Plenarvorträge - DPG-Tagungen

Oberflächenphysik Montag
sured for pure Au(111) as well as for the surface covered with a monolayer
Kr and Xe and it is analyzed with an extended Kronig-Penney model for
the 23 × √ 3 reconstruction. The first rare gas monolayer induces energetic
shifts of 50 to 150 meV increasing with the gas atomic number,
whereas a second monolayer results only in small additional shifts of 3
to 18 meV. Using an image potential model it is possible to characterize
these shifts qualitatively. For a semiquantitative analysis we use the
phase accumulation model.
O 14.20 Mo 18:00 Bereich C
Diffusion pathways of hydrogen across Si(001) double-height
steps — •M. Lawrenz, C. Schwalb, M. Dürr, and U. Höfer —
Fachbereich Physik, Philipps-Universität, D-35032 Marburg
Steps can play an important role for macroscopic diffusion processes
at surfaces. An interesting system to study diffusion across steps at the
atomic scale is hydrogen on vicinal Si(001). At room temperature, molecular
hydrogen dissociates selectively at the threefold coordinated DB-step
sites. The adsorption takes place on adjacent Si atoms aligned with the
dimer rows of the upper terrace. Surfaces consisting of long hydrogen
chains at steps and of clean terraces can be prepared easily in this way.
They provide a well-defined starting configuration for imaging hydrogen
diffusion with a variable-temperature STM in real time.
Hydrogen was found to diffuse across the steps and along the dimer
rows at temperatures in excess of 550 K. The hopping rates for different
pathways could be determined from successive scans of the same
300˚A×300˚A region. At 570 K, the rate for diffusion from the steps
sites to the dimers of the upper terrace is 2.2 × 10 −3 s −1 which is comparable
to the rate of intra-row diffusion on the terraces. In contrast,
the diffusion rate to the lower terrace is only 1 × 10 −4 s −1 . At the first
dimer of the upper terraces hydrogen has a preference to diffuse back
to the step sites. The overall step depletion rate could also be determined.
Its value of 1.25 × 10 −4 s −1 at 570 K is in good agreement with
Rdiff = 10 11 s −1 exp(−1.7 eV/kT) as measured previously with optical
second-harmonic generation [1].
[1] M. B. Raschke and U. Höfer, Phys. Rev. B 59, 2783 (1999)
O 14.21 Mo 18:00 Bereich C
Supramolecular solid-solid wetting: A locally guidable selfassembly
of organic semiconductor molecules — •Frank
Trixler 1 , Thomas Markert 1,2 , Ferdinand Jamitzky 1,3 , Axel
Gross 2 , and Wolfgang M. Heckl 1 — 1 Department für Geo- und
Umweltwissenschaften & Center for NanoScience, Ludwig-Maximilians-
Universität, 80333 München, Germany — 2 Physik-Department T30,
Technische Universität München, 85747 Garching, Germany —
3 Centre for Interdisciplinary Plasma Science, Max-Planck-Institut für
extraterrestrische Physik, 85748 Garching, Germany
An easy way to deposit unmodified organic semiconductor molecules
onto substrates and the local control of supramolecular self-assembly
for the realization of designed nanoscopic structures should be important
factors in a future supramolecular nanoelectronics. We show that
these requirements can easily be achieved by a process which we describe
as ” supramolecular solid/solid wetting“: Solid nanocrystals of organic
semiconductors behave similiar to liquid droplets when they get contact
to a crystal surface; this results in an one-dimensional self-assembly of
supramolecular nanowires driven by wetting. We demonstrate that this
self-assembly process is locally guidable at the nanometre scale and allows
to tap the full potential of supramolecular chemistry such as reversibility
and self-healing for organic nanoelectronics.
O 14.22 Mo 18:00 Bereich C
Self-assembly on Surfaces: Supramolecular [60]Fullerene/Porphyrin
Assemblies on Metals — •Hannes Spillmann 1 , Andreas
Kiebele 1 , H.-J. Güntherodt 1 , Thomas Jung 1,2 , Davide Bonifazi
3 , and Francois Diederich 3 — 1 Department of Physics, University
of Basel, Klingelbergstr. 82, CH-4056 Basel, Switzerland — 2 Paul
Scherrer Institute, CH-5232 Villigen, Switzerland — 3 Laboratory of Organic
Chemistry, ETH Zürich, Wolgang-Pauli-Str. 10, CH-8093 Zürich,
Switzerland
The unique electrochemical and photophysical properties of porphyrin
and [60]fullerene compounds makes them promising candidates for the
construction of two- and three-dimensional organic-based materials. An
important question is how pristine C60 (and C70) and self-assembled
monolayer of porphyrin arrays will organize on surfaces. Self-assemblies of
electron-rich flat aromatic molecules such as porphyrins shall enable the
selective formation of electron donating monolayer, which can be covered
by pristine C60. The chromophore interaction between the electron donor
(porphyrin) and acceptor (C60) should enhance a supramolecular multilayer
structure. Herein, we report first Scanning-Tunneling-Microscopy
investigations of the behaviour of several porphyrin derivatives on metal
surfaces wherein single porphyrin molecules are forcefully arranged in
self-assembled monolayer. The specific properties of the single porphyrin
derivatives make such molecularly modified surfaces suitable candidates
for patterned surfaces to allocate fullerenes. Consequently, first observations
of the interaction of the fabricated porphyrin-based assemblies with
fullerene molecules will be presented.
O 14.23 Mo 18:00 Bereich C
Tunnelling voltage dependent submolecular contrast and degenerate
LUMO of Naphthalocynines (Nc) on natural graphite
— •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
Phthalocyanines (Pc) are an interesting family of organic dye molecules
due to their structural analogy with important bio-molecules like porphyrin,
and also their applications in optoelectronic devices. We studied
the tunneling voltage dependancy of images of Nc monolayer deposited on
freshly cleaved natural graphite (0001) substrate using Organic Molecular
Beam Epitaxy (OMBE) under UHV conditions. The planar Nc molecules
formed monolayers with their molecular plane parallel to the substrate.
For the submolecular contrast we have observed a clear dependency on
tunnelling voltage especially at the centre of molecule. Parallel theoretical
calculations using Restricted Hartree-Fock (RHF) and Density Functional
Theory (DFT) reveal that this effect is due to the difference in
symmetry of molecular orbitals near the Fermi level of molecule. From
some single molecule STM images taken at a positive sample bias, we
have seen the fine structures of unfilled orbitals, which could be explained
only by the superposition of degenerate LUMO and LUMO+1 from the
calculation, which are mainly comprised of pi-electrons.
O 14.24 Mo 18:00 Bereich C
Aufbau von supramolekularen Ensembles an fest/flüssig Grenzflächen
— •Caroline Safarowsky 1 , Alexander Rang 2 , Christoph
Schalley 2 , Klaus Wandelt 1 und Peter Broekmann 1 —
1 Institut für Physikalische und Theoretische Chemie, Universität Bonn,
Wegelerstr. 12, 53115 Bonn — 2 Institut für Organische Chemie und Biochemie,
Universität Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn
Vorgestellt werden in-situ STM-Ergebnisse zum Phasenverhalten von
kationischen, organischen Molekülen auf einer chloridbedeckten Cu(100)-
Elektrodenoberfläche. Befindet sich die Kupferprobe im Kontakt mit einem
salzsauren Elektrolyten, so bildet sich eine c(2×2)-Cl Struktur, die
als anionisches Templat für die kationischen Moleküle dienen kann. Die
supramolekularen Aggregate können dabei entweder durch Selbstorganisation
kleiner Monomere (Dibenzylviologen) oder durch die Adsorption
großer metallorganischer Komplexe (Fujita-Square) gebildet werden.
Es entstehen molekulare Kavitäten, in die prinzipiell Gastmoleküle eingeschlossen
werden können. Die supramolekularen Quadrate sowie das
Dibenzylviologen ordnen sich quadratisch an und folgen damit der Symmetrie
der c(2×2)-Cl Struktur.
O 14.25 Mo 18:00 Bereich C
Investigation of perylene derivatives with STM — •Markus
Wahl 1 , Meike Stöhr 1 , Michael De Wild 1 , Hans-Joachim
Güntherodt 1 , Christian Galka 2 , Lutz Gade 2 , and Thomas
Andreas Jung 1,3 for the collaboration — 1 Institute of Physics,
University of Basel, Klingelbergstr. 82, 4056 Basel, Switzerland —
2 Laboratoire de Chimie Organometallique et de Catalyse, Institut Le
Bel, Universite Louis Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg,
France — 3 Laboratory for Micro- and Nanostructures, Paul Scherrer
Institut, 5232 Villigen, Switzerland
In the last years perylene derivatives have attracted much attention
because of possible and already existing applications in electronic and
optoelectronic devices. The derivative (4,9-diaminoperylenequinone-3,10diimine
= DAPQDI) we investigated belongs to a class of compounds
which is cited very often in the patent literature for photovoltaic devices.
From differential thermoanalysis and gravimetry it is known that the
DAPQDI can polymerize. With UHV-STM experiments we tried to figure
out if this polymerization can also take place on surfaces. Therefore,
thin films in the monolayer regime were prepared by OMBE (Organic
Molecular Beam Epitaxy) on Ag(111) and Cu(111) substrates. Additionally,
the samples could be annealed. In a first step, the arrangement of