Influence of the morphology on the electronic structure of hexa-peri-hexabenzocoronene thin films

Ultrathin films of hexa-peri-hexabenzocoronene (HBC), prepared in ultrahigh vacuum by sublimation and studied using a combination of atomic force micr oscopy, X-ray absorption spectroscopy, and photoelectron spectroscopy were interpreted with the help of results of quantum chemical calculations. The important role of the nature of the surface of the substrate as well as the effect of postdeposition annealing on the morphology and the subsequent el ectronic structure were studied. In particular, on the (0001) surfaces of h ighly oriented pyrolitic graphite (HOPG) or molybdenum disulfide (MoS2), th ere is a high degree of molecular order induced by the crystal structure of the substrate surface. When deposited at room temperature on HOPG, the mol ecules are epitaxially ordered, while crystals from the epitaxially ordered films nucleate and grow during heating. On oxidized silicon(001) or polycr ystalline gold surfaces, only totally disordered molecular solid films are formed. The epitaxial films assume a layered structure and exhibit a graphi te-like electronic structure. In particular, the relationship between elect ronic structure and topology is discussed. The results of these studies ind icate methods for the preparation of the crystalline seeds to molecular wir es.