Defect Structure of Localized Excitons in a ${\mathrm{WSe}}_{2}$ Monolayer

Defect Structure of Localized Excitons in a ${WSe}_{2}$ Monolayer

Shuai Zhang, Chen-Guang Wang, Ming-Yang Li, Di Huang, Lain-Jong Li, Wei Ji, and Shiwei Wu

Phys. Rev. Lett. 119, 046101 – Published 25 July 2017

Abstract

The atomic and electronic structure of intrinsic defects in a ${WSe}_{2}$ monolayer grown on graphite was revealed by low temperature scanning tunneling microscopy and spectroscopy. Instead of chalcogen vacancies that prevail in other transition metal dichalcogenide materials, intrinsic defects in ${WSe}_{2}$ arise surprisingly from single tungsten vacancies, leading to the hole ( $p$ -type) doping. Furthermore, we found these defects to dominate the excitonic emission of the ${WSe}_{2}$ monolayer at low temperature. Our work provided the first atomic-scale understanding of defect excitons and paved the way toward deciphering the defect structure of single quantum emitters previously discovered in the ${WSe}_{2}$ monolayer.

Received 25 February 2017

DOI:https://doi.org/10.1103/PhysRevLett.119.046101

Physics Subject Headings (PhySH)

Defects Excitons Luminescence Point defects Semiconductors

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Shuai Zhang¹, Chen-Guang Wang², Ming-Yang Li^3,4, Di Huang¹, Lain-Jong Li⁴, Wei Ji^2,*, and Shiwei Wu^1,5,†

¹State Key Laboratory of Surface Physics, Key Laboratory of Micro and Nano Photonic Structures (MOE), and Department of Physics, Fudan University, Shanghai 200433, China
²Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials and Micro-Nano Devices, Renmin University of China, Beijing 100872, China
³Research Center for Applied Sciences, Academia Sinica, Taipei 10617, Taiwan
⁴Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
⁵Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China

^*Corresponding author. wji@ruc.edu.cn
^†Corresponding author. swwu@fudan.edu.cn

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Vol. 119, Iss. 4 — 28 July 2017

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Figure 1
STM topographic images and $d I / d V$ spectrum of the ${WSe}_{2}$ monolayer grown on HOPG by CVD. (a),(b) STM images showing individual point defects in the ${WSe}_{2}$ monolayer for the same area but with different bias voltages of $- 2.5$ and $- 1.5 V$ , respectively. $I_{set} = 100 pA$ . The pitlike and trefoil-like structures in the images reflect the electronic distribution at different energy. (c),(d) Linear and logarithm plots of a typical $d I / d V$ spectrum acquired on defect-free monolayer area (black curves) and on the defect site (red curves), respectively. $V_{b} = - 2.5 V$ , $I_{set} = 100 pA$ . The edges (onsets) of the valence band at negative bias voltage were marked by arrows.
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Figure 2
Atomically resolved STM imaging and spectroscopy of individual point defects. (a),(b),(c) Close-up STM topographic images showing three identical point defects at different bias voltages of $- 1.8$ , $- 1.2$ , and $+ 1.5 V$ , respectively. When the bias voltage was set within the semiconducting band gap, the outmost selenium atoms in the ${WSe}_{2}$ monolayer exhibited as individual protrusions in (b). The point defects were centered on the tungsten atom sites, preserving the trigonal symmetry in the ${WSe}_{2}$ monolayer. (d) 2D plot of $d I / d V$ spectra along a line across the defect shown in (b). (e) A few $d I / d V$ spectra selected from (d). The corresponding positions were also marked as crosses in (b). All the spectra are shifted vertically for clarity. For comparison, a $d I / d V$ spectrum away from defects is also shown in the black curve. The peak shifts near the valence band edge (at negative bias voltage) were marked by two vertical dashed lines as $P_{1}$ ( $- 1.48 V$ , red) and $P_{2}$ ( $- 1.61 V$ , black), respectively.
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Figure 3
Calculated atomic structures and corresponding LDOS of four defect structures. (a)–(d) Top view of the fully relaxed atomic structures of the energetically most favorable configurations for single $V_{W}$ , $V_{Se 3}$ , $V_{WSe 3}$ , and ${Se}_{W}$ defects, respectively. Solid blue, orange, pink, and red balls and the hollow green ball represent the W, Se atoms in the first sublayer, Se atoms in the second sublayer, the Se antisite atom, and original sites for the removed atoms, respectively. (e)–(h) Corresponding LDOS of the defects above. Black ( $W_{0}$ ), blue ( ${Se}_{1}$ ), red ( ${Se}_{2}$ ), and green ( ${Se}_{3}$ ) curves correspond to LDOS on the W atoms and Se atoms as marked in (a)–(d), respectively. In the calculations, the vacuum level was aligned with that of multilayer graphene, and the charge neutral point of the aligned graphene was thus chosen as the Fermi level in these LDOSs.
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Figure 4
Localized exciton arising from tungsten vacancy defects. (a),(b) Electronic and excitonic schematic diagrams of the ${WSe}_{2}$ monolayer, respectively. For the sake of simplicity, only the band structure at the $K$ point is drawn. The spin configuration at the $K^{'}$ point is inverted because of time reversal symmetry, but would not affect the energy alignment of bright $| K_{e ↑} K_{h ↑} ⟩$ and dark $| K_{e ↓} K_{h ↑} ⟩$ free excitons as discussed in the text. (c) Temperature-dependent photoluminescence spectra of the ${WSe}_{2}$ monolayer on graphite. The data were obtained from the same location on the sample, and spectra are offset for clarity. The excitation wavelength and power were 532 nm and 1.60 mW, respectively.
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Figure 5
Valley polarization and valley coherence of the CVD-grown ${WSe}_{2}$ monolayer on sapphire. (a) Polarization-resolved photoluminescence spectra for $σ^{+}$ detection under circularly polarized excitation of laser at 1.96 eV and 26 K. The red and black lines correspond to $σ^{+}$ and $σ^{-}$ excitation, respectively. The circular polarization of the free exciton (1.72 eV) and the defect exciton (1.64 eV) was 49% and 16%, respectively. (b) Polarization-resolved photoluminescence spectra for horizontally polarized ( $H$ ) detection under the linearly polarized excitation of laser at 1.96 eV and 26 K. The red and black lines correspond to the horizontally ( $H$ ) and vertically ( $V$ ) polarized excitation, respectively. The valley coherence was only observed on the free exciton.
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Physical Review Letters

Defect Structure of Localized Excitons in a WSe2 Monolayer

Shuai Zhang, Chen-Guang Wang, Ming-Yang Li, Di Huang, Lain-Jong Li, Wei Ji, and Shiwei Wu

Phys. Rev. Lett. 119, 046101 – Published 25 July 2017

Abstract

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Defect Structure of Localized Excitons in a ${WSe}_{2}$ Monolayer