  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.
