Increase of trion and charged biexciton photoluminescence quantum yields by above-gap excitation in single-layer WSe2

Two-dimensional (2D) semiconductors stand at the forefront of a wide breadth of areas in nanoscale and quantum optoelectronics. In these materials, strong optical responses are married to many-body systems with intense coulombic interactions and a rich suite of excitonic phenomena. Here, enabled by meticulous excitation spectroscopies, we show that the photoluminescence quantum yields of charged biexcitons and trions in single-layer WSe2 are enhanced by up to 12× as the excitation energy is swept from below to above the quasiparticle bandgap. Insight into the origin of the enhancement is gained by power- and temperature-dependent measurements, elucidating key mechanisms including the formation of KQ excitons, direct formation of trions and biexcitons from an electron-hole gas/plasma, and defect-trapping. This dataset reveals a complexity of excited state relaxation in 2D semiconductors and highlights the potential to modulate excitonic yields and probe out-of-equilibrium physics with 2D semiconductors.