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Monolayer 2D semiconductors provide an attractive option for valleytronicsdue to the valley-addressability by helicity-specific light beam. But the shortvalley lifetime for excitons have hindered potential valleytronic applications.In this paper, we demonstrate a strategy for prolonging the valley lifetime byconverting excitons to trions through effective gate control and by takingadvantage of much longer valley lifetime for trions than for excitons. Incontinuous-wave experiments, we found the valley polarization increases as gatevoltage is tuned away from the charge neutrality, with the degree of valleypolarization increased from near zero to 38 % for excitons and to 33 % fortrions. This is the first successful observation of valley-polarization inMoTe2 without a magnetic field. In pump-probe experiments, we found that theintervalley scattering process of excitons is significantly suppressed as gatevoltage is tuned away from charge neutrality, with scattering time from 0.85 psto ~ 2.17 ps. In contrast, the intervalley scattering rate for trions increasesdue to increased availability of partner charges for trion spin flipping, withscattering time from 1.39 ns down to ~100 ps away from charge neutrality.Interestingly, our results show that, despite the accelerated intervalleyscattering, the trion polarization degree increases due to polarized triongeneration from the exciton-to-trion conversion overtaking the intervalleytrion scatterings. Importantly, the efficient exciton-to-trion conversionchanged the dominant depolarization mechanisms. As a result, the valleylifetime is dramatically improved by 1000 times from excitons to trions at thecharge neutrality. Our results shed new light into the depolarization dynamicsand the interplay of various depolarization channels for excitons and trionsand provide an effective strategy for prolonging the valley polarization.

Prolonging Valley Polarization Lifetime through Gate-Controlled Exciton-to-Trion Conversion in Monolayer Molybdenum Ditelluride