Identifying the Reason for Variations in Circularly Polarized Photoluminescence Values in Monolayer WS$_{\mathbf{2}}$

The unique electronic band structure in single layer WS$_{2}$ provides the ability to selectively populate a desired valley by exciting with circularly polarized light. The valley population is reflected through the circular polarization of photoluminescence (PL) and a high degree of circular polarization has been predicted in WS$_{2}$. Interestingly, experimental work has shown this is not always the case. In particular, recent experimental investigations of monolayer WS$_{2}$ find near zero valley polarized emission from the neutral exciton under near resonant excitation. We investigate the circularly polarized PL in over twenty WS$_{2}$ monolayer samples synthesized using chemical vapor deposition. The room temperature circularly polarized emission (P$_{circ})$ values vary from 0{\%} to 20{\%}. The samples also exhibit considerable variation in exciton lifetime, ranging from 300 ps to \textasciitilde 1.5 ns, as measured by time resolved photoluminescence. Comparing P$_{circ}$ with the exciton lifetimes ($\tau _{r})$ reveals an inverse relation between the $\tau_{r}$ and circular polarization, with samples exhibiting the longest $\tau_{r}$ having the lowest P$_{circ}$ and vice versa. Our findings suggest that 100{\%} circular polarization will be achieved in samples exhibiting short $\tau_{r}$.