Influence of defects on the valley polarization properties of monolayer MoS2 grown by chemical vapor deposition

Temperature-dependent polarization-resolved photoluminescence spectroscopy is carried out on as-grown, transferred, and coated 1L-${\mathrm{MoS}}_{2}$ samples grown by the chemical vapor deposition technique to explore the underlying mechanism behind the valley depolarization process. It has been found that the momentum scattering of the excitons due to the sulfur-vacancies-attached-with-air-molecules type of defects has a strong influence in the suppression of valley polarization. Our study reveals that at sufficiently low densities of such defects and temperatures, the long-range electron-hole exchange mediated intervalley transfer of excitons via the Maialle-Silva-Sham (MSS) mechanism, as suggested by a recent theory [Yu and Wu, Phys. Rev. B 89, 205303 (2014)], is indeed the most dominant spin-flip process. In the process, the momentum scattering of the excitons by the defects takes the central stage. Interestingly, the study finds the scattering rate to be proportional to the cube root of the density of the defects. The intervalley transfer process of excitons involving the $\mathrm{\ensuremath{\Gamma}}$ valley also has significance in valley depolarization, especially when the layer is either under a tensile strain or has a high density of ${V}_{S}$ defects, as these perturbations reduces $K$ to $\mathrm{\ensuremath{\Gamma}}$-energy separation. Band-structural calculations carried out within a density functional theory framework validate these findings. The study further suggests that exchange interactions with the physisorbed oxygen molecules can result in the intervalley spin-flip scattering of the excitons and this process gives an important contribution to valley depolarization, especially at the strong scattering regime.