Order/Disorder phenomena of sub-monolayer organic films
Ingo Kröger, Benjamin Stadtmüller, Christoph Kleimann, Christoph Stadler, Patrick Bayersdorfer, Christian Kumpf
The future development of organic electronic devices depends on the properties of organic thin-films and their interfaces with (metallic and insulating) substrates. The best results can be expected from well ordered films with large crystalline grains and "perfect" interfaces. Since thin film growth is dominantly influenced by the formation of the first molecular layer on the substrate (it represents a template for further growth), we put some special emphasis on the investigation of the adsorption of the molecules in the first layer. This is a result of the fine balance of molecule-substrate and molecule-molecule interaction. The latter usually is attractive due to van-der-Waals forces and causes island formation in the sub-monolayer regime. In thin films this results in relatively small crystalline grains.
The family of Metal-Phtalocyanine molecules behave differently when they adsorb on a Ag(111) surface. They exhibit repulsive intermolecular interaction which is mediated by the substrate and leads to a continuous rearrangement of the molecules when the coverage is increased. This was found in an extended series of high resolution low energy electron diffraction (SPA-LEED) experiments. Some results are shown in Fig. 1. It can be seen that the spots move continuously in the LEED pattern when the coverage is changed. The molecules always fill the surface terraces homogenously and hence do not form individual island or grains. The domain size corresponds to the terrace size of the substrate and hence depends only on the substrate surface properties.
The key for understanding the repulsive inter-molecular interaction is the charge transfer between molecule and substrate. From x-ray standing waves (XSW) measurements and simplified density functional theory (DFT) calculations we found strong indications for a significant overlap of molecular orbitals with the silver electronic states. The resulting exchange of electronic charge (donation/back-donation) is responsible for the repulsion since it leads to a competing effect between neighboring molecules for the charge transfer with the substrate (see Fig. 2). The effect was found for different molecules (SnPc, CuPc, TiOPc and H2Pc) on Ag(111) in different magnitude. By cooling an additional site-specific interaction with the substrate can be "switched on" which over-compensates the repulsion and leads to a rearrangement of the molecules into a commensurate pattern which (in the case of SnPc) consists in a mixed pattern of Sn-down and Sn-up oriented molecules. Hence, just by tuning the temperature (and/or the molecular coverage) this system can be switched from repulsive to attractive intermolecular interaction and back.
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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