  We examine different cases of heterostructures consisting of WS2 monolayers
grown by chemical vapor deposition (CVD) as the optically active material. We
show that the degree of valley polarization of WS2 is considerably influenced
by the material type used to form the heterostructure. Our results suggest the
interaction between WS2 and graphene (WS2/Gr) has a strong effect on the
temperature dependent depolarization (i.e. decrease of polarization with
increasing temperature), with polarization degrees reaching 24% at room
temperature under near-resonant excitation. This contrasts to hBN- encapsulated
WS2, which exhibits a room temperature polarization degree of only 11%. The
observed low depolarization rate in WS2/Gr heterostructure is attributed to the
nearly temperature independent scattering rate due to phonons and fast charge
and energy transfer processes from WS2 to graphene. Significant variations in
the degree of polarization are also observed at 4K between the different
heterostructure configurations. Intervalley hole scattering in the valence band
proximity between the K and {\Gamma} points of WS2 is sensitive to the
immediate environment, leading to the observed variations.
