Optical excitation and electron donation at the surface of MoS2

In this work, electronic structure variations of MoS2 under violet laser irradiation and potassium evaporation have been studied by angle-resolved photoemission spectroscopy (ARPES). By exposing violet laser on the fresh MoS2 surface, we found the shift of valence band to the higher binding energy of 140±10 meV. This behavior can be described by the optical excitation where electrons in the valence band are temporary pumped to the conduction band edge. By using potassium evaporation, the shift of valence band of 280±20 meV to higher binding energy as well as the emerging of conduction pockets at K point which are a result of electron donation from alkali metals. Another shift of valence band up to 40±10 meV have been observed after irradiating violet laser on the doped sample. The schematic band diagram which can be described the chemical shifts based on experimental ARPES data has been proposed. By using transport measurement, a variation of MoS2 electrical resistance under light irradiation supports the nonequilibrium electron donation induced by optical excitation.