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Transition metal dichalcogenides (TMDs) are known to support complexexcitonic states. Revealing the differences in relaxation dynamics amongdifferent excitonic species and elucidating the transition dynamics betweenthem may provide important guidelines for designing TMD-based excitonicdevices. Combining photoluminescence (PL) and reflectance contrast measurementswith ultrafast pump-probe spectroscopy under cryogenic temperatures, we hereinstudy the relaxation dynamics of neutral and charged excitons in aback-gate-controlled monolayer device. Pump-probe results reveal quitedifferent relaxation dynamics of excitonic states under different interfacialconditions: while neutral excitons experience much longer lifetime than trionsin monolayer WS2, the opposite is true in the WS2/h-BN heterostructure. It isfound that the insertion of h-BN layer between the TMD monolayer and thesubstrate has a great influence on the lifetimes of different excitonic states.The h-BN flakes can not only screen the effects of impurities and defects atthe interface, but also help establish a non-radiative transition from neutralexcitons to trions to be the dominant relaxation pathway, under cryogenictemperature. Our findings highlight the important role interface may play ingoverning the transient properties of carriers in 2D semiconductors, and mayalso have implications for designing light-emitting and photo-detecting devicesbased on TMDs.

Controlling relaxation dynamics of excitonic states in monolayer transition metal dichalcogenides WS2 through interface engineering