Interfacial electronic structure of vanadyl naphthalocyanine on highly ordered pyrolytic graphite

We present a core and valence region spectroscopic analysis of the interfacial electronic structure of thin films of vanadyl naphthalocyanine (VONc) deposited onto highly oriented pyrolytic graphite (HOPG). X-ray photoelectron spectroscopy indicates the predominantly ionic character of the vanadyl metal center coordinated by the heterocycle and affords the bandgap in the thin VONc films. Valence band photoelectron spectroscopy points to the existence of three different adsorption geometries of VONc on the HOPG surface. The distribution of the different geometries can be systematically influenced in a simple post-deposition processing step, with an immediate effect on the interfacial electronic environment. We find spectroscopic evidence in the valence levels that VONc grows on HOPG most likely in a 2D-gas fashion rather than by nucleation and growth of islands. These data allow us to predict accurately the interface dipole in the case of a broad class of dipolar organic semiconductors, based simply on molecular dipole moment, polarizability and molecular diameter. This ability provides an important step towards rational optimization of energy level alignment in organic electronics.