Robust room temperature valley polarization in monolayer and bilayer WS2

Abstract

We report robust room temperature valley polarization in chemical-vapor-deposition (CVD) grown monolayer and bilayer WS₂via polarization-resolved photoluminescence measurements using excitation below the bandgap. We show that excitation with energy slightly below the bandgap of the multi-valleyed transition metal chalcogenides can effectively suppress the random redistribution of excited electrons and, thereby, greatly enhance the efficiency of valley polarization at room temperature. Compared to mechanically exfoliated WS₂, our CVD grown WS₂ films also show enhancement in the coupling of spin, layer and valley degree of freedom and, therefore, provide improved valley polarization. At room temperature, using below-bandgap excitation and CVD grown monolayer and bilayer WS₂, we have reached a record-high valley polarization of 35% and 80%, respectively, exceeding the previously reported values of 10% and 65% for mechanically exfoliated WS₂ layers using resonant excitation. This observation provides a new direction to enhance valley control at room temperature.