Tracing the initial state of surface oxidation in black phosphorus

Black phosphorus has emerged as a class of two-dimensional semiconductors, but its degradation caused by surface oxidation upon exposure to ambient conditions has been a serious issue. A key to understanding the mechanism of surface oxidation is the initial-state structure that has remained elusive. We study the initial state of surface oxidation in black phosphorus by low-temperature core-level photoelectron spectroscopy with the in situ dosage of O-2 in the ultrahigh-vacuum condition. Our high-resolution P 2p core-level spectra show two clearly distinct initial-state components of P atoms that have one and two neighboring O atoms, respectively. It is followed by the rapid growth of other higher binding-energy components originating from incomplete P2O5 bonded to black phosphorus with one or two less bonds to O atoms. The variation in the proportion of these components reveals the initial-state structure of dissociative adsorption and its evolution to the final form of phosphorus oxides.