Room-Temperature Valley Polarization in Band Gap Engineered WS2xSe2(1–x) Monolayers: Implications for Spintronics and Valleytronics

The generation and manipulation of valley-spin polarization are essential for two-dimensional (2D) layered transition-metal dichalcogenides for spin-/valleytronic applications. Here, high crystal quality WS2xSe2(1–x) monolayers with sulfur composition tuning from 0 to 1 were prepared through a controlled chemical vapor deposition method. The crystal structure retains perfect C3-rotation symmetry, with the circular polarization degree of second harmonic generation achieving near unit. Both steady-state and time-resolved circular polarization-resolved photoluminescence (PL) characterizations demonstrate that the valley polarization degree of WS2xSe2(1–x) monolayers can be monotonically improved with gradually increasing sulfur concentration. A phenomenological model and the corresponding rate equations were established to describe the valley polarization dynamics of the bandgap engineered monolayer WS2xSe2(1–x), and a real band-edge intervalley scattering lifetime can be determined by fitting the circularly polarized PL decay curves using this model. The physical origin of the phenomenon of increasing degree of valley polarization with the decreasing of the hot electron energy has been revealed due to the continuous tuning of the initially injected polarization with varying the composition ratio. Our work gives insights into the underlying valley depolarization mechanism in 2D alloyed monolayers and provides a potential pathway for controllable synthesis of high-quality atomically thin alloys with tunable valley physics, which contribute to spintronic and valleytronic applications.