// Publications by members of the Surface Physics Group // Institut fuer Angewandte Physik, TU Wien, Austria. // http://www.iap.tuwien.ac.at/www/surface/ // // Complete since 1992. // Includes the full lists of publications of Ulrike Diebold and Michael Schmid // Version 2016-Feb-02 M. Schmid // // NOTE: Abstracts may include symbols defined in the textcomp package // (file textcomp.sty in your LaTeX distribution) // If you get a error message regarding a "\text..." item, add the // following to the beginning of your tex file: // \usepackage{textcomp} // Many journals are abbreviated here in their short and long forms. // Put the definitions with your preferred form below the one that you do not want, // it will override the first one // ------- LONG journal names ------- @string{ACSAMI = "ACS Applied Materials \& Interfaces"} @string{ACSCat = "{ACS} Catalysis"} @string{ACSEL = "{ACS} Energy Letters"} @string{AdvMatInt = "Advanced Materials Interfaces"} @string{AngChIE = "Angewandte Chemie International Edition"} @string{AnnRevPhCh = "Annual Review of Physical Chemistry"} @string{APL = "Applied Physics Letters"} @string{APLM = "{APL} Materials"} @string{ApPhA = "Applied Physics A: Materials Science \& Processing"} @string{APSS = "Applied Surface Science"} @string{BeilNano = "Beilstein Journal of Nanotechnology"} @string{CatLett = "Catalysis Letters"} @string{CatalTod = "Catalysis Today"} @string{ChemComm = "Chemical Communications"} @string{CPL = "Chemical Physics Letters"} @string{CSR = "Chemical Society Reviews"} @string{CMater = "Chemistry of Materials"} @string{ECAct = "Electrochimica Acta"} @string{ECSTr = "{ECS} Transactions"} @string{eJSSN = "e-Journal of Surface Science and Nanotechnology"} @string{EPL = "{EPL} {(Europhysics} Letters)"} @string{FreseniusJAC = "Fresenius' Journal of Analytical Chemistry"} @string{INLett = "International Nano Letters"} @string{JAcSocAm = "Journal of the Acoustical Society of America"} @string{JACS = "Journal of the American Chemical Society"} @string{JAP = "Journal of Applied Physics"} @string{JCP = "The Journal of Chemical Physics"} @string{JElSpec = "Journal of Electron Spectroscopy and Related Phenomena"} @string{JMCA = "Journal of Materials Chemistry A"} @string{JNuclMat = "Journal of Nuclear Materials"} @string{JPCB = "The Journal of Physical Chemistry B"} @string{JPCC = "The Journal of Physical Chemistry C"} @string{JPCL = "The Journal of Physical Chemistry Letters"} @string{JPCM = "Journal of Physics: Condensed Matter"} @string{JVSTA = "Journal of Vacuum Science and Technology A"} @string{JVSTB = "Journal of Vacuum Science and Technology B"} @string{nanoLett = "Nano Letters"} @string{NatChem = "Nature Chemistry"} @string{NatComm = "Nature Communications"} @string{NatMat = "Nature Materials"} @string{NatPhys = "Nature Physics"} @string{NatRMat = "Nature Reviews Materials"} @string{NIMB = "Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms"} @string{PCCP = "Physical Chemistry Chemical Physics"} @string{PhysStSolA = "physica status solidi (a)"} @string{PNAS = "Proceedings of the National Academy of Sciences"} @string{PRB = "Physical Review B"} @string{PRL = "Physical Review Letters"} @string{PRM = "Physical Review Materials"} @string{PRR = "Physical Review Research"} @string{PRX = "Physical Review X"} @string{ProcMRS = "Proceedings of the Materials Research Society"} @string{ProgSuSci = "Progress in Surface Science"} @string{RMP = "Reviews of Modern Physics"} @string{RSI = "Review of Scientific Instruments"} @string{SRL = "Surface Review and Letters"} @string{SuSci = "Surface Science"} @string{SuSciRep = "Surface Science Reports"} @string{TopCatal = "Topics in Catalysis"} // ------- SHORT journal names ------- @string{ACSAMI = "ACS Appl. Mater. Interfaces"} @string{ACSCat = "{ACS} Catal."} @string{ACSEL = "{ACS} Energy Lett."} @string{AdvMatInt = "Adv. Mater. Interfaces"} @string{AngChIE = "Angew. Chem. Int. Ed."} @string{AnnRevPhCh = "Annu. Rev. Phys. Chem."} @string{APL = "Appl. Phys. Lett."} @string{APLM = "{APL} Mater."} @string{ApPhA = "Appl. Phys. A"} @string{APSS = "Appl. Surf. Sci."} @string{BeilNano = "Beilstein J. Nanotechnol."} @string{CatLett = "Catal. Lett."} @string{CatalTod = "Catal. Today"} @string{ChemComm = "Chem. Commun."} @string{CPL = "Chem. Phys. Lett."} @string{CSR = "Chem. Soc. Rev."} @string{CMater = "Chem. Mater."} @string{ECAct = "Electrochim. Acta"} @string{ECSTr = "{ECS} Trans."} @string{eJSSN = "{e-J}. Surf. Sci. Nanotech."} @string{EPL = "Europhys. Lett."} @string{FreseniusJAC = "Fresenius J. Anal. Chem."} @string{INLett = "Int. Nano Lett."} @string{JAcSocAm = "J. Acoust. Soc. Am."} @string{JACS = "J. Am. Chem. Soc."} @string{JAP = "J. Appl. Phys."} @string{JCP = "J. Chem. Phys."} @string{JElSpec = "J. Electron Spectrosc. Relat. Phenom."} @string{JMCA = "J. Mater. Chem. A"} @string{JNuclMat = "J. Nucl. Mater."} @string{JPCB = "J. Phys. Chem. B"} @string{JPCC = "J. Phys. Chem. C"} @string{JPCL = "J. Phys. Chem. Lett."} @string{JPCM = "J. Phys.: Condens. Matter"} @string{JVSTA = "J. Vac. Sci. Technol. A"} @string{JVSTB = "J. Vac. Sci. Technol. B"} @string{nanoLett = "Nano Lett."} @string{NatChem = "Nat. Chem."} @string{NatComm = "Nat. Commun."} @string{NatMat = "Nat. Mater."} @string{NatPhys = "Nat. Phys."} @string{NatRMat = "Nat. Rev. Mater."} @string{NIMB = "Nucl. Instrum. Methods B"} @string{PCCP = "Phys. Chem. Chem. Phys."} @string{PhysStSolA = "Phys. Stat. Sol. A"} @string{PNAS = "Proc. Natl. Acad. Sci. USA"} @string{PRB = "Phys. Rev. B"} @string{PRL = "Phys. Rev. Lett."} @string{PRM = "Phys. Rev. Materials"} @string{PRR = "Phys. Rev. Research"} @string{PRX = "Phys. Rev. X"} @string{ProcMRS = "Proc. Mater. Res. Soc."} @string{ProgSuSci = "Progr. Surf. Sci."} @string{RMP = "Rev. Mod. Phys."} @string{RSI = "Rev. Sci. Instrum."} @string{SRL = "Surf. Rev. Lett."} @string{SuSci = "Surf. Sci."} @string{SuSciRep = "Surf. Sci. Rep."} @string{TopCatal = "Top. Catal."} @article{kraushofer_surface_2021, title = {Surface reduction state determines stabilization and incorporation of {Rh} on $\alpha{}$-{Fe$_2$O$_3$}(1$\bar{1}$02)}, volume = {8}, doi = {10.1002/admi.202001908}, abstract = {Iron oxides (FeOx) are among the most common support materials utilized in single atom catalysis. The support is nominally Fe2O3, but strongly reductive treatments are usually applied to activate the as-synthesized catalyst prior to use. Here, Rh adsorption and incorporation on the (1$\bar{1}$02) surface of hematite ($\alpha{}$-Fe2O3) are studied, which switches from a stoichiometric (1 $\times$ 1) termination to a reduced (2 $\times$ 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 $^\circ$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.}, number = {8}, journal = AdvMatInt, author = {Kraushofer, Florian and Resch, Nikolaus and Eder, Moritz and Rafsanjani-Abbasi, Ali and Tobisch, Sarah and Jakub, Zdenek and Franceschi, Giada and Riva, Michele and Meier, Matthias and Schmid, Michael and Diebold, Ulrike and Parkinson, Gareth S.}, year = {2021}, pages = {2001908} } @article{sokolovic_quest_2021, title = {Quest for a pristine unreconstructed {SrTiO$_3$}(001) surface: {An} atomically resolved study via noncontact atomic force microscopy}, volume = {103}, shorttitle = {Quest for a pristine unreconstructed \$\{{\textbackslash}mathrm\{{SrTiO}\}\}\_\{3\}(001)\$ surface}, doi = {10.1103/PhysRevB.103.L241406}, abstract = {The surfaces of perovskite oxides affect their functional properties, and while a bulk-truncated (1$\times$1) termination is generally assumed, its existence and stability is controversial. Here, such a surface is created by cleaving the prototypical SrTiO$_3$(001) in ultrahigh vacuum, and its response to thermal annealing is observed. Atomically resolved noncontact atomic force microscopy (nc-AFM) shows that intrinsic point defects on the as-cleaved surface migrate at temperatures above 200$\circ{}$C. At 400$\circ{}$C\textendash{}500$\circ{}$C, a disordered surface layer forms, albeit still with a (1$\times$1) pattern in low-energy electron diffraction (LEED). Purely TiO$_2$-terminated surfaces, prepared by wet-chemical treatment, are also disordered despite their (1$\times$1) periodicity in LEED.}, number = {24}, journal = PRB, author = {Sokolovi\'{c}, Igor and Franceschi, Giada and Wang, Zhichang and Xu, Jian and Pavelec, Ji\v{r}\'{\i} and Riva, Michele and Schmid, Michael and Diebold, Ulrike and Setv\'{\i}n, Martin}, month = jun, year = {2021}, pages = {L241406} } @article{mirabella_ni-modified_2021, title = {Ni-modified {Fe$_3$O$_4$}(001) surface as a simple model system for understanding the oxygen evolution reaction}, volume = {389}, doi = {10.1016/j.electacta.2021.138638}, abstract = {Electrochemical water splitting is an environmentally friendly technology to store renewable energy in the form of chemical fuels. Among the earth-abundant first-row transition metal-based catalysts, mixed Ni-Fe oxides have shown promising performance for effective and low-cost catalysis of the oxygen evolution reaction (OER) in alkaline media, but the synergistic roles of Fe and Ni cations in the OER mechanism remain unclear. In this work, we report how addition of Ni changes the reactivity of a model iron oxide catalyst, based on Ni deposited on and incorporated in a magnetite Fe$_3$O$_4$(001) single crystal, using a combination of surface science techniques in ultra-high vacuum such as low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and scanning tunneling microscopy (STM), as well as atomic force microscopy (AFM) in air, and electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in alkaline media. A significant improvement in the OER activity is observed when the top surface presents an iron fraction among the cations in the range of 20-40\%, which is in good agreement with what has been observed for powder catalysts. Furthermore, a decrease in the OER overpotential is observed following surface aging in electrolyte for three days. At higher Ni load, AFM shows the growth of a new phase attributed to an (oxy)-hydroxide phase which, according to CV measurements, does not seem to correlate with the surface activity towards OER. EIS suggests that the OER precursor species observed on the clean and Ni-modified surfaces are similar and Fe-centered, but form at lower overpotentials when the surface Fe:Ni ratio is optimized. We propose that the well-defined Fe$_3$O$_4$(001) surface can serve as a model system for understanding the OER mechanism and establishing the structure-reactivity relation on mixed Fe-Ni oxides.}, journal = ECAct, author = {Mirabella, Francesca and M\"{u}llner, Matthias and Touzalin, Thomas and Riva, Michele and Jakub, Zdenek and Kraushofer, Florian and Schmid, Michael and Koper, Marc T. M. and Parkinson, Gareth S. and Diebold, Ulrike}, month = sep, year = {2021}, pages = {138638} } @article{wagner_direct_2021, title = {Direct assessment of the acidity of individual surface hydroxyls}, volume = {592}, doi = {10.1038/s41586-021-03432-3}, abstract = {The state of deprotonation/protonation of surfaces has far-ranging implications in chemistry, from acid\textendash{}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\textemdash{}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)\textemdash{}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.}, number = {7856}, journal = {Nature}, author = {Wagner, Margareta and Meyer, Bernd and Setvin, Martin and Schmid, Michael and Diebold, Ulrike}, month = apr, year = {2021}, pages = {722--725} } @article{franchini_polarons_2021, title = {Polarons in materials}, volume = {6}, doi = {10.1038/s41578-021-00289-w}, abstract = {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.}, journal = NatRMat, author = {Franchini, Cesare and Reticcioli, Michele and Setvin, Martin and Diebold, Ulrike}, month = mar, year = {2021}, pages = {560--586} } @article{hulva_unraveling_2021, title = {Unraveling {CO} adsorption on model single-atom catalysts}, volume = {371}, doi = {10.1126/science.abe5757}, abstract = {Understanding how the local environment of a \textquotedblleft{}single-atom\textquotedblright{} 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 Fe$_3$O$_4$(001), a model support in which all metals occupy the same twofold-coordinated adsorption site upon deposition at room temperature. Surface science techniques revealed that CO adsorption strength at single metal sites differs from the respective metal surfaces and supported clusters. Charge transfer into the support modifies the d-states of the metal atom and the strength of the metal\textendash{}CO bond. These effects could strengthen the bond (as for Ag1\textendash{}CO) or weaken it (as for Ni1\textendash{}CO), but CO-induced structural distortions reduce adsorption energies from those expected on the basis of electronic structure alone. The extent of the relaxations depends on the local geometry and could be predicted by analogy to coordination chemistry.}, number = {6527}, journal = {Science}, author = {Hulva, Jan and Meier, Matthias and Bliem, Roland and Jakub, Zdenek and Kraushofer, Florian and Schmid, Michael and Diebold, Ulrike and Franchini, Cesare and Parkinson, Gareth S.}, month = jan, year = {2021}, pmid = {33479148}, pages = {375--379} } @article{grumelli_electrochemical_2020, title = {Electrochemical stability of the reconstructed {Fe$_3$O$_4$}(001) surface}, volume = {59}, doi = {https://doi.org/10.1002/anie.202008785}, abstract = {Establishing the atomic-scale structure of metal-oxide surfaces during electrochemical reactions is a key step to modeling this important class of electrocatalysts. Here, we demonstrate that the characteristic ($\surd{}$2$\times$$\surd{}$2)R45$^\circ$ 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 Fe$_3$O$_4$ 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.}, number = {49}, journal = AngChIE, author = {Grumelli, Doris and Wiegmann, Tim and Barja, Sara and Reikowski, Finn and Maroun, Fouad and Allongue, Philippe and Balajka, Jan and Parkinson, Gareth S. and Diebold, Ulrike and Kern, Klaus and Magnussen, Olaf M.}, year = {2020}, pages = {21904--21908} } @article{marcinkowski_adsorption_2020, title = {Adsorption and reaction of methanol on {Fe$_3$O$_4$}(001)}, volume = {152}, doi = {10.1063/1.5139418}, abstract = {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 Fe$_3$O$_4$(001) at cryogenic temperatures using a combination of temperature programmed desorption, x-ray photoelectron spectroscopy, and scanning tunneling microscopy. The methanol desorption profile from Fe$_3$O$_4$(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 $\sim{}$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 $\sim{}$350 K and a disproportionation reaction to form methanol and formaldehyde at $\sim{}$500 K. Only 20\% of the methoxy species undergo the disproportionation reaction, with most of them reacting via the 350 K pathway.}, number = {6}, journal = JCP, author = {Marcinkowski, Matthew D. and Adamsen, Kr\ae{}n C. and Doudin, Nassar and Sharp, Marcus A. and Smith, R. Scott and Wang, Yang and Wendt, Stefan and Lauritsen, Jeppe V. and Parkinson, Gareth S. and Kay, Bruce D. and Dohn\'{a}lek, Zdenek}, month = feb, year = {2020}, pages = {064703} } @article{stadlmayr_high_2020, title = {A high temperature dual-mode quartz crystal microbalance technique for erosion and thermal desorption spectroscopy measurements}, volume = {91}, doi = {10.1063/5.0012028}, abstract = {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.}, number = {12}, journal = RSI, author = {Stadlmayr, Reinhard and Szabo, Paul Stefan and Biber, Herbert and Koslowski, Hans Rudolf and Kadletz, Elisabeth and Cupak, Christian and Wilhelm, Richard Arthur and Schmid, Michael and Linsmeier, Christian and Aumayr, Friedrich}, month = dec, year = {2020}, pages = {125104} } @article{timmermann_iro2_2020, title = {{IrO$_2$} surface complexions identified through machine learning and surface investigations}, volume = {125}, doi = {10.1103/PhysRevLett.125.206101}, abstract = {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$\times$1) terminations competitive with (110) in reducing environments. Experiments on single crystals find that (101) facets dominate and exhibit the theoretically predicted (1$\times$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.}, number = {20}, journal = PRL, author = {Timmermann, Jakob and Kraushofer, Florian and Resch, Nikolaus and Li, Peigang and Wang, Yu and Mao, Zhiqiang and Riva, Michele and Lee, Yonghyuk and Staacke, Carsten and Schmid, Michael and Scheurer, Christoph and Parkinson, Gareth S. and Diebold, Ulrike and Reuter, Karsten}, month = nov, year = {2020}, pages = {206101} } @article{franceschi_atomically_2020, title = {Atomically resolved surface phases of {La}$_{0.8}${Sr}$_{0.2}${MnO}$_3$(110) thin films}, volume = {8}, doi = {10.1039/D0TA07032G}, abstract = {The atomic-scale properties of lanthanum\textendash{}strontium manganite (La1-xSrxMnO3-$\delta{}$, 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 \textendash{} LEED, X-ray photoelectron spectroscopy \textendash{} XPS, and low-energy He+ ion scattering \textendash{} 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 SrTiO$_3$(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 $^\circ$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.}, number = {43}, journal = JMCA, author = {Franceschi, Giada and Schmid, Michael and Diebold, Ulrike and Riva, Michele}, month = nov, year = {2020}, pages = {22947--22961} } @article{mirabella_atomic-scale_2020, title = {Atomic-scale studies of {Fe$_3$O$_4$}(001) and {TiO$_2$}(110) surfaces following immersion in {CO$_2$}-acidified water}, volume = {21}, doi = {10.1002/cphc.202000471}, abstract = {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 TiO$_2$ and Fe$_3$O$_4$ surfaces. Herein, we describe a simple approach to vary the pH through the partial pressure of CO$_2$ (p\_CO2 ) in the surrounding vacuum chamber and use this to study how these surfaces react to an acidic solution. The TiO$_2$(110) surface is unaffected by the acidic solution, except for a small amount of carbonaceous contamination. The Fe$_3$O$_4$(001)-($\surd{}$2 $\times$ $\surd{}$2)R45$^\circ$ surface begins to dissolve at a pH 4.0\textendash{}3.9 ( p\_CO2 = 0.8\textendash{}1 bar) and, although it is significantly roughened, the atomic-scale structure of the Fe$_3$O$_4$(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.}, number = {16}, journal = {ChemPhysChem}, author = {Mirabella, Francesca and Balajka, Jan and Pavelec, Jiri and G\"{o}bel, Markus and Kraushofer, Florian and Schmid, Michael and Parkinson, Gareth S. and Diebold, Ulrike}, month = aug, year = {2020}, pages = {1788--1796} } @article{kopfle_carbide-modified_2020, title = {Carbide-modified {Pd} on {ZrO$_2$} as active phase for {CO$_2$}-reforming of methane\textemdash{}{A} model phase boundary approach}, volume = {10}, doi = {10.3390/catal10091000}, abstract = {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\textendash{}ZrO$_2$ phase boundary under realistic DRM conditions, whereby the oxidative segregation of ZrO$_2$ from bulk intermetallic PdxZry leads to a highly active composite layer of carbide-modified Pd0 metal nanoparticles in contact with tetragonal ZrO$_2$. This active state exhibits reaction rates exceeding those of a conventional supported Pd\textendash{}ZrO$_2$ 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/ZrO$_2$ phase boundary, which serves the role of providing efficient CO$_2$ activation sites. In contrast, the near-surface intermetallic precursor decomposes toward ZrO$_2$ 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 ZrO$_2$\textendash{}Pd0 state.}, number = {9}, journal = {Catalysts}, author = {K\"{o}pfle, Norbert and Ploner, Kevin and Lackner, Peter and G\"{o}tsch, Thomas and Thurner, Christoph and Carbonio, Emilia and H\"{a}vecker, Michael and Knop-Gericke, Axel and Schlicker, Lukas and Doran, Andrew and Kober, Delf and Gurlo, Aleksander and Willinger, Marc and Penner, Simon and Schmid, Michael and Kl\"{o}tzer, Bernhard}, month = sep, year = {2020}, pages = {1000} } @article{meusel_atomic_2020, title = {Atomic force and scanning tunneling microscopy of ordered ionic liquid wetting layers from 110 {K} up to room temperature}, volume = {14}, doi = {10.1021/acsnano.0c03841}, abstract = {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\textendash{}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.}, number = {7}, journal = {ACS Nano}, author = {Meusel, Manuel and Lexow, Matthias and Gezmis, Afra and Sch\"{o}tz, Simon and Wagner, Margareta and Bayer, Andreas and Maier, Florian and Steinr\"{u}ck, Hans-Peter}, month = jul, year = {2020}, pages = {9000--9010} } @article{baker_oxide_2020, title = {Oxide chemistry and catalysis}, volume = {153}, doi = {10.1063/5.0021819}, abstract = {Editorial of a JCP special issue focused on oxide chemistry and catalysis}, number = {5}, journal = JCP, author = {Baker, L. Robert and Diebold, Ulrike and Park, Jeong Young and Selloni, Annabella}, month = aug, year = {2020}, pages = {050401} } @article{wojewoda_propagation_2020, title = {Propagation of spin waves through a {N\'{e}el} domain wall}, volume = {117}, doi = {10.1063/5.0013692}, abstract = {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 ($\mu{}$BLS). The 2D spin-wave intensity maps reveal that spin-wave transmission through a N\'{e}el 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 $\mu{}$BLS 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.}, number = {2}, journal = APL, author = {Wojewoda, O. and Hula, T. and Flaj\v{s}man, L. and Va\v{n}atka, M. and Gloss, J. and Holobr\'{a}dek, J. and Sta\v{n}o, M. and Stienen, S. and K\"{o}rber, L. and Schultheiss, K. and Schmid, M. and Schultheiss, H. and Urb\'{a}nek, M.}, month = jul, year = {2020}, pages = {022405} } @article{sokolovic_resolving_2020, title = {Resolving the adsorption of molecular {O$_2$} on the rutile {TiO$_2$}(110) surface by noncontact atomic force microscopy}, volume = {117}, doi = {10.1073/pnas.1922452117}, abstract = {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 TiO$_2$(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.}, number = {26}, journal = PNAS, author = {Sokolovi\'{c}, Igor and Reticcioli, Michele and \v{C}alkovsk\'{y}, Martin and Wagner, Margareta and Schmid, Michael and Franchini, Cesare and Diebold, Ulrike and Setv\'{\i}n, Martin}, month = jun, year = {2020}, pmid = {32527857}, pages = {14827--14837} } @article{franceschi_movable_2020, title = {Movable holder for a quartz crystal microbalance for exact growth rates in pulsed laser deposition}, volume = {91}, doi = {10.1063/5.0007643}, abstract = {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.}, number = {6}, journal = RSI, author = {Franceschi, Giada and Schmid, Michael and Diebold, Ulrike and Riva, Michele}, month = jun, year = {2020}, pages = {065003} } @article{stubian_fast_2020, title = {Fast low-noise transimpedance amplifier for scanning tunneling microscopy and beyond}, volume = {91}, doi = {10.1063/5.0011097}, abstract = {A transimpedance amplifier has been designed for scanning tunneling microscopy (STM). The amplifier features low noise (limited by the Johnson noise of the 1 G$\Omega{}$ 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\textendash{}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.}, number = {7}, journal = RSI, author = {\v{S}tubian, Martin and Bobek, Juraj and Setvin, Martin and Diebold, Ulrike and Schmid, Michael}, month = jul, year = {2020}, pages = {074701} } @article{franceschi_model_2020, title = {A model system for photocatalysis: {Ti}-doped $\alpha{}$-{Fe$_2$O$_3$}(1$\bar{1}$02) single-crystalline films}, volume = {32}, shorttitle = {A {Model} {System} for {Photocatalysis}}, doi = {10.1021/acs.chemmater.9b04908}, abstract = {Hematite ($\alpha{}$-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 $\alpha{}$-Fe2O3(1$\bar{1}$02) 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.}, number = {9}, journal = CMater, author = {Franceschi, Giada and Kraushofer, Florian and Meier, Matthias and Parkinson, Gareth S. and Schmid, Michael and Diebold, Ulrike and Riva, Michele}, month = may, year = {2020}, pages = {3753--3764} } @article{arndt_orderdisorder_2020, title = {Order\textendash{}disorder phase transition of the subsurface cation vacancy reconstruction on {Fe$_3$O$_4$}(001)}, volume = {22}, doi = {10.1039/D0CP00690D}, abstract = {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.}, number = {16}, journal = PCCP, author = {Arndt, Bj\"{o}rn and Lechner, Barbara A. J. and Bourgund, Alexander and Gr\aa{}n\"{a}s, Elin and Creutzburg, Marcus and Krausert, Konstantin and Hulva, Jan and Parkinson, Gareth S. and Schmid, Michael and Vonk, Vedran and Esch, Friedrich and Stierle, Andreas}, month = apr, year = {2020}, pages = {8336--8343} } @article{ryan_probing_2020, title = {Probing structural changes upon carbon monoxide coordination to single metal adatoms}, volume = {152}, doi = {10.1063/1.5137904}, abstract = {In this work, the adsorption height of Ag adatoms on the Fe$_3$O$_4$(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 \AA{} $\pm{}$ 0.08 \AA{}, compared to the previously measured height of 0.96 \AA{} $\pm{}$ 0.03 \AA{} 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 \AA{}, in remarkably good agreement with the experimental results.}, number = {5}, journal = JCP, author = {Ryan, P. T. P. and Meier, M. and Jakub, Z. and Balajka, J. and Hulva, J. and Payne, D. J. and Lee, T.-L. and Franchini, C. and Allegretti, F. and Parkinson, G. S. and Duncan, D. A.}, month = feb, year = {2020}, pages = {051102} } @article{jakub_adsorbate-induced_2020, title = {Adsorbate-induced structural evolution changes the mechanism of {CO} oxidation on a {Rh}/{Fe$_3$O$_4$}(001) model catalyst}, volume = {12}, doi = {10.1039/C9NR10087C}, abstract = {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/Fe$_3$O$_4$(001) \textquotedblleft{}single-atom\textquotedblright{} 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\textendash{}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.}, number = {10}, journal = {Nanoscale}, author = {Jakub, Zdenek and Hulva, Jan and Ryan, Paul T. P. and Duncan, David A. and Payne, David J. and Bliem, Roland and Ulreich, Manuel and Hofegger, Patrick and Kraushofer, Florian and Meier, Matthias and Schmid, Michael and Diebold, Ulrike and Parkinson, Gareth S.}, month = mar, year = {2020}, pages = {5866--5875} } @article{franceschi_two-dimensional_2021, title = {Two-dimensional surface phase diagram of a multicomponent perovskite oxide: {La}$_{0.8}${Sr}$_{0.2}${MnO}$_3$(110)}, volume = {5}, shorttitle = {Two-dimensional surface phase diagram of a multicomponent perovskite oxide}, doi = {10.1103/PhysRevMaterials.5.L092401}, abstract = {The many surface reconstructions of (110)-oriented lanthanum strontium manganite (La0.8Sr0.2MnO3, LSMO) were followed as a function of the oxygen chemical potential ($\mu{}$O) and the surface cation composition. Decreasing $\mu{}$O 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.}, number = {9}, journal = PRM, author = {Franceschi, Giada and Schmid, Michael and Diebold, Ulrike and Riva, Michele}, month = sep, year = {2021}, pages = {L092401} } @article{parkinson_adding_2021, title = {Adding oxides to the {2D} toolkit}, volume = {20}, doi = {10.1038/s41563-021-01048-6}, abstract = {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.}, number = {8}, journal = NatMat, author = {Parkinson, Gareth S.}, month = aug, year = {2021}, pages = {1041--1042} } @article{jakub_rapid_2021, title = {Rapid oxygen exchange between hematite and water vapor}, volume = {12}, doi = {10.1038/s41467-021-26601-4}, abstract = {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 \textquotedblleft{}r-cut\textquotedblright{} (\$\$1{\textbackslash}bar\{1\}02\$\$) surface of hematite ($\alpha{}$-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70\hspace{0.167em}$^\circ$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.}, number = {1}, journal = NatComm, author = {Jakub, Zdenek and Meier, Matthias and Kraushofer, Florian and Balajka, Jan and Pavelec, Jiri and Schmid, Michael and Franchini, Cesare and Diebold, Ulrike and Parkinson, Gareth S.}, month = nov, year = {2021}, pages = {6488} } @article{wagner_oxygen-rich_2021, title = {Oxygen-rich tetrahedral surface phase on high-temperature rutile {VO$_2$(110)$_\mathrm{T}$} single crystals}, volume = {5}, doi = {10.1103/PhysRevMaterials.5.125001}, abstract = {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 $\times$ 2) superstructure that coexists with small patches of the underlying unreconstructed (110)-(1$\times$1) surface when the crystal is annealed {\textgreater}600$\circ{}$C. The best structural model for the (2 $\times$ 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.}, number = {12}, journal = PRM, author = {Wagner, Margareta and Planer, Jakub and Heller, Bettina S. J. and Langer, Jens and Limbeck, Andreas and Boatner, Lynn A. and Steinr\"{u}ck, Hans-Peter and Redinger, Josef and Maier, Florian and Mittendorfer, Florian and Schmid, Michael and Diebold, Ulrike}, month = dec, year = {2021}, pages = {125001} } @article{kraushofer_single_2022, title = {Single {Rh} adatoms stabilized on $\alpha{}$-{Fe$_2$O$_3$}(1$\bar{1}$02) by coadsorbed water}, volume = {7}, doi = {10.1021/acsenergylett.1c02405}, abstract = {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 $\alpha{}$-Fe2O3(1$\bar{1}$02) in the presence of 2 $\times$ 10\textendash{}8 mbar of water at room temperature, in marked contrast to the rapid sintering observed under UHV conditions. Annealing to 50 $^\circ$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.}, number = {1}, journal = ACSEL, author = {Kraushofer, Florian and Haager, Lena and Eder, Moritz and Rafsanjani-Abbasi, Ali and Jakub, Zden\v{e}k and Franceschi, Giada and Riva, Michele and Meier, Matthias and Schmid, Michael and Diebold, Ulrike and Parkinson, Gareth S.}, month = jan, year = {2022}, pages = {375--380} } @article{cui_reversible_2017, title = {Reversible anion-driven switching of an organic {2D} crystal at a solid\textendash{}liquid interface}, volume = {13}, doi = {10.1002/smll.201702379}, abstract = {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\textendash{}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\textendash{}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\textendash{}adsorbate interactions in heterogeneous catalysis.}, number = {46}, journal = {Small}, author = {Cui, Kang and Mali, Kunal S. and Wu, Dongqing and Feng, Xinliang and M\"{u}llen, Klaus and Walter, Michael and De Feyter, Steven and Mertens, Stijn F. L.}, month = dec, year = {2017}, pages = {1702379} } @article{kraushofer_atomic-scale_2018, title = {Atomic-scale structure of the hematite $\alpha$-{Fe}$_2${O}$_3$(1$\bar{1}$02) \textquotedblleft{}r-cut\textquotedblright{} surface}, volume = {122}, doi = {10.1021/acs.jpcc.7b10515}, abstract = {The $\alpha{}$-{Fe}$_2${O}$_3$(1$\bar{1}$02) 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 $\times$ 1) surface can be prepared by annealing at 450 $^\circ$C in $\approx{}$10\textendash{}6 mbar O2, and a reduced (2 $\times$ 1) reconstruction is formed by UHV annealing at 540 $^\circ$C. The (1 $\times$ 1) surface is close to an ideal bulk termination, and the undercoordinated surface Fe atoms reduce the surface bandgap by $\approx{}$0.2 eV with respect to the bulk. The work function is measured to be 5.7 $\pm{}$ 0.2 eV, and the VBM is located 1.5 $\pm{}$ 0.1 eV below EF. The images obtained from the (2 $\times$ 1) reconstruction cannot be reconciled with previously proposed models, and a new \textquotedblleft{}alternating trench\textquotedblright{} 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 $\times$ 1) termination is 5.4 $\pm{}$ 0.2 eV.}, number = {3}, journal = JPCC, author = {Kraushofer, Florian and Jakub, Zdenek and Bichler, Magdalena and Hulva, Jan and Drmota, Peter and Weinold, Michael and Schmid, Michael and Setvin, Martin and Diebold, Ulrike and Blaha, Peter and Parkinson, Gareth S.}, month = jan, year = {2018}, pages = {1657--1669} } @article{hulva_adsorption_2018, title = {Adsorption of {CO} on the {Fe$_3$O$_4$}(001) surface}, volume = {122}, doi = {10.1021/acs.jpcb.7b06349}, abstract = {The interaction of CO with the Fe$_3$O$_4$(001)-($\surd{}$2 $\times$ $\surd{}$2)R45$^\circ$ 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\textendash{}Fe$_3$O$_4$ 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 ($\surd{}$2 $\times$ $\surd{}$2)R45$^\circ$ 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 ($\surd{}$2 $\times$ $\surd{}$2)R45$^\circ$ unit cell, a much sharper desorption feature emerges at $\sim{}$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.}, number = {2}, journal = JPCB, author = {Hulva, Jan and Jakub, Zden\v{e}k and Novotny, Zbynek and Johansson, Niclas and Knudsen, Jan and Schnadt, Joachim and Schmid, Michael and Diebold, Ulrike and Parkinson, Gareth S.}, month = jan, year = {2018}, pages = {721--729} } @article{meier_probing_2018, title = {Probing the geometry of copper and silver adatoms on magnetite: quantitative experiment versus theory}, volume = {10}, doi = {10.1039/C7NR07319D}, abstract = {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 Fe$_3$O$_4$(001). Both adatoms occupy bulk-continuation cation sites, but with a markedly different height above the surface (0.43 $\pm{}$ 0.03 \AA{} (Cu1) and 0.96 $\pm{}$ 0.03 \AA{} (Ag1)). HSE-based calculations accurately predict the experimental geometry, but the more common PBE + U and PBEsol + U approaches perform poorly.}, number = {5}, journal = {Nanoscale}, author = {Meier, Matthias and Jakub, Zden\v{e}k and Balajka, Jan and Hulva, Jan and Bliem, Roland and Thakur, Pardeep K. and Lee, Tien-Lin and Franchini, Cesare and Schmid, Michael and Diebold, Ulrike and Allegretti, Francesco and Duncan, David A. and Parkinson, Gareth S.}, year = {2018}, pages = {2226--2230} } @article{setvin_polarity_2018, title = {Polarity compensation mechanisms on the perovskite surface {KTaO}$_3$(001)}, volume = {359}, doi = {10.1126/science.aar2287}, abstract = {Compensating a polar surface An ionic crystal surface can be electrostatically unstable, and the surface must reconstruct in some way to avoid this \textquotedblleft{}polar catastrophe.\textquotedblright{} 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\textemdash{}a \textquotedblleft{}polar catastrophe\textquotedblright{}\textemdash{}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.}, number = {6375}, journal = {Science}, author = {Setvin, Martin and Reticcioli, Michele and Poelzleitner, Flora and Hulva, Jan and Schmid, Michael and Boatner, Lynn A. and Franchini, Cesare and Diebold, Ulrike}, month = feb, year = {2018}, pages = {572--575} } @article{wagner_prototypical_2018, title = {Prototypical organic\textendash{}oxide interface: {Intramolecular} resolution of sexiphenyl on {In2O3}(111)}, volume = {10}, doi = {10.1021/acsami.8b02177}, abstract = {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 $^\circ$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 $\times$ 1) structure with one 6P per In2O3(111) substrate unit cell, that is, a molecular density of 5.64 $\times$ 1013 cm\textendash{}2.}, number = {16}, journal = ACSAMI, author = {Wagner, Margareta and Hofinger, Jakob and Setv\'{\i}n, Martin and Boatner, Lynn A. and Schmid, Michael and Diebold, Ulrike}, month = mar, year = {2018}, pages = {14175--14182} } @article{halwidl_full_2018, title = {A full monolayer of superoxide: oxygen activation on the unmodified {Ca}$_3${Ru}$_2${O}$_7$(001) surface}, volume = {6}, doi = {10.1039/C8TA00265G}, abstract = {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\textendash{}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.}, number = {14}, journal = JMCA, author = {Halwidl, Daniel and Mayr-Schm\"{o}lzer, Wernfried and Setvin, Martin and Fobes, David and Peng, Jin and Mao, Zhiqiang and Schmid, Michael and Mittendorfer, Florian and Redinger, Josef and Diebold, Ulrike}, month = apr, year = {2018}, pages = {5703--5713} } @article{urbanek_research_2018, title = {Research update: {Focused} ion beam direct writing of magnetic patterns with controlled structural and magnetic properties}, volume = {6}, doi = {10.1063/1.5029367}, abstract = {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.}, number = {6}, journal = APLM, author = {Urb\'{a}nek, Michal and Flaj\v{s}man, Luk\'{a}\v{s} and K\v{r}i\v{z}\'{a}kov\'{a}, Viola and Gloss, Jon\'{a}\v{s} and Hork\'{y}, Michal and Schmid, Michael and Varga, Peter}, month = jun, year = {2018}, pages = {060701} } @article{reticcioli_formation_2018, title = {Formation and dynamics of small polarons on the rutile {TiO}$_2$(110) surface}, volume = {98}, doi = {10.1103/PhysRevB.98.045306}, abstract = {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.}, number = {4}, journal = PRB, author = {Reticcioli, Michele and Setvin, Martin and Schmid, Michael and Diebold, Ulrike and Franchini, Cesare}, month = jul, year = {2018}, pages = {045306} } @article{meier_water_2018, title = {Water agglomerates on {Fe}$_3${O}$_4$(001)}, volume = {115}, doi = {10.1073/pnas.1801661115}, abstract = {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 Fe$_3$O$_4$(001); a complex mineral surface relevant in both modern technology and the natural environment. Strong OH\textendash{}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.}, number = {25}, journal = PNAS, author = {Meier, Matthias and Hulva, Jan and Jakub, Zden\v{e}k and Pavelec, Ji\v{r}\'{\i} and Setvin, Martin and Bliem, Roland and Schmid, Michael and Diebold, Ulrike and Franchini, Cesare and Parkinson, Gareth S.}, month = jun, year = {2018}, pages = {E5642--E5650} } @article{balajka_high-affinity_2018, title = {High-affinity adsorption leads to molecularly ordered interfaces on {TiO}$_2$ in air and solution}, volume = {361}, doi = {10.1126/science.aat6752}, abstract = {Researchers around the world have observed the formation of molecularly ordered structures of unknown origin on the surface of titanium dioxide (TiO$_2$) photocatalysts exposed to air and solution. Using a combination of atomic-scale microscopy and spectroscopy, we show that TiO$_2$ 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 TiO$_2$. This finding is relevant to TiO$_2$ 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.}, number = {6404}, journal = {Science}, author = {Balajka, Jan and Hines, Melissa A. and DeBenedetti, William J. I. and Komora, Mojmir and Pavelec, Jiri and Schmid, Michael and Diebold, Ulrike}, month = aug, year = {2018}, pages = {786--789} } @article{cui_interfacial_2018, title = {Interfacial supramolecular electrochemistry}, volume = {8}, doi = {10.1016/j.coelec.2018.06.002}, abstract = {Supramolecular chemistry at solid\textendash{}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.}, journal = {Current Opinion in Electrochemistry}, author = {Cui, Kang and Dorner, Iris and Mertens, Stijn F. L.}, month = mar, year = {2018}, pages = {156--163} } @incollection{mertens_adsorption_2018, title = {Adsorption and self-organization of organic molecules under electrochemical control}, volume = {4}, isbn = {978-0-12-809894-3}, abstract = {Adsorption and self-organization of organic molecules on solid substrates have been studied extensively in vacuum and at solid\textendash{}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\textendash{}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.}, booktitle = {Encyclopedia of {Interfacial} {Chemistry}: {Surface} {Science} and {Electrochemistry}}, publisher = {Elsevier}, author = {Mertens, Stijn F. L. }, editor = {{Klaus Wandelt}}, year = {2018}, pages = {13--23} } @article{balajka_apparatus_2018, title = {Apparatus for dosing liquid water in ultrahigh vacuum}, volume = {89}, doi = {10.1063/1.5046846}, abstract = {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 TiO$_2$(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.}, number = {8}, journal = RSI, author = {Balajka, Jan and Pavelec, Jiri and Komora, Mojmir and Schmid, Michael and Diebold, Ulrike}, month = aug, year = {2018}, pages = {083906} } @article{riva_influence_2018, title = {Influence of surface atomic structure demonstrated on oxygen incorporation mechanism at a model perovskite oxide}, volume = {9}, doi = {10.1038/s41467-018-05685-5}, abstract = {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\hspace{0.167em}$\times$\hspace{0.167em}1) and (2\hspace{0.167em}$\times$\hspace{0.167em}5) symmetry on 0.5 wt.\% Nb-doped {SrTiO}$_3$(110) were subjected to isotopically labeled oxygen exchange at 450\hspace{0.167em}$^\circ$C. The oxygen incorporation rate is three times higher on the (4\hspace{0.167em}$\times$\hspace{0.167em}1) surface phase compared to the (2\hspace{0.167em}$\times$\hspace{0.167em}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.}, number = {1}, journal = NatComm, author = {Riva, Michele and Kubicek, Markus and Hao, Xianfeng and Franceschi, Giada and Gerhold, Stefan and Schmid, Michael and Hutter, Herbert and Fleig, Juergen and Franchini, Cesare and Yildiz, Bilge and Diebold, Ulrike}, month = sep, year = {2018}, pages = {3710} } @article{lackner_water_2018, title = {Water adsorption at zirconia: from the {ZrO}$_2$(111)/{Pt}$_3${Zr}(0001) model system to powder samples}, volume = {6}, doi = {10.1039/C8TA04137G}, abstract = {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 $\pm{}$ 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 CO$_2$ TPD. Nevertheless, the defect density is much smaller than in previous studies of H2O/ZrO$_2$. 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.}, number = {36}, journal = JMCA, author = {Lackner, Peter and Hulva, Jan and K\"{o}ck, Eva-Maria and Mayr-Schm\"{o}lzer, Wernfried and Choi, Joong Il J. and Penner, Simon and Diebold, Ulrike and Mittendorfer, Florian and Redinger, Josef and Kl\"{o}tzer, Bernhard and Parkinson, Gareth S. and Schmid, Michael}, month = sep, year = {2018}, pages = {17587--17601} } @article{dvorak_imaging_2017, title = {Imaging of near-field interference patterns by aperture-type {SNOM} \textendash{} influence of illumination wavelength and polarization state}, volume = {25}, doi = {10.1364/OE.25.016560}, abstract = {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 \textendash{} one of the most challenging tasks of near field interference SNOM measurements \textendash{} is not yet fully resolved.}, number = {14}, journal = {Optics Express}, author = {Dvo\v{r}\'{a}k, Petr and \'{E}des, Zolt\'{a}n and Kvapil, Michal and \v{S}amo\v{r}il, Tom\'{a}\v{s} and Ligmajer, Filip and Hrto\v{n}, Martin and Kalousek, Radek and K\v{r}\'{a}pek, Vlastimil and Dub, Petr and Spousta, Ji\v{r}\'{\i} and Varga, Peter and \v{S}ikola, Tom\'{a}\v{s}}, month = jul, year = {2017}, pages = {16560--16573} } @article{mayr-schmolzer_adsorption_2019, title = {Adsorption of {CO} on the {Ca}$_3${Ru}$_2${O}$_7$(001) surface}, volume = {680}, doi = {10.1016/j.susc.2018.10.004}, abstract = {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.}, journal = SuSci, author = {Mayr-Schm\"{o}lzer, Wernfried and Halwidl, Daniel and Mittendorfer, Florian and Schmid, Michael and Diebold, Ulrike and Redinger, Josef}, month = feb, year = {2019}, pages = {18--23} } @article{lackner_surface_2019, title = {Surface structures of {ZrO}$_2$ films on {Rh}(111): {From} two layers to bulk termination}, volume = {679}, doi = {10.1016/j.susc.2018.09.004}, abstract = {We have studied zirconia films on a Rh(111) substrate with thicknesses in the range of 2\textendash{}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\textendash{}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\,$\times$\,1) or a distorted (2\,$\times$\,2) surface structure with respect to cubic ZrO$_2$(111); these structures correspond to tetragonal and monoclinic zirconia, respectively. The tetragonal phase occurs for annealing temperatures of up to 730\,$^\circ$C; transformation to the thermodynamically stable monoclinic phase occurs after annealing at 850\,$^\circ$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 ZrO$_2$ films on Rh(111) as a well-suited model system for surface-science studies on ZrO$_2$, 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 ZrO$_2$/Pt(111) system.}, journal = SuSci, author = {Lackner, Peter and Zou, Zhiyu and Mayr, Sabrina and Choi, Joong-Il Jake and Diebold, Ulrike and Schmid, Michael}, month = jan, year = {2019}, pages = {180--187} } @article{wagner_sexiphenyl_2018, title = {Sexiphenyl on {Cu}(100): nc-{AFM} tip functionalization and identification}, volume = {678}, doi = {10.1016/j.susc.2018.03.004}, abstract = {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$^\circ$ off the $langle$010$rangle$ 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.}, journal = SuSci, author = {Wagner, Margareta and Setv\'{\i}n, Martin and Schmid, Michael and Diebold, Ulrike}, month = dec, year = {2018}, pages = {124--127} } @article{gloss_growth_2019, title = {The growth of metastable fcc {Fe}$_78${Ni}$_22$ thin films on {H}-{Si}(100) substrates suitable for focused ion beam direct magnetic patterning}, volume = {469}, doi = {10.1016/j.apsusc.2018.10.263}, abstract = {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\,$^\circ$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.}, journal = APSS, author = {Gloss, Jonas and Hork\'{y}, Michal and K\v{r}i\v{z}\'{a}kov\'{a}, Viola and Flaj\v{s}man, Luk\'{a}\v{s} and Schmid, Michael and Urb\'{a}nek, Michal and Varga, Peter}, month = mar, year = {2019}, pages = {747--752} } @article{reticcioli_interplay_2019, title = {Interplay between adsorbates and polarons: {CO} on rutile {TiO}$_2$(110)}, volume = {122}, doi = {10.1103/PhysRevLett.122.016805}, abstract = {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 {TiO}$_2$(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 {TiO}$_2$(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.}, number = {1}, journal = PRL, author = {Reticcioli, Michele and Sokolovi\'{c}, Igor and Schmid, Michael and Diebold, Ulrike and Setvin, Martin and Franchini, Cesare}, month = jan, year = {2019}, pages = {016805} } @article{parkinson_unravelling_2017, title = {Unravelling single atom catalysis: {The} surface science approach}, volume = {38}, doi = {10.1016/S1872-2067(17)62878-X}, abstract = {This perspective discusses how studies of idealised model systems can shed light on the fundamental mechanisms of single-atom catalysis.}, number = {9}, journal = {Chinese Journal of Catalysis}, author = {Parkinson, Gareth S.}, month = sep, year = {2017}, pages = {1454--1459} } @article{parkinson_single-atom_2019, title = {Single-{Atom} {Catalysis}: {How} {Structure} {Influences} {Catalytic} {Performance}}, doi = {10.1007/s10562-019-02709-7}, abstract = {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 \textquotedblleft{}single-atom\textquotedblright{} 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.Graphical Abstract Open image in new window}, journal = CatLett, author = {Parkinson, Gareth S.}, month = feb, year = {2019} } @incollection{reticcioli_small_2019, address = {Cham}, title = {Small {Polarons} in {Transition} {Metal} {Oxides}}, isbn = {978-3-319-50257-1}, abstract = {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 {TiO}$_2$. 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.}, booktitle = {Handbook of {Materials} {Modeling}: {Applications}: {Current} and {Emerging} {Materials}}, publisher = {Springer International Publishing}, author = {Reticcioli, Michele and Diebold, Ulrike and Kresse, Georg and Franchini, Cesare}, editor = {Andreoni, Wanda and Yip, Sidney}, year = {2019}, doi = {10.1007/978-3-319-50257-1_52-1}, pages = {1--39} } @article{sokolovic_incipient_2019, title = {Incipient ferroelectricity: {A} route towards bulk-terminated {SrTiO}$_3$}, volume = {3}, doi = {10.1103/PhysRevMaterials.3.034407}, abstract = {An investigation of bulk-terminated (001) surfaces of {SrTiO}$_3$, a prototypical cubic perovskite, was made possible with a novel cleaving procedure. Controlled application of strain on a {SrTiO}$_3$ single-crystal results in a flat cleavage with $\mu{}$m-size domains of SrO and {TiO}$_2$. 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$\pm{}$2)\%; Sr vacancies form at the SrO termination and complementary Sr adatoms appear at the {TiO}$_2$ termination. These intrinsic defects stabilize the surface by balancing the interplay between ferroelectricity, surface polarity, and surface charge.}, number = {3}, journal = PRM, author = {Sokolovi\'{c}, Igor and Schmid, Michael and Diebold, Ulrike and Setvin, Martin}, month = mar, year = {2019}, pages = {034407} } @article{riva_pushing_2019, title = {Pushing the detection of cation nonstoichiometry to the limit}, volume = {3}, doi = {10.1103/PhysRevMaterials.3.043802}, abstract = {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 {SrTiO}$_3$(110), and use the established relation between those reconstructions and the surface composition to assess the cation excess deposited in pulsed-laser grown {SrTiO}$_3$(110) films. We demonstrate that a {\textless}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.}, number = {4}, journal = PRM, author = {Riva, Michele and Franceschi, Giada and Lu, Qiyang and Schmid, Michael and Yildiz, Bilge and Diebold, Ulrike}, month = apr, year = {2019}, pages = {043802} } @article{mullner_stability_2019, title = {Stability and catalytic performance of reconstructed {Fe}$_3${O}$_4$(001) and {Fe}$_3${O}$_4$(110) surfaces during oxygen evolution reaction}, volume = {123}, doi = {10.1021/acs.jpcc.8b08733}, abstract = {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 Fe$_3$O$_4$ single crystal surfaces, prepared in ultrahigh vacuum (UHV) are stable in alkaline electrolytein the range pH 7\textendash{}14 and under OER conditions in 1 M NaOH. Fe$_3$O$_4$(001) and Fe$_3$O$_4$(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. Fe$_3$O$_4$(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 (Fe$_3$O$_4$) as a promising model system for further mechanistic studies of electrochemical reactions in alkaline media and under highly oxidizing conditions.}, number = {13}, journal = JPCC, author = {M\"{u}llner, Matthias and Riva, Michele and Kraushofer, Florian and Schmid, Michael and Parkinson, Gareth S. and Mertens, Stijn F. L. and Diebold, Ulrike}, month = apr, year = {2019}, pages = {8304--8311} } @article{lackner_using_2019, title = {Using photoelectron spectroscopy to observe oxygen spillover to zirconia}, volume = {21}, doi = {10.1039/C9CP03322J}, abstract = {X-ray photoelectron spectroscopy (XPS) of five-monolayer-thick ZrO$_2$ 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 ($\approx{}$5\textendash{}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 ZrO$_2$ 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.}, number = {32}, journal = PCCP, author = {Lackner, Peter and Zou, Zhiyu and Mayr, Sabrina and Diebold, Ulrike and Schmid, Michael}, month = aug, year = {2019}, pages = {17613--17620} } @article{doudin_understanding_2019, title = {Understanding Heterolytic {H}$_2$ Cleavage and Water-Assisted Hydrogen Spillover on {Fe}$_3${O}$_4$(001)-Supported Single Palladium Atoms}, doi = {10.1021/acscatal.9b01425}, abstract = {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 Fe$_3$O$_4$(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 Fe$_3$O$_4$ 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.}, journal = ACSCat, author = {Doudin, Nassar and Yuk, Simuck F. and Marcinkowski, Matthew D. and Nguyen, Manh-Thuong and Liu, Jin-Cheng and Wang, Yang and Novotny, Zbynek and Kay, Bruce D. and Li, Jun and Glezakou, Vassiliki-Alexandra and Parkinson, Gareth S. and Rousseau, Roger and Dohn\'{a}lek, Zdenek}, month = jul, year = {2019}, pages = {7876--7887} } @article{bourgund_influence_2019, title = {Influence of Local Defects on the Dynamics of {O}\textendash{}{H} Bond Breaking and Formation on a Magnetite Surface}, volume = {123}, doi = {10.1021/acs.jpcc.9b05547}, abstract = {The transport of H adatoms across oxide supports plays an important role in many catalytic reactions. We investigate the dynamics of H/Fe$_3$O$_4$(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\textendash{}O lattice shortening the diffusion path. Quantitative analysis yields an apparent activation barrier of 0.94 $\pm{}$ 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.}, number = {32}, journal = JPCC, author = {Bourgund, Alexander and Lechner, Barbara A. J. and Meier, Matthias and Franchini, Cesare and Parkinson, Gareth S. and Heiz, Ueli and Esch, Friedrich}, month = aug, year = {2019}, pages = {19742--19747} } @article{jakub_nickel_2019, title = {Nickel Doping Enhances the Reactivity of {Fe}$_3${O}$_4$(001) to Water}, volume = {123}, doi = {10.1021/acs.jpcc.9b02993}, abstract = {We studied how nickel doping affects water adsorption at the Fe$_3$O$_4$(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\textendash{}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 Fe$_3$O$_4$(001).}, number = {24}, journal = JPCC, author = {Jakub, Zdenek and Hulva, Jan and Mirabella, Francesca and Kraushofer, Florian and Meier, Matthias and Bliem, Roland and Diebold, Ulrike and Parkinson, Gareth S.}, month = jun, year = {2019}, pages = {15038--15045} } @article{zaki_water_2019, title = {Water Ordering on the Magnetite {Fe}$_3${O}$_4$ Surfaces}, volume = {10}, doi = {10.1021/acs.jpclett.9b00773}, abstract = {The interaction of water with the most prominent surfaces of Fe$_3$O$_4$, (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 ($\surd{}$2 $\times$ $\surd{}$2)R45$^\circ$-reconstructed surface of Fe$_3$O$_4$(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 $\times$ 2) superstructure. This finding is discussed in the context of a similar (2 $\times$ 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\textendash{}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.}, number = {10}, journal = JPCL, author = {Zaki, Eman and Jakub, Zdenek and Mirabella, Francesca and Parkinson, Gareth S. and Shaikhutdinov, Shamil and Freund, Hans-Joachim}, month = may, year = {2019}, pages = {2487--2492} } @article{parkinson_atomic_2018, title = {Atomic Scale Insights into Single-Atom Catalysis}, volume = {30}, doi = {10.1002/vipr.201800695}, abstract = {\textquotedblleft{}Single-Atom\textquotedblright{} 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.}, number = {5}, journal = {Vakuum in Forschung und Praxis}, author = {Parkinson, Gareth S.}, year = {2018}, pages = {45--49} } @article{kraushofer_self-limited_2019, title = {Self-limited growth of an oxyhydroxide phase at the {Fe}$_3${O}$_4$(001) surface in liquid and ambient pressure water}, volume = {151}, doi = {10.1063/1.5116652}, abstract = {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 Fe$_3$O$_4$(001)-($\surd{}$2 $\times$ $\surd{}$2)R45$^\circ$ 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 $\times$ 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.}, number = {15}, journal = JCP, author = {Kraushofer, Florian and Mirabella, Francesca and Xu, Jian and Pavelec, Ji\v{r}\'{\i} and Balajka, Jan and M\"{u}llner, Matthias and Resch, Nikolaus and Jakub, Zden\v{e}k and Hulva, Jan and Meier, Matthias and Schmid, Michael and Diebold, Ulrike and Parkinson, Gareth S.}, month = oct, year = {2019}, pages = {154702} } @article{jakub_local_2019, title = {Local Structure and Coordination Define Adsorption in a Model {Ir}$_1$/{Fe}$_3${O}$_4$ Single-Atom Catalyst}, volume = {58}, doi = {10.1002/anie.201907536}, abstract = {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 Fe$_3$O$_4$(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.}, number = {39}, journal = AngChIE, author = {Jakub, Zdenek and Hulva, Jan and Meier, Matthias and Bliem, Roland and Kraushofer, Florian and Setvin, Martin and Schmid, Michael and Diebold, Ulrike and Franchini, Cesare and Parkinson, Gareth S.}, year = {2019}, pages = {13961--13968} } @article{franceschi_growth_2019, title = {Growth of {In}$_2${O}$_3$(111) thin films with optimized surfaces}, volume = {3}, doi = {10.1103/PhysRevMaterials.3.103403}, abstract = {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.}, number = {10}, journal = PRM, author = {Franceschi, Giada and Wagner, Margareta and Hofinger, Jakob and Kraj\v{n}\'{a}k, Tom\'{a}\v{s} and Schmid, Michael and Diebold, Ulrike and Riva, Michele}, month = oct, year = {2019}, pages = {103403} } @article{riva_epitaxial_2019, title = {Epitaxial growth of complex oxide films: {Role} of surface reconstructions}, volume = {1}, doi = {10.1103/PhysRevResearch.1.033059}, abstract = {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 {SrTiO}$_3$(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 {SrTiO}$_3$(110) films and affects their stoichiometry.}, number = {3}, journal = PRR, author = {Riva, Michele and Franceschi, Giada and Schmid, Michael and Diebold, Ulrike}, month = oct, year = {2019}, pages = {033059} } @article{lackner_substoichiometric_2019, title = {Substoichiometric ultrathin zirconia films cause strong metal\textendash{}support interaction}, volume = {7}, doi = {10.1039/C9TA08438J}, abstract = {The strong metal\textendash{}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 {TiO}$_2$ or Fe$_3$O$_4$. Zirconia (ZrO$_2$) 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 ($\approx{}$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.}, number = {43}, journal = JMCA, author = {Lackner, Peter and Choi, Joong Il Jake and Diebold, Ulrike and Schmid, Michael}, month = nov, year = {2019}, pages = {24837--24846} } @article{flajsman_zero-field_2020, title = {Zero-field propagation of spin waves in waveguides prepared by focused ion beam direct writing}, volume = {101}, doi = {10.1103/PhysRevB.101.014436}, abstract = {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.}, number = {1}, journal = PRB, author = {Flaj\v{s}man, Luk\'{a}\v{s} and Wagner, Kai and Va\v{n}atka, Marek and Gloss, Jon\'{a}\v{s} and K\v{r}i\v{z}\'{a}kov\'{a}, Viola and Schmid, Michael and Schultheiss, Helmut and Urb\'{a}nek, Michal}, month = jan, year = {2020}, pages = {014436} } @article{lackner_few-monolayer_2020, title = {Few-monolayer yttria-doped zirconia films: {Segregation} and phase stabilization}, volume = {152}, doi = {10.1063/1.5140266}, abstract = {For most applications, zirconia (ZrO$_2$) 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 ZrO$_2$ 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 $\pm{}$ 4 kJ/mol.}, number = {6}, journal = JCP, author = {Lackner, Peter and Brandt, Amy J. and Diebold, Ulrike and Schmid, Michael}, month = feb, year = {2020}, pages = {064709} } @article{jakub_partially_2019, title = {Partially Dissociated Water Dimers at the Water\textendash{}Hematite Interface}, volume = {4}, doi = {10.1021/acsenergylett.8b02324}, abstract = {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\textendash{}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\textendash{}H2O dimer species form at the (1$\bar{1}$02) 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.}, number = {2}, journal = ACSEL, author = {Jakub, Zdenek and Kraushofer, Florian and Bichler, Magdalena and Balajka, Jan and Hulva, Jan and Pavelec, Jiri and Sokolovi\'{c}, Igor and M\"{u}llner, Matthias and Setvin, Martin and Schmid, Michael and Diebold, Ulrike and Blaha, Peter and Parkinson, Gareth S.}, month = feb, year = {2019}, pages = {390--396} } @article{schneider_highlights_2020, title = {Highlights of the Science and Life of {Peter} {Varga} (1946\textemdash{}2018)}, volume = {18}, doi = {10.1380/ejssnt.2020.8}, abstract = {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.}, number = {0}, journal = {e-Journal of Surface Science and Nanotechnology}, author = {Schneider, Wolf-Dieter and Aumayr, Friedrich and Diebold, Ulrike}, month = feb, year = {2020}, pages = {8--11} } @article{pavelec_multi-technique_2017, title = {A multi-technique study of {CO}$_2$ adsorption on {Fe}$_3${O}$_4$ magnetite}, volume = {146}, doi = {10.1063/1.4973241}, abstract = {The adsorption of CO$_2$ on the Fe$_3$O$_4$(001)-($\sqrt{2} \times \sqrt{2}$)R45{\textdegree} surface was studied experimentally using temperature programmed desorption (TPD), photoelectron spectroscopies (UPS and XPS), and scanning tunneling microscopy. CO$_2$ binds most strongly at defects related to Fe2+, including antiphase domain boundaries in the surface reconstruction and above incorporated Fe interstitials. At higher coverages,CO$_2$ adsorbs at fivefold-coordinated Fe3+ sites with a binding energy of 0.4 eV. Above a coverage of 4 molecules per ($\sqrt{2} \times \sqrt{2}$)R45{\textdegree} unit cell, further adsorption results in a compression of the first monolayer up to a density approaching that of a CO$_2$ ice layer. Surprisingly, desorption of the second monolayer occurs at a lower temperature (?84 K) than CO$_2$ 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.}, number = {1}, journal = JCP, author = {Pavelec, Jiri and Hulva, Jan and Halwidl, Daniel and Bliem, Roland and Gamba, Oscar and Jakub, Zdenek and Brunbauer, Florian}, month = jan, year = {2017}, pages = {014701}, } @article{setvin_surface_2017, title = {Surface point defects on bulk oxides: atomically-resolved scanning probe microscopy}, volume = {46}, doi = {10.1039/C7CS00076F}, abstract = {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 ({TiO}$_2$), iron oxides (Fe$_3$O$_4$), 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.}, number = {7}, journal = CSR, author = {Setv{\'i}n, Martin and Wagner, Margareta and Schmid, Michael and Parkinson, Gareth S. and Diebold, Ulrike}, month = apr, year = {2017}, pages = {1772--1784}, } @article{setvin_formaldehyde_2017, title = {Formaldehyde Adsorption on the Anatase {TiO}$_2$(101) Surface: {Experimental} and Theoretical Investigation}, volume = {121}, doi = {10.1021/acs.jpcc.7b01434}, abstract = {Formaldehyde (CH2O) adsorption on the anatase {TiO}$_2$(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 ({\textless}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.}, number = {16}, journal = JPCC, author = {Setvin, Martin and Hulva, Jan and Wang, Honghong and Simschitz, Thomas and Schmid, Michael and Parkinson, Gareth S. and Di Valentin, Cristiana and Selloni, Annabella and Diebold, Ulrike}, month = apr, year = {2017}, pages = {8914--8922}, } @article{reiner_physical-chemical_2017, title = {Physical-chemical stability of fluorinated {III}-{N} surfaces: {Towards} the understanding of the (0001) {Al}$_x${Ga}$_{1-x}${N} surface donor modification by fluorination}, volume = {121}, doi = {10.1063/1.4985345}, abstract = {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 {\textdegree}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.}, number = {22}, journal = JAP, author = {Reiner, Maria and Schellander, Josef and Denifl, G{\"u}nter and Stadtmueller, Michael and Schmid, Michael and Frischmuth, Tobias and Schmid, Ulrich and Pietschnig, Rudolf and Ostermaier, Clemens}, month = jun, year = {2017}, pages = {225704}, } @article{setvin_electron_2017, title = {Electron transfer between anatase {TiO}$_2$ and an {O}$_2$ molecule directly observed by atomic force microscopy}, volume = {114}, doi = {10.1073/pnas.1618723114}, abstract = {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 {TiO}$_2$ 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 {TiO}$_2$ 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{\textendash}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.}, number = {13}, journal = PNAS, author = {Setvin, Martin and Hulva, Jan and Parkinson, Gareth S. and Schmid, Michael and Diebold, Ulrike}, month = mar, year = {2017}, pmid = {28289217}, pages = {E2556--E2562}, } @article{halwidl_ordered_2017, title = {Ordered hydroxyls on {Ca}$_3${Ru}$_2${O}$_7$(001)}, volume = {8}, doi = {10.1038/s41467-017-00066-w}, abstract = {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 {\texttimes} 1), c(2 {\texttimes} 6), (1 {\texttimes} 3), and (1 {\texttimes} 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.}, number = {1}, journal = JAP, author = {Halwidl, Daniel and Mayr-Schm{\"o}lzer, Wernfried and Fobes, David and Peng, Jin and Mao, Zhiqiang and Schmid, Michael and Mittendorfer, Florian and Redinger, Josef and Diebold, Ulrike}, month = jun, year = {2017}, pages = {23}, } @article{gamba_role_2017, title = {The Role of Surface Defects in the Adsorption of Methanol on {Fe}$_3${O}$_4$(001)}, volume = {60}, doi = {10.1007/s11244-016-0713-9}, abstract = {The adsorption of methanol (CH3OH) at the Fe$_3$O$_4$(001)-($\sqrt{2} \times \sqrt{2}$)R45{\textdegree} 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 ($\sqrt{2} \times \sqrt{2}$)R45{\textdegree} 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.}, number = {6-7}, journal = TopCatal, author = {Gamba, Oscar and Hulva, Jan and Pavelec, Jiri and Bliem, Roland and Schmid, Michael and Diebold, Ulrike and Parkinson, Gareth S.}, month = may, year = {2017}, pages = {420--430}, } @article{diebold_perspective:_2017, title = {Perspective: {A} controversial benchmark system for water-oxide interfaces: {H}$_2${O}/{TiO}$_2$(110)}, volume = {147}, doi = {10.1063/1.4996116}, abstract = {The interaction of water with the single-crystalline rutile {TiO}$_2$(110) surface has been the object of intense investigations with both experimental and computational methods. Not only is {TiO}$_2$(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 {TiO}$_2$(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{\textendash}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 {TiO}$_2$(110) arises as the next controversial frontier.}, number = {4}, journal = JCP, author = {Diebold, Ulrike}, month = jul, year = {2017}, pages = {040901}, } @article{reticcioli_polaron-driven_2017, title = {Polaron-driven surface reconstructions}, volume = {7}, doi = {10.1103/PhysRevX.7.031053}, abstract = {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{\texttimes}1) to (1{\texttimes}2) transition in rutile {TiO}$_2$(110).}, number = {3}, journal = PRX, author = {Reticcioli, Michele and Setvin, Martin and Hao, Xianfeng and Flauger, Peter and Kresse, Georg and Schmid, Michael and Diebold, Ulrike and Franchini, Cesare}, month = sep, year = {2017}, pages = {031053}, } @article{mullner_self-limiting_2017, title = {Self-limiting adsorption of {WO}$_3$ oligomers on oxide substrates in solution}, volume = {121}, doi = {10.1021/acs.jpcc.7b04076}, abstract = {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 {TiO}$_2$(110) and magnetite Fe$_3$O$_4$(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{\textendash}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.}, number = {36}, journal = JPCC, author = {M{\"u}llner, Matthias and Balajka, Jan and Schmid, Michael and Diebold, Ulrike and Mertens, Stijn F. L.}, month = sep, year = {2017}, pages = {19743--19750} } @article{lackner_construction_2017, title = {Construction and evaluation of an ultrahigh-vacuum-compatible sputter deposition source}, volume = {88}, doi = {10.1063/1.4998700}, abstract = {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{\texttimes}10{\textasciicircum}-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.}, number = {10}, journal = RSI, author = {Lackner, Peter and Choi, Joong Il Jake and Diebold, Ulrike and Schmid, Michael}, month = oct, year = {2017}, pages = {103904}, } @article{setvin_methanol_2017, title = {Methanol on Anatase {TiO}$_2$(101): Mechanistic Insights into Photocatalysis}, volume = {7}, doi = {10.1021/acscatal.7b02003}, abstract = {The photoactivity of methanol adsorbed on the anatase {TiO}$_2$ (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.}, number = {10}, journal = ACSCat, author = {Setvin, Martin and Shi, Xiao and Hulva, Jan and Simschitz, Thomas and Parkinson, Gareth S. and Schmid, Michael and Di Valentin, Cristiana and Selloni, Annabella and Diebold, Ulrike}, month = oct, year = {2017}, pages = {7081--7091}, } @article{balajka_surface_2017, title = {Surface Structure of {TiO}$_2$ Rutile (011) Exposed to Liquid Water}, volume = {121}, doi = {10.1021/acs.jpcc.7b09674}, abstract = {The rutile {TiO}$_2$(011) surface exhibits a (2 {\texttimes} 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 {\texttimes} 1) symmetry was observed containing two dissociated water molecules per unit cell. The two OH groups yield an apparent {\textquotedblleft}c(2 {\texttimes} 1){\textquotedblright} 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 {\texttimes} 1) surface. Investigation of possible adsorption structures and analysis of the domain boundaries in this structure provide strong evidence that the original (2 {\texttimes} 1) reconstruction is lifted. Unlike the (2 {\texttimes} 1) reconstruction, the (1 {\texttimes} 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.}, number = {47}, journal = JPCC, author = {Balajka, Jan and Aschauer, Ulrich and Mertens, Stijn F. L. and Selloni, Annabella and Schmid, Michael and Diebold, Ulrike}, month = nov, year = {2017}, pages = {26424--26431}, } @article{kopfle_zirconium-palladium_2017, title = {Zirconium-Palladium Interactions during Dry Reforming of Methane}, volume = {78}, doi = {10.1149/07801.2419ecst}, abstract = {Catalytic investigations on chemical-vapor-deposition (CVD)-prepared Pd/Zr{\textasciicircum}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 CO$_2$ 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{\textasciicircum}0 is located exclusively in subsurface layers after annealing in vacuum and prior to reaction.}, number = {1}, journal = ECSTr, author = {K{\"o}pfle, Norbert and Mayr, Lukas and Lackner, Peter and Schmid, Michael and Schmidmair, Daniela and G{\"o}tsch, Thomas and Penner, Simon and Kloetzer, Bernhard}, month = may, year = {2017}, pages = {2419--2430}, } @article{huynh_nanoconfined_2016, title = {Nanoconfined self-assembly on a grafted graphitic surface under electrochemical control}, volume = {9}, doi = {10.1039/C6NR07519C}, abstract = {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 ({\textgreater}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.}, number = {1}, journal = {Nanoscale}, author = {Huynh, Thi Mien Trung and Phan, Thanh Hai and Ivasenko, Oleksandr and Mertens, Stijn F. L. and Feyter, Steven De}, month = dec, year = {2016}, pages = {362--368}, } @article{li_area-selective_2017, title = {Area-selective passivation of sp$^2$ carbon surfaces by supramolecular self-assembly}, volume = {9}, doi = {10.1039/C7NR00022G}, abstract = {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.}, number = {16}, journal = {Nanoscale}, author = {Li, Zhi and Gorp, Hans Van and Walke, Peter and Phan, Thanh Hai and Fujita, Yasuhiko and Greenwood, John and Ivasenko, Oleksandr and Tahara, Kazukuni and Tobe, Yoshito and Uji-i, Hiroshi and Mertens, Stijn F. L. and Feyter, Steven De}, month = apr, year = {2017}, pages = {5188--5193}, } @article{mertens_copper_2017, title = {Copper underpotential deposition on boron nitride nanomesh}, volume = {246}, doi = {10.1016/j.electacta.2017.06.082}, abstract = {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{\textendash}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.}, number = {Supplement C}, journal = {Electrochimica Acta}, author = {Mertens, Stijn F. L.}, month = aug, year = {2017}, pages = {730--736}, } @article{mertens_wetting_2017, title = {Wetting, Adhesion and Stiction of {2D} Materials}, volume = {80}, doi = {10.1149/08002.0023ecst}, abstract = {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.}, number = {2}, journal = ECSTr, author = {Mertens, Stijn F. L.}, month = aug, year = {2017}, pages = {23--27}, } @article{arndt_atomic_2016, title = {Atomic structure and stability of magnetite {Fe}$_3${O}$_4$(001): {An} {X}-ray view}, volume = {653}, doi = {10.1016/j.susc.2016.06.002}, abstract = {The structure of the Fe$_3$O$_4$(001) surface was studied using surface X-ray diffraction in both ultra-high vacuum, and higher-pressure environments relevant to water{\textendash}gas shift catalysis. The experimental X-ray structure factors from the 2 x 2 R 4 5 o 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{\textendash}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 {\texttimes} 2 R 4 5 o 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.}, journal = SuSci, author = {Arndt, Bj{\"o}rn and Bliem, Roland and Gamba, Oscar and van der Hoeven, Jessi E. S. and Noei, Heshmat and Diebold, Ulrike and Parkinson, Gareth S. and Stierle, Andreas}, month = nov, year = {2016}, pages = {76--81}, } @article{gargallo-caballero_co_2016, title = {Co on {Fe}$_3${O}$_4$(001): {Towards} precise control of surface properties}, volume = {144}, doi = {10.1063/1.4942662}, abstract = {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 CO$_2$+. 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.}, number = {9}, journal = JCP, author = {Gargallo-Caballero, Raquel and Mart{\'i}n-Garc{\'i}a, Laura and Quesada, Adri{\'a}n and Granados-Miralles, Cecilia and Foerster, Michael and Aballe, Luc{\'i}a and Bliem, Roland and Parkinson, Gareth S. and Blaha, Peter and Marco, Jos{\'e} F. and de la Figuera, Juan}, month = mar, year = {2016}, pages = {094704}, } @article{halwidl_adsorption_2016, title = {Adsorption of water at the {SrO} surface of ruthenates}, volume = {15}, doi = {10.1038/nmat4512}, abstract = {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{\textendash}Sr bridge positions, circling the surface OH with a measured activation energy of 187 {\textpm} 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.}, journal = NatMat, author = {Halwidl, Daniel and St{\"o}ger, Bernhard and Mayr-Schm{\"o}lzer, Wernfried and Pavelec, Jiri and Fobes, David and Peng, Jin and Mao, Zhiqiang and Parkinson, Gareth S. and Schmid, Michael and Mittendorfer, Florian and Redinger, Josef and Diebold, Ulrike}, year = {2016}, pages = {450--455}, } @article{parkinson_fe3o41101_2016, title = {Fe$_3${O}$_4$(110)--$(1 \times 3)$ revisited: {Periodic} (111) nanofacets}, volume = {649}, doi = {10.1016/j.susc.2016.02.020}, abstract = {The structure of the Fe$_3$O$_4$(110){\textendash}(1 {\texttimes} 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 {\textasciimacron} 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.}, journal = SuSci, author = {Parkinson, Gareth S. and Lackner, Peter and Gamba, Oscar and Maa{\ss}, Sebastian and Gerhold, Stefan and Riva, Michele and Bliem, Roland and Diebold, Ulrike and Schmid, Michael}, month = jul, year = {2016}, pages = {120--123}, } @article{gerhold_adjusting_2016, title = {Adjusting island density and morphology of the {SrTiO}$_3$(110)-($4 \times 1$) surface: {Pulsed} laser deposition combined with scanning tunneling microscopy}, volume = {651}, doi = {10.1016/j.susc.2016.03.010}, abstract = {The first stages of homoepitaxial growth of the (4 {\texttimes} 1) reconstructed surface of {SrTiO}$_3$(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 {SrTiO}$_3$. 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.}, journal = SuSci, author = {Gerhold, Stefan and Riva, Michele and Yildiz, Bilge and Schmid, Michael and Diebold, Ulrike}, month = sep, year = {2016}, pages = {76--83}, } @article{wang_transition_2016, title = {Transition from reconstruction toward thin film on the (110) surface of strontium titanate}, volume = {16}, doi = {10.1021/acs.nanolett.5b05211}, abstract = {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 {SrTiO}$_3$ (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 {SrTiO}$_3$ (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.}, number = {4}, journal = nanoLett, author = {Wang, Z. and Loon, A. and Subramanian, A. and Gerhold, S. and McDermott, E. and Enterkin, J. A. and Hieckel, M. and Russell, B. C. and Green, R. J. and Moewes, A. and Guo, J. and Blaha, P. and Castell, M. R. and Diebold, U. and Marks, L. D.}, month = apr, year = {2016}, pages = {2407--2412}, } @article{miccio_interplay_2016, title = {Interplay between steps and oxygen vacancies on curved {TiO}$_2$(110)}, volume = {16}, doi = {10.1021/acs.nanolett.5b05286}, abstract = {A vicinal rutile {TiO}$_2$(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.}, number = {3}, journal = nanoLett, author = {Miccio, Luis A. and Setvin, Martin and M{\"u}ller, Moritz and Abad{\'i}a, Mikel and Piquero, Ignacio and Lobo-Checa, Jorge and Schiller, Frederik and Rogero, Celia and Schmid, Michael and S{\'a}nchez-Portal, Daniel and Diebold, Ulrike and Ortega, J. Enrique}, month = mar, year = {2016}, pages = {2017--2022}, } @article{choi_metal_2016, title = {Metal adatoms and clusters on ultrathin zirconia films}, volume = {120}, doi = {10.1021/acs.jpcc.6b03061}, abstract = {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 ZrO$_2$ 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 ZrO$_2$ films as well as the (1?11) surface of monoclinic ZrO$_2$. STM shows decreasing cluster size, indicative of increasing metal{\textendash}oxide interaction, in the sequence Ag {\textless} Pd ? Au {\textless} Ni ? Fe. Ag and Pd nucleate mostly at steps and domain boundaries of ZrO$_2$/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{\textendash}oxide interaction strength increases in the sequence Ag {\textless} Au {\textless} Pd {\textless} Ni on monoclinic ZrO$_2$, and Ag ? Au {\textless} Pd {\textless} Ni on the supported ultrathin ZrO$_2$ 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 ZrO$_2$, 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, ZrO$_2$\{111\} surfaces are more reactive than many other oxygen-terminated oxide surfaces.}, number = {18}, journal = JPCC, author = {Choi, Joong Il Jake and Mayr-Schm{\"o}lzer, Wernfried and Valenti, Ilaria and Luches, Paola and Mittendorfer, Florian and Redinger, Josef and Diebold, Ulrike and Schmid, Michael}, month = may, year = {2016}, pages = {9920--9932}, } @article{riss_imaging_2016, title = {Imaging single-molecule reaction intermediates stabilized by surface dissipation and entropy}, volume = {8}, doi = {10.1038/nchem.2506}, abstract = {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.}, journal = NatChem, author = {Riss, Alexander and Paz, Alejandro P{\'e}rez and Wickenburg, Sebastian and Tsai, Hsin-Zon and De Oteyza, Dimas G. and Bradley, Aaron J. and Ugeda, Miguel M. and Gorman, Patrick and Jung, Han Sae and Crommie, Michael F. and Rubio, Angel and Fischer, Felix R.}, month = jul, year = {2016}, pages = {678--683}, } @article{mertens_switching_2016, title = {Switching stiction and adhesion of a liquid on a solid}, volume = {534}, doi = {10.1038/nature18275}, abstract = {When a gecko moves on a ceiling it makes use of adhesion and stiction. Stiction{\textemdash}static friction{\textemdash}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{\textendash}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{\textendash}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.}, number = {7609}, journal = {Nature}, author = {Mertens, Stijn F. L. and Hemmi, Adrian and Muff, Stefan and Gr{\"o}ning, Oliver and De Feyter, Steven and Osterwalder, J{\"u}rg and Greber, Thomas}, month = jun, year = {2016}, pages = {676--679}, } @article{parkinson_iron_2016, title = {Iron oxide surfaces}, volume = {71}, doi = {10.1016/j.surfrep.2016.02.001}, abstract = {The current status of knowledge regarding the surfaces of the iron oxides, magnetite (Fe$_3$O$_4$), maghemite ($\gamma$-Fe2O3), haematite ($\alpha$-Fe2O3), and w{\"u}stite (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, $\alpha$-Fe2O3 adopts Fe$_3$O$_4$-like surfaces in reducing conditions, and Fe$_3$O$_4$ 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. Fe$_3$O$_4$ 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 Fe$_3$O$_4$(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{\textendash}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 (($\sqrt{2} \times \sqrt{2}$))R45{\textdegree} 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{\textendash}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 Fe$_3$O$_4$ and Fe2O3 phases. The Fe$_3$O$_4$(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 Fe$_3$O$_4$(100) and Fe$_3$O$_4$(111) surfaces exhibit Fe-rich terminations as the sample selvedge becomes reduced. The Fe$_3$O$_4$(110) surface forms a one-dimensional (1{\texttimes}3) reconstruction linked to nanofaceting, which exposes the more stable Fe$_3$O$_4$(111) surface. $\alpha$-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 {\textquotedblleft}bi-phase{\textquotedblright} structure is formed, which eventually transforms to a Fe$_3$O$_4$(111)-like termination. The structure of the bi-phase surface is controversial; a largely accepted model of coexisting Fe1-xO and $\alpha$-Fe2O3(0001) islands was recently challenged and a new structure based on a thin film of Fe$_3$O$_4$(111) on $\alpha$-Fe2O3(0001) was proposed. The merits of the competing models are discussed. The $\alpha$-Fe2O3($1\bar{1}02$) {\textquotedblleft}R-cut{\textquotedblright} 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, CO$_2$, 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{\textendash}gas shift, Fischer{\textendash}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 Fe$_3$O$_4$ 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.}, number = {1}, journal = SuSciRep, author = {Parkinson, Gareth S.}, month = mar, year = {2016}, pages = {272--365}, } @article{wang_tailoring_2016, title = {Tailoring the nature and strength of electron-phonon interactions in the {SrTiO}$_3$(001) {2D} electron liquid}, volume = {15}, doi = {10.1038/nmat4623}, abstract = {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 {SrTiO}$_3$(001) surface to reveal a remarkably complex evolution of electron{\textendash}phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO3/{SrTiO}$_3$ 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{\textendash}phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in {SrTiO}$_3$-based 2DELs.}, number = {8}, journal = NatMat, author = {Wang, Z. and McKeown Walker, S. and Tamai, A. and Wang, Y. and Ristic, Z. and Bruno, F. Y. and de la Torre, A. and Ricc{\`o}, S. and Plumb, N. C. and Shi, M. and Hlawenka, P. and S{\'a}nchez-Barriga, J. and Varykhalov, A. and Kim, T. K. and Hoesch, M. and King, P. D. C. and Meevasana, W. and Diebold, U. and Mesot, J. and Moritz, B. and Devereaux, T. P. and Radovic, M. and Baumberger, F.}, month = aug, year = {2016}, pages = {835--839}, } @article{setvin_following_2016, title = {Following the reduction of oxygen on {TiO}$_2$ anatase (101) step by step}, volume = {138}, doi = {10.1021/jacs.6b04004}, abstract = {We have investigated the reaction between O2 and H2O, coadsorbed on the (101) surface of a reduced {TiO}$_2$ 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.}, number = {30}, journal = JACS, author = {Setvin, Martin and Aschauer, Ulrich and Hulva, Jan and Simschitz, Thomas and Daniel, Benjamin and Schmid, Michael and Selloni, Annabella and Diebold, Ulrike}, month = aug, year = {2016}, pages = {9565--9571}, } @article{bliem_dual_2016, title = {Dual role of {CO} in the stability of subnano {Pt} clusters at the {Fe}$_3${O}$_4$(001) surface}, volume = {113}, doi = {10.1073/pnas.1605649113}, abstract = {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 Fe$_3$O$_4$(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{\textendash}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.}, number = {32}, journal = PNAS, author = {Bliem, Roland and Hoeven, Jessi E. S. van der and Hulva, Jan and Pavelec, Jiri and Gamba, Oscar and Jongh, Petra E. de and Schmid, Michael and Blaha, Peter and Diebold, Ulrike and Parkinson, Gareth S.}, month = aug, year = {2016}, pmid = {27457953}, pages = {8921--8926}, } @article{wagner_well-ordered_2016, title = {Well-ordered {In} adatoms at the {In}$_2${O}$_3$(111) surface created by {Fe} deposition}, volume = {117}, doi = {10.1103/PhysRevLett.117.206101}, abstract = {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.}, number = {20}, journal = PRL, author = {Wagner, Margareta and Lackner, Peter and Seiler, Steffen and Gerhold, Stefan and Osiecki, Jacek and Schulte, Karina and Boatner, Lynn A. and Schmid, Michael and Meyer, Bernd and Diebold, Ulrike}, month = nov, year = {2016}, pages = {206101}, } @article{ugeda_characterization_2016, title = {Characterization of collective ground states in single-layer {NbSe}$_2$}, volume = {12}, doi = {10.1038/nphys3527}, abstract = {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 $\times$ 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 $\Delta{}$ = 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.}, number = {1}, journal = NatPhys, author = {Ugeda, Miguel M. and Bradley, Aaron J. and Zhang, Yi and Onishi, Seita and Chen, Yi and Ruan, Wei and Ojeda-Aristizabal, Claudia and Ryu, Hyejin and Edmonds, Mark T. and Tsai, Hsin-Zon and Riss, Alexander and Mo, Sung-Kwan and Lee, Dunghai and Zettl, Alex and Hussain, Zahid and Shen, Zhi-Xun and Crommie, Michael F.}, month = jan, year = {2016}, pages = {92--97} } @article{yu_no_2015, title = {{NO} adsorption and diffusion on hydroxylated rutile {TiO}$_2$(110)}, volume = {17}, doi = {10.1039/C5CP04584C}, abstract = {We report a computational study of NO adsorption and diffusion on the hydroxylated rutile TiO$_2$(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 2$\pi{}$* 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 \textquotedblleft{}bright-dark-bright\textquotedblright{} configuration corresponding to diffusing NO at different positions are consistent with the experimental STM results.}, number = {40}, journal = PCCP, author = {Yu, Yan-Yan and Diebold, Ulrike and Gong, Xue-Qing}, month = oct, year = {2015}, pages = {26594--26598} } @article{setvin_aggregation_2015, title = {Aggregation and electronically induced migration of oxygen vacancies in {TiO}$_2$ anatase}, volume = {91}, doi = {10.1103/PhysRevB.91.195403}, abstract = {The influence of the electric field and electric current on the behavior of oxygen vacancies (VOs) in TiO$_2$ 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 $\approx{}$+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, \textquotedblleft{}hot\textquotedblright{} electrons in this process. The findings are closely related to the memristive behavior of oxides and oxygen diffusion in solid-oxide membranes.}, number = {19}, journal = PRB, author = {Setvin, Martin and Schmid, Michael and Diebold, Ulrike}, month = may, year = {2015}, pages = {195403} } @article{setvin_multitechnique_2015, title = {A multitechnique study of {CO} adsorption on the {TiO}$_2$ anatase (101) surface}, volume = {119}, doi = {10.1021/acs.jpcc.5b07999}, abstract = {The adsorption of carbon monoxide on the anatase TiO$_2$ (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 $\approx{}$2181 cm{\textasciicircum}-1 for both stoichiometric and reduced TiO$_2$(101) surfaces. From TPD, an adsorption energy of 0.37 $\pm{}$ 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 TiO$_2$(110) surface shows that the tendency for excess electron localization in anatase is much weaker than in rutile.}, number = {36}, journal = JPCC, author = {Setvin, Martin and Buchholz, Maria and Hou, Weiyi and Zhang, Cui and St\"{o}ger, Bernhard and Hulva, Jan and Simschitz, Thomas and Shi, Xiao and Pavelec, Jiri and Parkinson, Gareth S. and Xu, Mingchun and Wang, Yuemin and Schmid, Michael and W\"{o}ll, Christof and Selloni, Annabella and Diebold, Ulrike}, month = sep, year = {2015}, pages = {21044--21052} } @article{serrano_situ_2015, title = {In situ scanning tunneling microscopy study of {Ca}-modified rutile {TiO}$_2$(110) in bulk water}, volume = {6}, doi = {10.3762/bjnano.6.44}, abstract = {Despite the rising technological interest in the use of calcium-modified TiO$_2$ surfaces in biomedical implants, the Ca/TiO$_2$ 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 TiO$_2$(110) surfaces immersed in high purity water. The TiO$_2$ 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 TiO$_2$ 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 TiO$_2$ 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.}, journal = BeilNano, author = {Serrano, Giulia and Bonanni, Beatrice and Kosmala, Tomasz and Di Giovannantonio, Marco and Diebold, Ulrike and Wandelt, Klaus and Goletti, Claudio}, month = feb, year = {2015}, pages = {438--443} } @article{serrano_molecular_2015, title = {Molecular ordering at the interface between liquid water and rutile {TiO}$_2$(110)}, volume = {2}, doi = {10.1002/admi.201500246}, abstract = {The pivotal importance of TiO$_2$ as a technological material involves most applications in an aqueous environment, but the single-crystal TiO$_2$/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 TiO$_2$(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.}, number = {17}, journal = AdvMatInt, author = {Serrano, Giulia and Bonanni, Beatrice and Di Giovannantonio, Marco and Kosmala, Tomasz and Schmid, Michael and Diebold, Ulrike and Di Carlo, Aldo and Cheng, Jun and VandeVondele, Joost and Wandelt, Klaus and Goletti, Claudio}, month = nov, year = {2015}, pages = {1500246} } @article{li_growth_2015, title = {Growth of an ultrathin zirconia film on {Pt}$_3${Zr} examined by high-resolution x-ray photoelectron spectroscopy, temperature-programmed desorption, scanning tunneling microscopy, and density functional theory}, volume = {119}, doi = {10.1021/jp5100846}, abstract = {Ultrathin ({\textasciitilde}3 \AA{}) zirconium oxide films were grown on a single-crystalline Pt$_3$Zr(0001) substrate by oxidation in 1 x 10{\textasciicircum}-7 mbar of O2 at 673 K, followed by annealing at temperatures up to 1023 K. The ZrO$_2$ films are intended to serve as model supports for reforming catalysts and fuel cell anodes. The atomic and electronic structure and composition of the ZrO$_2$ 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 ZrO$_2$ clusters (thickness {\textasciitilde}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 Pt$_3$Zr substrate by ZrO$_2$, that is, formation of a closed oxide overlayer. Experiments and DFT calculations show that the core level shifts of Zr in the trilayer ZrO$_2$ films are between those of metallic Zr and thick (bulklike) ZrO$_2$. 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 ZrO$_2$ films or metal/ZrO$_2$ 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.}, number = {5}, journal = JPCC, author = {Li, Hao and Choi, Joong-Il Jake and Mayr-Schm\"{o}lzer, Wernfried and Weilach, Christian and Rameshan, Christoph and Mittendorfer, Florian and Redinger, Josef and Schmid, Michael and Rupprechter, G\"{u}nther}, year = {2015}, pages = {2462--2470} } @article{hao_coexistence_2015, title = {Coexistence of trapped and free excess electrons in {SrTiO}$_3$}, volume = {91}, doi = {10.1103/PhysRevB.91.085204}, abstract = {The question whether excess electrons in SrTiO$_3$ 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 $\approx{}$10{\textasciicircum}20 cm{\textasciicircum}-3. Below this value, the electrons stay delocalized or become large polarons. For oxygen-deficient SrTiO$_3$, 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.}, number = {8}, journal = PRB, author = {Hao, Xianfeng and Wang, Zhiming and Schmid, Michael and Diebold, Ulrike and Franchini, Cesare}, month = feb, year = {2015}, pages = {085204} } @article{gerhold_nickel-oxide-modified_2015, title = {Nickel-oxide-modified {SrTiO}$_3$ (110)-$(4 \times 1)$ surfaces and their interaction with water}, volume = {119}, doi = {10.1021/acs.jpcc.5b06144}, abstract = {Nickel oxide (NiO), deposited onto the strontium titanate (SrTiO$_3$) (110)-(4 $\times$ 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 $\times$ 1) reconstructed SrTiO$_3$(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.}, number = {35}, journal = JPCC, author = {Gerhold, Stefan and Riva, Michele and Wang, Zhiming and Bliem, Roland and Wagner, Margareta and Osiecki, Jacek and Schulte, Karina and Schmid, Michael and Diebold, Ulrike}, month = sep, year = {2015}, pages = {20481--20487} } @article{gamba_adsorption_2015, title = {Adsorption of formic acid on the {Fe}$_3${O}$_4$(001) surface}, volume = {119}, doi = {10.1021/acs.jpcc.5b05560}, abstract = {The adsorption of formic acid (HCOOH) on the Fe$_3$O$_4$(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\textendash{}) and hydroxyl groups, facilitated by the close proximity of undercoordinated Fe3+/O2\textendash{} cation/anion pairs at the Fe$_3$O$_4$(001) surface. Bidentate formate species are observed in IRRAS, and their position on Fe\textendash{}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 Fe$_3$O$_4$ single crystals with good sensitivity but that unusual peak shapes occur because the substrate is midway between a perfect conductor and a perfect dielectric.}, number = {35}, journal = JPCC, author = {Gamba, Oscar and Noei, Heshmat and Pavelec, Ji\v{r}\'{\i} and Bliem, Roland and Schmid, Michael and Diebold, Ulrike and Stierle, Andreas and Parkinson, Gareth S.}, month = sep, year = {2015}, pages = {20459--20465} } @article{bradley_probing_2015, title = {Probing the Role of Interlayer coupling and {Coulomb} Interactions on Electronic Structure in Few-Layer {MoSe}$_2$ Nanostructures}, volume = {15}, doi = {10.1021/acs.nanolett.5b00160}, abstract = {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\textendash{}electron interactions on van der Waals heterostructures and helps to clarify how their electronic properties might be tuned in future 2D nanodevices.}, number = {4}, journal = nanoLett, author = {Bradley, Aaron J. and M. Ugeda, Miguel and da Jornada, Felipe H. and Qiu, Diana Y. and Ruan, Wei and Zhang, Yi and Wickenburg, Sebastian and Riss, Alexander and Lu, Jiong and Mo, Sung-Kwan and Hussain, Zahid and Shen, Zhi-Xun and Louie, Steven G. and Crommie, Michael F.}, month = apr, year = {2015}, pages = {2594--2599} } @article{bliem_atomic-scale_2015, title = {An atomic-scale view of {CO} and {H}$_2$ oxidation on a {Pt}/{Fe}$_3${O}$_4$ model catalyst}, volume = {54}, doi = {10.1002/anie.201507368}, abstract = {Metal\textendash{}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/Fe$_3$O$_4$ model catalyst exposed to CO, H2, O2, and mixtures thereof at 550\hspace{0.25em}K. CO extracts lattice oxygen atoms at the cluster perimeter to form CO$_2$, 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 Fe$_3$O$_4$(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.}, number = {47}, journal = AngChIE, author = {Bliem, Roland and van\hspace{0.25em}der\hspace{0.25em}Hoeven, Jessi and Zavodny, Adam and Gamba, Oscar and Pavelec, Jiri and de\hspace{0.25em}Jongh, Petra E. and Schmid, Michael and Diebold, Ulrike and Parkinson, Gareth S.}, month = nov, year = {2015}, pages = {13999--14002} } @article{bliem_adsorption_2015, title = {Adsorption and incorporation of transition metals at the magnetite {Fe}$_3${O}$_4$(001) surface}, volume = {92}, doi = {10.1103/PhysRevB.92.075440}, abstract = {The adsorption of Ni, Co, Mn, Ti, and Zr at the ($\surd{}$2$\times$$\surd{}$2)R45$^\circ$-reconstructed Fe$_3$O$_4$(001) surface was studied by scanning tunneling microscopy, x-ray and ultraviolet photoelectron spectroscopy, low-energy electron diffraction (LEED), and density functional theory (DFT). Following deposition at room temperature, metals are either adsorbed as isolated adatoms or fill the subsurface cation vacancy sites responsible for the ($\surd{}$2$\times$$\surd{}$2)R45$^\circ$ reconstruction. Both configurations coexist, but the ratio of adatoms to incorporated atoms depends on the metal; Ni prefers the adatom configuration, Co and Mn form adatoms and incorporated atoms in similar numbers, and Ti and Zr are almost fully incorporated. With mild annealing, all adatoms transition to the incorporated cation configuration. At high coverage, the ($\surd{}$2$\times$$\surd{}$2)R45$^\circ$ reconstruction is lifted because all subsurface cation vacancies become occupied with metal atoms, and a (1$\times$1) LEED pattern is observed. DFT+U calculations for the extreme cases, Ni and Ti, confirm the energetic preference for incorporation, with calculated oxidation states in good agreement with photoemission experiments. Because the site preference is analogous to bulk ferrite (XFe2O4) compounds, similar behavior is likely to be typical for elements forming a solid solution with Fe$_3$O$_4$.}, number = {7}, journal = PRB, author = {Bliem, Roland and Pavelec, Jiri and Gamba, Oscar and McDermott, Eamon and Wang, Zhiming and Gerhold, Stefan and Wagner, Margareta and Osiecki, Jacek and Schulte, Karina and Schmid, Michael and Blaha, Peter and Diebold, Ulrike and Parkinson, Gareth S.}, month = aug, year = {2015}, pages = {075440} } @article{riss_imaging_2014, title = {Imaging and Tuning Molecular Levels at the Surface of a Gated Graphene Device}, volume = {8}, doi = {10.1021/nn501459v}, abstract = {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}/{SiO}2 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.}, number = {6}, journal = {{ACS} Nano}, author = {Riss, Alexander and Wickenburg, Sebastian and Tan, Liang Z. and Tsai, Hsin-Zon and Kim, Youngkyou and Lu, Jiong and Bradley, Aaron J. and Ugeda, Miguel M. and Meaker, Kacey L. and Watanabe, Kenji and Taniguchi, Takashi and Zettl, Alex and Fischer, Felix R. and Louie, Steven G. and Crommie, Michael F.}, month = apr, year = {2014}, pages = {5395--5401} } @article{riss_local_2014, title = {Local Electronic and Chemical Structure of Oligo-acetylene Derivatives Formed Through Radical Cyclizations at a Surface}, volume = {14}, doi = {10.1021/nl403791q}, abstract = {Semiconducting pi-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 pi-conjugated states in these oligomers is significantly lower than the energy of the corresponding localized monomer orbitals, consistent with theoretical predictions.}, number = {5}, journal = nanoLett, author = {Riss, Alexander and Wickenburg, Sebastian and Gorman, Patrick and Tan, Liang Z. and Tsai, Hsin-Zon and de Oteyza, Dimas G. and Chen, Yen-Chia and Bradley, Aaron J. and Ugeda, Miguel M. and Etkin, Grisha and Louie, Steven G. and Fischer, Felix R. and Crommie, Michael F.}, month = may, year = {2014}, pages = {2251--2255} } @article{cui_squeezing_2014, title = {Squeezing, Then Stacking: From Breathing Pores to Three-Dimensional Ionic Self-Assembly under Electrochemical Control}, volume = {53}, doi = {10.1002/anie.201406246}, abstract = {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.}, number = {47}, journal = AngChIE, author = {Cui, Kang and Mali, Kunal S. and Ivasenko, Oleksandr and Wu, Dongqing and Feng, Xinliang and Walter, Michael and M{\"u}llen, Klaus and De Feyter, Steven and Mertens, Stijn F. L.}, month = nov, year = {2014}, pages = {12951--12954} } @article{cui_potential-driven_2014, title = {Potential-driven molecular tiling of a charged polycyclic aromatic compound}, volume = {50}, doi = {10.1039/C4CC04189E}, abstract = {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.}, number = {72}, journal = ChemComm, author = {Cui, Kang and Ivasenko, Oleksandr and Mali, Kunal S. and Wu, Dongqing and Feng, Xinliang and M{\"u}llen, Klaus and De Feyter, Steven and Mertens, Stijn F. L.}, month = jul, year = {2014}, pages = {10376} } @article{wang_anisotropic_2014, title = {Anisotropic two-dimensional electron gas at {SrTiO}$_3$(110)}, volume = {111}, doi = {10.1073/pnas.1318304111}, abstract = {Two-dimensional electron gases (2DEGs) at oxide heterostructures are attracting considerable attention, as these might one day substitute conventional semiconductors at least for some functionalities. Here we present a minimal setup for such a 2DEG--the {SrTiO}3(110)-$(4 \times 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.}, number = {11}, journal = PNAS, author = {Wang, Zhiming and Zhong, Zhicheng and Hao, Xianfeng and Gerhold, Stefan and St{\"o}ger, Bernhard and Schmid, Michael and S{\'a}nchez-Barriga, Jaime and Varykhalov, Andrei and Franchini, Cesare and Held, Karsten and Diebold, Ulrike}, month = mar, year = {2014}, pmid = {24591596}, pages = {3933--3937} } @article{wang_vacancy_2014, title = {Vacancy clusters at domain boundaries and band bending at the {SrTiO}$_3$(110) surface}, volume = {90}, doi = {10.1103/PhysRevB.90.035436}, abstract = {Antiphase domain boundaries ({APDBs}) in the (n1)(n=4,5) reconstructions of the {SrTiO}3(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.}, number = {3}, journal = PRB, author = {Wang, Zhiming and Hao, Xianfeng and Gerhold, Stefan and Schmid, Michael and Franchini, Cesare and Diebold, Ulrike}, month = jul, year = {2014}, pages = {035436} } @article{wang_stabilizing_2014, title = {Stabilizing Single {Ni} Adatoms on a Two-Dimensional Porous Titania Overlayer at the {SrTiO}$_3$(110) Surface}, volume = {118}, doi = {10.1021/jp506234r}, abstract = {Nickel vapor-deposited on the {SrTiO}3(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.}, number = {34}, journal = JPCC, author = {Wang, Zhiming and Hao, Xianfeng and Gerhold, Stefan and Mares, Petr and Wagner, Margareta and Bliem, Roland and Schulte, Karina and Schmid, Michael and Franchini, Cesare and Diebold, Ulrike}, month = aug, year = {2014}, pages = {19904--19909} } @article{wagner_reducing_2014, title = {Reducing the {In$_2$O$_3$}(111) Surface Results in Ordered Indium Adatoms}, volume = {1}, doi = {10.1002/admi.201400289}, abstract = {When the {In$_2$O$_3$} 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.}, number = {8}, journal = AdvMatInt, author = {Wagner, Margareta and Seiler, Steffen and Meyer, Bernd and Boatner, Lynn A. and Schmid, Michael and Diebold, Ulrike}, month = nov, year = {2014}, pages = {1400289} } @article{stoger_point_2014, title = {Point defects at cleaved {Sr}$_{n+1}${Ru}$_n${O}$_{3n+1}$(001) surfaces}, volume = {90}, doi = {10.1103/PhysRevB.90.165438}, abstract = {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}/$(1 \times 1)$ 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.}, number = {16}, journal = PRB, author = {St{\"o}ger, Bernhard and Hieckel, Marcel and Mittendorfer, Florian and Wang, Zhiming and Schmid, Michael and Parkinson, Gareth S. and Fobes, David and Peng, Jin and Ortmann, John E. and Limbeck, Andreas and Mao, Zhiqiang and Redinger, Josef and Diebold, Ulrike}, month = oct, year = {2014}, pages = {165438} } @article{stoger_high_2014, title = {High Chemical Activity of a Perovskite Surface: Reaction of {CO} with {Sr}$_3${Ru}$_2${O}$_7$}, volume = {113}, doi = {10.1103/PhysRevLett.113.116101}, abstract = {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 {CO}2 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.}, number = {11}, journal = PRL, author = {St{\"o}ger, Bernhard and Hieckel, Marcel and Mittendorfer, Florian and Wang, Zhiming and Fobes, David and Peng, Jin and Mao, Zhiqiang and Schmid, Michael and Redinger, Josef and Diebold, Ulrike}, month = sep, year = {2014}, pages = {116101} } @article{spivey_synthesis_2014, title = {Synthesis, Characterization, and Computation of Catalysts at the Center for Atomic-Level Catalyst Design}, volume = {118}, doi = {10.1021/jp502556u}, abstract = {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.}, number = {35}, journal = JPCC, author = {Spivey, James J. and Krishna, Katla Sai and Kumar, Challa S.S.R. and Dooley, Kerry M. and Flake, John C. and Haber, Louis H. and Xu, Ye and Janik, Michael J. and Sinnott, Susan B. and Cheng, Yu-Ting and Liang, Tao and Sholl, David S. and Manz, Thomas A. and Diebold, Ulrike and Parkinson, Gareth S. and Bruce, David A. and de Jongh, Petra}, month = sep, year = {2014}, pages = {20043--20069} } @article{setvin_charge_2014, title = {Charge Trapping at the Step Edges of {TiO}$_2$ Anatase (101)}, volume = {53}, doi = {10.1002/anie.201309796}, abstract = {A combination of photoemission, atomic force, and scanning tunneling microscopy/spectroscopy measurements shows that excess electrons in the {TiO}2 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.}, number = {18}, journal = AngChIE, author = {Setvin, Martin and Hao, Xianfeng and Daniel, Benjamin and Pavelec, Jiri and Novotny, Zbynek and Parkinson, Gareth S. and Schmid, Michael and Kresse, Georg and Franchini, Cesare and Diebold, Ulrike}, year = {2014}, pages = {4714--4716} } @article{setvin_direct_2014, title = {Direct View at Excess Electrons in {TiO}$_2$ Rutile and Anatase}, volume = {113}, doi = {10.1103/PhysRevLett.113.086402}, abstract = {A combination of scanning tunneling microscopy and spectroscopy and density functional theory is used to characterize excess electrons in {TiO}2 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.}, number = {8}, journal = PRL, author = {Setvin, Martin and Franchini, Cesare and Hao, Xianfeng and Schmid, Michael and Janotti, Anderson and Kaltak, Merzuk and Van de Walle, Chris G. and Kresse, Georg and Diebold, Ulrike}, month = aug, year = {2014}, pages = {086402} } @article{setvin_surface_2014, title = {Surface preparation of {TiO}$_2$ anatase (101): Pitfalls and how to avoid them}, volume = {626}, doi = {10.1016/j.susc.2014.04.001}, abstract = {{TiO}2 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 {TiO}2 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.}, journal = SuSci, author = {Setv{\'\i}n, Martin and Daniel, Benjamin and Mansfeldova, Vera and Kavan, Ladislav and Scheiber, Philipp and Fidler, Martin and Schmid, Michael and Diebold, Ulrike}, month = aug, year = {2014}, note = {Setvin}, pages = {61--67} } @article{setvin_identification_2014, title = {Identification of adsorbed molecules via {STM} tip manipulation: {CO}, {H$_2$O}, and {O}$_2$ on {TiO}$_2$ anatase (101)}, volume = {16}, doi = {10.1039/C4CP03212H}, abstract = {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 {TiO}2 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.}, number = {39}, journal = PCCP, author = {Setvin, Martin and Daniel, Benjamin and Aschauer, Ulrich and Hou, Weiyi and Li, Ye-Fei and Schmid, Michael and Selloni, Annabella and Diebold, Ulrike}, month = sep, year = {2014}, pages = {21524--21530} } @article{sanches_hybrid_2014, title = {Hybrid exchange density functional study of vicinal anatase {TiO}$_2$ surfaces}, volume = {89}, doi = {10.1103/PhysRevB.89.245309}, abstract = {The observation of photocatalytic water splitting on the surface of anatase {TiO}2 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 {TiO}2 nanostructures and the relative positions of its band edges suggests that it will play a significant role in the water-{TiO}2 reactions in solar water splitting.}, number = {24}, journal = PRB, author = {Sanches, F. F. and Mallia, G. and Liborio, L. and Diebold, U. and Harrison, N. M.}, month = jun, year = {2014}, pages = {245309} } @article{gerhold_stoichiometry-driven_2014, title = {Stoichiometry-driven switching between surface reconstructions on {SrTiO}$_3$(001)}, volume = {621}, doi = {10.1016/j.susc.2013.10.015}, abstract = {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 {SrTiO}3(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 $^\circ$C in 4 10{\textasciicircum}-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.}, journal = SuSci, author = {Gerhold, Stefan and Wang, Zhiming and Schmid, Michael and Diebold, Ulrike}, month = mar, year = {2014}, pages = {L1--L4} } @article{choi_growth_2014, title = {The growth of ultra-thin zirconia films on {Pd}$_3${Zr}(0001)}, volume = {26}, doi = {10.1088/0953-8984/26/22/225003}, abstract = {Despite its importance in many areas of industry, such as catalysis, fuel cell technology and microelectronics, the surface structure and physical properties of {ZrO}2 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 {ZrO}2 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 {ZrO}2 film was obtained. This is an O-Zr-O trilayer based on cubic {ZrO}2(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 {ZrO}2 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 {ZrO}2 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.}, number = {22}, journal = JPCM, author = {Choi, J. I. J. and Mayr-Schm{\"o}lzer, W. and Mittendorfer, F. and Redinger, J. and Diebold, U. and Schmid, M.}, month = jun, year = {2014}, pages = {225003} } @article{bliem_cluster_2014, title = {Cluster Nucleation and Growth from a Highly Supersaturated Adatom Phase: Silver on Magnetite}, volume = {8}, doi = {10.1021/nn502895s}, abstract = {The atomic-scale mechanisms underlying the growth of Ag on the ($\sqrt{2} \times \sqrt{2}$)R45$^\circ$-Fe$_3$O$_4$(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 ($\sqrt{2} \times \sqrt{2}$)R45$^\circ$ 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 ($\sqrt{2}$?$\sqrt{2}$)R45$^\circ$ 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.}, number = {7}, journal = {{ACS} Nano}, author = {Bliem, Roland and Kosak, Rukan and Perneczky, Lukas and Novotny, Zbynek and Gamba, Oscar and Fobes, David and Mao, Zhiqiang and Schmid, Michael and Blaha, Peter and Diebold, Ulrike and Parkinson, Gareth S.}, month = jul, year = {2014}, pages = {7531--7537} } @article{bliem_subsurface_2014, title = {Subsurface cation vacancy stabilization of the magnetite (001) surface}, volume = {346}, doi = {10.1126/science.1260556}, abstract = {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 ($\sqrt{2} \times \sqrt{2}$)R45$^\circ$ reconstruction of Fe$_3$O$_4$(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.}, number = {6214}, journal = {Science}, author = {Bliem, R. and McDermott, E. and Ferstl, P. and Setvin, M. and Gamba, O. and Pavelec, J. and Schneider, M. A. and Schmid, M. and Diebold, U. and Blaha, P. and Hammer, L. and Parkinson, G. S.}, month = dec, year = {2014}, pages = {1215--1218} } @article{wang_strain-induced_2013, title = {Strain-Induced Defect Superstructure on the {SrTiO$_3$(110)} Surface}, volume = {111}, doi = {10.1103/PhysRevLett.111.056101}, abstract = {We report on a combined scanning tunneling microscopy and density functional theory calculation study of the {SrTiO$_3$(110)-$(4\times1)$} surface. It is found that antiphase domains are formed along the [1$\bar{1}$0]-oriented stripes on the surface. The domain boundaries are decorated by defect pairs consisting of {Ti$_2$O$_3$} 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.}, number = {5}, journal = PRL, author = {Wang, Zhiming and Li, Fengmiao and Meng, Sheng and Zhang, Jiandi and Plummer, E. W. and Diebold, Ulrike and Guo, Jiandong}, month = aug, year = {2013}, pages = {056101} }, @article{vogel_role_2013, title = {The Role of Defects in the Local Reaction Kinetics of {CO} Oxidation on Low-Index {Pd} Surfaces}, volume = {117}, doi = {10.1021/jp312510d}, abstract = {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 $\mu$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.}, number = {23}, journal = JPCC, author = {Vogel, D. and Spiel, C. and Schmid, M. and St\"{o}ger-Pollach, M. and Schl\"{o}gl, R. and Suchorski, Y. and Rupprechter, G.}, month = jun, year = {2013}, pages = {12054--12060} }, @article{uddin_tailoring_2013, title = {Tailoring the photocatalytic reaction rate of a nanostructured {TiO$_2$} matrix using additional gas phase oxygen}, volume = {3}, doi = {10.1186/2228-5326-3-16}, abstract = {Nanostructured {{TiO}$_2$} 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 {TiO$_2$} samples were prepared using various temperatures and were studied for the photocatalytic degradation of methylene blue. The photocatalytic efficiency of {{TiO}$_2$} was observed to increase by activating the {TiO$_2$} surface using nanoporous {PS}. The photocatalytic performance of the synthesized samples showed a higher performance using molecular O2, which was purged through the reactor.}, number = {1}, journal = INLett, author = {Uddin, Mohammed Jasim and Alam, Md Mohibul and Islam, Md Akhtarul and Snigda, Sharmin Rahman and Das, Sreejon and Rahman, Mohammed Mastabur and Uddin, Md Nizam and Morris, Cindy A. and Gonzalez, Richard D. and Diebold, Ulrike and Dickens, Tarik J. and Okoli, Okenwa I.}, month = mar, year = {2013}, pages = {16} }, @article{setvin_reaction_2013, title = {Reaction of {O}$_2$ with Subsurface Oxygen Vacancies on {TiO$_2$} Anatase (101)}, volume = {341}, doi = {10.1126/science.1239879}, abstract = {Oxygen (O$_2$) 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 O$_2$ molecules on the (101) surface of anatase (titanium oxide, {TiO$_2$)} 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. O$_2$ adsorbed as superoxo (O$_2^-$) at fivefold-coordinated Ti sites was transformed to peroxo (O$_2^{2-}$) and, via reaction with a {VO}, placed into an anion surface lattice site as an ({O$_2$})O species. This so-called bridging dimer also formed when O$_2$ directly reacted with {VOs} at or below the surface.}, number = {6149}, journal = {Science}, author = {Setv\'{\i}n, Martin and Aschauer, Ulrich and Scheiber, Philipp and Li, Ye-Fei and Hou, Weiyi and Schmid, Michael and Selloni, Annabella and Diebold, Ulrike}, month = aug, year = {2013}, pages = {988--991} }, @article{parkinson_carbon_2013, title = {Carbon monoxide-induced adatom sintering in a {Pd--Fe$_3$O$_4$} model catalyst}, volume = {12}, doi = {10.1038/nmat3667}, abstract = {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 {Fe$_3$O$_4$(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/Fe$_3$O$_4$} 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.}, number = {8}, journal = {Nature Materials}, author = {Parkinson, Gareth S. and Novotny, Zbynek and Argentero, Giacomo and Schmid, Michael and Pavelec, Ji\v{r}\'{\i} and Kosak, Rukan and Blaha, Peter and Diebold, Ulrike}, month = aug, year = {2013}, pages = {724--728} }, @article{novotny_probing_2013, title = {Probing the surface phase diagram of {Fe$_3$O$_4$(001)} towards the {Fe}-rich limit: Evidence for progressive reduction of the surface}, volume = {87}, doi = {10.1103/PhysRevB.87.195410}, abstract = {Reduced terminations of the {Fe$_3$O$_4$(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 $(\sqrt 2 \times \sqrt 2)$R45$^\circ$ 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 Fe$^{3+}$ to Fe$^{2+}$ upon Fe deposition. The antiferromagnetic {FeO} layer on top of ferromagnetic {Fe$_3$O$_4$(001)} suggests possible exchange bias in this system.}, number = {19}, journal = PRB, author = {Novotny, Zbynek and Mulakaluri, Narasimham and Edes, Zoltan and Schmid, Michael and Pentcheva, Rossitza and Diebold, Ulrike and Parkinson, Gareth S.}, month = may, year = {2013}, pages = {195410} }, @article{de_la_figuera_real-space_2013, title = {Real-space imaging of the {Verwey} transition at the (100) surface of magnetite}, volume = {88}, doi = {10.1103/PhysRevB.88.161410}, abstract = {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 $T_\mathrm V$, the initially flat surface undergoes a rooflike distortion with a periodicity of $\sim{}0.5 \mu$m 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 $(\sqrt 2$\times \sqrt 2)$R45$\circ{}$ 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 $(\sqrt 2$\times \sqrt 2)$R45$\circ{}$ reconstruction. Although conceptually similar, the charge-ordered phases of the (100) surface and sub-$T_\mathrm V$ bulk of magnetite are unrelated phenomena.}, number = {16}, journal = PRB, author = {de la Figuera, Juan and Novotny, Zbynek and Setvin, Martin and Liu, Tijiang and Mao, Zhiqiang and Chen, Gong and {N'Diaye}, Alpha T. and Schmid, Michael and Diebold, Ulrike and Schmid, Andreas K. and Parkinson, Gareth S.}, month = oct, year = {2013}, pages = {161410(R)} } @article{zenkyu_composition_2012, title = {Composition and local atomic arrangement of decagonal {Al-Co-Cu} quasicrystal surfaces}, volume = {86}, doi = {10.1103/PhysRevB.86.115422}, abstract = {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.}, number = {11}, journal = PRB, author = {Zenkyu, R. and Yuhara, J. and Matsui, T. and Shah Zaman, S. and Schmid, M. and Varga, P.}, month = sep, year = {2012}, pages = {115422} }, @article{scheiber_subsurface_2012, title = {({Sub)Surface} Mobility of Oxygen Vacancies at the {TiO$_2$} Anatase (101) Surface}, volume = {109}, doi = {10.1103/PhysRevLett.109.136103}, abstract = {Anatase is a metastable polymorph of {TiO$_2$.} In contrast to the more widely studied {TiO$_2$} 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 $\geq 200$ 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.}, number = {13}, journal = PRL, author = {Scheiber, Philipp and Fidler, Martin and Dulub, Olga and Schmid, Michael and Diebold, Ulrike and Hou, Weiyi and Aschauer, Ulrich and Selloni, Annabella}, month = sep, year = {2012}, pages = {136103} }, @article{parkinson_antiphase_2012, title = {Antiphase domain boundaries at the {Fe$_3$O$_4$(001)} surface}, volume = {85}, doi = {10.1103/PhysRevB.85.195450}, abstract = {Antiphase domain boundaries ({APDBs)} in the $(\sqrt 2 \times \sqrt 2)${R}45$\circ{}$ reconstruction of the {Fe$_3$O$_4$(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 $(\sqrt 2 \times \sqrt 2)${R}45$\circ{}$ reconstruction in which a lattice distortion couples to charge order in the subsurface layers. The {APDBs} are observed after prolonged annealing at 700 {$^\circ$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.}, number = {19}, journal = PRB, author = {Parkinson, Gareth S. and Manz, Thomas A. and Novotn\'{y}, Zbyn\v{e}k and Sprunger, Phillip T. and Kurtz, Richard L. and Schmid, Michael and Sholl, David S. and Diebold, Ulrike}, month = may, year = {2012}, pages = {195450} }, @incollection{parkinson_tailoring_2012, address = {Rijeka}, title = {Tailoring the Interface Properties of Magnetite for Spintronics}, isbn = {978-953-51-0637-1}, booktitle = {Advanced Magnetic Materials}, publisher = {{InTech}}, author = {Parkinson, Gareth S. and Diebold, Ulrike and Tang, Jinke and Malkinski, Leszek}, editor = {Malkinski, Leszek}, month = may, year = {2012}, pages = {61--88} }, @article{novotny_ordered_2012, title = {Ordered Array of Single Adatoms with Remarkable Thermal Stability: {Au/Fe$_3$O$_4$(001)}}, volume = {108}, doi = {10.1103/PhysRevLett.108.216103}, abstract = {Gold deposited on the {Fe$_3$O$_4$(001)} surface at room temperature was studied using scanning tunneling microscopy ({STM}) and x-ray photoelectron spectroscopy ({XPS}). This surface forms a $(\sqrt 2 \times \sqrt 2)${R45$^\circ$} 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 {$^\circ$C.} We propose the strong site preference to be related to charge and orbital ordering within the first subsurface layer of {Fe$_3$O$_4$(001)-$(\sqrt 2 \times \sqrt 2)$R45$^\circ$} Because of its high thermal stability, this could prove an ideal model system for probing the chemical reactivity of single atomic species.}, number = {21}, journal = PRL, author = {Novotn\'{y}, Zbyn\v{e}k and Argentero, Giacomo and Wang, Zhiming and Schmid, Michael and Diebold, Ulrike and Parkinson, Gareth S.}, month = may, year = {2012}, pages = {216103} }, @article{losovyj_evidence_2012, title = {Evidence for $s$--$d$ Hybridization in {Au}$_{38}$ Clusters}, volume = {116}, doi = {10.1021/jp3010508}, abstract = {Resonant enhancement is seen in some gold clusters at the gold $4f_{7/2}$ and $4f_{5/2}$ 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 $5d^{10}6s^1$ system. Here we report, for the first time, the presence of a strong resonant enhancement in photoemission at the $4f_{7/2}$ threshold and a weaker resonant enhancement at the $4f_{5/2}$ threshold, in the open (undressed) {Au}$_{38}$ 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 {Au}$_{38}$ are not a result of localized orbitals, which is the case in a molecular system.}, number = {9}, journal = JPCC, author = {Losovyj, Yaroslav B. and Li, Shao-Chun and Lozova, Natalia and Katsiev, Khabibulakh and Stellwagen, Daniel and Diebold, Ulrike and Kong, Lingmei and Kumar, Challa S. S. R.}, year = {2012}, pages = {5857--5861} }, @article{li_trapping_2012, title = {Trapping Nitric Oxide by Surface Hydroxyls on Rutile {TiO$_2$(110)}}, volume = {116}, doi = {10.1021/jp209290a}, abstract = {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 {TiO$_2$(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 {TiO$_2$(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.}, number = {2}, journal = JPCC, author = {Li, Shao-Chun and Jacobson, Peter and Zhao, Shu-Lei and Gong, Xue-Qing and Diebold, Ulrike}, year = {2012}, pages = {1887--1891} }, @article{jacobson_nickel_2012, title = {Nickel Carbide as a Source of Grain Rotation in Epitaxial Graphene}, volume = {6}, doi = {10.1021/nn300625y}, abstract = {Graphene has a close lattice match to the Ni(111) surface, resulting in a preference for 1 $\times$ 1 configurations. We have investigated graphene grown by chemical vapor deposition ({CVD)} on the nickel carbide ({Ni$_2$C)} reconstruction of Ni(111) with scanning tunneling microscopy ({STM}). The presence of excess carbon, in the form of {Ni$_2$C}, prevents graphene from adopting the preferred 1 $\times$ 1 configuration and leads to grain rotation. {STM} measurements show that residual {Ni$_2$C} domains are present under rotated graphene. Nickel vacancy islands are observed at the periphery of rotated grains and indicate {Ni$_2$C} dissolution after graphene growth. Density functional theory ({DFT)} calculations predict a very weak (van der Waals type) interaction of graphene with the underlying {Ni$_2$C}, which should facilitate a phase separation of the carbide into metal-supported graphene. These results demonstrate that surface phases such as {Ni$_2$C} can play a major role in the quality of epitaxial graphene.}, number = {4}, journal = {{ACS} Nano}, author = {Jacobson, Peter and St\"{o}ger, Bernhard and Garhofer, Andreas and Parkinson, Gareth S. and Schmid, Michael and Caudillo, Roman and Mittendorfer, Florian and Redinger, Josef and Diebold, Ulrike}, month = apr, year = {2012}, pages = {3564--3572} }, @article{jacobson_disorder_2012, title = {Disorder and Defect Healing in Graphene on {Ni(111)}}, volume = {3}, doi = {10.1021/jz2015007}, abstract = {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 {$^\circ$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).}, journal = JPCL, author = {Jacobson, Peter and St\"{o}ger, Bernhard and Garhofer, Andreas and Parkinson, Gareth S. and Schmid, Michael and Caudillo, Roman and Mittendorfer, Florian and Redinger, Josef and Diebold, Ulrike}, year = {2012}, pages = {136--139} }, @article{hagleitner_bulk_2012, title = {Bulk and surface characterization of {In$_2$O$_3$(001)} single crystals}, volume = {85}, doi = {10.1103/PhysRevB.85.115441}, abstract = {A comprehensive bulk and surface investigation of high-quality {In$_2$O$_3$(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 $2.0 \pm 0.5 \times 10^5 \Omega{}$ cm, which translates into a carrier concentration of $\approx 10^{12}$ cm$^{-3}$. The results from x-ray diffraction ({XRD)} measurements revise the lattice constant to 10.1150(5) \AA{} from the previously accepted value of 10.117 \AA{}. Scanning tunneling microscopy ({STM)} images of a reduced (sputtered/annealed) and oxidized (exposure to atomic oxygen at 300 {$^\circ$C)} surface show a step height of 5 \AA{}, 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 \AA{} 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.}, number = {11}, journal = PRB, author = {Hagleitner, Daniel R. and Menhart, Manfred and Jacobson, Peter and Blomberg, Sara and Schulte, Karina and Lundgren, Edvin and Kubicek, Markus and Fleig, J\"{u}rgen and Kubel, Frank and Puls, Christoph and Limbeck, Andreas and Hutter, Herbert and Boatner, Lynn A. and Schmid, Michael and Diebold, Ulrike}, month = mar, year = {2012}, pages = {115441} }, @article{gustafson_rh100-3_2012, title = {The {Rh}(100)-(3 $\times$ 1)-{2O} structure}, volume = {24}, doi = {10.1088/0953-8984/24/22/225006}, abstract = {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 $\times$ 2), (2 $\times$ 2)-pg and c(8 $\times$ 2) structures at coverages of 0.25, 0.5 and 1.75 {ML} respectively, an intermediate (3 $\times$ 1) structure with a coverage of 2/3 {ML} is identified.}, number = {22}, journal = JPCM, author = {Gustafson, J. and Lundgren, E. and Mikkelsen, A. and Borg, M. and Klikovits, J. and Schmid, M. and Varga, P. and Andersen, J. N.}, month = jun, year = {2012}, pages = {225006} }, @article{bonnell_imaging_2012, title = {Imaging physical phenomena with local probes: From electrons to photons}, volume = {84}, doi = {10.1103/RevModPhys.84.1343}, abstract = {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.}, number = {3}, journal = RMP, author = {Bonnell, Dawn A. and Basov, D. N. and Bode, Matthias and Diebold, Ulrike and Kalinin, Sergei V. and Madhavan, Vidya and Novotny, Lukas and Salmeron, Miquel and Schwarz, Udo D. and Weiss, Paul S.}, month = sep, year = {2012}, pages = {1343--1381} }, @article{antlanger_pt3zr0001_2012, title = {{Pt$_3$Zr(0001):} A substrate for growing well-ordered ultrathin zirconia films by oxidation}, volume = {86}, doi = {10.1103/PhysRevB.86.035451}, abstract = {We have studied the surface of pure and oxidized {Pt$_3$Zr(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. {Pt$_3$Zr} 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 {ZrO$_2$} film is obtained by oxidation in $10^{-7}$ mbar O$_2$ at 400 {$^\circ$C} and post-annealing at $\approx 800 ^\circ${C}. The oxide consists of an O-Zr-O trilayer, equivalent to a (111) trilayer of the fluorite structure of cubic {ZrO$_2$}, but contracted laterally. The oxide forms a $(\sqrt{19} \times \sqrt{19})${R}23$\circ{}$ 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 $\approx{}$2.3 {eV} above $E_\mathrm F$.}, number = {3}, journal = PRB, author = {Antlanger, Moritz and Mayr-Schm\"{o}lzer, Wernfried and Pavelec, Ji\v{r}\'{\i} and Mittendorfer, Florian and Redinger, Josef and Varga, Peter and Diebold, Ulrike and Schmid, Michael}, month = jul, year = {2012}, pages = {035451} }, @article{ait-mansour_interface-confined_2012, title = {Interface-confined mixing and buried partial dislocations for {Ag} bilayer on {Pt}(111)}, volume = {86}, doi = {10.1103/PhysRevB.86.085404}, abstract = {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.}, number = {8}, journal = PRB, author = {A\"{\i}t-Mansour, Kamel and Brune, Harald and Passerone, Daniele and Schmid, Michael and Xiao, Wende and Ruffieux, Pascal and Buchsbaum, Andreas and Varga, Peter and Fasel, Roman and Gr\"{o}ning, Oliver}, month = aug, year = {2012}, pages = {085404} }, @article{yuhara_growth_2011, title = {Growth and structure of an ultrathin tin oxide film on {Rh(111)}}, volume = {109}, doi = {10.1063/1.3537871}, abstract = {The oxidation of submonolayer tin films on a Rh(111) surface by O$_2$ 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 {$^\circ$C} and a partial pressure of $2 \times 10 ^{-7}$ mbar O$_2$. The tin oxide film had (2 $\times$ 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.}, number = {2}, journal = JAP, author = {Yuhara, J. and Tajima, D. and Matsui, T. and Tatsumi, K. and Muto, S. and Schmid, M. and Varga, P.}, month = jan, year = {2011}, pages = {024903} } @article{de_santis_growth_2011, title = {Growth of ultrathin cobalt oxide films on {Pt}(111)}, volume = {84}, doi = {10.1103/PhysRevB.84.125430}, abstract = {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)} moir\'{e} 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 $\pm{}$ 0.6\% in the surface plane with respect to bulk {CoO.} An almost-flawless moir\'{e} pattern is obtained after a final step of annealing at 740 K in oxygen. Insufficient oxidation leads to defects in the moir\'{e} pattern, consisting of triangular dislocation loops of different sizes; the smaller ones occupy half of the moir\'{e} 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 moir\'{e} pattern after annealing at 740 K, whereas the zigzag phase was observed after low-temperature annealing. The second monolayer consists of a moir\'{e} 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 $\times$ 3) reconstruction. Structure models are presented for all phases observed, and we argue that some of the moir\'{e}like structures might be useful as templates for metal cluster growth.}, number = {12}, journal = PRB, author = {De Santis, Maurizio and Buchsbaum, Andreas and Varga, Peter and Schmid, Michael}, year = {2011}, pages = {125430} }, @article{parkinson_room_2011, title = {Room Temperature Water Splitting at the Surface of Magnetite}, volume = {133}, doi = {10.1021/ja203432e}, abstract = {An array of surface science measurements has revealed novel water adsorption behavior at the {Fe$_3$O$_4$(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\textendash{}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 O$_2$.}, number = {32}, journal = JACS, author = {Parkinson, Gareth S. and Novotn\'{y}, Zbyn\v{e}k and Jacobson, Peter and Schmid, Michael and Diebold, Ulrike}, year = {2011}, pages = {12650--12655} }, @article{diebold_photocatalysts_2011, title = {Photocatalysts: Closing the gap}, volume = {3}, doi = {10.1038/nchem.1019}, abstract = {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.}, number = {4}, journal = NatChem, author = {Diebold, Ulrike}, month = apr, year = {2011}, pages = {271--272} }, @article{liu_growth_2011, title = {Growth and Organization of an Organic Molecular Monolayer on {TiO$_2$:} Catechol on Anatase (101)}, volume = {133}, doi = {10.1021/ja200001r}, abstract = {Anatase {TiO$_2$} 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\textendash{}D2} or {D2\textendash{}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.}, number = {20}, journal = JACS, author = {Liu, Li-Min and Li, Shao-Chun and Cheng, Hongzhi and Diebold, Ulrike and Selloni, Annabella}, month = may, year = {2011}, pages = {7816--7823} }, @article{li_photoemission_2011, title = {Photoemission Study of Azobenzene and Aniline Adsorbed on {TiO$_2$} Anatase (101) and Rutile (110) Surfaces}, volume = {115}, doi = {10.1021/jp202029a}, abstract = {The electronic structure of azobenzene and aniline, adsorbed on two {TiO$_2$} 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\textendash{}cis isomer conversion is proposed to facilitate the azobenzene cleavage. These results confirm that the {N$=$N} double bond of azobenzene is cleaved by {TiO$_2$} 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 $\leftrightarrow{}$ aniline conversion at {{TiO}$_2$} surfaces {[Li}, {S.-C.} ; Diebold, U. J. Am. Chem. Soc. 2010, 132, 64].}, number = {20}, journal = JPCC, author = {Li, Shao-Chun and Losovyj, Yaroslav and Paliwal, Vinod Kumar and Diebold, Ulrike}, month = may, year = {2011}, pages = {10173--10179} }, @article{schmid_unusual_2011, title = {Unusual Cluster Shapes and Directional Bonding of an fcc Metal: {Pt/Pt(111)}}, volume = {107}, doi = {10.1103/PhysRevLett.107.016102}, abstract = {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 $d_{z^2}$-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.}, number = {1}, journal = PRL, author = {Schmid, Michael and Garhofer, Andreas and Redinger, Josef and Wimmer, Florian and Scheiber, Philipp and Varga, Peter}, month = jun, year = {2011}, pages = {016102} }, @article{uddin_vitro_2011, title = {An in vitro controlled release study of valproic acid encapsulated in a titania ceramic matrix}, volume = {257}, doi = {16/j.apsusc.2011.03.079}, abstract = {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 {$\approx ^\circ$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.}, number = {18}, journal = APSS, author = {Uddin, M. J. and Mondal, D. and Morris, C. A. and Lopez, T. and Diebold, U. and Gonzalez, R. D.}, month = jul, year = {2011}, pages = {7920--7927} }, @article{li_adsorption-site-dependent_2011, title = {{Adsorption-Site-Dependent} Electronic Structure of Catechol on the Anatase {TiO$_2$(101)} Surface}, volume = {27}, doi = {10.1021/la201553k}, abstract = {The adsorption of catechol (1,2-benzendiol) on the anatase {TiO$_2$(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 $\times$ 1 superstructure at full coverage. A gap state of $\sim{}$1 {eV} above the valence band maximum is observed for dosages in excess of $\sim{}$0.4 Langmuir, but such a state is absent for lower coverages. The formation of the band gap states thus correlates with the adsorption at regular lattice sites and the onset of self-assembled superstructures.}, number = {14}, journal = {Langmuir}, author = {Li, Shao-Chun and Losovyj, Yaroslav and Diebold, Ulrike}, month = jul, year = {2011}, pages = {8600--8604} }, @article{shah_zaman_in-situ_2011, title = {In-situ magnetic nano-patterning of {Fe} films grown on {Cu}(100)}, volume = {110}, doi = {10.1063/1.3609078}, abstract = {Metastable paramagnetic face-centered cubic (fcc) Fe films grown on a Cu(100) single crystal at room temperature can be transformed to the ferromagnetic body-centered cubic (bcc) structure by ion irradiation. We have employed this technique to write small ferromagnetic patches by Ar+ irradiation through a gold coated {SiN} mask with regularly arranged 80-nm diameter holes, which was placed on top of the as-prepared fcc Fe films. Nanopatterning was performed on both 8-monolayer {(ML)} Fe films grown in ultrahigh vacuum as well as {22-ML} films stabilized by dosing carbon monoxide during growth. The structural transformation of these nano-patterned films was investigated using scanning tunneling microscopy. In both 8 and {22-ML} fcc Fe films, the bcc needles are found to protrude laterally out of the irradiated part of the sample, limiting the resolution of the technique to a few 10 nm. The magnetic transformation was confirmed by magnetic force microscopy.}, number = {2}, journal = JAP, author = {Shah Zaman, Sameena and Dvo\v{r}\'{a}k, Petr and Ritter, Robert and Buchsbaum, Andreas and Stickler, Daniel and Oepen, Hans Peter and Schmid, Michael and Varga, Peter}, month = jul, year = {2011}, pages = {024309} }, @article{parkinson_metastable_2011, title = {A metastable {Fe(A)} termination at the {Fe$_3$O$_4$(001)} surface}, volume = {605}, doi = {10.1016/j.susc.2011.05.018}, abstract = {A metastable {Fe(A)} terminated {Fe$_3$O$_4$(001)} surface was prepared by tailoring the surface preparation conditions. {STM}, {LEIS} and {LEED} are utilized to demonstrate that annealing the Ar$^+$ sputtered surface to 350~{$^\circ$C} produces an {Fe(A)} terminated surface with a ($\sqrt{2} \times \sqrt{2}$)R45$^\circ$ 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~{$^\circ$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.}, number = {15-16}, journal = SuSci, author = {Parkinson, Gareth S. and Novotn\'{y}, Zbyn\v{e}k and Jacobson, Peter and Schmid, Michael and Diebold, Ulrike}, month = aug, year = {2011}, pages = {L42--L45} }, @article{mittendorfer_oxygen-stabilized_2011, title = {Oxygen-Stabilized {Rh} Adatoms: {0D} Oxides on a Vicinal Surface}, doi = {10.1021/jz2011308}, abstract = {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.}, volume = {2}, number = {21}, journal = JPCL, author = {Mittendorfer, Florian and Franz, Thomas and Klikovits, Jan and Schmid, Michael and Merte, Lindsay R. and Shah Zaman, Sameena and Varga, Peter and Westerstr\"{o}m, Rasmus and Resta, Andrea and Andersen, Jesper N. and Gustafson, Johan and Lundgren, Edvin}, month = oct, year = {2011}, pages = {2747--2751} }, @article{ostermaier_metal-related_2010, title = {Metal-related gate sinking due to interfacial oxygen layer in {Ir/InAlN} high electron mobility transistors}, volume = {96}, doi = {10.1063/1.3458700}, abstract = {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\,{$^\circ$C} the diffusion is inhomogeneous and seems to occur along grain boundaries, which is consistent with the capacitance-voltage analysis. Annealing at 700\,{$^\circ$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}}, number = {26}, journal = APL, author = {C. Ostermaier and G. Pozzovivo and B. Basnar and W. Schrenk and M. Schmid and L. To\'th and B. Pe\'cz and {J.-F.} Carlin and M. Gonschorek and N. Grandjean and G. Strasser and D. Pogany and J. Kuzmik}, year = {2010}, pages = {263515} } @article{parkinson_semiconductor-half_2010, title = {Semiconductor-half metal transition at the {Fe$_3$O$_4$(001)} surface upon hydrogen adsorption}, volume = {82}, doi = {10.1103/PhysRevB.82.125413}, abstract = {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 {($\ssqrt{2}\timessqrt{2})$R45^\circ$} 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.}, number = {12}, journal = PRB, author = {Gareth S. Parkinson and Narasimham Mulakaluri and Yaroslav Losovyj and Peter Jacobson and Rossitza Pentcheva and Ulrike Diebold}, year = {2010}, pages = {125413} }, @article{shah_zaman_ion-beam-induced_2010, title = {Ion-beam-induced magnetic transformation of {CO}-stabilized fcc {Fe} films on {Cu(100)}}, volume = {82}, doi = {10.1103/PhysRevB.82.235401}, abstract = {We have grown {22-ML-thick} Fe films on a Cu(100) single crystal. The films were stabilized in the face-centered-cubic (fcc) $\gamma{}$ phase by adsorption of carbon monoxide during growth, preventing the transformation to the body-centered-cubic (bcc) $\alpha{}$ 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.}, number = {23}, journal = PRB, author = {Sameena {Shah Zaman} and Hinnerk O\ss{}mer and Jakub Jonner and Zbyn\v{e}k Novotn\'{y} and Andreas Buchsbaum and Michael Schmid and Peter Varga}, month = dec, year = {2010}, pages = {235401} }, @article{scheiber_observation_2010, title = {Observation and Destruction of an Elusive Adsorbate with {STM:} {O$_2$/TiO$_2$(110)}}, volume = {105}, doi = {10.1103/PhysRevLett.105.216101}, abstract = {When a slightly defective rutile {TiO$_2$(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 {TiO$_2$(110)} at a sample temperature of $\approx{}$100 K and imaged at 17 K, shows a molecular precursor at {VO} as a faint change in contrast. The adsorbed O2 easily dissociates during the {STM} measurements, and the formation of Oad's at both sides of the original {VO} is observed.}, number = {21}, journal = PRL, author = {Philipp Scheiber and Alexander Riss and Michael Schmid and Peter Varga and Ulrike Diebold}, month = nov, year = {2010}, pages = {216101} }, @article{li_hydrogen_2010, title = {Hydrogen bonding controls the dynamics of catechol adsorbed on a {TiO$_2$(110)} surface}, volume = {328}, doi = {10.1126/science.1188328}, abstract = {Direct studies of how organic molecules diffuse on metal oxide surfaces can provide insights into catalysis and molecular assembly processes. We studied individual catechol molecules, {C6H4(OH)2,} on a rutile {TiO$_2$(110)} surface with scanning tunneling microscopy. Surface hydroxyls enhanced the diffusivity of adsorbed catecholates. The capture and release of a proton caused individual molecules to switch between mobile and immobile states within a measurement period of minutes. Density functional theory calculations showed that the transfer of hydrogen from surface hydroxyls to the molecule and its interaction with surface hydroxyls substantially lowered the activation barrier for rotational motion across the surface. Hydrogen bonding can play an essential role in the initial stages of the dynamics of molecular assembly.}, number = {5980}, journal = {Science}, author = {Shao-Chun Li and Li-Na Chu and Xue-Qing Gong and Ulrike Diebold}, month = may, year = {2010}, pages = {882--884} }, @article{buchsbaum_highly_2010, title = {Highly ordered {Pd}, {Fe}, and {Co} clusters on alumina on {Ni$_3$Al(111)}}, volume = {81}, doi = {10.1103/PhysRevB.81.115420}, abstract = {Template-mediated growth of metals has attracted much interest due to the remarkable magnetic and catalytic properties of clusters in the nanometer range and provides the opportunity to grow clusters with narrow size distributions. We have grown well-ordered {Fe} and {Co} clusters on the ultrathin aluminum oxide on {Ni$_3$Al(111),} a template with a 4.1 nm lattice. The structure of the $\approx{}$0.5-nm-thick oxide film exhibits holes reaching down to the metal substrate at the corners of the {($\sqrt{67} \times \sqrt{67})R12.2$^\circ$} 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.}, number = {11}, journal = PRB, author = {Andreas Buchsbaum and Maurizio {De Santis} and Helio C. N. Tolentino and Michael Schmid and Peter Varga}, month = mar, year = {2010}, pages = {115420} }, @article{vlad_metastable_2010, title = {Metastable surface oxide on {CoGa(100):} Structure and stability}, volume = {81}, doi = {10.1103/PhysRevB.81.115402}, abstract = {We investigated the structure and formation of a surface oxide and bulk {$\beta{}$-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 $\sim{}$1.6 {eV} above the stability limit for bulk {$\beta{}$-Ga2O3.} The formation of the bulk oxide is kinetically hindered by the presence of the oxygen-terminated surface oxide, which most likely hampers dissociative oxygen chemisorption. We observe that below 620 K, the surface oxide is surprisingly stable at 1 bar oxygen pressure. Substrate faceting accompanies the bulk oxide formation at temperatures higher than 1020 K.}, number = {11}, journal = PRB, author = {A. Vlad and A. Stierle and M. Marsman and G. Kresse and I. Costina and H. Dosch and M. Schmid and P. Varga}, month = mar, year = {2010}, pages = {115402} }, @article{diebold_oxide_2010, title = {Oxide surfaces: Surface science goes inorganic}, volume = {9}, doi = {10.1038/nmat2708}, number = {3}, journal = NatMat, author = {Ulrike Diebold}, month = mar, year = {2010}, pages = {185--187} }, @article{aschauer_influence_2010, title = {Influence of Subsurface Defects on the Surface Reactivity of {TiO$_2$:} Water on Anatase (101)}, volume = {114}, doi = {10.1021/jp910492b}, abstract = {The adsorption of water on a reduced {TiO$_2$} anatase (101) surface is investigated with scanning tunneling microscopy and density functional theory calculations. The presence of subsurface defects, which are prevalent on reduced anatase (101), leads to a higher desorption temperature of adsorbed water, indicating an enhanced binding due to the defects. Theoretical calculations of water adsorption on anatase (101) surfaces containing subsurface oxygen vacancies or titanium interstitials show a strong selectivity for water binding to sites in the vicinity of the subsurface defects. Moreover, the water adsorption energy at these sites is considerably higher than that on the stoichiometric surface, thus giving an explanation for the experimental observations. The calculations also predict facile water dissociation at these sites, confirming the important role of defects in the surface chemistry of {TiO$_2$.}}, number = {2}, journal = JPCC, author = {Ulrich Aschauer and Yunbin He and Hongzhi Cheng and Shao-Chun Li and Ulrike Diebold and Annabella Selloni}, year = {2010}, pages = {1278--1284} }, @article{li_reactivity_2010, title = {Reactivity of {TiO$_2$} Rutile and Anatase Surfaces toward Nitroaromatics}, volume = {132}, doi = {10.1021/ja907865t}, abstract = {The {Au-TiO$_2$} system is a promising catalyst for the synthesis of nitro-aromatic compounds. The adsorption of azobenzene {(C6H5N$=$NH5C6)} and aniline {(C6H5NH$_2$)} on two single-crystalline {TiO$_2$} surfaces, anatase (101) and rutile (110), has been investigated with scanning tunneling microscopy {(STM),} low energy electron diffraction {(LEED),} and X-ray photoemission spectroscopy {(XPS).} While azobenzene adsorbs as an intact molecule at low coverages, ordered overlayers of phenyl imide {(C6H5N)} form at saturation coverage, indicating that {TiO$_2$} surfaces cleave the {N$=$N} bond even without the presence of Au. The same superstructures, p(1 $\times$ 2) on anatase and c(2 $\times$ 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 $\leftrightarrow{}$ azobenzene conversion is the activation of {O$_2$/H$_2$} for de/hydrogenation reactions.}, number = {1}, journal = JACS, author = {Shao-Chun Li and Ulrike Diebold}, year = {2010}, pages = {64--66} }, @article{dulub_preparation_2010, title = {Preparation of a pristine {TiO$_2$} anatase (101) surface by cleaving}, volume = {22}, doi = {10.1088/0953-8984/22/8/084014}, abstract = {A natural {TiO$_2$} 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 $\approx$ 3 nm high and 10\textendash{}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 {TiO$_2$} anatase is discussed.}, number = {8}, journal = JPCM, author = {Olga Dulub and Ulrike Diebold}, year = {2010}, pages = {084014} }, @article{diebold_oxide_2010-1, title = {Oxide Surface Science}, volume = {61}, doi = {10.1146/annurev.physchem.012809.103254}, abstract = {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\textendash{}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 {TiO$_2$.} 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.}, number = {1}, journal = AnnRevPhCh, author = {Ulrike Diebold and Shao-Chun Li and Michael Schmid}, year = {2010}, pages = {129--148} }, @article{schmid_high_2009, title = {High Island Densities in Pulsed Laser Deposition: Causes and Implications}, volume = {103}, doi = {10.1103/PhysRevLett.103.076101}, abstract = {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 ($\lessequivlnt{}$100 {eV)} of the deposited ions, high island densities are caused by the high instantaneous flux of arriving particles. Additional nuclei are formed at high ion energies ($\greaterequivlnt{}$200 {eV)} by adatoms created by the impinging ions. For heteroepitaxy, the island density is also increased by intermixing (deposited material implanted in the surface), creating an inhomogeneous potential energy surface for diffusing atoms. We discuss implications for layer-by-layer growth and sputter deposition.}, number = {7}, journal = PRL, author = {M. Schmid and C. Lenauer and A. Buchsbaum and F. Wimmer and G. Rauchbauer and P. Scheiber and G. Betz and P. Varga}, month = aug, year = {2009}, pages = {076101} }, @article{gustafson_structure_2009, title = {Structure and catalytic reactivity of {Rh} oxides}, volume = {145}, doi = {10.1016/j.cattod.2008.11.011}, abstract = {Using a combination of experimental and theoretical techniques, we show that a thin {RhO$_2$} surface oxide film forms prior to the bulk {Rh2O3} corundum oxide on all close-packed single crystal {Rh} surfaces. Based on previous reports, we argue that the {RhO$_2$} surface oxide also forms on vicinal {Rh} surfaces as well as on {Rh} nanoparticles. The detailed structure of this film was previously determined using {UHV} based techniques and density functional theory. In the present paper, we also examine the structure of the bulk {Rh2O3} corundum oxide using surface X-ray diffraction. Being armed with this structural information, we have explored the {CO} oxidation reaction over Rh(111), Rh(100) and {Pt25Rh75(100)} at realistic pressures using in situ surface X-ray diffraction and online mass spectrometry. In all three cases we find that an increase of the {CO$_2$} production coincides with the formation of the thin {RhO$_2$} surface oxide film. In the case of {Pt25Rh75(100),} our measurements demonstrate that the formation of bulk {Rh2O3} corundum oxide poisons the reaction, and argue that this is also valid for all other {Rh} surfaces. Our study implies that the {CO} oxidation reaction over {Rh} surfaces at realistic conditions is insensitive to the exact {Rh} substrate orientation, but is rather governed by the formation of a specific surface oxide phase.}, number = {3-4}, journal = CatalTod, author = {J. Gustafson and R. Westerstr\"{o}m and A. Resta and A. Mikkelsen and {J.N.} Andersen and O. Balmes and X. Torrelles and M. Schmid and P. Varga and B. Hammer and G. Kresse and {C.J.} Baddeley and E. Lundgren}, month = jul, year = {2009}, pages = {227--235} }, @article{he_local_2009, title = {Local ordering and electronic signatures of submonolayer water on anatase {TiO$_2$(101)}}, volume = {8}, doi = {10.1038/nmat2466}, abstract = {The interaction of water with metal oxide surfaces is of fundamental importance to various fields of science, ranging from geophysics to catalysis and biochemistry. In particular, the discovery that {TiO$_2$} photocatalyses the dissociation of water has triggered broad interest and intensive studies of water adsorption on {TiO$_2$} over decades. So far, these studies have mostly focused on the (110) surface of the most stable polymorph of {TiO$_2$,} rutile, whereas it is the metastable anatase form that is generally considered photocatalytically more efficient. The present combined experimental (scanning tunnelling microscopy) and theoretical (density functional theory and first-principles molecular dynamics) study gives atomic-scale insights into the adsorption of water on anatase (101), the most frequently exposed surface of this {TiO$_2$} polymorph. Water adsorbs as an intact monomer with a computed binding energy of 730 {meV.} The charge rearrangement at the molecule\textendash{}anatase interface affects the adsorption of further water molecules, resulting in short-range repulsive and attractive interactions along the [010] and [11-1]/[1-1-1] directions, respectively, and a locally ordered (2$\times$2) superstructure of molecular water.}, number = {7}, journal = NatMat, author = {Yunbin He and Antonio Tilocca and Olga Dulub and Annabella Selloni and Ulrike Diebold}, month = jul, year = {2009}, pages = {585--589} }, @article{he_nucleation_2009, title = {Nucleation and Growth of {1D} Water Clusters on Rutile {TiO$_2$}(011)-2$\times$1}, volume = {113}, doi = {10.1021/jp903017x}, abstract = {We present a combined experimental and theoretical study of the adsorption of water on the rutile {TiO$_2$(011)-2$\times$1} surface, whose \textquotedblleft{}brookite (001)-like\textquotedblright{} 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.}, number = {24}, journal = JPCC, author = {Yunbin He and Wei-Kun Li and Xue-Qing Gong and Olga Dulub and Annabella Selloni and Ulrike Diebold}, month = jun, year = {2009}, pages = {10329--10332} }, @article{golczewski_ion-induced_2009, title = {Ion-induced erosion of tungsten surfaces studied by a sensitive quartz-crystal-microbalance technique}, volume = {390-391}, doi = {10.1016/j.jnucmat.2009.01.279}, abstract = {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{\textasciicircum}-5 $\mathrm{\mu}$g/s, which corresponds to a removal (or deposition) of only 10{\textasciicircum}-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.}, journal = JNuclMat, author = {A. Golczewski and A. Kuzucan and K. Schmid and J. Roth and M. Schmid and F. Aumayr}, month = jun, year = {2009}, pages = {1102--1105} }, @article{he_evidence_2009, title = {Evidence for the Predominance of Subsurface Defects on Reduced Anatase {TiO$_2$(101)}}, volume = {102}, doi = {10.1103/PhysRevLett.102.106105}, abstract = {Scanning tunneling microscopy {(STM)} images taken on a freshly cleaved anatase {TiO$_2$(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.}, number = {10}, journal = PRL, author = {Yunbin He and Olga Dulub and Hongzhi Cheng and Annabella Selloni and Ulrike Diebold}, month = mar, year = {2009}, pages = {106105--4} }, @article{honolka_magnetism_2009, title = {Magnetism of {FePt} Surface Alloys}, volume = {102}, doi = {10.1103/PhysRevLett.102.067207}, abstract = {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.}, number = {6}, journal = PRL, author = {J. Honolka and T. Y. Lee and K. Kuhnke and A. Enders and R. Skomski and S. Bornemann and S. Mankovsky and J. Minar and J. Staunton and H. Ebert and M. Hessler and K. Fauth and G. Schutz and A. Buchsbaum and M. Schmid and P. Varga and K. Kern}, month = feb, year = {2009}, pages = {067207--4} }, @article{golczewski_quartz-crystal-microbalance_2009, title = {A quartz-crystal-microbalance technique to investigate ion-induced erosion of fusion relevant surfaces}, volume = {267}, doi = {10.1016/j.nimb.2008.10.088}, abstract = {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.}, number = {4}, journal = NIMB, author = {A. Golczewski and K. Dobes and G. Wachter and M. Schmid and F. Aumayr}, month = feb, year = {2009}, pages = {695--699} }, @article{li_correlation_2009, title = {Correlation between Bonding Geometry and Band Gap States at Organic-Inorganic Interfaces: Catechol on Rutile {TiO$_2$(110)}}, volume = {131}, doi = {10.1021/ja803595u}, abstract = {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 {TiO$_2$} is presented. Using the organic catechol on rutile {TiO$_2$(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 $\times$ 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 {TiO$_2$.}}, number = {3}, journal = JACS, author = {Shao-Chun Li and Jian-Guo Wang and Peter Jacobson and X.-Q. Gong and Annabella Selloni and Ulrike Diebold}, year = {2009}, pages = {980--984} }, @article{gong_2_2009, title = {The 2 $\times$ 1 reconstruction of the rutile {TiO$_2$(011)} surface: A combined density functional theory, X-ray diffraction, and scanning tunneling microscopy study}, volume = {603}, doi = {10.1016/j.susc.2008.10.034}, abstract = {An extensive search for possible structural models of the (2 $\times$ 1)-reconstructed rutile {TiO$_2$(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 {TiO$_2$(0} 0 1) surface, we call it the [`]brookite (0 0 1)-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.}, number = {1}, journal = SuSci, author = {Xue-Qing Gong and Navid Khorshidi and Andreas Stierle and Vedran Vonk and Claus Ellinger and Helmut Dosch and Hongzhi Cheng and Annabella Selloni and Yunbin He and Olga Dulub and Ulrike Diebold}, year = {2009}, pages = {138--144} }, @article{morales_structure_2009, title = {The structure of the polar Sn-doped indium oxide (001) surface}, volume = {95}, doi = {10.1063/1.3275716}, abstract = {Epitaxial Sn-doped {In$_2$O$_3$} {(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$\times$1)-structure with p4g symmetry. Atomically-resolved Scanning Tunneling Microscopy suggests that surface oxygen atoms undergo dimerization; possible adsorption sites are identified. The density of surface oxygen depends on the {Sn} concentration and it is suggested that both, dimerization and doping stabilize the polar {ITO(001)} surface.}, number = {25}, journal = APL, author = {Erie H. Morales and Ulrike Diebold}, year = {2009}, pages = {253105} }, @article{li_direction-dependent_2009, title = {Direction-dependent intermolecular interactions: catechol on {TiO$_2$(110)-1} x 1}, volume = {7396}, doi = {doi:10.1117/12.828204}, abstract = {The adsorption of a submonolayer of catechol {(C6H6O$_2$)} on the rutile {TiO$_2$(110)-1$\times$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.}, journal = {Proceedings of the {SPIE}}, author = {Shao-Chun Li and Ulrike Diebold}, year = {2009}, pages = {73960P--7} }, @article{kostelnik_leed_2009, title = {A {LEED} study of {NO} superstructures on the {Pd}(111) surface}, volume = {21}, doi = {10.1088/0953-8984/21/13/134005}, abstract = {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.}, number = {13}, journal = JPCM, author = {Petr Kosteln\'{\i}k and Tomas \v{S}ikola and Peter Varga and Michael Schmid}, year = {2009}, pages = {134005} }, @article{fathalla_water-soluble_2009, title = {Water-soluble nanorods self-assembled via pristine {C}$_{60}$ and porphyrin moieties}, volume = {2009}, doi = {10.1039/b908050c}, abstract = {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.}, number = {28}, journal = ChemComm, author = {Maher Fathalla and Shao-Chun Li and Ulrike Diebold and Alina Alb and Janarthanan Jayawickramarajah}, year = {2009}, pages = {4209--4211} }, @article{honolka_complex_2009, title = {Complex magnetic phase in submonolayer {Fe} stripes on {Pt(997)}}, volume = {79}, doi = {10.1103/PhysRevB.79.104430}, abstract = {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.}, number = {10}, journal = PRB, author = {Honolka, J. and Lee, T. Y. and Kuhnke, K. and Repetto, D. and Sessi, V. and Wahl, P. and Buchsbaum, A. and Varga, P. and Gardonio, S. and Carbone, C. and Krishnakumar, S. R. and Gambardella, P. and Komelj, M. and Singer, R. and F\"{a}hnle, M. and Fauth, K. and Sch\"{u}tz, G. and Enders, A. and Kern, K.}, month = mar, year = {2009}, pages = {104430} }, @article{klikovits_step-orientation-dependent_2008, title = {{Step-Orientation-Dependent} Oxidation: From {1D} to {2D} Oxides}, volume = {101}, doi = {10.1103/PhysRevLett.101.266104}, abstract = {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.}, number = {26}, journal = PRL, author = {J Klikovits and M Schmid and {LR} Merte and P Varga and R Westerstr\"{o}m and A Resta and {JN} Andersen and J Gustafson and A Mikkelsen and E Lundgren and F Mittendorfer and G Kresse}, month = dec, year = {2008}, pages = {266104} }, @article{jacobson_decomposition_2008, title = {Decomposition of catechol and carbonaceous residues on {TiO}$_2$(110): A model system for cleaning of extreme ultraviolet lithography optics}, volume = {26}, doi = {10.1116/1.3002566}, abstract = {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 {TiO$_2$(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 O$_2$) 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.}, number = {6}, journal = JVSTB, author = {Peter Jacobson and Shao-Chun Li and Chuandao Wang and Ulrike Diebold}, month = nov, year = {2008}, pages = {2236--2240} }, @article{li_scanning_2008, title = {Scanning Tunneling Microscopy Study of a Vicinal Anatase {TiO$_2$} Surface}, volume = {112}, doi = {10.1021/jp806383c}, abstract = {Using scanning tunneling microscopy {(STM)} and low-energy electron diffraction {(LEED),} the structure of the anatase {TiO$_2$} (5 -1 4) surface, $\sim{}$10$^\circ$ 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.}, number = {42}, journal = JPCC, author = {Shao-Chun Li and Olga Dulub and Ulrike Diebold}, month = oct, year = {2008}, pages = {16166--16170} }, @article{katsiev_characterization_2008, title = {Characterization of individual {SnO$_2$} nanobelts with {STM}}, volume = {602}, doi = {10.1016/j.susc.2008.05.042}, abstract = {The surface morphology of tin oxide nanobelts {(NB)} was studied with scanning tunneling microscopy {(STM),} dry-deposited on {TiO$_2$} substrates. {XPS} shows carbon contamination on as-grown, air-exposed {SnO$_2$} 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,} O$_2$ atmosphere, and air, were determined. Atomically resolved {STM} images show a {SnO$_2$(1} 0 1)-(1 $\times$ 1) structure on the top {NB} surface.}, number = {14}, journal = SuSci, author = {Khabibulakh Katsiev and Andrei Kolmakov and Minghu Fang and Ulrike Diebold}, month = jul, year = {2008}, pages = {L112--L114} }, @article{ait-mansour_fabrication_2008, title = {Fabrication of a Well-Ordered Nanohole Array Stable at Room Temperature}, volume = {8}, doi = {10.1021/nl8013378}, abstract = {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.}, number = {7}, journal = nanoLett, author = {K. {Ai\"t-Mansour} and A. Buchsbaum and P. Ruffieux and M. Schmid and P. Gro\"ning and P. Varga and R. Fasel and O. Gro\"ning}, month = jul, year = {2008}, pages = {2035--2040} }, @article{napetschnig_ultrathin_2008, title = {Ultrathin alumina film on {Cu-9at\%Al(111)}}, volume = {602}, doi = {10.1016/j.susc.2008.02.040}, abstract = {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.}, number = {10}, journal = SuSci, author = {E Napetschnig and M Schmid and P Varga}, month = may, year = {2008}, pages = {1750--1756} }, @article{katsiev_defects_2008, title = {Defects and {Pd} growth on the reduced {SnO$_2$(100)} surface}, volume = {602}, doi = {10.1016/j.susc.2008.03.003}, abstract = {Scanning tunneling microscopy experiments on a clean, reduced {SnO$_2$(100)}-(1 $\times$ 1) surface reveal surface defects with zero-, one-, and two-dimensions. Point defects consist of missing {SnO/SnO$_2$} 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 $\times$ 1) structure and a half-unit cell deep, form at [0 0 1]-oriented step edges.}, number = {9}, journal = SuSci, author = {Khabibulakh Katsiev and Matthias Batzill and Lynn A. Boatner and Ulrike Diebold}, month = may, year = {2008}, pages = {1699--1704} }, @article{rauchbauer_ultra-thin_2008, title = {Ultra-thin {Fe} films grown on {Cu} by pulsed laser deposition: Intermixing and bcc-like structures}, volume = {602}, doi = {10.1016/j.susc.2008.02.024}, abstract = {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.}, number = {8}, journal = SuSci, author = {G. Rauchbauer and A. Buchsbaum and H. Schiechl and P. Varga and M. Schmid and A. Biedermann}, month = apr, year = {2008}, pages = {1589--1598} }, @article{buchsbaum_time-of-flight_2008, title = {Time-of-flight spectroscopy of the energy distribution of laser-ablated atoms and ions}, volume = {79}, doi = {10.1063/1.2901607}, abstract = {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{\textasciicircum}-4 m, densities of 10{\textasciicircum}15\textendash{}10{\textasciicircum}16 m{\textasciicircum}-3 and ion energies up to 150 {eV.} Neutral spectra have shown an unexpectedly low fraction of neutrals (10{\textasciicircum}-3\textendash{}10{\textasciicircum}-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.}, number = {4}, journal = RSI, author = {A. Buchsbaum and G. Rauchbauer and P. Varga and M. Schmid}, month = apr, year = {2008}, pages = {043301--8} }, @article{diebold_wigglingway_2008, title = {Wiggling its way out of surface polarity: {Fe$_3$O$_4$}(100) {(A} Perspectives on the article: ``{A} combined {DFT/LEED} approach for complex oxide surface structure determination: {Fe$_3$O$_4$}(011)'' by {R. Pentcheva, W. Moritz, J. Rundgren, S. Frank, D. Schrupp, M. Scheffler})}, volume = {602}, doi = {10.1016/j.susc.2008.01.039}, number = {7}, journal = SuSci, author = {Ulrike Diebold}, month = apr, year = {2008}, pages = {1297--1298} }, @article{gong_small_2008, title = {Small {Au} and {Pt} Clusters at the Anatase {TiO$_2$(101)} Surface: Behavior at Terraces, Steps, and Surface Oxygen Vacancies}, volume = {130}, doi = {10.1021/ja0773148}, abstract = {The adsorption properties of Au and {Pt} metal nanoclusters on {TiO$_2$} anatase (101) were calculated using density functional theory. Structures and energetics of adsorbed Au and {Pt} monomers, dimers, and trimers at clean anatase {TiO$_2$(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.}, number = {1}, journal = JACS, author = {Xue-Qing Gong and Annabella Selloni and Olga Dulub and Peter Jacobson and Ulrike Diebold}, year = {2008}, pages = {370--381} }, @article{lazcano_oxygen_2008, title = {Oxygen adsorption on {Cu/ZnO(0001)-Zn}}, volume = {77}, doi = {10.1103/PhysRevB.77.035435}, abstract = {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 {$^\circ$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.}, number = {3}, journal = PRB, author = {Paola Lazcano and Matthias Batzill and Ulrike Diebold and Patricio H\"{a}berle}, year = {2008}, pages = {035435} }, @article{westerstrom_stressing_2008, title = {Stressing {Pd} atoms: Initial oxidation of the {Pd}(110) surface}, volume = {602}, doi = {10.1016/j.susc.2008.05.033}, abstract = {We have investigated the oxygen induced structures of the Pd(110) surface in the pressure range of 10{\textless}sup{\textgreater}-5{\textless}/sup{\textgreater}-10{\textless}sup{\textgreater}-3{\textless}/sup{\textgreater} mbar of oxygen, at a sample temperature of around 300 {$^\circ$C.} These structures, denoted as "(7x$\surd{}$3)" and "(9x$\surd{}$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.}, number = {14}, journal = SuSci, author = {R. Westerstr\"{o}m and C. J. Weststrate and A. Resta and A. Mikkelsen and J. Schnadt and J. N. Andersen and E. Lundgren and M. Schmid and N. Seriani and J. Harl and F. Mittendorfer and G. Kresse}, year = {2008}, pages = {2440--2447} }, @article{rupp_ion-beam_2008, title = {Ion-beam induced fcc-bcc transition in ultrathin {Fe} films for ferromagnetic patterning}, volume = {93}, doi = {10.1063/1.2969795}, abstract = {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.}, number = {6}, journal = APL, author = {W. Rupp and A. Biedermann and B. Kamenik and R. Ritter and Ch. Klein and E. Platzgummer and M. Schmid and P. Varga}, year = {2008}, pages = {063102--3} }, @article{morales_surface_2008, title = {Surface structure of {Sn}-doped {In$_2$O$_3$} (111) thin films by {STM}}, volume = {10}, doi = {10.1088/1367-2630/10/12/125030}, abstract = {High-quality Sn-doped {In$_2$O$_3$} {(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}, $\sim$ 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 {In$_2$O$_3$}(111)-1x1 surface is analyzed in terms of their possible role in surface chemical reactions.}, number = {12}, journal = {New Journal of Physics}, author = {Erie H. Morales and Yunbin He and Mykola Vinnichenko and Bernard Delley and Ulrike Diebold}, year = {2008}, pages = {125030} }, @article{schmid_nanotemplate_2007, title = {Nanotemplate with holes: Ultrathin alumina on {Ni$_3$Al(111)}}, volume = {99}, doi = {10.1103/PhysRevLett.99.196104}, abstract = {We have determined the structure of the ultrathin (root 67 x root {67)R12.2} degrees aluminum oxide on {Ni$_3$Al(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.}, number = {19}, journal = PRL, author = {M Schmid and G Kresse and A Buchsbaum and E Napetschnig and S Gritschneder and M Reichling and P Varga}, month = nov, year = {2007}, pages = {196104} }, @article{westerstrom_oxidation_2007, title = {Oxidation of {Pd(553)}: From ultrahigh vacuum to atmospheric pressure}, volume = {76}, doi = {10.1103/PhysRevB.76.155410}, abstract = {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 ($\surd{}$6 x $\surd{}$6) {Pd5O4} surface oxide can be observed on (111) terraces larger than those of the (332) terraces. Increasing the {O} pressure above 1 mbar results in the disappearance of the (332) facets and the formation of {PdO} bulk oxide.}, number = {15}, journal = PRB, author = {R. Westerstr\"{o}m and J. Gustafson and A. Resta and A. Mikkelsen and J. N. Andersen and E. Lundgren and N. Seriani and F. Mittendorfer and M. Schmid and J. Klikovits and P. Varga and M. D. Ackermann and J. W. M. Frenken and N. Kasper and A. Stierle}, month = oct, year = {2007}, pages = {155410--9} }, @article{di_valentin_doping_2007, title = {Doping and functionalization of photoactive semiconducting metal oxides}, volume = {339}, doi = {10.1016/j.chemphys.2007.09.022}, number = {1-3}, journal = {Chemical Physics}, author = {Cristiana Di Valentin and Ulrike Diebold and Annabella Selloni}, month = oct, year = {2007}, pages = {vii--viii} }, @article{batzill_surface_2007, title = {Surface studies of nitrogen implanted {TiO$_2$}}, volume = {339}, doi = {10.1016/j.chemphys.2007.07.037}, abstract = {Rutile {TiO$_2$(1} 1 0) single crystals have been doped by nitrogen-ion implantation. The change in the valence band and in the core level peak shapes are characterized by photoemission spectroscopy. Surface morphologies are characterized by scanning tunneling microscopy. N-dopants are observed to be in a 3- charge state and to substitute for O-anions in the {TiO$_2$} lattice for N-concentrations up to $\approx$5\% of the anions. The higher valency of the N-dopants compared to the host O-anions is proposed to be compensated by the formation of O-vacancies and/or Ti-interstitials. Two chemically shifted components arise in the Ti-2p core level upon N-doping. These components, shifted by 0.9 {eV} and 2.1 {eV,} are assigned to Ti-bound to N-ligands and possibly due to O-vacancies in the lattice. The Ti-3d band gap state observed in {UPS} is initially suppressed upon room temperature N-implantation and recovers a similar intensity as for undoped {TiO$_2$} samples upon annealing. This indicates that electrons left behind upon creation of O-vacancies are filling the N-2p level rather than Ti-3d states. The filled N-2p state is found at the top of the {TiO$_2$} valence band and is believed to be responsible for the band gap narrowing of N-doped {TiO$_2$} that shifts the photoactivity of {TiO$_2$} into the visible spectrum.}, number = {1-3}, journal = {Chemical Physics}, author = {Matthias Batzill and Erie H. Morales and Ulrike Diebold}, month = oct, year = {2007}, pages = {36--43} }, @article{dulub_electron-induced_2007, title = {Electron-Induced Oxygen Desorption from the {TiO$_2$(011)-$2\times 1$} Surface Leads to Self-Organized Vacancies}, volume = {317}, doi = {10.1126/science.1144787}, abstract = {When low-energy electrons strike a titanium dioxide surface, they may cause the desorption of surface oxygen. Oxygen vacancies that result from irradiating a {TiO$_2$(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.}, number = {5841}, journal = {Science}, author = {Olga Dulub and Matthias Batzill and Sergey Solovev and Elena Loginova and Alim Alchagirov and Theodore E. Madey and Ulrike Diebold}, month = aug, year = {2007}, pages = {1052--1056} }, @article{napetschnig_pd_2007, title = {Pd, {Co} and {Co-Pd} clusters on the ordered alumina film on {NiAl(110):} Contact angle, surface structure and composition}, volume = {601}, doi = {10.1016/j.susc.2007.05.047}, abstract = {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.}, number = {15}, journal = SuSci, author = {E Napetschnig and M Schmid and P Varga}, month = aug, year = {2007}, pages = {3233--3245} }, @article{klikovits_surface_2007, title = {Surface oxides on {Pd}(111): {STM} and density functional calculations}, volume = {76}, doi = {10.1103/PhysRevB.76.045405}, abstract = {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,} {Pd23O$_2$1,} {Pd19O18,} {Pd8O8,} and {Pd32O32.} All structures are one-layer oxides, in which oxygen atoms form a rectangular lattice, and all structures follow the same rules of favorable alignment of the oxide layer on the Pd(111) substrate. {DFT} calculations were used to simulate {STM} images as well as to determine the stability of the surface oxide structures. Simulated and measured {STM} images are in excellent agreement, indicating that the structural models are correct. Since the newly found surface oxides are clearly less stable than {Pd5O4,} we conclude that {Pd5O4} is the only thermodynamically stable phase, whereas all newly found structures are only kinetically stabilized. We also discuss possible mechanisms for the formation of these oxide structures.}, number = {4}, journal = PRB, author = {J Klikovits and E Napetschnig and M Schmid and N Seriani and O Dubay and G Kresse and P Varga}, month = jul, year = {2007}, pages = {045405} }, @article{saraf_surface_2007, title = {Surface and Interface Properties of Metal-Organic Chemical Vapor Deposition Grown a-Plane {Mg}$_x${Zn}$_{1-x}${O} (0 $\leq{}$ x $\leq{}$ 0.3) Films}, volume = {36}, doi = {10.1007/s11664-006-0052-x}, abstract = {The a-plane {Mg}\_x Zn\_1-x {O} (0 $\leq{}$ x $\leq{}$ 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 {$^\circ$C} to 500 {$^\circ$C,} with a typical growth rate of $\sim{}$500 nm/h. Field emission scanning electron microscopy {(FESEM)} images show that the films are smooth and dense. X-ray diffraction {(XRD)} scans confirm good crystallinity of the films. The interface of {Mg}\_x Zn\_1-x {O} films with r-sapphire was found to be semicoherent as characterized by high-resolution transmission electron microscopy {(HRTEM).} The {Mg}\_x Zn\_1-x {O} surfaces were characterized using scanning tunneling microscopy {(STM)} in ultrahigh vacuum {(UHV).} Low-energy electron diffraction {(LEED)} shows well-ordered and single-crystalline surfaces. The films have a characteristic wavelike surface morphology with needle-shaped domains running predominantly along the crystallographic c-direction. Photoluminescence {(PL)} measurements show a strong near-band-edge emission without observable deep level emission, indicating a low defect concentration. In-plane optical anisotropic transmission was observed by polarized transmission measurements.}, number = {4}, journal = {Journal of Electronic Materials}, author = {Gaurav Saraf and Jian Zhong and Olga Dulub and Ulrike Diebold and Theo Siegrist and Yicheng Lu}, month = apr, year = {2007}, pages = {446--451} }, @article{kostelnik_pd100-5_2007, title = {The {Pd}(100)-($\sqrt{5} \times \sqrt{5}$){R}27$^\circ$-{O} surface oxide: A {LEED,} {DFT} and {STM} study}, volume = {601}, doi = {10.1016/j.susc.2007.01.026}, abstract = {Using low energy electron diffraction {(LEED),} density functional theory {(DFT)} and scanning tunneling microscopy {(STM),} we have re-analyzed the Pd(100)-($\surd{}$5 x {$\surd{}$5)R27$^\circ$-O} surface oxide structure consisting, in the most recent model, of a strained {PdO(101)} layer on top of the Pd(100) surface {[M.} Todorova et al., Surf. Sci. 541 (2003) 101]. Both, {DFT} simulations using the Vienna Ab initio Simulation Package {(VASP)} and tensor {LEED} {I(V)} analysis of newly acquired {LEED} experimental data, show that the {PdO(101)} model is essentially correct. However, compared to the previous study, there is a horizontal shift of the {PdO(101)} layer with respect to the Pd(100) substrate. The atomic coordinates derived by {DFT} and {LEED} {(RP} = 0.162) are in excellent agreement with each other. We also present {STM} images with atomic resolution showing domain boundaries on the surface oxide and discuss the bonding geometry between the surface oxide and the substrate.}, number = {6}, journal = SuSci, author = {Petr Kosteln\'{\i}k and Nicola Seriani and Georg Kresse and Anders Mikkelsen and Edvin Lundgren and Volker Blum and Tom\'{a}s \v{S}ikola and Peter Varga and Michael Schmid}, month = mar, year = {2007}, pages = {1574--1581} }, @article{katsiev_growth_2007, title = {Growth of {One-Dimensional} {Pd} Nanowires on the Terraces of a Reduced {SnO$_2$(101)} Surface}, volume = {98}, doi = {10.1103/PhysRevLett.98.186102}, number = {18}, journal = PRL, author = {Khabibulakh Katsiev and Matthias Batzill and Ulrike Diebold and Alexander Urban and Bernd Meyer}, year = {2007}, }, @article{ventrice_jr_aresurfaces_2007, title = {Are the surfaces of {CrO$_2$} metallic?}, volume = {19}, doi = {10.1088/0953-8984/19/31/315207}, abstract = {Previous photoelectron spectroscopy studies of {CrO$_2$} have found either no density of states or a very low density of states at the Fermi level, suggesting that {CrO$_2$} is a semiconductor or a semi-metal. This is in contradiction to calculations that predict that {CrO$_2$} should be a half-metallic ferromagnet. Recently, techniques have been developed to grow high-quality epitaxial films of {CrO$_2$} on {TiO$_2$} substrates by chemical vapour deposition. We present photoelectron spectroscopy measurements of epitaxial {CrO$_2$(110)/TiO$_2$(110)} and {CrO$_2$(100)/TiO$_2$(100)} grown using a {CrO3} precursor. In addition, measurements of epitaxial {Cr2O3(0001)/{Pt}(111)} films grown by thermal evaporation of {Cr} in an oxygen atmosphere are presented as a reference for reduced {CrO$_2$} films. The measurements of the {CrO$_2$} surfaces show no emission at the Fermi level after sputtering and annealing the surfaces in oxygen, even though our soft core photoemission data and low-energy electron diffraction measurements provide evidence that stoichiometric {CrO$_2$} is present. The consequence of this is that neither surface of {CrO$_2$} is metallic. This behaviour could result from a metal to semiconductor transition at the (110) and (100) surfaces.}, number = {31}, journal = JPCM, author = {C. A. Ventrice Jr and D. R. Borst and H. Geisler and J. van Ek and Y. B. Losovyj and P. S. Robbert and U. Diebold and J. A. Rodriguez and G. X. Miao and A. Gupta}, year = {2007}, pages = {315207} }, @article{batzill_surface_2007-1, title = {Surface studies of gas sensing metal oxides}, volume = {9}, doi = {10.1039/b617710g}, abstract = {The relation of surface science studies of single crystal metal oxides to gas sensing applications is reviewed. Most metal oxide gas sensors are used to detect oxidizing or reducing gases and therefore this article focuses on surface reduction processes and the interaction of oxygen with these surfaces. The systems that are discussed are: (i) the oxygen vacancy formation on the surface of the ion conductor {CeO$_2$(111);} (ii) interaction of oxygen with {TiO$_2$} (both adsorption processes and the incorporation of oxygen into the {TiO$_2$(110)} lattice are discussed); (iii) the varying surface composition of {SnO$_2$(101)} and its consequence for the adsorption of water; and (iv) {Cu} modified {ZnO(0001)-Zn} surfaces and its interaction with oxygen. These examples are chosen to give a comprehensive overview of surface science studies of different kinds of gas sensing materials and to illustrate the potential that surface science studies have to give fundamental insight into gas sensing phenomena.}, number = {19}, journal = PCCP, author = {Matthias Batzill and Ulrike Diebold}, year = {2007}, pages = {2307--2318} }, @article{ondracek_chemical_2006, title = {Chemical ordering and composition fluctuations at the (001) surface of the {Fe$_{64}$Ni$_{36}$} Invar alloy}, volume = {74}, doi = {10.1103/PhysRevB.74.235437}, abstract = {We report on a study of (001) oriented fcc {Fe-Ni} alloy surfaces which combines first-principles calculations and low-temperature scanning tunneling microscopy {(STM)} experiments. Density functional theory calculations show that {Fe-Ni} alloy surfaces are buckled with the {Fe} atoms slightly shifted outwards and the {Ni} atoms inwards. This is consistent with the observation that the atoms in the surface layer can be chemically distinguished in the {STM} image: brighter spots (corrugation maxima with increased apparent height) indicate iron atoms, darker ones nickel atoms. This chemical contrast reveals a c(2$\times$2) chemical order (50\% {Fe}) with frequent {Fe}-rich defects on the {Fe64Ni36(001)} surface. The calculations also indicate that subsurface composition fluctuations may additionally modulate the apparent height of the surface atoms. The {STM} images show that this effect is pronounced compared to the surfaces of other disordered alloys, which suggests that some chemical order and corresponding concentration fluctuations exist also in the subsurface layers of Invar alloy. In addition, detailed electronic structure calculations allow us to identify the nature of a distinct peak below the Fermi level observed in the tunneling spectra. This peak corresponds to a surface resonance band which is particularly pronounced in iron-rich surface regions and provides a second type of chemical contrast with less spatial resolution but one that is essentially independent of the subsurface composition.}, number = {23}, journal = PRB, author = {M. Ondr\'{a}\v{c}ek and F. M\'{a}ca and J. Kudrnovsk\'{y} and J. Redinger and A. Biedermann and C. Fritscher and M. Schmid and P. Varga}, month = dec, year = {2006}, pages = {235437--7} }, @article{dulub_structure_2006, title = {Structure, defects, and impurities at the rutile {TiO$_2$}(011)-(2 $\times$ 1) surface: A scanning tunneling microscopy study}, volume = {600}, doi = {10.1016/j.susc.2006.06.042}, abstract = {The titanium dioxide rutile (0 1 1) (equivalent to (1 0 1)) surface reconstructs to a stable (2 $\times$ 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 $\times$ 1) phase. Anti-phase domain boundaries in the (2 $\times$ 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 $\times$ 1) periodicity in {STM.}}, number = {19}, journal = SuSci, author = {Olga Dulub and Cristiana Di Valentin and Annabella Selloni and Ulrike Diebold}, month = oct, year = {2006}, pages = {4407--4417} }, @article{schmid_oxygen-deficient_2006, title = {Oxygen-deficient line defects in an ultrathin aluminum oxide film}, volume = {97}, doi = {10.1103/PhysRevLett.97.046101}, abstract = {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.}, number = {4}, journal = PRL, author = {M. Schmid and M. Shishkin and G. Kresse and E. Napetschnig and P. Varga and M. Kulawik and N. Nilius and {H.-P.} Rust and {H.-J.} Freund}, month = jul, year = {2006}, pages = {046101--4} }, @article{gustafson_oxygen-induced_2006, title = {Oxygen-induced step bunching and faceting of {Rh(553)}: Experiment and ab initio calculations}, volume = {74}, doi = {10.1103/PhysRevB.74.035401}, abstract = {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\textendash{}6 mbar and a temperature of 380 {$^\circ$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.}, number = {3}, journal = PRB, author = {J. Gustafson and A. Resta and A. Mikkelsen and R. Westerstr\"{o}m and J. N. Andersen and E. Lundgren and J. Weissenrieder and M. Schmid and P. Varga and N. Kasper and X. Torrelles and S. Ferrer and F. Mittendorfer and G. Kresse}, month = jul, year = {2006}, pages = {035401--7} }, @article{knapp_unusual_2006, title = {Unusual Process of Water Formation on {RuO$_2$(110)} by Hydrogen Exposure at Room Temperature}, volume = {110}, doi = {10.1021/jp0626622}, abstract = {The reduction mechanism of the {RuO$_2$(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 {RuO$_2$(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.}, number = {29}, journal = JPCB, author = {M. Knapp and D. Crihan and A. P. Seitsonen and A. Resta and E. Lundgren and J. N. Andersen and M. Schmid and P. Varga and H. Over}, month = jul, year = {2006}, pages = {14007--14010} }, @article{klikovits_kinetics_2006, title = {Kinetics of the Reduction of the {Rh}(111) Surface Oxide: Linking Spectroscopy and Atomic-Scale Information}, volume = {110}, doi = {10.1021/jp0611875}, abstract = {The reduction of the surface oxide on Rh(111) by H$_2$ was observed in situ by scanning tunneling microscopy {(STM)} and high-resolution core level spectroscopy {(HRCLS).} At room temperature, H$_2$ 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 H$_2$ 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.}, number = {20}, journal = JPCB, author = {J. Klikovits and M. Schmid and J. Gustafson and A. Mikkelsen and A. Resta and E. Lundgren and J. N. Andersen and P. Varga}, month = may, year = {2006}, pages = {9966--9975} }, @article{biedermann_coexistence_2006, title = {Coexistence of fcc- and bcc-like crystal structures in ultrathin {Fe} films grown on {Cu}(111)}, volume = {73}, doi = {10.1103/PhysRevB.73.165418}, abstract = {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\textendash{}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.}, number = {16}, journal = PRB, author = {A. Biedermann and W. Rupp and M. Schmid and P. Varga}, month = apr, year = {2006}, pages = {165418--16} }, @article{schmid_structure_2006, title = {Structure of {Ag}(111)-p(4 $\times$ 4)-{O:} No Silver Oxide}, volume = {96}, doi = {10.1103/PhysRevLett.96.146102}, abstract = {The structure of the oxygen-induced p(4$\times$4) reconstruction of Ag(111) is determined by a combination of scanning tunneling microscopy, surface x-ray diffraction, core level spectroscopy, and density functional theory. We demonstrate that all previous models of this surface structure are incorrect and propose a new model which is able to explain all our experimental findings but has no resemblance to bulk silver oxide. We also shed some light on the limitations of current density functional theories and the potential role of van der Waals interactions in the stabilization of oxygen-induced surface reconstructions of noble metals.}, number = {14}, journal = PRL, author = {M. Schmid and A. Reicho and A. Stierle and I. Costina and J. Klikovits and P. Kosteln\'{\i}k and O. Dubay and G. Kresse and J. Gustafson and E. Lundgren and J. N. Andersen and H. Dosch and P. Varga}, month = apr, year = {2006}, pages = {146102--4} }, @article{batzill_tuningchemical_2006, title = {Tuning the chemical functionality of a gas sensitive material: Water adsorption on {SnO$_2$(101)}}, volume = {600}, doi = {10.1016/j.susc.2005.11.034}, abstract = {Photoemission and density functional theory studies show that water adsorbs dissociatively on the {SnO$_2$(1} 0 1) surface in the presence of terminating oxygen atoms and molecularly if these surface oxygen atoms are removed. The different chemical surface responses of these two bulk terminations of {SnO$_2$} also change the water induced band bending and consequently the conductivity of the gas sensing material.}, number = {4}, journal = SuSci, author = {Matthias Batzill and Wolfgang Bergermayer and Isao Tanaka and Ulrike Diebold}, month = feb, year = {2006}, pages = {29--32} }, @article{costina_combined_2006, title = {Combined {STM,} {LEED} and {DFT} study of {Ag}(100) exposed to oxygen near atmospheric pressures}, volume = {600}, doi = {10.1016/j.susc.2005.11.020}, abstract = {We have investigated the interaction of molecular oxygen with the Ag(100) surface in a temperature range from 130 K to 470 K and an oxygen partial pressure ranging up to 10 mbar by scanning tunneling microscopy, low electron energy diffraction, Auger electron spectroscopy and ab initio density functional calculations. We find that at 130 K, following oxygen exposures of 6000 Langmuirs O$_2$, the individual oxygen atoms are randomly distributed on the surface. When the sample is exposed to 10 mbar O$_2$ at room temperature, small, p(2 $\times$ 2) reconstructed patches are formed on the surface. After oxidation at [approximate]470 K and 10 mbar O$_2$ pressure the surface undergoes a c(4 $\times$ 6) reconstruction coexisting with a (6 $\times$ 6) superstructure. By ab initio thermodynamic calculations it is shown that the c(4 $\times$ 6) reconstruction is an oxygen adsorption induced superstructure which is thermodynamically stable for an intermediate range of oxygen chemical potential.}, number = {3}, journal = SuSci, author = {I. Costina and M. Schmid and H. Schiechl and M. Gajdos and A. Stierle and S. Kumaragurubaran and J. Hafner and H. Dosch and P. Varga}, month = feb, year = {2006}, pages = {617--624} }, @article{gabasch_growth_2006, title = {Growth and decay of the {Pd(111)-Pd$_5$O$_4$} surface oxide: Pressure-dependent kinetics and structural aspects}, volume = {600}, doi = {10.1016/j.susc.2005.09.052}, abstract = {Growth and decomposition of the {Pd$_5$O$_4$} surface oxide on Pd(111) were studied at sample temperatures between 573 and 683 K and O$_2$ gas pressures between 10{\textless}sup{\textgreater}-7{\textless}/sup{\textgreater} and 6 $\times$ 10{\textless}sup{\textgreater}-5{\textless}/sup{\textgreater} mbar, by means of an effusive O$_2$ beam from a capillary array doser, scanning tunnelling microscopy {(STM)} and thermal desorption spectrometry {(TDS).} Exposures beyond the p(2 $\times$ {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 $\times$ 10{\textless}sup{\textgreater}-6{\textless}/sup{\textgreater} 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 O$_2$ pressure pulse and then following further growth kinetics in the lower (10{\textless}sup{\textgreater}-6{\textless}/sup{\textgreater} 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 $\times$ {2)O} structure and these disordered seams. Among the intermediate phases one specifically stable phase was detected both during growth and decomposition of the {Pd$_5$O$_4$} layer. It is hexagonal with a distance of about 0.62 nm between the protrusions. Its well-ordered form is a ($\surd{}$67 x {$\surd{}$67)R12.1$^\circ$} superstructure. Isothermal decay of the {Pd$_5$O$_4$} oxide layer at 693 K involves at first a rearrangement into the ($\surd{}$67 x {$\surd{}$67)R12.1$^\circ$} 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 $\times$ {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.}, number = {1}, journal = SuSci, author = {Harald Gabasch and Werner Unterberger and Konrad Hayek and Bernhard Kl\"{o}tzer and Georg Kresse and Christof Klein and Michael Schmid and Peter Varga}, year = {2006}, pages = {205--218} }, @article{wang_enhanced_2006, title = {Enhanced tunneling magnetoresistance and high-spin polarization at room temperature in a polystyrene-coated {Fe$_3$O$_4$} granular system}, volume = {73}, doi = {10.1103/PhysRevB.73.134412}, abstract = {Polystyrene-coated {Fe$_3$O$_4$} 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 {Fe$_3$O$_4$.} The estimated spin polarization P is about 54\% and 83\% at room temperature and 110 K, respectively.}, number = {13}, journal = PRB, author = {Wendong Wang and Minhui Yu and Matthias Batzill and Jibao He and Ulrike Diebold and Jinke Tang}, year = {2006}, pages = {134412} }, @article{batzill_influence_2006, title = {Influence of Nitrogen Doping on the Defect Formation and Surface Properties of {TiO$_2$} Rutile and Anatase}, volume = {96}, doi = {10.1103/PhysRevLett.96.026103}, abstract = {Nitrogen doping-induced changes in the electronic properties, defect formation, and surface structure of {TiO$_2$} 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$\times$2 reconstruction of the rutile (110) surface. This thermal instability may degrade the catalyst during applications.}, number = {2}, journal = PRL, author = {Matthias Batzill and Erie Morales and Ulrike Diebold}, year = {2006}, pages = {026103} }, @article{lundgren_surface_2006, title = {Surface oxides on close-packed surfaces of late transition metals}, volume = {18}, abstract = {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.}, number = {30}, journal = JPCM, author = {Edvin Lundgren and Anders Mikkelsen and Jesper N. Andersen and Georg Kresse and Michael Schmid and Peter Varga}, year = {2006}, pages = {R481--R499} }, @article{gong_stepsanatase_2006, title = {Steps on anatase {TiO$_2$(101)}}, volume = {5}, doi = {10.1038/nmat1695}, abstract = {Surface defects strongly influence the surface chemistry of metal oxides, and a detailed picture of defect structures may help to understand reactivity and overall materials performance in many applications. We report first-principles calculations of step edges, the most common intrinsic defects on surfaces (and probably the predominant ones on nanoparticles). We have determined the structure, energetics, and chemistry of step edges on the (101) surface of {TiO$_2$} anatase, an important photocatalytic material. Scanning tunnelling microscopy measurements of step-edge configurations and the contrast in atomically resolved images agree remarkably well with the theoretical predictions. Step-edge formation energies as well as the adsorption energies of water scale with the surface energy of the step facet, a trend that is expected to generally hold for metal oxide surfaces. Depending on the terrace/step configuration, this can lead to a situation where a step is less reactive than the flat terrace.}, number = {8}, journal = NatMat, author = {Xue-Qing Gong and Annabella Selloni and Matthias Batzill and Ulrike Diebold}, year = {2006}, pages = {665--670} }, @article{batzill_tuning_2006, title = {Tuning surface properties of {SnO$_2$(101)} by reduction}, volume = {67}, doi = {10.1016/j.jpcs.2006.05.042}, abstract = {The {SnO$_2$(1} 0 1) surface can be prepared with a {SnO$_2$} or {SnO} composition and consequently the surface Sn-atoms are either in a {Sn(II)} or {Sn(IV)} charge state. For a {Sn(II)} surface, Sn-5s derived surface states are identified by resonant, angle resolved photoemission spectroscopy {(ARUPS).} The differences in the interface properties of the {Sn(II)} and {Sn(IV)} surfaces of {SnO$_2$(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 $\approx$1 {eV} with respect to the Fermi-level of the substrate. Density functional theory calculations predict dissociation of water on the stoichiometric {(Sn(IV))} surface but only weak molecular adsorption on the reduced {Sn(II)} surface. These predictions are in agreement with {ARUPS} measurements that show that at 160 K no water adsorbs on the reduced surface but adsorbs dissociatively on the stoichiometric surface. A strong adsorbate induced band bending is also observed for water adsorption on the stoichiometric surface that is likely associated with the formation of surface hydroxyls.}, number = {9-10}, journal = {Journal of Physics and Chemistry of Solids}, author = {Matthias Batzill and Khabibulakh Katsiev and James M. Burst and Yaroslav Losovyj and Wolfgang Bergermayer and Isao Tanaka and Ulrike Diebold}, year = {2006}, pages = {1923--1929} }, @article{batzill_characterizing_2006, title = {Characterizing solid state gas responses using surface charging in photoemission: water adsorption on {SnO$_2$(101)}}, volume = {18}, doi = {10.1088/0953-8984/18/8/L03}, abstract = {A novel experimental approach for studying the gas response mechanism of semiconducting gas sensor materials is demonstrated using the example of water adsorption on {SnO$_2$(101).} In this approach, valence band photoemission as a chemical probe is combined with photocurrent induced surface charging as a basis for contactless sample conductivity measurement.}, number = {8}, journal = JPCM, author = {Matthias Batzill and Ulrike Diebold}, year = {2006}, pages = {L129--L134} }, @article{wang_one-dimensional_2005, title = {{One-Dimensional} {PtO$_2$} at {Pt} Steps: Formation and Reaction with {CO}}, volume = {95}, doi = {10.1103/PhysRevLett.95.256102}, abstract = {Using core-level spectroscopy and density functional theory we show that a one-dimensional {(1D)} {PtO$_2$} oxide structure forms at the steps of the {Pt}(332) surface after O$_2$ 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.}, number = {25}, journal = PRL, author = {J. G. Wang and W. X. Li and M. Borg and J. Gustafson and A. Mikkelsen and T. M. Pedersen and E. Lundgren and J. Weissenrieder and J. Klikovits and M. Schmid and B. Hammer and J. N. Andersen}, month = dec, year = {2005}, pages = {256102} }, @article{schiechl_growth_2005, title = {Growth of ultrathin {Fe} films on {Cu}(111) by pulsed laser deposition}, volume = {594}, doi = {10.1016/j.susc.2005.07.016}, abstract = {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.}, number = {1-3}, journal = SuSci, author = {H. Schiechl and G. Rauchbauer and A. Biedermann and M. Schmid and P. Varga}, month = dec, year = {2005}, pages = {120--131} }, @article{beck_mixed_2005, title = {Mixed dissociated/molecular monolayer of water on the {TiO$_2$}(011)-(2 $\times$ 1) surface}, volume = {591}, doi = {10.1016/j.susc.2005.06.021}, abstract = {A combined theoretical and experimental approach is used to study water on the {TiO$_2$(0} 1 1)-(2 $\times$ 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.}, number = {1-3}, journal = SuSci, author = {T. J. Beck and Andreas Klust and Matthias Batzill and Ulrike Diebold and Cristiana Di Valentin and Antonio Tilocca and Annabella Selloni}, month = oct, year = {2005}, pages = {L267--L272} }, @article{batzill_pure_2005, title = {Pure and cobalt-doped {SnO$_2$(101)} films grown by molecular beam epitaxy on {Al$_2$O$_3$}}, volume = {484}, doi = {10.1016/j.tsf.2005.02.016}, abstract = {Pure and Co-doped epitaxial {SnO$_2$} 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 ($\approx$ 800 {$^\circ$C)} only a submonolayer amount of {Sn} adsorbs, indicating a strong adhesion of the first monolayer of tin on the alumina surface. {SnO$_2$} films grown at $\approx$ 400-600 {$^\circ$C} substrate temperature exhibit a {SnO$_2$(101)[010]{\textbar}{\textbar}Al2O3(-1012)[12-10]} epitaxial relationship. Subtle differences in the {XPD} data of {SnO$_2$} films compared to measurements on {SnO$_2$(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 {SnO$_2$} film to compensate for the more than 10\% lattice mismatch between the {SnO$_2$} films and the alumina substrate along the {SnO$_2$[-101]} direction. {CoxSn1-xO$_2$} 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.}, number = {1-2}, journal = {Thin Solid Films}, author = {Matthias Batzill and James M. Burst and Ulrike Diebold}, month = jul, year = {2005}, pages = {132--139} }, @article{di_valentin_adsorption_2005, title = {Adsorption of Water on Reconstructed Rutile {TiO$_2$(011)-(2$\times$1):} {TiO} Double Bonds and Surface Reactivity}, volume = {127}, doi = {10.1021/ja0511624}, abstract = {Recent combined experimental and theoretical studies {(Beck} et al., Phys. Rev. Lett. 2004, 93, 036104) have provided evidence for {TiO} double-bonded titanyl groups on the reconstructed rutile {TiO$_2$(011)-(21)} surface. The adsorption of water on the same surface is now investigated to further probe the properties of these groups, as well as to confirm their existence. Ultraviolet photoemission experiments show that water is adsorbed in molecular form at a sample temperature of 110 K. At the same time, the presence of a 3sigma state in the photoemission spectra and work function measurements indicate a significant amount of hydroxyls within the first monolayer of water. At room temperature, scanning tunneling microscopy {(STM)} suggests that dissociated water is present, and about 30\% of the surface active sites are hydroxylated. These findings are well explained by total energy density functional theory calculations and {CarParrinello} molecular dynamics simulations for water adsorption on the titanyl model of {TiO$_2$(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.}, number = {27}, journal = JACS, author = {Cristiana Di Valentin and Antonio Tilocca and Annabella Selloni and T. J. Beck and Andreas Klust and Matthias Batzill and Yaroslav Losovyj and Ulrike Diebold}, month = jul, year = {2005}, pages = {9895--9903} }, @article{kresse_structure_2005, title = {Structure of the Ultrathin Aluminum Oxide Film on {NiAl(110)}}, volume = {308}, doi = {10.1126/science.1107783}, abstract = {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.}, number = {5727}, journal = {Science}, author = {Georg Kresse and Michael Schmid and Evelyn Napetschnig and Maxim Shishkin and Lukas K\"{o}hler and Peter Varga}, month = jun, year = {2005}, pages = {1440--1442} }, @article{lundgren_surface_2005, title = {The surface oxide as a source of oxygen on {Rh}(111)}, volume = {144-147}, doi = {10.1016/j.elspec.2005.01.004}, abstract = {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.}, journal = JElSpec, author = {E. Lundgren and J. Gustafson and A. Resta and J. Weissenrieder and A. Mikkelsen and J. N. Andersen and L. K\"{o}hler and G. Kresse and J. Klikovits and A. Biederman and M. Schmid and P. Varga}, month = jun, year = {2005}, pages = {367--372} }, @article{cutrufello_optimization_2005, title = {Optimization of synthesis variables in the preparation of active sulfated zirconia catalysts}, volume = {101}, doi = {10.1007/s10562-005-3740-x}, abstract = {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 {H$_2$SO4} solution used in the sulfation step was found to be the most important variable. A systematic change in the concentration of the {H$_2$SO4} solution showed that the optimum acid concentration was 0.25 {N}. When a catalyst prepared with this acid concentration was used, the conversion of n-butane at 200 {$^\circ$C} was 35\% at 5 min t-o-s. This was close to the thermodynamic equilibrium value of 56\% conversion. This maximum was coincident with a catalyst with the highest specific surface area. An increase in the concentration of the {H$_2$SO4} solution above 0.25 {N} resulted in a decrease in both surface area and zirconia crystallinity. {XPS} studies showed a linear relationship between the {H$_2$SO4} solution concentration and the surface sulfur concentration. Bulk concentrations were determined by elemental analysis. The surface area increased to a maximum for a {H$_2$SO4} concentration of 0.25 {N}, while the concentration of bulk sulfur continued to increase when the acid concentration was progressively increased to 2.00 {N}. The use of a mordenite trap in the reactant stream resulted in an increase in n-butane conversion and a decrease in the rate of catalyst deactivation. {XPS} studies showed that the sulfur was present as sulfate species and that the oxidation state was not affected by the reaction.}, number = {1}, journal = CatLett, author = {M. Cutrufello and U. Diebold and R. Gonzalez}, month = may, year = {2005}, pages = {5--13} }, @article{gustafson_structure_2005, title = {Structure of a thin oxide film on {Rh}(100)}, volume = {71}, doi = {10.1103/PhysRevB.71.115442}, abstract = {The initial oxidation of Rh(100) has been studied using high resolution core level spectroscopy, low energy electron diffraction, surface x-ray diffraction, scanning tunneling microscopy, and density functional theory. We report a structural study of an oxygen induced structure displaying a c(8$\times$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.}, number = {11}, journal = PRB, author = {J. Gustafson and A Mikkelsen and M. Borg and J. N. Andersen and E. Lundgren and C. Klein and W. Hofer and M. Schmid and P. Varga and L. K\"{o}hler and G. Kresse and N. Kasper and A. Stierle and H. Dosch}, month = mar, year = {2005}, pages = {115442} }, @article{engelhardt_stm_2005, title = {An {STM} study of growth and alloying of {Cr} on {Ru}(0001) and {CO} adsorption on the alloy}, volume = {578}, doi = {10.1016/j.susc.2005.01.022}, abstract = {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.}, number = {1-3}, journal = SuSci, author = {M. P. Engelhardt and M. Schmid and A. Biedermann and R. Denecke and {H.-P.} Steinr\"{u}ck and P. Varga}, month = mar, year = {2005}, pages = {124--135} }, @article{diebold_dispersed_2005, title = {Dispersed {Au} atoms, supported on {TiO$_2$}(110)}, volume = {578}, doi = {10.1016/j.susc.2005.01.029}, number = {1-3}, journal = SuSci, author = {Ulrike Diebold}, month = mar, year = {2005}, pages = {1--3} }, @article{batzill_gas-phase-dependent_2005, title = {Gas-phase-dependent properties of {SnO$_2$} (110), (100), and (101) single-crystal surfaces: Structure, composition, and electronic properties}, volume = {72}, doi = {10.1103/PhysRevB.72.165414}, abstract = {The dependence of the surface structure, composition, and electronic properties of three low index {SnO$_2$} surfaces on the annealing temperature in vacuum has been investigated experimentally by low energy {He}$^+$ 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 {SnO$_2$} (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 {SnO$_2$} 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 {SnO$_2$} surface, leaving a {SnO} surface layer with a 1$\times$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$\times$1 reconstruction that forms after sputter and annealing cycles was also investigated. For this surface, states that span almost the entire band gap were observed. Resonant photoemission spectroscopy identified all the surface states on the reduced {SnO$_2$} surfaces as {Sn} derived.}, number = {16}, journal = PRB, author = {Matthias Batzill and Khabibulakh Katsiev and James Burst and Ulrike Diebold and Anne Chaka and Bernard Delley}, year = {2005}, pages = {165414}, }, @article{dulub_observation_2005, title = {Observation of the Dynamical Change in a Water Monolayer Adsorbed on a {ZnO} Surface}, volume = {95}, doi = {10.1103/PhysRevLett.95.136101}, abstract = {A combined scanning tunneling microscopy and density-functional theory {(DFT)} study shows a rich structure of water monolayers adsorbed on {ZnO(1} 0 -1 0) at room temperature. Most of the water is in a lowest-energy configuration where every second molecule is dissociated. It coexists with an energetically almost degenerate configuration consisting of a fully molecular water monolayer. Parts of the layer continuously switch back and forth between these two states. {DFT} calculations reveal that water molecules repeatedly associate and dissociate in this sustained dynamical process.}, number = {13}, journal = PRL, author = {Olga Dulub and Bernd Meyer and Ulrike Diebold}, year = {2005}, pages = {136101} }, @article{dulub_growth_2005, title = {Growth of Copper on Single Crystalline {ZnO:} Surface Study of a Model Catalyst}, volume = {36}, doi = {10.1007/s11244-005-7863-5}, abstract = {Copper, vapor-deposited on the polar, Zn-terminated {ZnO(0001)} surface is investigated in view of its suitability as model system for the technologically important {Cu/ZnO} catalyst. The structure and electronic properties of {Cu} clusters on {ZnO(0001)\textendash{}Zn} have been studied with scanning tunneling microscopy {(STM),} low energy electron diffraction {(LEED),} ultraviolet photoelectron spectroscopy {(UPS),} and low-energy {He}$^+$ ion scattering {(LEIS).} At room temperature copper grows as two-dimensional {(2D)} clusters only at very low coverages of 0.001\textendash{}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)\textendash{}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\textendash{}5 {ML} {Cu} deposits on the {ZnO(0001)\textendash{}Zn} surface in 10-6 mbar O$_2$ results in the formation of a ($\surd{}$3 $\times$ {$\surd{}$3)R30$^\circ$} superstructure with respect to the {ZnO} lattice. This superstructure likely contains {Cu}+ sites. The suitability of the different surface morphologies to probe specific sites that are thought to be active for catalytic processes is discussed.}, number = {1}, journal = TopCatal, author = {Olga Dulub and Matthias Batzill and Ulrike Diebold}, year = {2005}, pages = {65--76} }, @article{batzill_surface_2005, title = {The surface and materials science of tin oxide}, volume = {79}, doi = {10.1016/j.progsurf.2005.09.002}, abstract = {The study of tin oxide is motivated by its applications as a solid state gas sensor material, oxidation catalyst, and transparent conductor. This review describes the physical and chemical properties that make tin oxide a suitable material for these purposes. The emphasis is on surface science studies of single crystal surfaces, but selected studies on powder and polycrystalline films are also incorporated in order to provide connecting points between surface science studies with the broader field of materials science of tin oxide. The key for understanding many aspects of {SnO$_2$} surface properties is the dual valency of Sn. The dual valency facilitates a reversible transformation of the surface composition from stoichiometric surfaces with Sn4+ surface cations into a reduced surface with Sn2+ surface cations depending on the oxygen chemical potential of the system. Reduction of the surface modifies the surface electronic structure by formation of {Sn} 5s derived surface states that lie deep within the band gap and also cause a lowering of the work function. The gas sensing mechanism appears, however, only to be indirectly influenced by the surface composition of {SnO$_2$.} Critical for triggering a gas response are not the lattice oxygen concentration but chemisorbed (or ionosorbed) oxygen and other molecules with a net electric charge. Band bending induced by charged molecules cause the increase or decrease in surface conductivity responsible for the gas response signal. In most applications tin oxide is modified by additives to either increase the charge carrier concentration by donor atoms, or to increase the gas sensitivity or the catalytic activity by metal additives. Some of the basic concepts by which additives modify the gas sensing and catalytic properties of {SnO$_2$} are discussed and the few surface science studies of doped {SnO$_2$} are reviewed. Epitaxial {SnO$_2$} films may facilitate the surface science studies of doped films in the future. To this end film growth on titania, alumina, and {Pt}(111) is reviewed. Thin films on alumina also make promising test systems for probing gas sensing behavior. Molecular adsorption and reaction studies on {SnO$_2$} surfaces have been hampered by the challenges of preparing well-characterized surfaces. Nevertheless some experimental and theoretical studies have been performed and are reviewed. Of particular interest in these studies was the influence of the surface composition on its chemical properties. Finally, the variety of recently synthesized tin oxide nanoscopic materials is summarized.}, number = {2-4}, journal = ProgSuSci, author = {Matthias Batzill and Ulrike Diebold}, year = {2005}, pages = {47--154} }, @article{assmann_understandingstructural_2005, title = {Understanding the Structural Deactivation of Ruthenium Catalysts on an Atomic Scale under both Oxidizing and Reducing Conditions}, volume = {44}, doi = {10.1002/anie.200461805}, abstract = {The surface-science approach coupled with industrial catalyst research offers a synergistic strategy to improve the performance of industrial catalysts. The poorly understood microscopic processes that determine the structural deactivation of ruthenium-based catalysts during {CO} oxidation have been elucidated. Based on these results measures are proposed to improve the performance of ruthenium catalysts.}, number = {6}, journal = AngChIE, author = {Jens A\ss{}mann and Daniela Crihan and Marcus Knapp and Edvin Lundgren and Elke L\"{o}ffler and Martin Muhler and Vijay Narkhede and Herbert Over and Michael Schmid and Ari P. Seitsonen and Peter Varga}, year = {2005}, pages = {917--920} }, @article{koehler_high-coverage_2004, title = {{High-Coverage} Oxygen Structures on {Rh}(111): Adsorbate Repulsion and Site Preference Is Not Enough}, volume = {93}, doi = {10.1103/PhysRevLett.93.266103}, abstract = {A new {O} induced structure on Rh(111) displaying a {(2sqrt[3]$\times$2sqrt[3])R30$^\circ$} periodicity with an oxygen coverage of 2/3 has been studied by high resolution core level spectroscopy, scanning tunneling microscopy, and density functional theory. Although {O} favors fcc hollow sites in all other known phases, it occupies both fcc and hcp sites in this structure, which cannot be explained by pairwise adsorbate repulsion only. Both the {(2sqrt[3]$\times$2sqrt[3])R30$^\circ$} and {(2$\times$2)-3O} structures also exemplify that density-of-states contrast can lead to oxygen adatoms appearing as protrusions in scanning tunneling microscopy images.}, number = {26}, journal = PRL, author = {L. K\"{o}hler and G. Kresse and M. Schmid and E. Lundgren and J. Gustafson and A. Mikkelsen and M. Borg and J. Yuhara and J. N. Andersen and M. Marsman and P. Varga}, month = dec, year = {2004}, pages = {266103} }, @article{batzill_tuningoxide/organic_2004, title = {Tuning the oxide/organic interface: Benzene on {SnO}$_2$(101)}, volume = {85}, doi = {10.1063/1.1831565}, abstract = {Two different {SnO$_2$(101)} bulk terminations have been prepared in order to demonstrate the impact of the oxide surface composition on the interface properties between {SnO$_2$(101)} and an organic film. The change in work function causes a rigid shift of the molecular orbitals of the condensed organic film by 1 {eV} with respect to the valence band of {SnO$_2$.} 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.}, number = {23}, journal = APL, author = {Matthias Batzill and Khabibulakh Katsiev and Ulrike Diebold}, month = dec, year = {2004}, pages = {5766--5768} }, @article{klein_structure_2004, title = {Structure of the cobalt-filled missing-row reconstruction of {Pt}(110)}, volume = {70}, doi = {10.1103/PhysRevB.70.153403}, abstract = {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.}, number = {15}, journal = PRB, author = {C. Klein and R. Koller and E. Lundgren and F. M\'{a}ca and J. Redinger and M. Schmid and P. Varga}, month = oct, year = {2004}, pages = {153403} }, @article{diebold_atomic-scale_2004, title = {Atomic-scale properties of low-index {ZnO} surfaces}, volume = {237}, doi = {10.1016/j.apsusc.2004.06.040}, abstract = {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.}, number = {1-4}, journal = APSS, author = {Ulrike Diebold and Lynn Vogel Koplitz and Olga Dulub}, month = oct, year = {2004}, pages = {336--342} }, @article{biedermann_reconstruction_2004, title = {Reconstruction of the clean and {H} covered "magnetic live surface layer" of {Fe} films grown on {Cu}(100)}, volume = {563}, doi = {10.1016/j.susc.2004.06.150}, abstract = {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 $\times$ 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 H$_2$ decorates the p4g(2 $\times$ 2) surface reconstruction, while higher hydrogen doses transform the surface reconstruction to p(2 $\times$ 1).}, number = {1-3}, journal = SuSci, author = {Albert Biedermann and Rupert Tscheliessnig and Christof Klein and Michael Schmid and Peter Varga}, month = aug, year = {2004}, pages = {110--126} }, @article{yuhara_atomic_2004, title = {Atomic structure of an {Al-Co-Ni} decagonal quasicrystalline surface}, volume = {70}, doi = {10.1103/PhysRevB.70.024203}, abstract = {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$\pm{}$1)$\times$10{\textasciicircum}14 cm-2 . The surface densities of {Al} and {TM} (transition metal, i.e., {Co} and {Ni}) are determined as (8$\pm{}$1)$\times$10{\textasciicircum}14 cm-2 and (1.0$\pm{}$0.2)$\times$10{\textasciicircum}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 $\approx{}$0.12 nm high protrusions in the {STM} images are attributed to local oxide clusters due to their {STS} spectra different from the corrugation maxima and through in situ {STM} observations during exposure to O$_2$ gas at 2$\times$10{\textasciicircum}-6 Pa at {RT.}}, number = {2}, journal = PRB, author = {J. Yuhara and J. Klikovits and M. Schmid and P. Varga and Y. Yokoyama and T. Shishido and K. Soda}, month = jul, year = {2004}, pages = {024203} }, @article{napetschnig_growth_2004, title = {Growth of Ce on {Rh}(111)}, volume = {556}, doi = {10.1016/j.susc.2004.03.006}, abstract = {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 $\times$ 2) superstructure is observed. Above 0.4 {ML,} {Rh} is diffusing through pinholes into the film, forming an unstructured mixed layer. Annealing at 250 {$^\circ$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 $\times$ 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 $\times$ 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 $\times$ 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 \AA{} 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.}, number = {1}, journal = SuSci, author = {E. Napetschnig and M. Schmid and P. Varga}, month = may, year = {2004}, pages = {1--10} }, @article{gustafson_self-limited_2004, title = {Self-Limited Growth of a Thin Oxide Layer on {Rh}(111)}, volume = {92}, doi = {10.1103/PhysRevLett.92.126102}, abstract = {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.}, number = {12}, journal = PRL, author = {J. Gustafson and A. Mikkelsen and M. Borg and E. Lundgren and L. K\"{o}hler and G. Kresse and M. Schmid and P. Varga and J. Yuhara and X. Torrelles and C. Quir\'{o}s and J. N. Andersen}, month = mar, year = {2004}, pages = {126102} }, @article{biedermann_local_2004, title = {Local atomic structure of ultra-thin {Fe} films grown on {Cu}(100)}, volume = {78}, doi = {10.1007/s00339-003-2435-7}, abstract = {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\textendash{}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.}, number = {6}, journal = ApPhA, author = {A. Biedermann and R. Tscheliessnig and M. Schmid and P. Varga}, month = mar, year = {2004}, pages = {807--816} }, @article{beck_surface_2004, title = {Surface Structure of {TiO$_2$(011)-(2$\times$1)}}, volume = {93}, doi = {10.1103/PhysRevLett.93.036104}, abstract = {A combined experimental and first principles study of the (2$\times$1)-reconstructed rutile {TiO$_2$(011)} surface is presented. Our results provide evidence that the surface structure is described by a model that includes onefold coordinated (titanyl) oxygen atoms giving rise to double bonded {Ti=O} species. These species should play a special role in the enhanced photocatalytic activity of the {TiO$_2$(011)} surface.}, number = {3}, journal = PRL, author = {T. Beck and Andreas Klust and Matthias Batzill and Ulrike Diebold and Cristiana Di Valentin and Annabella Selloni}, year = {2004}, pages = {036104} }, @article{over_visualization_2004, title = {Visualization of Atomic Processes on Ruthenium Dioxide using Scanning Tunneling Microscopy}, volume = {5}, doi = {10.1002/cphc.200300833}, abstract = {The visualization of surface reactions on the atomic scale provides direct insight into the microscopic reaction steps taking place in a catalytic reaction at a (model) catalyst's surface. Employing the technique of scanning tunneling microscopy {(STM),} we investigated the {CO} oxidation reaction over the {RuO$_2$(110)} and {RuO$_2$(100)} surfaces. For both surfaces the protruding bridging {O} atoms are imaged in {STM} as bright features. The reaction mechanism is identical on both orientations of {RuO$_2$.} {CO} molecules adsorb on the undercoordinated surface Ru atoms from where they recombine with undercoordinated {O} atoms to form {CO$_2$} at the oxide surface. In contrast to the {RuO$_2$(110)} surface, the {RuO$_2$(100)} surface stabilizes also a catalytically inactive c(2$\times$2) surface phase onto which {CO} is not able to adsorb above 100 K. We argue that this inactive {RuO$_2$(100)-c(2$\times$2)} phase may play an important role in the deactivation of {RuO$_2$} catalysts in the electrochemical Cl2 evolution and other heterogeneous reactions.}, number = {2}, journal = {{ChemPhysChem}}, author = {H. Over and M. Knapp and E. Lundgren and A. P. Seitsonen and M. Schmid and P. Varga}, year = {2004}, pages = {167--174} }, @article{meyer_partial_2004, title = {Partial Dissociation of Water Leads to Stable Superstructures on the Surface of Zinc Oxide}, volume = {43}, doi = {10.1002/anie.200461696}, abstract = {Half-dissociated: Experimental and computational findings conclude that water forms a highly ordered superstructure on defect-free surfaces of zinc oxide, in which every second water molecule is dissociated (see picture). The results are of general relevance for heterogeneous catalysis.}, number = {48}, journal = AngChIE, author = {Bernd Meyer and Dominik Marx and Olga Dulub and Ulrike Diebold and Martin Kunat and Deler Langenberg and Christof W\"{o}ll}, year = {2004}, pages = {6641--6645} }, @article{batzill_surface_2004, title = {Surface oxygen chemistry of a gas-sensing material: {SnO$_2$(101)}}, volume = {65}, abstract = {Experimental techniques and density-functional theory have been employed to identify the surface composition and structure of {SnO$_2$(101).} The stoichiometric {Sn$^{4+}$O$_2^{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.}, number = {1}, journal = EPL, author = {M. Batzill and A. M. Chaka and U. Diebold}, year = {2004}, pages = {61--67} }, @article{klein_vanadium_2003, title = {Vanadium surface oxides on {Pd}(111): A structural analysis}, volume = {68}, doi = {10.1103/PhysRevB.68.235416}, abstract = {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 {$^\circ$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 $\approx{}$0.2 {ML} (monolayer). At 0.31 {ML} the entire surface is covered by this well-ordered open (4$\times$4) structure. Annealing this structure in H$_2$ atmosphere transforms the phase into a {V2O3} surface oxide with (2$\times$2) periodicity, whose optimal coverage is reached at 0.5 {ML} vanadium. Models for both ordered structures have been suggested before on the basis of ab initio density-functional theory {(DFT)} calculations and molecular-dynamics simulations and these models are now unambiguously confirmed by quantitative low-energy electron-diffraction {(LEED)} analyses. In the (4$\times$4) phase, the {V} atoms are surrounded by four oxygen atoms in an unusual tetrahedral coordination leading to a {V5O14} stoichiometry. This tetrahedral coordination allows the oxide to adopt open loosely packed two-dimensional {(2D)} and {1D} structures, which are stabilized by the surface-oxide interface energy. Furthermore, it is shown that state of the art {DFT} calculations can indeed predict complex structures exactly as well as that modern quantitative {LEED} is capable of dealing with very large unit cells.}, number = {23}, journal = PRB, author = {C. Klein and G. Kresse and S. Surnev and F. P. Netzer and M. Schmid and P. Varga}, month = dec, year = {2003}, pages = {235416} }, @article{gauthier_surface_2003, title = {Surface structure of the missing-row reconstruction of {VC$_{0.8}$(110):} a scanning tunneling microscopy analysis}, volume = {547}, doi = {10.1016/j.susc.2003.10.016}, abstract = {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 $\times$ 1) or (4 $\times$ 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 $\times$ 2) and (4 $\times$ 2) superstructure cells, which also show up in {LEED.} We attribute these additional periodicities to ordering of carbon vacancies in the surface rows.}, number = {3}, journal = SuSci, author = {Y. Gauthier and M. Schmid and W. Hebenstreit and P. Varga}, month = dec, year = {2003}, pages = {394--402} }, @article{kalousek_slowing_2003, title = {Slowing down adatom diffusion by an adsorbate: {Co} on {Pt}(111) with and without preadsorbed {CO}}, volume = {68}, doi = {10.1103/PhysRevB.68.233401}, abstract = {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 \textquotedblleft{}crowded\textquotedblright{} surface.}, number = {23}, journal = PRB, author = {R. Kalousek and M. Schmid and A. Hammerschmid and E. Lundgren and P. Varga}, month = dec, year = {2003}, pages = {233401} }, @article{diebold_one_2003, title = {One step towards bridging the materials gap: surface studies of {TiO$_2$} anatase}, volume = {85}, doi = {10.1016/S0920-5861(03)00378-X}, abstract = {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 {TiO$_2$} rutile. The anatase (1 0 1) surface is stable in a (1$\times$1) configuration. It exhibits twofold coordinated (bridging) oxygen atoms and fivefold coordinated {Ti} atoms with a density comparable to the one found on rutile (1 1 0). Step edges are terminated by fourfold coordinated {Ti} sites. In contrast to rutile (1 1 0), anatase (1 0 1) does not show a strong tendency for losing twofold coordinated oxygen atoms upon annealing in ultrahigh vacuum. The apparent lack of point defects is also reflected in the adsorption/desorption behavior of water and methanol. The anatase (1 0 0) surface has the second-lowest surface energy and tends to from a (1$\times$2) reconstruction. A model with (1 0 1)-oriented microfacets agrees with the observed features in atomically-resolved {STM} images. The (0 0 1) surface forms a (1$\times$4) reconstruction that is well explained by an [`]ad-molecule' model predicted from density functional theory calculations. A (1$\times$3) reconstruction was observed for the anatase (1 0 3) surface.}, number = {2-4}, journal = CatalTod, author = {U. Diebold and N. Ruzycki and G. S. Herman and A. Selloni}, month = oct, year = {2003}, pages = {93--100} }, @article{yuhara_two-dimensional_2003, title = {Two-dimensional alloy of immiscible metals: Single and binary monolayer films of {Pb} and {Sn} on {Rh}(111)}, volume = {67}, doi = {10.1103/PhysRevB.67.195407}, abstract = {The single and binary metal films of {Pb} and {Sn} on Rh(111) have been studied at room temperature by scanning tunneling microscopy {(STM),} low-energy electron diffraction, and Auger electron spectroscopy. Both {Pb} and {Sn} are mobile at low coverage and form commensurate overlayers of {(4$\times$4)-Pb} and {c(2$\times$4)-Sn,} respectively. From atomically resolved {STM} images, the atomic arrangements of {(4$\times$4)-Pb} and {c(2$\times$4)-Sn} have been identified to be hexagonal and rectangular structures, respectively. With increasing coverage, both commensurate phases change into incommensurate phases followed by island formation {(Stranski-Krastanov} growth). This shows that Rh(111) is static and inert enough to support two-dimensional {(2D)} phases of {Pb} and/or {Sn} without alloying at room temperature. The {Pb-Sn} bimetallic film on Rh(111) forms an ordered {2D} alloy of {PbSn3} with an incommensurate structure close to (sqrt[7]$\times$sqrt[7]), contrary to the immiscibility of {Pb} and {Sn} in the bulk. From atomically resolved {STM} images, the atomic arrangement of this {(sqrt[7]$\times$sqrt[7])-(Pb,Sn)} structure has been determined.}, number = {19}, journal = PRB, author = {J. Yuhara and M. Schmid and P. Varga}, month = may, year = {2003}, pages = {195407} }, @article{koller_surface_2003, title = {Surface structure and composition of {Pt$_{50}$Rh$_{50}$(110):} room temperature analysis of the (1 $\times$ 3) missing-row reconstruction}, volume = {530}, doi = {10.1016/S0039-6028(03)00381-9}, abstract = {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 $\times$ 2) missing-row structure, {Pt50Rh50(110)} exhibits a (1 $\times$ 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.}, number = {3}, journal = SuSci, author = {R. Koller and Y. Gauthier and C. Klein and M. {De Santis} and M. Schmid and P. Varga}, month = may, year = {2003}, pages = {121--135} }, @article{klein_when_2003, title = {When Scanning Tunneling Microscopy Gets the Wrong Adsorption Site: {H} on {Rh}(100)}, volume = {90}, doi = {10.1103/PhysRevLett.90.176101}, abstract = {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.}, number = {17}, journal = PRL, author = {C. Klein and A. Eichler and E. L. D. Hebenstreit and G. Pauer and R. Koller and A. Winkler and M. Schmid and P. Varga}, month = apr, year = {2003}, pages = {176101} }, @article{batzill_surface_2003, title = {Surface morphologies of {SnO$_2$(110)}}, volume = {529}, doi = {10.1016/S0039-6028(03)00357-1}, abstract = {Scanning tunneling microscopy {(STM),} ion scattering spectroscopy {(ISS),} and low energy electron diffraction {(LEED)} was used to investigate the surface morphology of {SnO$_2$(1} 1 0) for different preparation conditions. Annealing in 10-3 mbar oxygen results in a 1 $\times$ 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 $\times$ 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 $\times$ 2) and 4 $\times$ 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 $\times$ 1 periodicity. This implies that the c(2 $\times$ 2)-structure is associated with adislands at the surface. Annealing to 1100 K resulted in the formation of a 4 $\times$ 1 surface only. This surface exhibits terraces with meandering step edges and antiphase domain boundaries of the 4 $\times$ 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 $\times$ 1 structure and the surface looses its long-range order. This causes a 1 $\times$ 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.}, number = {3}, journal = SuSci, author = {Matthias Batzill and Khabibulakh Katsiev and Ulrike Diebold}, month = apr, year = {2003}, pages = {295--311} }, @article{ruzycki_scanning_2003, title = {Scanning tunneling microscopy study of the anatase (100) surface}, volume = {529}, doi = {10.1016/S0039-6028(03)00117-1}, abstract = {Scanning tunneling microscopy {(STM)} has been used to investigate the structure of the {TiO$_2$} anatase (1 0 0) surface. Natural single crystals of anatase were employed; and after several cycles of sputtering and annealing at T=450 {$^\circ$C,} the {TiO$_2$(1} 0 0) surface was free of impurities, and reconstructed to a (1$\times$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 (1 0 1)-oriented microfacets run along the anatase [0 1 0] direction.}, number = {1-2}, journal = SuSci, author = {Nancy Ruzycki and Gregory S. Herman and Lynn A. Boatner and Ulrike Diebold}, month = apr, year = {2003}, pages = {L239--L244} }, @article{kresse_complex_2003, title = {Complex surface reconstructions solved by ab initio molecular dynamics}, volume = {76}, doi = {10.1007/s00339-002-2007-2}, abstract = {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$\times$2) reconstruction of V(110).}, number = {5}, journal = ApPhA, author = {G. Kresse and W. Bergermayer and R. Podloucky and E. Lundgren and R. Koller and M. Schmid and P. Varga}, month = mar, year = {2003}, pages = {701--710} }, @article{diebold_structure_2003, title = {Structure and properties of {TiO$_2$} surfaces: a brief review}, volume = {76}, doi = {10.1007/s00339-002-2004-5}, abstract = {Titanium oxides are used in a wide variety of technological applications where surface properties play a role. {TiO$_2$} surfaces, especially the (110) face of rutile, have become prototypical model systems in the surface science of metal oxides. Reduced {TiO$_2$} single crystals are easy to work with experimentally, and their surfaces have been characterized with virtually all surface-science techniques. Recently, {TiO$_2$} has also been used to refine computational ab initio approaches and to calculate properties of adsorption systems. Scanning tunneling microscopy {(STM)} studies have shown that the surface structure of {TiO$_2$(110)} is more complex than originally anticipated. The reduction state of the sample, i.e. the number and type of bulk defects, as well as the surface treatment (annealing in vacuum vs. annealing in oxygen), can give rise to different structures, such as two different (1$\times$2) reconstructions, a `rosette' overlayer, and crystallographic shear planes. Single point defects can be identified with {STM} and influence the surface chemistry in a variety of ways; the adsorption of water is discussed as one example. The growth of a large number of different metal overlayers has been studied on {TiO$_2$(110).} Some of these studies have been instrumental in furthering the understanding of the `strong metal support interaction' between {group-VIII} metals and {TiO$_2$,} as well as low-temperature oxidation reactions on {TiO$_2$-supported} nanoscopic gold clusters. The growth morphology, interfacial oxidation/reduction reaction, thermal stability, and geometric structure of ultra-thin metal overlayers follow general trends where the most critical parameter is the reactivity of the overlayer metal towards oxygen. It has been shown recently that the technologically more relevant {TiO$_2$} anatase phase can also be made accessible to surface investigations.}, number = {5}, journal = ApPhA, author = {U. Diebold}, month = mar, year = {2003}, pages = {681--687} }, @article{marcus_locus_2003, title = {The Locus of Sulfate Sites on Sulfated Zirconia}, volume = {86}, doi = {10.1023/A:1022680421726}, abstract = {The surface of sulfated zirconia was probed using X-ray photoelectron spectroscopy. It was observed that the entire inventory of sulfur could be completely removed by sputtering the surface using an argon beam. Calibration using a {TiO$_2$(110)} standard resulted in a surface concentration of 2.85 sulfur atoms/nm2. This is in reasonable agreement with a value of 4.15 sulfur atoms/nm2 based on sulfur analysis on the assumption that all of the sulfur was located at the surface. These results suggest that most, if not all, of the sulfur is near or at the surface. When charging was taken into account, we observed that the oxidation state of sulfur did not change following catalyst deactivation during the isomerization of n-butane. We also determined that the entire inventory of sulfur was present as {SO42-.} These results reinforce previous studies suggesting that catalyst deactivation occurs as the result of carbon deposition and not a change in the oxidation state of sulfur.}, number = {4}, journal = CatLett, author = {Rachel Marcus and Ulrike Diebold and Richard D. Gonzalez}, month = mar, year = {2003}, pages = {151--156} }, @article{herman_experimental_2003, title = {Experimental Investigation of the Interaction of Water and Methanol with {Anatase-TiO$_2$(101)}}, volume = {107}, doi = {10.1021/jp0275544}, abstract = {The interaction of water and methanol with well-defined (1 1) terminated surfaces of {anatase-TiO$_2$(101)} were investigated with temperature-programmed desorption {(TPD)} and X-ray photoelectron spectroscopy {(XPS).} For water, three desorption states were observed in the {TPD} spectra at 160, 190, and 250 K. The three desorption peaks were assigned to multilayer water, water adsorbed to 2-fold-coordinated {O}, and water adsorbed to 5-fold-coordinated {Ti}, respectively. The {TPD} spectra for methanol were more complicated. For methanol, five desorption peaks were observed in the {TPD} spectra at 135, 170, 260, 410, and 610 K. The five desorption peaks were assigned to multilayer methanol, methanol adsorbed to 2-fold-coordinated {O}, methanol adsorbed to 5-fold-coordinated {Ti}, methoxy adsorbed to 5-fold-coordinated {Ti}, and methoxy adsorbed to {Ti} at step edges, respectively. The {XPS} results indicated that the adsorbed water and methanol were predominantly bound to the surface in a molecular state, with no evidence for dissociation. Furthermore, the {O} 1s core-level binding energies for water and methanol were found to shift to an 0.75 {eV} lower binding energy for coverages before multilayer desorption is observed in the {TPD} spectra. The {O} 1s core-level binding-energy shift appears to be linear in this region and corresponds to water and methanol bonding to {Ti} cation and {O} anion sites on the surface. The {C} 1s core-level binding energy for methanol was found to remain approximately constant in the same coverage regime.}, number = {12}, journal = JPCB, author = {G. S. Herman and Z. Dohnalek and N. Ruzycki and U. Diebold}, month = mar, year = {2003}, pages = {2788--2795} }, @article{dulub_novel_2003, title = {Novel Stabilization Mechanism on Polar Surfaces: {ZnO(0001)-Zn}}, volume = {90}, doi = {10.1103/PhysRevLett.90.016102}, abstract = {The (1$\times$1) terminated {(0001)-Zn} surface of wurtzite {ZnO} was investigated with scanning tunneling microscopy. The surface is characterized by the presence of nanosized islands with a size-dependent shape and triangular holes with single-height, O-terminated step edges. It is proposed that the resulting overall decrease of the surface Zn concentration stabilizes this polar surface. Ab initio calculations of test geometries predict triangularly shaped reconstructions over a wide range of oxygen and hydrogen chemical potentials. The formation of these reconstructions appears to be electrostatically driven.}, number = {1}, journal = PRL, author = {Olga Dulub and Ulrike Diebold and G. Kresse}, year = {2003}, pages = {016102} }, @article{batzill_influence_2003, title = {Influence of subsurface, charged impurities on the adsorption of chlorine at {TiO$_2$(110)}}, volume = {367}, doi = {10.1016/S0009-2614(02)01635-4}, abstract = {Adsorption of chlorine on {TiO$_2$(1} 1 0) surfaces modified by subsurface impurity atoms was studied by scanning tunneling microscopy. Adsorption was found to be suppressed in the vicinity of positively charged subsurface donor atoms. The reduced adsorption is explained by an increased electron affinity close to the impurity atoms that renders these sites energetically unfavorable for acceptor-like adsorbates.}, number = {3-4}, journal =CPL, author = {Matthias Batzill and Eleonore L. D. Hebenstreit and Wilhelm Hebenstreit and Ulrike Diebold}, year = {2003}, pages = {319--323} }, @article{diebold_surface_2003, title = {The surface science of titanium dioxide}, volume = {48}, doi = {10.1016/S0167-5729(02)00100-0}, abstract = {Titanium dioxide is the most investigated single-crystalline system in the surface science of metal oxides, and the literature on rutile (1 1 0), (1 0 0), (0 0 1), and anatase surfaces is reviewed. This paper starts with a summary of the wide variety of technical fields where {TiO$_2$} is of importance. The bulk structure and bulk defects (as far as relevant to the surface properties) are briefly reviewed. Rules to predict stable oxide surfaces are exemplified on rutile (1 1 0). The surface structure of rutile (1 1 0) is discussed in some detail. Theoretically predicted and experimentally determined relaxations of surface geometries are compared, and defects (step edge orientations, point and line defects, impurities, surface manifestations of crystallographic shear {planes--CSPs)} are discussed, as well as the image contrast in scanning tunneling microscopy {(STM).} The controversy about the correct model for the (1$\times$2) reconstruction appears to be settled. Different surface preparation methods, such as reoxidation of reduced crystals, can cause a drastic effect on surface geometries and morphology, and recommendations for preparing different {TiO$_2$(1} 1 0) surfaces are given. The structure of the {TiO$_2$(1} 0 0)-(1$\times$1) surface is discussed and the proposed models for the (1$\times$3) reconstruction are critically reviewed. Very recent results on anatase (1 0 0) and (1 0 1) surfaces are included. The electronic structure of stoichiometric {TiO$_2$} surfaces is now well understood. Surface defects can be detected with a variety of surface spectroscopies. The vibrational structure is dominated by strong {Fuchs-Kliewer} phonons, and high-resolution electron energy loss spectra often need to be deconvoluted in order to render useful information about adsorbed molecules. The growth of metals {(Li,} {Na}, K, Cs, Ca, {Al}, {Ti}, {V}, Nb, {Cr}, {Mo}, {Mn}, {Fe}, {Co}, {Rh}, {Ir}, {Ni}, {Pd}, {Pt}, {Cu}, Ag, Au) as well as some metal oxides on {TiO$_2$} is reviewed. The tendency to [`]wet' the overlayer, the growth morphology, the epitaxial relationship, and the strength of the interfacial oxidation/reduction reaction all follow clear trends across the periodic table, with the reactivity of the overlayer metal towards oxygen being the most decisive factor. Alkali atoms form ordered superstructures at low coverages. Recent progress in understanding the surface structure of metals in the [`]strong-metal support interaction' {(SMSI)} state is summarized. Literature is reviewed on the adsorption and reaction of a wide variety of inorganic molecules {(H$_2$,} O$_2$, {H$_2$O,} {CO,} {CO$_2$,} N2, {NH3,} {NOx,} sulfur- and halogen-containing molecules, rare gases) as well as organic molecules (carboxylic acids, alcohols, aldehydes and ketones, alkynes, pyridine and its derivates, silanes, methyl halides). The application of {TiO$_2$-based} systems in photo-active devices is discussed, and the results on {UHV-based} photocatalytic studies are summarized. The review ends with a brief conclusion and outlook of {TiO$_2$-based} surface science for the future.}, number = {5-8}, journal = SuSciRep, author = {Ulrike Diebold}, year = {2003}, pages = {53--229} }, @article{kuyanov_dynamics_2003, title = {Dynamics of the {TiO$_2$}(110) surface and step: Onset of defects in the ordered structure}, volume = {68}, doi = {10.1103/PhysRevB.68.233404}, abstract = {Molecular dynamics simulations are carried out on planar and stepped {TiO$_2$} (110) surfaces. The simulations focus on the development of defects in the initially ordered structure as the temperature increases. For the planar surface, bridging oxygen atoms are found to leave their equilibrium positions and subsequently roam one-dimensional lanes formed by adjacent rows of other bridging oxygen atoms, until they become stuck on a previously fivefold-coordinated titanium atom. For the stepped surface, the first defects to occur correspond to titanium-oxygen bonds breaking near the step edge.}, number = {23}, journal = PRB, author = {Igor Kuyanov and Daniel Lacks and Ulrike Diebold}, year = {2003}, pages = {233404} }, @article{kresse_competing_2003, title = {Competing stabilization mechanism for the polar {ZnO(0001)-Zn} surface}, volume = {68}, doi = {10.1103/PhysRevB.68.245409}, abstract = {Density-functional calculations for the {(0001)-Zn} surface of wurtzite {ZnO} are reported. Different stabilization mechanisms, such as metallization of the surface layer, adsorption of {OH} groups or {O} adatoms, the formation of Zn vacancies, and large scale triangular reconstructions are considered. The calculations indicate that isolated Zn vacancies or {O} adatoms are unfavorable compared to triangular reconstructions. In the absence of hydrogen, these triangular features are stable under any realistic temperature and pressure. When hydrogen is present, the reconstruction is lifted, and hydroxyl groups stabilize the ideal otherwise unreconstructed surface. The transition between the unreconstructed hydroxyl covered surface and the triangular shaped features occurs abruptly; {OH} groups lift the reconstruction, but their adsorption is energetically unfavorable on the triangularly reconstructed surface.}, number = {24}, journal = PRB, author = {Georg Kresse and Olga Dulub and Ulrike Diebold}, year = {2003}, pages = {245409} }, @article{koplitz_stm_2003, title = {{STM} Study of Copper Growth on {ZnO(0001)-Zn} and {ZnO(000$\bar{1}$)-O} Surfaces}, volume = {107}, doi = {10.1021/jp0352175}, abstract = {The room-temperature growth of {Cu} on the polar {(0001)-Zn} and {(000$\bar{1}$)-O} surfaces of zinc oxide has been studied with scanning tunneling microscopy {(STM).} Copper grows on the {(0001)-Zn} surface as three-dimensional clusters even at low coverages (0.050.25 monolayers {(ML));} two-dimensional {(2D)} islands are only observed at very low coverages (0.0010.05 {ML).} The average size of the {3D} clusters increases with coverage, and their density increases slowly. Surface roughness and sputter damage change the growth mode to more {2D-like.} The {Cu} clusters are well-separated and exhibit a well-defined hexagonal shape. Equilibrium crystal shape analysis yields an apparent work of adhesion of 3.4 +- 0.1 J/m2 for the largest clusters. On the {(0001\=)-O} surface, formation of two-dimensional {Cu} clusters was observed at coverages of less than 0.1 {ML.}}, number = {38}, journal = JPCB, author = {Lynn Vogel Koplitz and Olga Dulub and Ulrike Diebold}, year = {2003}, pages = {10583--10590} }, @article{batzill_variations_2002, title = {Variations of the local electronic surface properties of {TiO$_2$(110)} induced by intrinsic and extrinsic defects}, volume = {66}, doi = {10.1103/PhysRevB.66.235401}, abstract = {Variation of the local electronic structure at rutile {TiO$_2$(110)} surfaces was studied by scanning tunneling spectroscopy {(STS).} Structural surface features such as step edges, (1$\times$2) reconstructed strands, and their terminations were correlated to changes in tunneling spectra. In particular, band-gap states, associated with a reduced surface, showed characteristic variations. In addition, electronic variations due to extrinsic defects are discussed. Nanometer wide protrusions in constant current scanning tunneling microscopy images were identified in {STS} as local electronic alterations. These features are interpreted to be due to local band bending induced by individual, charged impurity atoms.}, number = {23}, journal = PRB, author = {Matthias Batzill and Khabibulakh Katsiev and Daniel J. Gaspar and Ulrike Diebold}, month = dec, year = {2002}, pages = {235401} }, @article{over_experimental_2002, title = {Experimental and simulated {STM} images of stoichiometric and partially reduced {RuO$_2$(110)} surfaces including adsorbates}, volume = {515}, doi = {10.1016/S0039-6028(02)01853-8}, abstract = {We present experimental and {DFT-simulated} {STM} images of ultrathin {RuO$_2$(110)} films on Ru(0001), including adsorbates such as oxygen and {CO.} We are able to identify the under-coordinated {O} atoms on the {RuO$_2$(110)} surface with {STM,} i.e. the bridging {O} atoms and the on-top {O} atoms. The partial reduction of the {RuO$_2$(110)} surface by {CO} exposure at room temperature leads to a surface where part of the bridging {O} atoms have been removed and some of the vacancies are occupied by bridging {CO.} When dosing 10 L of {CO} at room temperature, all the {RuO$_2$(110)} surface becomes mildly reduced in that all bridging {O} atoms are replaced by bridging {CO} molecules. Annealing the surface to 600 K produces holes on the terraces of such a mildly reduced {RuO$_2$(110)} surface. These pits are not generated by the recombination of lattice {O} with {CO,} but rather these pits are assigned to a complex temperature-induced rearrangement of surface atoms in the topmost {RuO$_2$} double layer of {RuO$_2$(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.}, number = {1}, journal = SuSci, author = {H. Over and A. P. Seitsonen and E. Lundgren and M. Schmid and P. Varga}, month = aug, year = {2002}, pages = {143--156} }, @article{bischoff_scanning_2002, title = {Scanning tunneling spectroscopy on clean and contaminated {V}(001)}, volume = {513}, doi = {10.1016/S0039-6028(02)01783-1}, abstract = {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.}, number = {1}, journal = SuSci, author = {M. M. J. Bischoff and C. Konvicka and A. J. Quinn and M. Schmid and J. Redinger and R. Podloucky and P. Varga and H. van Kempen}, month = jul, year = {2002}, pages = {9--25} }, @article{koller_structure_2002, title = {The structure of the oxygen-induced c(6$\times$2) reconstruction of {V}(110)}, volume = {512}, doi = {10.1016/S0039-6028(02)01722-3}, abstract = {The adsorption of 2.4 Langmuir oxygen on V(110) induces a c(6$\times$2) reconstruction with an oxygen coverage of 0.5 {ML.} Its structure was determined using {STM,} quantitative {LEED} and ab initio density functional calculations in combination with molecular dynamics. Driven by the strong vanadium-oxygen bonding, the vanadium atoms at the surface are significantly rearranged compared to their bulk positions. The reconstructed geometry offers threefold and fourfold coordinated hollow sites, which are partially occupied by oxygen. The large set of structural data derived from {LEED} {I-V} analysis {(R\_Pe=0.11)} and ab initio calculations, as well as the experimental and simulated {STM} images agree well. Additionally the structure of clean V(110) was determined.}, number = {1-2}, journal = SuSci, author = {R. Koller and W. Bergermayer and G. Kresse and C. Konvicka and M. Schmid and J. Redinger and R. Podloucky and P. Varga}, month = jun, year = {2002}, pages = {16--28} }, @article{lundgren_two-dimensional_2002, title = {Two-dimensional oxide on {Pd}(111)}, volume = {88}, doi = {10.1103/PhysRevLett.88.246103}, abstract = {The oxidation of Pd(111) leads to an incommensurate surface oxide, which was studied by the use of scanning tunneling microscopy, surface x-ray diffraction, high resolution core level spectroscopy, and density functional calculations. A combination of these methods reveals a two-dimensional structure having no resemblance to bulk oxides of Pd. Our study also demonstrates how the atomic arrangement of a nontrivial incommensurate surface can be solved by molecular dynamics in a case where experimental techniques alone give no solution.}, number = {24}, journal = PRL, author = {E. Lundgren and G. Kresse and C. Klein and M. Borg and J. N. Andersen and M. {De Santis} and Y. Gauthier and C. Konvicka and M. Schmid and P. Varga}, month = jun, year = {2002}, pages = {246103} }, @article{tang_current-controlled_2002, title = {Current-controlled channel switching and magnetoresistance in an {Fe$_3$C} island film supported on a {Si} substrate}, volume = {91}, doi = {10.1063/1.1447880}, abstract = {A film of magnetic {Fe3C} islands separated by nanochannels of graphite was prepared with pulsed laser deposition on a Si substrate with a native {SiO$_2$} surface. When the temperature is increased above 250 K the resistance suddenly drops because electron conduction switches from the film to the Si inversion layer underneath. The film shows a negative magnetoresistance. The inversion layer exhibits a large positive magnetoresistance. The transition to the low resistance channel can be reversed by applying a large measuring current, making possible current-controlled switching between two types of electron magnetotransport at room temperature.}, journal = JAP, author = {Jinke Tang and Jianbiao Dai and Kaiying Wang and Weilie Zhou and Nancy Ruzycki and Ulrike Diebold}, month = may, year = {2002}, pages = {8411--8413} }, @article{hebenstreit_adsorption_2002, title = {The adsorption of chlorine on {TiO$_2$}(110) studied with scanning tunneling microscopy and photoemission spectroscopy}, volume = {505}, doi = {10.1016/S0039-6028(02)01385-7}, journal = SuSci, author = {E. L. D. Hebenstreit and W. Hebenstreit and H. Geisler and C. A. Ventrice and D. A. Hite and P. T. Sprunger and U. Diebold}, month = may, year = {2002}, pages = {336--348} }, @article{dulub_stm_2002, title = {{STM} study of {Cu} growth on the {ZnO(10$\bar{1}$0)} surface}, volume = {504}, doi = {10.1016/S0039-6028(02)01107-X}, journal = SuSci, author = {Olga Dulub and Lynn A. Boatner and Ulrike Diebold}, month = apr, year = {2002}, pages = {271--281} }, @article{shaikhutdinov_interaction_2002, title = {Interaction of oxygen with palladium deposited on a thin alumina film}, volume = {501}, doi = {10.1016/S0039-6028(01)01850-7}, abstract = {The interaction of oxygen with {Pd} particles, vapor deposited onto a thin alumina film grown on a {NiAl(110)} substrate, was studied by {STM,} {AES,} {LEED,} {XPS,} {TPD} and molecular beam techniques. The results show that O$_2$ 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{\textasciicircum}-1} for 1 and 3-5 nm {Pd} particles studied.}, number = {3}, journal = SuSci, author = {Sh. Shaikhutdinov and M. Heemeier and J. Hoffmann and I. Meusel and B. Richter and M. B\"{a}umer and H. Kuhlenbeck and J. Libuda and H. {-J.} Freund and R. Oldman and S. D. Jackson and C. Konvicka and M. Schmid and P. Varga}, month = apr, year = {2002}, pages = {270--281} }, @article{lundgren_geometry_2002, title = {Geometry of the valence transition induced surface reconstruction of {Sm}(0001)}, volume = {88}, doi = {10.1103/PhysRevLett.88.136102}, abstract = {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.}, number = {13}, journal = PRL, author = {E. Lundgren and J. N. Andersen and R. Nyholm and X. Torrelles and J. Rius and A. Delin and A. Grechnev and O. Eriksson and C. Konvicka and M. Schmid and P. Varga}, month = mar, year = {2002}, pages = {136102} }, @article{lundgren_misfit_2002, title = {A misfit structure in the {Co/Pt(111)} system studied by scanning tunnelling microscopy and embedded atom method calculations}, volume = {498}, doi = {10.1016/S0039-6028(01)01754-X}, abstract = {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].}, number = {3}, journal = SuSci, author = {E. Lundgren and G. Leonardelli and M. Schmid and P. Varga}, month = feb, year = {2002}, pages = {257--265} }, @article{bergermayer_superstructures_2002, title = {Superstructures of carbon on {V}(100)}, volume = {497}, doi = {10.1016/S0039-6028(01)01659-4}, abstract = {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 {\textless}010{\textgreater} oriented rows of {C} atoms. At higher coverages of {ThetaC} = 0.41 {ML,} mainly {\textless}010{\textgreater} 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} $\approx{}$ 0.37 {ML} and {theta\_O} $\approx{}$ 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.}, number = {1-3}, journal = SuSci, author = {W. Bergermayer and R. Koller and C. Konvicka and M. Schmid and G. Kresse and J. Redinger and P. Varga and R. Podloucky}, year = {2002}, pages = {294--304} }, @article{konvicka_stabilizing_2002, title = {Stabilizing single metal adatoms at room temperature: {Pd} on {C}- and {O}-covered {V}(100)}, volume = {496}, doi = {10.1016/S0039-6028(01)01605-3}, abstract = {The initial stages of {Pd} thin film growth on clean and C- and O-precovered V(100) surfaces at room temperature {(RT)} and 200 {$^\circ$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 {$^\circ$C} leads to the formation of rectangular {Pd} islands (size $\approx{}$40$\times$40 \AA{}2) and to the compression of the initial {C} and {O} adlayer in the areas between the Pd. For higher {Pd} coverages we found that oxygen acts as an anti-surfactant, shifting the onset of second layer growth dramatically. The reason for the change of the growth mode of {Pd} on V(100) from {Stranski\textendash{}Krastanov} to a {Volmer\textendash{}Weber-type} growth in the presence of oxygen can be found in a higher free energy of the film/substrate interface compared to the clean surface.}, number = {3}, journal = SuSci, author = {C. Konvicka and A. Hammerschmid and M. Schmid and P. Varga}, year = {2002}, pages = {209--220} }, @article{jimenez-mier_decay_2002, title = {Decay channels for the {Ti}(2p$_{1/2}$) core hole excitations in {TiO$_2$} observed by x-ray {Raman} scattering}, volume = {65}, doi = {10.1103/PhysRevB.65.184105}, abstract = {We present high-resolution x-ray fluorescence spectra, corrected for self-absorption, following resonant excitation of a Ti(2p1/2) electron in {TiO$_2$.} Several transitions are studied that show complex behavior as a function of photon excitation energy, indicative of interaction in the excitation and decay channels. Three peaks are identified as transitions resulting from the excitation of a 2p1/2 electron, one of them corresponding to direct valence emission with a 3d spectator electron and the other two to participator emission following the {Coster-Kronig} decay of the 2p1/2 hole. Additional emission features at higher photon energies correspond to valence emission in the presence of a charge-transfer excitation that has been found in the 2p x-ray photoelectron spectroscopy spectrum, and also in the x-ray absorption spectrum. A detailed analysis of the energy dispersion versus excitation energy provides information about the dynamical processes involved. The results are interpreted in terms of the calculated band structure of the compound.}, number = {18}, journal = PRB, author = {J. {Jim\'{e}nez-Mier} and U. Diebold and D. Ederer and T. Callcott and M. Grush and R. Perera}, year = {2002}, pages = {184105} }, @article{vogtenhuber_ab_2002, title = {Ab initio and experimental studies of chlorine adsorption on the rutile {TiO$_2$}(110) surface}, volume = {65}, doi = {10.1103/PhysRevB.65.125411}, abstract = {We report a comprehensive study on the adsorption of {Cl} on the clean rutile {TiO$_2$} (110) (1$\times$1) surface. {STM} and photoemission spectroscopy results are compared to ab initio results. At room temperature, {Cl} adsorbs dissociatively and binds to the fivefold coordinated {Ti} atoms in an on-top configuration. The calculations predict energetically more favorable adsorption on a reduced surface, where bridging oxygen atoms are missing in a (1$\times$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.}, number = {12}, journal = PRB, author = {Doris Vogtenhuber and Raimund Podloucky and Josef Redinger and Eleonore Hebenstreit and Wilhelm Hebenstreit and Ulrike Diebold}, year = {2002}, pages = {125411} }, @incollection{schmid_segregation_2002, series = {The Chemical Physics of Solid Surfaces}, title = {Segregation and surface chemical ordering-an experimental view on the atomic scale}, volume = {Volume 10}, isbn = {1571-0785}, abstract = {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.}, booktitle = {Surface Alloys and Alloy Surfaces}, publisher = {Elsevier}, author = {M. Schmid and P. Varga}, editor = {{D.P.} Woodruff}, year = {2002}, pages = {118--151} }, @article{dulub_stm_2002-1, title = {{STM} study of the geometric and electronic structure of {ZnO(0001)-Zn,} {(000$\bar{1}$)-O,} (10$\bar{1}$0), and (11$\bar{2}$0) surfaces}, volume = {519}, doi = {10.1016/S0039-6028(02)02211-2}, abstract = {The geometric and electronic structure of clean (0001), (000-1), (11-20), and (10-10) faces of {ZnO} single crystals have been studied with scanning tunneling microscopy {(STM)} and spectroscopy {(STS),} low-energy electron diffraction {(LEED),} and low-energy {He}$^+$ ion scattering spectroscopy {(LEIS).} All surfaces exhibit a (1$\times$1) termination but distinctly different terrace and step structures. On the zinc-terminated {(0001)-Zn} surface, the terraces are covered with triangular islands and pits of different sizes, rotated by 180$^\circ$ with respect to those in the neighboring terraces. Single-layer steps with a height of $\approx{}$2.7 \AA{} are observed. Vicinal surfaces of {(0001)-Zn} consist of terraces separated by alternating straight and saw-tooth-shaped steps. On the oxygen-terminated {(Image} {)-O} surface, flat hexagonal terraces are separated by predominantly $\approx{}$5.3 \AA{} high-double-layer steps. The terraces are wide ($\approx{}$500 \AA{}) and smooth with no added islands and holes. They are not covered with a saturation coverage of hydrogen. Near-atomic-resolution images of the prism {(Image} ) surface show flat, rectangular terraces separated by single-layer steps ($\approx{}$3 \AA{}) running perpendicular to the{\textless}0001{\textgreater} and {\textless}1-210{\textgreater} directions. A high density of terraces with atomic rows running preferentially along the {\textless}0001{\textgreater} directions was observed on the as-grown (11-20) surface. This surface is the least stable and tends to form long grooves that are $\approx{}$250 \AA{} wide and $\approx{}$50 \AA{} deep along the {\textless}1-100{\textgreater} directions. {STS} measurements show semiconductor-like behavior of all the surfaces, but a slightly different {I\textendash{}V} characteristic of the {(000-1)-O} face. Based on these results, structural models for the different surfaces are proposed and related to the stability and reactivity of {ZnO} surfaces.}, number = {3}, journal = SuSci, author = {Olga Dulub and Lynn A Boatner and Ulrike Diebold}, year = {2002}, pages = {201--217} }, @article{chambers_epitaxial_2001, title = {Epitaxial growth and properties of ferromagnetic co-doped {TiO$_2$} anatase}, volume = {79}, doi = {10.1063/1.1420434}, abstract = {We have used oxygen-plasma-assisted molecular-beam epitaxy {(OPA-MBE)} to grow {CoxTi1\textendash{}xO$_2$} anatase on {SrTiO$_3$(001)} for x = $\approx$0.01\textendash{}0.10, and have measured the structural, compositional, and magnetic properties of the resulting films. Whether epitaxial or polycrystalline, these {CoxTi1\textendash{}xO$_2$} films are ferromagnetic semiconductors at and above room temperature. However, the magnetic and structural properties depend critically on the {Co} distribution, which varies widely with growth conditions. {Co} is substitutional in the anatase lattice and in the +2 formal oxidation state in ferromagnetic {CoxTi1\textendash{}xO$_2$.} The magnetic properties of {OPA-MBE} grown material are significantly better than those of analogous pulsed laser deposition-grown material.}, number = {21}, journal = APL, author = {S. A. Chambers and S. Thevuthasan and R. F. C. Farrow and R. F. Marks and J. U. Thiele and L. Folks and M. G. Samant and A. J. Kellock and N. Ruzycki and D. L. Ederer and U. Diebold}, month = nov, year = {2001}, pages = {3467--3469} }, @article{over_direct_2001, title = {Direct imaging of catalytically important processes in the oxidation of {CO} over {RuO$_2$(110)}}, volume = {123}, doi = {10.1021/ja016408t}, abstract = {Ruthenium dioxide {(RuO$_2$)} reveals unique and promising redox properties, making {RuO$_2$} a potential candidate for a versatile oxidation catalyst. Recently Zhang and Kisch reported, for instance, that {RuO$_2$} is a robust and efficient catalyst for room temperature {(RT)} oxidation of {CO} by humid air; recall that typical metal oxides do not tolerate humidity. In this contribution we present scanning tunneling microscopy {(STM)} data which directly image the catalytically important processes occurring on the {RuO$_2$(110)} surface after exposing the pristine surface to {CO} and O$_2$. These data are complemented by density functional theory {(DFT)} calculations. The following processes are governing the catalytic activity of {RuO$_2$} on atomic scale. The reactants from the gas phase encounter strongly binding adsorption sites on the {RuO$_2$(110)} surface in the form of the under-coordinated Ru atoms. For instance, {CO} adsorbs on the stoichiometric {RuO$_2$(110)} surface by 1.2 {eV} (over the {1f-cus-Ru),} while on the reduced {RuO$_2$(110)} surface the {CO} binding energies are 1.61 {eV} and 1.85 {eV} for adsorption over {1f-cus-Ru} and {2f-cus-Ru} atoms, respectively. The {RuO$_2$} surface provides an active oxygen species to react with {CO,} i.e. the under-coordinated (bridging) lattice oxygen atoms Obr. The recombination of adsorbed {CO} with Obr creates vacancies, which are identified with {STM.} At {RT,} oxygen molecules from the gas phase can efficiently dissociate on {RuO$_2$(110)} via the molecular precursor state. This leads to weakly held {O} atoms, which are grouped in pairs as imaged with {STM.} It is argued that the weakly held oxygen plays an important role in replenishing the consumed (bridging) lattice oxygen atoms on the (partially) reduced {RuO$_2$(110)} surface.}, number = {47}, journal = JACS, author = {H. Over and A. P. Seitsonen and E. Lundgren and M. Schmid and P. Varga}, month = nov, year = {2001}, pages = {11807--11808} }, @article{jennison_structure_2001, title = {Structure of an ultrathin {TiO$_x$} film, formed by the strong metal support interaction {(SMSI),} on {Pt} nanocrystals on {TiO$_2$(110)}}, volume = {492}, doi = {10.1016/S0039-6028(01)01460-1}, abstract = {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\textendash{}O} unit, produces {\textgreater}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 \textquotedblleft{}canyons\textquotedblright{} {(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.}, number = {1-2}, journal = SuSci, author = {D. R. Jennison and O. Dulub and W. Hebenstreit and U. Diebold}, month = oct, year = {2001}, pages = {L677--L687} }, @article{leonardelli_adatom_2001, title = {Adatom interlayer diffusion on {Pt}(111): an embedded atom method study}, volume = {490}, doi = {10.1016/S0039-6028(01)01212-2}, abstract = {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.}, number = {1-2}, journal = SuSci, author = {G. Leonardelli and E. Lundgren and M. Schmid}, month = sep, year = {2001}, pages = {29--42} }, @article{hayderer_sputtering_2001, title = {Sputtering of {Au} and {Al$_2$O$_3$} surfaces by slow highly charged ions}, volume = {182}, doi = {10.1016/S0168-583X(01)00668-1}, abstract = {A quartz crystal microbalance technique was used for measuring total sputtering yields for polycrystalline Au and {Al2O3} under impact of slow (100-1500 {eV)} multiply charged {Ar} and {Xe} ions. Up to the highest charge states investigated {(Xe25+),} the sputter yield for the Au target remains independent on the projectile charge state and can be well described by kinetic sputtering only. For {Al2O3,} on the contrary, a dramatic increase in total sputtering yield with increasing projectile charge state was found, showing that in this case potential sputtering {(PS),} i.e., sputtering due to the potential energy of the projectile, clearly dominates over kinetically induced sputtering. Results can be explained within the defect-mediated desorption model of {PS.}}, number = {1-4}, journal = NIMB, author = {G. Hayderer and S. Cernusca and V. Hoffmann and D. Niemann and N. Stolterfoht and M. Schmid and P. Varga and {HP.} Winter and F. Aumayr}, month = aug, year = {2001}, pages = {143--147} }, @article{hebenstreit_bulk-defect_2001, title = {Bulk-defect dependent adsorption on a metal oxide surface: {S/TiO$_2$(110)}}, volume = {486}, doi = {10.1016/S0039-6028(01)01067-6}, abstract = {The adsorption of molecular sulfur on {TiO$_2$(110)(1$\times$1)} has been studied with scanning tunneling microscopy and photoelectron spectroscopy. At room temperature {S} binds dissociatively to 5-fold coordinated {Ti} atoms and oxygen vacancies. At elevated temperatures {(120\textendash{}440$^\circ$C)} sulfur replaces surface oxygen atoms. Evidence was found that the reduction state of {TiO$_2$} crystals strongly affects the surface coverage of {S} at elevated temperatures. The rate of the {O\textendash{}S} site exchange is kinetically limited by the arrival of diffusing bulk defects at the surface.}, number = {3}, journal = SuSci, author = {E. L. D. Hebenstreit and W. Hebenstreit and H. Geisler and C. A. Ventrice Jr. and P. T. Sprunger and U. Diebold}, month = jul, year = {2001}, pages = {L467--L474} }, @article{gauthier_adsorption_2001, title = {Adsorption Sites and Ligand Effect for {CO} on an Alloy Surface: A Direct View}, volume = {87}, doi = {10.1103/PhysRevLett.87.036103}, abstract = {{CO} adsorption on a {PtCo(111)} surface was studied by scanning tunneling microscopy. Comparison of images with chemical contrast of {Pt} and {Co} and images showing the {CO} molecules indicates that {CO} resides exclusively on top of {Pt} sites and never on Co. {CO} bonding is highly sensitive to the chemical environment. The probability to find {CO} on a {Pt} atom increases drastically with the number of its {Co} nearest neighbors. Ab initio calculations show that this ligand effect is due to different positions of the center of the {Pt} d band.}, number = {3}, journal = PRL, author = {Y. Gauthier and M. Schmid and S. Padovani and E. Lundgren and V. Bu\v{s} and G. Kresse and J. Redinger and P. Varga}, month = jun, year = {2001}, pages = {036103} }, @article{koller_structure_2001, title = {The structure of the oxygen induced (1$\times$5) reconstruction of {V}(100)}, volume = {480}, doi = {10.1016/S0039-6028(01)00978-5}, abstract = {The adsorption of 1 Langmuir oxygen on the V(100) surface at 200 {$^\circ$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 $\approx$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.}, number = {1-2}, journal = SuSci, author = {R. Koller and W. Bergermayer and G. Kresse and E. L. D. Hebenstreit and C. Konvicka and M. Schmid and R. Podloucky and P. Varga}, month = may, year = {2001}, pages = {11--24} }, @article{schmid_oxygen_2001, title = {Oxygen adsorption on {Al}(111): low transient mobility}, volume = {478}, doi = {10.1016/S0039-6028(01)00967-0}, abstract = {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.}, number = {3}, journal = SuSci, author = {M. Schmid and G. Leonardelli and R. Tschelie\ss{}nig and A. Biedermann and P. Varga}, month = may, year = {2001}, pages = {L355--L362} }, @article{hayderer_kinetically_2001, title = {Kinetically Assisted Potential Sputtering of Insulators by Highly Charged Ions}, volume = {86}, doi = {10.1103/PhysRevLett.86.3530}, abstract = {A new form of potential sputtering has been found for impact of slow ( $\leq{}$1500 {eV)} multiply charged {Xe} ions (charge states up to q = 25) on {MgOx.} In contrast to alkali-halide or {SiO$_2$} 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.}, number = {16}, journal = PRL, author = {G. Hayderer and S. Cernusca and M. Schmid and P. Varga and {HP.} Winter and F. Aumayr and D. Niemann and V. Hoffmann and N. Stolterfoht and C. Lemell and L. Wirtz and J. Burgd\"{o}rfer}, month = apr, year = {2001}, pages = {3530} }, @article{schmid_exchange_2001, title = {Exchange processes in interlayer diffusion \textendash{} kinks, corners and the growth mode}, volume = {72}, doi = {10.1007/s003390100753}, abstract = {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\textendash{}step interaction, can lead to an instability of the growth mode.}, number = {4}, journal = ApPhA, author = {M. Schmid and E. Lundgren and G. Leonardelli and A. Hammerschmid and B. Stanka and P. Varga}, month = apr, year = {2001}, pages = {405--412} }, @article{bischoff_influence_2001, title = {Influence of Impurities on Localized Transition Metal Surface States: Scanning Tunneling Spectroscopy on {V}(001)}, volume = {86}, doi = {10.1103/PhysRevLett.86.2396}, abstract = {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 $\sim{}$0.2 {eV)} at contaminated areas. Because of the negative dispersion of this state, the upward shift cannot be described in a simple confinement picture. Rather, the surface state energy is governed by vanadium surface s- d interactions which are altered by carbon coverage.}, number = {11}, journal = PRL, author = {M. M. J. Bischoff and C. Konvicka and A. J. Quinn and M. Schmid and J. Redinger and R. Podloucky and P. Varga and H. van Kempen}, month = mar, year = {2001}, pages = {2396} }, @article{blum_segregation_2001, title = {Segregation and ordering at {Fe$_{1-x}$Al$_x$}(100) surfaces - a model case for binary alloys}, volume = {474}, doi = {10.1016/S0039-6028(00)00987-0}, abstract = {The geometrical structure, chemical order and composition of (100) oriented surfaces of the binary alloy system {Fe\_1-xAl\_x} were investigated in the {Fe}-rich regime (x = 0.03, 0.15, and 0.30) using quantitative low-energy electron diffraction. Low-energy {He}$^+$ 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.}, number = {1-3}, journal = SuSci, author = {V. Blum and L. Hammer and W. Meier and K. Heinz and M. Schmid and E. Lundgren and P. Varga}, month = mar, year = {2001}, pages = {81--97} }, @article{biedermann_nucleation_2001, title = {Nucleation of bcc Iron in Ultrathin fcc Films}, volume = {86}, doi = {10.1103/PhysRevLett.86.464}, abstract = {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.}, number = {3}, journal = PRL, author = {Albert Biedermann and Michael Schmid and Peter Varga}, year = {2001}, pages = {464} }, @article{hebenstreit_structures_2001, title = {Structures of sulfur on {TiO$_2$(110)} determined by scanning tunneling microscopy, X-ray photoelectron spectroscopy and low-energy electron diffraction}, volume = {470}, doi = {10.1016/S0039-6028(00)00849-9}, abstract = {The temperature dependent adsorption of sulfur on {TiO$_2$(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 {$\approx$120$^\circ$C,} 80\% of the sulfur desorbs and the {S} 2p peak position changes from 164.3$\pm{}$0.1 to 162.5$\pm{}$0.1 {eV.} This peak shift corresponds to a change of the adsorption site to the position of the bridging oxygen atoms of {TiO$_2$(1} 1 0). Further heating causes little change in {S} coverage and {XPS} binding energies, up to a temperature of {$\approx$430$^\circ$C} where most of the {S} desorbs and the {S} 2p peak shifts back to higher binding energy. Sulfur adsorption at {150$^\circ$C,} {200$^\circ$C,} and {300$^\circ$C} leads to a rich variety of structures and adsorption sites as observed with {LEED} and {STM.} At low coverages, sulfur occupies the position of the bridging oxygen atoms. At {200$^\circ$C} these {S} atoms arrange in a (3$\times$1) superstructure. For adsorption between {300$^\circ$C} and {400$^\circ$C} a (3$\times$3) and (4$\times$1) {LEED} pattern is observed for intermediate and saturation coverage, respectively. Adsorption at elevated temperature reduces the substrate as indicated by a strong Ti3+ shoulder in the {XPS} {Ti} 2p3/2 peak, with up to 15.6\% of the total peak area for the (4$\times$1) structure. {STM} of different coverages adsorbed at {400$^\circ$C} indicates structural features consisting of two single {S} atoms placed next to each other along the [0 0 1] direction at the position of the in-plane oxygen atoms. The (3$\times$3) and the (4$\times$1) structure are formed by different arrangements of these {S} pairs.}, number = {3}, journal = SuSci, author = {E. L. D. Hebenstreit and W. Hebenstreit and U. Diebold}, year = {2001}, pages = {347--360} }, @article{varga_ultrathin_2001, title = {Ultrathin Films of {Co} on {Pt}(111): an {STM} View}, volume = {187}, doi = {10.1002/1521-396X(200109)187:1<97::AID-PSSA97>3.0.CO;2-A}, abstract = {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 moir\'{e} 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.}, number = {1}, journal = PhysStSolA, author = {P. Varga and E. Lundgren and J. Redinger and M. Schmid}, year = {2001}, pages = {97--112} }, @article{hayderer_stm_2001, title = {{STM} Studies of {HCI-induced} Surface Damage on Highly Oriented Pyrolytic Graphite}, volume = {T92}, doi = {10.1238/Physica.Topical.092a00156}, abstract = {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 ($\surd{}$3 x $\surd{}$3) R 30$^\circ$ 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.}, journal = {Physica Scripta}, author = {G. Hayderer and S. Cernusca and M. Schmid and P. Varga and {HP} Winter and F. Aumayr}, year = {2001}, pages = {156--157} }, @incollection{diebold_structure_2001, series = {The Chemical Physics of Solid Surfaces}, title = {The structure of {TiO$_2$} surfaces}, volume = {9}, isbn = {978-0-444-50745-7}, booktitle = {Oxide Surfaces}, publisher = {Elsevier}, author = {Ulrike Diebold}, editor = {{D.P.} Woodruff}, year = {2001}, pages = {443--480} }, @article{diebold_understanding_2001, title = {Understanding Metal Oxide Surfaces at the Atomic Scale: {STM} Investigations of Bulk-defect Dependent Surface Processes}, volume = {654}, abstract = {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 {TiO$_2$(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 {TiO$_2$} 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.}, journal = ProcMRS, author = {Ulrike Diebold}, year = {2001}, pages = {AA5.1} }, @article{biedermann_crystallographic_2001, title = {Crystallographic Structure of Ultrathin {Fe} Films on {Cu}(100)}, volume = {87}, doi = {10.1103/PhysRevLett.87.086103}, abstract = {We report the observation of bcc-like crystal structures in 2\textendash{}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.}, number = {8}, journal = PRL, author = {Albert Biedermann and Rupert Tschelie\ss{}nig and Michael Schmid and Peter Varga}, year = {2001}, pages = {086103} }, @article{hebenstreit_sulfur_2001, title = {Sulfur on {TiO$_2$(110)} studied with resonant photoemission}, volume = {64}, doi = {10.1103/PhysRevB.64.115418}, abstract = {Adsorption of sulfur on {TiO$_2$(110)} at room temperature {(RT)} and {350$^\circ$C} has been studied with ultraviolet photoelectron spectroscopy. A {TiO$_2$(110)} (1$\times$1) surface with a small amount of oxygen vacancies was prepared by sputtering and annealing in ultrahigh vacuum. Oxygen vacancies induce a defect state that pins the Fermi level just below the conduction-band minimum. Sulfur adsorption at room temperature leads to the disappearance of this vacancy-related band-gap state, indicating that the surface oxygen vacancies are filled by sulfur. Sulfur-induced valence-band features are identified at binding energies of 3.4 and 8 {eV.} Adsorption of S at {350$^\circ$C} forms a (4$\times$1) superstructure at high coverages [$\approx{}$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.}, number = {11}, journal = PRB, author = {Hebenstreit, E. and Hebenstreit, W. and Geisler, H. and Thornburg, S. and Ventrice, C. and Hite, D. and Sprunger, P. and Diebold, U.}, month = aug, year = {2001}, pages = {115418} }, @article{hebenstreit_bulk_2000, title = {Bulk Terminated {NaCl(111)} on Aluminum: A Polar Surface of an Ionic Crystal?}, volume = {85}, doi = {10.1103/PhysRevLett.85.5376}, abstract = {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.}, number = {25}, journal = PRL, author = {W. Hebenstreit and M. Schmid and J. Redinger and R. Podloucky and P. Varga}, month = dec, year = {2000}, pages = {5376} }, @article{gauthier_segregation_2000, title = {Segregation and chemical ordering in the surface layers of {Pt$_{25}$Co$_{75}$(111):} a {LEED/STM} study}, volume = {466}, doi = {10.1016/S0039-6028(00)00751-2}, abstract = {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 \AA{} for {Pt} and {Co} sites. {LEED} shows that the {Pt} sublattice in the top layer actually resides 0.1 \AA{} 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.}, number = {1-3}, journal = SuSci, author = {Y. Gauthier and R. {Baudoing-Savois} and J. M. Bugnard and W. Hebenstreit and M. Schmid and P. Varga}, month = nov, year = {2000}, pages = {155--166} }, @article{dai_characterization_2000, title = {Characterization of the natural barriers of intergranular tunnel junctions: {Cr}$_2${O}$_3$ surface layers on {CrO}$_2$ nanoparticles}, volume = {77}, doi = {10.1063/1.1320845}, abstract = {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\textendash{}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.}, number = {18}, journal = APL, author = {Jianbiao Dai and Jinke Tang and Huiping Xu and Leonard Spinu and Wendong Wang and Kaiying Wang and Amar Kumbhar and Min Li and Ulrike Diebold}, month = oct, year = {2000}, pages = {2840--2842} }, @article{konvicka_surface_2000, title = {Surface and subsurface alloy formation of vanadium on {Pd}(111)}, volume = {463}, doi = {10.1016/S0039-6028(00)00643-9}, abstract = {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 {$^\circ$C} only a small amount of {V} is observed in the top-most layer. By using {STM} a ($\surd{}$3 x {$\surd{}$3)R30$^\circ$} superstructure is observed to be formed and {XPD} measurements demonstrate that this structure is due to {V} atoms incorporated in the second layer. This finding is confirmed by ab-initio calculations. Further, a model for the ($\surd{}$3 x {$\surd{}$3)R30$^\circ$} structure based on the experiments and the ab-initio calculations is given.}, number = {3}, journal = SuSci, author = {Ch. Konvicka and Y. Jeanvoine and E. Lundgren and G. Kresse and M. Schmid and J. Hafner and P. Varga}, month = sep, year = {2000}, pages = {199--210} }, @article{hebenstreit_adsorption_2000, title = {Adsorption of sulfur on {TiO$_2$(110)} studied with {STM,} {LEED} and {XPS:} temperature-dependent change of adsorption site combined with {O-S} exchange}, volume = {461}, doi = {10.1016/S0039-6028(00)00538-0}, number = {1-3}, journal = SuSci, author = {E. L. D. Hebenstreit and W. Hebenstreit and U. Diebold}, month = aug, year = {2000}, pages = {87--97} }, @article{schmid_stm_2000, title = {{STM} and {STS} of bulk electron scattering by subsurface objects}, volume = {109}, doi = {10.1016/S0368-2048(00)00108-0}, abstract = {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.}, number = {1-2}, journal = JElSpec, author = {M. Schmid and S. Crampin and P. Varga}, month = aug, year = {2000}, pages = {71--84} }, @article{lundgren_thin_2000, title = {Thin films of {Co} on {Pt}(111): Strain relaxation and growth}, volume = {62}, doi = {10.1103/PhysRevB.62.2843}, abstract = {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 moir\'{e} 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.}, number = {4}, journal = PRB, author = {E. Lundgren and B. Stanka and M. Schmid and P. Varga}, month = jul, year = {2000}, pages = {2843} }, @article{platzgummer_temperature-dependent_2000, title = {Temperature-dependent segregation reversal and (1$\times$3) missing-row structure of {Pt$_{90}$Co$_{10}$(110)}}, volume = {453}, doi = {10.1016/S0039-6028(00)00351-4}, abstract = {The surface structure and composition of the clean {Pt90Co10(110)} surface is investigated by low energy electron diffraction {(LEED)} and low energy ion scattering {(LEIS).} Through {LEED} {I-V} analysis we find a (1x3) missing-row reconstruction on the equilibrated {Pt90Co10(110)} surface - comparable with the pure {Pt}(110) (1x3) surface - in which all atomic positions in the topmost layer and in the (111) oriented micro facets are {Pt}-enriched. Due to the fact that the unreconstructed {Pt25Co75(110)} surface is known to exhibit an almost pure {Co} top layer the {Pt} segregation reported in this study is undoubtedly connected to the existence of the missing-row reconstruction. The proposed structural influence on the composition is confirmed by {LEIS} experiments performed on the hot {Pt90Co10(110)} surface, in which simultaneously temperature induced changes of the surface composition and qualitative changes in the surface structure are monitored. The measured low energy ion spectra do not only reproduce the calculated first-layer composition of the {LEED} analysis, they also show a less pronounced {Pt} segregation at temperatures around {750$^\circ$C,} and eventually a reversed {Pt} segregation above {750$^\circ$C,} i.e. {Co} enrichment of the {Pt90Co10(110)} surface with respect to the bulk concentration. We find a clear correlation between the thermal deconstruction and the surface composition. The striking segregation reversal during temperature variation is attributed to the high excess value of the mixing enthalpy, which implies a structure-dominated segregation behavior.}, number = {1-3}, journal = SuSci, author = {E. Platzgummer and M. Sporn and R. Koller and M. Schmid and W. Hofer and P. Varga}, month = may, year = {2000}, pages = {214--224} }, @article{li_influence_2000, title = {The Influence of the Bulk Reduction State on the Surface Structure and Morphology of Rutile {TiO$_2$(110)} Single Crystals}, volume = {104}, doi = {10.1021/jp9943272}, abstract = {We have investigated the relationship between different types and amounts of bulk defects and the surface morphology of {TiO$_2$(110)} single crystals prepared by annealing in ultrahigh vacuum and in oxygen. Rutile {TiO$_2$(110)} specimens were cut from the same crystal and were heated in a furnace to different temperatures which resulted in different states of reduction (colors of the crystals). After characterization of the bulk defects with electron paramagnetic resonance {(EPR),} the specimens were studied with scanning tunneling microscopy {(STM),} low-energy {He}$^+$ 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., {TiO$_2$} clusters on blue crystals and rosette networks on dark blue crystals.}, number = {20}, journal = JPCB, author = {Min Li and Wilhelm Hebenstreit and Ulrike Diebold and Alexei M. Tyryshkin and Michael K. Bowman and Glen G. Dunham and Michael A. Henderson}, month = may, year = {2000}, pages = {4944--4950} }, @article{wirtz_curve-crossing_2000, title = {Curve-crossing analysis for potential sputtering of insulators}, volume = {451}, doi = {10.1016/S0039-6028(00)00027-3}, abstract = {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.}, number = {1-3}, journal = SuSci, author = {L. Wirtz and G. Hayderer and C. Lemell and J. Burgd\"{o}rfer and L. H\"{a}gg and {C.O.} Reinhold and P. Varga and P. Winter and F. Aumayr}, month = apr, year = {2000}, pages = {197--202} }, @article{hayderer_observation_2000, title = {Observation of a threshold in potential sputtering of {LiF} surfaces}, volume = {164-165}, doi = {10.1016/S0168-583X(99)01070-8}, abstract = {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.}, journal = NIMB, author = {G. Hayderer and C. Lemell and L. Wirtz and M. Schmid and J. Burgd\"{o}rfer and P. Varga and {HP.} Winter and F. Aumayr}, month = apr, year = {2000}, pages = {517--521} }, @article{stanka_surface_2000, title = {Surface reconstruction of {Fe$_3$O$_4$(001)}}, volume = {448}, doi = {10.1016/S0039-6028(99)01182-6}, abstract = {We have investigated the surface termination, structure, morphology and composition of {Fe$_3$O$_4$(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 \AA{} thick epitaxial films of {Fe$_3$O$_4$(001)} grown by oxygen-plasma-assisted molecular-beam epitaxy {(OPA-MBE)} on {MgO(001)} substrates. The ($\surd{}$2 $\times$ {$\surd{}$2)R45$^\circ$} surface reconstruction that is present on the as-grown surface is recovered by heating the sample in oxygen (10-6\textendash{}10-7 mbar) at temperatures between 420 and 770 K after a through-air transfer from the {MBE} chamber. {STM} results are best interpreted by assuming an autocompensated B-layer termination, which consists of a layer of octahedrally coordinated iron and tetrahedrally coordinated oxygen, along with one oxygen vacancy per unit cell. Evidence for a vacancy-induced lateral relaxation of the adjacent octahedral iron ions is presented. Further annealing in ultrahigh vacuum causes a transformation to either a (1$\times$n) or a The ($\surd{}$2 $\times$ {$\surd{}$2)R45$^\circ$} structure. These surfaces can be reproducibly transformed back to the The ($\surd{}$2 $\times$ {$\surd{}$2)R45$^\circ$} reconstruction by annealing in oxygen. Interestingly, at no time do we observe the other autocompensated termination, which consists of one-half of a monolayer of tetrahedrally coordinated {Fe(III),} despite its observation on the as-grown surface. Thus, it appears that the surface termination is critically dependent on the method of surface preparation.}, number = {1}, journal = SuSci, author = {B. Stanka and W. Hebenstreit and U. Diebold and S. A. Chambers}, month = mar, year = {2000}, pages = {49--63} }, @article{over_atomic-scale_2000, title = {Atomic-Scale Structure and Catalytic Reactivity of the {RuO$_2$(110)} Surface}, volume = {287}, doi = {10.1126/science.287.5457.1474}, abstract = {Exposure of a Ru(0001) surface to large doses of O2 at elevated temperatures leads to the growth of an epitaxial layer of {RuO$_2$} with (110) surface orientation whose structure was analyzed quantitatively by combination of low energy electron diffraction, scanning tunneling microscopy and density functional calculations. The surface exposes essentially bridging {O} atoms and Ru atoms not capped by oxygen. The latter play the role of coordinatively unsaturated sites (cus) - a hypothesis introduced long ago to account for the catalytic activity of oxide surfaces - onto which {CO} may become chemisorbed and from where it may react with neighboring {lattice-O} to {CO$_2$.} The distortion of the surface lattice thereby caused is restored by uptake of oxygen from the gas phase, i.e., the oxide surface itself is actively participating in the catalytic reaction. In this way a general mechanism originally proposed by Mars and van Krevelen could be verified.}, number = {5457}, journal = {Science}, author = {H. Over and Y. D. Kim and A. P. Seitsonen and S. Wendt and E. Lundgren and M. Schmid and P. Varga and A. Morgante and G. Ertl}, month = feb, year = {2000}, pages = {1474--1476} }, @article{hebenstreit_scanning_2000, title = {Scanning tunneling microscopy investigation of the {TiO$_2$} anatase (101) surface}, volume = {62}, doi = {10.1103/PhysRevB.62.R16334}, abstract = {We report the first scanning tunneling microscopy {(STM)} study of single-crystalline anatase. Atomically resolved images of the (101) surface are consistent with a bulk-truncated (1$\times$1) termination. Step edges run predominantly in the [010], [-111], and [-1-11] directions. The surface is stable with very few point defects. Fourfold-coordinated {Ti} atoms at step edges are preferred adsorption sites and allow the identification of tunneling sites in {STM.}}, number = {24}, journal = PRB, author = {Wilhelm Hebenstreit and Nancy Ruzycki and Gregory Herman and Yufei Gao and Ulrike Diebold}, year = {2000}, pages = {R16334--R16336} }, @article{dulub_imaging_2000, title = {Imaging Cluster Surfaces with Atomic Resolution: The Strong {Metal-Support} Interaction State of {Pt} Supported on {TiO$_2$(110)}}, volume = {84}, doi = {10.1103/PhysRevLett.84.3646}, abstract = {Nanosized platinum clusters were grown on a {TiO$_2$(110)} surface and annealed in ultrahigh vacuum at high temperatures. This leads to the so-called strong metal-support interaction {(SMSI)} state, characterized by a complete encapsulation of the clusters with a reduced titanium oxide layer. We present atomically resolved scanning tunneling microscopy measurements of the cluster surfaces and an atomic model of the {SMSI} state. The ability to resolve the cluster surface geometry with atomistic detail may help to identify the active sites responsible for the {SMSI.}}, number = {16}, journal = PRL, author = {Olga Dulub and Wilhelm Hebenstreit and Ulrike Diebold}, year = {2000}, pages = {3646--3649} }, @article{li_morphology_2000, title = {Morphology change of oxygen-restructured {TiO$_2$(110)} surfaces by {UHV} annealing: Formation of a low-temperature (1$\times$2) structure}, volume = {61}, doi = {10.1103/PhysRevB.61.4926}, abstract = {When reduced {TiO$_2$(110)} single crystals are oxidized at moderate temperatures (470\textendash{}660 K), the surfaces restructure. Interstitial {Ti} atoms from the bulk diffuse to the surface where they react with gaseous oxygen and form new, added {TiO$_2$} layers. These are characterized by three structural elements: small (1$\times$1) islands, irregular networks of pseudohexagonal rosettes, and [001]-oriented strands. The strands exhibit the same structural characteristics as the (1$\times$2) surface reconstruction, which forms upon annealing at higher temperatures. Atomic-resolution scanning tunneling microscopy images of the strands are consistent with the {added-Ti2O3-row} model. {UHV} annealing of oxygen-restructured surfaces smooths the surfaces and converts the rosette networks into strands and finally into regular (1$\times$1) terraces. The composition of these oxygen-induced phases is quantified using {18O2} gas in combination with low-energy {He}$^+$ ion scattering measurements. Dynamic processes for the conversion from rosette networks into (1$\times$2) strands and ultimately into (1$\times$1) terraces are discussed.}, number = {7}, journal = PRB, author = {Min Li and Wilhelm Hebenstreit and Ulrike Diebold}, year = {2000}, pages = {4926--4933} }, @article{varga_hochauflsende_2000, title = {Hochaufl\"{o}sende {Rastertunnelmikroskopie} unterscheidet {Atome}}, volume = {31}, doi = {10.1002/1521-3943(200009)31:5<215::AID-PIUZ215>3.0.CO;2-0}, abstract = {Rastertunnelmikroskopie mit atomarer Aufl\"{o}sung erm\"{o}glicht die chemische Analyse an Oberfl\"{a}chen von metallischen Legierungen und ultrad\"{u}nnen Filmen auf atomarer Skala und tr\"{a}gt damit zum besseren Verst\"{a}ndnis von katalytischen Prozessen bei.}, number = {5}, journal = {Physik in unserer Zeit}, author = {Peter Varga and Michael Schmid and Josef Redinger}, year = {2000}, pages = {215--221} }, @article{lundgren_role_2000, title = {On the role of kinks and strain in heteroepitaxial growth: An {STM} study}, volume = {7}, doi = {10.1142/S0218625X00000750}, abstract = {Interlayer diffusion of {Co} over steps of vacancy islands on the {Pt}(111) surface as studied by scanning tunneling microscopy is presented. It is demonstrated that {Co} atoms descend {Pt} steps by an exchange diffusion process at the step edge with the {Pt} atoms. Further, the exchange diffusion process is observed to occur at the corners (kinks) of the vacancy islands. The importance of kinks concerning whether the growth mode of a heteropitaxial film is two-dimensional or three-dimensional is demonstrated for the case of thin {Co} films on {Pt}(111). We argue that the strain in the {Co} film is to a large extent responsible for the kink formation.}, number = {5-6}, journal = SRL, author = {E. Lundgren and M. Schmid and G. Leonardelli and A. Hammerschmid and B. Stanka and P. Varga}, year = {2000}, pages = {673--677} }, @article{diebold_relationship_2000, title = {The relationship between bulk and surface properties of rutile {TiO$_2$(110)}}, volume = {7}, doi = {10.1142/S0218625X0000052X}, abstract = {We report scanning tunneling microscopy and complementary spectroscopic measurements on {TiO$_2$(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.}, number = {5-6}, journal = SRL, author = {Ulrike Diebold and Min Li and Olga Dulub and Eleonore L. D. Hebenstreit and Wilhelm Hebenstreit}, year = {2000}, pages = {613--617} }, @article{hayderer_threshold_1999, title = {Threshold for Potential Sputtering of {LiF}}, volume = {83}, doi = {10.1103/PhysRevLett.83.3948}, abstract = {We have measured total sputtering yields for impact of slow ( $\leq{}$100 {eV)} singly and doubly charged ions on {LiF.} The minimum potential energy necessary to induce potential sputtering {(PS)} from {LiF} was determined to be about 10 {eV.} This threshold coincides with the energy necessary to produce a cold hole in the valence band of {LiF} by resonant neutralization. This allows the first unambiguous identification of {PS} induced by cold holes. Further stepwise increase of the sputtering yield with higher projectile potential energies provides evidence for additional defect-mediated sputtering mechanisms operative in alkali halides.}, number = {19}, journal = PRL, author = {G. Hayderer and M. Schmid and P. Varga and H P. Winter and F. Aumayr and L. Wirtz and C. Lemell and J. Burgd\"{o}rfer and L. H\"{a}gg and C. O. Reinhold}, month = nov, year = {1999}, pages = {3948} }, @article{hebenstreit_pt25rh75111_1999, title = {{Pt$_{25}$Rh$_{75}$(111),} (110), and (100) studied by scanning tunnelling microscopy with chemical contrast}, volume = {441}, doi = {10.1016/S0039-6028(99)00880-8}, abstract = {Scanning tunneling microscopy images with chemical contrast allowed the direct determination of the composition and short-range order behaviour of the clean {Pt25Rh75(111),} (110), and (100) alloy surfaces. All measurements were performed at room temperature and showed a strong platinum enrichment depending on the preparation temperature. In the top layers of both {Pt25Rh75(111)} and {Pt25Rh75(110)-(1x2)} we find a preference for unlike nearest neighbors. {Pt25Rh75(100)} exhibits after a preparation temperature of {900$^\circ$C} a preference for clustering whereas after a preparation temperature of {600$^\circ$C,} comparable to {Pt25Rh75(111)} and (110) ordering tendencies appear. Additional investigations of the chemical identity of atoms surrounding hollow positions show mostly small deviations from a random distribution. However, the number of hollow sites surrounded by {Rh} atoms only can be significantly affected by the short range order. {Pt25Rh75(110)} exhibits a (1x2) missing-row reconstruction after annealing above {700$^\circ$C.} After the first annealing of the sputtered surface it is accompanied by mesoscopic long range "waves" with a height of approximately 2 nm and a wavelength up to 200 nm depending on the preparation temperature.}, number = {2-3}, journal = SuSci, author = {E. L. D. Hebenstreit and W. Hebenstreit and M. Schmid and P. Varga}, month = nov, year = {1999}, pages = {441--453} }, @article{li_oxygen-induced_1999, title = {Oxygen-induced restructuring of the {TiO$_2$(110)} surface: a comprehensive study}, volume = {437}, doi = {10.1016/S0039-6028(99)00720-7}, number = {1-2}, journal = SuSci, author = {Min Li and Wilhelm Hebenstreit and Leo Gross and Ulrike Diebold and M. A. Henderson and D. R. Jennison and P. A. Schultz and M. P. Sears}, month = aug, year = {1999}, pages = {173--190} }, @article{duisberg_high_1999, title = {High temperature growth of {Pt} on the {Rh}(111) surface}, volume = {433-435}, doi = {10.1016/S0039-6028(99)00037-0}, abstract = {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.}, journal = SuSci, author = {M. Duisberg and M. Dr\"{a}ger and K. Wandelt and E. L. D. Gruber and M. Schmid and P. Varga}, month = aug, year = {1999}, pages = {554--558} }, @article{lundgren_interlayer_1999, title = {Interlayer Diffusion of Adatoms: A Scanning-Tunneling Microscopy Study}, volume = {82}, doi = {10.1103/PhysRevLett.82.5068}, abstract = {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.}, number = {25}, journal = PRL, author = {E. Lundgren and B. Stanka and G. Leonardelli and M. Schmid and P. Varga}, month = jun, year = {1999}, pages = {5068} }, @article{henderson_interaction_1999, title = {Interaction of Molecular Oxygen with the {Vacuum-Annealed} {TiO$_2$(110)} Surface: Molecular and Dissociative Channels}, volume = {103}, doi = {10.1021/jp990655q}, abstract = {We have examined the interaction of molecular oxygen with the {TiO$_2$(110)} surface using temperature-programmed desorption {(TPD),} isotopic labeling studies, sticking probability measurements, and electron energy loss spectroscopy {(ELS).} Molecular oxygen does not adsorb on the {TiO$_2$(110)} surface in the temperature range between 100 and 300 K unless surface oxygen vacancy sites are present. These vacancy defects are generated by annealing the crystal at 850 K, and can be quantified reliably using water {TPD.} Adsorption of O2 at 120 K on a {TiO$_2$(110)} surface with 8\% oxygen vacancies (about 4*10{\textasciicircum}13 sites/cm2) occurs with an initial sticking probability of 0.5-0.6 that diminishes as the surface is saturated. The saturation coverage at 120 K, as estimated by {TPD} uptake measurements, is approximately three times the surface vacancy population. Coverage-dependent {TPD} shows little or no O2 desorption below a coverage of 4*10{\textasciicircum}13 molecules/cm2 (the vacancy population), presumably due to dissociative filling of the vacancy sites in a 1:1 ratio. Above a coverage of 4*10{\textasciicircum}13 molecules/cm2, a first-order O2 {TPD} peak appears at 410 K. Oxygen molecules in this peak do not scramble oxygen atoms with either the surface or with other coadsorbed oxygen molecules. Sequential exposures of {16O2} and {18O2} at 120 K indicate that each adsorbed O2 molecule, irrespective of its adsorption sequence, has equivalent probabilities with respect to its neighbors to follow the two channels (molecular and dissociative), suggesting that O2 adsorption is not only precursor-mediated, as the sticking probability measurements indicate, but that all O2 molecules reside in this precursor state at 120 K. This precursor state may be associated with a weak 145 K O2 {TPD} state observed at high O2 exposures. {ELS} measurements suggest charge transfer from the surface to the O2 molecule based on disappearance of the vacancy loss feature at 0.8 {eV,} and the appearance of a 2.8 {eV} loss that can be assigned to an adsorbed O2- species based on comparisons with {TiO$_2$} inorganic complexes in the literature. Utilizing results from recent spin-polarized {DFT} calculations in the literature, we propose a model where three O2 molecules are bound in the vicinity of each vacancy site at 120 K. For adsorption temperatures above 150 K, the dissociation channel completely dominates and the surface adsorbs oxygen in a 1:1 ratio with each vacancy site. {ELS} measurements indicate that the vacancies are filled, and the remaining oxygen adatom, which is apparent in {TPD,} is transparent in {ELS.} On the basis of the variety of oxygen adsorption states observed in this study, further work is needed in order to determine which oxygen-related species play important roles in chemical and photochemical oxidation processes on {TiO$_2$} surfaces.}, number = {25}, journal = JPCB, author = {Michael A. Henderson and William S. Epling and Craig L. Perkins and Charles H. F. Peden and Ulrike Diebold}, month = jun, year = {1999}, pages = {5328--5337} }, @article{hebenstreit_atomic_1999, title = {Atomic resolution by {STM} on ultra-thin films of alkali halides: experiment and local density calculations}, volume = {424}, doi = {10.1016/S0039-6028(99)00095-3}, abstract = {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.}, number = {2-3}, journal = SuSci, author = {W. Hebenstreit and J. Redinger and Z. Horozova and M. Schmid and R. Podloucky and P. Varga}, month = apr, year = {1999}, pages = {L321--L328} }, @article{lundgren_atomic-scale_1999, title = {An atomic-scale study of the {Co} induced dendrite formation on {Pt}(111)}, volume = {423}, doi = {10.1016/S0039-6028(98)00931-5}, abstract = {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 Gr\"{u}tter and D\"{u}rig, 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.}, number = {2-3}, journal = SuSci, author = {E. Lundgren and B. Stanka and W. Koprolin and M. Schmid and P. Varga}, month = mar, year = {1999}, pages = {357--363} }, @article{wouda_adsorbate_1999, title = {Adsorbate migration on {PdAg(111)}}, volume = {423}, doi = {10.1016/S0039-6028(98)00937-6}, abstract = {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.}, number = {1}, journal = SuSci, author = {P. T. Wouda and M. Schmid and B. E. Nieuwenhuys and P. Varga}, month = mar, year = {1999}, pages = {L229--L235} }, @article{platzgummer_temperature-dependent_1999, title = {Temperature-dependent segregation and (1$\times$2) missing-row reconstruction of {Pt$_{25}$Rh$_{75}$(110)}}, volume = {423}, doi = {10.1016/S0039-6028(98)00924-8}, abstract = {The surface structure and composition of the clean {Pt25Rh75(110)} surface is investigated by low energy electron diffraction {(LEED)} and low energy ion scattering {(LEIS).} For the equilibrated {Pt25Rh75(110)} surface we observe a (1x2) missing-row reconstruction in analogy to the pure {Pt}(110) surface, and a significant {Pt} enrichment of the topmost atomic layer (up to 80 at.\% {Pt}). As the same strong surface enrichment in {Pt} was found in a previous study on the (100) and (111) surface of the same bulk composition, this means that in contrast to {Pt-Ni} and {Pt-Co} alloys, for {Pt25Rh75} alloys the segregation behavior is not influenced extensively by the surface orientation. In addition to the structure analysis by {LEED} we performed {LEIS} experiments to determine the temperature induced changes of the surface composition and structure. Since the {Pt} segregation is less pronounced at elevated temperature, the surface reveals a temperature induced deconstruction of the (1x2) structure around {750$^\circ$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.}, number = {1}, journal = SuSci, author = {E. Platzgummer and M. Sporn and R. Koller and M. Schmid and W. Hofer and P. Varga}, month = mar, year = {1999}, pages = {134--143} }, @article{varga_chemical_1999, title = {Chemical discrimination on atomic level by {STM}}, volume = {141}, doi = {10.1016/S0169-4332(98)00514-5}, abstract = {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.}, number = {3-4}, journal = APSS, author = {P. Varga and M. Schmid}, month = mar, year = {1999}, pages = {287--293} }, @article{schmid_oxygen-induced_1999, title = {Oxygen-induced vacancy formation on a metal surface}, volume = {82}, doi = {10.1103/PhysRevLett.82.355}, abstract = {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.}, number = {2}, journal = PRL, author = {M. Schmid and G. Leonardelli and M. Sporn and E. Platzgummer and W. Hebenstreit and M. Pinczolits and P. Varga}, year = {1999}, pages = {355} }, @article{platzgummer_temperature-dependent_1999-1, title = {Temperature-dependent segregation on {Pt$_{25}$Rh$_{75}$(111)} and (100)}, volume = {419}, doi = {10.1016/S0039-6028(98)00800-0}, abstract = {Surface segregation is studied on {Pt25Rh75(111)} and {Pt25Rh75(100)} by {LEED} intensity analysis and {LEIS.} Although both equilibrated surfaces are strongly {Pt}-enriched (up to 80 at.\%), we find an interesting difference in the segregation behavior when annealing the sputtered surfaces. The {Pt} concentration grows continuously on {Pt25Rh75(111)} until {1000$^\circ$C,} whereas it reaches a maximum enrichment around {500$^\circ$C} on {Pt25Rh75(100)} and decreases thereafter. This contrasting behavior results solely from the kinetic limitation in the low-temperature regime, and is not due to energetic reasons. From temperature-dependent composition profiles we determine the segregation kinetics as well as the annealing temperature necessary for thermodynamic equilibration. We find that an equilibrium is acquired on the {Pt25Rh75(100)} surface by the interchange of {Pt} and {Rh} atoms within the near-surface layers, and on the {Pt25Rh75(111)} surface by a diffusion of {Pt} atoms from bulk to the near-surface region. The latter leads to an overall {Pt} enrichment of several layers, and is only observed after annealing at {1100$^\circ$C.} The presence of carbon contamination on the {Pt25Rh75(100)} surface causes a significant reduction of the {Pt} segregation. There is excellent agreement between the top-layer concentrations derived by {LEIS} and quantitative {LEED.}}, number = {2-3}, journal = SuSci, author = {E. Platzgummer and M. Sporn and R. Koller and S. Forsthuber and M. Schmid and W. Hofer and P. Varga}, year = {1999}, pages = {236--248} }, @article{zhang_spatial_1999, title = {Spatial self-organization of a nanoscale structure on the {Pt}(111) surface}, volume = {59}, doi = {10.1103/PhysRevB.59.5837}, abstract = {A self-organized nanostructure has been observed on the {Cr/Pt(111)} system. {Cr} overlayers with various coverages were deposited on a {Pt}(111) surface. Upon annealing to different temperatures, metastable surface alloys characteristic for a miscible metallic system have been observed. For a limited coverage range from 1.5 to 3.0 {ML} and an annealing temperature of 800 K, a spatial self-organization takes place on the bimetallic surface, forming a highly ordered hexagonal superstructure. The scanning tunnel microscope contrast of the superstructure is sensitive to the bias voltage, resulting in different patterns: either a mosaic structure and/or a network consisting of one-atom-wide lines. Relative to the {Pt}(111) surface, the superstructure has a {($\surd{}$39$\times$$\surd{}$39)R16.1$^\circ$} unit cell with a dimension of 17.3 \AA{}. 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.}, number = {8}, journal = PRB, author = {L. Zhang and J. van Ek and U. Diebold}, year = {1999}, pages = {5837--5846} }, @article{li_oxygen-induced_1999-1, title = {Oxygen-induced restructuring of rutile {TiO$_2$(110):} formation mechanism, atomic models, and influence on surface chemistry}, volume = {114}, doi = {10.1039/a903598b}, abstract = {The rutile {TiO$_2$(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 \AA{} wide), one-unit cell wide strands, and small ($\approx{}$tens of \AA{}) (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 {TiO$_2$} and other bulk-reduced oxide materials is briefly discussed.}, journal = {Faraday Discussions}, author = {Min Li and Wilhelm Hebenstreit and Ulrike Diebold and Michael A. Henderson and Dwight R. Jennison}, year = {1999}, pages = {245--258} }, @article{hayderer_highly_1999, title = {A highly sensitive quartz-crystal microbalance for sputtering investigations in slow ion--surface collisions}, volume = {70}, doi = {10.1063/1.1149979}, abstract = {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.}, number = {9}, journal = RSI, author = {G. Hayderer and M. Schmid and P. Varga and {HP.} Winter and F. Aumayr}, year = {1999}, pages = {3696--3700} }, @incollection{czanderna_specimen_1999, series = {Methods of Surface Characterization}, title = {Specimen Treatment: Preparation of Metal Compound Materials {(Mainly} Oxides)}, volume = {4}, isbn = {{030645887X,} 9780306458873}, booktitle = {Specimen handling, preparation, and treatments in surface characterization}, publisher = {Springer}, author = {Ulrike Diebold}, editor = {Alvin Warren Czanderna and Cedric John Powell and Theodore E. Madey}, year = {1999}, pages = {145--172} }, @article{wouda_stm_1998, title = {{STM} study of the (111) and (100) surfaces of {PdAg}}, volume = {417}, doi = {10.1016/S0039-6028(98)00673-6}, abstract = {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.}, number = {2-3}, journal = SuSci, author = {P. T. Wouda and M. Schmid and B. E. Nieuwenhuys and P. Varga}, month = nov, year = {1998}, pages = {292--300} }, @article{sporn_accuracy_1998, title = {The accuracy of quantitative {LEED} in determining chemical composition profiles of substitutionally disordered alloys: a case study}, volume = {416}, doi = {10.1016/S0039-6028(98)00596-2}, abstract = {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.}}, number = {3}, journal = SuSci, author = {M. Sporn and E. Platzgummer and S. Forsthuber and M. Schmid and W. Hofer and P. Varga}, month = oct, year = {1998}, pages = {423--429} }, @article{sporn_quantitative_1998, title = {A quantitative {LEED} analysis of the oxygen-induced p(3$\times$1) reconstruction of {Pt$_{25}$Rh$_{75}$(100)}}, volume = {416}, doi = {10.1016/S0039-6028(98)00574-3}, abstract = {{Pt25Rh75(100)} forms a p(3$\times$1) reconstruction at saturation coverage of oxygen (23 L O2, {600$^\circ$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 \AA{}. The first substrate layer relaxes outward by 8.8\% of the bulk value to 2.08 \AA{}, but we do not observe any significant relaxations of deeper layer spacings. The shifted rows pop out of the surface by 0.38 \AA{}. 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$\times$1)-O/Pt25Rh75(100)} is enriched in {Rh} (90\%). Oxygen adsorption at a moderate temperature {(600$^\circ$C)} and formation of the p(3$\times$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.}, number = {3}, journal = SuSci, author = {M. Sporn and E. Platzgummer and E. L. D. Gruber and M. Schmid and W. Hofer and P. Varga}, month = oct, year = {1998}, pages = {384--395} }, @article{aschoff_unreconstructed_1998, title = {Unreconstructed {Au}(100) monolayers on a {Au$_3$Pd(100)} single-crystal surface}, volume = {415}, doi = {10.1016/S0039-6028(98)00564-0}, abstract = {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 \AA{} as evaluated by X-ray analysis. The surface lattice constant of the gold layer on the alloy surface is 0.08 \AA{} smaller than that of bulk gold.}, number = {3}, journal = SuSci, author = {M. Aschoff and S. Speller and J. Kuntze and W. Heiland and E. Platzgummer and M. Schmid and P. Varga and B. Baretzky}, month = oct, year = {1998}, pages = {L1051--L1054} }, @article{li_oxygen-induced_1998, title = {Oxygen-induced restructuring of the rutile {TiO$_2$(110)(1$\times$1)} surface}, volume = {414}, doi = {10.1016/S0039-6028(98)00549-4}, abstract = {We report scanning tunneling microscopy {(STM)} results of bulk-reduced rutile {TiO$_2$} single crystals. {TiO$_2$(110)} surfaces, prepared by sputtering and annealing at 850 K in {UHV,} exhibit a (1$\times$1) surface termination and flat, several-hundred-angstrom-wide terraces. After exposure to oxygen at elevated temperatures (onset $\approx$470 K), the surfaces are covered with small (typically tens of angstroms wide) terraces with monoatomic step height and the same (1$\times$1) structure. On top and in between these terraces appear patches of an irregular network consisting of interconnected rosettes (width $\approx$7 \AA{}) with pseudohexagonal symmetry. The positions of atoms within the network are consistent with an incomplete {TiO$_2$(110)} layer. This is a kinetically limited, metastable phase that easily transforms into the regular (1$\times$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 {TiO$_2$(110)(1$\times$1)} surface.}, number = {1-2}, journal = SuSci, author = {M Li and W Hebenstreit and U Diebold}, month = sep, year = {1998}, pages = {L951--L956} }, @article{zhang_characterization_1998, title = {Characterization of {Ca} impurity segregation on the {TiO$_2$(110)} surface}, volume = {412-413}, doi = {10.1016/S0039-6028(98)00432-4}, abstract = {{TiO$_2$(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 {TiO$_2$(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.}, journal = SuSci, author = {L. P. Zhang and M. Li and U. Diebold}, month = sep, year = {1998}, pages = {242--251} }, @article{platzgummer_trajectory-dependent_1998, title = {Trajectory-dependent neutralization of 1 {keV} {He}$^+$ ions scattered from {Pb}(111) and {Pb} films on {Cu}(100)}, volume = {412-413}, doi = {10.1016/S0039-6028(98)00388-4}, abstract = {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$^\circ$. This corresponds to ion-neighbor distances between 2.3 and 1 \AA{}. 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 \AA{}. 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$\times$4) {Pb/Cu(100)} and the existence of a {Pb} overlayer on the c(2$\times$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.}, journal = SuSci, author = {E. Platzgummer and M. Borrell and C. Nagl and M. Schmid and P. Varga}, month = sep, year = {1998}, pages = {202--212} }, @article{epling_evidence_1998, title = {Evidence for oxygen adatoms on {TiO$_2$(110)} resulting from {O$_2$} dissociation at vacancy sites}, volume = {412-413}, doi = {10.1016/S0039-6028(98)00446-4}, abstract = {Annealing {TiO$_2$(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 ({\textless}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 {TiO$_2$(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 ({\textgreater}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 {TiO$_2$(110).} These results also detail the importance of sample preparation techniques on the chemistry which can occur at a solid surface.}, journal = SuSci, author = {William S. Epling and Charles H. F. Peden and Michael A. Henderson and Ulrike Diebold}, month = sep, year = {1998}, pages = {333--343} }, @article{diebold_intrinsic_1998, title = {Intrinsic defects on a {TiO$_2$(110)(1$\times$1)} surface and their reaction with oxygen: a scanning tunneling microscopy study}, volume = {411}, doi = {10.1016/S0039-6028(98)00356-2}, abstract = {We report a scanning tunneling microscopy {(STM)} study of the rutile {TiO$_2$(110)} surface. The surface was prepared by sputtering and annealing in an ultrahigh vacuum {(UHV).} After annealing to 1100 K in {UHV,} a (1$\times$1) surface with a terrace width of $\approx$100 \AA{} is obtained. The terraces are separated by monoatomic step edges running predominantly parallel to {\textless}001{\textgreater} and type directions. Approximately half of the {\textless}001{\textgreater}-type steps have a kinked appearance that is attributed to a (4$\times$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$\pm{}$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.}, number = {1-2}, journal = SuSci, author = {Ulrike Diebold and Jeremiah Lehman and Talib Mahmoud and Markus Kuhn and Georg Leonardelli and Wilhelm Hebenstreit and Michael Schmid and Peter Varga}, month = aug, year = {1998}, pages = {137--153} }, @article{diebold_high_1998, title = {High Transient Mobility of Chlorine on {TiO$_2$(110):} Evidence for {``Cannon-Ball''} Trajectories of Hot Adsorbates}, volume = {81}, doi = {10.1103/PhysRevLett.81.405}, abstract = {Scanning tunneling microscopy was used to study the initial stages of {Cl}2 adsorption on {TiO}2(110). {Cl} atoms adsorb on the rows of fivefold coordinated surface {Ti} atoms, and mostly form well separated pairs (average distance 26 \AA{}, 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 \textquotedblleft{}cannon-ball\textquotedblright{}-like trajectory. Channeling along the {Ti} rows leads to large average {Cl-Cl} distances.}, number = {2}, journal = PRL, author = {Ulrike Diebold and Wilhelm Hebenstreit and Georg Leonardelli and Michael Schmid and Peter Varga}, month = jul, year = {1998}, pages = {405} }, @article{hofer_scanning_1998, title = {Scanning tunneling microscopy of binary-alloy surfaces: is chemical contrast a consequence of alloying?}, volume = {405}, doi = {10.1016/S0039-6028(98)00140-X}, abstract = {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.}, number = {2-3}, journal = SuSci, author = {W. A. Hofer and G. Ritz and W. Hebenstreit and M. Schmid and P. Varga and J. Redinger and R. Podloucky}, month = may, year = {1998}, pages = {L514--L519} }, @article{robbert_novel_1998, title = {Novel electronic and magnetic properties of ultrathin chromium oxide films grown on {Pt}(111)}, volume = {16}, doi = {10.1116/1.581283}, abstract = {The growth of epitaxial metal\textendash{}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 $\times$ 2) symmetry through the first two monolayers of growth which is followed by a ($\surd{}$3 $\times$ {$\surd{}$3)R30$^\circ$} 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 $\approx$ 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 $\times$ 2) phase beyond $\approx$ 0.2 monolayers is more consistent with either a {gamma-Cr2O3(111)} overlayer or possibly a reconstructed {Cr2O3(0001)} overlayer.}, journal = JVSTA, author = {P. S. Robbert and H. Geisler and Jr. Ventrice and J. van Ek and S. Chaturvedi and J. A. Rodriguez and M. Kuhn and U. Diebold}, month = may, year = {1998}, pages = {990--995} }, @article{zhang_highly_1998, title = {Highly ordered nanoscale surface alloy formed through Cr-induced {Pt}(111) reconstruction}, volume = {57}, doi = {10.1103/PhysRevB.57.R4285}, abstract = {We present scanning tunneling microscopy {(STM)} results of an ordered {Cr/Pt} surface alloy formed upon annealing 1.5\textendash{}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 \AA{}; 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.}, number = {8}, journal = PRB, author = {Lanping Zhang and J. van Ek and Ulrike Diebold}, month = feb, year = {1998}, pages = {R4285--R4288} }, @article{sporn_anti-corrugation_1998, title = {Anti-corrugation and nitrogen c(2 $\times$ 2) on {Cr}(100): {STM} on atomic scale and quantitative {LEED}}, volume = {396}, doi = {10.1016/S0039-6028(97)00660-2}, abstract = {We present a {LEED} {I-V} analysis of c(2 $\times$ {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 \AA{} above the first chromium layer. The interlayer spacing between the first and the second chromium layer is expanded to 1.55 \AA{} (7.5\% with respect to the bulk value of 1.44 \AA{}). The interlayer spacing between the second and the third layer is contracted to 1.41 \AA{}. The second chromium layer is buckled (0.13 \AA{}). 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 \AA{}, the bond length to the second-layer chromium atom amounts to 1.97 \AA{}. The nitrogen position in c(2 $\times$ {2)-N} determined by {LEED} is used to identify hollow sites in scanning tunnelling microscopy images. We found that hollow sites in p(1 $\times$ {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 $\times$ {1)-N/Cr(100)} at a (local) nitrogen coverage of 1 monolayer.}, number = {1-3}, journal = SuSci, author = {M. Sporn and E. Platzgummer and M. Pinczolits and W. Hebenstreit and M. Schmid and W. Hofer and P. Varga}, year = {1998}, pages = {78--86} }, @article{gauthier_chemical_1998, title = {Chemical ordering and reconstruction of {Pt$_{25}$Co$_{75}$(100):} an {LEED/STM} study}, volume = {396}, doi = {10.1016/S0039-6028(97)00665-1}, abstract = {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 $\times$ 5), (1 $\times$ 6) and (1 $\times$ 7) patterns. Segregation and chemical ordering lead to the formation of c(2 $\times$ 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 \AA{} above that of {Co}, whereas {LEED} shows that {Pt} atoms are geometrically $\approx$0.04 \AA{} higher. The composition was determined down to the fourth layer. An oscillatory segregation profile is observed, with {Pt}-rich layers ({\textless}c1{\textgreater} = 62.6\% {Pt}, {\textless}c3{\textgreater} = 53.5\%) and {Pt}-depleted layers ({\textless}c2{\textgreater} = 6.9\%, {\textless}c4{\textgreater} = 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.}, number = {1-3}, journal = SuSci, author = {Y. Gauthier and P. Dolle and R. {Baudoing-Savois} and W. Hebenstreit and E. Platzgummer and M. Schmid and P. Varga}, year = {1998}, pages = {137--155} }, @article{rodriguez_h2s_1997, title = {{H$_2$S} adsorption on chromium, chromia, and gold/chromia surfaces: Photoemission studies}, volume = {107}, doi = {10.1063/1.475319}, abstract = {The reaction of {H$_2$S} 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, {H$_2$S} 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 {H$_2$S} on {Cr3O4} and {Cr2O3} films at room temperature produces a decrease of 0.3\textendash{}0.8 {eV} in the work function of the surface and significant binding-energy shifts (0.2\textendash{}0.6 {eV)} in the {Cr} 3p core levels and {Cr} 3d features in the valence region. The rate of dissociation of {H$_2$S} increases following the sequence: {Cr2O3{\textless}Cr3O4{\textless}Cr.} For chromium, the density of states near the Fermi level is large, and these states offer a better match in energy for electron acceptor or donor interactions with the frontier orbitals of {H$_2$S} than the valence and conduction bands of the chromium oxides. This leads to a large dissociation probability for {H$_2$S} on the metal, and a low dissociation probability for the molecule on the oxides. In the case of {Cr3O4} and {Cr2O3,} there is a correlation between the size of the band gap in the oxide and its reactivity toward {H$_2$S.} The uptake of sulfur by the oxides significantly increases when they are "promoted" with gold. The {Au/Cr2O3} surfaces exhibit a unique electronic structure in the valence region and a larger ability to dissociate {H$_2$S} than polycrystalline Au or pure {Cr2O3.} The results of ab initio {SCF} calculations for the adsorption of {H$_2$S} on {AuCr4O6} and {AuCr10O15} clusters show a shift of electrons from the gold toward the oxide unit that enhances the strength of the {Au(6s){\textless}--{\textgreater}H$_2$S(5a1,2b1)} bonding interactions and facilitates the decomposition of the molecule.}, number = {21}, journal = JCP, author = {J. A. Rodriguez and S. Chaturvedi and M. Kuhn and J. van Ek and U. Diebold and P. S. Robbert and H. Geisler and Jr. Ventrice}, month = dec, year = {1997}, pages = {9146--9156} }, @article{schmid_nitrogen-induced_1997, title = {The nitrogen-induced herringbone reconstruction of {Cr}(110)}, volume = {389}, doi = {10.1016/S0039-6028(97)00474-3}, abstract = {Segregation of nitrogen causes herringbone-like (3 $\times$ n) surface reconstructions, which were studied by atomically resolved scanning tunneling microscopy {(STM).} The main building blocks of the reconstruction are domains of ($\surd{}$6 $\times$ {$\surd{}$6)R} $\pm{}$ 35$^\circ$ 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.}, number = {1-3}, journal = SuSci, author = {M. Schmid and M. Pinczolits and W. Hebenstreit and P. Varga}, month = nov, year = {1997}, pages = {L1140--L1146} }, @article{wouda_interaction_1997, title = {Interaction of oxygen with {PtRh(100)} studied with {STM}}, volume = {388}, doi = {10.1016/S0039-6028(97)00375-0}, abstract = {The adsorption of oxygen at {500$^\circ$C} on a {Pt50Rh50(100)} single crystal surface was studied using {UHV-STM} and Auger electron spectroscopy. Images were taken of the p(3 $\times$ 1) phase; of a mixed phase with p(2 $\times$ 2), c(2 $\times$ 2) and (3 $\times$ 3) units; and of rhodium oxide patches. Possible models for these structures involving surface reconstruction are presented. Exposure of the p(3 $\times$ {1)O/PtRh(100)} to H$_2$ at room temperature led to the conversion to the p(1 $\times$ 1) substrate structure. The ordering and composition of this substrate structure after reduction is discussed.}, number = {1-3}, journal = SuSci, author = {P. T. Wouda and M. Schmid and W. Hebenstreit and P. Varga}, month = oct, year = {1997}, pages = {63--70} }, @article{hebenstreit_segregation_1997, title = {Segregation and reconstructions of {Pt$_x$Ni$_{1 - x}$(100)}}, volume = {388}, doi = {10.1016/S0039-6028(97)00392-0}, abstract = {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 $\times$ 1) superstructures (n = 7, 12 and 19) consisting of similar (7 $\times$ 1) and (5 $\times$ 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.}, number = {1-3}, journal = SuSci, author = {W. Hebenstreit and G. Ritz and M. Schmid and A. Biedermann and P. Varga}, month = oct, year = {1997}, pages = {150--161} }, @article{ritz_pt100_1997, title = {{Pt}(100) quasihexagonal reconstruction: A comparison between scanning tunneling microscopy data and effective medium theory simulation calculations}, volume = {56}, doi = {10.1103/PhysRevB.56.10518}, abstract = {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 \textquotedblleft{}fingerprint\textquotedblright{} 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.}, number = {16}, journal = PRB, author = {G. Ritz and M. Schmid and P. Varga and A. Borg and M. R\o{}nning}, month = oct, year = {1997}, pages = {10518} }, @article{zhang_thermal_1997, title = {Thermal Stability of Ultrathin {Cr} Films on {Pt}(111)}, volume = {101}, doi = {10.1021/jp9627786}, abstract = {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.}, number = {23}, journal = JPCB, author = {Lanping Zhang and Markus Kuhn and Ulrike Diebold and Jose A. Rodriguez}, month = jun, year = {1997}, pages = {4588--4596} }, @article{zhang_epitaxial_1997, title = {Epitaxial growth of ultrathin films of chromium and its oxides on {Pt}(111)}, volume = {15}, doi = {10.1116/1.580635}, abstract = {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 $\times$ 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 $\times$ [square root {of]3)R30$^\circ$} structure appears, due to the formation of the stable {Cr2O3} phase.}, journal = JVSTA, author = {Lanping Zhang and Markus Kuhn and Ulrike Diebold}, month = may, year = {1997}, pages = {1576--1580} }, @article{schmid_segregation_1997, title = {Segregation of impurities on {Cr}(100) studied by {AES} and {STM}}, volume = {377-379}, doi = {10.1016/S0039-6028(96)01539-7}, abstract = {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 $\times$ 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$\surd{}$2 $\times$ {$\surd{}$2)R} $\pm{}$ 45$^\circ$ structure forms, followed by a first-order phase transformation to a p(1 $\times$ 1) structure. The existence of patches of the N-rich p(1 $\times$ 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.}, journal = SuSci, author = {M. Schmid and M. Pinczolits and W. Hebenstreit and P. Varga}, month = apr, year = {1997}, pages = {1023--1027} }, @article{shaw_magnesium_1997, title = {Magnesium outdiffusion through magnetite films grown on magnesium oxide (001) (abstract)}, volume = {81}, doi = {10.1063/1.365161}, abstract = {Scanning tunneling microscopy {(STM)} studies of 1 $\mathrm{\mu}$m thick films of single crystalline {Fe$_3$O$_4$} grown on {MgO(001)} indicate that repeated annealing of the sample in {UHV} causes {Mg} diffusion through the {Fe$_3$O$_4$} film. The onset of this effect was clearly seen by {STM} at room temperature for samples raised above 400\textendash{}430 {$^\circ$C.} It appears that the annealing process causes the migration of {Mg} from the substrate entirely through the {Fe$_3$O$_4$} 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, {Fe$_3$O$_4$,} (8.396 \AA{}), which is already strained in thin-film growth on a substrate, further toward the cubic lattice constant of magnesioferrite, {MgFe2O4,} (8.375 \AA{}) 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 {Fe$_3$O$_4$} film of the same thickness and growth parameters, which shows no magnesium migration into the film during growth up to substrate temperatures of 300 {$^\circ$C.}}, number = {8}, journal = JAP, author = {K. A. Shaw and E. Lochner and D. M. Lind and J. F. Anderson and M. Kuhn and U. Diebold}, month = apr, year = {1997}, pages = {5176} }, @article{tao_decomposition_1997, title = {Decomposition of {P(CH$_3$)$_3$} on {Ru}(0001): comparison with {PH$_3$} and {PCl$_3$}}, volume = {375}, doi = {10.1016/S0039-6028(96)01280-0}, abstract = {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} --{\textgreater} {P(CH3)2} --{\textgreater} {P(CH3)} --{\textgreater} {P}, which is complete around $\approx$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} --{\textgreater} {PHx} --{\textgreater} {P}, which is complete around $\approx$190 K. The conversion of chemisorbed phosphorus to ruthenium phosphide is observed and is complete around $\approx$700 K on Ru(0001). {PCl3} also follows a step-wise decomposition reaction, {PCl3} --{\textgreater} {PCl2} --{\textgreater} {PCl} --{\textgreater} {P}, which is complete around $\approx$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.}, number = {2-3}, journal = SuSci, author = {{H.-S.} Tao and U. Diebold and {N.D.} Shinn and {T.E.} Madey}, month = apr, year = {1997}, pages = {257--267} }, @article{zhang_growth_1997, title = {Growth, structure and thermal properties of chromium oxide films on {Pt}(111)}, volume = {375}, doi = {10.1016/S0039-6028(96)01256-3}, abstract = {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 $\times$ 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 $\times$ 2) structure is observed and attributed to the epitaxial growth of metastable {Cr3O4.} At higher coverage, a ([radical sign]3 $\times$ [radical {sign]3)R30$^\circ$} structure appears, due to the formation of the stable {Cr2O3} phase. After heating in vacuum to about 700 K, the p(2 $\times$ 2) film begins to decompose and agglomerate, and finally becomes a ([radical sign]3 $\times$ [radical {sign]3)R30$^\circ$} 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.}, number = {1}, journal = SuSci, author = {Lanping Zhang and Markus Kuhn and Ulrike Diebold}, month = mar, year = {1997}, pages = {1--12} }, @article{borrell_study_1997, title = {Study of silica supported {Pt$_x$Ni$_{1-x}$} catalysts by ion scattering spectroscopy}, volume = {125}, doi = {10.1007/BF01246216}, abstract = {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 H$_2$ up to 500 {$^\circ$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 {$^\circ$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.}, number = {1}, journal = {Microchimica Acta}, author = {Martine Borrell and Andreas Jentys and Peter Varga}, month = mar, year = {1997}, pages = {389--393} }, @article{zhang_growth_1997-1, title = {Growth and structure of ultrathin {Cr} films on {Pt}(111)}, volume = {371}, doi = {10.1016/S0039-6028(96)01001-1}, abstract = {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, {\textless}111{\textgreater}, aligned with the close-packed {Pt}(111) directions, {\textless}110{\textgreater}. 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.}, number = {2-3}, journal = SuSci, author = {Lanping Zhang and Markus Kuhn and Ulrike Diebold}, month = feb, year = {1997}, pages = {223--234} }, @article{anderson_surface_1997, title = {Surface structure and morphology of {Mg}-segregated epitaxial {Fe$_3$O$_4$(001)} thin films on {MgO(001)}}, volume = {56}, doi = {10.1103/PhysRevB.56.9902}, abstract = {We have investigated the structural and compositional changes that are induced by the segregation of substrate {Mg} to the surface of 1$\mu{}$m-thick {Fe$_3$O$_4$(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 {Fe$_3$O$_4$} 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\textendash{}grown samples are flat and show a {($\surd{}$2$\times$$\surd{}$2)R45$^\circ$} reconstruction with respect to the {Fe$_3$O$_4$} surface unit cell. Onset of {Mg} segregation to the surface occurs around 670 K, with long, narrow extensions of terraces growing along the [110] and [11\textasciimacron{}0] directions. Heating in an oxygen atmosphere induces a 1$\times$4 surface reconstruction, and extremely long ($\approx{}$1000 \AA{}), wide terraces. We attribute this annealing stage to the formation of a {MgFe2O4} surface phase, exhibiting highly anisotropic surface diffusion and step formation energy.}, number = {15}, journal = PRB, author = {J. Anderson and Markus Kuhn and Ulrike Diebold and K. Shaw and P. Stoyanov and D. Lind}, year = {1997}, pages = {9902--9909} }, @article{varga_sputter_1997, title = {Sputter yields of insulators bombarded with hyperthermal multiply charged ions}, volume = {T73}, doi = {10.1088/0031-8949/1997/T73/100}, abstract = {The total sputter yield for Au, Si, {GaAs,} {SiO2,} {MgO,} {LiF} and {NaCl} bombarded with hyperthermal highly charged {Ar}{\textasciicircum}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.}, journal = {Physica Scripta}, author = {P. Varga and T. Neidhart and M. Sporn and G. Libiseller and M. Schmid and F. Aumayr and H. P. Winter}, year = {1997}, pages = {307--310} }, @article{sporn_potential_1997, title = {Potential Sputtering of Clean {SiO$_2$} by Slow Highly Charged Ions}, volume = {79}, doi = {10.1103/PhysRevLett.79.945}, abstract = {The recently discovered phenomenon of potential sputtering, i.e., the efficient removal of neutral and ionized target particles from certain insulator surfaces due to the potential rather than the kinetic energy of impinging slow highly charged ions, has now also been observed for stoichiometric {SiO2} surfaces. Using a sensitive quartz crystal microbalance technique, total sputter yields induced by {Ar}{\textasciicircum}q+ (q$\leq{}$14) and {Xe}{\textasciicircum}q+ (q$\leq{}$27) ions have been determined for {LiF} and {SiO2} surfaces. The primary mechanisms for potential sputtering (defect mediated sputtering) and its considerable practical relevance for highly charged ion-induced surface modification of insulators are discussed.}, number = {5}, journal = PRL, author = {M. Sporn and G. Libiseller and T. Neidhart and M. Schmid and F. Aumayr and {HP.} Winter and P. Varga and M. Grether and D. Niemann and N. Stolterfoht}, year = {1997}, pages = {945} }, @incollection{kuhn_intrinsic_1997, address = {Warrendale, {PA}}, series = {Mater. Res. Soc. Symp. Proc.}, title = {Intrinsic Point Defects on {TiO$_2$(110)} Surface and Their Reaction With Oxygen: A Scanning-Tunneling-Microscopy Study}, volume = {474}, isbn = {1-55899-378-9}, abstract = {The interaction of molecular oxygen, at room temperature, with a reduced {TiO$_2$(110)} surface has been studied in situ by scanning tunneling microscopy {(STM).} Oxygen vacancies (point defects) were created on a clean {TiO$_2$(110)} surface by annealing in ultra-high vacuum and successfully imaged on the atomic scale. These point defect sites were stable under ultrahigh vacuum conditions. During exposure to molecular oxygen, new point defects appear at different locations on the surface although their overall number is reduced. A mechanism for this dynamic healing process is proposed.}, booktitle = {Epitaxial Oxide Thin Films {III}}, author = {K. Kuhn and J. F. Anderson and J. Lehman and T. Mahmoud and U. Diebold}, editor = {C. Foster and {J.S.} Speck and D. Schlom and {C.-B.} Eom and {M.E.} Hawley}, year = {1997}, pages = {359} }, @incollection{anderson_surface_1997-1, address = {Warrendale, {PA}}, series = {Mater. Res. Soc. Symp. Proc.}, title = {Surface structure and morphology of {Mg}-segregated, epitaxial {Fe$_3$O$_4$} thin films on {MgO(001)}}, volume = {474}, isbn = {1-55899-378-9}, abstract = {{Fe$_3$O$_4$} films on {MgO(001).} The thin films have been grown with plasma-assisted {MBE,} and characterization with {RHEED} (reflection high-energy electron diffraction), x-ray diffraction {(XRD),} and Superconducting Quantum Interference Device {(SQUID)} magnetometry show slightly strained, singlecrystalline Fe$_3$O$_4$ 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 $(\sqrt{2} \times \sqrt{2})${R}$45^\circ$ reconstruction with respect to the Fe$_3$O$_4$ 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 ($\approx$ 1000 A), wide terraces.}, booktitle = {Epitaxial Oxide Thin Films {III}}, author = {J. F. Anderson and K. Kuhn and U. Diebold and K. Shaw}, editor = {C. Foster and {J.S.} Speck and D. Schlom and {C.-B.} Eom and {M.E.} Hawley}, year = {1997}, pages = {265} }, @article{nagl_inverse_1996, title = {Inverse corrugation and corrugation enhancement of {Pb} superstructures on {Cu}(111) and (110)}, volume = {369}, doi = {10.1016/S0039-6028(96)00907-7}, abstract = {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 $\times$ 1) superstructure of {Pb/Cu(110),} similarly, the lowest coordinated {Pb} atoms also show the lowest apparent height in {STM} images.}, number = {1-3}, journal = SuSci, author = {C. Nagl and M. Schmid and P. Varga}, month = dec, year = {1996}, pages = {159--168} }, @article{tsong_scanning_1996, title = {Scanning tunneling microscopy studies of niobium carbide (100) and (110) surfaces}, volume = {366}, doi = {10.1016/0039-6028(96)00804-7}, abstract = {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 $\times$ 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 $\times$ 1) and (5 $\times$ 1) geometries was observed over large areas. The nanometer-scale faceting phenomenon may be common to the (110) surfaces of most transition-metal carbides.}, number = {1}, journal = SuSci, author = {R. M. Tsong and M. Schmid and C. Nagl and P. Varga and R. F. Davis and I. S. T. Tsong}, month = oct, year = {1996}, pages = {85--92} }, @article{wouda_chemically_1996, title = {Chemically resolved {STM} on a {PtRh(100)} surface}, volume = {359}, doi = {10.1016/0039-6028(96)00022-2}, abstract = {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 ({\textless}500 k$\Omega{}$), 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.}, number = {1-3}, journal = SuSci, author = {P. T. Wouda and B. E. Nieuwenhuys and M. Schmid and P. Varga}, month = jul, year = {1996}, pages = {17--22} }, @article{diebold_tio2_1996, title = {{TiO$_2$} by {XPS}}, volume = {4}, doi = {10.1116/1.1247794}, abstract = {The surfaces of titanium oxide belong to the most-studied oxide systems in the surface science literature. This is in part because {TiO$_2$} surfaces and interfaces play a major role in several technological applications, e.g., as promoters in catalysis, as photocatalysts, and as gas sensors. {TiO$_2$} 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)]. {TiO$_2$} 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 {TiO$_2$} surface can reproducibly be prepared through sputtering and annealing in oxygen (2 $\times$ 10 \textendash{} 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.}}, number = {3}, journal = {Surface Science Spectra}, author = {Ulrike Diebold and T. E. Madey}, month = jul, year = {1996}, pages = {227--231} }, @article{anderson_epitaxially_1996, title = {Epitaxially Grown {Fe$_3$O$_4$} Thin Films: An {XPS} Study}, volume = {4}, doi = {10.1116/1.1247796}, abstract = {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 $\mathrm{\mu}$m thick {Fe$_3$O$_4$} 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 $\times$ 10 \textendash{} 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.}, number = {3}, journal = {Surface Science Spectra}, author = {J. F. Anderson and Markus Kuhn and Ulrike Diebold}, month = jul, year = {1996}, pages = {266--272} }, @article{biedermann_scanning_1996, title = {Scanning Tunneling Spectroscopy of {One-Dimensional} Surface States on a Metal Surface}, volume = {76}, doi = {10.1103/PhysRevLett.76.4179}, abstract = {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$\times$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.}, number = {22}, journal = PRL, author = {A. Biedermann and O. Genser and W. Hebenstreit and M. Schmid and J. Redinger and R. Podloucky and P. Varga}, month = may, year = {1996}, pages = {4179} }, @article{nagl_subsurface_1996, title = {Subsurface islands and superstructures of {Cu} on {Pb}(111)}, volume = {352-354}, doi = {10.1016/0039-6028(95)01200-1}, abstract = {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.}, journal = SuSci, author = {C. Nagl and E. Platzgummer and M. Schmid and P. Varga and S. Speller and W. Heiland}, month = may, year = {1996}, pages = {540--545} }, @article{diebold_surface_1996, title = {Surface segregation of silicon in platinum(111)}, volume = {14}, doi = {10.1116/1.580318}, abstract = {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 \textendash{}(105 $\pm{}$ 30) {kJ/mole.} A well-ordered ([square root of]19 $\times$ [square root {of]19)R23.41$^\circ$} or {\textbar}{\textless}sub{\textgreater} - 2{\textless}/sub{\textgreater}{\textless}sup{\textgreater}3{\textless}/sup{\textgreater} {\textless}sup{\textgreater}2{\textless}/sup{\textgreater}{\textless}sub{\textgreater}5{\textless}/sub{\textgreater}{\textbar} 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 {\textbar}{\textless}sub{\textgreater} - 2{\textless}/sub{\textgreater}{\textless}sup{\textgreater}3{\textless}/sup{\textgreater} {\textless}sup{\textgreater}2{\textless}/sup{\textgreater}{\textless}sub{\textgreater}5{\textless}/sub{\textgreater}{\textbar} and {\textbar}{\textless}sub{\textgreater} - 3{\textless}/sub{\textgreater}{\textless}sup{\textgreater}2{\textless}/sup{\textgreater} {\textless}sup{\textgreater}3{\textless}/sup{\textgreater}{\textless}sub{\textgreater}5{\textless}/sub{\textgreater}{\textbar} 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.}, journal = JVSTA, author = {Ulrike Diebold and Lanping Zhang and John F. Anderson and Pawel Mrozek}, month = may, year = {1996}, pages = {1679--1683} }, @article{schmid_quantum_1996, title = {Quantum Wells and Electron Interference Phenomena in {Al} due to Subsurface Noble Gas Bubbles}, volume = {76}, doi = {10.1103/PhysRevLett.76.2298}, abstract = {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\textendash{}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.}, number = {13}, journal = PRL, author = {M. Schmid and W. Hebenstreit and P. Varga and S. Crampin}, month = mar, year = {1996}, pages = {2298} }, @article{diebold_evidence_1996, title = {Evidence for the Tunneling Site on Transition-Metal Oxides: {TiO$_2$(110)}}, volume = {77}, doi = {10.1103/PhysRevLett.77.1322}, abstract = {We present atomically resolved scanning tunneling microscopy {(STM)} images from {TiO$_2$(110)} surfaces. After annealing nearly perfect stoichiometric 1$\times$1 surfaces to elevated temperatures in ultrahigh vacuum, randomly distributed oxygen vacancies are observed. The apparent shape of these defects provides strong evidence that the {STM} is imaging undercoordinated {Ti} atoms, as do first-principles pseudopotential calculations of the electronic states accessible to tunneling. We thus resolve a controversy as to whether {STM} imaging on this surface is dominated by geometric-structure or electronic effects.}, number = {7}, journal = PRL, author = {Ulrike Diebold and J. Anderson and {Kwok-On} Ng and David Vanderbilt}, year = {1996}, pages = {1322--1325} }, @article{ritz_strain-induced_1996, title = {Strain-induced local surface chemical ordering observed by {STM}}, volume = {53}, doi = {10.1103/PhysRevB.53.16019}, abstract = {{PtNi} alloys are known to exhibit a tendency towards chemical ordering, which also effects surface segregation. Scanning tunneling microscopy results obtained in the strain field of dislocations on {PtxNi1-x(110)} surfaces show a (2$\times$1) superstructure of alternating {Pt} and {Ni} atoms in some regions close to the dislocation core. In other regions, the apparent height of all surface atoms is equal, in agreement with low energy ion scattering results yielding a surface concentration of almost 100\% Ni. This indicates that the strain present in the vicinity of dislocations influences both the surface composition and chemical order. The experimental results are compared to simulation calculations of chemical ordering and segregation, using embedded atom method potentials and linear elasticity theory. The simulations indicate that the (2$\times$1) superstructure is due to an L10 ordered phase in regions where the tetragonal distortion of the L10 phase with respect to the cubic substrate can alleviate stress. It is argued that this dislocation-induced ordering can immobilize dislocations.}, number = {23}, journal = PRB, author = {G. Ritz and M. Schmid and A. Biedermann and P. Varga}, year = {1996}, pages = {16019--16026} }, @article{nagl_superstructures_1995, title = {$p(n \times 1)$ superstructures of {Pb} on {Cu}(110)}, volume = {52}, doi = {10.1103/PhysRevB.52.16796}, abstract = {The structures of the p(n$\times$1) superstructures (n=4, 5, and 9) of {Pb} on {Cu}(110) in the coverage range between {FTHETA=0.75} and 0.8 are revealed by atomically resolved scanning tunneling microscopy. All three superstructures are formed by substitution of every nth row of {Cu} atoms (n=4, 5, and 9) in the [001] direction by {Pb} atoms. The {Pb} atoms in between are lined up in the [1\textasciimacron{}10] direction. The p(4$\times$1) structure appears in two different modifications: one with substitutional rows of {Pb} atoms and one with a simple overlayer structure without substituted rows of {Cu} atoms. Alternating succession of these two modifications results in p(12$\times$1) domains. It is further shown that the p(9$\times$1) structure is not a succession of p(4$\times$1) and p(5$\times$1) but a superstructure on its own. The p(5$\times$1) structure proposed here agrees with previous x-ray-diffraction data at least as well as the quasihexagonal model proposed earlier. We have, in addition, identified the nature of the phase that has been described incommensurate obtained by desorption of {Pb} upon annealing above 600 K.}, number = {23}, journal = PRB, author = {C. Nagl and M. Pinczolits and M. Schmid and P. Varga and I. K. Robinson}, month = dec, year = {1995}, pages = {16796} }, @article{diebold_adsorption_1995, title = {Adsorption and thermal stability of {Mn} on {TiO$_2$(110):} $2p$ X-ray absorption spectroscopy and soft X-ray photoemission}, volume = {343}, doi = {10.1016/0039-6028(95)00780-6}, abstract = {X-ray absorption and photoelectron spectroscopies are used to study the adsorption and reaction of {Mn} films deposited on {TiO$_2$(110)} at {25$^\circ$C} and after annealing to {$\approx$650$^\circ$C.} Fractional monolayer coverages of {Mn} at {25$^\circ$C} produce a reactive, disordered interface consisting of reduced {Ti} cations and oxidized {Mn} overlayer atoms. Metallic {Mn} is found only for thicker layers at {25$^\circ$C.} Annealing {Mn} films to {$\approx$650$^\circ$C} leads to several changes that are largely independent of initial overlayer coverage: Metallic {Mn} thermally desorbs leaving only {Mn}2+ ions; interfacial {Ti} cations are largely re-oxidized to the {Ti}4+ state; and the local order is increased at the interface. The formation of a crystalline ternary surface oxide, {MnTiOx,} is proposed to account for these chemical and structural changes.}, number = {1-2}, journal = SuSci, author = {Ulrike Diebold and Neal D. Shinn}, month = dec, year = {1995}, pages = {53--60} }, @article{boehmig_deconvolution_1995, title = {Deconvolution of {STM} images using entropy as a regularization functional}, volume = {353}, doi = {10.1007/s0021653530439}, abstract = {The Maximum Entropy approach is applied to restore and sharpen scanning tunneling microscopy {(STM)} images with atomic resolution. Based on the {STM} theory of Tersoff and Hamann the process of data acquisition can be approximated by the convolution of a localised atomic density of states (i.e. narrow spots in the reconstruction) of the sample and a Gaussian resolution function which limits the resolution. In {STM} practice a good and robust estimation of the atomic core positions is necessary for different reasons, such as to be able to calculate the characteristics of the atomic lattice or to study non-periodicities.}, number = {3}, journal = FreseniusJAC, author = {S. B\"{o}hmig and M. Schmid and H. St\"{o}ri}, month = oct, year = {1995}, pages = {439--442} }, @article{biedermann_competitive_1995, title = {Competitive segregation of {Si} and {P} on {Fe$_{96.5}$Si$_{3.5}$} (100) and (110)}, volume = {353}, doi = {10.1007/s0021653530259}, abstract = {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 $\times$ 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 $\times$ 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.}, number = {3}, journal = FreseniusJAC, author = {A. Biedermann and M. Schmid and B. Reichl and P. Varga}, month = oct, year = {1995}, pages = {259--262} }, @article{nagl_direct_1995, title = {Direct Observation of a New Growth Mode: Subsurface Island Growth of {Cu} on {Pb}(111)}, volume = {75}, doi = {10.1103/PhysRevLett.75.2976}, abstract = {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\textendash{}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.}, number = {16}, journal = PRL, author = {C. Nagl and E. Platzgummer and M. Schmid and P. Varga and S. Speller and W. Heiland}, month = oct, year = {1995}, note = {The atomically resolved {STM} images have been reproduced incorrectly in this article. The correct versions of fig. 2 and fig. 3 are shown in the errata, {PRL} vol. 76, p. 3240 (1996).}, pages = {2976} }, @article{speller_stm_1995, title = {An {STM} study of the step structure of {Pb}(110) and {Pb}(111)}, volume = {331-333}, doi = {10.1016/0039-6028(95)00257-X}, abstract = {The (110) and (111) surfaces of lead are investigated by scanning tunneling microscopy. They are atomically resolved with corrugations of 0.1 \AA{} and 0.6 \AA{} 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.}, number = {Part 2}, journal = SuSci, author = {S. Speller and W. Heiland and A. Biedermann and E. Platzgummer and C. Nagl and M. Schmid and P. Varga}, month = jul, year = {1995}, pages = {1056--1061} }, @article{nagl_surface_1995, title = {Surface alloying and superstructures of {Pb} on {Cu}(100)}, volume = {331-333}, doi = {10.1016/0039-6028(95)00387-8}, abstract = {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 $\times$ 4) superstructure is observed. The atomic arrangement of the c(4 $\times$ 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 $\times$ 2) structure can be observed. The transition to the c(5$\surd{}$2 $\times$ {$\surd{}$2)R45$^\circ$} superstructure of the dense overlayer proceeds via the insertion of antiphase domain boundary. The model for the c(5$\surd{}$2 $\times$ {$\surd{}$2)R45$^\circ$} superstructure presented in literature is confirmed.}, number = {Part 1}, journal = SuSci, author = {C. Nagl and E. Platzgummer and O. Haller and M. Schmid and P. Varga}, month = jul, year = {1995}, pages = {831--837} }, @article{diebold_ultrathin_1995, title = {Ultrathin metal film growth on {TiO$_2$(110):} an overview}, volume = {331-333}, doi = {10.1016/0039-6028(95)00124-7}, abstract = {The interface between an oxide substrate and a metal overlayer may crucially influence the macroscopic behavior of technological devices such as sensors, catalysts, ceramics, and semiconductor chips. Hence investigations of adsorption, nucleation and growth of ultrathin metal films on metal oxide surfaces have attracted increasing interest during recent years. Experimental results on the growth of metal overlayers on a model oxide, {TiO$_2$(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.}, number = {Part 2}, journal = SuSci, author = {Ulrike Diebold and {Jian-Mei} Pan and Theodore E. Madey}, month = jul, year = {1995}, pages = {845--854} }, @article{biedermann_domain_1995, title = {Domain wall structures in an ordered {Si/Fe(110)} surface alloy}, volume = {331-333}, doi = {10.1016/0039-6028(95)00339-8}, abstract = {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 $\times$ {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 $\times$ n) supercells can be observed.}, number = {Part 1}, journal = SuSci, author = {A. Biedermann and M. Schmid and P. Varga}, month = jul, year = {1995}, pages = {787--793} }, @article{neidhart_potential_1995, title = {Potential Sputtering of Lithium Fluoride by Slow Multicharged Ions}, volume = {74}, doi = {10.1103/PhysRevLett.74.5280}, abstract = {Thin polycrystalline {LiF} films have been bombarded by slow ($\leq{}$1 {keV)} multicharged Arq+ ions (q$\leq{}$9), in order to study the resulting total sputter yields by means of a quartz crystal microbalance. More than 99\% of sputtered particles are neutral and show yields, at given impact energy, in proportion to the potential energy of projectile ions. The respective \textquotedblleft{}potential sputtering\textquotedblright{} process already takes place far below 100 {eV} impact energy. It can be related to defect production in {LiF} following electron capture by the multicharged ions, and removes about one {LiF} molecule per 100 {eV} of projectile potential energy.}, number = {26}, journal = PRL, author = {T. Neidhart and F. Pichler and F. Aumayr and {HP.} Winter and M. Schmid and P. Varga}, month = jun, year = {1995}, pages = {5280} }, @article{neidhart_determination_1995, title = {Determination of electron-induced total sputter yield of {LiF}}, volume = {101}, doi = {10.1016/0168-583X(95)00062-3}, abstract = {We present measurements of the total sputter yield for {LiF} at {150$^\circ$C} induced by electrons with an impact energy between 10 and 500 {eV.} A nearly linear increase with kinetic energy has been measured. The decrease of the yield with increasing electron dose for energies below 100 {eV} is also shown. For very high electron dose the yield even becomes zero and simultaneously a red coloration of the surface is observed. Low energy ion scattering {(LEIS)} measurements at such a surface showed a {Li} enrichment to more than 90\%. To reach such a composition with electrons between 40 and 80 {eV} kinetic energy sputtering of about 20 monolayers {LiF} is necessary.}, number = {1-2}, journal = NIMB, author = {T. Neidhart and M. Sporn and M. Schmid and P. Varga}, month = jun, year = {1995}, pages = {127--130} }, @article{mayer_titanium_1995, title = {Titanium and reduced titania overlayers on titanium dioxide(110)}, volume = {73}, doi = {10.1016/0368-2048(94)02258-5}, abstract = {The adsorption of titanium on titanium dioxide {TiO$_2$(110)} has been studied by X-ray photoelectron spectroscopy {(XPS)} and low energy ion scattering {(LEIS).} The {XPS} data for {Ti} overlayers are interpreted using peak fitting based on experimental standard spectra. 4 \AA{} of {Ti} deposited at 150 K reacts with the substrate to produce [approximate] 12 \AA{} of intermediate oxidation state {Ti}. Adsorption of neutral metal begins on top of this interface oxide film, but 20 \AA{} of deposited {Ti} are needed to cover the oxide completely. {LEIS} data indicate a tendency for clustering of {Ti} on top of the interface oxide. {Ar}$^+$ sputtering of stoichiometric {TiO$_2$} 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 $\times$ 1017 ions cm-2, at which point the reduced overlayer is 17 \AA{} thick.}, number = {1}, journal = JElSpec, author = {J. T. Mayer and U. Diebold and T. E. Madey and E. Garfunkel}, month = may, year = {1995}, pages = {1--11} }, @article{neidhart_secondary_1995, title = {Secondary ion emission from lithium fluoride under impact of slow multicharged ions}, volume = {98}, doi = {10.1016/0168-583X(95)00169-7}, abstract = {Secondary ion emission has been investigated for bombardment of polycrystalline lithium fluoride by slow multicharged {Ar} ions (charge state q $\leq{}$ 9, impact energy E\_k $\leq{}$ 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.}}, number = {1-4}, journal = NIMB, author = {T. Neidhart and F. Pichler and F. Aumayr and H. P. Winter and M. Schmid and P. Varga}, month = may, year = {1995}, pages = {465--468} }, @article{benes_sensors_1995, title = {Sensors based on piezoelectric resonators}, volume = {48}, doi = {10.1016/0924-4247(95)00846-2}, abstract = {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.}, number = {1}, journal = {Sensors and Actuators A: Physical}, author = {E. Benes and M. Gr\"{o}schl and W. Burger and M. Schmid}, month = may, year = {1995}, pages = {1--21} }, @article{schmid_surface_1995, title = {Surface stress, surface elasticity, and the size effect in surface segregation}, volume = {51}, doi = {10.1103/PhysRevB.51.10937}, abstract = {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.}, number = {16}, journal = PRB, author = {M. Schmid and W. Hofer and P. Varga and P. Stoltze and K. W. Jacobsen and J. K. N\o{}rskov}, month = apr, year = {1995}, pages = {10937} }, @article{dong_search_1995, title = {A search for surface alloy formation in faceting induced by monolayer metal films: {Pd/W}(111) and {Ni/W}(111)}, volume = {322}, doi = {10.1016/0039-6028(95)90032-2}, abstract = {A monolayer of {Pd} on a {W}(111) surface will cause faceting of the surface upon annealing to T {\textgreater} 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 \AA{} {Pd} deposited at room temperature almost completely covers the substrate W. In the temperature range from room temperature to $\approx$ 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.}, number = {1-3}, journal = SuSci, author = {Chengzhi Dong and Lizhong Zhang and Ulrike Diebold and Theodore E. Madey}, year = {1995}, pages = {221--229} }, @article{tao_radiation-induced_1995, title = {Radiation-induced decomposition of {PF$_3$} on {Ru}(0001)}, volume = {13}, doi = {10.1116/1.579502}, abstract = {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} --{\textgreater} {PF2} --{\textgreater} {PF} --{\textgreater} {P}, is observed. The dissociation cross section for {PF3} --{\textgreater} {PF2} induced by 550 {eV} electrons is measured to be on the order of $\approx$ 1 $\times$ 10 \textendash{}16 cm2. The main channels that lead to {PF3} dissociation involve the excitation of {F} 2s as well as valence states of {PF3.}}, number = {5}, journal = JVSTA, author = {{H.-S.} Tao and U. Diebold and V. Chakarian and D. K. Shuh and J. A. Yarmoff and N. D. Shinn and T. E. Madey}, year = {1995}, pages = {2553--2557} }, @article{nagl_submonolayer_1994, title = {Submonolayer growth of {Pb} on {Cu}(111): surface alloying and de-alloying}, volume = {321}, doi = {10.1016/0039-6028(94)90189-9}, abstract = {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.}, number = {3}, journal = SuSci, author = {C. Nagl and O. Haller and E. Platzgummer and M. Schmid and P. Varga}, month = dec, year = {1994}, pages = {237--248} }, @article{biedermann_segregated_1994, title = {Segregated {Si} on {Fe$_{96.5}$Si$_{3.5}$(110):} Domain-wall structures in a two-dimensional alloy}, volume = {50}, doi = {10.1103/PhysRevB.50.17518}, abstract = {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$\times$3)Si} superstructure weakly disordered by $langle$110$rangle$ 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$\times$3) structure at least up to 40\% corresponding to a {c(1$\times$5)Si} commensurate domain-wall structure. Correlation analysis of the disordered phase at 15\% coverage, obtained by short annealing, allows a qualitative determination of the interaction forces among individual silicon atoms on the surface, yielding attraction of third-nearest neighbors and repulsion of nearest neighbors.}, number = {23}, journal = PRB, author = {A. Biedermann and M. Schmid and P. Varga}, month = dec, year = {1994}, pages = {17518} }, @article{diebold_electronic_1994, title = {Electronic structure of ultrathin {Fe} films on {TiO$_2$(110)} studied with soft-x-ray photoelectron spectroscopy and resonant photoemission}, volume = {50}, doi = {10.1103/PhysRevB.50.14474}, abstract = {We report on soft-x-ray photoelectron spectroscopy {(SXPS)} of a {TiO$_2$(110)} surface during deposition of {Fe} in the monolayer regime. At low fractional monolayer coverages, the adsorbed {Fe} atoms are oxidized and {Ti} cations at the interface become reduced due to {Fe} adsorption. {SXPS} from shallow core levels and valence bands show that {Fe} starts to exhibit metallic character at a coverage of approximately 0.7 equivalent monolayers. Two well separated defect states appear in the band gap of {TiO$_2$} at iron coverages well below one monolayer. We use resonant photoemission to obtain information on the partial density of states, and we assign these defect states as being {Fe}-derived and Ti-derived states, located at the {Fe} and {Ti} sites, respectively. We suggest that a position change of oxygen is involved in the bonding of {Fe} on the {TiO$_2$(110)} surface.}, number = {19}, journal = PRB, author = {Ulrike Diebold and Hui-Shu Tao and Neal D. Shinn and Theodore E. Madey}, month = nov, year = {1994}, pages = {14474} }, @article{schmid_shifted-row_1994, title = {The shifted-row reconstruction of {Pt$_x$Ni$_{1-x}$(100)}}, volume = {318}, doi = {10.1016/0039-6028(94)90103-1}, abstract = {Scanning tunneling microscopy on {Pt10Ni90(100)} and {Pt25Ni75(100)} single crystals reveals close-packed rows of atoms, which are shifted by {\textless}110{\textgreater} 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 $\times$ 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.}, number = {3}, journal = SuSci, author = {M. Schmid and A. Biedermann and S. D. B\"{o}hmig and P. Weigand and P. Varga}, month = oct, year = {1994}, pages = {289--298} }, @article{boehmig_enhancement_1994, title = {Enhancement of {STM} images and estimation of atomic positions based on maximum entropy deconvolution}, volume = {313}, doi = {10.1016/0039-6028(94)91152-5}, abstract = {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).}}, number = {1-2}, journal = SuSci, author = {S. D. B\"{o}hmig and M. Schmid and H. St\"{o}ri}, month = jun, year = {1994}, pages = {6--16} }, @article{tao_surface_1994, title = {Surface chemistry of {PH$_3$,} {PF$_3$} and {PCl$_3$} on {Ru}(0001)}, volume = {312}, doi = {10.1016/0039-6028(94)90725-0}, abstract = {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 {\textless} x {\textless} 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}, {F} and {Cl)} 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.}, number = {3}, journal = SuSci, author = {H.-S. Tao and U. Diebold and N. D. Shinn and T. E. Madey}, month = jun, year = {1994}, pages = {323--344} }, @article{neidhart_total_1994, title = {Total sputter yield of {LiF} induced by hyperthermal ions measured by a quartz microbalance}, volume = {90}, doi = {10.1016/0168-583X(94)95601-4}, abstract = {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.}}, number = {1-4}, journal = NIMB, author = {T. Neidhart and Z. Toth and M. Hochhold and M. Schmid and P. Varga}, month = may, year = {1994}, pages = {496--500} }, @incollection{varga_sputtering_1994, address = {Chichester}, series = {Wiley Series in Ion Chemistry and Physics}, title = {Sputtering of metals and insulators with hyperthermal singly and doubly charged rare gas ions}, volume = {3}, isbn = {0471938912}, abstract = {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$^\circ$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.}, booktitle = {Low Energy {Ion-Surface} Interactions}, publisher = {Wiley}, author = {P. Varga and U. Diebold}, editor = {J. Wayne Rabalais}, month = apr, year = {1994}, pages = {355--386} }, @article{diebold_supression_1994, title = {Supression of electron-induced positive ion emission by a molecular overlayer: Ion-molecule charge exchange at a surface}, volume = {72}, doi = {10.1103/PhysRevLett.72.1116}, abstract = {We report the suppression of electron-stimulated desorption of positive ions {(O+} and {F}+) from a {TiO$_2$} (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($\pm{}$0.5) and 2.7($\pm{}$1) ($\times$10-15) cm2 have been determined for {O}+ and {F}+ ions, respectively.}, number = {7}, journal = PRL, author = {Ulrike Diebold and Theodore E. Madey}, month = feb, year = {1994}, pages = {1116--1119} }, @article{biedermann_chemical_1994, title = {Chemical analysis of {Pt$_x$Ni$_{1-x}$} alloy single crystal surfaces by scanning tunnelling microscopy}, volume = {349}, doi = {10.1007/BF00323272}, abstract = {Two {STM} investigations are presented, in which irregular tip conditions enable direct access to chemical and structural information of a surface on an atomic scale, otherwise invisible for the {STM.} They allow a study of surface ordering of a {Pt25Ni75(111)} crystal by chemical contrast between the alloy components, and a study of carbon superstructures on a {Pt10Ni90(100)} surface by simultaneous imaging of substrate lattice and carbon atoms. All these images were obtained at very low tunnelling resistances and thus at small tip-sample distances. A chemical interaction between the probably adsorbate covered tip and the sample is proposed to explain these images.}, number = {1}, journal = FreseniusJAC, author = {A. Biedermann and M. Schmid and P. Varga}, year = {1994}, pages = {201--203} }, @incollection{pan_local_1994, title = {Local structural study of ultrathin metal films on {TiO$_2$(110)} using {ARXPS} and {MEED}}, isbn = {9810215517}, booktitle = {The Structure of Surfaces {IV:} Proceedings of the 4th International Conference on the Structure of Surfaces Shanghai, China August 16-19, 1993}, publisher = {World Scientific}, author = {J.-M. Pan and B. L. Maschhoff and U. Diebold and T. E. Madey}, editor = {S. Y. Tong and Xide Xie and {Hsi-Te} Hsieh and M. A. Van Hove}, year = {1994}, pages = {328} }, @article{chakarian_influence_1994, title = {The influence of preadsorbed {K} on the adsorption of {PF$_3$} on {Ru}(0001) studied by soft x-ray photoelectron spectroscopy}, volume = {100}, doi = {10.1063/1.467195}, number = {7}, journal = JCP, author = {Varoujan Chakarian and David K. Shuh and Jory A. Yarmoff and Hui-Shu Tao and Ulrike Diebold and Brian L. Maschhoff and Theodore E. Madey and Neal D. Shinn}, year = {1994}, pages = {5301} }, @article{stadler_surface_1993, title = {Surface effects on {Pt-Ni} single crystals calculated with the embedded-atom method}, volume = {48}, doi = {10.1103/PhysRevB.48.11352}, abstract = {Recent scanning-tunneling-microscopy {(STM)} experiments on {Pt25Ni75(111)} and {Pt10Ni90(110)} surfaces showing lattice mismatch dislocations near the surface and surface ordering phenomena have been verified in simulation calculations using the embedded-atom method {(EAM).} It was found that the {EAM} can be successfully applied for the calculation of ordering effects and crystal defects such as dislocations using energy minimization routines and Monte Carlo calculations. By comparing experimental data and {EAM} results, it was possible to determine the depth of the dislocations and therefore the number of layers with {Pt} enrichment on the surface. For the (111) face exhibiting surface ordering phenomena, the calculated short-range order is in agreement with {STM} data where the two species could be clearly distinguished.}, number = {15}, journal = PRB, author = {H. Stadler and W. Hofer and M. Schmid and P. Varga}, month = oct, year = {1993}, pages = {11352} }, @article{pan_ultrathin_1993, title = {Ultrathin reactive metal films on {TiO$_2$(110):} growth, interfacial interaction and electronic structure of chromium films}, volume = {295}, doi = {10.1016/0039-6028(93)90288-U}, abstract = {A study of ultrathin {Cr} films on {TiO$_2$(110)} surfaces is reported. The combination of low energy ion scattering {(LEIS)} and X-ray photoelectron spectroscopy {(XPS)} enables us to study quantitatively the growth of reactive metal films on metal oxides, and the chemical interactions at the interface. Ultrathin {Cr} films grow initially in a quasi-two-dimensional fashion at room temperature. Changes in oxidation states of both {Ti} and {Cr} during film formation suggest that strong chemical interactions at the interface are of great importance in the growth of reactive metal films. {Cr} "wets" the surface more effectively at 300 K than ultrathin films of less reactive metals, {Fe} and {Cu}. This suggests that a relation exists between the metal reactivity with oxygen and the wetting of metal films on oxides: The more reactive the metal towards oxygen, the better is the wetting ability. The chemical interaction is accompanied by charge transfer at the interface, causing a reduction of the surface work function. The interfacial interaction also causes the interfacial {Cr} layer to behave differently from the top metallic {Cr} atoms upon thermal annealing. The top layers of metallic {Cr} show a strong clustering tendency at {500$^\circ$C,} while the interfacial {Cr} seems to have less surface mobility and slow bulk diffusion.}, number = {3}, journal = SuSci, author = {Jian-Mei Pan and Ulrike Diebold and Lizhong Zhang and Theodore E. Madey}, month = oct, year = {1993}, pages = {411--426} }, @article{schmid_segregated_1993, title = {Segregated carbon on {Pt$_{10}$Ni$_{90}$(100)} studied by scanning tunneling microscopy}, volume = {294}, doi = {10.1016/0039-6028(93)90103-Q}, abstract = {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 $\times$ 2) and 40 to 70 pm in the c(2 $\times$ 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 $\times$ 2) superstructure was observed, exhibiting areas of the p4g structure known from earlier {LEED} studies of Ni(100).}, number = {3}, journal = SuSci, author = {M. Schmid and A. Biedermann and P. Varga}, month = sep, year = {1993}, pages = {L952--L958} }, @incollection{neidhart_ionization_1993, address = {New York}, edition = {1}, series = {{NATO} {ASI} Series}, title = {Ionization of {LiF} by hyperthermal multiply charged ions}, volume = {B 306}, isbn = {0306444895}, abstract = {Sputtering of a {LiF} thin film (0.25 $\mathrm{\mu}$m) 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.}, booktitle = {Ionization of Solids by Heavy Particles}, publisher = {Plenum}, author = {T. Neidhart and M. Schmid and P. Varga}, editor = {Raul A. Baragiola}, month = aug, year = {1993}, pages = {447--453} }, @article{pan_structural_1993, title = {Structural study of ultrathin metal films on {TiO$_2$} using {LEED,} {ARXPS} and {MEED}}, volume = {291}, doi = {10.1016/0039-6028(93)90455-S}, abstract = {A structural study of ultrathin {Cr}, {Fe} and {Cu} metal films on stoichiometric and {Ar}$^+$ sputtered {TiO$_2$(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 {TiO$_2$(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 {TiO$_2$(110)} yielding pronounced forward scattering features in {ARXPS} and {MEED} data. {Cu} overlayers grow with fcc(111) structures exhibiting two equivalent domain orientations on {TiO$_2$(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 {TiO$_2$(110)} surfaces.}, number = {3}, journal = SuSci, author = {Jian-Mei Pan and Brian L. Maschhoff and Ulrike Diebold and Theodore E. Madey}, month = jul, year = {1993}, pages = {381--394} }, @article{schmid_preferential_1993, title = {Preferential sputtering of {Pt-Ni} alloy single crystals studied by scanning tunneling microscopy}, volume = {82}, doi = {10.1016/0168-583X(93)96028-B}, abstract = {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.}, number = {2}, journal = NIMB, author = {M. Schmid and A. Biedermann and C. Slama and H. Stadier and P. Weigand and P. Varga}, month = jul, year = {1993}, pages = {259--268} }, @article{stadler_embedded-atom_1993, title = {Embedded-atom method calculations applied to surface segregation of {Pt-Ni} single crystals}, volume = {287-288}, doi = {10.1016/0039-6028(93)90804-S}, abstract = {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.}, number = {Part 1}, journal = SuSci, author = {H. Stadler and W. Hofer and M. Schmid and P. Varga}, month = may, year = {1993}, pages = {366--370} }, @article{diebold_ultrathin_1993, title = {Ultrathin metal films on {TiO$_2$(110):} metal overlayer spreading and surface reactivity}, volume = {287-288}, doi = {10.1016/0039-6028(93)91095-7}, abstract = {We have studied the growth of ultrathin ({\textless} 50 \AA{}) {Cu} overlayers on stoichiometric {TiO$_2$(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 {TiO$_2$} cations takes place upon contact with a {Cu} overlayer.}, number = {Part 2}, journal = SuSci, author = {U. Diebold and J.-M. Pan and T. E. Madey}, month = may, year = {1993}, pages = {896--900} }, @article{schmid_direct_1993, title = {Direct observation of surface chemical order by scanning tunneling microscopy}, volume = {70}, doi = {10.1103/PhysRevLett.70.1441}, abstract = {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 \AA{}. 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.}, number = {10}, journal = PRL, author = {M. Schmid and H. Stadler and P. Varga}, month = mar, year = {1993}, pages = {1441} }, @article{diebold_growth_1993, title = {Growth mode of ultrathin copper overlayers on {TiO$_2$(110)}}, volume = {47}, doi = {10.1103/PhysRevB.47.3868}, abstract = {Copper overlayers with thicknesses up to several tens of angstroms have been vapor deposited at various substrate temperatures onto rutile {TiO$_2$(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 \AA{} 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.}, number = {7}, journal = PRB, author = {U. Diebold and J.-M. Pan and T. E. Madey}, month = feb, year = {1993}, pages = {3868} }, @article{weigand_surface_1993, title = {Surface composition of {Pt$_{25}$Ni$_{75}$(111)} investigated by {ISS} and {STM}}, volume = {346}, doi = {10.1007/BF00321431}, abstract = {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.}, number = {1}, journal = FreseniusJAC, author = {P. Weigand and C. Nagl and M. Schmid and P. Varga}, year = {1993}, pages = {281--283} }, @incollection{neidhart_desorption_1993, address = {Berlin}, series = {Springer Series in Surface Sciences}, title = {Desorption from {LiF(100)} by singly- and doubly-charged hyperthermal {He} ions}, volume = {31}, isbn = {978-3540564737}, abstract = {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.}, booktitle = {Desorption Induced by Electronic Transitions {DIET} V}, publisher = {Springer}, author = {T. Neidhart and M Schmid and P. Varga}, editor = {A. R. Burns and E. B. Stechel and D. R. Jennison}, year = {1993}, pages = {129--132} }, @incollection{madey_electron_1993, address = {Berlin}, series = {Springer Series in Surface Sciences}, title = {Electron stimulated desorption {(ESD)} of ammonia on {TiO$_2$(110):} The influence of substrate defect structure}, volume = {33}, isbn = {978-3540564737}, booktitle = {Adsorption on Ordered Surfaces on Ionic Solids and Thin Films}, publisher = {Springer}, author = {T. E. Madey and U. Diebold and J.-M. Pan}, editor = {E. Umbach and {H.-J.} Freund}, year = {1993}, pages = {147--155} }, @incollection{madey_structure_1993, address = {Berlin}, series = {Springer Series in Surface Sciences}, title = {Structure and kinetics of electron beam damage in a chemisorbed monolayer: {PF$_3$} on {Ru}(0001)}, volume = {31}, isbn = {978-3540564737}, booktitle = {Desorption Induced by Electronic Transitions {DIET} {V}}, publisher = {Springer}, author = {T. E. Madey and H. S. Tao and L. Nair and U. Diebold and S. M. Shivaprasad and A. L. Johnson and A. Poradzsiz and N. D. Shinn and J. A. Yarmoff and V. Chakarian and D. Shuh}, editor = {A. R. Burns and E. B. Stechel and D. R. Jennison}, year = {1993}, pages = {182--188} }, @incollection{diebold_electron_1993, address = {Berlin}, series = {Springer Series in Surface Sciences}, title = {Electron stimulated desorption {(ESD)} of ammonia on {TiO$_2$(110):} The influence of substrate defect structure}, volume = {31}, isbn = {978-3540564737}, booktitle = {Desorption Induced by Electronic Transitions {DIET} {V}}, publisher = {Springer}, author = {U. Diebold and T. E. Madey}, editor = {A. R. Burns and E. B. Stechel and D. R. Jennison}, year = {1993}, pages = {284--288} }, @article{schmid_mismatch_1992, title = {Mismatch dislocations caused by preferential sputtering of a platinum-nickel alloy surface}, volume = {55}, doi = {10.1007/BF00348334}, abstract = {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.}, number = {5}, journal = ApPhA, author = {M. Schmid and A. Biedermann and H. Stadler and C. Slama and P. Varga}, month = nov, year = {1992}, pages = {468--475} }, @article{schmid_analysis_1992, title = {Analysis of vibration-isolating systems for scanning tunneling microscopes}, volume = {42-44}, doi = {10.1016/0304-3991(92)90493-4}, abstract = {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.}, number = {Part 2}, journal = {Ultramicroscopy}, author = {M. Schmid and P. Varga}, month = jul, year = {1992}, pages = {1610--1615} }, @article{pan_interaction_1992, title = {Interaction of water, oxygen, and hydrogen with {TiO$_2$(110)} surfaces having different defect densities}, volume = {10}, doi = {10.1116/1.577986}, abstract = {We have studied the stoichiometric (annealed in oxygen), the slightly oxygen-deficient (annealed in vacuum), and the highly defective (sputtered with {Ar}$^+$) {TiO$_2$(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 {H{\textless}sub{\textgreater}2{\textless}/sub{\textgreater}{\textless}sup{\textgreater}18{\textless}/sup{\textgreater}O} exposure. Water adsorbs on both stoichiometric and slightly O-deficient surfaces [with oxygen vacancies $\approx$0 and 0.08 monolayer {(ML),} respectively] at room temperature. There is little or no dependence of saturation water coverage (lower limit of $\approx$0.07 {ML} for both surfaces) on the concentration of surface oxygen vacancies. On the highly defective surfaces, the saturation water coverage increases to a lower limit of 0.15 {ML} and the saturation {18O} coverage increases to 0.4 {ML.} The interaction of hydrogen with the stoichiometric surface creates defect states that can be observed by {XPS} and by subsequent adsorption of {18O.}}, number = {4}, journal = JVSTA, author = {J.-M. Pan and B. L. Maschhoff and U. Diebold and T. E. Madey}, month = jul, year = {1992}, pages = {2470--2476} }, @article{diebold_adsorption_1992, title = {Adsorption and electron stimulated desorption of {NH}$_3$/{TiO}$_2$(110)}, volume = {10}, doi = {10.1116/1.577939}, abstract = {Adsorption and electron stimulated processes of {NH3} on rutile {TiO$_2$(110)} have been studied by means of x-ray photoemission spectroscopy {(XPS)} and low-energy ion scattering {(LEIS).} Defects, mostly surface oxygen vacancies, can be produced easily on {TiO$_2$} surfaces by thermal treatment or by rare gas ion sputtering. We have used three differently prepared {TiO$_2$} 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 $\approx$0.16\textendash{}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 $\approx$ 1 $\times$ 10\textendash{}16 cm2. When the highly oxygen-deficient {TiO$_2$} 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.}, number = {4}, journal = JVSTA, author = {U. Diebold and T. E. Madey}, month = jul, year = {1992}, pages = {2327--2335} }, @article{wutte_sputtering_1992, title = {Sputtering of {LiF(100)} with low energetic {Ne}$^+$ and {Ne}$^{2+}$ ions}, volume = {65}, doi = {10.1016/0168-583X(92)95029-Q}, abstract = {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 \AA{}ngstr\"{o}ms 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.}, number = {1-4}, journal = NIMB, author = {D. Wutte and U. Diebold and M. Schmid and P. Varga}, month = mar, year = {1992}, pages = {167--172} }, @article{schmid_lattice_1992, title = {Lattice mismatch dislocations in a preferentially sputtered alloy studied by scanning tunneling microscopy}, volume = {69}, doi = {10.1103/PhysRevLett.69.925}, abstract = {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.}, number = {6}, journal = PRL, author = {M. Schmid and A. Biedermann and H. Stadler and P. Varga}, year = {1992}, pages = {925} }, @article{varga_electronic_1991, title = {Electronic effects in low-energy ion sputtering of {LiF}}, volume = {58}, doi = {10.1016/0168-583X(91)95879-I}, abstract = {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.}, number = {3-4}, journal = NIMB, author = {P. Varga and U. Diebold and D. Wutte}, month = jun, year = {1991}, pages = {417--421} }, @article{diebold_low-energy_1991, title = {Low-energy ion impact desorption cross sections of carbon monoxide from {Ni}(111)}, volume = {248}, doi = {10.1016/0039-6028(91)90068-4}, abstract = {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.} M\"{o}ller, 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.}, number = {1-2}, journal = SuSci, author = {U. Diebold and W. M\"{o}ller and P. Varga}, month = may, year = {1991}, pages = {147--157} }, @article{schmid_motional_1991, title = {Motional capacitance of layered piezoelectric thickness-mode resonators}, volume = {38}, doi = {10.1109/58.79604}, abstract = {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.}, number = {3}, journal = {{IEEE} Transactions on Ultrasonics, Ferroelectrics and Frequency Control}, author = {M. Schmid and E. Benes and W. Burger and V. Kravchenko}, month = may, year = {1991}, pages = {199--206} }, @article{diebold_influence_1991, title = {Influence of the primary ion charge state on secondary ion production: bombardment of {CO/Ni(111)} with {Ne}$^+$, {Ne}$^{2+}$, {Kr}$^+$ and {Kr}$^{2+}$ at low impact energies}, volume = {241}, doi = {10.1016/0167-2584(91)91044-W}, abstract = {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 ({\textless} 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.}, number = {1-2}, journal = SuSci, author = {U. Diebold and P. Varga}, year = {1991}, pages = {L6--L10} }, @article{nowotny_layered_1991, title = {Layered piezoelectric resonators with an arbitrary number of electrodes (general one-dimensional treatment)}, volume = {90}, doi = {10.1121/1.401915}, abstract = {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.}, number = {3}, journal = JAcSocAm, author = {Helmut Nowotny and Ewald Benes and Michael Schmid}, year = {1991}, pages = {1238--1245} }, @incollection{diebold_desorption_1991, address = {Berlin}, series = {Springer Series in Surface Sciences}, title = {Desorption and secondary ion production during bombardment of {CO/Ni(111)} with {Ne}$^+$ and {Ne}$^{++}$ ions at very low impact energies}, volume = {19}, isbn = {0387523863}, abstract = {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.}, booktitle = {Desorption Induced by Electronic Transitions {DIET} {IV}}, publisher = {{Springer-Verlag}}, author = {U Diebold and P. Varga}, editor = {G. Betz and P. Varga}, year = {1991}, pages = {193--203} }, @article{benes_vibration_1991, title = {Vibration modes of mass-loaded planoconvex quartz crystal resonators}, volume = {90}, doi = {10.1121/1.401940}, abstract = {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.}, number = {2}, journal = JAcSocAm, author = {Ewald Benes and Michael Schmid and Victor Kravchenko}, year = {1991}, pages = {700--706} }, @article{schmid_computer-controlled_1990, title = {A computer-controlled system for the measurement of complete admittance spectra of piezoelectric resonators}, volume = {1}, doi = {10.1088/0957-0233/1/9/021}, abstract = {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.}, number = {9}, journal = {Measurement Science and Technology}, author = {M Schmid and E. Benes and R. Sedlaczek}, year = {1990}, pages = {970--975} }, @article{diebold_determination_1990, title = {Determination of cross-sections for {CO} desorption from {Ni}(111) induced by {Ar}-ions of very low impact energy}, volume = {41}, doi = {10.1016/0042-207X(90)90312-M}, abstract = {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.}, number = {1-3}, journal = {Vacuum}, author = {U Diebold and P Varga}, year = {1990}, pages = {210--212} }, @article{benes_progress_1989, title = {Progress in monitoring thin film thickness by use of quartz crystals}, volume = {174}, doi = {10.1016/0040-6090(89)90907-3}, abstract = {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\textregistered{}} 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\textregistered{}} 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.}, number = {Part 1}, journal = {Thin Solid Films}, author = {E. Benes and M. Schmid and G. Thorn}, month = jul, year = {1989}, pages = {307--314} }, @article{diebold_angle_1988, title = {Angle resolved electron energy loss spectroscopy on graphite}, volume = {197}, doi = {10.1016/0039-6028(88)90638-3}, abstract = {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].}, number = {3}, journal = SuSci, author = {U. Diebold and A. Preisinger and P. Schattschneider and P. Varga}, year = {1988}, pages = {430--443} }, @article{ebel_reduced_1984, title = {Reduced thickness of contamination layers determined from {C} 1{\em s}- and {C{\em KVV}-lines}}, volume = {34}, doi = {10.1016/0368-2048(84)80076-6}, number = {3}, journal = JElSpec, author = {Maria F. Ebel and M. Schmid and H. Ebel and A. Vogel}, year = {1984}, pages = {313--316} }, @INPROCEEDINGS{hertl_investigation_1985, author={Hertl, S. and Benes, E. and Wimmer, L. and Schmid, M.}, booktitle={39th Annual Symposium on Frequency Control. 1985}, title={Investigation of Quartz Crystal Thickness Shear and Twist Modes Using a New Noninterferometric Laser Speckle Measurement Method}, year={1985}, month={}, volume={}, number={}, pages={ 535 - 543}, keywords={}, doi={}, }, @article{varga_generation_1975, title = {Generation of high intensity electron beams from a duoplasmatron discharge}, volume = {25}, doi = {10.1016/0042-207X(75)90488-1}, abstract = {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\textendash{}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.}, number = {9-10}, journal = {Vacuum}, author = {Varga, P and Bruck, M and Bruckm\"{u}ller, R}, month = oct, year = {1975}, pages = {421--424} }, @article{hofer_adsorbate_1984, title = {Adsorbate dependent neutralization of ions near a surface}, volume = {2}, doi = {10.1016/0168-583X(84)90228-3}, abstract = {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.}, number = {1-3}, journal = NIMB, author = {Hofer, W. and Varga, P.}, month = mar, year = {1984}, pages = {391--395} }, @article{varga_depth_1984, title = {Depth profiling of the altered layer in {Ta$_2$O$_5$} produced by sputtering with {He} ions}, volume = {2}, doi = {10.1016/0168-583X(84)90318-5}, abstract = {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.}, number = {1-3}, journal = NIMB, author = {Varga, P. and Taglauer, E.}, month = mar, year = {1984}, pages = {800--803} }, @article{hetzendorf_preferential_1986, title = {Preferential sputtering and surface segregation in {Au-Pd} alloys}, volume = {18}, doi = {10.1016/S0168-583X(86)80076-3}, abstract = {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$^\circ$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.}, number = {1-6}, journal = NIMB, author = {Hetzendorf, G. and Varga, P.}, year = {1986}, pages = {501--503} }, @article{fehringer_potential_1987, title = {Potential emission for multicharged ion impact on clean tungsten above the kinetic emission threshold}, volume = {23}, doi = {10.1016/0168-583X(87)90455-1}, abstract = {Apparent electron emission yields for impact of N$^{q+}$ (q = 4-6) and Ne$^{q+}$ (q = 4-7) on polycrystalline tungsten at impact velocities of 0.5 - 3 $\times$ 10$^5$ ms$^{-1}$ are presented. For $v$ {\textgreater}$v_t \approx 10^5$ ms$^{-1}$ with $v_t$ 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.}, number = {1-2}, journal = NIMB, author = {Fehringer, M. and Delaunay, M. and Geller, R. and Varga, P. and Winter, H.}, month = apr, year = {1987}, pages = {245--247} }, @article{weigand_surface_1992, title = {Surface composition of {Pt$_x$Ni$_{1-x}$} single crystal alloys}, volume = {64}, doi = {10.1016/0168-583X(92)95444-V}, abstract = {We present results of the surface composition of the topmost layer of three low index planes of {Pt$_x$Ni$_{1-x}$} 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.}, number = {1-4}, journal = NIMB, author = {Weigand, P. and Novacek, P. and van Husen, G. and Neidhart, T. and Mezey, L. Z. and Hofer, W. and Varga, P.}, month = feb, year = {1992}, pages = {93--97} }, @article{weigand_surface_1994, title = {Surface composition of {Pt$_{10}$Ni$_{90}$(110)}}, volume = {85}, doi = {10.1016/0168-583X(94)95857-2}, abstract = {Low energy ion scattering spectroscopy results are presented which allow to derive the first and second layer's composition of the {Pt$_{10}$Ni$_{90}$(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.}, number = {1-4}, journal = NIMB, author = {Weigand, P. and Jelinek, B. and Hofer, W. and Varga, P.}, month = mar, year = {1994}, pages = {424--428} }, @article{vana_electron_1995, title = {Electron emission from polycrystalline lithium fluoride bombarded by slow multicharged ions}, volume = {100}, doi = {10.1016/0168-583X(94)00814-0}, abstract = {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. Ion impact energies have been varied from almost zero up to 10 $\times q$ {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.}, number = {2-3}, journal = NIMB, author = {Vana, M. and Aumayr, F. and Varga, P. and Winter, H. P.}, month = jun, year = {1995}, pages = {284--289} }, @article{seifert_defect_1995, title = {Defect mediated sputtering of amorphous {LiF} induced by low-energy ion bombardment}, volume = {101}, doi = {10.1016/0168-583X(95)00052-6}, abstract = {Sputter yields of amorphous {LiF} caused by low-energy Ar$^+$, Ar$^{2+}$, 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.}, number = {1-2}, journal = NIMB, author = {Seifert, N. and Yan, Q. and Barnes, A. V. and Tolk, N. H. and Neidhart, T. and Varga, P. and Husinsky, W. and Betz, G.}, month = jun, year = {1995}, pages = {131--136} },