Regensburg 2010 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 56: Metal substrates: Adsorption of organic / bio molecules VI
O 56.9: Vortrag
Mittwoch, 24. März 2010, 17:00–17:15, H36
Repulsion Between Molecules on a Metal: (Sub-) Monolayers of Hexa-peri-hexabenzocoronene on Au(111) — •Christian Wagner1,3, Daniel Kasemann1, Roman Forker1,2, and Torsten Fritz1,2 — 1Inst. für Angewandte Photophysik, TU Dresden — 2Inst. für Festkörperphysik, FSU Jena — 3Inst. für Bio- und Nanosysteme 3, FZ Jülich, JARA Fundamentals of Future Information Technology
We investigate the growth of hexa-peri-hexabenzocoronene (HBC) on Au(111) for monolayer (ML) and sub-ML coverage by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). A transition from a disordered isotropic phase at low coverage to a highly ordered phase with a coverage-dependent lattice constant at higher coverage is found and attributed to a repulsive intermolecular force. To deduce the origin of this repulsion a model is developed, containing the Coulomb and van der Waals interaction between the molecules. The metal substrate is accounted for by the inclusion of mirror charges and a dipole created by the pushback effect, while the van der Waals interaction is modeled on a force field level. The Coulomb repulsion is caused by the partially positively charged hydrogen atoms. The effect can, however, only be explained under the postulate of a screening of the attractive London forces by the metal substrate as a result of the non-additivity of dispersion forces. As the mechanisms described are rather universal they should be able to explain a similar repulsion observed for other hydrocarbons [1,2].
[1] C. Seidel et al., Phys. Rev. B 64, 195418 (2001)
[2] S. Müllegger and A. Winkler, Surf. Sci. 600, 1290 (2006)