Effects of defects and impurities on the optical properties and the valley polarization in monolayer MoS2

摘要

We study the effects of charged defects and impurities on the optical properties and the valley polarization in monolayer MoS2. Both the defects generated by vacuum annealing and the impurities introduced by AuCl3 chemical doping dramatically increase the photoluminescence (PL) intensity. This is due to a p-doping effect, which increases the exciton lifetime by suppressing the non-radiative trionic decay in the n-type, as-exfoliated monolayer MoS2. The PL from vacuum-annealed MoS2 can be controlled reversibly. It decreases upon laser irradiation or pumping in a vacuum due to the desorption of air molecules adsorbed at S-vacancy sites and recovers slowly with the re-adsorption of air molecules. Both the vacuum annealing and the AuCl3 doping reduce the valley polarization significantly. A systematic study on the PL and the circular polarization reveals that the reduction in the circular polarization cannot be explained by the increased exciton lifetime alone. The valley polarization almost recovers to its pre-annealing value after pumping in a vacuum, indicating that S-vacancies with 'charged' air molecules affect the valley properties more strongly than neutral ones. The valley polarization decrease is more significant at high temperatures, which excludes the increased intervalley scattering due to defects or impurities as its main origin. Relaxation of the valley-contrasting optical selection rule due to defects or impurities, which was reported recently, seems to explain our experimental results better.