sokolović, Igor AU - Reticcioli, Michele AU - Čalkovský, Martin AU - Wagner, Margareta AU - Schmid, Michael AU - Franchini, Cesare AU - Diebold, Ulrike AU - Setvín, Martin T2 - Proceedings of the National Academy of Sciences AB - Interaction of molecular oxygen with semiconducting oxide surfaces plays a key role in many technologies. The topic is difficult to approach both by experiment and in theory, mainly due to multiple stable charge states, adsorption configurations, and reaction channels of adsorbed oxygen species. Here we use a combination of noncontact atomic force microscopy (AFM) and density functional theory (DFT) to resolve O2 adsorption on the rutile TiO2(110) surface, which presents a longstanding challenge in the surface chemistry of metal oxides. We show that chemically inert AFM tips terminated by an oxygen adatom provide excellent resolution of both the adsorbed species and the oxygen sublattice of the substrate. Adsorbed O2 molecules can accept either one or two electron polarons from the surface, forming superoxo or peroxo species. The peroxo state is energetically preferred under any conditions relevant for applications. The possibility of nonintrusive imaging allows us to explain behavior related to electron/hole injection from the tip, interaction with UV light, and the effect of thermal annealing. DA - 2020/06/30/ PY - 2020 DO - 10.1073/pnas.1922452117 VL - 117 IS - 26 SP - 14827 EP - 14837 J2 - Proc. Natl. Acad. Sci. USA SN - 0027-8424, 1091-6490 ER - TY - JOUR TI - Movable holder for a quartz crystal microbalance for exact growth rates in pulsed laser deposition AU - Franceschi, Giada AU - Schmid, Michael AU - Diebold, Ulrike AU - Riva, Michele T2 - Review of Scientific Instruments AB - Controlling the amount of material deposited by pulsed laser deposition (PLD) down to fractions of one atomic layer is crucial for nanoscale technologies based on thin-film heterostructures. Albeit unsurpassed for measuring growth rates with high accuracy, the quartz crystal microbalance (QCM) suffers from some limitations when applied to PLD. The strong directionality of the PLD plasma plume and its pronounced dependence on deposition parameters (e.g., background pressure and fluence) require that the QCM is placed at the same position as the substrate during growth. However, QCM sensors are commonly fixed off to one side of the substrate. This also entails fast degradation of the crystal, as it is constantly exposed to the ablated material. The design for a movable QCM holder discussed in this work overcomes these issues. The holder is compatible with standard transfer arms, enabling easy insertion and transfer between a PLD chamber and other adjoining vacuum chambers. The QCM can be placed at the same position as the substrate during PLD growth. Its resonance frequency is measured in vacuum at any location where it can be in contact with an electrical feedthrough, before and after deposition. We tested the design for the deposition of hematite (Fe2O3), comparing the rates derived from the QCM and from reflection high-energy electron diffraction oscillations during homoepitaxial growth. DA - 2020/06/01/ PY - 2020 DO - 10.1063/5.0007643 VL - 91 IS - 6 SP - 065003 J2 - Rev. Sci. Instrum. SN - 0034-6748 ER - TY - JOUR TI - Fast low-noise transimpedance amplifier for scanning tunneling microscopy and beyond AU - Štubian, Martin AU - Bobek, Juraj AU - Setvin, Martin AU - Diebold, Ulrike AU - Schmid, Michael T2 - Review of Scientific Instruments AB - A transimpedance amplifier has been designed for scanning tunneling microscopy (STM). The amplifier features low noise (limited by the Johnson noise of the 1 GΩ feedback resistor at low input current and low frequencies), sufficient bandwidth for most STM applications (50 kHz at 35 pF input capacitance), a large dynamic range (0.1 pA–50 nA without range switching), and a low input voltage offset. The amplifier is also suited for placing its first stage into the cryostat of a low-temperature STM, minimizing the input capacitance and reducing the Johnson noise of the feedback resistor. The amplifier may also find applications for specimen current imaging and electron-beam-induced current measurements in scanning electron microscopy and as a photodiode amplifier with a large dynamic range. This paper also discusses the sources of noise including the often neglected effect of non-balanced input impedance of operational amplifiers and describes how to accurately measure and adjust the frequency response of low-current transimpedance amplifiers. DA - 2020/07/01/ PY - 2020 DO - 10.1063/5.0011097 VL - 91 IS - 7 SP - 074701 J2 - Rev. Sci. Instrum. SN - 0034-6748 ER - TY - JOUR TI - Ultrasmooth metallic foils for growth of high quality graphene by chemical vapor deposition AU - Procházka, Pavel AU - Mach, Jindřich AU - Bischoff, Dominik AU - Lišková, Zuzana AU - Dvořák, Petr AU - Vaňatka, Marek AU - Pauline Simonet AU - Varlet, Anastasia AU - Hemzal, Dušan AU - Petrenec, Martin AU - Kalina, Lukáš AU - Bartošík, Miroslav AU - Klaus Ensslin AU - Varga, Peter AU - Čechal, Jan AU - Šikola, Tomáš T2 - Nanotechnology AB - Synthesis of graphene by chemical vapor deposition is a promising route for manufacturing large-scale high-quality graphene for electronic applications. The quality of the employed substrates plays a crucial role, since the surface roughness and defects alter the graphene growth and cause difficulties in the subsequent graphene transfer. Here, we report on ultrasmooth high-purity copper foils prepared by sputter deposition of Cu thin film on a SiO 2 /Si template, and the subsequent peeling off of the metallic layer from the template. The surface displays a low level of oxidation and contamination, and the roughness of the foil surface is generally defined by the template, and was below 0.6 nm even on a large scale. The roughness and grain size increase occurred during both the annealing of the foils, and catalytic growth of graphene from methane (≈1000 °C), but on the large scale still remained far below the roughness typical for commercial foils. The micro-Raman spectroscopy and transport measurements proved the high quality of graphene grown on such foils, and the room temperature mobility of the graphene grown on the template stripped foil was three times higher compared to that of one grown on the commercial copper foil. The presented high-quality copper foils are expected to provide large-area substrates for the production of graphene suitable for electronic applications. DA - 2014/// PY - 2014 DO - 10.1088/0957-4484/25/18/185601 VL - 25 IS - 18 SP - 185601 J2 - Nanotechnology SN - 0957-4484 ER - TY - JOUR TI - A model system for photocatalysis: Ti-doped α-Fe2O3(11̅02) single-crystalline films AU - Franceschi, Giada AU - Kraushofer, Florian AU - Meier, Matthias AU - Parkinson, Gareth S. AU - Schmid, Michael AU - Diebold, Ulrike AU - Riva, Michele T2 - Chemistry of Materials AB - Hematite (α-Fe2O3) is one of the most investigated anode materials for photoelectrochemical water splitting. Its efficiency improves by doping with Ti, but the underlying mechanisms are not understood. One hurdle is separating the influence of doping on conductivity, surface states, and morphology, which all affect performance. To address this complexity, one needs well-defined model systems. We build such a model system by growing single-crystalline, atomically flat Ti-doped α-Fe2O3(11̅02) films by pulsed laser deposition (PLD). We characterize their surfaces, combining in situ scanning tunneling microscopy (STM) with density functional theory (DFT), and reveal how dilute Ti impurities modify the atomic-scale structure of the surface as a function of the oxygen chemical potential and Ti content. Ti preferentially substitutes subsurface Fe and causes a local restructuring of the topmost surface layers. Based on the experimental quantification of Ti-induced surface modifications and the structural model we have established, we propose a strategy that can be used to separate the effects of Ti-induced modifications to the surface atomic and electronic structures and bulk conductivity on the reactivity of Ti-doped hematite. DA - 2020/05/12/ PY - 2020 DO - 10.1021/acs.chemmater.9b04908 VL - 32 IS - 9 SP - 3753 EP - 3764 J2 - Chem. Mater. SN - 0897-4756 ST - A Model System for Photocatalysis ER - TY - JOUR TI - Order–disorder phase transition of the subsurface cation vacancy reconstruction on Fe3O4(001) AU - Arndt, Björn AU - Lechner, Barbara A. J. AU - Bourgund, Alexander AU - Grånäs, Elin AU - Creutzburg, Marcus AU - Krausert, Konstantin AU - Hulva, Jan AU - Parkinson, Gareth S. AU - Schmid, Michael AU - Vonk, Vedran AU - Esch, Friedrich AU - Stierle, Andreas T2 - Physical Chemistry Chemical Physics AB - We present surface X-ray diffraction and fast scanning tunneling microscopy results to elucidate the nature of the surface phase transition on magnetite (001) from a reconstructed to a non-reconstructed surface around 720 K. In situ surface X-ray diffraction at a temperature above the phase transition, at which long-range order is lost, gives evidence that the subsurface cation vacancy reconstruction still exists as a local structural motif, in line with the characteristics of a 2D second-order phase transition. Fast scanning tunneling microscopy results across the phase transition underpin the hypothesis that the reconstruction lifting is initiated by surplus Fe ions occupying subsurface octahedral vacancies. The reversible near-surface iron enrichment and reduction of the surface to stoichiometric composition is further confirmed by in situ low-energy ion scattering, as well as ultraviolet and X-ray photoemission results. DA - 2020/04/29/ PY - 2020 DO - 10.1039/D0CP00690D VL - 22 IS - 16 SP - 8336 EP - 8343 J2 - Phys. Chem. Chem. Phys. SN - 1463-9084 ER - TY - JOUR TI - The growth of ultra-thin zirconia films on Pd3Zr(0001) AU - Choi, J. I. J. AU - Mayr-Schmölzer, W. AU - Mittendorfer, F. AU - Redinger, J. AU - Diebold, U. AU - Schmid, M. T2 - Journal of Physics: Condensed Matter AB - Despite its importance in many areas of industry, such as catalysis, fuel cell technology and microelectronics, the surface structure and physical properties of ZrO2 are not well understood. Following the successful growth of ultra-thin zirconia on Pt3Zr(0001) (Antlanger et al 2012 Phys. Rev. B 86 035451), we report on recent progress into ZrO2 thin films, which were prepared by oxidation of a Pd3Zr(0001) crystal. Results from scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS) as well as density-functional theory (DFT) are presented. Many sputter-annealing cycles are required for preparation of the clean Pd3Zr alloy surface, because oxygen easily dissolves in the bulk. By oxidation and post-annealing, a homogeneous ultra-thin ZrO2 film was obtained. This is an O-Zr-O trilayer based on cubic ZrO2(1 1 1). Using STM images corrected for distortion and creep of the piezo scanner the in-plane lattice parameter was determined as (351.2 ± 0.4) pm, slightly contracted with respect to the cubic ZrO2 bulk phase. The oxide forms an overlayer that is either incommensurate or has a very large superstructure cell (a = 8.3 nm); nevertheless its rotational orientation is always the same. In contrast to ultra-thin zirconia on Pt3Zr(0001), where the uppermost substrate layer is pure (but reconstructed) Pt, STM and XPS suggest a stoichiometric Pd3Zr below the oxide. The oxide film binds to the substrate mainly via bonds between oxygen and the Zr atoms in the substrate. The ultra-thin oxide shows large buckling in STM, confirmed by DFT calculations, where the buckling of the Zr layer can exceed 100 pm. Compared to the ZrO2 film on Pt3Zr(0001), the oxide on Pd3Zr(0001) has the advantage that the substrate below does not reconstruct, leading to a homogeneous oxide film. DA - 2014/06/04/ PY - 2014 DO - 10.1088/0953-8984/26/22/225003 VL - 26 IS - 22 SP - 225003 J2 - J. Phys.: Condens. Matter SN - 0953-8984 ER - TY - CHAP TI - Small polarons in transition metal oxides AU - Reticcioli, Michele AU - Diebold, Ulrike AU - Kresse, Georg AU - Franchini, Cesare T2 - Handbook of Materials Modeling: Applications: Current and Emerging Materials A2 - Andreoni, Wanda A2 - Yip, Sidney AB - The formation of polarons is a pervasive phenomenon in transition metal oxide compounds, with a strong impact on the physical properties and functionalities of the hosting materials. In its original formulation, the polaron problem considers a single charge carrier in a polar crystal interacting with its surrounding lattice. Depending on the spatial extension of the polaron quasiparticle, originating from the coupling between the excess charge and the phonon field, one speaks of small or large polarons. This chapter discusses the modeling of small polarons in real materials, with a particular focus on the archetypal polaron material TiO2. After an introductory part, surveying the fundamental theoretical and experimental aspects of the physics of polarons, the chapter examines how to model small polarons using first-principles schemes in order to predict, understand, and interpret a variety of polaron properties in bulk phases and surfaces. Following the spirit of this handbook, different types of computational procedures and prescriptions are presented with specific instructions on the setup required to model polaron effects. CY - Cham DA - 2019/// PY - 2019 SP - 1 EP - 39 PB - Springer International Publishing SN - 978-3-319-50257-1 ER - TY - JOUR TI - Atomic scale insights into single-atom catalysis AU - Parkinson, Gareth S. T2 - Vakuum in Forschung und Praxis AB - “Single-Atom” Catalysis (SAC) is a rapidly emerging field aimed at minimizing the amount of precious metals required to perform important catalytic reactions. Modern heterogeneous catalysts already utilize nanoparticles containing 100s to 1000s of atoms on an inexpensive support, but the dream of SAC is to do the same chemistry with single atoms. The concept is firmly entrenched, and SAC systems have demonstrated activity for a variety of reaction, metal, and support combinations. Nevertheless, the topic remains controversial because it is extremely difficult to characterize a catalyst based on single atom active sites, and even harder to figure out how they work. In our group in Vienna, we study model SAC systems in a highly controlled ultrahigh vacuum environment using a variety of state-of-the-art surface-science techniques to discover what makes a stable single atom catalyst, the mechanisms underlying their catalytic activity, and the processes leading to their deactivation. DA - 2018/// PY - 2018 DO - 10.1002/vipr.201800695 VL - 30 IS - 5 SP - 45 EP - 49 J2 - Vakuum in Forschung und Praxis SN - 1522-2454 ER - TY - JOUR TI - Influence of local defects on the dynamics of O–H bond breaking and formation on a magnetite surface AU - Bourgund, Alexander AU - Lechner, Barbara A. J. AU - Meier, Matthias AU - Franchini, Cesare AU - Parkinson, Gareth S. AU - Heiz, Ueli AU - Esch, Friedrich T2 - The Journal of Physical Chemistry C AB - The transport of H adatoms across oxide supports plays an important role in many catalytic reactions. We investigate the dynamics of H/Fe3O4(001) between 295 and 382 K. By scanning tunneling microscopy at frame rates of up to 19.6 fps, we observe the thermally activated switching of H between two O atoms on neighboring Fe rows. This switching rate changes in proximity to a defect, explained by density functional theory as a distortion in the Fe–O lattice shortening the diffusion path. Quantitative analysis yields an apparent activation barrier of 0.94 ± 0.07 eV on a pristine surface. The present work highlights the importance of local techniques in the study of atomic-scale dynamics at defective surfaces such as oxide supports. DA - 2019/08/15/ PY - 2019 DO - 10.1021/acs.jpcc.9b05547 VL - 123 IS - 32 SP - 19742 EP - 19747 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - Understanding heterolytic H2 cleavage and water-assisted hydrogen spillover on Fe3O4(001)-supported single palladium atoms AU - Doudin, Nassar AU - Yuk, Simuck F. AU - Marcinkowski, Matthew D. AU - Nguyen, Manh-Thuong AU - Liu, Jin-Cheng AU - Wang, Yang AU - Novotny, Zbynek AU - Kay, Bruce D. AU - Li, Jun AU - Glezakou, Vassiliki-Alexandra AU - Parkinson, Gareth S. AU - Rousseau, Roger AU - Dohnálek, Zdenek T2 - ACS Catalysis AB - The high specific activity and cost-effectiveness of single-atom catalysts (SACs) hold great promise for numerous catalytic chemistries. In hydrogenation reactions, the mechanisms of critical steps such as hydrogen activation and spillover are far from understood. Here, we employ a combination of scanning tunneling microscopy and density functional theory to demonstrate that on a model SAC comprised of single Pd atoms on Fe3O4(001), H2 dissociates heterolytically between Pd and surface oxygen. The efficient hydrogen spillover allows for continuous hydrogenation to high coverages, which ultimately leads to the lifting of Fe3O4 reconstruction and Pd reduction and destabilization. Water plays an important role in reducing the proton diffusion barrier, thereby facilitating the redistribution of hydroxyls away from Pd. Our study demonstrates a distinct H2 activation mechanism on single Pd atoms and corroborates the importance of charge transport on reducible support away from the active site. DA - 2019/07/17/ PY - 2019 DO - 10.1021/acscatal.9b01425 SP - 7876 EP - 7887 J2 - ACS Catal. ER - TY - JOUR TI - Local structure and coordination define adsorption in a model Ir1/Fe3O4 single-atom catalyst AU - Jakub, Zdenek AU - Hulva, Jan AU - Meier, Matthias AU - Bliem, Roland AU - Kraushofer, Florian AU - Setvin, Martin AU - Schmid, Michael AU - Diebold, Ulrike AU - Franchini, Cesare AU - Parkinson, Gareth S. T2 - Angewandte Chemie International Edition AB - Single-atom catalysts (SACs) bridge homo- and heterogeneous catalysis because the active site is a metal atom coordinated to surface ligands. The local binding environment of the atom should thus strongly influence how reactants adsorb. Now, atomically resolved scanning-probe microscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, and DFT are used to study how CO binds at different Ir1 sites on a precisely defined Fe3O4(001) support. The two- and five-fold-coordinated Ir adatoms bind CO more strongly than metallic Ir, and adopt structures consistent with square-planar IrI and octahedral IrIII complexes, respectively. Ir incorporates into the subsurface already at 450 K, becoming inactive for adsorption. Above 900 K, the Ir adatoms agglomerate to form nanoparticles encapsulated by iron oxide. These results demonstrate the link between SAC systems and coordination complexes, and that incorporation into the support is an important deactivation mechanism. DA - 2019/// PY - 2019 DO - 10.1002/anie.201907536 VL - 58 IS - 39 SP - 13961 EP - 13968 J2 - Angew. Chem. Int. Ed. SN - 1521-3773 ER - TY - JOUR TI - Nickel doping enhances the reactivity of Fe3O4(001) to water AU - Jakub, Zdenek AU - Hulva, Jan AU - Mirabella, Francesca AU - Kraushofer, Florian AU - Meier, Matthias AU - Bliem, Roland AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - The Journal of Physical Chemistry C AB - We studied how nickel doping affects water adsorption at the Fe3O4(001) surface to understand the enhanced performance of spinel ferrites for the water-gas shift and oxygen evolution reactions. Two different configurations were prepared: 2-fold-coordinated Ni adatoms on top of the surface and Ni atoms incorporated into the octahedral sites of the support. Using temperature-programmed desorption, X-ray photoemission spectroscopy, and scanning tunneling microscopy, we show that water is adsorbed and dissociated on the nickel adatoms at room temperature, resulting in Ni–OH species and surface hydroxyl groups. Nickel atoms incorporated into the support do not directly adsorb water but modify the electronic properties of the surface Fe, and water adsorbed on these sites has similar characteristics as that of water adsorbed on intrinsic surface defects. In both cases, the presence of Ni significantly alters the growth of the first water monolayer on Fe3O4(001). DA - 2019/06/20/ PY - 2019 DO - 10.1021/acs.jpcc.9b02993 VL - 123 IS - 24 SP - 15038 EP - 15045 J2 - J. Phys. Chem. C SN - 1932-7447 ER - TY - JOUR TI - Partially dissociated water dimers at the water–hematite interface AU - Jakub, Zdenek AU - Kraushofer, Florian AU - Bichler, Magdalena AU - Balajka, Jan AU - Hulva, Jan AU - Pavelec, Jiri AU - Sokolović, Igor AU - Müllner, Matthias AU - Setvin, Martin AU - Schmid, Michael AU - Diebold, Ulrike AU - Blaha, Peter AU - Parkinson, Gareth S. T2 - ACS Energy Letters AB - The oxygen evolution reaction (OER) is thought to occur via a four-step mechanism with *O, *OH, and *OOH as adsorbed intermediates. Linear scaling of the *OH and **OOH adsorption energies is proposed to limit the oxides’ efficiency as OER catalysts, but the use of simple descriptors to screen candidate materials neglects potentially important water–water interactions. Here, we use a combination of temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), noncontact atomic force microscopy (nc-AFM), and density functional theory (DFT)-based computations to show that highly stable HO–H2O dimer species form at the (11̅02) facet of hematite; a promising anode material for photoelectrochemical water splitting. The UHV-based results are complemented by measurements following exposure to liquid water and are consistent with prior X-ray scattering results. The presence of strongly bound water agglomerates is generally not taken into account in OER reaction schemes but may play a role in determining the required OER overpotential on metal oxides. DA - 2019/02/08/ PY - 2019 DO - 10.1021/acsenergylett.8b02324 VL - 4 IS - 2 SP - 390 EP - 396 J2 - ACS Energy Lett. ER - TY - JOUR TI - Single-atom catalysis: How structure influences catalytic performance AU - Parkinson, Gareth S. T2 - Catalysis Letters AB - It now seems clear that supported metal adatoms can be effective catalysts for some reactions, but how to make best use of this phenomenon remains an open question. Most studies to date have focussed on synthesizing stable “single-atom” variants of functioning supported nanoparticle systems, but there is mounting evidence that the properties of supported adatoms do not scale from those of larger nanoparticles in a simple way. The sensitivity of the adsorption properties to the charge state and coordination environment of the adatom has led researchers to dream that single atom catalysis (SAC) can bridge the gap between heterogeneous and homogeneous catalysis, opening the door to a new generation of highly-selective catalysts for difficult reactions. To make this dream a reality, a fundamental understanding of how the structure of the active site affects the adsorption properties of adatoms is essential. Since the active site geometry cannot be unambiguously determined from realistic SAC systems at present, experiments on precisely-defined model systems must work hand-in-hand with theory to provide a fundamental basis for this rapidly growing field. DA - 2019/05/01/ PY - 2019 DO - 10.1007/s10562-019-02709-7 VL - 149 IS - 5 SP - 1137 EP - 1146 J2 - Catal. Lett. SN - 1572-879X ST - Single-Atom Catalysis ER - TY - JOUR TI - Water ordering on the magnetite Fe3O4 surfaces AU - Zaki, Eman AU - Jakub, Zdenek AU - Mirabella, Francesca AU - Parkinson, Gareth S. AU - Shaikhutdinov, Shamil AU - Freund, Hans-Joachim T2 - The Journal of Physical Chemistry Letters AB - The interaction of water with the most prominent surfaces of Fe3O4, (001) and (111), is directly compared using a combination of temperature-programmed desorption, temperature-programmed low energy electron diffraction (TP LEED), and scanning probe microscopies. Adsorption on the (√2 × √2)R45°-reconstructed surface of Fe3O4(001) is strongly influenced by the surface reconstruction, which remains intact at all coverages. Close to the completion of the first monolayer, however, the ad-layer adopts a longer-range (2 × 2) superstructure. This finding is discussed in the context of a similar (2 × 2) superstructure recently observed on the (111) facet, which exists over a significantly larger range of temperatures and coverages. In both cases, the long-range order is evidence that water–water interactions exert a significant influence on the structure already prior to the nucleation of the second layer. We conclude that the stability differences stem from the smaller unit cell on the (111) surface, and the ability of water to more easily form stable hexagonal ice-like structures on the hexagonal substrate. DA - 2019/05/16/ PY - 2019 DO - 10.1021/acs.jpclett.9b00773 VL - 10 IS - 10 SP - 2487 EP - 2492 J2 - J. Phys. Chem. Lett. ER - TY - JOUR TI - Highlights of the science and life of Peter Varga (1946—2018) AU - Schneider, Wolf-Dieter AU - Aumayr, Friedrich AU - Diebold, Ulrike T2 - e-Journal of Surface Science and Nanotechnology AB - Peter Varga has passed on October 27, 2018. His pioneering discoveries of chemical resolution at the atomic scale on surface alloys, atomic resolution of ultrathin alkali halides, nucleation of bcc iron in ultrathin films, and the microscopic structure of ultrathin alumina films stimulated worldwide research. In recognition of his outstanding scientific contributions, in December 2017 the Japanese Society for the Promotion of Science (JSPS) awarded him a prize for his distinguished contribution on the clarification of surface phenomena by atomic level investigations and the development of novel functional materials. This contribution highlights the life of Peter Varga as a scientist and as a person. With his elegance, his energy, his wit, and his generosity he was a close friend and role model to many of us, and showed us how to combine scientific curiosity and creativity with the lightness of being. DA - 2020/02/06/ PY - 2020 DO - 10.1380/ejssnt.2020.8 VL - 18 IS - 0 SP - 8 EP - 11 J2 - e-J. Surf. Sci. Nanotech. SN - 1348-0391 ER - TY - JOUR TI - Probing structural changes upon carbon monoxide coordination to single metal adatoms AU - Ryan, P. T. P. AU - Meier, M. AU - Jakub, Z. AU - Balajka, J. AU - Hulva, J. AU - Payne, D. J. AU - Lee, T.-L. AU - Franchini, C. AU - Allegretti, F. AU - Parkinson, G. S. AU - Duncan, D. A. T2 - The Journal of Chemical Physics AB - In this work, the adsorption height of Ag adatoms on the Fe3O4(001) surface after exposure to CO was determined using normal incidence x-ray standing waves. The Ag adatoms bound to CO (AgCO1Ag1CO) are found to be pulled out of the surface to an adsorption height of 1.15 Å ± 0.08 Å, compared to the previously measured height of 0.96 Å ± 0.03 Å for bare Ag adatoms and clusters. Utilizing DFT+vdW+U calculations with the substrate unit cell dimension fixed to the experimental value, the predicted adsorption height for AgCO1Ag1CO was 1.16 Å, in remarkably good agreement with the experimental results. DA - 2020/02/04/ PY - 2020 DO - 10.1063/1.5137904 VL - 152 IS - 5 SP - 051102 J2 - J. Chem. Phys. SN - 0021-9606 ER - TY - JOUR TI - Adsorbate-induced structural evolution changes the mechanism of CO oxidation on a Rh/Fe3O4(001) model catalyst AU - Jakub, Zdenek AU - Hulva, Jan AU - Ryan, Paul T. P. AU - Duncan, David A. AU - Payne, David J. AU - Bliem, Roland AU - Ulreich, Manuel AU - Hofegger, Patrick AU - Kraushofer, Florian AU - Meier, Matthias AU - Schmid, Michael AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - Nanoscale AB - The structure of a catalyst often changes in reactive environments, and following the structural evolution is crucial for the identification of the catalyst's active phase and reaction mechanism. Here we present an atomic-scale study of CO oxidation on a model Rh/Fe3O4(001) “single-atom” catalyst, which has a very different evolution depending on which of the two reactants, O2 or CO, is adsorbed first. Using temperature-programmed desorption (TPD) combined with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), we show that O2 destabilizes Rh atoms, leading to the formation of RhxOy clusters; these catalyze CO oxidation via a Langmuir–Hinshelwood mechanism at temperatures as low as 200 K. If CO adsorbs first, the system is poisoned for direct interaction with O2, and CO oxidation is dominated by a Mars-van-Krevelen pathway at 480 K. DA - 2020/03/12/ PY - 2020 DO - 10.1039/C9NR10087C VL - 12 IS - 10 SP - 5866 EP - 5875 J2 - Nanoscale SN - 2040-3372 ER - TY - JOUR TI - Two-dimensional surface phase diagram of a multicomponent perovskite oxide: La0.8Sr0.2MnO3(110) AU - Franceschi, Giada AU - Schmid, Michael AU - Diebold, Ulrike AU - Riva, Michele T2 - Physical Review Materials AB - The many surface reconstructions of (110)-oriented lanthanum strontium manganite (La0.8Sr0.2MnO3, LSMO) were followed as a function of the oxygen chemical potential (μO) and the surface cation composition. Decreasing μO causes Mn to migrate across the surface, enforcing phase separation into A-site-rich areas and a variety of composition-related, structurally diverse B-site-rich reconstructions. The composition of these phase-separated structures was quantified with scanning tunneling microscopy, and these results were used to build a two-dimensional phase diagram of the LSMO(110) equilibrium surface structures. DA - 2021/09/24/ PY - 2021 DO - 10.1103/PhysRevMaterials.5.L092401 VL - 5 IS - 9 SP - L092401 J2 - Phys. Rev. Materials ST - Two-dimensional surface phase diagram of a multicomponent perovskite oxide ER - TY - JOUR TI - Adding oxides to the 2D toolkit AU - Parkinson, Gareth S. T2 - Nature Materials AB - Two-dimensional (2D) metal oxides that can be exfoliated are produced via direct oxidation of their elemental metals, providing a simple and easy way to incorporate these materials in van der Waals heterostructures. DA - 2021/08// PY - 2021 DO - 10.1038/s41563-021-01048-6 VL - 20 IS - 8 SP - 1041 EP - 1042 J2 - Nat. Mater. SN - 1476-4660 ER - TY - JOUR TI - Rapid oxygen exchange between hematite and water vapor AU - Jakub, Zdenek AU - Meier, Matthias AU - Kraushofer, Florian AU - Balajka, Jan AU - Pavelec, Jiri AU - Schmid, Michael AU - Franchini, Cesare AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - Nature Communications AB - Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surprisingly reactive. Specifically, we show that all the 3-fold coordinated lattice oxygen atoms on a defect-free single-crystalline “r-cut” ($$1\bar{1}02$$) surface of hematite (α-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70 °C, while the atomic-scale surface structure is unperturbed by the process. A similar behavior is observed after liquid-water exposure, but the experimental data clearly show most of the exchange happens during desorption of the final monolayer, not during immersion. Density functional theory computations show that the exchange can happen during on-surface diffusion, where the cost of the lattice oxygen extraction is compensated by the stability of an HO-HOH-OH complex. Such insights into lattice oxygen stability are highly relevant for many research fields ranging from catalysis and hydrogen production to geochemistry and paleoclimatology. DA - 2021/11/10/ PY - 2021 DO - 10.1038/s41467-021-26601-4 VL - 12 IS - 1 SP - 6488 J2 - Nat. Commun. SN - 2041-1723 ER - TY - JOUR TI - Oxygen-rich tetrahedral surface phase on high-temperature rutile VO2(110)_T single crystals AU - Wagner, Margareta AU - Planer, Jakub AU - Heller, Bettina S. J. AU - Langer, Jens AU - Limbeck, Andreas AU - Boatner, Lynn A. AU - Steinrück, Hans-Peter AU - Redinger, Josef AU - Maier, Florian AU - Mittendorfer, Florian AU - Schmid, Michael AU - Diebold, Ulrike T2 - Physical Review Materials AB - Vanadium dioxide undergoes a metal-insulator transition from an insulating (monoclinic) to a metallic (tetragonal) phase close to room temperature, which makes it a promising functional material for many applications, e.g., as chemical sensors. Not much is known about its surface and interface properties, although these are critical in many applications. In this paper, we present an atomic-scale investigation of the tetragonal rutile VO2(110)T single-crystal surface and report results obtained with scanning tunneling microscopy, low-energy electron diffraction, and x-ray photoelectron spectroscopy, supported by density-functional-theory-based calculations. The surface reconstructs into an oxygen-rich (2 × 2) superstructure that coexists with small patches of the underlying unreconstructed (110)−(1×1) surface when the crystal is annealed >600∘C. The best structural model for the (2 × 2) surface termination, conceptually derived from a vanadium pentoxide (001) monolayer, consists of rings of corner-shared tetrahedra. Over a wide range of oxygen chemical potentials, this reconstruction is more stable than the unreconstructed (110) surface and models proposed in the literature. DA - 2021/12/02/ PY - 2021 DO - 10.1103/PhysRevMaterials.5.125001 VL - 5 IS - 12 SP - 125001 J2 - Phys. Rev. Materials ER - TY - JOUR TI - Single Rh adatoms stabilized on α-Fe2O3(11̅02) by coadsorbed water AU - Kraushofer, Florian AU - Haager, Lena AU - Eder, Moritz AU - Rafsanjani-Abbasi, Ali AU - Jakub, Zdeněk AU - Franceschi, Giada AU - Riva, Michele AU - Meier, Matthias AU - Schmid, Michael AU - Diebold, Ulrike AU - Parkinson, Gareth S. T2 - ACS Energy Letters AB - Oxide-supported single-atom catalysts are commonly modeled as a metal atom substituting surface cation sites in a low-index surface. Adatoms with dangling bonds will inevitably coordinate molecules from the gas phase, and adsorbates such as water can affect both stability and catalytic activity. Herein, we use scanning tunneling microscopy (STM), noncontact atomic force microscopy (ncAFM), and X-ray photoelectron spectroscopy (XPS) to show that high densities of single Rh adatoms are stabilized on α-Fe2O3(11̅02) in the presence of 2 × 10–8 mbar of water at room temperature, in marked contrast to the rapid sintering observed under UHV conditions. Annealing to 50 °C in UHV desorbs all water from the substrate leaving only the OH groups coordinated to Rh, and high-resolution ncAFM images provide a direct view into the internal structure. We provide direct evidence of the importance of OH ligands in the stability of single atoms and argue that their presence should be assumed when modeling single-atom catalysis systems. DA - 2022/01/14/ PY - 2022 DO - 10.1021/acsenergylett.1c02405 VL - 7 IS - 1 SP - 375 EP - 380 J2 - ACS Energy Lett. ER -