Plenarvorträge - DPG-Tagungen

Oberflächenphysik Freitag
where Cu dissolution starts. At higher potentials thicker and still epitaxial
Au islands are emerging on the surface. The applied potential range
is although always well below the so-called critical potential, where the
passivation breaks down and massive Cu dissolution starts. AFM images
reveal a surface that is densely packed with Au islands of a homogeneous
size distribution. Only with longer timescales a more porous morphology
of the surface evolves.
O 46.2 Fr 11:30 H45
X-ray reflectivity of thin liquid films — •Robert Fendt 1 , Christian
Gutt 1 , Michael Sprung 1 , Tuana Ghaderi 1 , Oliver Seeck 2 ,
and Metin Tolan 1 — 1 Experimentelle Physik I, Universität Dortmund,
44221 Dortmund — 2 IFF, Forschungszentrum Jülich, 52425 Jülich
Physical properties of confined liquids may differ significantly from
those of the corresponding bulk liquids. The confinement of a liquid between
a hard wall and the liquid-gas interface on a nanometer thick region
is expected to alter the liquid structure drastically.
We investigated the structure of thin liquid heptane and heptanol films
(thickness of 2-12 nm) adsorbed on silicon substrates by means of x-ray
reflectivity. The aim of the experiment was to study (i) the liquid-solid
interface, (ii) the influence of substrate-adsorbate interaction and (iii)
the capillary wave spectrum of the films. The experiments were carried
out using a newly constructed sample cell which allows to cover a substrate
by a liquid film via gas adsorption. Experiments were carried out
at the W1 diffractometer at Hasylab, Hamburg and at Dortmund using
a laboratory diffractometer.
The results show that the density of thin liquid heptane films is well
described by a homogenous electron density and a capillary wave broadened
interface to the gas phase. In contrast thin films of the corresponding
alcohol heptanol show pronounced oscillations in the electron density
profile, indicating a layering of the molecules close to the hard wall.
O 46.3 Fr 11:45 H45
Nanofluidics: Viscous Dissipation in Layered Liquid Films —
•Thomas Becker and Frieder Mugele — Angewandte Physik, Universität
Ulm, Albert Einstein Allee 11, 89081 Ulm
Using a temperature-controlled 2D-imaging Surface Forces Apparatus
(SFA) we investigated the layer-by-layer collapse of molecularly thin films
of a model liquid between two atomically smooth substrates as a function
of temperature. The dynamics of the consecutive expulsion of four
molecular layers were found to slow down with decreasing film thickness
but showed no evidence for confinement introduced solidification. Using a
new hydrodynamic model, we show that the sliding friction of the liquid
layers on top of the solid substrates is approximately 35 times higher than
the mutual friction between adjacent liquid layers. The latter was independent
of film thickness and in close agreement with the bulk viscosity
[1]. [1] T. Becker and F. Mugele, Phys. Rev. Lett. 91 , p. 166104/1-4
(2003)
O 46.4 Fr 12:00 H45
Interactions Between Seeds and Silicon Nanorods Grown by
Thermal CVD — •Vladislav Spassov, Alan Savan, and Henry
Haefke — CSEM Swiss Center for Electronics and Microtechnology,
Inc., Rue Jaquet-Droz 1, CH 2000, Neuchatel, Switzerland
Growth of silicon nanorods or whiskers by thermal chemical vapor deposition
(CVD) often takes place by the vapor-liquid-solid (VLS) mechanism.
A catalytic seed, for example gold, liquefies on the surface of a
supporting substrate, decomposing the source material in the gas phase
and increasing the epitaxial growth rate of a crystal below it. In the case
of gold, the metal cap remains at the top surface, with the pure, crystalline
silicon nanorod growing up from below. While silicon is the most
widely-used semiconductor material used for the fabrication of electronic
devices to date, gold is normally avoided during device processing, because
it migrates rapidly into silicon and creates deep levels which can
degrade device performance. In this investigation, we use high resolution
transmission electron microscopy (HR-TEM) to examine the interface between
the seed and the Si nanorod. Further studies look for the presence
and placement of catalyst or catalyst-silicon alloy or eutectic mixtures in
or along the nanorod.
O 46.5 Fr 12:15 H45
Structural and electronic effects in the electrochemical properties
of UHV prepared bimetallic Pt/Ru(0001) electrodes
— •H.E. Hoster, E. Filonenko, and R.J. Behm — Abteilung
Oberflächenchemie und Katalyse, Universität Ulm, 89069 Ulm
Combining UHV-STM with cyclic voltammetry in an electrochemical
prechamber, we have investigated the adsorption of H and OH in HClO4
solution on two different, well-defined types of bimetallic Pt/Ru(0001)
surfaces. On pure Ru(0001) the removal of OHad in the cathodic scan,
together with the simultaneous adsorption of Hupd, and its counterpart
in the reverse scan direction exhibit a significant hysteresis. In the presence
of vapor deposited Pt monolayer islands, these processes proceed
much more rapidly in a couple of sharp peaks around 100 mV RHE,
with no apparent hysteresis. We explain the enhanced H adsorption by
a kinetic promotion effect, including facile hydrogen adsorption on the
Pt islands and subsequent spill-over to the Ru areas, where the adsorbed
hydrogen can react with OHad. In the anodic scan, the Pt sites in turn
allow for a faster oxidation of H adsorbed at Ru. On atomically smooth
PtxRuy/Ru(0001) surface alloys, which are formed by subsequent annealing
of the Pt covered substrate to higher temperatures, adsorption of OH
and H are again modified significantly. Comparing results for different Pt
surface contents, where the concentration and ensemble size distribution
of the Pt surface atoms are determined by atomically resolved STM images
with chemical contrast, allows conclusions on the effect of specific
adsorption ensembles and adsorption sites.
O 46.6 Fr 12:30 H45
Adsorption of lead on copper surfaces — •Ari P Seitsonen
und Jürg Hutter — Physikalisch Chemisches Institut der Universität
Zürich
Adsorption of Pb on Cu(100) and Cu(111) leads to a variety of interesting
phenomena. We use molecular dynamics methods to simulate
the Pb monolayer on the copper surfaces, employing both density functional
theory (DFT) to describe the electronic wave functions explicitly,
and embedded atom method (EAM) for an empirical, fitted interaction
between the atoms. We study the melting of the overlayer, and perform
simulations at various different temperature regions.