Plenarvorträge - DPG-Tagungen

Symposium Organic and Hybrid Systems for Future Electronics Donnerstag
Fachsitzungen
– Haupt-, Fach-, Kurzvorträge und Posterbeiträge –
SYOH 1 Geometric Organic Structures on Surfaces
Zeit: Donnerstag 14:00–14:45 Raum: H37
SYOH 1.1 Do 14:00 H37
Self-assembled molecular architectures — •Elena Mena-
Osteritz and Peter Baeuerle — Organic Chemistry II. University
of Ulm. Albert-Einstein-allee 11 89081 ULM
Well-defined pi-conjugated macrocycles are of interest as modular
building blocks for the assembly of new materials and supramolecular
chemistry. They will play an essential role as key components for coming
nanoelectronic devices. However, the assembly of molecular materials
in nanoscale architectures will be a crucial step for the future molecular
scale electronics. On the other side due to their toroidal structure,
p-conjugated macrocycles could represent intriguing molecular circuits
which would additionally include sites for recognition and selective complexation.
Two years ago our group successfully achieved the synthesis of the first
class of fully conjugated macrocycles: the cyclo[n]thiophene. In this contribution
we will present the different 2D-arrangements of the macrocycles
at the liquid/HOPG interface re-vealed by in-situ scanning tunnelling
macroscopy (STM). The data will be analysed with the help of theoretical
conformational and MO analyses. By means of STM, we also investigated
epitaxy and interactions of C60-fullerenes with 2D crystalline monolayers
of cyclo[12]thiophene resulting in perfectly ordered 1:1 complexes.
STM-analyses of the nature, specificity and dynamics of the processes
taking place at the surface, supported by theoretical calculations, will be
discussed.
SYOH 1.2 Do 14:15 H37
Growth of perylene films on Cu(110) — •Kathrin Hänel, Sandra
Söhnchen, Thomas Strunskus, Gregor Witte, Alexander
Birkner, and Christof Wöll — Lehrstuhl für Physikalische Chemie
I, Ruhr Universität Bochum, Universitätstr. 150, 44780 Bochum
Organic semiconductors such as polycyclic hydrocarbons have attracted
a considerable amount of attention because of their promising
potential for electronic and optoelectronic devices. Of particular interest
for the fabrication of such devices is the molecular microstructure in het-
SYOH 2 Spectroscopy
erostructures of thin organic films deposited on inorganic substrates. Here
we present a comprehensive study of perylene (C20H12) films grown under
UHV conditions on Cu(110). The growth and structure of the films
were characterized by means of STM, LEED, HAS, TDS, XPS, NEX-
AFS. While the perylene molecules form an ordered monolayer with an
almost planar orientation, they continue to grow in an upright orientation
without a preferential azimuthal ordering. The multilayer films
reveal a pronounced tendency for dewetting leading to the formation of
descrete islands. The terrace structure of such islands could be imaged
by non contact AFM measurements under ambient condition and reveals
the presence of molecular steps.
SYOH 1.3 Do 14:30 H37
Spectro-microscopic study of organic film growth: PTCDA/
Ag(111) — •Th. Schmidt 1 , U. Groh 1 , H. Marchetto 2 , R. Fink 3 ,
and E. Umbach 1 for the SMART collaboration — 1 Exp. Physik II,
Universität Würzburg, 97074 Würzburg — 2 Fritz-Haber-Institut, 14195
Berlin — 3 Phys. Chemie II, Univ. Erlangen, 91058 Erlangen
We report on first dynamic growth studies of PTCDA on Ag(111) using
a PEEM with photon excitation by either UV-light or polarized monoenergetic
synchrotron radiation. The PEEM instrument is the SMART
spectro-microscope in the actual version, an energy-filtered, but not yet
aberration-corrected microscope. At elevated temperatures between 350
and 450 K we observe the layer-by-layer growth of two to three layers, followed
by the growth of three-dimensional islands (Stranski-Krastanov).
At room temperature a transition from Stranski-Krastanov to layer-bylayer
growth (Franck-van der Merwe) occurs. By laterally resolved NEX-
AFS spectroscopy and by switching the linear polarization of the light
one can clearly derive the molecular orientation as flat-lying in both, the
islands and the bilayer. Additionally, the circular polarization reveals a
natural circular dichroism for the PTCDA crystallites, which is unique
and unexpectedly large for these planar and non-chiral molecules. Funded
by BMBF under contract no. 05-KS1WW2-0.
Zeit: Donnerstag 14:45–15:45 Raum: H37
SYOH 2.1 Do 14:45 H37
Relation between optical transition energies and the transport
gap in α-PTCDA — •Reinhard Scholz and Thorsten U Kampen
— Institut für Physik, Technische Universität Chemnitz
Recently, the slowest photoluminescence (PL) channels in α-PTCDA
were assigned to pairs of oppositely charged molecules [1]. Based on detailed
model calculations of molecular dimers with time-dependent density
functional theory, we can determine the Stokes shift arising from the
internal deformation of the ionized molecules, resulting in an improved estimate
for the energy of the charge transfer states in the periodic crystal.
These findings are compared with the results of high-resolution photoemission
spectroscopy and electrical characterization techniques applied
to PTCDA films grown on n-doped GaAs(100) substrates. Depending
on the substrate treatment, the ionization potential of the GaAs surfaces
varies by more than 1 eV, resulting in a sign change of the interface dipole
within this interval. From a linear fit of the interface dipole vs. the electron
affinity of the substrate, an electron affinity of 4.12 ± 0.10 eV can
be deduced for the organic layer. This procedure allows for an estimate
of the energy offset between the LUMO of PTCDA and the conduction
band minimum of the substrate, and the resulting HOMO-LUMO gap of
2.44 - 2.55 eV is in agreement with the lower limit estimated from the
electrical measurements. The energetic difference between the observed
PL energy and the transport gap can be related to the energy required
for the separation of the two opposite charges to infinite distance.
[1] A. Yu. Kobitski, R. Scholz, H. P. Wagner, and D. R. T. Zahn, Phys.
Rev. B 68, 155201 (2003).
SYOH 2.2 Do 15:00 H37
Femtosecond near-field spectroscopy of semiconducting polymers
— •Kerstin Müller 1 , Christoph Lienau 1 , Dario Polli 2 ,
Giulio Cerullo 2 , and Guglielmo Lanzani 2 — 1 Max-Born-Institut
für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin —
2 Politecnico di Milano, Dipartimento di Fisica, 20133 Milano, Italy
During recent years, ultrafast spectroscopy has provided a wealth of
new insight into the dynamics of optical excitations of semiconducting
polymers and thus into the function of polymer-based optoelectronic devices.
So far from these studies little is known about spatial inhomogeneities
of optical nonlinearities in such systems. Such information is
crucial, however, for a detailed microscopic understanding of the optolectronic
properties of polymers, which are strongly influenced by effects
of disorder, e.g, fluctuations in size and orientation of polymer chains
on nanoscopic and mesoscopic length scales. Here, we introduce a novel
technique, femtosecond near-field pump-probe spectroscopy, for spatially
mapping the temporal dynamics of polymer optical nonlinearities on a
100 nm length scale with sub-picosecond resolution. We report first experimental
studies of the time- and space-resolved photo-induced absorption
of thin films of methyl-substituted poly(para-phenylene) ladder-type
polymers (m-LPPP) and of mixed phase segregated polyfluorene/dye