The electronic structure and energy level alignment of porphyrin/metal interfaces studied by ultraviolet photoelectron spectroscopy

Electronic structures of 5, 10, 15, 20-zinctetraphenylporphyrin (ZnTPP)/metal (Au, Ag, Al, Mg) interfaces prepared in ultrahigh vacuum were investigated by ultraviolet photoelectron spectroscopy (UPS). We found that the electronic energy levels of ZnTPP align to the vacuum level of substrate metal, with a constant energy shift of vacuum levels across the interface. These findings cannot be explained by the simple models assuming vacuum level alignment at the interface. We also found that sample exposure to oxygen induces energy level shift in close relation with change of substrate work function at oxygen exposure.