Interface Properties of VOPc on Ni(111) and Graphene/Ni(111): Orientation-Dependent Charge Transfer

Interface properties of VOPc films on Ni(111) and graphene/Ni(111) were investigated by X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and X-ray absorption spectroscopy (XAS). The XAS spectra reveal that the molecules grow highly ordered on both substrates, flat lying on the substrate surface. On Ni(111), strong interactions between vanadium ions of the VOPc molecules and the substrate were observed. It seems, however, that not all molecules of the first monolayer interact with the substrate. Scanning tunneling microscopy (STM) suggests that the strength of the interaction depends on the molecular orientation (oxygen-up or oxygen-down). The interaction can be completely prevented by graphene as an intermediate layer. Graphene affects not only the strength of the interaction at the interface, but also the energy level alignment.