Hexa-peri-hexabenzocoronene on Ag(111): Monolayer/multilayer transition of molecular orientation and electronic structure

The molecular orientation and electronic structure of hexa-peri-hexabenzocoronene (HBC) deposited on Ag(111) was investigated in the mono- and multilayer regime by near edge X-ray absorption fine structure spectroscopy (NEXAFS), scanning tunneling and scanning force microscopy (STM and SFM), and ultraviolet photoelectron spectroscopy (UPS). We found that the molecular orientation changed as a function of coverage, exhibiting a transition from flat lying HBC in the monolayer to molecules being 43° inclined relative to the substrate plane in the multilayer. This structural transition was accompanied by a rigid shift of molecular energy levels by 0.3 eV toward higher binding energy between mono- and multilayer. In addition, broadening of the highest molecular levels by ca. 0.35 eV was observed in the multilayer, suggesting the formation of extended intermolecular energy bands.