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




Lin WH1, Tseng WS2, Went C2, Teague ML2, Rossman GR3, Atwater HA1, Yeh NC,2
 
  1Department of Applied Physics
California Institute of Technology
Pasadena, CA  91125, USA

2 Department of Physics
California Institute of Technology
Pasadena, CA 91125, USA

3 Division of Geological and Planetary Sciences
California Institute of Technology
Pasadena, CA  91125-2500, USA


ABSTRACT

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 (Pcirc), we found unprecedentedly large circular polarization (Pcirc ~ 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.