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Nano Research 2015, 8(8): 2562-2572 https://doi.org/10.1007/s12274-015-0762-6
Research Article | Issue | Published: 29 August 2015

Strain-induced direct–indirect bandgap transition and phonon modulation in monolayer WS2

Show Author's Information Yanlong Wang1,2,§,  Chunxiao Cong2,§,  Weihuang Yang1,2,  Jingzhi Shang2,  Namphung Peimyoo2,  Yu Chen2,  Junyong Kang3,  Jianpu Wang1,4,  Wei Huang1,4,5( ),  Ting Yu2( )
Nanyang Technological University-Nanjing Tech Center of Research and DevelopmentNanjing Tech UniversityNanjing211816China
Division of Physics and Applied PhysicsSchool of Physical and Mathematical SciencesNanyang Technological University637371Singapore City, Singapore
Fujian Key Laboratory of Semiconductor Materials and ApplicationsDepartment of PhysicsXiamen UniversityXiamen361005China
Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM)Jiangsu National Synergetic Center for Advanced Materials (SICAM)Nanjing Tech University (NanjingTech)Nanjing211816China
Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM)Nanjing University of Posts & Telecommunications9 Wenyuan RoadNanjing210046China

§ These authors contributed equally to this work.

Keywords:
strain, monolayer WS2, light-emission tuning, indirect transition, trion, crystallographic orientation
Cite this article:
Wang Y, Cong C, Yang W, et al. Strain-induced direct–indirect bandgap transition and phonon modulation in monolayer WS2. Nano Research, 2015, 8(8): 2562-2572. https://doi.org/10.1007/s12274-015-0762-6
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Abstract Full text Electronic supplementary material About this article

In situ strain photoluminescence (PL) and Raman spectroscopy have been employed to exploit the evolutions of the electronic band structure and lattice vibrational responses of chemical vapor deposition (CVD)-grown monolayer tungsten disulphide (WS2) under uniaxial tensile strain. Observable broadening and appearance of an extra small feature at the longer-wavelength side shoulder of the PL peak occur under 2.5% strain, which could indicate the direct-indirect bandgap transition and is further confirmed by our density-functional-theory calculations. As the strain increases further, the spectral weight of the indirect transition gradually increases. Over the entire strain range, with the increase of the strain, the light emissions corresponding to each optical transition, such as the direct bandgap transition (K-K) and indirect bandgap transition (Γ-K, ≥2.5%), exhibit a monotonous linear redshift. In addition, the binding energy of the indirect transition is found to be larger than that of the direct transition, and the slight lowering of the trion dissociation energy with increasing strain is observed. The strain was used to modulate not only the electronic band structure but also the lattice vibrations. The softening and splitting of the in-plane E' mode is observed under uniaxial tensile strain, and polarization-dependent Raman spectroscopy confirms the observed zigzag-oriented edge of WS2 grown by CVD in previous studies. These findings enrich our understanding of the strained states of monolayer transition-metal dichalcogenide (TMD) materials and lay a foundation for developing applications exploiting their strain-dependent optical properties, including the strain detection and light-emission modulation of such emerging two-dimensional TMDs.


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Electronic supplementary material
About this article

Strain-induced direct–indirect bandgap transition and phonon modulation in monolayer WS2

Show Author's information Yanlong Wang1,2,§, Chunxiao Cong2,§, Weihuang Yang1,2, Jingzhi Shang2, Namphung Peimyoo2, Yu Chen2, Junyong Kang3, Jianpu Wang1,4, Wei Huang1,4,5( ), Ting Yu2( )
Nanyang Technological University-Nanjing Tech Center of Research and DevelopmentNanjing Tech UniversityNanjing211816China
Division of Physics and Applied PhysicsSchool of Physical and Mathematical SciencesNanyang Technological University637371Singapore City, Singapore
Fujian Key Laboratory of Semiconductor Materials and ApplicationsDepartment of PhysicsXiamen UniversityXiamen361005China
Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM)Jiangsu National Synergetic Center for Advanced Materials (SICAM)Nanjing Tech University (NanjingTech)Nanjing211816China
Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM)Nanjing University of Posts & Telecommunications9 Wenyuan RoadNanjing210046China

§ These authors contributed equally to this work.

Abstract

In situ strain photoluminescence (PL) and Raman spectroscopy have been employed to exploit the evolutions of the electronic band structure and lattice vibrational responses of chemical vapor deposition (CVD)-grown monolayer tungsten disulphide (WS2) under uniaxial tensile strain. Observable broadening and appearance of an extra small feature at the longer-wavelength side shoulder of the PL peak occur under 2.5% strain, which could indicate the direct-indirect bandgap transition and is further confirmed by our density-functional-theory calculations. As the strain increases further, the spectral weight of the indirect transition gradually increases. Over the entire strain range, with the increase of the strain, the light emissions corresponding to each optical transition, such as the direct bandgap transition (K-K) and indirect bandgap transition (Γ-K, ≥2.5%), exhibit a monotonous linear redshift. In addition, the binding energy of the indirect transition is found to be larger than that of the direct transition, and the slight lowering of the trion dissociation energy with increasing strain is observed. The strain was used to modulate not only the electronic band structure but also the lattice vibrations. The softening and splitting of the in-plane E' mode is observed under uniaxial tensile strain, and polarization-dependent Raman spectroscopy confirms the observed zigzag-oriented edge of WS2 grown by CVD in previous studies. These findings enrich our understanding of the strained states of monolayer transition-metal dichalcogenide (TMD) materials and lay a foundation for developing applications exploiting their strain-dependent optical properties, including the strain detection and light-emission modulation of such emerging two-dimensional TMDs.

Keywords: strain, monolayer WS2, light-emission tuning, indirect transition, trion, crystallographic orientation

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Publication history
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Acknowledgements

Publication history

Received: 12 January 2015
Revised: 25 February 2015
Accepted: 04 March 2015
Published: 29 August 2015
Issue date: August 2015

Copyright

© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2015

Acknowledgements

Acknowledgements

This work is supported by the Singapore National Research Foundation NRF RF Award No. NRFRF2010- 07, MOE Tier 2 MOE2012-T2-2-049, A*Star SERC PSF grant No. 1321202101, and MOE Tier 1 MOE2013- T1-2-235. W. Huang acknowledges the support of the National Basic Research Program of China (973 Program) (No. 2015CB932200), the National Natural Science Foundation of China (NSFC) (Grant Nos. 21144004, 20974046, 21101095, 21003076, 20774043, 51173081, 50428303, 61136003, and 50428303), the Ministry of Education of China (No. IRT1148), the NSF of Jiangsu Province (Grant Nos. SBK201122680, 11KJB510017, BK2008053, 11KJB510017, BK2009025, 10KJB510013, and BZ2010043), and NUPT (Nos. NY210030 and NY211022). J. P. Wang is grateful for the NSFC (No. 11474164), NSF of Jiangsu province (No. BK20131413), and the Jiangsu Specially-Appointed Professor program. Y. L. Wang thanks Luqing Wang, Dr. Xiaolong Zou, and Dr. Alex Kutana for the constructive discussion.

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