Nearly 90% circularly polarized emission in monolayer WS₂ single crystals by chemical vapor deposition (CVD)

Monolayer transition-metal dichalcogenides in the 2H-phase are semiconductors promising for opto-valleytronic/spintronic applications. Here we report novel opto-valleytronic properties of heterogeneous domains in CVD-grown monolayer WS₂ single crystals. By illuminating WS₂ with off-resonance circularly-polarized light and measuring the resulting spatially resolved circularly-polarized emission (P_circ), we found unprecedentedly large circular polarization (P_circ ~ 60% and ~ 45% for α- and β-domains, respectively) at 300 K, which further increased to ~ 90% in the α-domains at 80 K. Spatially resolved photoluminescence, Raman spectroscopy, xray photoelectron spectroscopy, Kelvin-probe force microscopy and conductive atomic force microscopy revealed direct correlation among the photoluminescence intensity, nanoscale defect densities, and chemical potential, with the α-domains showing lower defect densities
and a smaller (~ 0.15 eV) work function. These findings together with atomically resolved scanning tunneling microscopic/spectroscopic studies strongly suggested that tungstenvacancy nano-clusters were the primary non-radiative defects responsible for suppressed photoluminescence and circular polarization in WS₂.