  Monolayer 2D semiconductors provide an attractive option for valleytronics
due to the valley-addressability by helicity-specific light beam. But the short
valley lifetime for excitons have hindered potential valleytronic applications.
In this paper, we demonstrate a strategy for prolonging the valley lifetime by
converting excitons to trions through effective gate control and by taking
advantage of much longer valley lifetime for trions than for excitons. In
continuous-wave experiments, we found the valley polarization increases as gate
voltage is tuned away from the charge neutrality, with the degree of valley
polarization increased from near zero to 38 % for excitons and to 33 % for
trions. This is the first successful observation of valley-polarization in
MoTe2 without a magnetic field. In pump-probe experiments, we found that the
intervalley scattering process of excitons is significantly suppressed as gate
voltage is tuned away from charge neutrality, with scattering time from 0.85 ps
to ~ 2.17 ps. In contrast, the intervalley scattering rate for trions increases
due to increased availability of partner charges for trion spin flipping, with
scattering time from 1.39 ns down to ~100 ps away from charge neutrality.
Interestingly, our results show that, despite the accelerated intervalley
scattering, the trion polarization degree increases due to polarized trion
generation from the exciton-to-trion conversion overtaking the intervalley
trion scatterings. Importantly, the efficient exciton-to-trion conversion
changed the dominant depolarization mechanisms. As a result, the valley
lifetime is dramatically improved by 1000 times from excitons to trions at the
charge neutrality. Our results shed new light into the depolarization dynamics
and the interplay of various depolarization channels for excitons and trions
and provide an effective strategy for prolonging the valley polarization.
