Electrical and Chemical Tuning of Exciton Lifetime in Monolayer MoS2 for Field-Effect Transistors

We report the room temperature tuning of excitonic lifetime in pristine and hole-doped monolayer MoS2 based field effect transistor (FET) devices by systematically controlling the free carrier density. We observed that in pristine MoS2 devices, with intrinsic electron doping, an exciton dominant regime with an exciton lifetime of 3 ns exists, when doped electrostatically with holes. Interestingly we observe a sharp decrease in exciton lifetime and population with an increase of the electron density by electrostatic doping, with a corresponding increase in negative trion population. With increased hole doping by a chemical method, the exciton lifetime decreases, but it remains almost constant with electrostatic carrier density tuning. This decrease in lifetime, compared to that of the pristine case, might be due to the exciton-exciton annihilation mechanism which is proposed to be existent in a high exciton density regime. Further hole doping by a chemical method leads to a transition to a positive trion dominated regime, in which the exciton lifetime decreases further due to nonradiative energy transfer to the positive trions. We observe a slight increase in exciton lifetime due to partial neutralization of positive trions at high electrostatic electron doping and a corresponding increase in the probability of excitons. We suggest that when calculating the lifetime of excitons, the exciton-to-trions formation and exciton-exciton annihilation mechanisms should be considered. These fine-tunings of excitons in monolayer MoS2 can provide a platform for probing the excitonic physics and photonic applications.