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O: Oberflächenphysik

O 14: Postersitzung (Adsorption an Oberflächen, Epitaxie und Wachstum, Organische Dünnschichten, Oxide und Isolatoren, Phasenübergänge, Rastersondentechniken, Struktur und Dynamik reiner Oberflächen)

O 14.23: Poster

Montag, 8. März 2004, 18:00–21:00, Bereich C

Tunnelling voltage dependent submolecular contrast and degenerate LUMO of Naphthalocynines (Nc) on natural graphite — •T. G. Gopakumar, M. Lackinger, F. Müller, and M. Hietschold — Chemnitz University of Technology, Institute of Physics, Solid Surfaces Analysis Group, D-09107, Chemnitz, Germany

Phthalocyanines (Pc) are an interesting family of organic dye molecules due to their structural analogy with important bio-molecules like porphyrin, and also their applications in optoelectronic devices. We studied the tunneling voltage dependancy of images of Nc monolayer deposited on freshly cleaved natural graphite (0001) substrate using Organic Molecular Beam Epitaxy (OMBE) under UHV conditions. The planar Nc molecules formed monolayers with their molecular plane parallel to the substrate. For the submolecular contrast we have observed a clear dependency on tunnelling voltage especially at the centre of molecule. Parallel theoretical calculations using Restricted Hartree-Fock (RHF) and Density Functional Theory (DFT) reveal that this effect is due to the difference in symmetry of molecular orbitals near the Fermi level of molecule. From some single molecule STM images taken at a positive sample bias, we have seen the fine structures of unfilled orbitals, which could be explained only by the superposition of degenerate LUMO and LUMO+1 from the calculation, which are mainly comprised of pi-electrons.