Charge Dynamics and Electronic Structures of Monolayer MoS<sub>2</sub>Films Grown by Chemical Vapor Deposition

Shen, Chien Chang; Hsu, Yu-Te; Li, Lain‐Jong; Liu, Hsiang Lin

doi:10.7567/apex.6.125801

Cited by 88 publications

(76 citation statements)

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“…Deep in the ultraviolet (UV) region, the dielectric function shows a series of absorption peaks or fine features separated by about 0.7 eV. Our high precision dielectric function data facilitates the accurate optical properties analysis, and the results agree well with previous work including ellipsometry [24], absorption measurements [8], PL measurements [9], and theoretical calculation [16][17][18].…”

Section: Introductionsupporting

confidence: 83%

“…The reported experimental studies focused on photoluminescence (PL), absorbance, optical contrast, and photoconductivity in the vicinity of the K point in its Brillouin zone, and the corresponding spectral ranges are rather limited [8,9,19,[21][22][23]. On the other hand, there are limited reports on the broad band optical dielectric function of monolayer MoS 2 in the published literature [24], and it is expected to be distinct from that of bulk MoS 2 . Not until very recently could large-area monolayer MoS 2 with sample sizes up to millimeters be grown by chemical vapor deposition (CVD) [25][26][27][28].…”

Section: Introductionmentioning

confidence: 99%

“…Yim et al [32] have found that, in their four-phase ellipsometric model, the refraction index (n) and the extinction coefficient (k) of their CVD MoS 2 exhibit a dependence on the MoS 2 film thickness. Shen et al [24], using terahertz (THz) absorption and SE, investigated the charge dynamics to show the semiconductor behavior and a blueshift of ß0.2 eV of charge transfer bands compared with those of the bulk material and also determine the dielectric function from 0.73 to 6.42 eV.…”

Section: Introductionmentioning

confidence: 99%

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Broadband optical properties of large-area monolayer CVD molybdenum disulfide

et al. 2014

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Recently emerging large-area single-layer MoS 2 grown by chemical vapor deposition has triggered great interest due to its exciting potential for applications in advanced electronic and optoelectronic devices. Unlike gapless graphene, MoS 2 has an intrinsic band gap in the visible which crosses over from an indirect to a direct gap when reduced to a single atomic layer. In this paper, we report a comprehensive study of fundamental optical properties of MoS 2 revealed by optical spectroscopy of Raman, photoluminescence, and vacuum ultraviolet spectroscopic ellipsometry. A band gap of 1.42 eV is determined by the absorption threshold of bulk MoS 2 that shifts to 1.83 eV in monolayer MoS 2 . We extracted the high precision dielectric function up to 9.0 eV, which leads to the identification of many unique interband transitions at high symmetry points in the MoS 2 momentum space. The positions of the so-called A and B excitons in single layers are found to shift upwards in energy compared with those of the bulk form and have smaller separation because of the decreased interactions between the layers. A very strong optical critical point predicted to correspond to a quasiparticle gap is observed at 2.86 eV, which is attributed to optical transitions along the parallel bands between the M and points in the reduced Brillouin zone. The absence of the bulk MoS 2 spin-orbit interaction peak at ß3.0 eV in monolayer MoS 2 is, as predicted, the consequence of the coalescence of nearby excitons. A higher energy optical transition at 3.98 eV, commonly occurring in bulk semiconductors, is associated with a combination of several critical points. Additionally, extending into the vacuum ultraviolet energy spectrum are a series of newly observed oscillations representing optical transitions from valence bands to higher conduction bands of the monolayer MoS 2 complex band structure. These optical transitions herein reported enhance our understanding of monolayer MoS 2 as well as of two-dimensional systems in general and thus provide informative guidelines for MoS 2 optical device designs and theoretical considerations.

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Section: Introductionsupporting

confidence: 83%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Broadband optical properties of large-area monolayer CVD molybdenum disulfide

et al. 2014

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“…4c) likely arises from dielectric environment differences between bulk and single layer specimens. The extinction coefficient, k, in the region 3 to 4.5 eV is about ∼2.5…3.5, which is comparable with that (∼3.2) of Yim et al [14] for ultrathin CVD MoS 2 and with the data of Li et al [15] for TMDs, but much lower than that (∼5.1) of Shen et al [39]. The higher coefficient, k, could be a result of sample structural quality or the way it was extracted from their four-phase ellipsometric model [39].…”

Section: Resultssupporting

confidence: 79%

“…The extinction coefficient, k, in the region 3 to 4.5 eV is about ∼2.5…3.5, which is comparable with that (∼3.2) of Yim et al [14] for ultrathin CVD MoS 2 and with the data of Li et al [15] for TMDs, but much lower than that (∼5.1) of Shen et al [39]. The higher coefficient, k, could be a result of sample structural quality or the way it was extracted from their four-phase ellipsometric model [39]. It is well known that the optical conductivity spectra σ(ω) reflect the main details of the density of electron states within energies close to critical points (CPs) [40].…”

Section: Resultssupporting

confidence: 79%

Ellipsometry and optical spectroscopy of low-dimensional family TMDs

Kravets¹

2017

Semicond. Phys. Quantum Electron. Optoelectron.

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Abstract. Here, we report a comprehensive study of fundamental optical properties of two-dimensional materials. These properties have been ascertained using spectroscopic ellipsometry, optical spectroscopy of Raman scattering, and photoluminescence. We have focused on optical properties of the chemically exfoliated layered TMDs: MoS 2 , MoSe 2 , WS 2 , and WSe 2 . The complex refractive index and optical conductivity within the region 1 to 4.5 eV were extracted, which lead to identification of many unique interband transitions at high symmetry points in the electron band structure. The positions of the so-called A and B excitons in monolayers are found to shift upwards in energy as compared with those of the bulk form and have smaller separation because of the decreased interactions between the layers. For monolayer TMDs, the valence-band spin-orbit splitting at the K point was estimated from the energy difference between the A and B exciton peaks. Our findings of the spin-orbit coupling of ∼0.16, ∼0.26, ∼0.37, and ∼0.55 eV in monolayers MoS 2 , MoSe 2 , WS 2 , and WSe 2 , respectively. All these findings not only extend our understanding the novel electronic structures of mono-and few-layers TMDs but also provide foundation for future technological applications of optoelectronic and spintronic device components.

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Progress, Challenges, and Opportunities for 2D Material Based Photodetectors

Long

Wang

Fang

et al. 2018

Adv Funct Materials

1,130

872

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2D material based photodetectors have attracted many research projects due to their unique structures and excellent electronic and optoelectronic properties. These 2D materials, including semimetallic graphene, semiconducting black phosphorus, transition metal dichalcogenides, insulating hexagonal boron nitride, and their various heterostructures, show a wide distribution in bandgap values. To date, hundreds of photodetectors based on 2D materials have been reported. Here, a review of photodetectors based on 2D materials covering the detection spectrum from ultraviolet to infrared is presented. First, a brief insight into the detection mechanisms of 2D material photodetectors as well as introducing the figure-of-merits which are key factors for a reasonable comparison between different photodetectors is provided. Then, the recent progress on 2D material based photodetectors is reviewed. Particularly, the excellent performances such as broadband spectrum detection, ultrahigh photoresponsivity and sensitivity, fast response speed and high bandwidth, polarization-sensitive detection are pointed out on the basis of the state-of-the-art 2D photodetectors. Initial applications based on 2D material photodetectors are mentioned. Finally, an outlook is delivered, the challenges and future directions are discussed, and general advice for designing and realizing novel high-performance photodetectors is given to provide a guideline for the future development of this fast-developing field.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201803807. atomic layer, while these atomically thin layers are bonded together by weak van der Waals interactions along the third dimension perpendicular to the 2D plane. The weak interlayer interaction makes it possible to exfoliate bulk crystals into isolated thin 2D flakes and even a single atomically thin layer.A photodetector is a device that can convert a light signal into an electrical signal. High performance photodetectors play important roles in many fields of our daily life, including electro-optical displays, imaging, environment monitoring, optical communication, military, security checking and so forth. 2D materials, as one of most competitive materials for designing photodetectors, have been demonstrated with remarkable characteristics, including broad detection waveband covering the wavelengths from UV to terahertz frequencies (THz, 10 12 Hz), ultrahigh photoresponsivity, polarization sensitive photodetection, high-speed photoresponse, high spatially resolved imaging, etc. Thanks to the recently developed dry transfer technique, [1] various heterostructures based on 2D materials have been designed and fabricated with desirable band alignments. Photodetectors based on these heterostructures have exhibited enhanced performances like high photovoltaic external quantum efficiency up to 30% for graphene-WS 2 -graphene, [2] ultrahigh photoresponsivity up to ≈10 10 A W −1 for graphene-MoS 2 , [3] ultrahigh photogain ...

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Charge Dynamics and Electronic Structures of Monolayer MoS₂Films Grown by Chemical Vapor Deposition

Cited by 88 publications

References 50 publications

Broadband optical properties of large-area monolayer CVD molybdenum disulfide

Broadband optical properties of large-area monolayer CVD molybdenum disulfide

Ellipsometry and optical spectroscopy of low-dimensional family TMDs

Progress, Challenges, and Opportunities for 2D Material Based Photodetectors

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Charge Dynamics and Electronic Structures of Monolayer MoS2Films Grown by Chemical Vapor Deposition

Cited by 88 publications

References 50 publications

Broadband optical properties of large-area monolayer CVD molybdenum disulfide

Broadband optical properties of large-area monolayer CVD molybdenum disulfide

Ellipsometry and optical spectroscopy of low-dimensional family TMDs

Progress, Challenges, and Opportunities for 2D Material Based Photodetectors

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Charge Dynamics and Electronic Structures of Monolayer MoS₂Films Grown by Chemical Vapor Deposition