  Platinum diselenide (PtSe_2) field-effect transistors with ultrathin channel
regions exhibit p-type electrical conductivity that is sensitive to temperature
and environmental pressure. Exposure to a supercontinuum white light source
reveals that positive and negative photoconductivity coexists in the same
device. The dominance of one type of photoconductivity over the other is
controlled by environmental pressure. Indeed, positive photoconductivity
observed in high vacuum converts to negative photoconductivity when the
pressure is rised. Density functional theory calculations confirm that
physisorbed oxygen molecules on the PtSe_2 surface act as acceptors. The
desorption of oxygen molecules from the surface, caused by light irradiation,
leads to decreased carrier concentration in the channel conductivity. The
understanding of the charge transfer occurring between the physisorbed oxygen
molecules and the PtSe_2 film provides an effective route for modulating the
density of carriers and the optical properties of the material.
