Electronic properties of 2<i>H</i>-stacking bilayer MoS<sub>2</sub> measured by terahertz time-domain spectroscopy

doi:10.1007/s11467-023-1295-1

Front. Phys.

2023, Vol. 18

Issue (5) : 53303 https://doi.org/10.1007/s11467-023-1295-1

RESEARCH ARTICLE

Electronic properties of 2H-stacking bilayer MoS₂ measured by terahertz time-domain spectroscopy

Xingjia Cheng^1,², Wen Xu^3,^1,⁴(

), Hua Wen^1,², Jing Zhang^1,², Heng Zhang^1,², Haowen Li³, Francois M. Peeters^3,⁵, Qingqing Chen^1,²

¹. Key Laboratory of Materials Physics, Institute of Solid State Physics, HFIPS, Chinese Academy of Sciences, Hefei 230031, China
². University of Science and Technology of China, Hefei 230026, China
³. Micro Optical Instruments Inc., Shenzhen 518118, China
⁴. School of Physics and Astronomy and Yunnan Key Laboratory of Quantum Information of Yunnan Province, Yunnan University, Kunming 650091, China
⁵. Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium

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Abstract

Bilayer (BL) molybdenum disulfide (MoS₂) is one of the most important electronic structures not only in valleytronics but also in realizing twistronic systems on the basis of the topological mosaics in moiré superlattices. In this work, BL MoS₂ on sapphire substrate with 2H-stacking structure is fabricated. We apply the terahertz (THz) time-domain spectroscopy (TDS) for examining the basic optoelectronic properties of this kind of BL MoS₂. The optical conductivity of BL MoS₂ is obtained in temperature regime from 80 K to 280 K. Through fitting the experimental data with the theoretical formula, the key sample parameters of BL MoS₂ can be determined, such as the electron density, the electronic relaxation time and the electronic localization factor. The temperature dependence of these parameters is examined and analyzed. We find that, similar to monolayer (ML) MoS₂, BL MoS₂ with 2H-stacking can respond strongly to THz radiation field and show semiconductor-like optoelectronic features. The theoretical calculations using density functional theory (DFT) can help us to further understand why the THz optoelectronic properties of BL MoS₂ differ from those observed for ML MoS₂. The results obtained from this study indicate that the THz TDS can be applied suitably to study the optoelectronic properties of BL MoS₂ based twistronic systems for novel applications as optical and optoelectronic materials and devices.

Keywords terahertz time-domain spectroscopy bilayer MoS₂ optoelectronics

Corresponding Author(s): Wen Xu

Issue Date: 22 May 2023

Cite this article:

Xingjia Cheng,Wen Xu,Hua Wen, et al. Electronic properties of 2H-stacking bilayer MoS₂ measured by terahertz time-domain spectroscopy[J]. Front. Phys. , 2023, 18(5): 53303.

URL:

https://academic.hep.com.cn/fop/EN/10.1007/s11467-023-1295-1
https://academic.hep.com.cn/fop/EN/Y2023/V18/I5/53303

Fig.1 Schematic diagram of the sample preparation. i) The ML MoS

2

was grown on sapphire wafer by using the CVD; ii) The as-grown ML-MoS

2

/sapphire wafer was cut into small pieces; iii) The ML MoS

2

film on a small piece was peeled off from sapphire substrate; and iv) the ML MoS

2

film was transferred onto the small ML-MoS

2

/sapphire piece, where the twisting angle between two ML MoS

2

films was set to be at

θ = 60 ∘

to obtain the 2H-stacking BL MoS

2

. The bottom figures show lattice structure of 2H-stacking BL MoS

2

Fig.2 (a) AFM image of BL MoS

2

on sapphire substrate. The inset shows the hight of the step-change between MoS

2

layer and the substrate along the line marked as white straight line, which corresponds to the thickness of the BL MoS

2

. (b) Raman spectra of BL MoS

2

on sapphire substrate (black curve), measured at room-temperature by a 532 nm laser excitation. Two characteristic peaks for BL MoS

2

A 1 g

and

E 2 g 1

with a spacing about 22.1 cm^?1, can be clearly identified. By spectral decomposition (red and blue curves), the Raman peaks

A 1 g

and

E 2 g 1

along with “a” peak induced characteristically by the 2H-stacking type and the characteristic peak for sapphire can be found. (c) Photoluminescence (PL) spectra of ML (black curve) and BL (red curve) MoS

2

, measured at room-temperature using a 532 nm laser excitation, where the results are shown after deducting the signals from sapphire substrate. The inset shows the electronic band structure of ML MoS

2

obtained from DFT calculation. (d) Electronic band structure of 2H-stacking BL MoS

2

, obtained from the DFT calculation. Here the

Σ m i n

-point in the conduction band is lower than that in the K-point, with a difference about 197 meV.

Fig.3 Schematic diagram of the THz TDS system for optical transmission measurement. Here PCA is the photoconductance antenna.

Fig.4 (a) Amplitude and phase angle of the THz electric field

E m s (ω)

transmitted through a BL MoS

2

-sapphire sample as a function of radiation frequency

f = ω / (2 π)

at different temperatures. The inset shows the electric field strength

E m s (t)

of the THz beam transmitted through a BL MoS

2

/sapphire sample as a function of delay time at different temperatures. (b) Real

σ 1 (ω)

(upper panel) and imaginary

σ 2 (ω)

(lower panel) parts of the optical conductivity for BL MoS

2

as a function of radiation frequency f =

ω / (2 π)

at different temperatures. The dots are experimental results and the curves are obtained from Drude?Smith formula given by Eq. (3). Here

Σ 0 = e 2 / (4 ?) = 6.07 × 10 ? 5

Fig.5 Electron density n_e (blue), electronic relaxation time

τ

(red), and electronic localization factor c (inset) in 2H-stacking BL MoS

2

as a function of temperature. For n_e and c, the curves are drawn to guide the eye, and for

τ

the curve is obtained by using Eq. (4).

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