Valley excitons: From monolayer semiconductors to moiré superlattices

Abstract

Group-VIB transition metal dichalcogenides have recently emerged as a novel class of two-dimensional semiconductors. Monolayers feature direct bandgap in the visible range, with band-edges at the two degenerate corners of the hexagonal Brillouin zone, known as valleys. The optical properties are dominated by tightly bound excitons formed at the valleys, which has intriguing properties associated with the valley degree of freedom. Stacking two different monolayers into van der Waals heterostructures is a powerful approach to tailor the optoelectronic properties, where interlayer excitons become the ground state configurations in the type-II band alignment. The inevitable lattice mismatch and twisting leads to the formation of moiré pattern which has profound effect on the exciton physics, introducing spatially-varying optical properties and energy landscape in the nanoscale. In this chapter we give an overview of the theoretical foundation and experimental progresses of the valley-related phenomena for excitons in monolayers, and heterobilayer moiré superlattices.