Probing band structure renormalization in 2D semiconductors induced by external dielectric screening through angle-resolved photoemission spectroscopy

We investigate the effect of dielectric screening by the external environment on the electronic states in the two-dimensional (2D) semiconductor WS₂ using angle-resolved photoemission spectroscopy (ARPES). As has been previously reported from optical spectroscopy of monolayer WS₂ [1], an increase in environmental screening from the presence of graphene can lead to a ~100 meV reduction of the bandgap. Our ARPES study of monolayer WS₂ partially placed on h-BN and graphene, however, reveals that the dispersion of the valence band is essentially unchanged (< 10 meV shifts) when screening by graphene is present. Thus the screening-induced band renormalization appears to lead only to a rigid shift, rather than a restructuring of the valence band. We compare this experimental finding with theory based on GΔW calculations, including material-realistic frequency and momentum dependent screening due to the substrate. In agreement with experiment, theory predicts the primary effect of bandgap renormalization along with a scissor-like shift of all electronic states. We discuss the physical origin of this result and its implication for the use of controlled changes in the environment to tune the band structure of 2D semiconductors.
[1] A. Raja et al. Nature Commun., 8, 15251 (2017)