Role of hydrogen in the chemical vapor deposition growth of MoS2 atomic layers

In Situ Defect Engineering of Controllable Carrier Types in WSe2 for Homomaterial Inverters and Self-Powered Photodetectors

WSe2 has a high mobility of electrons and holes, which is an ideal choice as active channels of electronics in extensive fields. However, carrier-type tunability of WSe2 still has enormous challenges, which are essential to overcome for practical applications. In this work, the direct growth of n-doped few-layer WSe2 is realized via in situ defect engineering. The n-doping of WSe2 is attributed to Se vacancies induced by the H2 flow purged in the cooling process. The electrical measurements based on field effect transistors demonstrate that the carrier type of WSe2 synthesized is successfully transferred from the conventional p-type to the rarely reported n-type. The electron carrier concentration is efficiently modulated by the concentration of H2 during the cooling process. Furthermore, homomaterial inverters and self-powered photodetectors are fabricated based on the doping-type-tunable WSe2. This work reveals a significant way to realize the controllable carrier type of two-dimensional (2D) materials, exhibiting great potential in future 2D electronics engineering.

MoS2 Photoelectrodes for Hydrogen Production: Tuning the S-Vacancy Content in Highly Homogeneous Ultrathin Nanocrystals

Tuning the electrocatalytic properties of MoS2 layers can be achieved through different paths, such as reducing their thickness, creating edges in the MoS2 flakes, and introducing S-vacancies. We combine these three approaches by growing MoS2 electrodes by using a special salt-assisted chemical vapor deposition (CVD) method. This procedure allows the growth of ultrathin MoS2 nanocrystals (1-3 layers thick and a few nanometers wide), as evidenced by atomic force microscopy and scanning tunneling microscopy. This morphology of the MoS2 layers at the nanoscale induces some specific features in the Raman and photoluminescence spectra compared to exfoliated or microcrystalline MoS2 layers. Moreover, the S-vacancy content in the layers can be tuned during CVD growth by using Ar/H2 mixtures as a carrier gas. Detailed optical microtransmittance and microreflectance spectroscopies, micro-Raman, and X-ray photoelectron spectroscopy measurements with sub-millimeter spatial resolution show that the obtained samples present an excellent homogeneity over areas in the cm2 range. The electrochemical and photoelectrochemical properties of these MoS2 layers were investigated using electrodes with relatively large areas (0.8 cm2). The prepared MoS2 cathodes show outstanding Faradaic efficiencies as well as long-term stability in acidic solutions. In addition, we demonstrate that there is an optimal number of S-vacancies to improve the electrochemical and photoelectrochemical performances of MoS2.

Selective Chemical Vapor Deposition Growth of WS2/MoS2 Vertical and Lateral Heterostructures on Gold Foils

Vertical and lateral heterostructures consisting of atomically layered two-dimensional (2D) materials exhibit intriguing properties, such as efficient charge/energy transfer, high photoresponsivity, and enhanced photocatalytic activities. However, the controlled fabrication of vertical or lateral heterojunctions on metal substrates remains challenging. Herein, we report a facile and controllable method for selective growth of WS₂/MoS₂ vertical or lateral heterojunctions on polycrystalline gold (Au) foil by tuning the gas flow rate of hydrogen (H₂). We find that lateral growth is favored without H₂, whereas vertical growth mode can be switched on by introducing 8–10 sccm H₂. In addition, the areal coverage of the WS₂/MoS₂ vertical heterostructures is tunable in the range of 12–25%. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) results demonstrate the quality and absence of cross-contamination of the as-grown heterostructures. Furthermore, we investigate the effects of the H₂ flow rate on the morphology of the heterostructures. These pave the way to develop unprecedented 2D heterostructures towards applications in (opto)electronic devices.

Remote Plasma-Induced Synthesis of Self-Assembled MoS2/Carbon Nanowall Nanocomposites and Their Application as High-Performance Active Materials for Supercapacitors

The objective of this study is to investigate the synthesis and influence of MoS2 on carbon nanowalls (CNWs) as supercapacitor electrodes. The synthesis of MoS2 on CNW was achieved by the introduction of hydrogen remote plasma from ammonium tetrathiomolybdate (ATTM) without deterioration of the CNWs. The topographical surface structures and electrochemical characteristics of the MoS2–CNW composite electrodes were explored using two ATTM-dispersed organic solvents—acetonitrile and dimethylformamide (DMF). In this study, CNW and MoS2 were synthesized using an electron cyclotron resonance plasma. However, hydrogen radicals, which transform ATTM into MoS2, were provided in the form of a remote plasma source. The electrochemical performances of MoS2–CNW hybrid electrodes with various morphologies—depending on the solvent and ATTM concentration—were evaluated using a three-electrode system. The results revealed that the morphology of the synthesized MoS2 was influenced by the organic solvent used and affected both the electrochemical performance and topographical characteristics. Notably, considerable enhancement of the specific capacitance was observed for the MoS2 with open top edges synthesized from DMF. These encouraging results may motivate additional research on hybrid supercapacitor electrodes and the rapid synthesis of MoS2 and other transition metal dichalcogenides.

A Review on Chemical Vapour Deposition of Two-Dimensional MoS2 Flakes

Two-dimensional (2D) materials such as graphene, transition metal dichalcogenides, and boron nitride have recently emerged as promising candidates for novel applications in sensing and for new electronic and photonic devices. Their exceptional mechanical, electronic, optical, and transport properties show peculiar differences from those of their bulk counterparts and may allow for future radical innovation breakthroughs in different applications. Control and reproducibility of synthesis are two essential, key factors required to drive the development of 2D materials, because their industrial application is directly linked to the development of a high-throughput and reliable technique to obtain 2D layers of different materials on large area substrates. Among various methods, chemical vapour deposition is considered an excellent candidate for this goal thanks to its simplicity, widespread use, and compatibility with other processes used to deposit other semiconductors. In this review, we explore the chemical vapour deposition of MoS2, considered one of the most promising and successful transition metal dichalcogenides. We summarize the basics of the synthesis procedure, discussing in depth: (i) the different substrates used for its deposition, (ii) precursors (solid, liquid, gaseous) available, and (iii) different types of promoters that favour the growth of two-dimensional layers. We also present a comprehensive analysis of the status of the research on the growth mechanisms of the flakes.

Rational design for high-yield monolayer WS2films in confined space under fast thermal processing

Tungsten Disulfide (WS₂) films, as one of the most attractive members in the family of transition metal dichalcogenides, were synthesized typically on SiO₂/Si substrate by confine-spaced chemical vapor deposition method. The whole process could be controlled efficiently by precursor concentration and fast thermal process. To be priority, the effect of fast heating-up to cooling-down process and source ratio-dependent rule for WS₂structure have been systematically studied, leading to high-yield and fine structure of monolayer WS₂films with standard triangular morphology and average edge length of 92.4μm. The growth time of the samples was regulated within 3 min, and the optimal source ratio of sulfur to tungsten oxide is about 200:3. The whole experimental duration was about 50 min, which is only about quarter in comparison to relevant reports. We assume one type of 'multi-nucleation dynamic process' to provide a potential way for fast synthesis of the samples. Finally, the good performance of as-fabricated field-effect transistor on WS₂film was achieved, which exhibits high electron mobility of 4.62 cm²V^-1s^-1, fast response rate of 42 ms, and remarkable photoresponsivity of 3.7 × 10^-3A W^-1. Our work will provide a promising robust way for rapid synthesis of high-quality monolayer TMDs films and pave the way for the potential applications of TMDCs.

One-Step Synthesis of NbSe2/Nb-Doped-WSe2 Metal/Doped-Semiconductor van der Waals Heterostructures for Doping Controlled Ohmic Contact

van der Waals heterostructures (vdWHs) of metallic (m-) and semiconducting (s-) transition-metal dichalcogenides (TMDs) exhibit an ideal metal/semiconductor (M/S) contact in a field-effect transistor. However, in the current two-step chemical vapor deposition process, the synthesis of m-TMD on pregrown s-TMD contaminates the van der Waals (vdW) interface and hinders the doping of s-TMD. Here, NbSe₂/Nb-doped-WSe₂ metal-doped-semiconductor (M/d-S) vdWHs are created via a one-step synthesis approach using a niobium molar ratio-controlled solution-phase precursor. The one-step growth approach synthesizes Nb-doped WSe₂ with a controllable doping concentration and metal/doped-semiconductor vdWHs. The hole carrier concentration can be precisely controlled by controlling the Nb/(W + Nb) molar ratio in the precursor solution from ∼3 × 10¹¹/cm² at Nb-0% to ∼1.38 × 10¹²/cm² at Nb-60%; correspondingly, the contact resistance R_C value decreases from 10 888.78 at Nb-0% to 70.60 kΩ.μm at Nb-60%. The Schottky barrier height measurement in the Arrhenius plots of ln(I_sat/T²) versus q/K_BT demonstrated an ohmic contact in the NbSe₂/W_xNb_1-xSe₂ vdWHs. Combining p-doping in WSe₂ and M/d-S vdWHs, the mobility (27.24 cm² V^-1 s^-1) and on/off ratio (2.2 × 10⁷) are increased 1238 and 4400 times, respectively, compared to that using the Cr/pure-WSe₂ contact (0.022 cm² V^-1 s^-1 and 5 × 10³, respectively). Together, the R_C value using the NbSe₂ contact shows 2.46 kΩ.μm, which is ∼29 times lower than that of using a metal contact. This method is expected to guide the synthesis of various M/d-S vdWHs and applications in future high-performance integrated circuits.

Effects of Deposition and Annealing Temperature on the Structure and Optical Band Gap of MoS2 Films

In this study, molybdenum disulfide (MoS₂) film samples were prepared at different temperatures and annealed through magnetron sputtering technology. The surface morphology, crystal structure, bonding structure, and optical properties of the samples were characterized and analyzed. The surface of the MoS₂ films prepared by radio frequency magnetron sputtering is tightly coupled and well crystallized, the density of the films decreases, and their voids and grain size increase with the increase in deposition temperature. The higher the deposition temperature is, the more stable the MoS₂ films deposited will be, and the 200 °C deposition temperature is an inflection point of the film stability. Annealing temperature affects the structure of the films, which is mainly related to sulfur and the growth mechanism of the films. Further research shows that the optical band gaps of the films deposited at different temperatures range from 0.92 eV to 1.15 eV, showing semiconductor bandgap characteristics. The optical band gap of the films deposited at 200 °C is slightly reduced after annealing in the range of 0.71–0.91 eV. After annealing, the optical band gap of the films decreases because of the two exciton peaks generated by the K point in the Brillouin zone of MoS₂. The blue shift of the K point in the Brillouin zone causes a certain change in the optical band gap of the films.

Inherent Resistance of Seed-Mediated Grown MoSe2 Monolayers to Defect Formation

Recent progress in the chemical vapor deposition technique toward growing large-area and single-crystalline two-dimensional (2D) transition metal dichalcogenides (TMDs) has resulted in an electronic/optoelectronic device performance that rivals that of their top-down counterparts, despite the extensive use of hydrogen, a common reducing agent that readily generates defects in TMDs. Herein, we report that 2D MoSe₂ domains containing oxide seeds are resistant to hydrogen-induced defect generation. Specifically, we observed that the etching of the edges of seed-containing MoSe₂ was significantly less than that of pristine MoSe₂, without apparent seed particles, under the same H₂ annealing conditions. Our systematic approach for controlling the H₂ exposure time indicates that the oxidation of Mo and the edge roughening of seedless MoSe₂ coincidentally increase after H₂ exposure owing to the formation of Se vacancy followed by Mo oxidation, which is not the case with seed-containing MoSe₂. An ab initio calculation indicates that hydrogen preferentially adsorbs more onto O bonded to Mo than onto Se, providing further evidence of the resistance of seeded MoSe₂ to hydrogen etching. This finding provides an insight into controlling defect formation in 2D TMDs by employing sacrificial adsorption sites for reactive species (i.e., hydrogen).

The Large-Scale Preparation and Optical Properties of MoS2/WS2 Vertical Hetero-Junction

A variety of hetero-junctions can be constructed to form the basic structural units in the different optoelectronic devices, such as the photo-detectors, solar cells, sensors and light-emitting diodes. In our research, the large-area high-quality MoS₂/WS₂ vertical hetero-junction are prepared by the two-step atmospheric pressure chemical vapor deposition (APCVD) methods and the dry transfer method, and the corresponding optimal reaction conditions of MoS₂/WS₂ vertical hetero-junction are obtained. The morphology, composition and optical properties of MoS₂/WS₂ vertical hetero-junction are systematically characterized by the optical microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy and the field emission scanning electron microscopy. Compared to the mechanical transfer method, the MoS₂/WS₂ vertical hetero-junction sample obtained by the APCVD and dry transfer methods have lower impurity content, cleaner interfaces and tighter interlayer coupling. Besides, the strong interlayer coupling and effective interlayer charge transfer of MoS₂/WS₂ vertical hetero-junction are also further studied. The photoluminescence intensity of MoS₂/WS₂ vertical hetero-junction is significantly reduced compared to the single MoS₂ or WS₂ material. In general, this research can help to achieve the large-scale preparation of various Van der Waals hetero-junctions, which can lay the foundation for the new application of optoelectronic devices.

Methane-Mediated Vapor Transport Growth of Monolayer WSe2 Crystals

The electrical and optical properties of semiconducting transition metal dichalcogenides (TMDs) can be tuned by controlling their composition and the number of layers they have. Among various TMDs, the monolayer WSe₂ has a direct bandgap of 1.65 eV and exhibits p-type or bipolar behavior, depending on the type of contact metal. Despite these promising properties, a lack of efficient large-area production methods for high-quality, uniform WSe₂ hinders its practical device applications. Various methods have been investigated for the synthesis of large-area monolayer WSe₂, but the difficulty of precisely controlling solid-state TMD precursors (WO₃, MoO₃, Se, and S powders) is a major obstacle to the synthesis of uniform TMD layers. In this work, we outline our success in growing large-area, high-quality, monolayered WSe₂ by utilizing methane (CH₄) gas with precisely controlled pressure as a promoter. When compared to the catalytic growth of monolayered WSe₂ without a gas-phase promoter, the catalytic growth of the monolayered WSe₂ with a CH₄ promoter reduced the nucleation density to 1/1000 and increased the grain size of monolayer WSe₂ up to 100 μm. The significant improvement in the optical properties of the resulting WSe₂ indicates that CH₄ is a suitable candidate as a promoter for the synthesis of TMD materials, because it allows accurate gas control.

Atomic Layer Deposition of High-Quality Al2O3 Thin Films on MoS2 with Water Plasma Treatment

Atomic layer deposition (ALD) of ultrathin dielectric films on two-dimensional (2D) materials for electronic device applications remains one of the key challenges because of the lack of dangling bonds on the 2D material surface. In this work, a new technique to deposit uniform and high-quality Al₂O₃ films with thickness down to 1.5 nm on MoS₂ is introduced. By treating the surface using water plasma prior to the ALD process, hydroxyl groups are introduced to the MoS₂ surface, facilitating the chemisorption of trimethylaluminum in a conventional water-based ALD system. Raman and X-ray photoelectron spectroscopy measurements show that the water plasma treatment does not induce noticeable material degradation. The deposited Al₂O₃ films show excellent device-related electrical performance characteristics, including low interface trap density and outstanding gate controllability.

Probing the Field-Effect Transistor with Monolayer MoS2 Prepared by APCVD

The two-dimensional materials can be used as the channel material of transistor, which can further decrease the size of transistor. In this paper, the molybdenum disulfide (MoS₂) is grown on the SiO₂/Si substrate by atmospheric pressure chemical vapor deposition (APCVD), and the MoS₂ is systematically characterized by the high-resolution optical microscopy, Raman spectroscopy, photoluminescence spectroscopy, and the field emission scanning electron microscopy, which can confirm that the MoS₂ is a monolayer. Then, the monolayer MoS₂ is selected as the channel material to complete the fabrication process of the back-gate field effect transistor (FET). Finally, the electrical characteristics of the monolayer MoS₂-based FET are tested to obtain the electrical performance. The switching ratio is 10³, the field effect mobility is about 0.86 cm²/Vs, the saturation current is 2.75 × 10^-7 A/μm, and the lowest gate leakage current is 10^-12 A. Besides, the monolayer MoS₂ can form the ohmic contact with the Ti/Au metal electrode. Therefore, the electrical performances of monolayer MoS₂-based FET are relatively poor, which requires the further optimization of the monolayer MoS₂ growth process. Meanwhile, it can provide the guidance for the application of monolayer MoS₂-based FETs in the future low-power optoelectronic integrated circuits.

Selective Engineering of Chalcogen Defects in MoS2 by Low-Energy Helium Plasma

Structural defects in two-dimensional transition-metal dichalcogenides can significantly modify the material properties. Previous studies have shown that chalcogen defects can be created by physical sputtering, but the energetic ions can potentially displace transition-metal atoms at the same time, leading to ambiguous results and in some cases, degradation of material quality. In this work, selective sputtering of S atoms in monolayer MoS₂ without damaging the Mo sublattice is demonstrated with low-energy helium plasma treatment. Based on X-ray photoelectron spectroscopy analysis, wide-range tuning of S defect concentration is achieved by controlling the ion energy and sputtering time. Furthermore, characterization with scanning transmission electron microscopy confirms that by keeping the ion energy low, the Mo sublattice remains intact. The properties of MoS₂ at different defect concentrations are also characterized. In situ device measurement shows that the flake can be tuned from a semiconducting to metallic-like behavior by introducing S defects due to the creation of mid-gap states. When the defective MoS₂ is exposed to air, the S defects are soon passivated, with oxygen atoms filling the defect sites.

Research on the Factors Affecting the Growth of Large-Size Monolayer MoS₂ by APCVD

The transition-metal chalcogenides (TMDs) are gaining increased attention from many scientists recently. Monolayer MoS₂ is an emerging layered TMD material with many excellent physical and electrical properties. It can be widely used in catalysis, transistors, optoelectronics and integrated circuits. Here, the large-sized monolayer MoS₂ is grown on the silicon substrate with a 285-nm-thick oxide layer by atmospheric pressure chemical vapor deposition (APCVD) of sulfurized molybdenum trioxide. This method is simple and it does not require vacuum treatment. In addition, the effects of growth conditions, such as sulfur source, molybdenum source, growth temperature, and argon flow rate on the quality and area of MoS₂ are further studied systematically. These analysis results help to master the morphology and optical properties of monolayer MoS₂. The high quality, excellent performance, and large-size monolayer MoS₂ under the optimal growth condition is characterized by optical microscopy, AFM, XPS, photoluminescence, and Raman spectroscopy. The Raman spectrum and PL mapping show that the grown MoS₂ is a uniform triangular monolayer with a side length of 100 μm, which can pave the way for the applications of photodetectors and transistors.

Electronic-dimensionality reduction of bulk MoS2 by hydrogen treatment

A reduction in the electronic-dimensionality of materials is one method for achieving improvements in material properties. Here, a reduction in electronic-dimensionality is demonstrated using a simple hydrogen treatment technique. Quantum well states from hydrogen-treated bulk 2H-MoS2 are observed using angle resolved photoemission spectroscopy (ARPES). The electronic states are confined within a few MoS2 layers after the hydrogen treatment. A significant reduction in the band-gap can also be achieved after the hydrogen treatment, and both phenomena can be explained by the formation of sulfur vacancies generated by the chemical reaction between sulfur and hydrogen.

A General Method for the Chemical Synthesis of Large-Scale, Seamless Transition Metal Dichalcogenide Electronics

The capability to directly build atomically thin transition metal dichalcogenide (TMD) devices by chemical synthesis offers important opportunities to achieve large-scale electronics and optoelectronics with seamless interfaces. Here, a general approach for the chemical synthesis of a variety of TMD (e.g., MoS₂ , WS₂ , and MoSe₂ ) device arrays over large areas is reported. During chemical vapor deposition, semiconducting TMD channels and metallic TMD/carbon nanotube (CNT) hybrid electrodes are simultaneously formed on CNT-patterned substrate, and then coalesce into seamless devices. Chemically synthesized TMD devices exhibit attractive electrical and mechanical properties. It is demonstrated that chemically synthesized MoS₂ -MoS₂ /CNT devices have Ohmic contacts between MoS₂ /CNT hybrid electrodes and MoS₂ channels. In addition, MoS₂ -MoS₂ /CNT devices show greatly enhanced mechanical stability and photoresponsivity compared with conventional gold-contacted devices, which makes them suitable for flexible optoelectronics. Accordingly, a highly flexible pixel array based on chemically synthesized MoS₂ -MoS₂ /CNT photodetectors is applied for image sensing.

Adsorption of H2, O2, H2O, OH and H on monolayer MoS2

Hydrogen and hydrogen-containing gases are commonly used as reductants in chemical vapor deposition growth of MoS₂. Here, we consider the defects resulting from the presence of hydrogen during growth and the resulting electronically active defects. In particular, we find that the interstitial hydrogen defect is a negative-U center with amphoteric donor and acceptor properties. Additionally, we consider the effects of interaction with water and oxygen. The defects are analysed using density functional theory calculations.

A Novel and Facile Route to Synthesize Atomic-Layered MoS2 Film for Large-Area Electronics

High-quality and large-area molybdenum disulfide (MoS₂ ) thin film is highly desirable for applications in large-area electronics. However, there remains a challenge in attaining MoS₂ film of reasonable crystallinity due to the absence of appropriate choice and control of precursors, as well as choice of suitable growth substrates. Herein, a novel and facile route is reported for synthesizing few-layered MoS₂ film with new precursors via chemical vapor deposition. Prior to growth, an aqueous solution of sodium molybdate as the molybdenum precursor is spun onto the growth substrate and dimethyl disulfide as the liquid sulfur precursor is supplied with a bubbling system during growth. To supplement the limiting effect of Mo (sodium molybdate), a supplementary Mo is supplied by dissolving molybdenum hexacarbonyl (Mo(CO)₆ ) in the liquid sulfur precursor delivered by the bubbler. By precisely controlling the amounts of precursors and hydrogen flow, full coverage of MoS₂ film is readily achievable in 20 min. Large-area MoS₂ field effect transistors (FETs) fabricated with a conventional photolithography have a carrier mobility as high as 18.9 cm² V^-1 s^-1 , which is the highest reported for bottom-gated MoS₂ -FETs fabricated via photolithography with an on/off ratio of ≈10⁵ at room temperature.

In Situ Transmission Electron Microscopy Characterization and Manipulation of Two-Dimensional Layered Materials beyond Graphene

Two-dimensional (2D) ultra-thin materials beyond graphene with rich physical properties and unique layered structures are promising for applications in electronics, chemistry, energy, and bioscience, etc. The interaction mechanisms among the structures, chemical compositions and physical properties of 2D layered materials are critical for fundamental nanosciences and the practical fabrication of next-generation nanodevices. Transmission electron microscopy (TEM), with its high spatial resolution and versatile external fields, is undoubtedly a powerful tool for the static characterization and dynamic manipulation of nanomaterials and nanodevices at the atomic scale. The rapid development of thin-film and precision microelectromechanical systems (MEMS) techniques allows 2D layered materials and nanodevices to be probed and engineered inside TEM under external stimuli such as thermal, electrical, mechanical, liquid/gas environmental, optical, and magnetic fields at the nanoscale. Such advanced technologies leverage the traditional static TEM characterization into an in situ and interactive manipulation of 2D layered materials without sacrificing the resolution or the high vacuum chamber environment, facilitating exploration of the intrinsic structure-property relationship of 2D layered materials. In this Review, the dynamic properties tailored and observed by the most advanced and unprecedented in situ TEM technology are introduced. The challenges in spatial, time and energy resolution are discussed also.

Two-dimensional WS2 nanoribbon deposition by conversion of pre-patterned amorphous silicon

We present a method for area selective deposition of 2D WS₂ nanoribbons with tunable thickness on a dielectric substrate. The process is based on a complete conversion of a pre-patterned, H-terminated Si layer to metallic W by WF₆, followed by in situ sulfidation by H₂S. The reaction process, performed at 450 °C, yields nanoribbons with lateral dimension down to 20 nm and with random basal plane orientation. The thickness of the nanoribbons is accurately controlled by the thickness of the pre-deposited Si layer. Upon rapid thermal annealing at 900 °C under inert gas, the WS₂ basal planes align parallel to the substrate.

Vacuum ultraviolet excitation luminescence spectroscopy of few-layered MoS2

We report on vacuum ultraviolet (VUV) excited photoluminescence (PL) spectra emitted from a chemical vapor deposited MoS2 few-layered film. The excitation spectrum was recorded by monitoring intensities of PL spectra at ~1.9 eV. A strong wide excitation band peaking at 7 eV was found in the excitation. The PL excitation band is most intensive at liquid helium temperature and completely quenched at 100 K. Through first-principles calculations of photoabsorption in MoS2, the excitation was explicated and attributed to transitions of electrons from p- and d- type states in the valence band to the d- and p-type states in the conduction band. The obtained photon-in/photon-out results clarify the excitation and emission behavior of the low dimensional MoS2 when interacting with the VUV light sources.