Positively Charged Biexcitons in Monolayer WSe₂ in Type-I GaSe/WSe₂ van der Waals Heterostructures: Implications for the Biexciton Laser

Monolayer two-dimensional transition metal dichalcogenides (TMDCs) exhibit strong Coulomb interactions, which provide an ideal platform for the fundamental studies of many-body correlated excitonic states. As a typical representative of many-body problems, the neutral exciton and charged exciton have been widely studied. However, some high-order excitonic states involving more particles, such as four-body biexciton and five-body charged biexciton, remain elusive because of the requirement of a high exciton density. Herein, we present an observation of a five-particle positively charged biexciton in monolayer WSe2, which is rarely reported to the best of our knowledge, by achieving a high exciton density in the GaSe/WSe2 heterostructure with a type-I band alignment. The nature of the positively charged biexciton in monolayer WSe2 is confirmed by the studies of the excitation-power-dependent and gate-voltage-dependent photoluminescence measurements. The mechanism of formation of the positively charged biexciton is also elaborated in the view of the band alignment of the GaSe/WSe2 heterostructure. In the GaSe/WSe2 heterostructure with type-I band alignment, the excessive hole carriers transfer into WSe2 and facilitate the formation of the positively charged exciton and further facilitate the formation of the positively charged biexciton in WSe2. The influence of charge transfer on the excitonic properties in the type-I band heterostructure by funneling excitons into one layer is very different from that in most TMDCs van der Waals heterostructures with type-II band alignments in which the electrons and holes are confined in two separate layers. These investigations provide a better understanding of the high-order excitonic states in monolayer TMDCs and offer some basis for exploration of the related phenomena of the many-body effect and the biexciton laser based on van der Waals heterostructures.