Electronic structure of PTCDA on Sn/Si(111)-3×3

Abstract

The electronic structures of PTCDA films on Sn/Si(111)-√3×√3 have been studied by high-resolution photoelectron spectroscopy (PES) and near-edge x-ray absorption fine structure (NEXAFS). There is a clear chemical interaction between PTCDA molecules and the Sn/Si(111)-√3×√3 surface, as indicated by new components in Sn 4d core levels that are shifted to higher binding energies. The chemical interaction is also evidenced by new components in the O-1s and C-1s core-level spectra. The new components are shifted to lower binding energies and have their origins from two reactions between Sn dangling bonds and the PTCDA molecules. One reaction is located at the anhydride O atom and the other is at the carbonyl C atom. These reactions also induce modifications in the HOMO and LUMO levels seen in the valence band and NEXAFS spectra. For thin films, the NEXAFS results suggest a tilting molecular configuration with respect to the substrate, while for the thicker films, there is an improved ordering of the molecular orientation to the substrate.