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@PHDTHESIS{Stadtmller:209708,
author = {Stadtmüller, Benjamin},
othercontributors = {Kumpf, Christian},
title = {{S}tudy of intermolecular interactions in hetero-organic
thin films},
volume = {61},
address = {Jülich},
publisher = {Forschungszentrum Jülich, Zentralbibliothek},
reportid = {RWTH-CONV-143514},
series = {Schriften des Forschungszentrums Jülich : Reihe
Schlüsseltechnologien},
pages = {VIII, 198 S. : Ill., graph. Darst.},
year = {2013},
note = {Zugl.: Aachen, Techn. Hochsch., Diss., 2013},
abstract = {The interest in organic semiconductors is based on their
great potential to serve as active materials in electronic
devices such as organic light-emitting diodes or organic
photovoltaic cells. The performance of these molecular
assemblies does not only depend on the properties of the
organic bulk materials but also on the interfaces formed by
the contact between different materials. Therefore, the
physical properties of interfaces between metal contacts and
organic materials have been studied in detail by adsorbing
aromatic prototype molecules on highly crystalline surfaces
which resulted in a comprehensive understanding of the
intermolecular interactions between equal types of
molecules. In contrast, only a few aspects of the
interaction between different types of molecules have been
revealed yet. This is surprising since, i.e.,
organic-organic interfaces play a major role for the
efficiency of organic electronics. In order to tackle this
gap in our knowledge, this work presents a systematic study
of the fundamental interaction mechanisms in hetero-organic
thin films containing the prototype molecules
3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and
copper-II-phthalocyanine (CuPc) adsorbed on the
(111)-oriented silver surface (Ag(111)). The investigation
of laterally mixed hetero-organic monolayer films give an
insight into the direct and substrate mediated
intermolecular interactions between CuPc and PTCDA. On the
other hand, vertically stacked bilayer films allow to
determine the intermolecular interactions in vertical
stacking direction. CuPc and PTCDA are most suited for this
study since they have been studied extensively on the
Ag(111) surface. In addition, they show a different growth
behavior on Ag(111). While the attractive interaction
between PTCDA molecules leads to a clustering of molecules
on the surface and to the formation of islands, the
repulsion between CuPc molecules results in a diluted 2D gas
phase with a continuously increasing molecular density with
rising coverage. A comprehensive characterization of the
interactions in hetero-organic thin films was achieved by a
combination of various experimental approaches. The
geometric structure of these films was determined by low
energy electron diffraction (LEED), scanning tunneling
microscopy (STM) and the X-ray standing waves technique. The
electronic structure of the individual molecular species was
accessed by angle resolved photoemission spectroscopy
(ARPES) which was combined with the orbital tomography
approach. For the laterally mixed CuPc and PTCDA monolayer
films on Ag(111), the attractive electrostatic interaction
between both types of molecules results in the formation of
three ordered phases each having a different ratio of CuPc
to PTCDA. The structural parameters of these hetero-organic
films can be tuned by changing the relative coverage of the
molecules on the surface. Consequently, this allows to
control the lateral order of these heteromolecular films.
Furthermore, an electronic coupling between CuPc and PTCDA
could be revealed which results in an effective charge
transfer from CuPc to PTCDA as well as in a leveling of the
adsorption heights of both molecular species above the
surface. This coupling between CuPc and PTCDA is attributed
to an unoccupied hybrid state expanding over all molecules
in the hetero-organic structure. In the subsequent part of
this work, the vertically stacked bilayer films of CuPc and
PTCDA adsorbed on Ag(111) are discussed. For the adsorption
of CuPc on PTCDA, a smooth interface is formed between the
CuPc and the PTCDA layer. On PTCDA, CuPc behaves like a 2D
gas for a coverage below the first closed layer. A phase
transition to an ordered phase can be observed when closing
the first CuPc layer at room temperature or when decreasing
the sample temperature below 160K. The vertical bonding
distance between the organic layers is comparable to the
lattice spacing for molecular crystals and hence points to a
mainly van der Waals and electrostatic interaction across
the CuPc/PTCDA interface. Most remarkable, however, the
chemical bond between PTCDA and the Ag(111) surface is
modified by the adsorption of CuPc. This leads to an
altering of the adsorption height of PTCDA on Ag(111) and to
an enhanced charge transfer into the PTCDA LUMO. The latter
is attributed to an additional screening effect induced by
the CuPc layer. For the reversed hetero-organic bilayer
system PTCDA on a monolayer CuPc on Ag(111), no smooth
interface is formed. Instead, PTCDA replaces CuPc molecules
and forms a disordered mixed film with CuPc. This finding
can be correlated with the intermolecular interactions in
the CuPc and PTCDA monolayer film and the interaction
strength of these molecules with the Ag(111) surface.},
keywords = {Oberfläche (SWD) / Adsorption (SWD) / Grenzfläche (SWD) /
Struktur (SWD) / Elektronenstruktur (SWD) /
Photoelektronenspektroskopie (SWD)},
cin = {130000 / 134110},
ddc = {530},
cid = {$I:(DE-82)130000_20140620$ / $I:(DE-82)134110_20140620$},
shelfmark = {73.63.-b * 68.43.h * 79.60.Dp * 68.49.Uv * 61.05.jh},
typ = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
urn = {urn:nbn:de:hbz:82-opus-45768},
url = {https://publications.rwth-aachen.de/record/209708},
}
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