TY  - JOUR
ID  - 2424
T1  - Disorder and Defect Healing in Graphene on Ni(111)
JF  - J. Phys. Chem. Lett.
JA  - J. Phys. Chem. Lett.
M3  - 10.1021/jz2015007
A1  - Jacobson,Peter
A1  - Stöger,Bernhard
A1  - Garhofer,Andreas
A1  - Parkinson,Gareth S.
A1  - Schmid,Michael
A1  - Caudillo,Roman
A1  - Mittendorfer,Florian
A1  - Redinger,Josef
A1  - Diebold,Ulrike
VL  - 3
PY  - 2012///
N2  - The structural evolution of graphene on Ni(111) is investigated as a function of growth temperature by scanning tunneling microscopy (STM). Low temperature (400?500 °C) growth results in a continuous but highly defective film with small ordered graphene domains and disordered domains composed of Stone?Wales (SW)-like defects. As the growth temperature is increased, the disordered domains shrink leaving small clusters of defects alongside epitaxially matched graphene. Density functional theory (DFT) calculations indicate the crucial role of the metallic support for the healing of SW defects, as the interaction with the substrate leads to a stabilization of the reaction intermediate. This work highlights the effect of the graphene-substrate interaction on the temperature dependence of the defect concentration in epitaxial graphene on Ni(111).
The structural evolution of graphene on Ni(111) is investigated as a function of growth temperature by scanning tunneling microscopy (STM). Low temperature (400-500 °C) growth results in a continuous but highly defective film with small ordered graphene domains and disordered domains composed of Stone-Wales (SW)-like defects. As the growth temperature is increased, the disordered domains shrink leaving small clusters of defects alongside epitaxially matched graphene. Density functional theory (DFT) calculations indicate the crucial role of the metallic support for the healing of SW defects, as the interaction with the substrate leads to a stabilization of the reaction intermediate. This work highlights the effect of the graphene-substrate interaction on the temperature dependence of the defect concentration in epitaxial graphene on Ni(111).
KW  - DFT
KW  - Graphene
KW  - Nickel
KW  - STM
SP  - 136
EP  - 139
SN  - 1948-7185
UR  - http://dx.doi.org/10.1021/jz2015007
ER  - 

TY  - JOUR
ID  - 792
T1  - Oxygen-Stabilized Rh Adatoms: 0D Oxides on a Vicinal Surface
JF  - J. Phys. Chem. Lett.
JA  - J. Phys. Chem. Lett.
M3  - doi: 10.1021/jz2011308
A1  - Mittendorfer,Florian
A1  - Franz,Thomas
A1  - Klikovits,Jan
A1  - Schmid,Michael
A1  - Merte,Lindsay R.
A1  - Shah Zaman,Sameena
A1  - Varga,Peter
A1  - Westerström,Rasmus
A1  - Resta,Andrea
A1  - Andersen,Jesper N.
A1  - Gustafson,Johan
A1  - Lundgren,Edvin
VL  - 2
IS  - 21
PY  - 2011/10/19/
N2  - 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.
KW  - DFT
KW  - Oxygen
KW  - Rhodium
KW  - STM
SP  - 2747
EP  - 2751
SN  - 1948-7185
UR  - http://dx.doi.org/10.1021/jz2011308
ER  - 

TY  - JOUR
ID  - 90
T1  - A metastable Fe(A) termination at the Fe3O4(001) surface
JF  - Surface Science
JA  - Surf. Sci.
M3  - 16/j.susc.2011.05.018
A1  - Parkinson,Gareth S.
A1  - Novotný,Zbynek
A1  - Jacobson,Peter
A1  - Schmid,Michael
A1  - Diebold,Ulrike
VL  - 605
IS  - 15-16
PY  - 2011/08//
N2  - A metastable Fe(A) terminated Fe3O4(001) surface was prepared by tailoring the surface preparation conditions. STM, LEIS and LEED are utilized to demonstrate that annealing the Ar+ sputtered surface to 350 °C produces an Fe(A) terminated surface with a (√2 × √2)R45° superstructure. Within the superstructure both single Fe atoms and Fe dimer species are observed. The surface is reoxidized upon annealing to higher temperatures, eventually leading to the recovery of the energetically favorable Jahn-Teller distorted surface at 700 °C. The ability to reproducibly prepare the Fe(A) termination in this simple manner will allow investigations into the structure-function relationship for this important technological material.
KW  - Iron oxide
KW  - ISS
KW  - STM
SP  - L42-L45
EP  - L42-L45
SN  - 0039-6028
UR  - http://www.sciencedirect.com/science/article/pii/S0039602811002081
ER  - 

TY  - JOUR
ID  - 305
T1  - In-situ magnetic nano-patterning of Fe films grown on Cu(100)
JF  - Journal of Applied Physics
JA  - J. Appl. Phys.
M3  - 10.1063/1.3609078
A1  - Shah Zaman,Sameena
A1  - Dvořák,Petr
A1  - Ritter,Robert
A1  - Buchsbaum,Andreas
A1  - Stickler,Daniel
A1  - Oepen,Hans Peter
A1  - Schmid,Michael
A1  - Varga,Peter
VL  - 110
IS  - 2
PY  - 2011/07/27/
N2  - 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.
KW  - Carbon monoxide
KW  - Copper
KW  - ion bombardment
KW  - Iron
KW  - Magnetism
SP  - 024309
EP  - 024309
SN  - 10897550
ER  - 

TY  - JOUR
ID  - 337
T1  - Unusual Cluster Shapes and Directional Bonding of an fcc Metal: Pt/Pt(111)
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.107.016102
A1  - Schmid,Michael
A1  - Garhofer,Andreas
A1  - Redinger,Josef
A1  - Wimmer,Florian
A1  - Scheiber,Philipp
A1  - Varga,Peter
VL  - 107
IS  - 1
PY  - 2011/06/29/
N2  - Small clusters of Pt adatoms grown on Pt(111) exhibit a preference for the formation of linear chains, which cannot be explained by simple diffusion-limited aggregation. Density functional theory calculations show that short chains are energetically favorable to more compact configurations due to strong directional bonding by dz2-like orbitals, explaining the stability of the chains. The formation of the chains is governed by substrate distortions, leading to funneling towards the chain ends.
KW  - DFT
KW  - Growth
KW  - Platinum
KW  - STM
SP  - 016102
EP  - 016102
UR  - http://link.aps.org/doi/10.1103/PhysRevLett.107.016102
ER  - 

TY  - JOUR
ID  - 606
T1  - Room Temperature Water Splitting at the Surface of Magnetite
JF  - Journal of the American Chemical Society
JA  - J. Am. Chem. Soc.
M3  - 10.1021/ja203432e
A1  - Parkinson,Gareth S.
A1  - Novotný,Zbyněk
A1  - Jacobson,Peter
A1  - Schmid,Michael
A1  - Diebold,Ulrike
VL  - 133
IS  - 32
PY  - 2011///
N2  - An array of surface science measurements has revealed novel water adsorption behavior at the Fe3O4(001) surface. Following room temperature exposure to water, a low coverage of hydrogen atoms is observed, with no associated water hydroxyl group. Mild annealing of the hydrogenated surface leads to desorption of water via abstraction of surface oxygen atoms, leading to a reduction of the surface. These results point to an irreversible splitting of the water molecule. The observed phenomena are discussed in the context of recent DFT calculations ( Mulakaluri, N. ; Pentcheva, R. ; Scheffler, M. J. Phys. Chem. C 2010, 114, 11148 ), which show that the Jahn–Teller distorted surface isolates adsorbed H in a geometry that could kinetically hinder recombinative desorption. In contrast, the adsorption geometry facilitates interaction between water hydroxyl species, which are concluded to leave the surface following a reactive desorption process, possibly via the creation of O2.
KW  - H2O
KW  - Hydrogen
KW  - Iron oxide
KW  - STM
SP  - 12650
EP  - 12655
UR  - http://dx.doi.org/10.1021/ja203432e
ER  - 

TY  - JOUR
ID  - 233
T1  - Growth of ultrathin cobalt oxide films on Pt(111)
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.84.125430
A1  - De Santis,Maurizio
A1  - Buchsbaum,Andreas
A1  - Varga,Peter
A1  - Schmid,Michael
VL  - 84
IS  - 12
PY  - 2011///
N2  - Cobalt surface oxides were grown on Pt(111) by depositing Co and dosing with molecular oxygen at temperatures ranging between 300 and 740 K. Oxidation of 1 monolayer (ML) Co results in a two-dimensional (2D) moiré structure, observed using both low-energy electron diffraction and scanning tunneling microscopy and interpreted as a polar (oxygen terminated) CoO(111) atomic bilayer. It is expanded by 2.7 ± 0.6% in the surface plane with respect to bulk CoO. An almost-flawless moiré pattern is obtained after a final step of annealing at 740 K in oxygen. Insufficient oxidation leads to defects in the moiré pattern, consisting of triangular dislocation loops of different sizes; the smaller ones occupy half of the moiré cell. Low-temperature annealing (450 K) can be used to create a zigzag phase, which is mainly observed in 1-ML-thick areas after several cycles of Co deposition (1 ML each) and oxidation at 10−7 mbar. The CoO films obtained by deposition/oxidation cycles exhibit Stranski-Krastanov growth; the structure of the 2D layer between the islands depends on the thermal treatment. It exhibits the moiré pattern after annealing at 740 K, whereas the zigzag phase was observed after low-temperature annealing. The second monolayer consists of a moiré pattern different from that of the first layer, presumably a wurtzite-like structure. Above the third layer, we observe only small three-dimensional islands, which exhibit a bandgap. We have also studied oxidation of surface alloys obtained by depositing Co and annealing. On these surfaces, we found a quasi-(3 × 3) reconstruction. Structure models are presented for all phases observed, and we argue that some of the moirélike structures might be useful as templates for metal cluster growth.
KW  - Cobalt
KW  - Cobalt oxide
KW  - Platinum
KW  - STM
SP  - 125430
EP  - 125430
UR  - http://link.aps.org/doi/10.1103/PhysRevB.84.125430
ER  - 

TY  - JOUR
ID  - 517
T1  - Ion-beam-induced magnetic transformation of CO-stabilized fcc Fe films on Cu(100)
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.82.235401
A1  - Shah Zaman,Sameena
A1  - Oßmer,Hinnerk
A1  - Jonner,Jakub
A1  - Novotný,Zbyněk
A1  - Buchsbaum,Andreas
A1  - Schmid,Michael
A1  - Varga,Peter
VL  - 82
IS  - 23
PY  - 2010/12/01/
N2  - We have grown 22-ML-thick Fe films on a Cu(100) single crystal. The films were stabilized in the face-centered-cubic (fcc) γ phase by adsorption of carbon monoxide during growth, preventing the transformation to the body-centered-cubic (bcc) α phase. A structural transformation of these films from fcc to bcc can be induced by Ar+ ion irradiation. Scanning-tunneling microscopy images show the nucleation of bcc crystallites, which grow with increasing Ar+ ion dose and eventually result in complete transformation of the film to bcc. Surface magneto-optic Kerr effect measurements confirm the transformation of the Fe film from paramagnetic (fcc) to ferromagnetic (bcc) with an in-plane easy axis. The transformation can also be observed by low-energy electron diffraction. We find only very few nucleation sites of the bcc phase and argue that nucleation of the bcc phase happens under special circumstances during resolidification of the molten iron in the thermal spike after ion impact. Intermixing with the Cu substrate impedes the transformation. We also demonstrate the transformation of films coated with Au to protect them from oxidation at ambient conditions.
KW  - Copper
KW  - ion bombardment
KW  - Iron
KW  - Magnetism
SP  - 235401
EP  - 235401
UR  - http://link.aps.org/doi/10.1103/PhysRevB.82.235401
ER  - 

TY  - JOUR
ID  - 540
T1  - Observation and Destruction of an Elusive Adsorbate with STM: O2/TiO2(110)
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.105.216101
A1  - Scheiber,Philipp
A1  - Riss,Alexander
A1  - Schmid,Michael
A1  - Varga,Peter
A1  - Diebold,Ulrike
VL  - 105
IS  - 21
PY  - 2010/11/15/
N2  - When a slightly defective rutile TiO2(110) surface is exposed to O2 at elevated temperatures, the molecule dissociates at defects, filling O vacancies (VO) and creating O adatoms (Oad) on Ti5c rows. The adsorption of molecular O2 at low temperatures has remained controversial. Low-temperature scanning tunneling microscopy of O2, dosed on TiO2(110) at a sample temperature of ≈100 K and imaged at 17 K, shows a molecular precursor at VO as a faint change in contrast. The adsorbed O2 easily dissociates during the STM measurements, and the formation of Oad’s at both sides of the original VO is observed.
KW  - Oxygen
KW  - STM
KW  - TiO2
SP  - 216101
EP  - 216101
UR  - http://link.aps.org/doi/10.1103/PhysRevLett.105.216101
ER  - 

TY  - JOUR
ID  - 638
T1  - Highly ordered Pd, Fe, and Co clusters on alumina on Ni3Al(111)
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.81.115420
A1  - Buchsbaum,Andreas
A1  - De Santis,Maurizio
A1  - Tolentino,Helio C. N.
A1  - Schmid,Michael
A1  - Varga,Peter
VL  - 81
IS  - 11
PY  - 2010/03/11/
N2  - Template-mediated growth of metals has attracted much interest due to the remarkable magnetic and catalytic properties of clusters in the nanometer range and provides the opportunity to grow clusters with narrow size distributions. We have grown well-ordered Fe and Co clusters on the ultrathin aluminum oxide on Ni3Al(111), a template with a 4.1 nm lattice. The structure of the ≈0.5-nm-thick oxide film exhibits holes reaching down to the metal substrate at the corners of the (√67×√67)R12.2° unit cell. Pd atoms trapped in these corner holes create metallic nucleation sites where Fe as well as Co clusters can nucleate and form a well-ordered hexagonal arrangement on the oxide nanomesh. We have studied these Fe and Co clusters and applied different methods such as scanning tunneling microscopy and surface x-ray diffraction to determine the morphology and crystallography of the clusters. For Fe, we found cluster growth in either bcc[110] or bcc[100] direction, depending on the deposition temperature and for Co we found close-packed planes on top of the clusters and random stacking of close-packed planes. Pd clusters grow with fcc(111) orientation.
KW  - Alumina
KW  - Cobalt
KW  - Growth
KW  - Iron
KW  - Palladium
SP  - 115420
EP  - 115420
UR  - http://link.aps.org/doi/10.1103/PhysRevB.81.115420
ER  - 

TY  - JOUR
ID  - 635
T1  - Metastable surface oxide on CoGa(100): Structure and stability
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.81.115402
A1  - Vlad,A.
A1  - Stierle,A.
A1  - Marsman,M.
A1  - Kresse,G.
A1  - Costina,I.
A1  - Dosch,H.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 81
IS  - 11
PY  - 2010/03/02/
N2  - We investigated the structure and formation of a surface oxide and bulk β-Ga2O3 on CoGa(100) from ultrahigh vacuum to 1 bar oxygen pressure in a temperature range from 300 to 1040 K. We combined in situ surface x-ray diffraction with scanning tunneling microscopy, atomic force microscopy, and density-functional theory calculations. We find that the two-dimensional epitaxial surface oxide layer exhibits a p2mm symmetry with an additional mirror plane as compared to the bulk oxide. The surface oxide layer is found to form under metastable conditions at an oxygen chemical potential ∼1.6 eV above the stability limit for bulk β-Ga2O3. The formation of the bulk oxide is kinetically hindered by the presence of the oxygen-terminated surface oxide, which most likely hampers dissociative oxygen chemisorption. We observe that below 620 K, the surface oxide is surprisingly stable at 1 bar oxygen pressure. Substrate faceting accompanies the bulk oxide formation at temperatures higher than 1020 K.
KW  - Cobalt
KW  - DFT
KW  - Ga2O3
KW  - Gallium
KW  - STM
KW  - SXRD
SP  - 115402
EP  - 115402
UR  - http://link.aps.org/doi/10.1103/PhysRevB.81.115402
ER  - 

TY  - JOUR
ID  - 836
T1  - Oxide Surface Science
JF  - Annual Review of Physical Chemistry
JA  - Annu. Rev. Phys. Chem.
M3  - 10.1146/annurev.physchem.012809.103254
A1  - Diebold,Ulrike
A1  - Li,Shao-Chun
A1  - Schmid,Michael
VL  - 61
IS  - 1
PY  - 2010/03//
N2  - Most metals are oxidized under ambient conditions, and metal oxides show interesting and technologically promising properties. This has motivated much recent research on oxide surfaces. The combination of scanning tunneling microscopy with first-principles density functional theory–based computational techniques provides an atomic-scale view of the properties of metal-oxide materials. Surface polarity is a key concept for predicting the stability of oxide surfaces and is discussed using ZnO as an example. This review also highlights the role of surface defects for surface reactivity, and their interplay with defects in the bulk, for the case of TiO2. Ultrathin metal-oxide films, grown either through reactive evaporation on metal single crystals or through oxidation of metal alloys (such as Al2O3/NiAl), have gained popularity as supports for planar model catalysts. The surface oxides that form upon oxidation on Pt-group metals (e.g., Ru, Rh, Pd, and Pt) are considered as model systems for CO oxidation.
KW  - Alumina
KW  - Oxygen
KW  - Palladium
KW  - Rhodium
KW  - TiO2
KW  - Vanadium
KW  - ZnO
SP  - 129
EP  - 148
SN  - 0066-426X
UR  - http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.physchem.012809.103254
ER  - 

TY  - JOUR
ID  - 396
T1  - Metal-related gate sinking due to interfacial oxygen layer in Ir/InAlN high electron mobility transistors
JF  - Applied Physics Letters
JA  - Appl. Phys. Lett.
M3  - 10.1063/1.3458700
A1  - Ostermaier,C.
A1  - Pozzovivo,G.
A1  - Basnar,B.
A1  - Schrenk,W.
A1  - Schmid,M.
A1  - Tóth,L.
A1  - Pécz,B.
A1  - Carlin,J.-F.
A1  - Gonschorek,M.
A1  - Grandjean,N.
A1  - Strasser,G.
A1  - Pogany,D.
A1  - Kuzmik,J.
VL  - 96
IS  - 26
PY  - 2010///
N2  - We report on an annealing-induced “gate sinking” effect in a 2-nm-thin In0.17Al0.83N/AlN barrier high electron mobility transistor with Ir gate. Investigations by transmission electron microscopy linked the effect to an oxygen containing interlayer between the gate metal and the InAlN layer and revealed diffusion of oxygen into iridium during annealing. Below 700 °C  the diffusion is inhomogeneous and seems to occur along grain boundaries, which is consistent with the capacitance-voltage analysis. Annealing at 700 °C increased the gate capacitance over a factor 2, shifted the threshold voltage from +0.3 to +1 V and increased the transconductance from 400 to 640 mS/mm
KW  - Iridium
SP  - 263515
EP  - 263515
SN  - 00036951
UR  - http://link.aip.org/link/APPLAB/v96/i26/p263515/s1&Agg=doi
ER  - 

TY  - JOUR
ID  - 1184
T1  - High Island Densities in Pulsed Laser Deposition: Causes and Implications
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.103.076101
A1  - Schmid,M.
A1  - Lenauer,C.
A1  - Buchsbaum,A.
A1  - Wimmer,F.
A1  - Rauchbauer,G.
A1  - Scheiber,P.
A1  - Betz,G.
A1  - Varga,P.
VL  - 103
IS  - 7
PY  - 2009/08/10/
N2  - By studying metal growth on Pt(111), we determine the reasons for the high island densities observed in pulsed laser deposition (PLD) compared to conventional thermal deposition. For homoepitaxy by PLD with moderate energies (≲100 eV) of the deposited ions, high island densities are caused by the high instantaneous flux of arriving particles. Additional nuclei are formed at high ion energies (≳200 eV) by adatoms created by the impinging ions. For heteroepitaxy, the island density is also increased by intermixing (deposited material implanted in the surface), creating an inhomogeneous potential energy surface for diffusing atoms. We discuss implications for layer-by-layer growth and sputter deposition.
KW  - Cobalt
KW  - Growth
KW  - Platinum
SP  - 076101
EP  - 076101
UR  - http://link.aps.org/doi/10.1103/PhysRevLett.103.076101
ER  - 

TY  - JOUR
ID  - 48
T1  - Structure and catalytic reactivity of Rh oxides
JF  - Catalysis Today
JA  - Catal. Today
M3  - 10.1016/j.cattod.2008.11.011
A1  - Gustafson,J.
A1  - Westerström,R.
A1  - Resta,A.
A1  - Mikkelsen,A.
A1  - Andersen,J.N.
A1  - Balmes,O.
A1  - Torrelles,X.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Hammer,B.
A1  - Kresse,G.
A1  - Baddeley,C.J.
A1  - Lundgren,E.
VL  - 145
IS  - 3-4
PY  - 2009/07/30/
N2  - Using a combination of experimental and theoretical techniques, we show that a thin RhO2 surface oxide film forms prior to the bulk Rh2O3 corundum oxide on all close-packed single crystal Rh surfaces. Based on previous reports, we argue that the RhO2 surface oxide also forms on vicinal Rh surfaces as well as on Rh nanoparticles. The detailed structure of this film was previously determined using UHV based techniques and density functional theory. In the present paper, we also examine the structure of the bulk Rh2O3 corundum oxide using surface X-ray diffraction. Being armed with this structural information, we have explored the CO oxidation reaction over Rh(111), Rh(100) and Pt25Rh75(100) at realistic pressures using in situ surface X-ray diffraction and online mass spectrometry. In all three cases we find that an increase of the CO2 production coincides with the formation of the thin RhO2 surface oxide film. In the case of Pt25Rh75(100), our measurements demonstrate that the formation of bulk Rh2O3 corundum oxide poisons the reaction, and argue that this is also valid for all other Rh surfaces. Our study implies that the CO oxidation reaction over Rh surfaces at realistic conditions is insensitive to the exact Rh substrate orientation, but is rather governed by the formation of a specific surface oxide phase.
KW  - Carbon monoxide
KW  - Oxygen
KW  - Platinum
KW  - Rhodium
KW  - STM
KW  - SXRD
SP  - 227
EP  - 235
SN  - 0920-5861
ER  - 

TY  - JOUR
ID  - 712
T1  - Ion-induced erosion of tungsten surfaces studied by a sensitive quartz-crystal-microbalance technique
JF  - Journal of Nuclear Materials
JA  - J. Nucl. Mater.
M3  - 10.1016/j.jnucmat.2009.01.279
A1  - Golczewski,A.
A1  - Kuzucan,A.
A1  - Schmid,K.
A1  - Roth,J.
A1  - Schmid,M.
A1  - Aumayr,F.
VL  - 390-391
PY  - 2009/06/15/
N2  - A highly sensitive quartz-crystal-microbalance (QCM) technique was used to study erosion of polycrystalline tungsten films due to impact of deuterium, carbon and argon ions, as well as retention of deuterium in these films. Polycrystalline tungsten films coated onto a SC-cut quartz crystal were bombarded by ions with impact energies from 100 eV up to a few keV and the frequency change due to mass loss (sputtering, desorption) or mass gain (implantation, adsorption) during bombardment was determined. Our setup was capable of detecting mass-changes as small as 10^-5 µg/s, which corresponds to a removal (or deposition) of only 10^-4 W monolayers/s. While our total sputtering yields for deuterium and argon projectiles compare well with the results of previous work, we derive new data on sputtering of tungsten by carbon ions. In addition we demonstrate that our setup is well suited for determining deuterium retention rates in tungsten.
KW  - ion bombardment
KW  - QCM
KW  - Tungsten
SP  - 1102
EP  - 1105
SN  - 0022-3115
ER  - 

TY  - JOUR
ID  - 743
T1  - Magnetism of FePt Surface Alloys
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.102.067207
A1  - Honolka,J.
A1  - Lee,T. Y.
A1  - Kuhnke,K.
A1  - Enders,A.
A1  - Skomski,R.
A1  - Bornemann,S.
A1  - Mankovsky,S.
A1  - Minar,J.
A1  - Staunton,J.
A1  - Ebert,H.
A1  - Hessler,M.
A1  - Fauth,K.
A1  - Schutz,G.
A1  - Buchsbaum,A.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Kern,K.
VL  - 102
IS  - 6
PY  - 2009/02/13/
N2  - 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.
KW  - DFT
KW  - Iron
KW  - Magnetism
KW  - Platinum
KW  - STM
SP  - 067207
EP  - 4
UR  - http://link.aps.org/abstract/PRL/v102/e067207
ER  - 

TY  - JOUR
ID  - 1176
T1  - A quartz-crystal-microbalance technique to investigate ion-induced erosion of fusion relevant surfaces
JF  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
JA  - Nucl. Instrum. Methods B
M3  - 10.1016/j.nimb.2008.10.088
A1  - Golczewski,A.
A1  - Dobes,K.
A1  - Wachter,G.
A1  - Schmid,M.
A1  - Aumayr,F.
VL  - 267
IS  - 4
PY  - 2009/02//
N2  - 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.
KW  - instrumental
KW  - ion bombardment
KW  - QCM
KW  - Tungsten
SP  - 695
EP  - 699
SN  - 0168-583X
ER  - 

TY  - JOUR
ID  - 564
T1  - A LEED study of NO superstructures on the Pd(111) surface
JF  - Journal of Physics: Condensed Matter
JA  - J. Phys.: Condens. Matter
M3  - 10.1088/0953-8984/21/13/134005
A1  - Kostelník,Petr
A1  - Šikola,Tomas
A1  - Varga,Peter
A1  - Schmid,Michael
VL  - 21
IS  - 13
PY  - 2009///
N2  - 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.
KW  - LEED
KW  - Nitrogen oxides
KW  - NO
KW  - Palladium
SP  - 134005
EP  - 134005
SN  - 0953-8984
ER  - 

TY  - JOUR
ID  - 18
T1  - Step-Orientation-Dependent Oxidation: From 1D to 2D Oxides
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.101.266104
A1  - Klikovits,J
A1  - Schmid,M
A1  - Merte,LR
A1  - Varga,P
A1  - Westerström,R
A1  - Resta,A
A1  - Andersen,JN
A1  - Gustafson,J
A1  - Mikkelsen,A
A1  - Lundgren,E
A1  - Mittendorfer,F
A1  - Kresse,G
VL  - 101
IS  - 26
PY  - 2008/12/31/
N2  - 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.
KW  - DFT
KW  - Oxygen
KW  - Rhodium
KW  - STM
SP  - 266104
EP  - 266104
SN  - 0031-9007
UR  - http://link.aps.org/doi/10.1103/PhysRevLett.101.266104
ER  - 

TY  - JOUR
ID  - 117
T1  - Fabrication of a Well-Ordered Nanohole Array Stable at Room Temperature
JF  - Nano Letters
JA  - Nano Lett.
M3  - 10.1021/nl8013378
A1  - Aït-Mansour,K.
A1  - Buchsbaum,A.
A1  - Ruffieux,P.
A1  - Schmid,M.
A1  - Gröning,P.
A1  - Varga,P.
A1  - Fasel,R.
A1  - Gröning,O.
VL  - 8
IS  - 7
PY  - 2008/07/01/
N2  - 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.
KW  - ion bombardment
KW  - ISS
KW  - Platinum
KW  - Silver
KW  - STM
SP  - 2035
EP  - 2040
UR  - http://dx.doi.org/10.1021/nl8013378
ER  - 

TY  - JOUR
ID  - 1168
T1  - Ultrathin alumina film on Cu-9at%Al(111)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/j.susc.2008.02.040
A1  - Napetschnig,E
A1  - Schmid,M
A1  - Varga,P
VL  - 602
IS  - 10
PY  - 2008/05/15/
N2  - 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.
KW  - Alumina
KW  - Aluminum
KW  - Copper
KW  - ISS
KW  - STM
SP  - 1750
EP  - 1756
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 351
T1  - Ultra-thin Fe films grown on Cu by pulsed laser deposition: Intermixing and bcc-like structures
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/j.susc.2008.02.024
A1  - Rauchbauer,G
A1  - Buchsbaum,A
A1  - Schiechl,H
A1  - Varga,P
A1  - Schmid,M
A1  - Biedermann,A
VL  - 602
IS  - 8
PY  - 2008/04/15/
N2  - 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.
KW  - Copper
KW  - Iron
KW  - Magnetism
KW  - PLD
KW  - STM
SP  - 1589
EP  - 1598
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 660
T1  - Time-of-flight spectroscopy of the energy distribution of laser-ablated atoms and ions
JF  - Review of Scientific Instruments
JA  - Rev. Sci. Instrum.
M3  - 10.1063/1.2901607
A1  - Buchsbaum,A.
A1  - Rauchbauer,G.
A1  - Varga,P.
A1  - Schmid,M.
VL  - 79
IS  - 4
PY  - 2008/04/04/
N2  - The growth of ultrathin films, deposited by laser ablation, crucially depends on the energy of the ablated species. Therefore, a time-of-flight (TOF) spectrometer has been constructed and measurements have been carried out in order to determine the energy distribution of laser-ablated Fe and Pt atoms and ions in the plasma created by nanosecond pulses of a frequency-doubled neodymium doped yttrium aluminum garnet laser. The experiments have been performed in ultrahigh vacuum at relatively low laser power. For measuring the spectra of the neutrals, a cross-beam electron source for postionization and electric as well as magnetic fields for repelling the ions are employed. Nevertheless, measurements of neutral particles are restricted to low plasma densities due to electrostatic shielding within the plasma, leading to an inefficient deflection of charged particles by electrostatic and magnetic fields. Test measurements have been performed by utilizing the TOF spectrometer as a pressure gauge and also by chopping the electron beam, running the TOF spectrometer as a residual gas mass spectrometer. The spectra of the laser-ablated plasmas have shown plasma conditions with a Debye length of approximately 10^−4  m, densities of 10^15–10^16  m^−3 and ion energies up to 150  eV. Neutral spectra have shown an unexpectedly low fraction of neutrals (10^−3–10^−4) and hyperthermal energies up to several 10 eV, possibly contributed by recombination of ions and electrons in the plasma. Even though gas spectra had demonstrated the expected sensitivity of the TOF spectrometer for low-energy neutrals, no thermally evaporated neutral atoms could be found.
KW  - Growth
KW  - Iron
KW  - Platinum
KW  - PLD
SP  - 043301
EP  - 043301
UR  - http://link.aip.org/link/?RSI/79/043301/1
ER  - 

TY  - JOUR
ID  - 79
T1  - Stressing Pd atoms: Initial oxidation of the Pd(110) surface
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/j.susc.2008.05.033
A1  - Westerström,R.
A1  - Weststrate,C.J.
A1  - Resta,A.
A1  - Mikkelsen,A.
A1  - Schnadt,J.
A1  - Andersen,J.N.
A1  - Lundgren,E.
A1  - Schmid,M.
A1  - Seriani,N.
A1  - Harl,J.
A1  - Mittendorfer,F.
A1  - Kresse,G.
VL  - 602
IS  - 14
PY  - 2008///
N2  - We have investigated the oxygen induced structures of the Pd(110) surface in the pressure range of 10<sup>-5</sup>-10<sup>-3</sup> mbar of oxygen, at a sample temperature of around 300 °C. These structures, denoted as "(7x√3)" and "(9x√3)", are studied in detail by the use of a combination of low-energy electron diffraction, scanning tunneling microscopy, high-resolution core level spectroscopy, and ab-initio simulations. Based on our data a model is proposed for these structures containing segments of Pd atoms in the [1 -1 0] direction, in which the Pd rows are decorated by O atoms in a zig-zag pattern. The segments are periodically separated by displaced Pd atoms. Density functional theory calculations show that the displacements reduce the oxygen induced stress significantly, as compared to a structure with no displacements. The calculations also suggest that the new structures are stabilized by domain formation.
KW  - DFT
KW  - Oxygen
KW  - Palladium
KW  - STM
SP  - 2440
EP  - 2447
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 203
T1  - Ion-beam induced fcc-bcc transition in ultrathin Fe films for ferromagnetic patterning
JF  - Applied Physics Letters
JA  - Appl. Phys. Lett.
M3  - 10.1063/1.2969795
A1  - Rupp,W.
A1  - Biedermann,A.
A1  - Kamenik,B.
A1  - Ritter,R.
A1  - Klein,Ch.
A1  - Platzgummer,E.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 93
IS  - 6
PY  - 2008///
N2  - 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.
KW  - Iron
KW  - Magnetism
KW  - STM
SP  - 063102
EP  - 3
UR  - http://link.aip.org/link/?APL/93/063102/1
ER  - 

TY  - JOUR
ID  - 387
T1  - Nanotemplate with holes: Ultrathin alumina on Ni3Al(111)
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.99.196104
A1  - Schmid,M
A1  - Kresse,G
A1  - Buchsbaum,A
A1  - Napetschnig,E
A1  - Gritschneder,S
A1  - Reichling,M
A1  - Varga,P
VL  - 99
IS  - 19
PY  - 2007/11/09/
N2  - We have determined the structure of the ultrathin (root 67 x root 67)R12.2 degrees aluminum oxide on Ni3Al(111) by a combination of scanning tunneling microscopy and density functional theory. In addition to other local defects, the main structural feature of the unit cell is a 0.4-nm-diameter hole reaching down to the metal substrate. Understanding the structure and metal growth on this oxide allows us to use it as a template for growing highly regular arrays of nanoparticles.
KW  - Alumina
KW  - Aluminum
KW  - DFT
KW  - Growth
KW  - Nickel
KW  - STM
SP  - 196104
EP  - 196104
SN  - 0031-9007
ER  - 

TY  - JOUR
ID  - 78
T1  - Oxidation of Pd(553): From ultrahigh vacuum to atmospheric pressure
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.76.155410
A1  - Westerström,R.
A1  - Gustafson,J.
A1  - Resta,A.
A1  - Mikkelsen,A.
A1  - Andersen,J. N.
A1  - Lundgren,E.
A1  - Seriani,N.
A1  - Mittendorfer,F.
A1  - Schmid,M.
A1  - Klikovits,J.
A1  - Varga,P.
A1  - Ackermann,M. D.
A1  - Frenken,J. W. M.
A1  - Kasper,N.
A1  - Stierle,A.
VL  - 76
IS  - 15
PY  - 2007/10/15/
N2  - The oxidation of a vicinal Pd(553) surface has been studied from ultrahigh vacuum (UHV) to atmospheric oxygen pressures at elevated sample temperatures. The investigation combines traditional electron based UHV techniques such as high resolution core level spectroscopy, low-energy electron diffraction, scanning tunneling microscopy with in situ surface x-ray diffraction, and ab initio simulations. In this way, we show that the O atoms preferentially adsorb at the step edges at oxygen pressures below 10(-6) mbar and that the (553) surface is preserved. In the pressure range between 10(-6) and 1 mbar and at a sample temperature of 300-400 degrees C, a surface oxide forms and rearranges the (553) surface facets and forming (332) facets. Most of the surface oxide can be described as a PdO(101) plane, similar to what has been found previously on other Pd surfaces. However, in the present case, the surface oxide is reconstructed along the step edges, and the stability of this structure is discussed. In addition, the (√6 x √6) Pd5O4 surface oxide can be observed on (111) terraces larger than those of the (332) terraces. Increasing the O pressure above 1 mbar results in the disappearance of the (332) facets and the formation of PdO bulk oxide.
KW  - DFT
KW  - Oxygen
KW  - Palladium
KW  - STM
SP  - 155410
EP  - 9
UR  - http://link.aps.org/abstract/PRB/v76/e155410
ER  - 

TY  - JOUR
ID  - 796
T1  - Pd, Co and Co-Pd clusters on the ordered alumina film on NiAl(110): Contact angle, surface structure and composition
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/j.susc.2007.05.047
A1  - Napetschnig,E
A1  - Schmid,M
A1  - Varga,P
VL  - 601
IS  - 15
PY  - 2007/08/01/
N2  - 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.
KW  - Alumina
KW  - Aluminum
KW  - Cobalt
KW  - ISS
KW  - Nickel
KW  - Palladium
KW  - STM
SP  - 3233
EP  - 3245
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 977
T1  - Surface oxides on Pd(111): STM and density functional calculations
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.76.045405
A1  - Klikovits,J
A1  - Napetschnig,E
A1  - Schmid,M
A1  - Seriani,N
A1  - Dubay,O
A1  - Kresse,G
A1  - Varga,P
VL  - 76
IS  - 4
PY  - 2007/07//
N2  - The formation of one-layer surface oxides on Pd(111) has been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). Besides the Pd5O4 structure determined previously, structural details of six different surface oxides on Pd(111) will be presented. These oxides are observed for preparation in oxygen-rich conditions, approaching the thermodynamic stability limit of the PdO bulk oxide at an oxygen chemical potential of -0.95 to -1.02 eV (570-605 K, 5x10(-4) mbar O-2). Sorted by increasing oxygen fraction in the primitive unit cell, the stoichiometry of the surface oxides is Pd5O4, Pd9O8, Pd20O18, Pd23O21, Pd19O18, Pd8O8, and Pd32O32. All structures are one-layer oxides, in which oxygen atoms form a rectangular lattice, and all structures follow the same rules of favorable alignment of the oxide layer on the Pd(111) substrate. DFT calculations were used to simulate STM images as well as to determine the stability of the surface oxide structures. Simulated and measured STM images are in excellent agreement, indicating that the structural models are correct. Since the newly found surface oxides are clearly less stable than Pd5O4, we conclude that Pd5O4 is the only thermodynamically stable phase, whereas all newly found structures are only kinetically stabilized. We also discuss possible mechanisms for the formation of these oxide structures.
KW  - DFT
KW  - Oxygen
KW  - Palladium
KW  - STM
SP  - 045405
EP  - 045405
SN  - 1098-0121
ER  - 

TY  - JOUR
ID  - 367
T1  - The Pd(100)-(√5 x √5)R27°-O surface oxide: A LEED, DFT and STM study
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/j.susc.2007.01.026
A1  - Kostelník,Petr
A1  - Seriani,Nicola
A1  - Kresse,Georg
A1  - Mikkelsen,Anders
A1  - Lundgren,Edvin
A1  - Blum,Volker
A1  - Šikola,Tomás
A1  - Varga,Peter
A1  - Schmid,Michael
VL  - 601
IS  - 6
PY  - 2007/03/15/
N2  - Using low energy electron diffraction (LEED), density functional theory (DFT) and scanning tunneling microscopy (STM), we have re-analyzed the Pd(100)-(√5 x √5)R27°-O surface oxide structure consisting, in the most recent model, of a strained PdO(101) layer on top of the Pd(100) surface [M. Todorova et al., Surf. Sci. 541 (2003) 101]. Both, DFT simulations using the Vienna Ab initio Simulation Package (VASP) and tensor LEED I(V) analysis of newly acquired LEED experimental data, show that the PdO(101) model is essentially correct. However, compared to the previous study, there is a horizontal shift of the PdO(101) layer with respect to the Pd(100) substrate. The atomic coordinates derived by DFT and LEED (RP = 0.162) are in excellent agreement with each other. We also present STM images with atomic resolution showing domain boundaries on the surface oxide and discuss the bonding geometry between the surface oxide and the substrate.
KW  - DFT
KW  - LEED
KW  - Oxygen
KW  - Palladium
KW  - STM
SP  - 1574
EP  - 1581
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 565
T1  - Chemical ordering and composition fluctuations at the (001) surface of the Fe64Ni36 Invar alloy
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.74.235437
A1  - Ondráček,M.
A1  - Máca,F.
A1  - Kudrnovský,J.
A1  - Redinger,J.
A1  - Biedermann,A.
A1  - Fritscher,C.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 74
IS  - 23
PY  - 2006/12/15/
N2  - We report on a study of (001) oriented fcc Fe-Ni alloy surfaces which combines first-principles calculations and low-temperature scanning tunneling microscopy (STM) experiments. Density functional theory calculations show that Fe-Ni alloy surfaces are buckled with the Fe atoms slightly shifted outwards and the Ni atoms inwards. This is consistent with the observation that the atoms in the surface layer can be chemically distinguished in the STM image: brighter spots (corrugation maxima with increased apparent height) indicate iron atoms, darker ones nickel atoms. This chemical contrast reveals a c(2×2) chemical order (50% Fe) with frequent Fe-rich defects on the Fe64Ni36(001) surface. The calculations also indicate that subsurface composition fluctuations may additionally modulate the apparent height of the surface atoms. The STM images show that this effect is pronounced compared to the surfaces of other disordered alloys, which suggests that some chemical order and corresponding concentration fluctuations exist also in the subsurface layers of Invar alloy. In addition, detailed electronic structure calculations allow us to identify the nature of a distinct peak below the Fermi level observed in the tunneling spectra. This peak corresponds to a surface resonance band which is particularly pronounced in iron-rich surface regions and provides a second type of chemical contrast with less spatial resolution but one that is essentially independent of the subsurface composition.
KW  - DFT
KW  - Iron
KW  - Nickel
KW  - STM
SP  - 235437
EP  - 7
UR  - http://link.aps.org/abstract/PRB/v74/e235437
ER  - 

TY  - JOUR
ID  - 1199
T1  - Oxygen-deficient line defects in an ultrathin aluminum oxide film
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.97.046101
A1  - Schmid,M.
A1  - Shishkin,M.
A1  - Kresse,G.
A1  - Napetschnig,E.
A1  - Varga,P.
A1  - Kulawik,M.
A1  - Nilius,N.
A1  - Rust,H.-P.
A1  - Freund,H.-J.
VL  - 97
IS  - 4
PY  - 2006/07/28/
N2  - 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.
KW  - Alumina
KW  - Aluminum
KW  - DFT
KW  - Nickel
KW  - STM
SP  - 046101
EP  - 4
UR  - http://link.aps.org/abstract/PRL/v97/e046101
ER  - 

TY  - JOUR
ID  - 1081
T1  - Oxygen-induced step bunching and faceting of Rh(553): Experiment and ab initio calculations
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.74.035401
A1  - Gustafson,J.
A1  - Resta,A.
A1  - Mikkelsen,A.
A1  - Westerström,R.
A1  - Andersen,J. N.
A1  - Lundgren,E.
A1  - Weissenrieder,J.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Kasper,N.
A1  - Torrelles,X.
A1  - Ferrer,S.
A1  - Mittendorfer,F.
A1  - Kresse,G.
VL  - 74
IS  - 3
PY  - 2006/07/15/
N2  - Using a combined experimental and theoretical approach, we show that the initial oxidation of a Rh(553) surface, a surface vicinal to (111), undergoes step bunching when exposed to oxygen, forming lower-index facets. At a pressure of about 10–6  mbar and a temperature of 380  °C this leads to (331) facets with one-dimensional oxide chains along the steps, coexisting with (111) facets. Further increase of the pressure and temperature results in (111) facets only, covered by an O-Rh-O surface oxide. Our density functional theory calculations provide an atomistic understanding of the observed behavior.
KW  - DFT
KW  - Oxygen
KW  - Rhodium
KW  - STM
SP  - 035401
EP  - 035401
UR  - http://link.aps.org/abstract/PRB/v74/e035401
ER  - 

TY  - JOUR
ID  - 799
T1  - Unusual Process of Water Formation on RuO2(110) by Hydrogen Exposure at Room Temperature
JF  - The Journal of Physical Chemistry B
JA  - J. Phys. Chem. B
M3  - 10.1021/jp0626622
A1  - Knapp,M.
A1  - Crihan,D.
A1  - Seitsonen,A. P.
A1  - Resta,A.
A1  - Lundgren,E.
A1  - Andersen,J. N.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Over,H.
VL  - 110
IS  - 29
PY  - 2006/07/01/
N2  - The reduction mechanism of the RuO2(110) surface by molecular hydrogen exposure is unraveled to an unprecedented level by a combination of temperature programmed reaction, scanning tunneling microscopy, high-resolution core level shift spectroscopy, and density functional theory calculations. We demonstrate that even at room temperature hydrogen exposure to the RuO2(110) surface leads to the formation of water. In a two-step process, hydrogen saturates first the bridging oxygen atoms to form (ObrH) species and subsequently part of these ObrH groups move to the undercoordinated Ru atoms where they form adsorbed water. This latter process is driven by thermodynamics leaving vacancies in the bridging O rows.
KW  - DFT
KW  - Hydrogen
KW  - Oxygen
KW  - RuO2
KW  - STM
SP  - 14007
EP  - 14010
UR  - http://dx.doi.org/10.1021/jp0626622
ER  - 

TY  - JOUR
ID  - 376
T1  - Kinetics of the Reduction of the Rh(111) Surface Oxide: Linking Spectroscopy and Atomic-Scale Information
JF  - The Journal of Physical Chemistry B
JA  - J. Phys. Chem. B
M3  - 10.1021/jp0611875
A1  - Klikovits,J.
A1  - Schmid,M.
A1  - Gustafson,J.
A1  - Mikkelsen,A.
A1  - Resta,A.
A1  - Lundgren,E.
A1  - Andersen,J. N.
A1  - Varga,P.
VL  - 110
IS  - 20
PY  - 2006/05/01/
N2  - The reduction of the surface oxide on Rh(111) by H2 was observed in situ by scanning tunneling microscopy (STM) and high-resolution core level spectroscopy (HRCLS). At room temperature, H2 does not adsorb on the oxide, only in reduced areas. Reduction starts in very few sites, almost exclusively in stepped areas. One can also initiate the reduction process by deliberately creating defects with the STM tip allowing us to examine the reduction kinetics in detail. Depending on the size of the reduced area and the hydrogen pressure, two growth regimes were found. At low H2 pressures or small reduced areas, the reduction rate is limited by hydrogen adsorption on the reduced area. For large reduced areas, the reduction rate is limited by the processes at the border of the reduced area. Since a near-random distribution of the reduction nuclei was found and the reduction process at defects starts at a random time, one can use JohnsonMehlAvramiKolmogoroff (JMAK) theory to describe the process of reduction. The microscopic data from STM agree well with spatially averaged data from HRCLS measurements.
KW  - Hydrogen
KW  - Oxygen
KW  - Rhodium
KW  - STM
SP  - 9966
EP  - 9975
UR  - http://dx.doi.org/10.1021/jp0611875
ER  - 

TY  - JOUR
ID  - 447
T1  - Coexistence of fcc- and bcc-like crystal structures in ultrathin Fe films grown on Cu(111)
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.73.165418
A1  - Biedermann,A.
A1  - Rupp,W.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 73
IS  - 16
PY  - 2006/04/15/
N2  - We report on bcc-like phases in ultrathin Fe films grown by thermal deposition on Cu(111) previously thought to consist exclusively of fcc phases distinguished only by their magnetic order. Our scanning tunneling microscopy and spectroscopy data together with published x-ray photoelectron diffraction results [M. T. Kief and W. F. Egelhoff, Jr., Phys. Rev. B 47, 10785 (1993)] provide us with sufficient detail to deduce the film structure. Two growth regimes are considered: (1) films with 1–2 monolayer average thickness grown near 200 K, which nucleate as bcc-like bilayer islands: Larger islands show bcc-like fringes coexisiting with an fcc center domain; i.e., the bcc-like phase is stable only within a certain distance to a step edge. The presence of a bcc-like bilayer phase provides a straightforward explanation for the ferromagnetism previously observed in these films. In addition we find that the bcc-like phase can be promoted by H adsorption at 80  K. The bcc domains form "displacement vortex" structures to simultaneously minimize film stress and interface energy. (2) In films grown at room temperature, between pseudomorphic fcc areas, we observe a more ideal but still strained bcc phase in regions with a local thickness of at least 4 monolayers. Also in this growth regime, the fcc-bcc transformation is facilitated by step edges, which are abundant due to the imperfect layer-by-layer growth.
KW  - Copper
KW  - Iron
KW  - Magnetism
KW  - STM
SP  - 165418
EP  - 16
UR  - http://link.aps.org/abstract/PRB/v73/e165418
ER  - 

TY  - JOUR
ID  - 467
T1  - Structure of Ag(111)-p(4 x 4)-O: No Silver Oxide
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.96.146102
A1  - Schmid,M.
A1  - Reicho,A.
A1  - Stierle,A.
A1  - Costina,I.
A1  - Klikovits,J.
A1  - Kostelník,P.
A1  - Dubay,O.
A1  - Kresse,G.
A1  - Gustafson,J.
A1  - Lundgren,E.
A1  - Andersen,J. N.
A1  - Dosch,H.
A1  - Varga,P.
VL  - 96
IS  - 14
PY  - 2006/04/14/
N2  - The structure of the oxygen-induced p(4×4) reconstruction of Ag(111) is determined by a combination of scanning tunneling microscopy, surface x-ray diffraction, core level spectroscopy, and density functional theory. We demonstrate that all previous models of this surface structure are incorrect and propose a new model which is able to explain all our experimental findings but has no resemblance to bulk silver oxide. We also shed some light on the limitations of current density functional theories and the potential role of van der Waals interactions in the stabilization of oxygen-induced surface reconstructions of noble metals.
KW  - DFT
KW  - Oxygen
KW  - Silver
KW  - STM
SP  - 146102
EP  - 4
UR  - http://link.aps.org/abstract/PRL/v96/e146102
ER  - 

TY  - JOUR
ID  - 543
T1  - Combined STM, LEED and DFT study of Ag(100) exposed to oxygen near atmospheric pressures
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/j.susc.2005.11.020
A1  - Costina,I.
A1  - Schmid,M.
A1  - Schiechl,H.
A1  - Gajdos,M.
A1  - Stierle,A.
A1  - Kumaragurubaran,S.
A1  - Hafner,J.
A1  - Dosch,H.
A1  - Varga,P.
VL  - 600
IS  - 3
PY  - 2006/02/01/
N2  - We have investigated the interaction of molecular oxygen with the Ag(100) surface in a temperature range from 130 K to 470 K and an oxygen partial pressure ranging up to 10 mbar by scanning tunneling microscopy, low electron energy diffraction, Auger electron spectroscopy and ab initio density functional calculations. We find that at 130 K, following oxygen exposures of 6000 Langmuirs O2, the individual oxygen atoms are randomly distributed on the surface. When the sample is exposed to 10 mbar O2 at room temperature, small, p(2 × 2) reconstructed patches are formed on the surface. After oxidation at [approximate]470 K and 10 mbar O2 pressure the surface undergoes a c(4 × 6) reconstruction coexisting with a (6 × 6) superstructure. By ab initio thermodynamic calculations it is shown that the c(4 × 6) reconstruction is an oxygen adsorption induced superstructure which is thermodynamically stable for an intermediate range of oxygen chemical potential.
KW  - Oxygen
KW  - Silver
KW  - STM
SP  - 617
EP  - 624
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 773
T1  - Growth and decay of the Pd(111)-Pd5O4 surface oxide: Pressure-dependent kinetics and structural aspects
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/j.susc.2005.09.052
A1  - Gabasch,Harald
A1  - Unterberger,Werner
A1  - Hayek,Konrad
A1  - Klötzer,Bernhard
A1  - Kresse,Georg
A1  - Klein,Christof
A1  - Schmid,Michael
A1  - Varga,Peter
VL  - 600
IS  - 1
PY  - 2006/01/01/
N2  - Growth and decomposition of the Pd<sub>5</sub>O<sub>4</sub> surface oxide on Pd(111) were studied at sample temperatures between 573 and 683 K and O2 gas pressures between 10<sup>-7</sup> and 6 × 10<sup>-5</sup> mbar, by means of an effusive O2 beam from a capillary array doser, scanning tunnelling microscopy (STM) and thermal desorption spectrometry (TDS). Exposures beyond the p(2 × 2)O adlayer (saturation coverage 0.25) at 683 K (near thermodynamic equilibrium with respect to Pd5O4 surface oxide formation) lead to incorporation of additional oxygen into the surface. To initiate the incorporation, a critical pressure beyond the thermodynamic stability limit of the surface oxide is required. This thermodynamic stability limit is near 8.9 × 10<sup>-6</sup> mbar at 683 K, in good agreement with calculations by density functional theory. A controlled kinetic study was feasible by generating nuclei by only a short O2 pressure pulse and then following further growth kinetics in the lower (10<sup>-6</sup> mbar) pressure range. Growth of the surface oxide layer at a lower temperature (573 K) studied by STM is characterized by a high degree of heterogeneity. Among various metastable local structures, a seam of disordered oxide formed at the step edges is a common structural feature characteristic of initial oxide growth. Further oxide nucleation appears to be favoured along the interface between the p(2 × 2)O structure and these disordered seams. Among the intermediate phases one specifically stable phase was detected both during growth and decomposition of the Pd<sub>5</sub>O<sub>4</sub> layer. It is hexagonal with a distance of about 0.62 nm between the protrusions. Its well-ordered form is a (√67 x √67)R12.1° superstructure. Isothermal decay of the Pd<sub>5</sub>O<sub>4</sub> oxide layer at 693 K involves at first a rearrangement into the  (√67 x √67)R12.1° structure, indicating its high-temperature stability. This structure can break up into small clusters of uniform size and leaves a free metal surface area covered by a p(2 × 2)O adlayer. The rate of desorption increases autocatalytically with increasing phase boundary metal-oxide. We propose that at close-to-equilibrium conditions (693 K) surface oxide growth and decay occur via this intermediate structure.
KW  - Oxygen
KW  - Palladium
KW  - STM
SP  - 205
EP  - 218
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 1216
T1  - Surface oxides on close-packed surfaces of late transition metals
JF  - Journal of Physics: Condensed Matter
JA  - J. Phys.: Condens. Matter
A1  - Lundgren,Edvin
A1  - Mikkelsen,Anders
A1  - Andersen,Jesper N.
A1  - Kresse,Georg
A1  - Schmid,Michael
A1  - Varga,Peter
VL  - 18
IS  - 30
PY  - 2006///
N2  - 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.
KW  - DFT
KW  - Oxygen
KW  - Palladium
KW  - Rhodium
KW  - STM
SP  - R481-R499
EP  - R481-R499
SN  - 0953-8984
ER  - 

TY  - JOUR
ID  - 768
T1  - One-Dimensional PtO2 at Pt Steps: Formation and Reaction with CO
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.95.256102
A1  - Wang,J. G.
A1  - Li,W. X.
A1  - Borg,M.
A1  - Gustafson,J.
A1  - Mikkelsen,A.
A1  - Pedersen,T. M.
A1  - Lundgren,E.
A1  - Weissenrieder,J.
A1  - Klikovits,J.
A1  - Schmid,M.
A1  - Hammer,B.
A1  - Andersen,J. N.
VL  - 95
IS  - 25
PY  - 2005/12/15/
N2  - Using core-level spectroscopy and density functional theory we show that a one-dimensional (1D) PtO2 oxide structure forms at the steps of the Pt(332) surface after O2 exposure. The 1D oxide is found to be stable in an oxygen pressure range, where bulk oxides are only metastable, and is therefore argued to be a precursor to the Pt oxidation. As an example of the consequences of such a precursor exclusively present at the steps, we investigate the reaction of CO with oxygen covered Pt(332). Albeit more strongly bound, the oxidic oxygen is found to react more easily with CO than oxygen chemisorbed on the Pt terraces.
KW  - Carbon monoxide
KW  - DFT
KW  - Oxygen
KW  - Platinum
SP  - 256102
EP  - 256102
UR  - http://link.aps.org/abstract/PRL/v95/e256102
ER  - 

TY  - JOUR
ID  - 1054
T1  - Growth of ultrathin Fe films on Cu(111) by pulsed laser deposition
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/j.susc.2005.07.016
A1  - Schiechl,H.
A1  - Rauchbauer,G.
A1  - Biedermann,A.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 594
IS  - 1-3
PY  - 2005/12/01/
N2  - 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.
KW  - Copper
KW  - Iron
KW  - ISS
KW  - PLD
KW  - STM
SP  - 120
EP  - 131
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 740
T1  - Structure of the Ultrathin Aluminum Oxide Film on NiAl(110)
JF  - Science
JA  - Science
M3  - 10.1126/science.1107783
A1  - Kresse,Georg
A1  - Schmid,Michael
A1  - Napetschnig,Evelyn
A1  - Shishkin,Maxim
A1  - Köhler,Lukas
A1  - Varga,Peter
VL  - 308
IS  - 5727
PY  - 2005/06/03/
N2  - 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.
KW  - Alumina
KW  - Aluminum
KW  - DFT
KW  - Nickel
KW  - STM
SP  - 1440
EP  - 1442
UR  - http://www.sciencemag.org/cgi/content/abstract/308/5727/1440
ER  - 

TY  - JOUR
ID  - 1006
T1  - The surface oxide as a source of oxygen on Rh(111)
JF  - Journal of Electron Spectroscopy and Related Phenomena
JA  - J Electron Spectrosc. Relat. Phenom.
M3  - 10.1016/j.elspec.2005.01.004
A1  - Lundgren,E.
A1  - Gustafson,J.
A1  - Resta,A.
A1  - Weissenrieder,J.
A1  - Mikkelsen,A.
A1  - Andersen,J.N.
A1  - Köhler,L.
A1  - Kresse,G.
A1  - Klikovits,J.
A1  - Biederman,A.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 144-147
PY  - 2005/06//
N2  - 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.
KW  - Carbon monoxide
KW  - Oxygen
KW  - Rhodium
KW  - STM
KW  - STM
SP  - 367
EP  - 372
SN  - 0368-2048
ER  - 

TY  - JOUR
ID  - 824
T1  - Structure of a thin oxide film on Rh(100)
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.71.115442
A1  - Gustafson,J.
A1  - Mikkelsen,A
A1  - Borg,M.
A1  - Andersen,J. N.
A1  - Lundgren,E.
A1  - Klein,C.
A1  - Hofer,W.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Köhler,L.
A1  - Kresse,G.
A1  - Kasper,N.
A1  - Stierle,A.
A1  - Dosch,H.
VL  - 71
IS  - 11
PY  - 2005/03/31/
N2  - The initial oxidation of Rh(100) has been studied using high resolution core level spectroscopy, low energy electron diffraction, surface x-ray diffraction, scanning tunneling microscopy, and density functional theory. We report a structural study of an oxygen induced structure displaying a c(8×2) periodicity at an oxygen pressure above 10−5 mbar and using a sample temperature of 700 K . Our experimental and theoretical data demonstrate that this structure is due to the formation of a thin surface oxide with a hexagonal trilayer O-Rh-O structure.
KW  - DFT
KW  - LEED
KW  - Oxygen
KW  - Rhodium
KW  - STM
SP  - 115442
EP  - 115442
UR  - http://link.aps.org/abstract/PRB/v71/e115442
ER  - 

TY  - JOUR
ID  - 151
T1  - An STM study of growth and alloying of Cr on Ru(0001) and CO adsorption on the alloy
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/j.susc.2005.01.022
A1  - Engelhardt,M.P.
A1  - Schmid,M.
A1  - Biedermann,A.
A1  - Denecke,R.
A1  - Steinrück,H.-P.
A1  - Varga,P.
VL  - 578
IS  - 1-3
PY  - 2005/03/10/
N2  - 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.
KW  - Carbon monoxide
KW  - Chromium
KW  - Growth
KW  - Ruthenium
KW  - STM
SP  - 124
EP  - 135
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 1062
T1  - Understanding the Structural Deactivation of Ruthenium Catalysts on an Atomic Scale under both Oxidizing and Reducing Conditions
JF  - Angewandte Chemie International Edition
JA  - Angew. Chem. Int. Ed.
M3  - 10.1002/anie.200461805
A1  - Aßmann,Jens
A1  - Crihan,Daniela
A1  - Knapp,Marcus
A1  - Lundgren,Edvin
A1  - Löffler,Elke
A1  - Muhler,Martin
A1  - Narkhede,Vijay
A1  - Over,Herbert
A1  - Schmid,Michael
A1  - Seitsonen,Ari P.
A1  - Varga,Peter
VL  - 44
IS  - 6
PY  - 2005///
N2  - No Abstract

The surface-science approach coupled with industrial catalyst research offers a synergistic strategy to improve the performance of industrial catalysts. The poorly understood microscopic processes that determine the structural deactivation of ruthenium-based catalysts during CO oxidation have been elucidated. Based on these results measures are proposed to improve the performance of ruthenium catalysts.
KW  - RuO2
KW  - STM
SP  - 917
EP  - 920
UR  - http://dx.doi.org/10.1002/anie.200461805
ER  - 

TY  - JOUR
ID  - 1132
T1  - High-Coverage Oxygen Structures on Rh(111): Adsorbate Repulsion and Site Preference Is Not Enough
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.93.266103
A1  - Köhler,L.
A1  - Kresse,G.
A1  - Schmid,M.
A1  - Lundgren,E.
A1  - Gustafson,J.
A1  - Mikkelsen,A.
A1  - Borg,M.
A1  - Yuhara,J.
A1  - Andersen,J. N.
A1  - Marsman,M.
A1  - Varga,P.
VL  - 93
IS  - 26
PY  - 2004/12/21/
N2  - A new O induced structure on Rh(111) displaying a (2sqrt[3]×2sqrt[3])R30° periodicity with an oxygen coverage of 2/3 has been studied by high resolution core level spectroscopy, scanning tunneling microscopy, and density functional theory. Although O favors fcc hollow sites in all other known phases, it occupies both fcc and hcp sites in this structure, which cannot be explained by pairwise adsorbate repulsion only. Both the (2sqrt[3]×2sqrt[3])R30° and (2×2)-3O structures also exemplify that density-of-states contrast can lead to oxygen adatoms appearing as protrusions in scanning tunneling microscopy images.
KW  - DFT
KW  - Oxygen
KW  - Rhodium
KW  - STM
SP  - 266103
EP  - 266103
UR  - http://link.aps.org/abstract/PRL/v93/e266103
ER  - 

TY  - JOUR
ID  - 886
T1  - Structure of the cobalt-filled missing-row reconstruction of Pt(110)
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.70.153403
A1  - Klein,C.
A1  - Koller,R.
A1  - Lundgren,E.
A1  - Máca,F.
A1  - Redinger,J.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 70
IS  - 15
PY  - 2004/10/18/
N2  - 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.
KW  - Cobalt
KW  - DFT
KW  - LEED
KW  - Platinum
KW  - STM
SP  - 153403
EP  - 153403
UR  - http://link.aps.org/abstract/PRB/v70/e153403
ER  - 

TY  - JOUR
ID  - 534
T1  - Reconstruction of the clean and H covered "magnetic live surface layer" of Fe films grown on Cu(100)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/j.susc.2004.06.150
A1  - Biedermann,Albert
A1  - Tscheliessnig,Rupert
A1  - Klein,Christof
A1  - Schmid,Michael
A1  - Varga,Peter
VL  - 563
IS  - 1-3
PY  - 2004/08/10/
N2  - The surface of 6-7 monolayer thick fcc Fe films grown at room temperature on a Cu(100) substrate is characterized by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The STM images show a p4g(2 × 2) structure at 5 and 80 K, but not at 300 K. LEED, however, indicates an expansion of the interlayer distance and lateral distortions of similar magnitude both at 150 K and at 300 K. No evidence for a significant change of the surface structure is detected by a LEED spot profile analysis between 150 and 300 K. We attribute the apparent absence of the reconstruction in the STM images at 300 K to surface dynamics caused by domain boundary motion. The particular surface structure with bond angles and distances similar to bcc Fe suggests a driving force of the reconstruction which is similar to that operative in the fcc-to-bcc transition of bulk Fe. Dosing less than 5 L H2 decorates the p4g(2 × 2) surface reconstruction, while higher hydrogen doses transform the surface reconstruction to p(2 × 1).
KW  - Hydrogen
KW  - Iron
KW  - LEED
KW  - STM
SP  - 110
EP  - 126
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 75
T1  - Atomic structure of an Al-Co-Ni decagonal quasicrystalline surface
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.70.024203
A1  - Yuhara,J.
A1  - Klikovits,J.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Yokoyama,Y.
A1  - Shishido,T.
A1  - Soda,K.
VL  - 70
IS  - 2
PY  - 2004/07/07/
N2  - We have analyzed the structure and composition of the first layer of an Al72Co16Ni12 tenfold surface by means of scanning tunneling microscopy (STM), ion scattering spectroscopy (ISS), and Auger electron spectroscopy (AES). High-resolution STM images reveal local structures that have decagonal symmetry in addition to the usual pentagonal symmetry of the surface. This quasicrystal surface resembles a random tiling instead of an ideal quasiperiodic tiling. After annealing at 1100 K , the total surface atomic density found by ISS is (9±1)×10^14 cm−2 . The surface densities of Al and TM (transition metal, i.e., Co and Ni) are determined as (8±1)×10^14 cm−2 and (1.0±0.2)×10^14 cm−2 , respectively from ISS, indicating a similar density of Al and much lower density of the TM atoms in the surface layer than in a truncated bulk. The Al surface atomic density agrees well with the number of corrugation maxima in the STM images. A model of the arrangement of the Al atoms in the top layer is presented. Scanning tunneling spectroscopy (STS) is performed to study the local electronic structure. The STS spectrum at the corrugation maxima is similar to that at the corrugation minima. A few ≈0.12 nm high protrusions in the STM images are attributed to local oxide clusters due to their STS spectra different from the corrugation maxima and through in situ STM observations during exposure to O2 gas at 2×10^−6 Pa at RT.
KW  - Aluminum
KW  - Cobalt
KW  - ISS
KW  - Nickel
KW  - STM
SP  - 024203
EP  - 024203
UR  - http://link.aps.org/abstract/PRB/v70/e024203
ER  - 

TY  - JOUR
ID  - 451
T1  - Growth of Ce on Rh(111)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/j.susc.2004.03.006
A1  - Napetschnig,E.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 556
IS  - 1
PY  - 2004/05/01/
N2  - We have studied the growth of cerium films on Rh(111) using STM (scanning tunneling microscopy), LEED (low energy electron diffraction), XPS (X-ray photoelectron spectroscopy) and AES (Auger electron spectroscopy). Measurements of the Ce films after room temperature deposition showed that Ce is initially forming nanoclusters in the low coverage regime. These clusters consist of 12 Ce atoms and have the shape of pinwheels. At a coverage of 0.25 ML (monolayer, ML) an adatom layer with a (2 × 2) superstructure is observed. Above 0.4 ML, Rh is diffusing through pinholes into the film, forming an unstructured mixed layer. Annealing at 250 °C leads to the formation of ordered Ce-Rh compounds based on the bulk compound CeRh3. At a coverage of 0.1 ML, small ordered (2 × 2) surface alloy domains are observed. The exchanged Rh atoms form additional alloy islands situated on the pure Rh(1 1 1) surface, showing the same (2 × 2) superstructure as the surface alloy. At a coverage of 0.25 ML, the surface is completely covered by the surface alloy and alloy islands. The (2 × 2) structure is equivalent to a (111)-plane of CeRh3, contracted by 6%. Annealing a 1 ML thick Ce layer leads to a flat surface consisting of different rotational domains of CeRh3(100). The Rh needed for alloy formation comes from 50 Å deep pits in the substrate. Finally we show that LEIS (low energy ion scattering) is not suitable for the characterization of Ce and CeRh films due to strong effects of neutralization.
KW  - Cerium
KW  - Growth
KW  - ISS
KW  - Rhodium
KW  - STM
SP  - 1
EP  - 10
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 482
T1  - Self-Limited Growth of a Thin Oxide Layer on Rh(111)
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.92.126102
A1  - Gustafson,J.
A1  - Mikkelsen,A.
A1  - Borg,M.
A1  - Lundgren,E.
A1  - Köhler,L.
A1  - Kresse,G.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Yuhara,J.
A1  - Torrelles,X.
A1  - Quirós,C.
A1  - Andersen,J. N.
VL  - 92
IS  - 12
PY  - 2004/03/26/
N2  - 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.
KW  - DFT
KW  - Oxygen
KW  - Rhodium
KW  - STM
SP  - 126102
EP  - 126102
UR  - http://link.aps.org/abstract/PRL/v92/e126102
ER  - 

TY  - JOUR
ID  - 1011
T1  - Local atomic structure of ultra-thin Fe films grown on Cu(100)
JF  - Applied Physics A: Materials Science & Processing
JA  - Appl. Phys. A
M3  - 10.1007/s00339-003-2435-7
A1  - Biedermann,A.
A1  - Tscheliessnig,R.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 78
IS  - 6
PY  - 2004/03/01/
N2  - Ultra-thin epitaxial Fe films grown by thermal deposition on Cu(100) are analyzed by scanning tunneling microscopy. Evidence is presented that the morphological characteristics and magnetic properties are a direct consequence of FCC-to-BCC transitions reminiscent of those occurring in bulk Fe. In contrast to the assumption of a ferromagnetic FCC phase in previous models of the Fe/Cu(100) system, we observe a tightly twinned and strained BCC-like phase termed nanomartensite in films below 5 ML thickness, which encompasses almost the entire film volume of 3 ML films. In addition, the surface of 7–8 ML films reconstructs by forming non-close-packed structures with BCC-like bond angles. The formation of these BCC-like phases is the reason for the expansion of the interlayer spacing observed in these films and correlates perfectly with their ferromagnetic ordering.
KW  - Copper
KW  - Iron
KW  - Magnetism
KW  - STM
SP  - 807
EP  - 816
UR  - http://dx.doi.org/10.1007/s00339-003-2435-7
ER  - 

TY  - JOUR
ID  - 814
T1  - Visualization of Atomic Processes on Ruthenium Dioxide using Scanning Tunneling Microscopy
JF  - ChemPhysChem
JA  - ChemPhysChem
M3  - 10.1002/cphc.200300833
A1  - Over,H.
A1  - Knapp,M.
A1  - Lundgren,E.
A1  - Seitsonen,A. P.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 5
IS  - 2
PY  - 2004///
N2  - The visualization of surface reactions on the atomic scale provides direct insight into the microscopic reaction steps taking place in a catalytic reaction at a (model) catalyst's surface. Employing the technique of scanning tunneling microscopy (STM), we investigated the CO oxidation reaction over the RuO2(110) and RuO2(100) surfaces. For both surfaces the protruding bridging O atoms are imaged in STM as bright features. The reaction mechanism is identical on both orientations of RuO2. CO molecules adsorb on the undercoordinated surface Ru atoms from where they recombine with undercoordinated O atoms to form CO2 at the oxide surface. In contrast to the RuO2(110) surface, the RuO2(100) surface stabilizes also a catalytically inactive c(2×2) surface phase onto which CO is not able to adsorb above 100 K. We argue that this inactive RuO2(100)-c(2×2) phase may play an important role in the deactivation of RuO2 catalysts in the electrochemical Cl2 evolution and other heterogeneous reactions.
KW  - Carbon monoxide
KW  - Oxygen
KW  - RuO2
KW  - STM
SP  - 167
EP  - 174
UR  - http://dx.doi.org/10.1002/cphc.200300833
ER  - 

TY  - JOUR
ID  - 318
T1  - Vanadium surface oxides on Pd(111): A structural analysis
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.68.235416
A1  - Klein,C.
A1  - Kresse,G.
A1  - Surnev,S.
A1  - Netzer,F. P.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 68
IS  - 23
PY  - 2003/12/30/
N2  - Scanning tunneling microscopy studies of vanadium oxides grown on Pd(111) show interesting structures especially in the low-coverage region. Evaporation of V in an oxygen background at elevated sample temperature (250 °C) results in the formation of a nonperiodic honeycomb-like structure growing from the steps, which starts to transform into an ordered phase at a vanadium coverage of ≈0.2 ML (monolayer). At 0.31 ML the entire surface is covered by this well-ordered open (4×4) structure. Annealing this structure in H2 atmosphere transforms the phase into a V2O3 surface oxide with (2×2) periodicity, whose optimal coverage is reached at 0.5 ML vanadium. Models for both ordered structures have been suggested before on the basis of ab initio density-functional theory (DFT) calculations and molecular-dynamics simulations and these models are now unambiguously confirmed by quantitative low-energy electron-diffraction (LEED) analyses. In the (4×4) phase, the V atoms are surrounded by four oxygen atoms in an unusual tetrahedral coordination leading to a V5O14 stoichiometry. This tetrahedral coordination allows the oxide to adopt open loosely packed two-dimensional (2D) and 1D structures, which are stabilized by the surface-oxide interface energy. Furthermore, it is shown that state of the art DFT calculations can indeed predict complex structures exactly as well as that modern quantitative LEED is capable of dealing with very large unit cells.
KW  - DFT
KW  - LEED
KW  - Palladium
KW  - STM
KW  - Vanadium
KW  - Vanadium oxide
SP  - 235416
EP  - 235416
UR  - http://link.aps.org/abstract/PRB/v68/e235416
ER  - 

TY  - JOUR
ID  - 1193
T1  - Surface structure of the missing-row reconstruction of VC0.8(110): a scanning tunneling microscopy analysis
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/j.susc.2003.10.016
A1  - Gauthier,Y.
A1  - Schmid,M.
A1  - Hebenstreit,W.
A1  - Varga,P.
VL  - 547
IS  - 3
PY  - 2003/12/20/
N2  - Scanning tunnelling microscopy (STM) was used to study the (110) surface of a VC0.8 sample. The surface shows a missing-row reconstruction, i.e., a grating structure with ridges and valleys oriented along the [001] direction and (100) and (010) facets. We did not find unreconstructed (110) terraces. The regular spacing of the ridges corresponds to a periodicity of (3 × 1) or (4 × 1), depending on preparation, presumably related to different concentrations of carbon vacancies. In the STM images, we can also observe apparent pairing of atoms in the rows, leading to the larger c(6 × 2) and (4 × 2) superstructure cells, which also show up in LEED. We attribute these additional periodicities to ordering of carbon vacancies in the surface rows.
KW  - STM
KW  - VC
SP  - 394
EP  - 402
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 242
T1  - Slowing down adatom diffusion by an adsorbate: Co on Pt(111) with and without preadsorbed CO
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.68.233401
A1  - Kalousek,R.
A1  - Schmid,M.
A1  - Hammerschmid,A.
A1  - Lundgren,E.
A1  - Varga,P.
VL  - 68
IS  - 23
PY  - 2003/12/05/
N2  - Submonolayer deposition of cobalt on Pt(111) was studied by scanning tunneling microscopy at room temperature. It was found that deposition on a surface with adsorbed carbon monoxide (saturation coverage) leads to an island density almost ten times as high as that resulting from deposition on the clean platinum surface. Based on nucleation theory, we explain this fact as the result of a reduction of the diffusion coefficient of the Co adatoms in the presence of CO by more than two orders of magnitude. This effect is attributed to the displacement and/or rearrangement of the CO molecules necessary when Co adatoms diffuse over the “crowded” surface.
KW  - Carbon monoxide
KW  - Cobalt
KW  - Platinum
KW  - STM
SP  - 233401
EP  - 233401
UR  - http://link.aps.org/abstract/PRB/v68/e233401
ER  - 

TY  - JOUR
ID  - 415
T1  - Two-dimensional alloy of immiscible metals: Single and binary monolayer films of Pb and Sn on Rh(111)
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.67.195407
A1  - Yuhara,J.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 67
IS  - 19
PY  - 2003/05/12/
N2  - The single and binary metal films of Pb and Sn on Rh(111) have been studied at room temperature by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. Both Pb and Sn are mobile at low coverage and form commensurate overlayers of (4×4)-Pb and c(2×4)-Sn, respectively. From atomically resolved STM images, the atomic arrangements of (4×4)-Pb and c(2×4)-Sn have been identified to be hexagonal and rectangular structures, respectively. With increasing coverage, both commensurate phases change into incommensurate phases followed by island formation (Stranski-Krastanov growth). This shows that Rh(111) is static and inert enough to support two-dimensional (2D) phases of Pb and/or Sn without alloying at room temperature. The Pb-Sn bimetallic film on Rh(111) forms an ordered 2D alloy of PbSn3 with an incommensurate structure close to (sqrt[7]×sqrt[7]), contrary to the immiscibility of Pb and Sn in the bulk. From atomically resolved STM images, the atomic arrangement of this (sqrt[7]×sqrt[7])-(Pb,Sn) structure has been determined.
KW  - Growth
KW  - Lead
KW  - Rhodium
KW  - STM
KW  - Tin
SP  - 195407
EP  - 195407
UR  - http://link.aps.org/abstract/PRB/v67/e195407
ER  - 

TY  - JOUR
ID  - 1013
T1  - Surface structure and composition of Pt50Rh50(110): room temperature analysis of the (1 × 3) missing-row reconstruction
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(03)00381-9
A1  - Koller,R.
A1  - Gauthier,Y.
A1  - Klein,C.
A1  - De Santis,M.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 530
IS  - 3
PY  - 2003/05/01/
N2  - The room temperature structure and composition of the clean Pt50Rh50(110) surface is investigated by low energy ion scattering (LEIS), scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and grazing incidence X-ray diffraction (GIXRD). While Pt25Rh75(1 1 0) reconstructs with a (1 × 2) missing-row structure, Pt50Rh50(110) exhibits a (1 × 3) structure in analogy with Pt80Fe20(110) and Pt90Co10(110). Three missing rows lead to the formation of (111) facets in which all atomic sites are enriched with platinum. Similarly all sites directly underneath are enriched with Rh leading to oscillation of the Pt concentrations similar to that of (111) surfaces. These composition changes are accompanied by a marked inwards relaxation of the top row (-10%) and large buckling in layers 3-5. Additionally the atomic positions of the facets are shifted laterally towards the valleys formed by the missing rows. Consistent pictures are derived from LEED, GIXRD, STM and LEIS concerning the composition while some discrepancy--similar to that recorded for pure Pt--are found concerning the interlayer distances between LEED and X-rays.
KW  - ISS
KW  - LEED
KW  - Platinum
KW  - Rhodium
SP  - 121
EP  - 135
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 205
T1  - When Scanning Tunneling Microscopy Gets the Wrong Adsorption Site: H on Rh(100)
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.90.176101
A1  - Klein,C.
A1  - Eichler,A.
A1  - Hebenstreit,E. L. D.
A1  - Pauer,G.
A1  - Koller,R.
A1  - Winkler,A.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 90
IS  - 17
PY  - 2003/04/29/
N2  - 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.
KW  - DFT
KW  - Hydrogen
KW  - Rhodium
KW  - STM
SP  - 176101
EP  - 176101
UR  - http://link.aps.org/abstract/PRL/v90/e176101
ER  - 

TY  - JOUR
ID  - 480
T1  - Complex surface reconstructions solved by ab initio molecular dynamics
JF  - Applied Physics A: Materials Science & Processing
JA  - Appl. Phys. A
M3  - 10.1007/s00339-002-2007-2
A1  - Kresse,G.
A1  - Bergermayer,W.
A1  - Podloucky,R.
A1  - Lundgren,E.
A1  - Koller,R.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 76
IS  - 5
PY  - 2003/03/15/
N2  - Abstract.   Complex surface reconstructions and surface oxides, in particular, often exhibit complicated atomic arrangements, which are difficult to resolve with traditional experimental methods, such as low energy electron diffraction (LEED), surface X-ray diffraction (SXRD) or scanning tunnelling microscopy (STM) alone. Therefore, ab initio density functional calculations are used as a supplement to the experimental techniques, but even then the structural determination usually relies on a simple trial and error procedure, in which conceivable models are first constructed and then tested for their stability in ab initio calculations. An exhaustive search of the configuration space is usually difficult and requires a significant human effort. Solutions to this problem, such as simulated annealing, have long been known, but are usually considered to be too time-consuming in combination with first principles methods. In this work, we show that ab initio density functional codes are now sufficiently fast to perform extensive finite temperature molecular dynamics. The merits of this approach are exemplified for two cases, for a complex two-dimensional surface oxide on Pd(111), and for the oxygen induced c(6×2) reconstruction of V(110).
KW  - DFT
KW  - Oxygen
KW  - Palladium
KW  - Vanadium
SP  - 701
EP  - 710
UR  - http://dx.doi.org/10.1007/s00339-002-2007-2
ER  - 

TY  - JOUR
ID  - 838
T1  - Experimental and simulated STM images of stoichiometric and partially reduced RuO2(110) surfaces including adsorbates
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(02)01853-8
A1  - Over,H.
A1  - Seitsonen,A. P.
A1  - Lundgren,E.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 515
IS  - 1
PY  - 2002/08//
N2  - We present experimental and DFT-simulated STM images of ultrathin RuO2(110) films on Ru(0001), including adsorbates such as oxygen and CO. We are able to identify the under-coordinated O atoms on the RuO2(110) surface with STM, i.e. the bridging O atoms and the on-top O atoms. The partial reduction of the RuO2(110) surface by CO exposure at room temperature leads to a surface where part of the bridging O atoms have been removed and some of the vacancies are occupied by bridging CO. When dosing 10 L of CO at room temperature, all the RuO2(110) surface becomes mildly reduced in that all bridging O atoms are replaced by bridging CO molecules. Annealing the surface to 600 K produces holes on the terraces of such a mildly reduced RuO2(110) surface. These pits are not generated by the recombination of lattice O with CO, but rather these pits are assigned to a complex temperature-induced rearrangement of surface atoms in the topmost RuO2 double layer of RuO2(110). With this process the bridging O atoms are again populated and surplus Ru atoms agglomerate in small islands at the rims of the holes.
KW  - Carbon monoxide
KW  - DFT
KW  - Oxygen
KW  - RuO2
KW  - STM
SP  - 143
EP  - 156
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 779
T1  - Scanning tunneling spectroscopy on clean and contaminated V(001)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(02)01783-1
A1  - Bischoff,M. M. J.
A1  - Konvicka,C.
A1  - Quinn,A. J.
A1  - Schmid,M.
A1  - Redinger,J.
A1  - Podloucky,R.
A1  - Varga,P.
A1  - van Kempen,H.
VL  - 513
IS  - 1
PY  - 2002/07//
N2  - 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.
KW  - Carbon
KW  - Oxygen
KW  - STM
KW  - STS
KW  - Vanadium
SP  - 9
EP  - 25
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 957
T1  - The structure of the oxygen-induced c(6×2) reconstruction of V(110)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(02)01722-3
A1  - Koller,R.
A1  - Bergermayer,W.
A1  - Kresse,G.
A1  - Konvicka,C.
A1  - Schmid,M.
A1  - Redinger,J.
A1  - Podloucky,R.
A1  - Varga,P.
VL  - 512
IS  - 1-2
PY  - 2002/06/20/
N2  - The adsorption of 2.4 Langmuir oxygen on V(110) induces a c(6×2) reconstruction with an oxygen coverage of 0.5 ML. Its structure was determined using STM, quantitative LEED and ab initio density functional calculations in combination with molecular dynamics. Driven by the strong vanadium-oxygen bonding, the vanadium atoms at the surface are significantly rearranged compared to their bulk positions. The reconstructed geometry offers threefold and fourfold coordinated hollow sites, which are partially occupied by oxygen. The large set of structural data derived from LEED I-V analysis (R_Pe=0.11) and ab initio calculations, as well as the experimental and simulated STM images agree well. Additionally the structure of clean V(110) was determined.
KW  - DFT
KW  - LEED
KW  - Oxygen
KW  - STM
KW  - Vanadium
SP  - 16
EP  - 28
SN  - 0039-6028
UR  - http://www.sciencedirect.com/science/article/B6TVX-45PTW4N-G/2/5c4458b53ece08324575a89b2684311a
ER  - 

TY  - JOUR
ID  - 1134
T1  - Two-dimensional oxide on Pd(111)
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.88.246103
A1  - Lundgren,E.
A1  - Kresse,G.
A1  - Klein,C.
A1  - Borg,M.
A1  - Andersen,J. N.
A1  - De Santis,M.
A1  - Gauthier,Y.
A1  - Konvicka,C.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 88
IS  - 24
PY  - 2002/06/03/
N2  - 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.
KW  - DFT
KW  - Oxygen
KW  - Palladium
KW  - STM
SP  - 246103
EP  - 246103
UR  - http://link.aps.org/abstract/PRL/v88/e246103
ER  - 

TY  - JOUR
ID  - 929
T1  - Interaction of oxygen with palladium deposited on a thin alumina film
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(01)01850-7
A1  - Shaikhutdinov,Sh.
A1  - Heemeier,M.
A1  - Hoffmann,J.
A1  - Meusel,I.
A1  - Richter,B.
A1  - Bäumer,M.
A1  - Kuhlenbeck,H.
A1  - Libuda,J.
A1  - Freund,H. -J.
A1  - Oldman,R.
A1  - Jackson,S. D.
A1  - Konvicka,C.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 501
IS  - 3
PY  - 2002/04/01/
N2  - The interaction of oxygen with Pd particles, vapor deposited onto a thin alumina film grown on a NiAl(110) substrate, was studied by STM, AES, LEED, XPS, TPD and molecular beam techniques. The results show that O2 exposure at 400-500 K strongly influences the oxide support. We suggest that the oxygen atoms formed by dissociation on the Pd surface can diffuse through the alumina film and react with the NiAl substrate underneath the Pd particles, thus increasing the thickness of the oxide film. The surface oxygen inhibits hydrogen adsorption, and readily reacts with CO at 300-500 K. For large and crystalline Pd particles, the system exhibits adsorption-desorption properties which are very similar to those of the Pd(111) single crystal surface. The molecular beam and TPD experiments reveal that, at low coverage, CO adsorbs slightly stronger on the smaller Pd particles, with an adsorption energy difference of ca. 5-7 kJmol^-1 for 1 and 3-5 nm Pd particles studied.
KW  - Alumina
KW  - Carbon monoxide
KW  - Oxygen
KW  - Palladium
SP  - 270
EP  - 281
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 1150
T1  - Geometry of the valence transition induced surface reconstruction of Sm(0001)
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.88.136102
A1  - Lundgren,E.
A1  - Andersen,J. N.
A1  - Nyholm,R.
A1  - Torrelles,X.
A1  - Rius,J.
A1  - Delin,A.
A1  - Grechnev,A.
A1  - Eriksson,O.
A1  - Konvicka,C.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 88
IS  - 13
PY  - 2002/03/18/
N2  - 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.
KW  - Samarium
KW  - STM
SP  - 136102
EP  - 136102
UR  - http://link.aps.org/abstract/PRL/v88/e136102
ER  - 

TY  - JOUR
ID  - 348
T1  - A misfit structure in the Co/Pt(111) system studied by scanning tunnelling microscopy and embedded atom method calculations
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(01)01754-X
A1  - Lundgren,E.
A1  - Leonardelli,G.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 498
IS  - 3
PY  - 2002/02/10/
N2  - 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].
KW  - Cobalt
KW  - Platinum
SP  - 257
EP  - 265
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 866
T1  - Superstructures of carbon on V(100)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(01)01659-4
A1  - Bergermayer,W.
A1  - Koller,R.
A1  - Konvicka,C.
A1  - Schmid,M.
A1  - Kresse,G.
A1  - Redinger,J.
A1  - Varga,P.
A1  - Podloucky,R.
VL  - 497
IS  - 1-3
PY  - 2002/01/20/
N2  - Carbon adsorption on V(100) was studied by both experimental methods and density functional theory. At low carbon coverages of theta_C = 0.18 ML and oxygen below the experimental detection limit, measured scanning tunneling microscopy (STM) images show both areas of local c(2x2) structure and <010> oriented rows of C atoms. At higher coverages of ThetaC = 0.41 ML, mainly <010> oriented C rows with some local p(1x2) patterns are formed. The observed c(2x2) pattern is attributed to the presence of oxygen, since a mixture of carbon and oxygen favours the c(2x2) superstructure according to both the STM and the ab initio results. The calculations show that for theta_C = 0.50 ML the p(1x2) structure is more stable than c(2x2) by 0.13 eV per adsorbed atom. From the ab initio results it is predicted that p(1x2) changes into c(2x2) at a mixed coverage of about theta_C ≈ 0.37 ML and theta_O ≈ 0.13 ML. The geometry of the c(2x2) structure was determined using quantitative LEED showing good agreement with the ab initio data. Also the simulated STM images agree well with the experimental STM data.
KW  - Carbon
KW  - DFT
KW  - LEED
KW  - Oxygen
KW  - STM
KW  - Vanadium
SP  - 294
EP  - 304
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 61
T1  - Stabilizing single metal adatoms at room temperature: Pd on C- and O-covered V(100)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(01)01605-3
A1  - Konvicka,C.
A1  - Hammerschmid,A.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 496
IS  - 3
PY  - 2002/01/10/
N2  - The initial stages of Pd thin film growth on clean and C- and O-precovered V(100) surfaces at room temperature (RT) and 200 °C have been studied by means of scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). In the presence of C and O, the adsorption of Pd atoms in the clean four-fold hollow sites of the V surface is strongly preferred. Upon deposition of Pd in the submonolayer range, it was possible to stabilize single Pd adatoms and small clusters at RT. Annealing such a surface at 200 °C leads to the formation of rectangular Pd islands (size ≈40×40 Å2) and to the compression of the initial C and O adlayer in the areas between the Pd. For higher Pd coverages we found that oxygen acts as an anti-surfactant, shifting the onset of second layer growth dramatically. The reason for the change of the growth mode of Pd on V(100) from Stranski–Krastanov to a Volmer–Weber-type growth in the presence of oxygen can be found in a higher free energy of the film/substrate interface compared to the clean surface.
KW  - Carbon
KW  - Growth
KW  - Oxygen
KW  - Palladium
KW  - STM
KW  - Vanadium
SP  - 209
EP  - 220
SN  - 0039-6028
ER  - 

TY  - CHAP
ID  - 677
T1  - Segregation and surface chemical ordering-an experimental view on the atomic scale
T2  - Surface Alloys and Alloy Surfaces
T3  - The Chemical Physics of Solid Surfaces
A1  - Schmid,M.
A1  - Varga,P.
ED  - Woodruff,D.P.
VL  - Volume 10
PB  - Elsevier
PY  - 2002///
N2  - 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.
KW  - REVIEW
KW  - STM
SP  - 118
EP  - 151
SN  - 1571-0785
UR  - http://dx.doi.org/10.1016/S1571-0785(02)80091-8
ER  - 

TY  - JOUR
ID  - 121
T1  - Direct imaging of catalytically important processes in the oxidation of CO over RuO2(110)
JF  - Journal of the American Chemical Society
JA  - J. Am. Chem. Soc.
M3  - 10.1021/ja016408t
A1  - Over,H.
A1  - Seitsonen,A. P.
A1  - Lundgren,E.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 123
IS  - 47
PY  - 2001/11/01/
N2  - Ruthenium dioxide (RuO2) reveals unique and promising redox properties, making RuO2 a potential candidate for a versatile oxidation catalyst. Recently Zhang and Kisch reported, for instance, that RuO2 is a robust and efficient catalyst for room temperature (RT) oxidation of CO by humid air; recall that typical metal oxides do not tolerate humidity. In this contribution we present scanning tunneling microscopy (STM) data which directly image the catalytically important processes occurring on the RuO2(110) surface after exposing the pristine surface to CO and O2. These data are complemented by density functional theory (DFT) calculations.

The following processes are governing the catalytic activity of RuO2 on atomic scale. The reactants from the gas phase encounter strongly binding adsorption sites on the RuO2(110) surface in the form of the under-coordinated Ru atoms. For instance, CO adsorbs on the stoichiometric RuO2(110) surface by 1.2 eV (over the 1f-cus-Ru), while on the reduced RuO2(110) surface the CO binding energies are 1.61 eV and 1.85 eV for adsorption over 1f-cus-Ru and 2f-cus-Ru atoms, respectively. The RuO2 surface provides an active oxygen species to react with CO, i.e. the under-coordinated (bridging) lattice oxygen atoms Obr. The recombination of adsorbed CO with Obr creates vacancies, which are identified with STM. At RT, oxygen molecules from the gas phase can efficiently dissociate on RuO2(110) via the molecular precursor state. This leads to weakly held O atoms, which are grouped in pairs as imaged with STM. It is argued that the weakly held oxygen plays an important role in replenishing the consumed (bridging) lattice oxygen atoms on the (partially) reduced RuO2(110) surface.
KW  - Carbon monoxide
KW  - Oxygen
KW  - RuO2
SP  - 11807
EP  - 11808
UR  - http://dx.doi.org/10.1021/ja016408t
ER  - 

TY  - JOUR
ID  - 1114
T1  - Adatom interlayer diffusion on Pt(111): an embedded atom method study
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(01)01212-2
A1  - Leonardelli,G.
A1  - Lundgren,E.
A1  - Schmid,M.
VL  - 490
IS  - 1-2
PY  - 2001/09/01/
N2  - 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.
KW  - Growth
KW  - Platinum
SP  - 29
EP  - 42
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 95
T1  - Sputtering of Au and Al2O3 surfaces by slow highly charged ions
JF  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
JA  - Nucl. Instrum. Methods B
M3  - 10.1016/S0168-583X(01)00668-1
A1  - Hayderer,G.
A1  - Cernusca,S.
A1  - Hoffmann,V.
A1  - Niemann,D.
A1  - Stolterfoht,N.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Winter,HP.
A1  - Aumayr,F.
VL  - 182
IS  - 1-4
PY  - 2001/08//
N2  - 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.
KW  - Alumina
KW  - Gold
KW  - ion bombardment
KW  - QCM
SP  - 143
EP  - 147
SN  - 0168-583X
ER  - 

TY  - JOUR
ID  - 227
T1  - Adsorption Sites and Ligand Effect for CO on an Alloy Surface: A Direct View
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.87.036103
A1  - Gauthier,Y.
A1  - Schmid,M.
A1  - Padovani,S.
A1  - Lundgren,E.
A1  - Buš,V.
A1  - Kresse,G.
A1  - Redinger,J.
A1  - Varga,P.
VL  - 87
IS  - 3
PY  - 2001/06/28/
N2  - 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.
KW  - Carbon monoxide
KW  - Cobalt
KW  - DFT
KW  - Platinum
KW  - STM
SP  - 036103
EP  - 036103
UR  - http://link.aps.org/abstract/PRL/v87/e036103
ER  - 

TY  - JOUR
ID  - 1091
T1  - The structure of the oxygen induced (1×5) reconstruction of V(100)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(01)00978-5
A1  - Koller,R.
A1  - Bergermayer,W.
A1  - Kresse,G.
A1  - Hebenstreit,E. L. D.
A1  - Konvicka,C.
A1  - Schmid,M.
A1  - Podloucky,R.
A1  - Varga,P.
VL  - 480
IS  - 1-2
PY  - 2001/05/30/
N2  - The adsorption of 1 Langmuir oxygen on the V(100) surface at 200 °C leads to a (1x5) reconstruction. The structure of this surface was determined by scanning tunneling microscopy (STM), low energy electron diffraction (quantitative LEED) and density functional theory calculations. The STM images show dark lines every fifth vanadium lattice constant. Between these (1x5) lines, oxygen resides in four-fold coordinated hollow sites with a coverage of ~70%. From the LEED analysis (Pendry R-factor = 0.17) it was found that in the dark lines oxygen adsorbs in bridge sites, in agreement with the ab initio calculations. The driving force behind this reconstruction is surface stress induced by the vanadium-oxygen bonds.
KW  - DFT
KW  - LEED
KW  - Oxygen
KW  - STM
KW  - Vanadium
SP  - 11
EP  - 24
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 865
T1  - Oxygen adsorption on Al(111): low transient mobility
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(01)00967-0
A1  - Schmid,M.
A1  - Leonardelli,G.
A1  - Tscheließnig,R.
A1  - Biedermann,A.
A1  - Varga,P.
VL  - 478
IS  - 3
PY  - 2001/05/10/
N2  - 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.
KW  - Aluminum
KW  - Aluminum
KW  - Oxygen
KW  - STM
SP  - L355-L362
EP  - L355-L362
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 554
T1  - Kinetically Assisted Potential Sputtering of Insulators by Highly Charged Ions
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.86.3530
A1  - Hayderer,G.
A1  - Cernusca,S.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Winter,HP.
A1  - Aumayr,F.
A1  - Niemann,D.
A1  - Hoffmann,V.
A1  - Stolterfoht,N.
A1  - Lemell,C.
A1  - Wirtz,L.
A1  - Burgdörfer,J.
VL  - 86
IS  - 16
PY  - 2001/04/16/
N2  - A new form of potential sputtering has been found for impact of slow ( ≤1500 eV) multiply charged Xe ions (charge states up to q = 25) on MgOx. In contrast to alkali-halide or SiO2 surfaces this mechanism requires the simultaneous presence of electronic excitation of the target material and of a kinetically formed collision cascade within the target in order to initiate the sputtering process. This kinetically assisted potential sputtering mechanism has been identified to be present for other insulating surfaces as well.
KW  - ion bombardment
KW  - MgO
KW  - QCM
SP  - 3530
EP  - 3530
UR  - http://link.aps.org/abstract/PRL/v86/p3530
ER  - 

TY  - JOUR
ID  - 663
T1  - Exchange processes in interlayer diffusion – kinks, corners and the growth mode
JF  - Applied Physics A: Materials Science & Processing
JA  - Appl. Phys. A
M3  - 10.1007/s003390100753
A1  - Schmid,M.
A1  - Lundgren,E.
A1  - Leonardelli,G.
A1  - Hammerschmid,A.
A1  - Stanka,B.
A1  - Varga,P.
VL  - 72
IS  - 4
PY  - 2001/04/15/
N2  - Abstract.   Interlayer diffusion, i.e. mass transport between different terraces, is known to be an essential process for obtaining layer-by-layer growth, avoiding formation of three-dimensional (3D) islands when growing thin films. We present experimental results for the growth of cobalt on Pt(111), which demonstrate the importance of kinks and corners for interlayer diffusion. We show that Co grows two-dimensionally as long as strain caused by the Pt-Co interface keeps the step edges rough, with a high kink density, and then transforms to 3D growth with straight steps. The results for growth with adsorbed carbon monoxide show that CO acts as a surfactant, causing two-dimensional growth unless heterogeneous nucleation occurs. Again, this process is related to roughening of the steps, being a new mechanism for the action of a surfactant. A scanning tunneling microscopy study at the atomic scale confirms the fact that step descent happens only at kinks and (concave) corners, and in conjunction with simulations allows us to identify some of the relevant atomic-exchange processes. We finally argue that the dependence of the growth mode on the step morphology, together with straightening of the steps by step–step interaction, can lead to an instability of the growth mode.
KW  - Cobalt
KW  - Growth
KW  - Platinum
KW  - STM
SP  - 405
EP  - 412
UR  - http://dx.doi.org/10.1007/s003390100753
ER  - 

TY  - JOUR
ID  - 1133
T1  - Influence of Impurities on Localized Transition Metal Surface States: Scanning Tunneling Spectroscopy on V(001)
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.86.2396
A1  - Bischoff,M. M. J.
A1  - Konvicka,C.
A1  - Quinn,A. J.
A1  - Schmid,M.
A1  - Redinger,J.
A1  - Podloucky,R.
A1  - Varga,P.
A1  - van Kempen,H.
VL  - 86
IS  - 11
PY  - 2001/03/12/
N2  - The first scanning tunneling spectroscopy measurements on V(001) are reported. A strong surface state is detected which is very sensitive to the presence of segregated carbon impurities. The surface state energy shifts from 0.03 eV below the Fermi level at clean areas towards higher energies (up to ∼0.2 eV) at contaminated areas. Because of the negative dispersion of this state, the upward shift cannot be described in a simple confinement picture. Rather, the surface state energy is governed by vanadium surface s- d interactions which are altered by carbon coverage.
KW  - STM
KW  - STS
KW  - Vanadium
SP  - 2396
EP  - 2396
UR  - http://link.aps.org/abstract/PRL/v86/p2396
ER  - 

TY  - JOUR
ID  - 801
T1  - Segregation and ordering at Fe1-xAlx(100) surfaces - a model case for binary alloys
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(00)00987-0
A1  - Blum,V.
A1  - Hammer,L.
A1  - Meier,W.
A1  - Heinz,K.
A1  - Schmid,M.
A1  - Lundgren,E.
A1  - Varga,P.
VL  - 474
IS  - 1-3
PY  - 2001/03/01/
N2  - 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.
KW  - Aluminum
KW  - Aluminum
KW  - Iron
KW  - ISS
KW  - LEED
KW  - STM
SP  - 81
EP  - 97
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 423
T1  - Nucleation of bcc Iron in Ultrathin fcc Films
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.86.464
A1  - Biedermann,Albert
A1  - Schmid,Michael
A1  - Varga,Peter
VL  - 86
IS  - 3
PY  - 2001/01/15/
N2  - 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.
KW  - Iron
KW  - STM
SP  - 464
EP  - 464
UR  - http://link.aps.org/abstract/PRL/v86/p464
ER  - 

TY  - JOUR
ID  - 115
T1  - Ultrathin Films of Co on Pt(111): an STM View
JF  - physica status solidi (a)
JA  - Physica Stat. Sol.
M3  - 10.1002/1521-396X(200109)187:1<97::AID-PSSA97>3.0.CO;2-A
A1  - Varga,P.
A1  - Lundgren,E.
A1  - Redinger,J.
A1  - Schmid,M.
VL  - 187
IS  - 1
PY  - 2001///
N2  - The growth, structure and morphology of ultrathin Co layers with a thickness up to 15 layers deposited at room temperature on Pt(111) have been studied by using scanning tunnelling microscopy (STM) with atomic resolution and chemical discrimination between Co and Pt. This chemical contrast has been confirmed by simulations with an FLAPW (Full Potential Linearized Augmented Plane Waves) ab-initio computer code based on density functional theory. By the help of this contrast between Pt and Co atoms in STM constant current images it is shown that in the early stages of submonolayer growth Co is incorporated into the Pt surface, thereby forming dislocation lines. We were also able to demonstrate that Co atoms descend from the upper terrace to the lower one by an exchange diffusion process with the Pt atoms at the step edges. It is shown that this interlayer diffusion does not take place at straight steps, but rather at corners or kinks. The first completed Co monolayer (ML) is almost pseudomorphic (Co in the Pt fcc lattice sites) with a high density of defects due to the lattice mismatch. The second layer exhibits a moiré structure, with the Co in-plane lattice distance close to that of bulk Co. The step edges which are very rough at a coverage of two monolayers become smoother with increasing Co deposition. The growth mode is two-dimensional (layer-by-layer) around two to three monolayers and changes afterward into three-dimensional growth (island growth). We observe that the change of the step edge morphology is also correlated to this change from 2D to 3D growth mode. The reason for the 2D growth at the beginning is attributed to the strained interface between the Co overlayer and the Pt(111) surface which hinders the formation of straight steps. Therefore, many kinks and corners are formed, increasing the probability for interlayer diffusion by the above mentioned exchange process. With increasing number of layers the strain decreases, steps become smoother, interlayer diffusion decreases and therefore island growth develops. Up to the highest coverage (15 ML) studied the growth is characterised by a mainly twinned fcc-like stacking. Only a small amount of hcp stacking has been observed. Further experiments showed that preadsorption of carbon monoxide acts as a surfactant which extends the layer-by-layer growth up to higher Co coverages.
KW  - Cobalt
KW  - Growth
KW  - Platinum
KW  - STM
SP  - 97
EP  - 112
UR  - http://dx.doi.org/10.1002/1521-396X(200109)187:1<97::AID-PSSA97>3.0.CO;2-A
ER  - 

TY  - JOUR
ID  - 313
T1  - STM Studies of HCI-induced Surface Damage on Highly Oriented Pyrolytic Graphite
JF  - Physica Scripta
JA  - Physica Scripta
M3  - 10.1238/Physica.Topical.092a00156
A1  - Hayderer,G.
A1  - Cernusca,S.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Winter,HP
A1  - Aumayr,F.
VL  - T92
PY  - 2001///
N2  - Scanning tunneling microscopy (STM) with atomic scale resolution has been applied to study surface defects in highly oriented pyrolytic graphite (HOPG) which have been produced by impact of 150 eV singly and multiply charged Ar ions (charge state up to 9+). The most prominent surface defects are protrusions. Their area density is in good agreement with the applied ion dose, implying that about every single ion impact causes one protrusion. A (√3 x √3) R 30° surface reconstruction, as characteristic for interstitial defects in HOPG, is observed in the vicinity of most defects. As the most remarkable result we find that the measured size of the hillocks (mean diameter and height) increases with projectile charge state.
KW  - Carbon
KW  - ion bombardment
KW  - STM
SP  - 156
EP  - 157
SN  - 1402-4896
ER  - 

TY  - JOUR
ID  - 845
T1  - Crystallographic Structure of Ultrathin Fe Films on Cu(100)
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.87.086103
A1  - Biedermann,Albert
A1  - Tscheließnig,Rupert
A1  - Schmid,Michael
A1  - Varga,Peter
VL  - 87
IS  - 8
PY  - 2001///
N2  - We report the observation of bcc-like crystal structures in 2–4 monolayer (ML) Fe films grown on fcc Cu(100) using scanning tunneling microscopy. The local bcc structure provides a straightforward explanation for their frequently reported outstanding magnetic properties, i.e., ferromagnetic ordering in all layers with a Curie temperature above 300 K. The nonpseudomorphic structure, which becomes pseudomorphic above 4 ML film thickness, is unexpected in terms of conventional rules of thin film growth and stresses the importance of finite thickness effects in ferromagnetic ultrathin films.
KW  - Growth
KW  - Iron
KW  - Magnetism
KW  - STM
SP  - 086103
EP  - 086103
UR  - http://link.aps.org/abstract/PRL/v87/e086103
ER  - 

TY  - JOUR
ID  - 1135
T1  - Bulk Terminated NaCl(111) on Aluminum: A Polar Surface of an Ionic Crystal?
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.85.5376
A1  - Hebenstreit,W.
A1  - Schmid,M.
A1  - Redinger,J.
A1  - Podloucky,R.
A1  - Varga,P.
VL  - 85
IS  - 25
PY  - 2000/12/18/
N2  - 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.
KW  - NaCl
KW  - STM
SP  - 5376
EP  - 5376
UR  - http://link.aps.org/abstract/PRL/v85/p5376
ER  - 

TY  - JOUR
ID  - 183
T1  - Segregation and chemical ordering in the surface layers of Pt25Co75(111): a LEED/STM study
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(00)00751-2
A1  - Gauthier,Y.
A1  - Baudoing-Savois,R.
A1  - Bugnard,J. M.
A1  - Hebenstreit,W.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 466
IS  - 1-3
PY  - 2000/11/01/
N2  - Segregation and chemical ordering on Pt25Co75(111) are studied by quantitative low energy electron diffraction (LEED) analysis and scanning tunnelling microscopy (STM). Although LEED patterns do not show any sign of superstructure, LEED calculations undoubtedly point to a surface which contains about the same amount of both species and reveal significant short range chemical ordering (down to the third layer). Pt and Co surface sites are locally arranged with a (1x2) unit cell, in the manner of the ordered tetragonal L10 phase. More direct evidence is given by STM images which exhibit parallel Pt and Co monoatomic chains a few lattice constants long and an apparent height difference of about 0.2 Å for Pt and Co sites. LEED shows that the Pt sublattice in the top layer actually resides 0.1 Å above the Co one. Otherwise the surface is bulklike, with weak relaxations of interlayer distances. The use of a chemically ordered model for the LEED analysis, in which sublattice occupancies are optimised, results in a remarkable improvement of the fit with experiment as compared to a totally random distribution; however, most interestingly, it does not modify the average layer concentration profile versus depth (55, 5 and 35 at% Pt respectively for the three outermost layers). The distortions needed for the tetragonal L1_0 phase with respect to the fcc L1_2 phase explains why chemical order does not extend over larger domains. Finally, both techniques yield complementary pictures and quite consistent results as to the top layer content and chemical order.
KW  - Cobalt
KW  - LEED
KW  - Platinum
KW  - STM
SP  - 155
EP  - 166
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 478
T1  - Surface and subsurface alloy formation of vanadium on Pd(111)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(00)00643-9
A1  - Konvicka,Ch.
A1  - Jeanvoine,Y.
A1  - Lundgren,E.
A1  - Kresse,G.
A1  - Schmid,M.
A1  - Hafner,J.
A1  - Varga,P.
VL  - 463
IS  - 3
PY  - 2000/09/10/
N2  - We have studied the submonolayer growth of vanadium on the Pd(111) surface at different substrate temperatures. By using LEIS (Low energy ion spectroscopy), AES (Auger electron spectroscopy), STM (Scanning tunneling microscopy), XPD (X-ray photoelectron diffraction) and ab-initio local-density-functional calculations we find that V atoms deposited at room temperature substitute surface Pd atoms. In addition, islands are formed on the surface, which consist mostly of the substituted Pd atoms. At higher temperatures, V diffuses into subsurface layers and at a temperature of 300 °C only a small amount of V is observed in the top-most layer. By using STM a (√3 x √3)R30° superstructure is observed to be formed and XPD measurements demonstrate that this structure is due to V atoms incorporated in the second layer. This finding is confirmed by ab-initio calculations. Further, a model for the (√3 x √3)R30° structure based on the experiments and the ab-initio calculations is given.
KW  - DFT
KW  - Growth
KW  - ISS
KW  - Palladium
KW  - STM
KW  - Vanadium
SP  - 199
EP  - 210
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 514
T1  - STM and STS of bulk electron scattering by subsurface objects
JF  - Journal of Electron Spectroscopy and Related Phenomena
JA  - J. Electron Spectrosc. Relat. Phenom.
M3  - 10.1016/S0368-2048(00)00108-0
A1  - Schmid,M.
A1  - Crampin,S.
A1  - Varga,P.
VL  - 109
IS  - 1-2
PY  - 2000/08//
N2  - 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.
KW  - Aluminum
KW  - Copper
KW  - STM
SP  - 71
EP  - 84
SN  - 0368-2048
ER  - 

TY  - JOUR
ID  - 188
T1  - Thin films of Co on Pt(111): Strain relaxation and growth
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.62.2843
A1  - Lundgren,E.
A1  - Stanka,B.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 62
IS  - 4
PY  - 2000/07/15/
N2  - The growth, structure and morphology of thin Co layers with a thickness ranging from 1 to 15 monolayers deposited at room temperature on Pt(111) have been studied by the use of scanning tunneling microscopy. We demonstrate that the first Co layer grows preferably in the Pt fcc lattice sites, with a high density of defects due to the lattice mismatch. The second Co layer is found to exhibit a moiré structure, with the Co in-plane lattice distance close to that of bulk Co. The growth of thin Co films is observed to be mostly in terms of flat layers (two dimensional) up to a Co coverage of about 3.5 ML. At higher coverages, we find that the Co grows in (three dimensional) islands and we show that the growth is characterized by a mainly twinned fcc-like stacking. We argue that the reason for the two dimensional growth mode at lower Co coverages is due to the strained interface between the Co overlayers and the Pt(111) surface, resulting in a large number of kinks and corners which facilitate interlayer diffusion. For higher coverage such sites become less common, due to the decreasing influence of the strained interface, resulting in no interlayer diffusion leading to a three dimensional growth mode. The implications by these observations on the magnetic properties of the Co/Pt(111) interface system are discussed.
KW  - Cobalt
KW  - Growth
KW  - Platinum
KW  - STM
SP  - 2843
EP  - 2843
UR  - http://link.aps.org/abstract/PRB/v62/p2843
ER  - 

TY  - JOUR
ID  - 815
T1  - Temperature-dependent segregation reversal and (1×3) missing-row structure of Pt90Co10(110)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(00)00351-4
A1  - Platzgummer,E.
A1  - Sporn,M.
A1  - Koller,R.
A1  - Schmid,M.
A1  - Hofer,W.
A1  - Varga,P.
VL  - 453
IS  - 1-3
PY  - 2000/05/10/
N2  - The surface structure and composition of the clean Pt90Co10(110) surface is investigated by low energy electron diffraction (LEED) and low energy ion scattering (LEIS). Through LEED I-V analysis we find a (1x3) missing-row reconstruction on the equilibrated Pt90Co10(110) surface - comparable with the pure Pt(110) (1x3) surface - in which all atomic positions in the topmost layer and in the (111) oriented micro facets are Pt-enriched. Due to the fact that the unreconstructed Pt25Co75(110) surface is known to exhibit an almost pure Co top layer the Pt segregation reported in this study is undoubtedly connected to the existence of the missing-row reconstruction. The proposed structural influence on the composition is confirmed by LEIS experiments performed on the hot Pt90Co10(110) surface, in which simultaneously temperature induced changes of the surface composition and qualitative changes in the surface structure are monitored. The measured low energy ion spectra do not only reproduce the calculated first-layer composition of the LEED analysis, they also show a less pronounced Pt segregation at temperatures around 750°C, and eventually a reversed Pt segregation above 750°C, i.e. Co enrichment of the Pt90Co10(110) surface with respect to the bulk concentration. We find a clear correlation between the thermal deconstruction and the surface composition. The striking segregation reversal during temperature variation is attributed to the high excess value of the mixing enthalpy, which implies a structure-dominated segregation behavior.
KW  - Cobalt
KW  - ISS
KW  - LEED
KW  - Platinum
KW  - STM
SP  - 214
EP  - 224
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 9
T1  - Observation of a threshold in potential sputtering of LiF surfaces
JF  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
JA  - Nucl. Instrum. Methods B
M3  - 10.1016/S0168-583X(99)01070-8
A1  - Hayderer,G.
A1  - Lemell,C.
A1  - Wirtz,L.
A1  - Schmid,M.
A1  - Burgdörfer,J.
A1  - Varga,P.
A1  - Winter,HP.
A1  - Aumayr,F.
VL  - 164-165
PY  - 2000/04//
N2  - 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.
KW  - ion bombardment
KW  - LiF
KW  - QCM
SP  - 517
EP  - 521
SN  - 0168-583X
ER  - 

TY  - JOUR
ID  - 32
T1  - Atomic-scale structure and catalytic reactivity of the RuO2(110) surface
JF  - Science
JA  - Science
M3  - 10.1126/science.287.5457.1474
A1  - Over,H.
A1  - Kim,Y. D.
A1  - Seitsonen,A. P.
A1  - Wendt,S.
A1  - Lundgren,E.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Morgante,A.
A1  - Ertl,G.
VL  - 287
IS  - 5457
PY  - 2000/02/25/
N2  - Exposure of a Ru(0001) surface to large doses of O2 at elevated temperatures leads to the growth of an epitaxial layer of RuO2 with (110) surface orientation whose structure was analyzed quantitatively by combination of low energy electron diffraction, scanning tunneling microscopy and density functional calculations. The surface exposes essentially bridging O atoms and Ru atoms not capped by oxygen. The latter play the role of coordinatively unsaturated sites (cus) - a hypothesis introduced long ago to account for the catalytic activity of oxide surfaces - onto which CO may become chemisorbed and from where it may react with neighboring lattice-O to CO2. The distortion of the surface lattice thereby caused is restored by uptake of oxygen from the gas phase, i.e., the oxide surface itself is actively participating in the catalytic reaction. In this way a general mechanism originally proposed by Mars and van Krevelen could be verified.
KW  - Carbon monoxide
KW  - Oxygen
KW  - RuO2
KW  - STM
SP  - 1474
EP  - 1476
UR  - http://www.sciencemag.org/cgi/content/abstract/287/5457/1474
ER  - 

TY  - JOUR
ID  - 244
T1  - Hochauflösende Rastertunnelmikroskopie unterscheidet Atome
JF  - Physik in unserer Zeit
JA  - Physik in unserer Zeit
M3  - 10.1002/1521-3943(200009)31:5<215::AID-PIUZ215>3.0.CO;2-0
A1  - Varga,Peter
A1  - Schmid,Michael
A1  - Redinger,Josef
VL  - 31
IS  - 5
PY  - 2000///
N2  - Rastertunnelmikroskopie mit atomarer Auflösung ermöglicht die chemische Analyse an Oberflächen von metallischen Legierungen und ultradünnen Filmen auf atomarer Skala und trägt damit zum besseren Verständnis von katalytischen Prozessen bei.
KW  - REVIEW
KW  - STM
SP  - 215
EP  - 221
UR  - http://dx.doi.org/10.1002/1521-3943(200009)31:5<215::AID-PIUZ215>3.0.CO;2-0
ER  - 

TY  - JOUR
ID  - 774
T1  - On the role of kinks and strain in heteroepitaxial growth: An STM study
JF  - Surface Review and Letters
JA  - Surf. Rev. Lett.
M3  - 10.1142/S0218625X00000750
A1  - Lundgren,E.
A1  - Schmid,M.
A1  - Leonardelli,G.
A1  - Hammerschmid,A.
A1  - Stanka,B.
A1  - Varga,P.
VL  - 7
IS  - 5-6
PY  - 2000///
N2  - 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.
KW  - Cobalt
KW  - Growth
KW  - Platinum
KW  - STM
SP  - 673
EP  - 677
ER  - 

TY  - JOUR
ID  - 885
T1  - Threshold for Potential Sputtering of LiF
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.83.3948
A1  - Hayderer,G.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Winter,H P.
A1  - Aumayr,F.
A1  - Wirtz,L.
A1  - Lemell,C.
A1  - Burgdörfer,J.
A1  - Hägg,L.
A1  - Reinhold,C. O.
VL  - 83
IS  - 19
PY  - 1999/11/08/
N2  - We have measured total sputtering yields for impact of slow ( ≤100 eV) singly and doubly charged ions on LiF. The minimum potential energy necessary to induce potential sputtering (PS) from LiF was determined to be about 10 eV. This threshold coincides with the energy necessary to produce a cold hole in the valence band of LiF by resonant neutralization. This allows the first unambiguous identification of PS induced by cold holes. Further stepwise increase of the sputtering yield with higher projectile potential energies provides evidence for additional defect-mediated sputtering mechanisms operative in alkali halides.
KW  - ion bombardment
KW  - LiF
KW  - QCM
SP  - 3948
EP  - 3948
UR  - http://link.aps.org/abstract/PRL/v83/p3948
ER  - 

TY  - JOUR
ID  - 481
T1  - Pt25Rh75(111), (110), and (100) studied by scanning tunnelling microscopy with chemical contrast
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(99)00880-8
A1  - Hebenstreit,E. L. D.
A1  - Hebenstreit,W.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 441
IS  - 2-3
PY  - 1999/11/01/
N2  - Scanning tunneling microscopy images with chemical contrast allowed the direct determination of the composition and short-range order behaviour of the clean Pt25Rh75(111), (110), and (100) alloy surfaces. All measurements were performed at room temperature and showed a strong platinum enrichment depending on the preparation temperature. In the top layers of both Pt25Rh75(111) and Pt25Rh75(110)-(1x2) we find a preference for unlike nearest neighbors. Pt25Rh75(100) exhibits after a preparation temperature of 900°C a preference for clustering whereas after a preparation temperature of 600°C, comparable to Pt25Rh75(111) and (110) ordering tendencies appear. Additional investigations of the chemical identity of atoms surrounding hollow positions show mostly small deviations from a random distribution. However, the number of hollow sites surrounded by Rh atoms only can be significantly affected by the short range order.
Pt25Rh75(110) exhibits a (1x2) missing-row reconstruction after annealing above 700°C. After the first annealing of the sputtered surface it is accompanied by mesoscopic long range "waves" with a height of approximately 2 nm and a wavelength up to 200 nm depending on the preparation temperature.
KW  - Platinum
KW  - Rhodium
KW  - STM
SP  - 441
EP  - 453
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 851
T1  - High temperature growth of Pt on the Rh(111) surface
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(99)00037-0
A1  - Duisberg,M.
A1  - Dräger,M.
A1  - Wandelt,K.
A1  - Gruber,E. L. D.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 433-435
PY  - 1999/08/02/
N2  - 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.
KW  - Growth
KW  - ISS
KW  - Platinum
KW  - Rhodium
KW  - STM
SP  - 554
EP  - 558
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 523
T1  - Interlayer Diffusion of Adatoms: A Scanning-Tunneling Microscopy Study
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.82.5068
A1  - Lundgren,E.
A1  - Stanka,B.
A1  - Leonardelli,G.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 82
IS  - 25
PY  - 1999/06/21/
N2  - 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.
KW  - Cobalt
KW  - Platinum
KW  - STM
SP  - 5068
EP  - 5068
UR  - http://link.aps.org/abstract/PRL/v82/p5068
ER  - 

TY  - JOUR
ID  - 441
T1  - Atomic resolution by STM on ultra-thin films of alkali halides: experiment and local density calculations
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(99)00095-3
A1  - Hebenstreit,W.
A1  - Redinger J.
A1  - Horozova Z.
A1  - Schmid M.
A1  - Podloucky R.
A1  - Varga P.
VL  - 424
IS  - 2-3
PY  - 1999/04/01/
N2  - 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.
KW  - DFT
KW  - NaCl
KW  - STM
SP  - L321-L328
EP  - L321-L328
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 234
T1  - An atomic-scale study of the Co induced dendrite formation on Pt(111)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(98)00931-5
A1  - Lundgren,E.
A1  - Stanka,B.
A1  - Koprolin,W.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 423
IS  - 2-3
PY  - 1999/03/10/
N2  - We have studied the initial growth of Co on the Pt(111) surface in a temperature range of 300-400 K with scanning tunneling microscopy, low energy ion scattering and Auger electron spectroscopy. In agreement with previous work by Grütter and Dürig, when depositing 0.1 ML Co on Pt(111) at 400 K, the formation of large dendrites is observed. These dendrites are formed in conjunction with a Co induced local reconstruction of the Pt(111) surface, resulting in dislocations. The dendrites, however, show no evidence for any dislocations below their surface, the local reconstruction is observed to be lifted by the formation of the dendrites. LEIS data suggest that the dendrites consist mainly of Pt, implying, together with the STM data, that Co is incorporated underneath the Pt. A model for this process is proposed.
KW  - Cobalt
KW  - Growth
KW  - ISS
KW  - Platinum
KW  - STM
SP  - 357
EP  - 363
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 377
T1  - Adsorbate migration on PdAg(111)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(98)00937-6
A1  - Wouda,P. T.
A1  - Schmid,M.
A1  - Nieuwenhuys,B. E.
A1  - Varga,P.
VL  - 423
IS  - 1
PY  - 1999/03/01/
N2  - 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.
KW  - Oxygen
KW  - Palladium
KW  - Silver
KW  - STM
SP  - L229-L235
EP  - L229-L235
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 199
T1  - Temperature-dependent segregation and (1×2) missing-row reconstruction of Pt25Rh75(110)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(98)00924-8
A1  - Platzgummer,E.
A1  - Sporn,M.
A1  - Koller,R.
A1  - Schmid,M.
A1  - Hofer,W.
A1  - Varga,P.
VL  - 423
IS  - 1
PY  - 1999/03/01/
N2  - The surface structure and composition of the clean Pt25Rh75(110) surface is investigated by low energy electron diffraction (LEED) and low energy ion scattering (LEIS). For the equilibrated Pt25Rh75(110) surface we observe a (1x2) missing-row reconstruction in analogy to the pure Pt(110) surface, and a significant Pt enrichment of the topmost atomic layer (up to 80 at.% Pt). As the same strong surface enrichment in Pt was found in a previous study on the (100) and (111) surface of the same bulk composition, this means that in contrast to Pt-Ni and Pt-Co alloys, for Pt25Rh75 alloys the segregation behavior is not influenced extensively by the surface orientation. In addition to the structure analysis by LEED we performed LEIS experiments to determine the temperature induced changes of the surface composition and structure. Since the Pt segregation is less pronounced at elevated temperature, the surface reveals a temperature induced deconstruction of the (1x2) structure around 750°C, resulting in an fcc(110) (1x1) surface at high temperature. Temperature dependent measurements further show a hysteresis-like behavior of the top-layer composition, which is attributed to an enhanced Pt segregation on the (1x2) reconstructed surface.
KW  - ISS
KW  - LEED
KW  - Platinum
KW  - Rhodium
KW  - STM
SP  - 134
EP  - 143
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 445
T1  - Chemical discrimination on atomic level by STM
JF  - Applied Surface Science
JA  - Appl. Surf. Sci.
M3  - 10.1016/S0169-4332(98)00514-5
A1  - Varga,P.
A1  - Schmid,M.
VL  - 141
IS  - 3-4
PY  - 1999/03//
N2  - 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.
KW  - STM
SP  - 287
EP  - 293
SN  - 0169-4332
ER  - 

TY  - JOUR
ID  - 27
T1  - Oxygen-induced vacancy formation on a metal surface
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.82.355
A1  - Schmid,M.
A1  - Leonardelli,G.
A1  - Sporn,M.
A1  - Platzgummer,E.
A1  - Hebenstreit,W.
A1  - Pinczolits,M.
A1  - Varga,P.
VL  - 82
IS  - 2
PY  - 1999/01/11/
N2  - 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.
KW  - Chromium
KW  - ISS
KW  - LEED
KW  - Oxygen
KW  - STM
SP  - 355
EP  - 355
UR  - http://link.aps.org/abstract/PRL/v82/p355
ER  - 

TY  - JOUR
ID  - 437
T1  - Temperature-dependent segregation on Pt25Rh75(111) and (100)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(98)00800-0
A1  - Platzgummer,E.
A1  - Sporn,M.
A1  - Koller,R.
A1  - Forsthuber,S.
A1  - Schmid,M.
A1  - Hofer,W.
A1  - Varga,P.
VL  - 419
IS  - 2-3
PY  - 1999/01/04/
N2  - Surface segregation is studied on Pt25Rh75(111) and Pt25Rh75(100) by LEED intensity analysis and LEIS. Although both equilibrated surfaces are strongly Pt-enriched (up to 80 at.%), we find an interesting difference in the segregation behavior when annealing the sputtered surfaces. The Pt concentration grows continuously on Pt25Rh75(111) until 1000°C, whereas it reaches a maximum enrichment around 500°C on Pt25Rh75(100) and decreases thereafter. This contrasting behavior results solely from the kinetic limitation in the low-temperature regime, and is not due to energetic reasons. From temperature-dependent composition profiles we determine the segregation kinetics as well as the annealing temperature necessary for thermodynamic equilibration. We find that an equilibrium is acquired on the Pt25Rh75(100) surface by the interchange of Pt and Rh atoms within the near-surface layers, and on the Pt25Rh75(111) surface by a diffusion of Pt atoms from bulk to the near-surface region. The latter leads to an overall Pt enrichment of several layers, and is only observed after annealing at 1100°C. The presence of carbon contamination on the Pt25Rh75(100) surface causes a significant reduction of the Pt segregation. There is excellent agreement between the top-layer concentrations derived by LEIS and quantitative LEED.
KW  - ion bombardment
KW  - ISS
KW  - LEED
KW  - Platinum
KW  - Rhodium
SP  - 236
EP  - 248
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 854
T1  - A highly sensitive quartz-crystal microbalance for sputtering investigations in slow ion--surface collisions
JF  - Review of Scientific Instruments
JA  - Rev. Sci. Instrum.
M3  - 10.1063/1.1149979
A1  - Hayderer,G.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Winter,HP.
A1  - Aumayr,F.
VL  - 70
IS  - 9
PY  - 1999///
N2  - 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.
KW  - instrumental
KW  - ion bombardment
KW  - LiF
KW  - QCM
SP  - 3696
EP  - 3700
UR  - http://link.aip.org/link/?RSI/70/3696/1
ER  - 

TY  - JOUR
ID  - 1167
T1  - STM study of the (111) and (100) surfaces of PdAg
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(98)00673-6
A1  - Wouda,P. T.
A1  - Schmid,M.
A1  - Nieuwenhuys,B. E.
A1  - Varga,P.
VL  - 417
IS  - 2-3
PY  - 1998/11/20/
N2  - 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.
KW  - Palladium
KW  - Silver
KW  - STM
SP  - 292
EP  - 300
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 1116
T1  - A quantitative LEED analysis of the oxygen-induced p(3×1) reconstruction of Pt25Rh75(100)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(98)00574-3
A1  - Sporn,M.
A1  - Platzgummer,E.
A1  - Gruber,E. L. D.
A1  - Schmid,M.
A1  - Hofer,W.
A1  - Varga,P.
VL  - 416
IS  - 3
PY  - 1998/10/30/
N2  - Pt25Rh75(100) forms a p(3×1) reconstruction at saturation coverage of oxygen (23 L O2, 600°C). A previous STM study on O/Pt50Rh50(100) suggests that every third row of the first substrate layer is shifted by half a lattice constant ("shifted rows"). We present a LEED I(E) analysis of Pt25Rh75(100) confirming the shifted-row model and find that oxygen resides in threefold-coordinated sites on both sides of the shifted rows. The adsorbate occupies those of the threefold-coordinated sites that are directly separated by the metal atom in the shifted row. Further I(E) calculations exclude the alternative threefold-coordinated adsorption site beside the shifted row as well as the fourfold-coordinated site symmetrically in between the shifted rows. We achieve a Pendry R-factor of 0.14 for the best-fit structure. Oxygen has equal bond lengths to its three metal neighbours, amounting to 1.95 Å. The first substrate layer relaxes outward by 8.8% of the bulk value to 2.08 Å, but we do not observe any significant relaxations of deeper layer spacings. The shifted rows pop out of the surface by 0.38 Å. After determination of the oxygen adsorption site with LEED, we examine local adsorption structures on Pt25Rh75(100) at low oxygen coverage with STM. We resolve the shifted rows in real-space, and for special tip conditions, we find maxima of apparent height at in-plane positions that coincide with the oxygen position as established by quantitative LEED. We determine chemical-composition depth-profiles by quantitative LEED for three surface preparations occurring during sample preparation. While the first substrate layer of clean and annealed Pt25Rh75(100) is enriched in Pt (76%) as compared to the bulk value (25%), that in p(3×1)-O/Pt25Rh75(100) is enriched in Rh (90%). Oxygen adsorption at a moderate temperature (600°C) and formation of the p(3×1) structure reverse segregation on Pt25Rh75(100). Finally, oxygen can be removed at room temperature by exposure of the surface to hydrogen. This lifts the reconstruction but keeps the Rh enrichment of the first substrate layer.
KW  - LEED
KW  - Platinum
KW  - Rhodium
SP  - 384
EP  - 395
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 455
T1  - The accuracy of quantitative LEED in determining chemical composition profiles of substitutionally disordered alloys: a case study
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(98)00596-2
A1  - Sporn,M.
A1  - Platzgummer,E.
A1  - Forsthuber,S.
A1  - Schmid,M.
A1  - Hofer,W.
A1  - Varga,P.
VL  - 416
IS  - 3
PY  - 1998/10/30/
N2  - 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.
KW  - LEED
KW  - Rhodium
SP  - 423
EP  - 429
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 590
T1  - Unreconstructed Au(100) monolayers on a Au3Pd(100) single-crystal surface
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(98)00564-0
A1  - Aschoff,M.
A1  - Speller,S.
A1  - Kuntze,J.
A1  - Heiland,W.
A1  - Platzgummer,E.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Baretzky,B.
VL  - 415
IS  - 3
PY  - 1998/10/09/
N2  - The Au3Pd(100) single-crystal surface was studied with ion scattering methods, low-energy electron diffraction (LEED) and scanning tunneling microscopy. The crystal is covered at room temperature with a pure, (100)-ordered gold layer. Palladium is found in the second layer only. The lattice constant of the gold surface as evaluated by ion scattering and a tensor low-energy electron diffraction (TLEED) analysis is equal to the bulk lattice constant of 4.017 Å as evaluated by X-ray analysis. The surface lattice constant of the gold layer on the alloy surface is 0.08 Å smaller than that of bulk gold.
KW  - Gold
KW  - ISS
KW  - Palladium
KW  - STM
SP  - L1051-L1054
EP  - L1051-L1054
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 340
T1  - Trajectory-dependent neutralization of 1 keV He+ ions scattered from Pb(111) and Pb films on Cu(100)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(98)00388-4
A1  - Platzgummer,E.
A1  - Borrell,M.
A1  - Nagl,C.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 412-413
PY  - 1998/09/03/
N2  - We observe trajectory-dependent neutralization of 1 keV He+ ions scattered from the Pb(111) surface. Varying the azimuthal angle of the ion trajectories we observe neutralization by charge densities of the next and second next neighbors of the target atom occurring at polar angles between 40 and 15°. This corresponds to ion-neighbor distances between 2.3 and 1 Å. We adjust a three-parameter neutralization model for Pb to fit the trajectory dependence of the survival probability for the well known Pb(111) surface geometry. Good agreement with experimental results is found only if a shell-like neutralization region is centered at a radius of 1.46 Å. In regard to the spatial distribution of the neutralization rate we consider the Pb 6sp electrons to be the source of the long distance neutralization that leads to the trajectory dependence. The same neutralization model is then applied for a structural analysis of ultrathin Pb films evaporated on Cu(100), where we confirm the existence of a Pb-Cu surface alloy on the c(4×4) Pb/Cu(100) and the existence of a Pb overlayer on the c(2×2) Pb/Cu(100). It turns out that the neutralization model developed for Pb atoms embedded in the (111) surface holds also for adsorbed Pb atoms.
KW  - Copper
KW  - Lead
SP  - 202
EP  - 212
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 685
T1  - Intrinsic defects on a TiO2(110)(1×1) surface and their reaction with oxygen: a scanning tunneling microscopy study
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(98)00356-2
A1  - Diebold,Ulrike
A1  - Lehman,Jeremiah
A1  - Mahmoud,Talib
A1  - Kuhn,Markus
A1  - Leonardelli,Georg
A1  - Hebenstreit,Wilhelm
A1  - Schmid,Michael
A1  - Varga,Peter
VL  - 411
IS  - 1-2
PY  - 1998/08/11/
N2  - We report a scanning tunneling microscopy (STM) study of the rutile TiO2(110) surface. The surface was prepared by sputtering and annealing in an ultrahigh vacuum (UHV). After annealing to 1100 K in UHV, a (1×1) surface with a terrace width of ~100 Å is obtained. The terraces are separated by monoatomic step edges running predominantly parallel to <001> and type directions. Approximately half of the <001>-type steps have a kinked appearance that is attributed to a (4×1)-reconstructed step edge. Atomic models for autocompensated step edges are presented. Oxygen vacancies (point defects) in the bridging oxygen rows are created by the high-temperature anneal in UHV. In STM images, these oxygen vacancies appear as bright features centered on dark rows. Their density is 7±3% per surface unit cell and is reduced upon exposure to molecular oxygen at room temperature. Dark features on bright rows are also seen; these are not affected by molecular oxygen and are tentatively assigned to subsurface defects. Hydroxyl groups from spurious water in the oxygen gas stream are observed to adsorb dissociatively at step edges and on the in-plane Ti rows on the terraces. The appearance of the surface oxygen vacancies depends on the state of the STM tip; asymmetric tips skew the appearance of the point defects and may even render images where they are invisible. Tip changes occur frequently, especially when the surface has been exposed to oxygen, and may lead to images that are hard to interpret. The "normal" tip state where the vacancies appear as bright spots connecting bright rows can be regained reproducibly by scanning with a high (up to +10 V) tip voltage; the tip is then possibly covered with substrate material. The oxygen vacancies show strong interactions with the STM tip, i.e. tip-induced oxidation and mobility. These interactions depend strongly on the state of the tip, and are enhanced by the presence of oxygen in the ambient. A model for the tip-induced oxidation is presented where oxygen atoms hop between the tip and sample to explain these effects.
KW  - STM
KW  - TiO2
SP  - 137
EP  - 153
SN  - 0039-6028
UR  - http://www.sciencedirect.com/science/article/B6TVX-3W9KNSW-2B/2/3b3138a49577b7aece49f27c371b974e
ER  - 

TY  - JOUR
ID  - 1049
T1  - High Transient Mobility of Chlorine on TiO2(110): Evidence for ``Cannon-Ball'' Trajectories of Hot Adsorbates
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.81.405
A1  - Diebold,Ulrike
A1  - Hebenstreit,Wilhelm
A1  - Leonardelli,Georg
A1  - Schmid,Michael
A1  - Varga,Peter
VL  - 81
IS  - 2
PY  - 1998/07/13/
N2  - Scanning tunneling microscopy was used to study the initial stages of Cl2 adsorption on TiO2(110). Cl atoms adsorb on the rows of fivefold coordinated surface Ti atoms, and mostly form well separated pairs (average distance 26 Å, atoms can be two or three rows apart). Abstractive adsorption results in 10% single Cl adatoms. We propose that Cl2 dissociates in an approximately upright position. The outer Cl atom is emitted along the bond axis and can surmount the substrate bridging oxygen rows in a “cannon-ball”-like trajectory. Channeling along the Ti rows leads to large average Cl-Cl distances.
KW  - Chlorine
KW  - STM
KW  - TiO2
SP  - 405
EP  - 405
UR  - http://link.aps.org/abstract/PRL/v81/p405
ER  - 

TY  - JOUR
ID  - 1070
T1  - Scanning tunneling microscopy of binary-alloy surfaces: is chemical contrast a consequence of alloying?
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(98)00140-X
A1  - Hofer,W. A.
A1  - Ritz,G.
A1  - Hebenstreit,W.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Redinger,J.
A1  - Podloucky,R.
VL  - 405
IS  - 2-3
PY  - 1998/05/15/
N2  - 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.
KW  - DFT
KW  - Nickel
KW  - Platinum
KW  - Rhodium
KW  - STM
SP  - L514-L519
EP  - L514-L519
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 172
T1  - Anti-corrugation and nitrogen c(2 × 2) on Cr(100): STM on atomic scale and quantitative LEED
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(97)00660-2
A1  - Sporn,M.
A1  - Platzgummer,E.
A1  - Pinczolits,M.
A1  - Hebenstreit,W.
A1  - Schmid,M.
A1  - Hofer,W.
A1  - Varga,P.
VL  - 396
IS  - 1-3
PY  - 1998/01/20/
N2  - We present a LEED I-V analysis of c(2 × 2)-N/Cr(100). We found nitrogen residing in fourfold hollow sites and exclude adsorption models in which nitrogne adsorbs on a metal site (on-top, substitutional or second-layer interstitial). We achieved a Pendry R-factor of 0.16 for the best-fit structure. Nitrogen resides at a vertical distance of 0.36 Å above the first chromium layer. The interlayer spacing between the first and the second chromium layer is expanded to 1.55 Å (7.5% with respect to the bulk value of 1.44 Å). The interlayer spacing between the second and the third layer is contracted to 1.41 Å. The second chromium layer is buckled (0.13 Å). The second-layer chromium atom beneath a nitrogen atom resides deeper in the bulk. The nitrogen bond length to the four first-layer chromium atoms amounts to 2.07 Å, the bond length to the second-layer chromium atom amounts to 1.97 Å. The nitrogen position in c(2 × 2)-N determined by LEED is used to identify hollow sites in scanning tunnelling microscopy images. We found that hollow sites in p(1 × 1)-Cr(100) are imaged as hillocks and chromium atoms as depressions. This is anti-corrugation of clean Cr(100). Anti-corrugation seems to be related to a surface state of clean Cr(100) and is lifted in p(1 × 1)-N/Cr(100) at a (local) nitrogen coverage of 1 monolayer.
KW  - Chromium
KW  - Nitrogen
KW  - STM
SP  - 78
EP  - 86
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 518
T1  - Chemical ordering and reconstruction of Pt25Co75(100): an LEED/STM study
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(97)00665-1
A1  - Gauthier,Y.
A1  - Dolle,P.
A1  - Baudoing-Savois,R.
A1  - Hebenstreit,W.
A1  - Platzgummer,E.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 396
IS  - 1-3
PY  - 1998/01/20/
N2  - The surface of a disordered Pt25Co75(100) alloy has been investigated using quantitative LEED, AES and UHV-STM at room temperature. Atomic-resolution images reveal that it reconstructs with close-packed rows shifted by half the interatomic distance, from hollow to bridge sites. The density of shifted rows increases with the surface Pt concentration, leading to (1 × 5), (1 × 6) and (1 × 7) patterns. Segregation and chemical ordering lead to the formation of c(2 × 2) domains between the shifted rows. Chemical resolution was achieved with STM: the apparent height of the Pt atoms in the STM topographs is about 0.1-0.4 Å above that of Co, whereas LEED shows that Pt atoms are geometrically ~0.04 Å higher. The composition was determined down to the fourth layer. An oscillatory segregation profile is observed, with Pt-rich layers (<c1> = 62.6% Pt, <c3> = 53.5%) and Pt-depleted layers (<c2> = 6.9%, <c4> = 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.
KW  - Cobalt
KW  - Platinum
KW  - STM
SP  - 137
EP  - 155
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 431
T1  - The nitrogen-induced herringbone reconstruction of Cr(110)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(97)00474-3
A1  - Schmid,M.
A1  - Pinczolits,M.
A1  - Hebenstreit,W.
A1  - Varga,P.
VL  - 389
IS  - 1-3
PY  - 1997/11/06/
N2  - Segregation of nitrogen causes herringbone-like (3 × n) surface reconstructions, which were studied by atomically resolved scanning tunneling microscopy (STM). The main building blocks of the reconstruction are domains of (√6 × √6)R ± 35° cells, separated by domain boundaries running in the [110] direction. Since the density of Cr atoms in the surface is reduced by the reconstruction, the driving force of the reconstruction is believed to be compressive stress caused by the incorporation of N atoms into the surface.
KW  - Chromium
KW  - Nitrogen
KW  - STM
SP  - L1140-L1146
EP  - L1140-L1146
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 542
T1  - Interaction of oxygen with PtRh(100) studied with STM
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(97)00375-0
A1  - Wouda,P. T.
A1  - Schmid,M.
A1  - Hebenstreit,W.
A1  - Varga,P.
VL  - 388
IS  - 1-3
PY  - 1997/10/23/
N2  - The adsorption of oxygen at 500°C on a Pt50Rh50(100) single crystal surface was studied using UHV-STM and Auger electron spectroscopy. Images were taken of the p(3 × 1) phase; of a mixed phase with p(2 × 2), c(2 × 2) and (3 × 3) units; and of rhodium oxide patches. Possible models for these structures involving surface reconstruction are presented. Exposure of the p(3 × 1)O/PtRh(100) to H2 at room temperature led to the conversion to the p(1 × 1) substrate structure. The ordering and composition of this substrate structure after reduction is discussed.
KW  - Platinum
KW  - Rhodium
KW  - STM
SP  - 63
EP  - 70
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 464
T1  - Segregation and reconstructions of PtxNi1-x(100)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(97)00392-0
A1  - Hebenstreit,W.
A1  - Ritz,G.
A1  - Schmid,M.
A1  - Biedermann,A.
A1  - Varga,P.
VL  - 388
IS  - 1-3
PY  - 1997/10/23/
N2  - It is known that on (100) surfaces of PtxNi1 - x single crystals Pt segregates. With increasing Pt concentration in the surface the transition from unreconstructed Ni(100) to the pseudo hexagonal Pt(100) reconstruction occurs via a shifted row reconstruction and several pseudo hexagonal (n × 1) superstructures (n = 7, 12 and 19) consisting of similar (7 × 1) and (5 × 1) subcells. This was revealed by atomically resolved scanning tunnelling microscopy (STM). From low energy ion scattering measurements it becomes clear that the formation of the pseudo hexagonal structure leads to strong amplification of Pt segregation. Chemically resolved STM on the atomic scale shows that Pt prefers the highly coordinated four-fold hollow sites in the pseudo hexagonal structures and Ni is pushed into nearly on-top or bridge sites. Therefore the strong tendency of Pt to increase its coordination is proposed as the driving force of the reconstructions. Corrugations and chemical ordering measured by STM within the pseudo hexagonal reconstructions are confirmed by simulations based on embedded atom method potentials.
KW  - ISS
KW  - Nickel
KW  - Platinum
KW  - STM
SP  - 150
EP  - 161
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 52
T1  - Pt(100) quasihexagonal reconstruction: A comparison between scanning tunneling microscopy data and effective medium theory simulation calculations
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.56.10518
A1  - Ritz,G.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Borg,A.
A1  - Rønning,M.
VL  - 56
IS  - 16
PY  - 1997/10/15/
N2  - The interpretation of scanning tunneling microscopy (STM) data is usually limited to first-layer effects, but with increasing resolution of the STM images deeper-layer effects may also become visible in the top-layer corrugations. We have investigated the clean Pt(100) surface, which is known to be pseudohexagonally reconstructed and for which there is some evidence for a second-layer reconstruction. The big unit cell makes it difficult to investigate the deeper layers by traditional methods like low-energy-electron diffraction (LEED). We have, therefore, used a “fingerprint” technique to compare highly resolved STM data of the clean Pt(100) surface to effective-medium-theory simulation calculations in order to determine the geometric structure of the second atomic layer. We were able to show that STM can be sensitive to deeper layer effects and that excellent agreement could only be achieved for an unreconstructed second layer. The simulation results also agree well with the corrugations determined by LEED whereas the maximum corrugation amplitude is higher than previously derived from helium-diffraction measurements.
KW  - Platinum
SP  - 10518
EP  - 10518
UR  - http://link.aps.org/abstract/PRB/v56/p10518
ER  - 

TY  - JOUR
ID  - 918
T1  - Segregation of impurities on Cr(100) studied by AES and STM
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(96)01539-7
A1  - Schmid,M.
A1  - Pinczolits,M.
A1  - Hebenstreit,W.
A1  - Varga,P.
VL  - 377-379
PY  - 1997/04/20/
N2  - With increasing annealing temperature, Auger electron spectroscopy (AES) of a Cr(100) single crystal shows segregation of C, N and O as the dominating segregating species, indicating competitive segregation of these elements. An STM study of N structures shows a c(2 × 2) superstructure at N coverages up to 1/2. The local N coverage can be increased by insertion of N-rich domain boundaries up to 2/3, where a c(3√2 × √2)R ± 45° structure forms, followed by a first-order phase transformation to a p(1 × 1) structure. The existence of patches of the N-rich p(1 × 1) structure at coverages below 2/3 is probably due to additional carbon impurities stabilizing this structure. The possibility of inverse corrugation on the pure Cr(100) surface is discussed.
KW  - Carbon
KW  - Chromium
KW  - Nitrogen
KW  - Oxygen
KW  - STM
SP  - 1023
EP  - 1027
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 872
T1  - Sputter yields of insulators bombarded with hyperthermal multiply charged ions
JF  - Physica Scripta
JA  - Physica Scripta
M3  - 10.1088/0031-8949/1997/T73/100
A1  - Varga,P.
A1  - Neidhart,T.
A1  - Sporn,M.
A1  - Libiseller,G.
A1  - Schmid,M.
A1  - Aumayr,F.
A1  - Winter,H. P.
VL  - T73
PY  - 1997///
N2  - The total sputter yield for Au, Si, GaAs, SiO2, MgO, LiF and NaCl bombarded with hyperthermal highly charged Ar^q+ ions (q = 1-9) has been measured. Only for alkali halides (LiF, NaCl) and to some extent for SiO2 potential sputtering (enhancement of the sputter yield with increasing charge state of the primary ion) has been observed. All other targets showed normal collision induced sputtering. From that result it is obvious that the mechanisms for sputtering can not be explained by the Coulomb explosion model, because in this model insulators like MgO and semiconductors like Si and GaAs should also show charge state dependence of the sputtering yield. Alkali halides and SiO2 are materials which are known for strong electron phonon coupling where electronic excitations in the valence band are localized by formation of self trapped excitons (STE) and/or self trapped holes (STH). During bombardment with highly charged ions the neutralization process in front of, at and below the surface causes the formation of STE and/or STH. Therefore the potential sputtering can be explained as a defect mediated sputtering process which is well known in electron stimulated desorption (ESD) where the decay of STH and/or STE into different colour centers leads at the end to the desorption of neutralized anions. The also created neutral cations are either evaporated (as it is the case for the alkali halides) or have to be removed by momentum transfer by the impinging projectiles. Therefore it is very likely that in the case of SiO2 for very low impact energy mainly only oxygen is enhanced sputtered, the surface is enriched in Si and the potential sputtering effect decreases with increasing ion dose.
KW  - GaAs
KW  - Gold
KW  - ion bombardment
KW  - LiF
KW  - MgO
KW  - NaCl
KW  - QCM
KW  - Silicon
KW  - SiO2
SP  - 307
EP  - 310
SN  - 1402-4896
ER  - 

TY  - JOUR
ID  - 732
T1  - Potential Sputtering of Clean SiO2 by Slow Highly Charged Ions
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.79.945
A1  - Sporn,M.
A1  - Libiseller,G.
A1  - Neidhart,T.
A1  - Schmid,M.
A1  - Aumayr,F.
A1  - Winter,HP.
A1  - Varga,P.
A1  - Grether,M.
A1  - Niemann,D.
A1  - Stolterfoht,N.
VL  - 79
IS  - 5
PY  - 1997///
N2  - The recently discovered phenomenon of potential sputtering, i.e., the efficient removal of neutral and ionized target particles from certain insulator surfaces due to the potential rather than the kinetic energy of impinging slow highly charged ions, has now also been observed for stoichiometric SiO2 surfaces. Using a sensitive quartz crystal microbalance technique, total sputter yields induced by Ar^q+ (q≤14) and Xe^q+ (q≤27) ions have been determined for LiF and SiO2 surfaces. The primary mechanisms for potential sputtering (defect mediated sputtering) and its considerable practical relevance for highly charged ion-induced surface modification of insulators are discussed.
KW  - ion bombardment
KW  - QCM
KW  - SiO2
SP  - 945
EP  - 945
UR  - http://link.aps.org/abstract/PRL/v79/p945
ER  - 

TY  - JOUR
ID  - 1163
T1  - Inverse corrugation and corrugation enhancement of Pb superstructures on Cu(111) and (110)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/S0039-6028(96)00907-7
A1  - Nagl,C.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 369
IS  - 1-3
PY  - 1996/12/20/
N2  - Contrary to intuitive expectation, a monatomic hexagonal close-packed Pb film on Cu(111) shows an "inverse" corrugation, i.e. the Pb atoms in on-top adsorption sites appear lowest in scanning tunneling microscopy (STM) images, as well as in effective medium theory (EMT) simulation. On small subsurface Cu islands on a Pb(111) substrate, which are also covered by a monatomic Pb film, the corrugation of this film was found to be strongly dependent on the thickness of the Cu island (the thinner the island, the larger the corrugation). EMT simulations reproduce this enhanced corrugation qualitatively and thus further confirm the formation of these subsurface Cu islands. On a p(8 × 1) superstructure of Pb/Cu(110), similarly, the lowest coordinated Pb atoms also show the lowest apparent height in STM images.
KW  - Copper
KW  - Lead
KW  - STM
SP  - 159
EP  - 168
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 1043
T1  - Scanning tunneling microscopy studies of niobium carbide (100) and (110) surfaces
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/0039-6028(96)00804-7
A1  - Tsong,R. M.
A1  - Schmid,M.
A1  - Nagl,C.
A1  - Varga,P.
A1  - Davis,R. F.
A1  - Tsong,I. S. T.
VL  - 366
IS  - 1
PY  - 1996/10/10/
N2  - Scanning tunneling microscopy (STM) studies were conducted on the surfaces of NbC0.75(100) and NbC0.865(110) single crystals after in situ cleaning treatments of sputtering and annealing cycles. STM images show atom-resolved structures of both surfaces. On NbC0.75(100), localized areas of a square (1 × 1) structure were observed, together with regions of hexagonal structure, indicative of a coexisting surface phase, possibly that of Nb4C3-x. On the NbC0.865(110) surface, a ridge-and-valley grating structure consisting of both (4 × 1) and (5 × 1) geometries was observed over large areas. The nanometer-scale faceting phenomenon may be common to the (110) surfaces of most transition-metal carbides.
KW  - Carbon
KW  - Niobium
KW  - STM
SP  - 85
EP  - 92
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 609
T1  - Chemically resolved STM on a PtRh(100) surface
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/0039-6028(96)00022-2
A1  - Wouda,P. T.
A1  - Nieuwenhuys,B. E.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 359
IS  - 1-3
PY  - 1996/07//
N2  - Scanning tunnelling microscopy on PtRh(100) (molar bulk composition 1 : 1) has revealed the possibility of direct determination of the surface of this system. During measurements at low tunnelling resistance (<500 kΩ), the Pt and Rh atoms appear with a clearly observable apparent height difference of more than 20 pm. No long range ordering has been found. Variation of the sample preparation method and comparison of STM and Auger electron spectroscopy measurements led to the conclusions that there is a preferential surface segregation of platinum, that rhodium atoms are the ones with the highest apparent height, and that there is limited tendency of clustering on the surface. Furthermore, it was found that platinum atoms preferentially populate the step edges on this crystal surface.
KW  - Platinum
KW  - Rhodium
KW  - STM
SP  - 17
EP  - 22
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 1038
T1  - Strain-induced local surface chemical ordering observed by STM
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.53.16019
A1  - Ritz,G.
A1  - Schmid,M.
A1  - Biedermann,A.
A1  - Varga,P.
VL  - 53
IS  - 23
PY  - 1996/06//
N2  - PtNi alloys are known to exhibit a tendency towards chemical ordering, which also effects surface segregation. Scanning tunneling microscopy results obtained in the strain field of dislocations on PtxNi1-x(110) surfaces show a (2×1) superstructure of alternating Pt and Ni atoms in some regions close to the dislocation core. In other regions, the apparent height of all surface atoms is equal, in agreement with low energy ion scattering results yielding a surface concentration of almost 100% Ni. This indicates that the strain present in the vicinity of dislocations influences both the surface composition and chemical order. The experimental results are compared to simulation calculations of chemical ordering and segregation, using embedded atom method potentials and linear elasticity theory. The simulations indicate that the (2×1) superstructure is due to an L10 ordered phase in regions where the tetragonal distortion of the L10 phase with respect to the cubic substrate can alleviate stress. It is argued that this dislocation-induced ordering can immobilize dislocations.
KW  - ISS
KW  - Nickel
KW  - Platinum
KW  - STM
SP  - 16019
EP  - 16026
SN  - 0163-1829
UR  - http://prb.aps.org/abstract/PRB/v53/i23/p16019_1
ER  - 

TY  - JOUR
ID  - 926
T1  - Scanning Tunneling Spectroscopy of One-Dimensional Surface States on a Metal Surface
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.76.4179
A1  - Biedermann,A.
A1  - Genser,O.
A1  - Hebenstreit,W.
A1  - Schmid,M.
A1  - Redinger,J.
A1  - Podloucky,R.
A1  - Varga,P.
VL  - 76
IS  - 22
PY  - 1996/05/27/
N2  - Scanning tunneling spectroscopy permits real-space observation of one-dimensional electronic states on a Fe(100) surface alloyed with Si. These states are localized along chains of Fe atoms in domain boundaries of the Fe(100) c(2×2)Si surface alloy, as confirmed by first-principles spin-polarized calculations. The calculated charge densities illustrate the d-like orbital character of the one-dimensional state and show its relationship to a two-dimensional state existing on the pure Fe(100) surface.
KW  - Iron
KW  - Silicon
KW  - STM
SP  - 4179
EP  - 4179
UR  - http://link.aps.org/abstract/PRL/v76/p4179
ER  - 

TY  - JOUR
ID  - 592
T1  - Subsurface islands and superstructures of Cu on Pb(111)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/0039-6028(95)01200-1
A1  - Nagl,C.
A1  - Platzgummer,E.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Speller,S.
A1  - Heiland,W.
VL  - 352-354
PY  - 1996/05/15/
N2  - 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.
KW  - Copper
KW  - Growth
KW  - Lead
KW  - STM
SP  - 540
EP  - 545
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 682
T1  - Quantum Wells and Electron Interference Phenomena in Al due to Subsurface Noble Gas Bubbles
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.76.2298
A1  - Schmid,M.
A1  - Hebenstreit,W.
A1  - Varga,P.
A1  - Crampin,S.
VL  - 76
IS  - 13
PY  - 1996/03/25/
N2  - Scanning tunneling microscopy on Ar ion bombarded and annealed aluminum surfaces shows electron interference between the surface and subsurface bubbles of implanted gas. The depth of the bubbles as determined from the energy dependence of the standing waves indicates a minimum around 6–7 layers on Al(111). The appearance and energy dependence of the interference pattern is in good agreement with scattering theory based on free electrons, and indicates the bubbles have a shape given by the Wulff construction.
KW  - Aluminum
KW  - Argon
KW  - ion bombardment
KW  - STM
SP  - 2298
EP  - 2298
UR  - http://link.aps.org/abstract/PRL/v76/p2298
ER  - 

TY  - JOUR
ID  - 186
T1  - p(n x 1) superstructures of Pb on Cu(110)
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.52.16796
A1  - Nagl,C.
A1  - Pinczolits,M.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Robinson,I. K.
VL  - 52
IS  - 23
PY  - 1995/12/15/
N2  - The structures of the p(n×1) superstructures (n=4, 5, and 9) of Pb on Cu(110) in the coverage range between FTHETA=0.75 and 0.8 are revealed by atomically resolved scanning tunneling microscopy. All three superstructures are formed by substitution of every nth row of Cu atoms (n=4, 5, and 9) in the [001] direction by Pb atoms. The Pb atoms in between are lined up in the [1¯10] direction. The p(4×1) structure appears in two different modifications: one with substitutional rows of Pb atoms and one with a simple overlayer structure without substituted rows of Cu atoms. Alternating succession of these two modifications results in p(12×1) domains. It is further shown that the p(9×1) structure is not a succession of p(4×1) and p(5×1) but a superstructure on its own. The p(5×1) structure proposed here agrees with previous x-ray-diffraction data at least as well as the quasihexagonal model proposed earlier. We have, in addition, identified the nature of the phase that has been described incommensurate obtained by desorption of Pb upon annealing above 600 K.
KW  - Copper
KW  - Lead
KW  - STM
SP  - 16796
EP  - 16796
UR  - http://link.aps.org/abstract/PRB/v52/p16796
ER  - 

TY  - JOUR
ID  - 1192
T1  - Deconvolution of STM images using entropy as a regularization functional
JF  - Fresenius' Journal of Analytical Chemistry
JA  - Fresenius J. Anal. Chem.
M3  - 10.1007/s0021653530439
A1  - Böhmig,S.
A1  - Schmid,M.
A1  - Störi,H.
VL  - 353
IS  - 3
PY  - 1995/10/25/
N2  - 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.
KW  - STM
SP  - 439
EP  - 442
UR  - http://dx.doi.org/10.1007/s0021653530439
ER  - 

TY  - JOUR
ID  - 1146
T1  - Competitive segregation of Si and P on Fe96.5Si3.5 (100) and (110)
JF  - Fresenius' Journal of Analytical Chemistry
JA  - Fresenius J. Anal. Chem.
M3  - 10.1007/s0021653530259
A1  - Biedermann,A.
A1  - Schmid,M.
A1  - Reichl,B.
A1  - Varga,P.
VL  - 353
IS  - 3
PY  - 1995/10/25/
N2  - Annealing an Fe96.5Si3.5 (100)/(110) bicrystal, containing 90 ppm P, leads immediately to a strong segregation of silicon. The Si atoms, however, desegregate subsequently and are displaced by P, whose segregation enthalpy is larger than that of silicon. The corresponding surface structures formed on both faces have been studied using complementary methods: Scanning tunneling microscopy (STM) to obtain atomically resolved geometrical information and Auger electron spectroscopy (AES) for the determination of the surface composition. Si substitutes surface Fe atoms on both faces and forms ordered surface alloys, whereas P occupies hollow sites on the surface. Si and P form c(2 × 2) superstructures on the (100) surface, whereby each segregated phosphorus atom blocks in the average one silicon segregation site. The (110) surface, on the other hand, is characterized by a c(1 × 3) Si superstructure. Due to the anisotropy of this surface the P/Si exchange proceeds by the formation of silicon coverage decreasing domain boundaries within the silicon structure, which are simultaneously occupied by P atoms. Furthermore the comparison of the AES and STM derived phosphorus coverages indicates a P multilayer segregation on the (110) surface.
KW  - Iron
KW  - Phosphorus
KW  - Silicon
KW  - STM
SP  - 259
EP  - 262
UR  - http://dx.doi.org/10.1007/s0021653530259
ER  - 

TY  - JOUR
ID  - 209
T1  - Direct Observation of a New Growth Mode: Subsurface Island Growth of Cu on Pb(111)
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.75.2976
A1  - Nagl,C.
A1  - Platzgummer,E.
A1  - Schmid,M.
A1  - Varga,P.
A1  - Speller,S.
A1  - Heiland,W.
VL  - 75
IS  - 16
PY  - 1995/10/16/
N2  - Atomically resolved scanning tunneling microscopy on Cu/Pb(111) reveals a new growth mode, contrary to the Volmer-Weber mode expected from the significantly lower surface energy of Pb. (111)-oriented Cu islands with a thickness of 3–11 layers are immersed in the Pb substrate and covered by a single close-packed Pb layer. This subsurface growth mode occurring at room temperature can be explained by simple thermodynamic considerations.
KW  - Copper
KW  - Growth
KW  - Lead
KW  - STM
SP  - 2976
EP  - 2976
UR  - http://link.aps.org/abstract/PRL/v75/p2976
ER  - 

TY  - JOUR
ID  - 1111
T1  - An STM study of the step structure of Pb(110) and Pb(111)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/0039-6028(95)00257-X
A1  - Speller,S.
A1  - Heiland,W.
A1  - Biedermann,A.
A1  - Platzgummer,E.
A1  - Nagl,C.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 331-333
IS  - Part 2
PY  - 1995/07/01/
N2  - The (110) and (111) surfaces of lead are investigated by scanning tunneling microscopy. They are atomically resolved with corrugations of 0.1 Å and 0.6 Å respectively. The STM images allow conclusions about the motion of the atoms at step edges and step step interaction. At room temperature the steps on both surfaces are below the roughening transition. The influence of impurities and tip surface interaction on the step fluctuation is discussed.
KW  - Lead
KW  - STM
SP  - 1056
EP  - 1061
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 683
T1  - Surface alloying and superstructures of Pb on Cu(100)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/0039-6028(95)00387-8
A1  - Nagl,C.
A1  - Platzgummer,E.
A1  - Haller,O.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 331-333
IS  - Part 1
PY  - 1995/07/01/
N2  - The growth of submonolayer and monolayer Pb films on Cu(100) has been investigated by STM. In the submonolayer region a disordered surface alloy is found in spite of the immiscibility of bulk Cu and Pb. At a coverage of theta = 3/8 a c(4 × 4) superstructure is observed. The atomic arrangement of the c(4 × 4) superstructure unit cell could be revealed; it is formed by linear chains of Pb atoms in two rotational domains. Increasing the coverage to theta = 0.5, a c(2 × 2) structure can be observed. The transition to the c(5√2 × √2)R45° superstructure of the dense overlayer proceeds via the insertion of antiphase domain boundary. The model for the c(5√2 × √2)R45° superstructure presented in literature is confirmed.
KW  - Copper
KW  - Growth
KW  - Lead
KW  - STM
SP  - 831
EP  - 837
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 943
T1  - Domain wall structures in an ordered Si/Fe(110) surface alloy
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/0039-6028(95)00339-8
A1  - Biedermann,A.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 331-333
IS  - Part 1
PY  - 1995/07/01/
N2  - During the annealing process of an Fe96.5Si3.5(100)/(110) bicrystal, silicon and impurity carbon segregate to the surface. The structures formed by the segregands on the (110) surface have been studied by STM (geometry) and AES (chemical information). Silicon substitutes iron surface atoms and forms a two-dimensional alloy, whereas carbon occupies hollow sites in the first monolayer, leading to a distortion of the substrate lattice. The structures are based on a c(1 × 3)Si [theta] = 1/3 ordered surface alloy. Additional silicon as well as the co-segregating impurity carbon are inserted into this structure by formation of domain walls. If the density of these nearly straight and parallel domain walls becomes high enough, commensurate domain wall structures with c(1 × n) supercells can be observed.
KW  - Iron
KW  - Silicon
KW  - STM
SP  - 787
EP  - 793
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 1021
T1  - Potential Sputtering of Lithium Fluoride by Slow Multicharged Ions
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.74.5280
A1  - Neidhart,T.
A1  - Pichler,F.
A1  - Aumayr,F.
A1  - Winter,HP.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 74
IS  - 26
PY  - 1995/06/26/
N2  - Thin polycrystalline LiF films have been bombarded by slow (≤1 keV) multicharged Arq+ ions (q≤9), in order to study the resulting total sputter yields by means of a quartz crystal microbalance. More than 99% of sputtered particles are neutral and show yields, at given impact energy, in proportion to the potential energy of projectile ions. The respective “potential sputtering” process already takes place far below 100 eV impact energy. It can be related to defect production in LiF following electron capture by the multicharged ions, and removes about one LiF molecule per 100 eV of projectile potential energy.
KW  - ion bombardment
KW  - LiF
KW  - QCM
SP  - 5280
EP  - 5280
UR  - http://link.aps.org/abstract/PRL/v74/p5280
ER  - 

TY  - JOUR
ID  - 439
T1  - Determination of electron-induced total sputter yield of LiF
JF  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
JA  - Nucl. Instrum. Methods B
M3  - 10.1016/0168-583X(95)00062-3
A1  - Neidhart,T.
A1  - Sporn,M.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 101
IS  - 1-2
PY  - 1995/06/03/
N2  - We present measurements of the total sputter yield for LiF at 150°C induced by electrons with an impact energy between 10 and 500 eV. A nearly linear increase with kinetic energy has been measured. The decrease of the yield with increasing electron dose for energies below 100 eV is also shown. For very high electron dose the yield even becomes zero and simultaneously a red coloration of the surface is observed. Low energy ion scattering (LEIS) measurements at such a surface showed a Li enrichment to more than 90%. To reach such a composition with electrons between 40 and 80 eV kinetic energy sputtering of about 20 monolayers LiF is necessary.
KW  - ESD
KW  - ion bombardment
KW  - ISS
KW  - LiF
KW  - QCM
SP  - 127
EP  - 130
SN  - 0168-583X
ER  - 

TY  - JOUR
ID  - 104
T1  - Secondary ion emission from lithium fluoride under impact of slow multicharged ions
JF  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
JA  - Nucl. Instrum. Methods B
M3  - 10.1016/0168-583X(95)00169-7
A1  - Neidhart,T.
A1  - Pichler,F.
A1  - Aumayr,F.
A1  - Winter,H. P.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 98
IS  - 1-4
PY  - 1995/05/03/
N2  - Secondary ion emission has been investigated for bombardment of polycrystalline lithium fluoride by slow multicharged Ar ions (charge state q ≤ 9, impact energy E_k ≤ 500 eV). The F- ions originate from collisional energy transfer, almost independent of the primary ion charge, whereas the F+ yield strongly increases with q. The F+ ions are produced via interatomic Auger transitions from the F- 2p valence band into projectile states, and their desorption from LiF is controlled by Coulomb interaction of F+ with Li+ and F- surface ions, and LiF lattice relaxation. At high impact energy, emission of Li+ is also mainly due to collisional energy transfer, but toward lower Ek the primary ion charge plays an increasingly important role. The present measurements demonstrate that secondary ions account for less than 0.1% of our earlier measured total sputter yields from LiF.
KW  - ion bombardment
KW  - LiF
SP  - 465
EP  - 468
SN  - 0168-583X
ER  - 

TY  - JOUR
ID  - 272
T1  - Sensors based on piezoelectric resonators
JF  - Sensors and Actuators A: Physical
JA  - Sensors Actuators A
M3  - 10.1016/0924-4247(95)00846-2
A1  - Benes,E.
A1  - Gröschl,M.
A1  - Burger,W.
A1  - Schmid,M.
VL  - 48
IS  - 1
PY  - 1995/05/01/
N2  - 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.
KW  - QCM
SP  - 1
EP  - 21
SN  - 0924-4247
UR  - http://www.sciencedirect.com/science/article/B6THG-3YVCYW5-R/2/d566996f363a2d89bad8f778602e3402
ER  - 

TY  - JOUR
ID  - 271
T1  - Surface stress, surface elasticity, and the size effect in surface segregation
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.51.10937
A1  - Schmid,M.
A1  - Hofer,W.
A1  - Varga,P.
A1  - Stoltze,P.
A1  - Jacobsen,K. W.
A1  - Nørskov,J. K.
VL  - 51
IS  - 16
PY  - 1995/04/15/
N2  - 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.
KW  - Nickel
KW  - Palladium
KW  - Platinum
SP  - 10937
EP  - 10937
UR  - http://link.aps.org/abstract/PRB/v51/p10937
ER  - 

TY  - JOUR
ID  - 60
T1  - Submonolayer growth of Pb on Cu(111): surface alloying and de-alloying
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/0039-6028(94)90189-9
A1  - Nagl,C.
A1  - Haller,O.
A1  - Platzgummer,E.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 321
IS  - 3
PY  - 1994/12/20/
N2  - 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.
KW  - Copper
KW  - Growth
KW  - Lead
SP  - 237
EP  - 248
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 470
T1  - Segregated Si on Fe96.5Si3.5(110): Domain-wall structures in a two-dimensional alloy
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.50.17518
A1  - Biedermann,A.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 50
IS  - 23
PY  - 1994/12/15/
N2  - The surface structures formed by segregated silicon on a bcc Fe96.5Si3.5(110) surface have been studied by scanning tunneling microscopy (STM). Additionally Auger-electron spectroscopy and low-energy ion-scattering spectroscopy have been used to obtain the chemical and structural information necessary to decide between possible configurations suggested by the STM images. Near the expected saturation coverage a substitutional c(1×3)Si superstructure weakly disordered by 〈110〉 aligned domain boundaries was observed. Their arrangement is equivalent to the incommensurate domain-wall structures of chemisorption systems like H/Fe(110), which allow a continuous increase of the surface coverage by successive insertion of equidistantly arranged heavy domain walls. Our observations suggest a possible increase of the silicon coverage above the 33.3% of the perfect c(1×3) structure at least up to 40% corresponding to a c(1×5)Si commensurate domain-wall structure. Correlation analysis of the disordered phase at 15% coverage, obtained by short annealing, allows a qualitative determination of the interaction forces among individual silicon atoms on the surface, yielding attraction of third-nearest neighbors and repulsion of nearest neighbors.
KW  - Iron
KW  - ISS
KW  - Silicon
KW  - STM
SP  - 17518
EP  - 17518
UR  - http://link.aps.org/abstract/PRB/v50/p17518
ER  - 

TY  - JOUR
ID  - 475
T1  - The shifted-row reconstruction of PtxNi1-x(100)
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/0039-6028(94)90103-1
A1  - Schmid,M.
A1  - Biedermann,A.
A1  - Böhmig,S.D.
A1  - Weigand,P.
A1  - Varga,P.
VL  - 318
IS  - 3
PY  - 1994/10/20/
N2  - Scanning tunneling microscopy on Pt10Ni90(100) and Pt25Ni75(100) single crystals reveals close-packed rows of atoms, which are shifted by <110> along the direction of the rows into a bridge position and slightly outward of the surface. Maximum entropy deconvolution of atomically resolved STM data shows that all atoms between the shifted rows are close to the unreconstructed positions. The density of the shifted rows increases with increasing Pt surface concentration up to a maximum value of each 5th row shifted. The reconstruction shows little dependence on the carbon contamination of the surface, but it is lifted by a full c(2 × 2) coverage of carbon monoxide, which can be imaged simultaneously with the substrate, indicating an on-top position of CO. The driving force of the shifted-row reconstruction is discussed.
KW  - ISS
KW  - Nickel
KW  - Platinum
KW  - STM
SP  - 289
EP  - 298
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 176
T1  - Enhancement of STM images and estimation of atomic positions based on maximum entropy deconvolution
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/0039-6028(94)91152-5
A1  - Böhmig,S.D.
A1  - Schmid,M.
A1  - Störi,H.
VL  - 313
IS  - 1-2
PY  - 1994/06/20/
N2  - 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).
KW  - Nickel
KW  - Platinum
KW  - STM
SP  - 6
EP  - 16
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 613
T1  - Total sputter yield of LiF induced by hyperthermal ions measured by a quartz microbalance
JF  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
JA  - Nucl. Instrum. Methods B
M3  - 10.1016/0168-583X(94)95601-4
A1  - Neidhart,T.
A1  - Toth,Z.
A1  - Hochhold,M.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 90
IS  - 1-4
PY  - 1994/05/02/
N2  - 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.
KW  - ion bombardment
KW  - LiF
KW  - QCM
SP  - 496
EP  - 500
SN  - 0168-583X
ER  - 

TY  - JOUR
ID  - 864
T1  - Chemical analysis of PtxNi1−x alloy single crystal surfaces by scanning tunnelling microscopy
JF  - Fresenius' Journal of Analytical Chemistry
JA  - Fresenius J. Anal. Chem.
M3  - 10.1007/BF00323272
A1  - Biedermann,A.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 349
IS  - 1
PY  - 1994/01/01/
N2  - 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.
KW  - Nickel
KW  - Platinum
KW  - STM
SP  - 201
EP  - 203
UR  - http://dx.doi.org/10.1007/BF00323272
ER  - 

TY  - JOUR
ID  - 651
T1  - Surface effects on Pt-Ni single crystals calculated with the embedded-atom method
JF  - Physical Review B
JA  - Phys. Rev. B
M3  - 10.1103/PhysRevB.48.11352
A1  - Stadler,H.
A1  - Hofer,W.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 48
IS  - 15
PY  - 1993/10/15/
N2  - 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.
KW  - Nickel
KW  - Platinum
SP  - 11352
EP  - 11352
UR  - http://link.aps.org/abstract/PRB/v48/p11352
ER  - 

TY  - JOUR
ID  - 110
T1  - Segregated carbon on Pt10Ni90(100) studied by scanning tunneling microscopy
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/0039-6028(93)90103-Q
A1  - Schmid,M.
A1  - Biedermann,A.
A1  - Varga,P.
VL  - 294
IS  - 3
PY  - 1993/09/10/
N2  - Scanning tunneling microscopy is used to study the arrangement of segregated carbon atoms with atomic resolution. Individual carbon atoms are visible only under special tip conditions, while they normally do not directly appear on STM topographs. Under all tip conditions, carbon atoms affect the corrugation of their metal neighbours, reducing the apparent height by 20 to 40 pm in the p(2 × 2) and 40 to 70 pm in the c(2 × 2) superstructure. Therefore the existence, structure and amount of carbon can be also derived from images without directly visible carbon atoms. A substrate lattice distortion in regions of the carbon c(2 × 2) superstructure was observed, exhibiting areas of the p4g structure known from earlier LEED studies of Ni(100).
KW  - Carbon
KW  - Nickel
KW  - Platinum
KW  - STM
SP  - L952-L958
EP  - L952-L958
SN  - 0039-6028
ER  - 

TY  - CHAP
ID  - 707
T1  - Ionization of LiF by hyperthermal multiply charged ions
T2  - Ionization of Solids by Heavy Particles
T3  - NATO ASI Series
CY  - New York
A1  - Neidhart,T.
A1  - Schmid,M.
A1  - Varga,P.
ED  - Baragiola,Raul A.
VL  - B 306
PB  - Plenum
PY  - 1993/08/31/
N2  - Sputtering of a LiF thin film (0.25 µm) surface by singly- (He+, Ne+ and Ar+) and doubly-charged (Ne++ and Ar++) ions with impact energies between 10 and 500 eV has been performed. The yield of sputtered Li+ and F- ions is only slightly higher for doubly-charged ions compared to singly-charged projectiles at impact energies below 100 eV, whereas the F+ yield is substantially increased if doubly-charged projectiles are used. The experimental data are explained within the framework of a model based on calculations by Walkup and Avouris. A comparison is given with former measurements on a LiF single crystal.
KW  - ion bombardment
SP  - 447
EP  - 453
SN  - 0306444895
ER  - 

TY  - JOUR
ID  - 608
T1  - Preferential sputtering of Pt-Ni alloy single crystals studied by scanning tunneling microscopy
JF  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
JA  - Nucl. Instrum. Methods B
M3  - 10.1016/0168-583X(93)96028-B
A1  - Schmid,M.
A1  - Biedermann,A.
A1  - Slama,C.
A1  - Stadier,H.
A1  - Weigand,P.
A1  - Varga,P.
VL  - 82
IS  - 2
PY  - 1993/07/02/
N2  - 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.
KW  - ion bombardment
KW  - ISS
KW  - Nickel
KW  - Platinum
KW  - STM
SP  - 259
EP  - 268
SN  - 0168-583X
ER  - 

TY  - JOUR
ID  - 251
T1  - Embedded-atom method calculations applied to surface segregation of Pt-Ni single crystals
JF  - Surface Science
JA  - Surf. Sci.
M3  - 10.1016/0039-6028(93)90804-S
A1  - Stadler,H.
A1  - Hofer,W.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 287-288
IS  - Part 1
PY  - 1993/05/10/
N2  - 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.
KW  - Nickel
KW  - Platinum
SP  - 366
EP  - 370
SN  - 0039-6028
ER  - 

TY  - JOUR
ID  - 1178
T1  - Direct observation of surface chemical order by scanning tunneling microscopy
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.70.1441
A1  - Schmid,M.
A1  - Stadler,H.
A1  - Varga,P.
VL  - 70
IS  - 10
PY  - 1993/03/08/
N2  - We present the first scanning tunneling microscopy (STM) study which allows clear discrimination of two chemical species in a metal alloy. Special tunneling conditions, which we attribute to an adsorbate at the STM tip, cause a difference in corrugation between Pt and Ni atoms of 0.3 Å. The STM data reveal chemical short-range order at the surface, which is in agreement with embedded atom simulations and can be understood as small domains of an L10 ordered phase.
KW  - Nickel
KW  - Platinum
KW  - STM
SP  - 1441
EP  - 1441
UR  - http://link.aps.org/abstract/PRL/v70/p1441
ER  - 

TY  - JOUR
ID  - 512
T1  - Surface composition of Pt25Ni75(111) investigated by ISS and STM
JF  - Fresenius' Journal of Analytical Chemistry
JA  - Fresenius J. Anal. Chem.
M3  - 10.1007/BF00321431
A1  - Weigand,P.
A1  - Nagl,C.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 346
IS  - 1
PY  - 1993/01/01/
N2  - 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.
KW  - ion bombardment
KW  - ISS
KW  - Nickel
KW  - Platinum
SP  - 281
EP  - 283
UR  - http://dx.doi.org/10.1007/BF00321431
ER  - 

TY  - CHAP
ID  - 373
T1  - Desorption from LiF(100) by singly- and doubly-charged hyperthermal He ions
T2  - Desorption Induced by Electronic Transitions DIET V
T3  - Springer Series in Surface Sciences
CY  - Berlin
A1  - Neidhart,T.
A1  - Schmid,M
A1  - Varga,P.
ED  - Burns,A. R.
ED  - Stechel,E. B.
ED  - Jennison,D. R.
VL  - 31
PB  - Springer
PY  - 1993///
N2  - 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.
KW  - ion bombardment
KW  - LiF
SP  - 129
EP  - 132
SN  - 978-3540564737
ER  - 

TY  - JOUR
ID  - 1143
T1  - Mismatch dislocations caused by preferential sputtering of a platinum-nickel alloy surface
JF  - Applied Physics A: Materials Science & Processing
JA  - Appl. Phys. A
M3  - 10.1007/BF00348334
A1  - Schmid,M.
A1  - Biedermann,A.
A1  - Stadler,H.
A1  - Slama,C.
A1  - Varga,P.
VL  - 55
IS  - 5
PY  - 1992/11/01/
N2  - 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.
KW  - ion bombardment
KW  - Nickel
KW  - Platinum
KW  - STM
SP  - 468
EP  - 475
UR  - http://dx.doi.org/10.1007/BF00348334
ER  - 

TY  - JOUR
ID  - 304
T1  - Analysis of vibration-isolating systems for scanning tunneling microscopes
JF  - Ultramicroscopy
JA  - Ultramicroscopy
M3  - 10.1016/0304-3991(92)90493-4
A1  - Schmid,M.
A1  - Varga,P.
VL  - 42-44
IS  - Part 2
PY  - 1992/07//
N2  - 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.
KW  - STM
SP  - 1610
EP  - 1615
SN  - 0304-3991
ER  - 

TY  - JOUR
ID  - 1141
T1  - Sputtering of LiF(100) with low energetic Ne+ and Ne2+ ions
JF  - Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
JA  - Nucl. Instrum. Methods B
M3  - 10.1016/0168-583X(92)95029-Q
A1  - Wutte,D.
A1  - Diebold,U.
A1  - Schmid,M.
A1  - Varga,P.
VL  - 65
IS  - 1-4
PY  - 1992/03/01/
N2  - Sputtering of a LiF(100) surface by singly and doubly charged Ne ions with impact energies between 10 and 500 eV has been performed. The emission of Li+, Li0, F+ F0 and LiF+ was measured by means of a quadrupole mass spectrometer. The yield of sputtered Li+ ions and Li0 atoms decreases with decreasing impact energy and is slightly higher for Ne2+ compared to Ne+ at impact energies below 100 eV. The situation changes very drastically regarding the emitted F particles. In the whole energy range investigated, the F+ and F0 yield was about one order of magnitude larger if Ne2+ projectiles were used instead of Ne+. According to the experimental data we propose the following model for the emission of F+ [1,2]: (1) The F ion of the solid surface is changed to F+ by Auger neutralization of the Ne2+ into Ne+ or by double resonance transition into doubly excited states of Ne0 with subsequent autoionization into Ne+. These processes take place several Ångströms in front of the surface. (2) The initial repulsive forces between the F+ ion and the surface are changed into attractive ones within a very short time by rearrangement of the alkali halide lattice [2]. (3) The neutralized projectile sputters the F+ as in a normal sputtering event on a surface with low surface binding energy, and F+ can he emitted as long as the kinetic energy of the projectile is sufficiently high.
KW  - ion bombardment
KW  - LiF
SP  - 167
EP  - 172
SN  - 0168-583X
UR  - http://www.sciencedirect.com/science/article/B6TJN-470WFPC-XP/2/564e0302ee695c6ddad1195d57e3626a
ER  - 

TY  - JOUR
ID  - 157
T1  - Lattice mismatch dislocations in a preferentially sputtered alloy studied by scanning tunneling microscopy
JF  - Physical Review Letters
JA  - Phys. Rev. Lett.
M3  - 10.1103/PhysRevLett.69.925
A1  - Schmid,M.
A1  - Biedermann,A.
A1  - Stadler,H.
A1  - Varga,P.
VL  - 69
IS  - 6
PY  - 1992///
N2  - 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.
KW  - ion bombardment
KW  - Nickel
KW  - Platinum
KW  - STM
SP  - 925
EP  - 925
UR  - http://link.aps.org/abstract/PRL/v69/p925
ER  - 

TY  - JOUR
ID  - 1188
T1  - Motional capacitance of layered piezoelectric thickness-mode resonators
JF  - IEEE Transactions on  Ultrasonics, Ferroelectrics and Frequency Control
JA  - IEEE Trans. UFC
M3  - 10.1109/58.79604
A1  - Schmid,M.
A1  - Benes,E.
A1  - Burger,W.
A1  - Kravchenko,V.
VL  - 38
IS  - 3
PY  - 1991/05//
N2  - 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.
SP  - 199
EP  - 206
SN  - 0885-3010
UR  - http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=79604&tag=1
ER  - 

TY  - JOUR
ID  - 1072
T1  - Layered piezoelectric resonators with an arbitrary number of electrodes (general one-dimensional treatment)
JF  - Journal of the Acoustical Society of America
JA  - J. Acoust. Soc. Am.
M3  - 10.1121/1.401915
A1  - Nowotny,Helmut
A1  - Benes,Ewald
A1  - Schmid,Michael
VL  - 90
IS  - 3
PY  - 1991///
N2  - 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.
SP  - 1238
EP  - 1245
UR  - http://link.aip.org/link/?JAS/90/1238/1
ER  - 

TY  - JOUR
ID  - 113
T1  - Vibration modes of mass-loaded planoconvex quartz crystal resonators
JF  - Journal of the Acoustical Society of America
JA  - J. Acoust. Soc. Am.
M3  - 10.1121/1.401940
A1  - Benes,Ewald
A1  - Schmid,Michael
A1  - Kravchenko,Victor
VL  - 90
IS  - 2
PY  - 1991///
N2  - 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.
KW  - QCM
SP  - 700
EP  - 706
UR  - http://link.aip.org/link/?JAS/90/700/1
ER  - 

TY  - JOUR
ID  - 935
T1  - A computer-controlled system for the measurement of complete admittance spectra of piezoelectric resonators
JF  - Measurement Science and Technology
JA  - Meas. Sci. Technol.
M3  - 10.1088/0957-0233/1/9/021
A1  - Schmid,M
A1  - Benes,E.
A1  - Sedlaczek,R.
VL  - 1
IS  - 9
PY  - 1990///
N2  - 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.
SP  - 970
EP  - 975
SN  - 0957-0233
UR  - http://iopscience.iop.org/0957-0233/1/9/021/
ER  - 

TY  - JOUR
ID  - 859
T1  - Progress in monitoring thin film thickness by use of quartz crystals
JF  - Thin Solid Films
JA  - Thin Sol. Films
M3  - 10.1016/0040-6090(89)90907-3
A1  - Benes,E.
A1  - Schmid,M.
A1  - Thorn,G.
VL  - 174
IS  - Part 1
PY  - 1989/07//
N2  - The sensor function of high-accuracy quartz crystal thin film thickness monitors is determined by the properties of the unloaded crystal and the acoustic impedance zF of the film material. This material constant zF is often not precisely known, but it can be determined by the introduced two-frequency or Auto-Z-Match® technique which uses the frequencies of two different quasiharmonic modes of the crystal. In comparison with standard crystal thickness measurement techniques as well as with atomic absorption spectrometry and the conventional microbalance, the superiority of the Auto-Z-Match® technique is demonstrated for the evaporation materials copper, titanium, aluminium and lead. As the two-frequency technique allows the determination of the effective acoustic impedance of the film and thus thickness, determinations with higher accuracy are possible in the case of poorly known or unknown z-values. Other contributions to the practical performance and accuracy of quartz crystal thin film thickness monitors, such as the influence of crystal temperature, thin films stresses and the relaxation phenomenon, as well as the occurrence of frequency jumps, are discussed on the basis of the most recent research results.
KW  - QCM
SP  - 307
EP  - 314
SN  - 0040-6090
UR  - http://www.sciencedirect.com/science/article/B6TW0-46X43PT-GF/2/c82a43f3f0b5ebc0e67f86439cf4a7bd
ER  - 

TY  - CONF
ID  - 1129
T1  - Investigation of Quartz Crystal Thickness Shear and Twist Modes Using a New Noninterferometric Laser Speckle Measurement Method
M3  - 10.1109/FREQ.1985.200895
A1  - Hertl,S.
A1  - Benes,E.
A1  - Wimmer,L.
A1  - Schmid,M.
PY  - 1985///
N2  - The measurement of the mechanical vibration amplitudes of quartz crystals has been carried out by a variety of different methods. Interferometric methods firstly were used to investigate out-of-plane vibrations. Enhanced interferometric setups also allow the measurement of in-plane vibrations. Holographic techniques present an instantaneous picture of the whole object, whereas the interferometric methods supply the vibration amplitude distribution point after point. 

A very powerful technique to study amplitude distributios of quartz crystals is the use of X-ray diffraction. Using this method, many quartz-crystal vibration modes have been investigated so far. The measurement setup, however, is complicated and the resolution of the graphic output is rather poor. 

Looking for a simple mechanical setup that is insensitive to environmental disturbances, we developed a new method which allows the measurement of in-plane vibration amplitudes of quartz crystals. The detection range is about one to some hundred nanometers. Due to the use of a microcomputer that controls the whole measurement the graphic output can easily be adapted to the user's needs.
SP  - 535
EP  - 543
UR  - http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1537835
ER  - 

TY  - JOUR
ID  - 301
T1  - Reduced thickness of contamination layers determined from C 1s- and CKVV-lines
JF  - Journal of Electron Spectroscopy and Related Phenomena
JA  - J. Electron Spectrosc. Relat. Phenom.
M3  - 10.1016/0368-2048(84)80076-6
A1  - Ebel,Maria F.
A1  - Schmid,M.
A1  - Ebel,H.
A1  - Vogel,A.
VL  - 34
IS  - 3
PY  - 1984///
KW  - Carbon
KW  - XPS
SP  - 313
EP  - 316
SN  - 0368-2048
UR  - http://www.sciencedirect.com/science/article/B6TGC-44CH3Y1-MV/2/381a0a272b75874c9dbd7a8c9aaa5278
ER  -