Plenarvorträge - DPG-Tagungen

Oberflächenphysik Donnerstag
with rates as high as 10 8 s −1 .
[1] M. Dürr et al., Science 296, 1838 (2002).
O 38.3 Do 16:15 H38
Simulation der Desorption von Wasserstoff von bimetallischen
PtRu-Oberflächen — •Thomas Diemant, Hubert Rauscher und
R. J. Behm — Abt. Oberflächenchemie und Katalyse, Universität Ulm,
D-89069 Ulm
Bimetallische PtRu-Systeme finden großen Einsatz als Anodenmaterial
in Niedertemperatur-Brennstoffzellen. Die Untersuchung der Adsorption
und Desorption von Wasserstoff von PtRu-Oberflächen ist deshalb
von besonderem Interesse. Wir haben hier die Desorption von Ptmodifizierten
Ru(0001)-Oberflächen untersucht, die mit pseudomorphen
Pt-Monolagen-Inseln bedeckt waren. Die Filme werden durch die Deposition
von bis zu einer Monolage Platin auf einem Ru(0001)-Einkristall
bei Raumtemperatur und anschließendem Erhitzen auf 900 K erzeugt.
Die Morphologie und Zusammensetzung dieser PtRu-Oberflächen ist aus
früheren STM-Studien bekannt. Durch Vergleich experimenteller TPD-
Daten mit Simulationen auf der Basis verschiedener Modelle wird versucht,
ein genaueres Szenario des Desorptionsverhaltens von diesen bimetallischen
Oberflächen zu entwickeln.
O 38.4 Do 16:30 H38
Nitrile-functionalized molecules adsorbed on Si(001)-2x1
— •Ralf Funke 1 , Stefan Kubsky 1 , Francois Rochet 2 ,
Sylvie Rangan 2 , Holger Stein 1 , and Ulrich Köhler 1 —
1 Experimentalphysik / Oberflächenphysik, Ruhr-Universität Bochum,
44780 Bochum, Germany — 2 Laboratoire de Chimie Physique Matière
et Rayonnement, Université Pierre et Marie Curie, Paris, France
A controlled adsorption behaviour of organic molecules on silicon is
essential for the deposition of organic layers as a first step for molecular
electronics. The adsorption of acrylonitrile and allylnitrile was studied
using STM at 300K and at 90K. These results were correlated with X-ray
photoemission and absorption spectroscopy data. Both molecules adsorb
in a non-dissociative mode. For low coverage a single adsorption site was
found. In the case of acrylonitrile the molecule bridges two adjacent silicon
dimer rows inducing a strong local dimer buckling in one of the rows.
Bias dependent images and MO calculations are used to determine the
adsorption geometry. Time resolved STM images at 90K show a flipping
between local configurations of the molecule on a time scale of seconds.
The four C-atoms of allylnitrile bond in a semicircle-like configuration
to the silicon atoms of two neighbouring dimers in one dimer row.
Whereas in the case of acrylonitrile no long range order in the adsorbate
layer was found, the allylnitrile layer shows a (2x2) periodicity. The above
results show that despite an identical functional CN-group in a molecule
the local bonding to the silicon surface can be completely different.
O 38.5 Do 16:45 H38
Compression of a silver adlayer on Pt single crystal electrode
— •Katrin F. Domke, Helmut Baltruschat , and Xaioyin
Xaio — Institut für Physikalische Chemie, Abteilung Elektrochemie,
Römerstrasse 164, 53117 Bonn
The effect of coadsorption of CO on an underpotentially deposited
(UPD) silver monolayer on Pt(111) and Pt(665) single crystals is investigated
by means of electrochemical scanning tunneling microscopy
(EC-STM) and cyclic voltammetry (CV) in 0.05 M sulfuric acid. Pure
electrochemical experiments suggest that coadsorbing CO onto Pt(111)
covered by a monolayer of Ag forces Ag atoms of the first UPD monolayer
into a second adlayer. The present STM studies reveal the formation of
Ag islands on Pt(111) and Pt(665) after coadsorption of CO. These small
clusters have a narrow size distribution centered around 10 nm diameter
and are of approximately 0.5 nm height. The desorption of the newly
formed second Ag UPD, the oxidation of CO, and the desorption of the
first Ag UPD can be detected in the corresponding CVs in three different
oxidation peaks. STM images recorded afterwards show the unchanged
Pt surfaces.
O 38.6 Do 17:00 H38
Metal diffusion on transition metal dichalcogenide surfaces —
•R. Kunz und R. Adelung — Chair for Multicomponent Materials, Faculty
of Engineering, Christian-Albrechts University, Kaiserstr. 2, 24143
Kiel
The large difference in condensation coefficient is an immanent property
of layered crystals like the transition metal dichalcogenide crystals
(TMDC) for many different adsorbates. It is well documented that after
metal evaporation on such surfaces in UHV, various structures can
be formed in a self organized processes. In order to understand why
the different structures occur, a systematic study of the growth parameters,
(nucleation, diffusion length, evaporated metal, influence of the
substrate-crystal), is necessary. We could show that in extreme cases (Cu
on metallic TaS2) diffusion length of more than 50 µm could be observed
combined with a nucleation probability of almost zero. In contrast, metal
diffusion (Cu) on the geometrically similar surfaces on the semiconducting
(WSe2) surface show much shorter diffusion length and no diffusion
limited aggregation (DLA) growth. We suggest a model to explain the
different diffusion behavior as a key to understand the growth of different
self organized structures on TMDC-crystals.
O 38.7 Do 17:15 H38
Adsorption geometry of Palladium Phthalocyanine (PdPc) on
Graphite: STM and LEED investigations — •T. G. Gopakumar,
M. Lackinger, F. Müller, and M. Hietschold — Chemnitz
University of Technology, Institute of Physics, Solid Surfaces Analysis
Group, D-09107, Chemnitz, Germany
Adsorption geometry of planar Palladium Phthalocyanine (PdPc)
molecules on natural graphite was studied using a variable temperature
STM and LEED. PdPc films were prepared on a freshly cleaved natural
graphite (0001) surface using Organic Molecular Beam Epitaxy (OMBE)
under UHV conditions at room temperature. It was observed that PdPc
molecules were adsorbed with their molecular planes parallel to the substrate
and forming highly ordered phases with quadratic lattices. Further
analysis to find the orientation of molecular super-lattice with respect to
the graphite lattice vectors shows a rotation of 10 degree. For a better
understanding of the submolecular contrast, the software package Gaussian
98 was used to calculate the electronic structure of isolated PdPc
molecules. The different STM contrast could be explained and electronic
analogies were found.
O 38.8 Do 17:30 H38
Stability of Pd Surface Oxides at Ambient Pressures — •M.
Todorova 1 , J. Rogal 1 , K. Reuter 1 , M. Scheffler 1 , E. Lundgren
2 , J. Gustafson 2 , J.N. Andersen 2 , and A. Stierle 3 — 1 Fritz-
Haber-Institut, Berlin — 2 Departement of Synchrotron Radiation Research,
University of Lund, Sweden — 3 Max-Planck-Institut für Metallforschung,
Stuttgart
Using density-functional theory we study and compare the oxide formation
at the Pd(111) and Pd(100) surfaces. We identify the stable
phases and determine their (T, p) stability range using the concept of
first-principles atomistic thermodynamics. The observed surface oxides,
which only vaguely resemble their bulk counterparts, represent the most
stable phase for a wide range of environmental conditions exceeding the
stability range of bulk PdO by far. The corresponding overall structure
of the surface phase diagram for the Pd(100) surface is in good agreement
with surface X-ray diffraction (SXRD) measurements in the pressure
range up to 1 bar. For T < 600K the comparison with the experimental
data additionally discerns a kinetic hindrance to the formation
of the bulk oxide, stabilizing the ( √ 5 × √ 5)R27 ◦ surface oxide even up
to ambient pressures.
O 38.9 Do 17:45 H38
Adsorption and interaction of methane and ethane on the stoichiometric
and oxygen rich RuO2(110) surface — •Undine Erlekam,
Ursula A. Paulus, Yuemin Wang, Peter Geng, Karl
Jacobi, and Gerhard Ertl — Fritz-Haber-Institut der Max-Planck-
Gesellschaft, Faradayweg 4-6, D-14195 Berlin
The RuO2(110) single crystal surface exposing coordinatively unsaturated
Ru (Ru-cus) and oxygen (O-bridge) atoms, have been proven to
be an efficient catalyst, e.g. for the oxidation of CO [1]. An O-enriched
RuO2(110) surface is obtained by dissociative adsorption of O2 on the
stoichiometric surface giving rise to weakly bonded O-cus atoms on top
of Ru-cus.
In this study we investigated the adsorption and interaction of low hydrocarbons
such as methane and ethane on the RuO2(110) surfaces both
the stoichiometric and the O-rich ones in order to determine whether
or not low hydrocarbons can be oxidized under UHV-conditions. Due
to the weak adsorption of low hydrocarbons (physisorption) the sample
was cooled with liquid nitrogen. The techniques applied are highresolution
electron energy-loss spectroscopy (HREELS) and thermal desorption
spectroscopy (TDS) in combination with isotopic labeling exper-