Charged excitons in monolayer WSe2: Experiment and theory

Charged excitons, or X-+/- trions, in monolayer transition-metal dichalcogenides have binding energies of several tens of meV. Together with the neutral exciton X-0 they dominate the emission spectrum at low and elevated temperatures. We use charge-tunable devices based on WSe2 monolayers encapsulated in hexagonal boron nitride to investigate the difference in binding energy between X+ and X- and the X- fine structure. We find in the charge-neutral regime, the X-0 emission accompanied at lower energy by a strong peak close to the longitudinal optical (LO) phonon energy. This peak is absent in reflectivity measurements, where only the X-0 and an excited state of the X-0 are visible. In the n-doped regime, we find a closer correspondence between emission and reflectivity as the trion transition with a well-resolved fine-structure splitting of 6 meV for X- is observed. We present a symmetry analysis of the different X+ and X- trion states and results of the binding energy calculations. We compare the trion binding energy for the n- and p-doped regimes with our model calculations for low carrier concentrations. We demonstrate that the splitting between the X+ and X- trions as well as the fine structure of the X(-)state can be related to the short-range Coulomb-exchange interaction between the charge carriers.