Nearly 90% Circularly Polarized Emission in Monolayer WS2 Single Crystals by Chemical Vapor Deposition

Wei Hsiang Lin, Wei Shiuan Tseng, Cora M. Went, Marcus L. Teague, George R. Rossman, Harry A. Atwater, Nai Chang Yeh*

*Corresponding author for this work

Research output: Contribution to journalArticle

3 Scopus citations

Abstract

Monolayer transition-metal dichalcogenides (TMDCs) in the 2H-phase are promising semiconductors for opto-valleytronic and opto-spintronic applications because of their strong spin-valley coupling. Here, we report detailed studies of 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 find significantly large circular polarization (Pcirc up to 60% and 45% for α- and β-domains, respectively) already at 300 K, which increases to nearly 90% in the α-domains at 80 K. Studies of spatially resolved photoluminescence (PL) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, Kelvin-probe force microscopy, and conductive atomic force microscopy reveal direct correlation among the PL intensity, defect densities, and chemical potential, with the α-domains showing lower defect densities and a smaller work function by 0.13 eV than the β-domains. This work function difference indicates the occurrence of type-two band alignments between the α- and β-domains. We adapt a classical model to explain how electronically active defects may serve as nonradiative recombination centers and find good agreement between experiments and the model. Scanning tunneling microscopic/spectroscopic (STM/STS) studies provide further evidence for tungsten vacancies (WVs) being the primary defects responsible for the suppressed PL and circular polarization in WS2. These results therefore suggest a pathway to control the opto-valleytronic properties of TMDCs by means of defect engineering.

Original languageEnglish
Pages (from-to)1350-1359
Number of pages10
JournalACS Nano
Volume14
Issue number2
DOIs
StatePublished - 25 Feb 2020

Keywords

  • CAFM
  • CVD
  • KPFM
  • STM
  • transition metal dichalcogenides
  • valley polarization
  • WS

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    Lin, W. H., Tseng, W. S., Went, C. M., Teague, M. L., Rossman, G. R., Atwater, H. A., & Yeh, N. C. (2020). Nearly 90% Circularly Polarized Emission in Monolayer WS2 Single Crystals by Chemical Vapor Deposition. ACS Nano, 14(2), 1350-1359. https://doi.org/10.1021/acsnano.9b05550