Interface charge-transfer induced intralayer excited-state biexcitons in graphene/WS₂ van der Waals heterostructures

Monolayer transition metal dichalcogenides (TMDCs) are an ideal platform for multi-carrier bound states, the excitons and trions of which have been well identified and investigated. However, the formation and identification of biexcitons with certain configurations are more complicated. Here, we report a strategy to generate the hole-trion bound state, i.e. excited-state biexcitons, in a graphene/WS₂ van der Waals heterostructure, the formation of which is attributed to the charge transfer and exciton dissociation at the hetero-interface. The biexciton nature is confirmed by excitation-power dependent, helicity-resolved, and time-resolved photoluminescence measurements. This hole-trion bound state features a thermal activation energy of ∼32 meV, rendering a stable excited-state biexciton emission up to 330 K. Moreover, the emission behavior of the excited-state biexcitons can be tuned by modifying the charge transfer process at the hetero-interface via electrostatic gating. Our results will benefit to further understanding the complex multi-carrier interactions in 2D semiconductors and related heterostructures.