Microscopic Mechanisms of Defect-Mediated Valley Depolarization in Monolayer WS ₂

Monolayer transition metal dichalcogenides (TMDs) show promising prospects for valleytronics applications, yet their atomic thinness makes valley polarization highly susceptible to defects. Nevertheless, the microscopic mechanism underlying the influence of defects formed during growth on the valley polarization dynamics of TMDs is still poorly understood. Here, we elucidate the microscopic mechanisms of defect-mediated valley depolarization in as-grown WS 2 monolayers by integrating scanning tunneling microscopy/spectroscopy with helicity-resolved ultrafast transient reflection spectroscopy. We identify oxygen-substituting sulfur (O S ) and molybdenum-substituting tungsten (Mo W ) as the predominant defects. O S defects substantially reduce the energy separation between the valence-band maxima at the K and Γ points (Δ ΓK ) to ∼146 meV, thereby activating a hole K → Γ → K′ intervalley scattering channel at room temperature and shortening the hole valley polarization lifetime to the femtosecond regime. Moreover, both O S and Mo W defects act as spin-conserving electron intervalley scattering centers, enhancing the electron intervalley scattering rate. These defect-induced scattering pathways synergistically accelerate valley depolarization in WS 2 . Our findings offer important insights for defect engineering toward practical valleytronic applications in monolayer TMDs.