Valley Polarized Excitonic Complexes in Monolayer TMD alloy*

Monolayer transition metal dichalcogenides (TMDs) have superior optical properties. Particularly, Tungsten based TMDs are known for their excellent valley polarization due to the strong spin-orbit coupling. However, the optical bandgap is limited by these two materials, which hinders the valleytronics applications that rely on the resonant excitation. We explore the possibility to overcome this limitation through the monolayer alloy, WS_2xSe_2(1-x), which promises an atomically thin semiconductor with tunable bandgap. We find that the high-quality BN encapsulated monolayer alloy WS_0.6Se_1.4 inherits the superior optical properties of tungsten-based TMDs, including a trion splitting of ~ 6 meV and valley polarization as high as ~60%. In particular, we demonstrate the emerging interlayer electron-phonon coupling in the BN/ WS_0.6Se_1.4 /BN van der Waals heterostructure. This coupling can be sensitively tuned by electrostatic gating and renders the otherwise optically silent Raman modes visible.