Enhanced multiexciton formation by an electron-hole plasma in 2D semiconductors*

Transition metal dichalcogenide semiconductors are layered van der Waals materials that exhibit exceptional optoelectronic properties in monolayer form. Their atomically thin nature and reduced long-range dielectric screening make them ideal systems in which to study a rich suite of many-body electronic states that emerges from intense coulomb interactions between quantum-confined charge carriers in a truly 2D system. Using photoluminescence action spectroscopy of monolayer WSe2, we find an enhancement of multiexciton formation with increasing excitation energy. This enhancement is attributed to the formation of excitons from a high-energy electron-hole plasma and generates 200% more multiexciton states than lower-energy excitation. By measuring the enhancement effect in multiple samples with varying doping levels, we observe increased enhancement of the charged biexciton with increased doping and observe a point of maximal charged biexciton generation at an elevated temperature, which we hypothesize arises from donor ionization. Furthermore, the onset of the enhancement coincides with the energy of the quasiparticle bandgap, corroborating the role of the electron-hole plasma and highlighting how the formation of excited states can be uniquely manipulated in 2D semiconductors. Understanding these formation and relaxation dynamics of the rich manifold of exciton states is critical for leveraging this new class of 2D semiconductors for advanced technologies.