Nearly 90% Circularly-Polarized Emission in Monolayer WS₂ Single Crystals by Chemical Vapor Deposition
Abstract
Monolayer transition-metal dichalcogenides (TMDCs) in the 2H-phase are semiconductors promising 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 (P_circ), we find significantly large circular polarization (P_(circ) 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 (XPS), Kelvin-probe force microscopy (KPFM) and conductive atomic force microscopy (CAFM) 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 non-radiative recombination centers, and find good agreement between experiments and the model. Scanning tunneling microscopic/spectroscopic (STM/STS) studies provide further evidences 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 via defect engineering.
Additional Information
© 2019 American Chemical Society. Received: July 15, 2019; Accepted: August 23, 2019; Published: August 23, 2019. The authors gratefully acknowledge joint support from the Army Research Office under the Multi-University Research Initiative (MURI) program, National Science Foundation under the Physics Frontier Center program for Institute for Quantum Information and Matter (IQIM), and Kavli Foundation. The authors thank Teddy Huang from the Bruker Nano Surface Company for helpful input regarding the CAFM data analysis, and Yen-Chun Chen for useful discussion on the CVD growth for TMDCs. The authors also acknowledge support from the Beckman Institute at the California Institute of Technology for access to facilities at the Molecular Materials Research Center. Author Contributions: W.-H. Lin and N.-C. Yeh conceived the research ideas. W.-H. Lin construct the CVD system for h-WS_2 growth and participated in all the measurements and data analysis. W.-S. Tseng contributed to the XPS measurement. C. M. Went and H. A. Atwater contributed to the TRPL measurement. G. R. Rossman contributed to the Raman and PL mapping measurements. W.-H. Lin and H. A. Atwater contributed to the STM/STS studies presented in this manuscript using an Omicron VT STM system. M. L. Teague contributed to the STM/STS studies with a homemade STM system in N.-C. Yeh's group to independently verify the STM/STS results obtained by the Omicron VT STM system. W.-H. Lin and N.-C. Yeh wrote the manuscript, and N.-C. Yeh supervised and coordinated the project. The authors declare no competing financial interests.Additional details
- Alternative title
- Nearly 90% Circularly-Polarized Emission in Monolayer WS2 Single Crystals by Chemical Vapor Deposition
- Eprint ID
- 98203
- DOI
- 10.1021/acsnano.9b05550
- Resolver ID
- CaltechAUTHORS:20190826-083214491
- Army Research Office (ARO)
- NSF
- Institute for Quantum Information and Matter (IQIM)
- Kavli Foundation
- Caltech Beckman Institute
- Created
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2019-08-26Created from EPrint's datestamp field
- Updated
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2021-11-16Created from EPrint's last_modified field
- Caltech groups
- Institute for Quantum Information and Matter, Kavli Nanoscience Institute, Division of Geological and Planetary Sciences