  Transition metal dichalcogenides (TMDs) are known to support complex
excitonic states. Revealing the differences in relaxation dynamics among
different excitonic species and elucidating the transition dynamics between
them may provide important guidelines for designing TMD-based excitonic
devices. Combining photoluminescence (PL) and reflectance contrast measurements
with ultrafast pump-probe spectroscopy under cryogenic temperatures, we herein
study the relaxation dynamics of neutral and charged excitons in a
back-gate-controlled monolayer device. Pump-probe results reveal quite
different relaxation dynamics of excitonic states under different interfacial
conditions: while neutral excitons experience much longer lifetime than trions
in monolayer WS2, the opposite is true in the WS2/h-BN heterostructure. It is
found that the insertion of h-BN layer between the TMD monolayer and the
substrate 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 at
the interface, but also help establish a non-radiative transition from neutral
excitons to trions to be the dominant relaxation pathway, under cryogenic
temperature. Our findings highlight the important role interface may play in
governing the transient properties of carriers in 2D semiconductors, and may
also have implications for designing light-emitting and photo-detecting devices
based on TMDs.
