h1

%0 Thesis
%A Stadtmüller, Benjamin
%T Study of intermolecular interactions in hetero-organic thin films
%V 61
%C Jülich
%I Forschungszentrum Jülich, Zentralbibliothek
%M RWTH-CONV-143514
%B Schriften des Forschungszentrums Jülich : Reihe Schlüsseltechnologien
%P VIII, 198 S. : Ill., graph. Darst.
%D 2013
%Z Zugl.: Aachen, Techn. Hochsch., Diss., 2013
%X 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.
%K Oberfläche (SWD)
%K Adsorption (SWD)
%K Grenzfläche (SWD)
%K Struktur (SWD)
%K Elektronenstruktur (SWD)
%K Photoelektronenspektroskopie (SWD)
%F PUB:(DE-HGF)11 ; PUB:(DE-HGF)3
%9 Dissertation / PhD ThesisBook
%U https://publications.rwth-aachen.de/record/209708