Prominent room temperature valley polarization in WS₂/graphene heterostructures grown by chemical vapor deposition

We examine different cases of heterostructures consisting of WS2 monolayers grown by chemical vapor deposition 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 that the interaction between WS2 and graphene (WS2/Gr) has a strong effect on the temperature-dependent depolarization (i.e., decrease in polarization with increasing temperature), with polarization degrees reaching 24% at room temperature under near-resonant excitation. This contrasts with hBN-encapsulated WS2, which exhibits a room temperature polarization degree of only 11%. The observed low depolarization rate in the 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 4 K between the different heterostructure configurations. Intervalley hole scattering in the valence band proximity between the K and Γ points of WS2 is sensitive to the immediate environment, leading to the observed variations.