Plenarvorträge - DPG-Tagungen

Oberflächenphysik Montag
zitätskonstanten widerspiegelt.
O 9.6 Mo 17:00 H36
Warum die ”shear force” Distanzregelung auch im UHV funktioniert
— •S. Hoppe, G. Ctistis, J.J. Paggel und P. Fumagalli
— Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee
14, 14195 Berlin
Die Messung der Kräfte, zwischen einer lateral oszillierenden Spitze
und der Substratoberfläche ist eine gängige Methode die Abstandskontrolle
eines Nahfeldmikroskops zu realisieren. Diese Methode gehört zwar
zum Standard, jedoch ist die Natur der Wechselwirkung weitgehend unbestimmt.
Obwohl die Spitze bei Messungen im Ultrahochvakuum in mechanischem
Kontakt zur Probe zu sein scheint, ist keine Beschädigung
der Probenoberfläche erkennbar. Die Kombination aus sogenannten Distanzkurvenmessungen
und Strommessung bietet die Möglichkeit, die
Wechselwirkung zwischen Probe und Spitze genauer zu untersuchen.
Mit Hilfe des Modells des harmonischen Oszillators werden Feder- und
Dämpfungskonstante bestimmt. Durch Messung des elektrischen Kontaktes
kann der Punkt der Probenberührung identifiziert werden. Die
Messungen wurden an metallischen Oberflächen durchgeführt. Diese Arbeit
wurde unterstützt durch die Deutsche Forschungsgemeinschaft im
Rahmen des SFB 290.
O 9.7 Mo 17:15 H36
Study of particle-substrate interaction by nanomanipulation experiments
with dynamic scanning force microscopy — •Claudia
Ritter 1 , Markus Heyde 2 , and Klaus Rademann 1 — 1 Humboldt-
Universität zu Berlin, Institute of Chemistry, Brook-Taylor-Str. 2, D-
12489 Berlin, Germany — 2 Fritz-Haber-Institute of the Max-Planck-
Society, Faradayweg 4-6, D-14195 Berlin, Germany
We utilise an advanced homebuilt SFM in the dynamic mode, in
conjunction with a special homebuilt software, to perform precise
nanomanipulation experiments. The corresponding experimental technique
should be denoted as Dynamic Surface Modification (DSM), comprising
both the dynamic technique of the SFM, as well as the manipulation
(translation, in-plane rotation, cutting) of structurally unchanged
particles on a given substrate surface. It is easily possible to switch between
imaging mode and DSM mode, enabling the direct manipulation
of nanoparticles under ambient conditions with high precision and simultaneously
studying particle-substrate interaction to give evidence about
motion and tribological values of the sample system. We have successfully
manipulated miscellaneous nanoparticles on surfaces, e.g. antimony
islands, gold islands, tin islands, nanotubes, small latex spheres as well
as cells.
O 9.8 Mo 17:30 H36
STM Imaging of PTCDA Multilayers with Submolecular Resolution
— •Daniel Braun, Andre Schirmeisen, and Harald
Fuchs — Physikalisches Institut and CeNTech, University of Muenster,
Wilhelm-Klemm Str. 10, 48149 Muenster, Germany
Organic semiconductors have attracted intensive research interest over
the last decade, ever since the demonstration of a low-voltage-powered
OLED [1]. Charge transport and luminescence properties are governed
by the structural properties of the thin films, like molecular aggregation,
packing and orientation [2]. Understanding and tuning the epitaxy
of large aromatic adsorbates by molecular design is a task, which has
attracted much attention in the last years [3].
We investigate the growth of the archetype molecular compound
PTCDA, a semiconducting organic molecule, using UHV-STM. On the
thin multilayer films we observe submolecular features of the PTCDA
not only from the top-layer but also from the next layer, allowing us to
study directly the quasi-epitaxial growth.
[1] C.W.Tang, S.A.VanSlyke, Appl.Phys.Lett.51 (1987) 913
[2] C.Seidel, A.Schaefer, H.Fuchs, Surf.Sci.459 (2000) 310
[3] M.Eremtchenko, J.A.Schaefer, F.S.Tautz, Nature 425 (2003) 602
O 9.9 Mo 17:45 H36
Damping mechanisms in dynamic force microscopy —
•Andre Schirmeisen, Hendrik Hölscher, and Harald Fuchs
— Physikalisches Institut und CeNTech, Universität Münster,
Wilhelm-Klemm-Str.10, 48149 Münster
Dynamic force microscopy (DFM) in ultrahigh vacuum (UHV) is a
powerful tool to measure interatomic forces with molecular resolution.
However, apart from conservative forces the DFM is also capable of measuring
dissipative tip sample interactions. A considerable dispute has
arisen, as to what the underlying physical mechanisms are for the observed
energy dissipation. Atomic instabilities, electric damping mechanisms
and even feedback artefacts have been argued to govern the dissipation.
We performed force and energy dissipation spectroscopy experiments
on HOPG in UHV, where our instrument is operated in two
different dynamic modes: The constant excitation (CE) and constant
amplitude (CA) mode. First, we show that spectroscopy measurements
from both modes yield equivalent quantitative results, which allows us
to exclude artefacts induced by the amplitude feedback system inherent
only to the CA mode. Secondly, we present a series of spectroscopy experiments
acquired with different oscillation amplitudes, which allows us
extract the velocity dependence of the dynamic friction coefficient. In
fact, we will show that the velocity dependence is negligible and we will
argue that hysteretic mechanisms based on atomic instabilities govern
the energy dissipation in our case.
O 9.10 Mo 18:00 H36
Observation of the complete graphite unit cell with a lowtemperature
atomic force microscope — •Stefan Hembacher 1 ,
Franz J. Giessibl 1 , Jochen Mannhart 1 , and Calvin F. Quate 2 —
1 Universität Augsburg, Lehrstuhl für Experimentalphysik VI, Zentrum
für Elektronische Korrelationen und Magnetismus — 2 Ginzton Laboratory,
Stanford University, Stanford CA 94305
A new helium-temperature scanning tunneling/dynamic force microscope
employing the qPlus sensor is introduced. First measurements on
HOPG (highly oriented pyrolytic graphite), where the benefits of combined
STM/AFM measurements at helium temperature are clearly evident,
are presented. At low temperatures, thermal drift is only of the
order of 25 pm/h enabling slow scanning in constant height mode. Because
the noise in ∆f measurements scales as B 3/2 , tiny forces can be
measured with good S/N ratio.
Graphite has a hexagonal structure with two atoms in the surface unit
cell. While the α-atoms have a neighbor directly underneath, the β-atoms
have no direct neighbor in the layer below the surface layer. In scanning
tunneling microscopy experiments, only the β-atoms are visible. In AFM,
with repulsive forces, both α- and β-atoms should appear. Simultaneously
recorded frequency shift and tunneling current images in constant height
mode show the α- and the β-atoms in the frequency shift channel, while
in the current channel only the β-atoms are observed.
O 9.11 Mo 18:15 H36
Atomic Force and Scanning Tunnelling Microscopy Measurements
at Low Temperatures — •Markus Heyde, Maria Kulawik,
Hans-Peter Rust, and Hans-Joachim Freund — Fritz-
Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195
Berlin, Germany
Atomic force and scanning tunneling microscopy (AFM/STM) are the
most important tools for the investigation of surfaces on the atomic scale
in real space. While the STM is sensitive to the local density of states
and requires a conductive surface, the AFM can be used also on insulating
samples. Essential for achieving atomic resolution with an AFM is
a force-detector with a low noise performance and enhanced sensitivity
to short-range forces. For a detailed analysis and interpretation of surface
structures, an image sensor with the capability to record AFM and
STM image at the same surface area is highly desirable. A double quartz
tuning fork sensor for low temperature ultra-high vacuum atomic force
and scanning tunneling microscopy is presented. The features of the new
sensor are discussed. In addition, a low temperature, low noise ac signal
amplifier has been developed to pick-up the oscillation amplitude of the
tuning fork. First atomic force measurements are shown, allowing for the
resolution of different domains on a thin Al2O3 film on NiAl(110).