Telling the difference between phonon-assisted and charged-exciton optical transitions in monolayer transition-metal dichalcogenides

The wide interest in monolayer transition-metal dichalcogenides (ML-TMDs) has led to numerous recent studies of their optical properties. Close examination of these studies show that in many cases, optical transitions are associated with charged-excitons (negative and positive trions) instead of being associated with phonon-assisted excitonic emission. This confusion was partly caused due to the similar energy scales of optical phonons and trion binding energies, as well as by the common belief that a gate voltage can only tune the intensity of the trion optical transition. In this brief talk, we will explain how the exciton-phonon interaction can also be tuned in gated structures of ML-TMDs, leading to a voltage-dependent Huang Rhys factor in these materials. We will discuss the important role of charged impurities and the regimes at which optical transitions are dominated by the exciton-phonon interaction or by trions. We will also discuss the differences between photoluminescence and reflectivity experiments in this context.