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Research Article

Reliable and broad-range layer identification of Au-assisted exfoliated large area MoS2 and WS2 using reflection spectroscopic fingerprints

Bo Zou1, Yu Zhou1, Yan Zhou2, Yanyan Wu1, Yang He1, Xiaonan Wang1, Jinfeng Yang1, Lianghui Zhang1, Yuxiang Chen1, Shi Zhou3, Huaixin Guo4, Huarui Sun1( )
School of Science and Ministry of Industry and Information Technology Key Laboratory of Micro-Nano Optoelectronic Information System, Harbin Institute of Technology Shenzhen, Shenzhen 518055, China Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
University of Science and Technology of China, Hefei 230026, China
Science and Technology on Monolithic Integrated Circuits and Modules Laboratory, Nanjing Electronic Devices Institute, Nanjing 210016, China
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Graphical Abstract

The authors propose to use the intensity of exciton-induced reflection peaks to quickly and accurately determine the layer numbers in Au-assisted exfoliated large-area MoS2 and WS2 films in a broad thickness range. The method is based on optical interference supported by Fresnel’s theory calculations and is generic to transition metal dichalcogenides (TMDs) supported on different substrates.

Abstract

The emerging Au-assisted exfoliation technique enables the production of a wealth of large-area and high-quality ultrathin two dimensional (2D) crystals. Fast, damage-free, and reliable determination of the layer number of such 2D films can greatly promote layer-dependent physical studies and device applications. Here, an optical method has been developed for simple, high throughput, and accurate determination of the layer number for Au-assisted exfoliated MoS2 and WS2 films in a broad thickness range. The method is based on quantitative analysis of layer-dependent white light reflection spectra (WLRS), revealing that the intensity of exciton-induced reflection peaks can be used as a clear indicator for identifying the layer number. The simple yet robust method will facilitate fundamental studies on layer-dependent optical, electrical, and thermal properties and device applications of 2D materials. The technique can also be readily combined with photoluminescence (PL) and Raman spectroscopies to study other layer-dependent physical properties of 2D materials.

Keywords

Au-assisted exfoliationMoS2 and WS2layer identificationwhite light reflection spectraexcitonic peaks

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References

1

Bertolazzi, S.; Brivio, J.; Kis, A. Stretching and breaking of ultrathin MoS2. ACS Nano 2011, 5, 9703–9709.
Crossref Google Scholar
2

Chernikov, A.; Berkelbach, T. C.; Hill, H. M.; Rigosi, A.; Li, Y. L.; Aslan, O. B.; Reichman, D. R.; Hybertsen, M. S.; Heinz, T. F. Exciton binding energy and nonhydrogenic Rydberg series in monolayer WS2. Phys. Rev. Lett. 2014, 113, 076802.
Crossref Google Scholar
3

He, K. L.; Kumar, N.; Zhao, L.; Wang, Z. F.; Mak, K. F.; Zhao, H.; Shan, J. Tightly bound excitons in monolayer WSe2. Phys. Rev. Lett. 2014, 113, 026803.
Crossref Google Scholar
4

Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically thin MoS2: A new direct-gap semiconductor. Phys. Rev. Lett. 2010, 105, 136805.
Crossref Google Scholar
5

Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS2 transistors. Nat. Nanotechnol. 2011, 6, 147–150.
Crossref Google Scholar
6

Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat. Nanotechnol. 2012, 7, 699–712.
Crossref Google Scholar
7

Xu, X. D.; Yao, W.; Xiao, D.; Heinz, T. F. Spin and pseudospins in layered transition metal dichalcogenides. Nat. Phys. 2014, 10, 343–350.
Crossref Google Scholar
8

Yan, R. S.; Simpson, J. R.; Bertolazzi, S.; Brivio, J.; Watson, M.; Wu, X. F.; Kis, A.; Luo, T. F.; Hight Walker, A. R.; Xing, H. G. Thermal conductivity of monolayer molybdenum disulfide obtained from temperature-dependent Raman spectroscopy. ACS Nano 2014, 8, 986–993.
Crossref Google Scholar
9

Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films. Science 2004, 306, 666–669.
Crossref Google Scholar
10

Huang, Y.; Pan, Y. H.; Yang, R.; Bao, L. H.; Meng, L.; Luo, H. L.; Cai, Y. Q.; Liu, G. D.; Zhao, W. J.; Zhou, Z. et al. Universal mechanical exfoliation of large-area 2D crystals. Nat. Commun. 2020, 11, 2453.
Crossref Google Scholar
11

Huang, Y.; Sutter, E.; Shi, N. N.; Zheng, J. B.; Yang, T. Z.; Englund, D.; Gao, H. J.; Sutter, P. Reliable exfoliation of large-area high-quality flakes of graphene and other two-dimensional materials. ACS Nano 2015, 9, 10612–10620.
Crossref Google Scholar
12

Huang, Y.; Wang, Y. K.; Huang, X. Y.; Zhang, G. H.; Han, X.; Yang, Y.; Gao, Y.; Meng, L.; Wang, Y. S.; Geng, G. Z. et al. An efficient route to prepare suspended monolayer for feasible optical and electronic characterizations of two‐dimensional materials. InfoMat 2021, 4, e12274.
Crossref Google Scholar
13
Li Z. W. Ren L. W. Wang S. Y. Huang X. X. Li Q. Y. Lu Z. Y. Ding S. M. Deng H. J. Chen P. A. Lin J. et al Dry exfoliation of large-area 2D monolayer and heterostructure arrays ACS Nano 2021 15 13839 13846 10.1021/acsnano.1c05734

Li, Z. W.; Ren, L. W.; Wang, S. Y.; Huang, X. X.; Li, Q. Y.; Lu, Z. Y.; Ding, S. M.; Deng, H. J.; Chen, P. A.; Lin, J. et al. Dry exfoliation of large-area 2D monolayer and heterostructure arrays. ACS Nano 2021, 15, 13839–13846.
Crossref Google Scholar
14

Velický, M.; Donnelly, G. E.; Hendren, W. R.; McFarland, S.; Scullion, D.; DeBenedetti, W. J. I.; Correa, G. C.; Han, Y. M.; Wain, A. J.; Hines, M. A. et al. Mechanism of gold-assisted exfoliation of centimeter-sized transition-metal dichalcogenide monolayers. ACS Nano 2018, 12, 10463–10472.
Crossref Google Scholar
15

Lee, H. S.; Min, S. W.; Chang, Y. G.; Park, M. K.; Nam, T.; Kim, H.; Kim, J. H.; Ryu, S.; Im, S. MoS2 nanosheet phototransistors with thickness-modulated optical energy gap. Nano Lett. 2012, 12, 3695–3700.
Crossref Google Scholar
16

Lin, J. D.; Li, H.; Zhang, H.; Chen, W. Plasmonic enhancement of photocurrent in MoS2 field-effect-transistor. Appl. Phys. Lett. 2013, 102, 203109.
Crossref Google Scholar
17

Wang, W. Y.; Klots, A.; Prasai, D.; Yang, Y. M.; Bolotin, K. I.; Valentine, J. Hot electron-based near-infrared photodetection using bilayer MoS2. Nano Lett. 2015, 15, 7440–7444.
Crossref Google Scholar
18

Yan, J. H.; Ma, C. R.; Liu, P.; Yang, G. W. Plasmon-induced energy transfer and photoluminescence manipulation in MoS2 with a different number of layers. ACS Photonics 2017, 4, 1092–1100.
Crossref Google Scholar
19

Zhou, W. J.; Yin, Z. Y.; Du, Y. P.; Huang, X.; Zeng, Z. Y.; Fan, Z. X.; Liu, H.; Wang, J. Y.; Zhang, H. Synthesis of few-layer MoS2 nanosheet-coated TiO2 nanobelt heterostructures for enhanced photocatalytic activities. Small 2013, 9, 140–147.
Crossref Google Scholar
20

Roddaro, S.; Pingue, P.; Piazza, V.; Pellegrini, V.; Beltram, F. The optical visibility of graphene: Interference colors of ultrathin graphite on SiO2. Nano Lett. 2007, 7, 2707–2710.
Crossref Google Scholar
21

Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. The structure of suspended graphene sheets. Nature 2007, 446, 60–63.
Crossref Google Scholar
22

Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-dimensional atomic crystals. Proc. Natl. Acad. Sci. USA 2005, 102, 10451–10453.
Crossref Google Scholar
23

Hao, Y. F.; Wang, Y. Y.; Wang, L.; Ni, Z. H.; Wang, Z. Q.; Wang, R.; Koo, C. K.; Shen, Z. X.; Thong, J. T. Probing layer number and stacking order of few-layer graphene by Raman spectroscopy. Small 2010, 6, 195–200.
Crossref Google Scholar
24

Lee, C.; Yan, H. G.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S. Anomalous lattice vibrations of single- and few-layer MoS2. ACS Nano 2010, 4, 2695–2700.
Crossref Google Scholar
25

Li, X. L.; Han, W. P.; Wu, J. B.; Qiao, X. F.; Zhang, J.; Tan, P. H. Layer-number dependent optical properties of 2D materials and their application for thickness determination. Adv. Funct. Mater. 2017, 27, 1604468.
Crossref Google Scholar
26
Li H. Wu J. Huang X. Lu G. Yang J. Lu X. Xiong Q. H. Zhang H. Rapid and reliable thickness identification of two-dimensional nanosheets using optical microscopy ACS Nano 2013 7 10344 10353 10.1021/nn4047474

Li, H.; Wu, J.; Huang, X.; Lu, G.; Yang, J.; Lu, X.; Xiong, Q. H.; Zhang, H. Rapid and reliable thickness identification of two-dimensional nanosheets using optical microscopy. ACS Nano 2013, 7, 10344–10353.
Crossref Google Scholar
27

Taghavi, N. S.; Gant, P.; Huang, P.; Niehues, I.; Schmidt, R.; Michaelis de Vasconcellos, S.; Bratschitsch, R.; García-Hernández, M.; Frisenda, R.; Castellanos-Gomez, A. Thickness determination of MoS2, MoSe2, WS2 and WSe2 on transparent stamps used for deterministic transfer of 2D materials. Nano Res. 2019, 12, 1691–1695.
Crossref Google Scholar
28

Zhang, H.; Ran, F. R.; Shi, X. T.; Fang, X. R.; Wu, S. Y.; Liu, Y.; Zheng, X. Q.; Yang, P.; Liu, Y.; Wang, L. et al. Optical thickness identification of transition metal dichalcogenide nanosheets on transparent substrates. Nanotechnology 2017, 28, 164001.
Crossref Google Scholar
29

Castellanos-Gomez, A.; Agraït, N.; Rubio-Bollinger, G. Optical identification of atomically thin dichalcogenide crystals. Appl. Phys. Lett. 2010, 96, 213116.
Crossref Google Scholar
30

Gorbachev, R. V.; Riaz, I.; Nair, R. R.; Jalil, R.; Britnell, L.; Belle, B. D.; Hill, E. W.; Novoselov, K. S.; Watanabe, K.; Taniguchi, T. et al. Hunting for monolayer boron nitride: Optical and Raman signatures. Small 2011, 7, 465–468.
Crossref Google Scholar
31

Hu, X. M.; Qiu, C. C.; Liu, D. M. Rapid thin-layer WS2 detection based on monochromatic illumination photographs. Nano Res. 2020, 14, 840–845.
Crossref Google Scholar
32

Ni, Z. H.; Wang, H. M.; Kasim, J.; Fan, H. M.; Yu, T.; Wu, Y. H.; Feng, Y. P.; Shen, Z. X. Graphene thickness determination using reflection and contrast spectroscopy. Nano Lett. 2007, 7, 2758–2763.
Crossref Google Scholar
33

Zhao, W. J.; Ghorannevis, Z.; Chu, L. Q.; Toh, M.; Kloc, C.; Tan, P. H.; Eda, G. Evolution of electronic structure in atomically thin sheets of WS2 and WSe2. ACS Nano 2013, 7, 791–797.
Crossref Google Scholar
34

Anzai, Y.; Yamamoto, M.; Genchi, S.; Watanabe, K.; Taniguchi, T.; Ichikawa, S.; Fujiwara, Y.; Tanaka, H. Broad range thickness identification of hexagonal boron nitride by colors. Appl. Phys. Express 2019, 12, 055007.
Crossref Google Scholar
35

Chen, H. Y.; Fei, W. W.; Zhou, J. X.; Miao, C. Y.; Guo, W. L. Layer identification of colorful black phosphorus. Small 2017, 13, 1602336.
Crossref Google Scholar
36

Jin, Y.; Rah, Y.; Park, J.; Shim, J.; Yu, K. Rapid and broad-range thickness estimation method of hexagonal boron nitride using Raman spectroscopy and optical microscope. Appl. Phys. Lett. 2020, 116, 081104.
Crossref Google Scholar
37

Ouyang, W. G.; Liu, X. Z.; Li, Q. Y.; Zhang, Y. Y.; Yang, J. R.; Zheng, Q. S. Optical methods for determining thicknesses of few-layer graphene flakes. Nanotechnology 2013, 24, 505701.
Crossref Google Scholar
38

Huang, Y.; Sutter, E.; Sadowski, J. T.; Cotlet, M.; Monti, O. L. A.; Racke, D. A.; Neupane, M. R.; Wickramaratne, D.; Lake, R. K.; Parkinson, B. A. et al. Tin disulfide—an emerging layered metal dichalcogenide semiconductor: Materials properties and device characteristics. ACS Nano 2014, 8, 10743–10755.
Crossref Google Scholar
39

Benameur, M. M.; Radisavljevic, B.; Héron, J. S.; Sahoo, S.; Berger, H.; Kis, A. Visibility of dichalcogenide nanolayers. Nanotechnology 2011, 22, 125706.
Crossref Google Scholar
40

Childres, I.; Jauregui, L. A.; Foxe, M.; Tian, J. F.; Jalilian, R.; Jovanovic, I.; Chen, Y. P. Effect of electron-beam irradiation on graphene field effect devices. Appl. Phys. Lett. 2010, 97, 173109.
Crossref Google Scholar
41

Zhang, L. L.; Yan, H.; Sun, X. Q.; Dong, M. H.; Yildirim, T.; Wang, B. W.; Wen, B.; Neupane, G. P.; Sharma, A.; Zhu, Y. et al. Modulated interlayer charge transfer dynamics in a monolayer TMD/metal junction. Nanoscale 2019, 11, 418–425.
Crossref Google Scholar
42

Jin, Y.; Yu, K. A review of optics-based methods for thickness and surface characterization of two-dimensional materials. J. Phys. D Appl. Phys. 2021, 54, 393001.
Crossref Google Scholar
43

Rani, R.; Kundu, A.; Hazra, K. S. Spectral dependent white light reflection mapping of MoS2 flake for improving accuracy of conventional optical thickness profiling. Opt. Mater. 2019, 90, 46–50.
Crossref Google Scholar
44

Li, L. K.; Kim, J.; Jin, C. H.; Ye, G. J.; Qiu, D. Y.; da Jornada, F. H.; Shi, Z. W.; Chen, L.; Zhang, Z. C.; Yang, F. Y. et al. Direct observation of the layer-dependent electronic structure in phosphorene. Nat. Nanotechnol. 2017, 12, 21–25.
Crossref Google Scholar
45

Mao, N. N.; Tang, J. Y.; Xie, L. M.; Wu, J. X.; Han, B. W.; Lin, J. J.; Deng, S. B.; Ji, W.; Xu, H.; Liu, K. et al. Optical anisotropy of black phosphorus in the visible regime. J. Am. Chem. Soc. 2016, 138, 300–305.
Crossref Google Scholar
46

Ugeda, M. M.; Bradley, A. J.; Shi, S. F.; da Jornada, F. H.; Zhang, Y.; Qiu, D. Y.; Ruan, W.; Mo, S. K.; Hussain, Z.; Shen, Z. X. et al. Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor. Nat. Mater. 2014, 13, 1091–1095.
Crossref Google Scholar
47

Zou, B.; Zhou, Y.; Zhang, X.; Zhang, M.; Liu, K.; Gong, M. M.; Sun, H. R. Thickness-dependent ultralow in-plane thermal conductivity of chemical vapor-deposited SnSe2 nanofilms: Implications for thermoelectrics. ACS Appl. Nano Mater. 2020, 3, 10543–10550.
Crossref Google Scholar
48

Campos-Delgado, J.; Algara-Siller, G.; Santos, C. N.; Kaiser, U.; Raskin, J. P. Twisted Bi-layer graphene: Microscopic rainbows. Small 2013, 9, 3247–3251.
Crossref Google Scholar
49

Qiao, X. F.; Wu, J. B.; Zhou, L. W.; Qiao, J. S.; Shi, W.; Chen, T.; Zhang, X.; Zhang, J.; Ji, W.; Tan, P. H. Polytypism and unexpected strong interlayer coupling in two-dimensional layered ReS2. Nanoscale 2016, 8, 8324–8332.
Crossref Google Scholar
50

Velický, M.; Rodriguez, A.; Bouša, M.; Krayev, A. V.; Vondráček, M.; Honolka, J.; Ahmadi, M.; Donnelly, G. E.; Huang, F. M.; Abruña, H. D. et al. Strain and charge doping fingerprints of the strong interaction between monolayer MoS2 and gold. J. Phys. Chem. Lett. 2020, 11, 6112–6118.
Crossref Google Scholar
51

Zhu, L.; Tang, J.; Li, B. C.; Hou, T. Y.; Zhu, Y.; Zhou, J. D.; Wang, Z.; Zhu, X. R.; Yao, Z. P.; Cui, X. et al. Artificial neuron networks enabled identification and characterizations of 2D materials and van der Waals Heterostructures. ACS Nano 2022, 16, 2721–2729.
Crossref Google Scholar
52

Veshapidze, G.; Trachy, M. L.; Shah, M. H.; DePaola, B. D. Reducing the uncertainty in laser beam size measurement with a scanning edge method. Appl. Opt. 2006, 45, 8197–8199.
Crossref Google Scholar
53

Zou, B.; Wei, Y. D.; Zhou, Y.; Ke, D. N.; Zhang, X.; Zhang, M.; Yip, C. T.; Chen, X. B.; Li, W. Q.; Sun, H. R. Unambiguous determination of crystal orientation in black phosphorus by angle-resolved polarized Raman spectroscopy. Nanoscale Horiz. 2021, 6, 809–818.
Crossref Google Scholar
54

Qiu, D. Y.; da Jornada, F. H.; Louie, S. G. Optical spectrum of MoS2: Many-body effects and diversity of exciton states. Phys. Rev. Lett. 2013, 111, 216805.
Crossref Google Scholar
55

Jia, G. Y.; Liu, Y.; Gong, J. Y.; Lei, D. Y.; Wang, D. L.; Huang, Z. X. Excitonic quantum confinement modified optical conductivity of monolayer and few-layered MoS2. J. Mater. Chem. C 2016, 4, 8822–8828.
Crossref Google Scholar
56

Zou, B.; Wu, Z. X.; Zhou, Y.; Zhou, Y.; Wang, J.; Zhang, L. H.; Cao, F.; Sun, H. R. Spectroscopic ellipsometry investigation of Au-assisted exfoliated large-area single-crystalline monolayer MoS2. Phys. Status Solidi 2021, 15, 2100385.
Crossref Google Scholar
57

Buckley, R. G.; Beaglehole, D. Absorptance of thin films. Appl. Opt. 1977, 16, 2495–2499.
Crossref Google Scholar
58

Wilson, J. A.; Yoffe, A. D. The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties. Adv. Phys. 1969, 18, 193–335.
Crossref Google Scholar
59

Li, Y. L.; Chernikov, A.; Zhang, X.; Rigosi, A.; Hill, H. M.; van der Zande, A. M.; Chenet, D. A.; Shih, E. M.; Hone, J.; Heinz, T. F. Measurement of the optical dielectric function of monolayer transition-metal dichalcogenides: MoS2, MoSe2, WS2, and WSe2. Phys. Rev. B 2014, 90, 205422.
Crossref Google Scholar
60
Zhang H. Y. Bao C. H. Jiang Z. Y. Zhang K. N. Li H. Chen C. Y. Avila J. Wu Y. Duan W. H. Asensio M. C. et al Resolving deep quantum-well states in atomically thin 2H-MoTe2 flakes by nanospot angle-resolved photoemission spectroscopy Nano Lett. 2018 18 4664 4668 10.1021/acs.nanolett.8b00589

Zhang, H. Y.; Bao, C. H.; Jiang, Z. Y.; Zhang, K. N.; Li, H.; Chen, C. Y.; Avila, J.; Wu, Y.; Duan, W. H.; Asensio, M. C. et al. Resolving deep quantum-well states in atomically thin 2H-MoTe2 flakes by nanospot angle-resolved photoemission spectroscopy. Nano Lett. 2018, 18, 4664–4668.
Crossref Google Scholar
61

Chen, W. J.; Zhang, R. J.; Zheng, R. X.; Liu, B. L. Out-of-plane resistance switching of 2D Bi2O2Se at the nanoscale. Adv. Funct. Mater. 2021, 31, 2105795.
Crossref Google Scholar
62

Li, S. L.; Miyazaki, H.; Song, H. S.; Kuramochi, H.; Nakaharai, S.; Tsukagoshi, K. Quantitative Raman spectrum and reliable thickness identification for atomic layers on insulating substrates. ACS Nano 2012, 6, 7381–7388.
Crossref Google Scholar
63

Frisenda, R.; Niu, Y.; Gant, P.; Molina-Mendoza, A. J.; Schmidt, R.; Bratschitsch, R.; Liu, J. X.; Fu, L.; Dumcenco, D.; Kis, A. et al. Micro-reflectance and transmittance spectroscopy: A versatile and powerful tool to characterize 2D materials. J. Phys. D Appl. Phys. 2017, 50, 074002.
Crossref Google Scholar
64

Niu, Y.; Gonzalez-Abad, S.; Frisenda, R.; Marauhn, P.; Drüppel, M.; Gant, P.; Schmidt, R.; Taghavi, N. S.; Barcons, D.; Molina-Mendoza, A. J. et al. Thickness-dependent differential reflectance spectra of monolayer and few-layer MoS2, MoSe2, WS2 and WSe2. Nanomaterials (Basel) 2018, 8, 725.
Crossref Google Scholar
Nano Research
Volume 15 Issue 9,
September 2022
Pages 8470-8478
DOI: 10.1007/s12274-022-4418-z
Cite this article:
Zou B, Zhou Y, Zhou Y, et al. Reliable and broad-range layer identification of Au-assisted exfoliated large area MoS2 and WS2 using reflection spectroscopic fingerprints. Nano Research, 2022, 15(9): 8470-8478. https://doi.org/10.1007/s12274-022-4418-z
Topics:
Thickness measurement

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Received: 06 January 2022
Revised: 10 April 2022
Accepted: 11 April 2022
Published: 03 June 2022
© Tsinghua University Press 2022
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