Evidence of indirect gap in monolayer WSe2

Wei-Ting Hsu, Li-Syuan Lu, Dean Wang, Jing-Kai Huang, Ming-Yang Li, Tay-Rong Chang, Yi-Chia Chou, Zhen-Yu Juang, Horng-Tay Jeng, Lain-Jong Li and Wen-Hao Chang ()
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Wei-Ting Hsu: National Chiao Tung University
Li-Syuan Lu: National Chiao Tung University
Dean Wang: National Chiao Tung University
Jing-Kai Huang: Physical Sciences and Engineering, King Abdullah University of Science and Technology
Ming-Yang Li: Research Center for Applied Sciences, Academia Sinica
Tay-Rong Chang: National Cheng Kung University
Yi-Chia Chou: National Chiao Tung University
Zhen-Yu Juang: National Chiao Tung University
Horng-Tay Jeng: National Tsing Hua University
Lain-Jong Li: Physical Sciences and Engineering, King Abdullah University of Science and Technology
Wen-Hao Chang: National Chiao Tung University

Nature Communications, 2017, vol. 8, issue 1, 1-7

Abstract: Abstract Monolayer transition metal dichalcogenides, such as MoS2 and WSe2, have been known as direct gap semiconductors and emerged as new optically active materials for novel device applications. Here we reexamine their direct gap properties by investigating the strain effects on the photoluminescence of monolayer MoS2 and WSe2. Instead of applying stress, we investigate the strain effects by imaging the direct exciton populations in monolayer WSe2–MoS2 and MoSe2–WSe2 lateral heterojunctions with inherent strain inhomogeneity. We find that unstrained monolayer WSe2 is actually an indirect gap material, as manifested in the observed photoluminescence intensity–energy correlation, from which the difference between the direct and indirect optical gaps can be extracted by analyzing the exciton thermal populations. Our findings combined with the estimated exciton binding energy further indicate that monolayer WSe2 exhibits an indirect quasiparticle gap, which has to be reconsidered in further studies for its fundamental properties and device applications.

Date: 2017
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DOI: 10.1038/s41467-017-01012-6

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