Bonding Geometry of Large Pi-Conjugated Molecules on Insulator Surfaces

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We want to determine the bonding geometry (adsorption site plus internal structural modifications) of large organic pi-conjugated molecules on surfaces of ionic insulating materials (dielectrics). In contrast to metal surfaces, very little quantitative information is available on the bonding geometries of large molecules on insulator surfaces so far. This information is essential for a fundamental understanding of molecular bonding on ionic wide bandgap materials in general. It will further allow for testing density functional theory (DFT) calculations performed for these systems against experimental data, which is not trivial since the relevant van der Waals interactions are presently described in DFT by different empirical corrections. In addition, the structural data obtained in this project will help to interpret results from many other experiments on molecules on ionic surfaces (e.g., by STM, photoemission, or optical spectroscopy). Particular interest is on the adsorption of the model molecule PTCDA on the (100) surfaces of different alkali halides (AH), namely, NaCl and KCl. Our questions are: What are the adsorption sites? What is the role of the Coulombic interactions between partially charged molecular functional groups and surface ions? How strongly do these interactions distort the originally planar molecule, and how does this bonding comply with that on metal surfaces? Concerning the methodology, the project will use the intensity versus energy analysis of low energy electron diffraction (LEED-IV) and normal-incidence x-ray standing waves (NIXSW) in combination with thin epitaxial films of the AH. The application and development of the LEED-IV technique will be conducted together with Prof. Dr. G Held (University of Reading/GB). Applying LEED-IV to surface structures of large organic molecules requires further development of this method and benchmarking using earlier results from NIXSW. This project is based on results of the project: Vertical bonding geometry of pi-conjugated molecules on surfaces (So407/6-1) of the applicant, which will be described.