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.