Strongly enhanced exciton-phonon coupling in two-dimensional $\mathrm{WS}{\mathrm{e}}_{2}$

Strongly enhanced exciton-phonon coupling in two-dimensional $WS e_{2}$

Luojun Du, Mengzhou Liao, Jian Tang, Qian Zhang, Hua Yu, Rong Yang, Kenji Watanabe, Takashi Taniguchi, Dongxia Shi, Qingming Zhang, and Guangyu Zhang

Phys. Rev. B 97, 235145 – Published 25 June 2018

Abstract

The recently emerging laminar transition metal dichalcogenides provide an unprecedented platform for exploring fascinating layer-dependent properties. Determining the dependence of exciton-phonon coupling (EPC) on dimensionality would set a foundation for these exotic thickness-dependent phenomena. Here we report the observation of layer-dependent EPC between the $A_{1 g} (Γ)$ phonon and A′ exciton in $WS e_{2}$ down to the monolayer limit. Our results uncover that the strength of EPC increases dramatically with a descent of layer thickness. Compared with their bulk counterparts, the strength of EPC in monolayer $WS e_{2}$ is enhanced by nearly an order of magnitude. Furthermore, our work demonstrates that the giant EPC in the monolayer plays a prominent role in the exotic interlayer EPC of $WS e_{2}$ /boron nitride heterostructures. The gigantic EPC in the two-dimensional limit provides a firm basis for understanding and manipulating the peculiar phenomena and novel optoelectronic applications based on atomically thin $WS e_{2}$ .

Received 23 April 2018
Revised 9 June 2018

DOI:https://doi.org/10.1103/PhysRevB.97.235145

Physics Subject Headings (PhySH)

Excitons Optical conductivity Phonons

Thin films Transition metal dichalcogenides

Raman spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Luojun Du^1,2, Mengzhou Liao¹, Jian Tang¹, Qian Zhang², Hua Yu¹, Rong Yang¹, Kenji Watanabe³, Takashi Taniguchi³, Dongxia Shi^1,4,5,*, Qingming Zhang^1,2, and Guangyu Zhang^1,4,5,6,†

¹Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
²Department of Physics, Renmin University of China, Beijing 100872, China
³Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
⁴School of Physical Sciences, University of Chinese Academy of Science, Beijing 100190, China
⁵Beijing Key Laboratory for Nanomaterials and Nanodevices, Beijing 100190, China
⁶Collaborative Innovation Center of Quantum Matter, Beijing 100190, China

^*Corresponding author: dxshi@iphy.ac.cn
^†Corresponding author: gyzhang@aphy.iphy.ac.cn

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Vol. 97, Iss. 23 — 15 June 2018

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Figure 1
Microscopy and characterization of $WS e_{2}$ with different layers. (a)–(c) White-light microscope images of 1L–6L $WS e_{2}$ samples with a $\times 100$ magnification. The scale bars shown in (a)–(c) are $10 μ m$ . (d) Light absorption as a function of the layer number. The dotted line is the linear fitting with a slope of $6 %$ . (e) Normalized PL spectra by the intensity of the A exciton of $WS e_{2}$ for layer numbers = 1L–9L. Feature A labels the direct-gap transitions, and I labels the luminescence from an indirect gap interband transition. The spectra were taken under the same conditions using 2.33 eV excitation. (f) Dependence on the layer number of the relative integrated PL intensity normalized by the intensity of the monolayer at 1 (right) and the peak positions of A and I excitons (left).
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Figure 2
Resonant and off-resonant Raman spectra of $WS e_{2}$ . (a) Off-resonant Raman spectra using 633-nm line in the range of $210 - 280 c m^{- 1}$ . Multipeak Lorentzian fittings of 1L $WS e_{2}$ are overlaid. (b) Frequencies of $E_{2 g}^{1} (Γ)$ and $A_{1 g} (Γ)$ Raman modes (left) obtained from Lorentzian fittings of (a) and their difference (right) as a function of layer number. (c), (d) Resonant Raman spectra with 532-nm laser in the range of $210 - 280 c m^{- 1}$ (c) and $295 - 320 c m^{- 1}$ (d). Inset in (c) is the Raman spectrum of bilayer $WS e_{2}$ obtained in two scattering configurations: copolarized $[Z (XX) \bar{Z}]$ and cross-polarized $[Z (XY) \bar{Z}]$ , where Z and $\bar{Z}$ indicate wave vectors of incident and scattered light.
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Figure 3
Layer-dependent exciton-phonon coupling in $WS e_{2}$ . (a) Optical image of other representative $WS e_{2}$ samples on 300-nm-thick $Si O_{2}$ . We overlay the layer number on top of the corresponding thickness. Scale bar: $15 μ m$ . (b) Raman intensity mapping of the $A_{1 g} (Γ)$ mode for $WS e_{2}$ shown in (a) with 2.33 eV excitation. (c), (d) Raman intensity normalized by the intensity of the bulk as a function of layer number with 1.96 eV excitation (c) and 2.33 eV excitation, on resonance with the A′ exciton (d). (e) Raman intensity normalized by the bulk versus exciton energy. The black theory line is described by Eq. (3). (f) Raman intensity ratio between the $A_{1 g} (Γ)$ mode and $B_{2 g} (M)$ band as a function of layer number.
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Figure 4
Layer-dependent interlayer exciton-phonon coupling from 1L $WS e_{2} / hBN$ (black), 2L $WS e_{2} / hBN$ (red), 4L $WS e_{2} / hBN$ (blue), and an adjacent atomically thin hBN (magenta) excited by 2.33 radiation. Inset is the optical micrograph of hBN/ $WS e_{2}$ heterostructures. Thin hBN with uniform thickness is highlighted by the dashed black line. Monolayer, bilayer, and quadrilayer $WS e_{2}$ are highlighted by red, yellow, and magenta lines, respectively. Scale bar: $10 μ m$ .
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Physical Review B

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Strongly enhanced exciton-phonon coupling in two-dimensional $WS e_{2}$

Luojun Du, Mengzhou Liao, Jian Tang, Qian Zhang, Hua Yu, Rong Yang, Kenji Watanabe, Takashi Taniguchi, Dongxia Shi, Qingming Zhang, and Guangyu Zhang

Phys. Rev. B 97, 235145 – Published 25 June 2018

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Physical Review B

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Strongly enhanced exciton-phonon coupling in two-dimensional WSe2

Luojun Du, Mengzhou Liao, Jian Tang, Qian Zhang, Hua Yu, Rong Yang, Kenji Watanabe, Takashi Taniguchi, Dongxia Shi, Qingming Zhang, and Guangyu Zhang

Phys. Rev. B 97, 235145 – Published 25 June 2018

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Strongly enhanced exciton-phonon coupling in two-dimensional $WS e_{2}$