Temperature dependent valley polarization in WS₂ heterostructures

The absence of degeneracy in the valley indices of monolayer-TMDs serves as an essential property for the development of valleytronic devices. One criterion for the realization of such a device is to attain high polarization at RT. We examine vertical WS₂ heterostructures and show that the material type used, significantly influences the valley polarization of WS₂. The interaction between WS₂ and graphene has a strong effect on the T-dependent depolarization, with a polarization of 24% at RT under near-resonant excitation. This contrasts with hBN- encapsulated WS₂, which exhibits a RT polarization of 11%. The low depolarization rate in WS₂/Graphene is attributed to a) rapid charge and energy transfer processes of the scattered excitons, b) absence of thermal dissociation of trions and thermally assisted dark-to-bright transitions and c) partial suppression of the T-dependent bandgap renormalization. Significant variations in the polarization are also observed at 4K. We propose that intervalley hole scattering in the VB proximity between the K and Γ points of WS₂ is sensitive to the immediate environment, leading to the observed variations.