Optical spectroscopy of novel excitonic states and electronic phases in 2D semiconductors and moiré superlattices

Two-dimensional (2D) valley semiconductors, such as MoS2, MoSe2, WS2 and WSe2, host robust excitonic states with remarkable optical, optoelectronic, and valleytronic properties for novel applications. We investigate the electronic and excitonic states in the 2D semiconductors and moiré superlattices by optical spectroscopy. In the monolayer, we observe a panoply of excitonic states, including bright and dark, ground and excited excitonic states. When two monolayers are stacked together, the bilayer can host excitons with vertical dipole, which enables effective control of the excitons by an electric field. Remarkably, if the two layers have different lattice constants and/or twist angle, they can form moiré superlattices. The moiré superlattices can significantly modify the characteristics of electrons, excitons and trions, giving rise to moiré trions and numerous correlated electronic phases with distinctive optical signatures. These rich results motivate further experimental and theoretical research on the novel correlated phenomena in 2D systems.