Interaction of π-conjugated organic molecules with surfaces
Benjamin Stadtmüller, Christoph Kleimann, Ingo Kröger, Christoph Stadler, Christian Kumpf
and Ruslan Temirov, Sergey Subach, Stefan Tautz
Already for several years there is an increasing interest in the adsorption behavior of large π-conjugated molecules on different surfaces. These systems are relevant in the context of organic electronics since they represent prototypical systems on which the interaction between molecules and substrate surfaces can be studied. Usually these adsorbate systems show a variety of different phases, the properties of which can be tuned by the proper choice of molecules, substrates, preparation conditions, temperature, etc. In particular the interaction between the surface and the first molecular layer plays a dominant role since it determines the molecular orientation and the ordering in the first layer. This has further influence on the growth behavior of thin films as well as the electronic interface properties such as band offsets, interface dipole, and band bending. The stronger the interaction, the more distinctive is the substrate for the properties of the entire system. In the case of weak interaction, however, the bulk properties of the organic material tend to dominate.
The most prominent molecules in this context are NTCDA and PTCDA, but also pentacene, tetracene and phthalocyanines. In cooperation with other groups we have investigated these molecules since several years using high resolution spot-profile analysis low energy electron diffraction (SPA-LEED) and (normal-incidence) x-ray standing waves (NIXSW). The latter method is applied at beamline ID32 of the European Synchrotron Radiation source (ESRF) in Grenoble and has revealed very precise and fundamental information on molecular orientation, bonding distances, distortions of the molecules upon adsorption and site specific adsorption.
One prominent example is the investigation of NTCDA/Ag(111). In the NIXSW experiment we could distinguish the two different oxygen species and hence measure their vertical adsorption position separately. Fig 1 shows the O1s PES data. The double peak structure stems from the different O-species. In addition the height of the carbon was measured and indicated a significant distortion of the molecules upon adsorption on the Ag surface (see Fig. 2). It compares very well to other investigations of PTCDA on Ag(111), Au(111) and Cu(111).
In addition to this scientific aspect we also developed a self-consistent method that significantly improves the reliability of the NIXSW technique. This method is able to find a consistent result from two independent XSW data sets (PES- and Auger yield), which both have to be corrected for secondary effects (non-dipolarities in the PES process and electron-induced Auger decays). The correction is done in a recursive algorithm and (beside the scientific result of the adsorption height) determines the so called asymmetry parameter for PES and the electron-induced fraction for Auger.
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M. Sokolowski, Univ. Bonn,
F. Reinert, A. Schöll, Univ. Würzburg,
M. Rohlfing, Univ. Osnabrück,
W. Moritz, LMU München,
J. Zegenhagen, ESRF, Grenoble, France,
T.-L. Lee, Diamond Light Source, Oxfordshire, UK
DFG Ku 1531 2-1