Influence of Defects on the Valley Polarization Properties of Monolayer MoS${}_2$ Grown by Chemical Vapor Deposition

Abstract

Here, the underlying mechanisms behind valley de-polarization is investigated in chemical vapor deposited 1L-MoS${}_2$. Temperature dependent polarization resolved photoluminescence spectroscopy was carried out on as-grown, transferred and capped samples. It has been found that the momentum scattering of the excitons due to the sulfur-vacancies attached with air-molecule defects has a strong influence on the valley de-polarization process. Our study reveals that at sufficiently low densities of such defects and temperatures, long range electron-hole exchange mediated intervalley transfer due to momentum scattering via Maialle-Silva-Sham (MSS) mechanism of excitons is indeed the most dominant spin-flip process as suggested by T. Yu et al. The rate of momentum scattering of the excitons due to these defects is found to be proportional to the cube root of the density of the defects. Intervalley transfer process of excitons involving $\Gamma$-valley also has significance in the valley de-polarization process specially when the layer has tensile strain or high density of $V_S$ defects as these perturbations reduce $K$ to $\Gamma$-energy separation. Band-structural calculations carried out within the density functional theory framework validate this finding. Experimental results further suggest that exchange interactions with the physisorbed air molecules can also result in the intervalley spin-flip scattering of the excitons, and this process gives an important contribution to valley depolarization, specially at the strong scattering regime.

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