Two-dimensional Transition Metal Dichalcogenides Research Articles

Phase Transformation on Multilayer 2M-WS2 for Improved Surface-enhanced Raman Scattering.

In recent years, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been widely recognized as an ideal platform for surface-enhanced Raman scattering (SERS). Given their rich structural phases, phase transformation in 2D TMDCs is an efficient strategy to tailor their SERS performance. In this paper, we present the great SERS performance of multilayer 2M-WS2 and then investigate the effect of its phase transformation on SERS performance. It is observed that multilayer 2M-WS2 nanosheets undergo a thermally induced single-crystal phase transition from 2M-WS2 to 2H-WS2 upon thermal annealing or laser treatment. Distinguishing from the commercially available pure 2H-WS2 (P-2H-WS2), 2H-WS2 obtained by annealing and laser treatment still retain SERS properties comparable to those of 2M-WS2, among which the detection limits for CV molecules (10-8 M) are 3 orders of magnitude lower than that of P-2H-WS2 and the Raman intensity enhancements are ∼10-37 times higher. In contrast to the charge transfer (CT) mechanism governed by the Fermi level in metallic-phase 2M-WS2, 2H-WS2 obtained by phase transition exhibits accelerated CT facilitated by the bandgap reduction and reorganization resulting from the abundance of vacancies. This study introduces an interesting perspective and potential avenue for enhancing SERS through metal-to-semiconductor phase transitions in 2D TMDCs materials.

Simulation of electrical rectification effect in two-dimensional MoSe2/WSe2 lateral heterostructures

Two-dimensional (2D) transition metal dichalcogenides lateral heterostructures exhibit excellent performance in electrics and optics. The electron transport of the heterostructures can be effectively regulated by ingenious design. In this study, we construct a monolayer MoSe2/WSe2 lateral heterostructure, covalently connecting monolayer MoSe2 and monolayer WSe2. Using the Extended Huckel Theory method, we explored current-voltage characteristics under varied conditions, including altering carrier density, atomic replacement and interface angles. Calculations demonstrate a significant electrical rectification ratio (ERR) ranging from 200 to 800. Additionally, Employing Density Functional Theory with non-equilibrium Green’s function method, we investigated electronic properties, attributing the rectification effect to electronic state distribution differences, asymmetric transmission coefficients and band bending of projected local density of states. The expandability of the interfacial energy barrier enhances the rectification effect through adjustments in carrier concentration, atomic replacements and interface size. However, these enhancements introduce challenges such as increased electron-boundary scattering and reduced ambipolarity, resulting in a lower ERR. This study provides valuable theoretical insights for optimizing 2D electronic diode devices, offering avenues for precise control of the rectification effect.

Machine-learning-empowered identification of initial growth modes for 2D transition metal dichalcogenide thin films

In situ reflection high-energy electron diffraction (RHEED) is a powerful technique for monitoring surface states and offers invaluable insights into thin film growth. However, extracting hidden features and subtle changes from its vast data remains as a challenge. This work bridges the gap by employing machine learning (ML)-empowered RHEED analysis to elucidate the growth dynamics of two-dimensional (2D) transition metal dichalcogenide (TMDC) thin films grown under two distinct growth modes. Principal component analysis (PCA) and its modified processes were used to separate contributions of the graphene substrate and the MoSe2 film in the RHEED video. The ML-empowered RHEED analysis allowed us to effectively filter out the strong substrate signal and reconstructing RHEED videos solely for the MoSe2 films. This approach enabled detailed monitoring of film growth with unique features, and clearly distinguishing between the layer-by-layer growth mode and the island one. This work demonstrates the potentials of ML-empowered RHEED analysis for revealing complex growth dynamics of 2D TMDC materials, paving the way for advanced thin film monitoring and autonomous control in wider scope of thin film technologies.

Flower-Branch-Like carbon Microtube/Vanadium Selenide: A highly efficient electromagnetic wave absorbing composite

The unique layered structure and adjustable band gap of transition metal dichalcogenides (TMDs) have prompted extensive investigations into their potential for electromagnetic wave absorption (EWA). Vanadium selenide (VSe2), a notable representative of TMDs, has a larger interlayer spacing (6.1 Å) and higher conductivity (1 × 10-3 S/m), which has high advantageous in EWA applications. However, the existing research on the use of VSe2 for EWA remains relatively limited. Herein, a flower-branch-like carbon microtube/vanadium selenide (CMT/VSe2) composite was successfully synthesized via a facile solvothermal method. This composite material is composed of flower-like VSe2 nanostructures integrated with carbon microtubes. Importantly, the EWA properties of this composite can be effectively tailored by adjusting the concentration of the VSe2 precursor. The CMT/VSe2 composite exhibited a remarkable minimum reflection loss (RL) of −60.54 dB in the Ku-band, with a matching thickness of 2.27 mm. Moreover, the effective absorption bandwidth reached 6.38 GHz. These results demonstrate the great potential of this CMT/VSe2 composite material as a highly efficient EWA material. Furthermore, this work broadens the application of two-dimensional TMDs in the area of EWA.

Rhodamine 6G/Transition Metal Dichalcogenide Hybrid Nanoscrolls for Enhanced Optoelectronic Performance.

The low light absorption efficiency has seriously hindered the application of two-dimensional transition metal dichalcogenide (TMDC) nanosheets in the field of optoelectronic devices. Various approaches have been used to improve the performance of TMDC nanosheets. Preparation of one-dimensional TMDC nanoscrolls in combination with photoactive materials has been a promising method to improve their properties recently. In this work, we report a facile method to enhance the optoelectronic performance of TMDC nanoscrolls by wrapping the photoactive organic dye rhodamine (R6G) into them. After R6G molecules were deposited on monolayer TMDC nanosheets by the solution method, the R6G/MoS2 nanoscrolls with lengths up to hundreds of microns were prepared in a short time by dropping a mixture of ammonia and ethanol solution on the R6G/MoS2 nanosheets. The as-obtained R6G/MoS2 nanoscrolls were well characterized by optical microscopy, atomic force microscopy, Raman spectroscopy, and transmission electron microscopy to prove the encapsulation of R6G. There are multiple type II heterojunction interfaces in the R6G/MoS2 nanoscrolls, which can promote the generation of photo-induced carriers and the following electron-hole separation. The separated electrons were transported rapidly along the axial direction of the R6G/MoS2 nanoscrolls, which greatly improves the efficiency of light absorption and photoresponse. Under the irradiation of an incident 405 nm laser, the photoresponsivity, carrier mobility, external quantum efficiency, and detectivity of R6G/MoS2 nanoscrolls were enhanced to 66.07 A/W, 132.93 cm2V-1s-1, 20,261%, and 1.25 × 1012 cm·Hz1/2W-1, which are four orders of magnitude higher than those of monolayer MoS2 nanosheets. Our work indicates that the R6G/TMDC hybrid nanoscrolls could be promising materials for high-performance optoelectronic devices.

Broadband tunable resonance modes from multi-composition monolayer MoS2(1−x)Se2x with SiO2 microsphere cavity

Two-dimensional (2D) monolayer transition metal dichalcogenides (TMDCs) that are compatible with Si-based substrates have already exhibited huge application potential in optoelectronics and photonics. The MoS2(1−x)Se2x ternary alloy consisting of two different chalcogens, as a class of lasing gain medium, enriches the family of 2D TMDC materials. Here, monolayer MoS2(1−x)Se2x ternary alloys with tunable composition have been synthesized via single-step chemical vapor deposition method. Raman and photoluminescence studies demonstrate that the bandgap of grown monolayer MoS2(1−x)Se2x alloys can be gradually tuned from 1.59 to 1.82 eV, indicating the continuous changes of the chemical composition x from 0.82 to 0. The oscillation characteristic is further investigated, where the MoS2(1−x)Se2x alloy provides optical gain for the SiO2 microsphere resonant cavity. The achieved resonance modes in a broadband range from 610 to 810 nm not only extend the range of potential TMDC-based lasers, but also drive the applications of alloy materials in various optoelectronics devices.

A Mini Review: Phase Regulation for Molybdenum Dichalcogenide Nanomaterials.

Atomically thin two-dimensional transition metal dichalcogenides (TMDCs) have been regarded as ideal and promising nanomaterials that bring broad application prospects in extensive fields due to their ultrathin layered structure, unique electronic band structure, and multiple spatial phase configurations. TMDCs with different phase structures exhibit great diversities in physical and chemical properties. By regulating the phase structure, their properties would be modified to broaden the application fields. In this mini review, focusing on the most widely concerned molybdenum dichalcogenides (MoX2: X = S, Se, Te), we summarized their phase structures and corresponding electronic properties. Particularly, the mechanisms of phase transformation are explained, and the common methods of phase regulation or phase stabilization strategies are systematically reviewed and discussed. We hope the review could provide guidance for the phase regulation of molybdenum dichalcogenides nanomaterials, and further promote their real industrial applications.

Raman scattering owing to magneto-polaron states in monolayer transition metal dichalcogenides

Magneto-optical measurements are fundamental research tools that allow for studying the hitherto unexplored optical transitions and the related applications of topological two-dimensional (2D) transition metal dichalcogenides (TMDs). A theoretical model is developed for the first-order magneto-resonant Raman scattering in a monolayer of TMD. A significant number of avoided crossing points involving optical phonons in the magneto-polaron (MP) spectrum, a superposition of the electron and hole states in the excitation branches, and their manifestations in optical transitions at various light scattering configurations are unique features for these 2D structures. The Raman intensity reveals three resonant splittings of double avoided-crossing levels. The three excitation branches are present in the MP spectrum provoked by the coupling of the Landau levels in the conduction and valence bands via an out-of-plane A1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$A_1$$\\end{document}-optical phonon mode. The energy gaps at the anticrossing points in the MP scattering spectrum are revealed as a function of the electron and hole optical deformation potential constants. The resonant MP Raman scattering efficiency profile allows for quantifying the relative contribution of the conduction and valence bands in the formation of MPs. The results obtained are a guideline for controlling MP effects on the magneto-optical properties of TMD semiconductors, which open pathways to novel optoelectronic devices based on 2D TMDs.

MoSe2 monolayer as a two-dimensional anode material for lithium-ion batteries: A first-principles study

Nowadays, finding applicable electrode materials with high energy density and high cycle stability is highly desired in the field of lithium-ion batteries. MoSe2 monolayer has been experimentally synthesized and shows satisfactory metal anchoring capability, making it a potential candidate for energy storage devices. In this work, first-principles calculations are employed to investigate the potential of MoSe2 monolayers with different phases (1T, 1T′ and 1H) as anode materials. The results show that 1T′-MoSe2 monolayer has excellent thermal, dynamical and mechanical stability. In addition, 1T′-MoSe2 monolayer exhibits metallic property under Li adsorption, ensuring good electrical conductivity for efficient electron transport. The Li atom has a low diffusion barrier of 0.299 eV on 1T′-MoSe2 monolayer, which results in a good charge-discharge rate. 1T′-MoSe2 monolayer has a maximum Li storage capacity of 422 mA h/g and an open-circuit voltage of 0.21 V. Our work reveals the application of 1T′-MoSe2 monolayer as an anode material for lithium-ion batteries and promotes the design of energy storage materials based on two-dimensional transition metal dichalcogenides.

Effect of molecular rotation and concentration on the adsorption of pentacene molecules on two-dimensional monolayer transition metal dichalcogenides

Heterostructures composed of pentacene (PEN) molecules and transition metal dichalchogenides (TMDs) are promising materials for small, flexible and lightweight photovoltaic devices and various other optoelectronic applications. The effects of changing concentration and orientation of adsorbed PEN molecules on two-dimensional monolayer substrates of TMDs, namely MoS2, MoSe2, WS2 and WSe2, were investigated using first-principles calculations based on density functional theory. We examined the structural and electronic properties of the corresponding PEN/TMD heterostructures and compared these between differing PEN concentrations and the orientations of PEN with respect to the underlying substrate crystal structure. We analyze the band alignment of the heterostructures and demonstrate a concentration-dependent staggered-to-straddling (typeII-I) band gap transition in PEN/MoSe2.

Intercalation of quaternary ammonium cations as a key factor of electron storage in MoS2 thin films

Electrochemical intercalation of cations within two-dimensional transition metal dichalcogenides presents a promising route for tailoring their optoelectronic properties. We have now succeeded in modulating the optical properties of MoS2 thin films through electrochemical intercalation of quaternary ammonium cations. The spectroelectrochemical experiments conducted with varying sizes of the intercalant revealed the size-dependent stability of the intercalated MoS2 nanosheets. The observed absorption change of the exciton bands is reversible and arises from the storage of electrons in MoS2 nanosheets and the subsequent weakening of interlayer van der Waals interactions following cation intercalation. This structural change is evidenced by the emergence of A*1g out-of-plane Raman mode. Additionally, the photoelectron spectroscopy reveals the emergence of a lower binding energy component of Mo 3d and the shift in Fermi level to higher energies, confirming the presence of stored electrons in cation intercalated-MoS2. The underlying mechanism of intercalation-induced property modifications in MoS2 discussed in the present study is useful in developing strategies for energy conversion devices.

InP Crystal Phase Heterojunction Transistor with a Vertical Gate-All-Around Structure.

Crystal phase transitions can form a new type of heterojunction with different atomic arrangements in the same material: crystal phase heterojunction (CPHJ). The CPHJ has an inherently strong impact on band engineering without concerns over critical thicknesses with misfit dislocations and a semiconductor-metal transition. In-plane CPHJ was recently demonstrated in two-dimensional (2D) transition-metal dichalcogenide (TMD) materials and utilized for conventional planar field-effect transistor applications. However, scalability such as gate electrostatic control, miniaturization, and multigate structure have been limited because of the geometrical issue. Here, we demonstrated a transistor using the CPHJ with a vertical gate-all-around structure by forming a CPHJ in conventional III-V semiconductors. The CPHJ, composed of wurtzite InP nanowires with zincblende InP substrates, showed an atomically flat heterojunction without dislocations and indicated a Type-II band discontinuity across the junction. The CPHJ transistor had moderate to good gate electrostatic controllability with high on-state currents and transconductance. The CPHJ offer will provide a new switching mechanism and add a new junction and device design choice to the long history of transistors.

Switching mechanisms of two-dimensional transition metal dichalcogenides(2D TMDCs)-based memristive devices for neuromorphic computing

The recent surge in artificial intelligence has escalated the demand for computing performance and storage capability in information processing hardware. However, in traditional von Neumann architecture, the processing and memory units are connected by a bus with limited bandwidth, which leads to excessive time and power consumption in data transfer. To address this issue, the emerging neuromorphic architecture co-locates the memory and processing functionalities by mimicking the information processing akin to the human brain. The inefficiency of traditional technology in neuromorphic hardware has led to extensive interest in novel memristive devices. The two-dimensional (2D) transition metal dichalcogenides (TMDCs) exhibit appealing characteristics for memristive devices, including tunable bandgap, high mobility at atomic thickness, and the rich possibility of defect engineering. This review focuses on the switching mechanisms of memristive devices based on 2D TMDCs. Additionally, properties and performances of 2D TMDCs-based memristive devices are also summarized in this review with respect to different switching mechanisms.

Strong nonlinear optical processes with extraordinary polarization anisotropy in inversion-symmetry broken two-dimensional PdPSe

Nonlinear optical activities, especially second harmonic generation (SHG), are key phenomena in inversion-symmetry-broken two-dimensional (2D) transition metal dichalcogenides (TMDCs). On the other hand, anisotropic nonlinear optical processes are important for unique applications in nano-nonlinear photonic devices with polarization functions, having become one of focused research topics in the field of nonlinear photonics. However, the strong nonlinearity and strong optical anisotropy do not exist simultaneously in common 2D materials. Here, we demonstrate strong second-order and third-order susceptibilities of 64 pm/V and 6.2×10−19 m2/V2, respectively, in the even-layer PdPSe, which has not been discovered in other common TMDCs (e.g., MoS2). Strikingly, it also simultaneously exhibited strong SHG anisotropy with an anisotropic ratio of ~45, which is the largest reported among all 2D materials to date, to the best of our knowledge. In addition, the SHG anisotropy ratio can be harnessed from 0.12 to 45 (375 times) by varying the excitation wavelength due to the dispersion of χ(2)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\chi }^{(2)}$$\\end{document} values. As an illustrative example, we further demonstrate polarized SHG imaging for potential applications in crystal orientation identification and polarization-dependent spatial encoding. These findings in 2D PdPSe are promising for nonlinear nanophotonic and optoelectronic applications.

Catalytic strategies for uniform monolayer MoSe2 growth in CVD processes

To facilitate the widespread utilization of devices based on two-dimensional transition metal dichalcogenide (TMDC) materials, addressing the challenge of producing large-area and uniform monolayers is crucial. In this study, we investigated the efficacy of various catalysts in promoting the growth of monolayer MoSe2 through chemical vapor deposition (CVD) as a potential solution to improve surface coverage and uniformity while maintaining the monolayer structure. Five distinct water-soluble catalysts (sodium chloride (NaCl), zinc chloride (ZnCl2), potassium hydroxide (KOH), potassium chloride (KCl), potassium oxalate (K2C2O4)) were employed in conjunction with a glass substrate for the CVD-based growth of monolayer MoSe2. We applied these catalyst solutions as thin layers onto SiO2/Si substrates to observe their specific catalytic effects on monolayer MoSe2 formation. MoSe2 was selected as the target material due to its promising characteristics within the TMDC family, particularly its high mobility. Our findings revealed that NaCl, ZnCl2, KCl, and K2C2O4 catalysts enhanced monolayer growth in the 2H phase on SiO2/Si substrates, while the glass substrate resulted in a hybrid 1T/2H MoSe2 structure, albeit with the highest surface coverage. Furthermore, the use of a catalyst allowed us to bypass the laborious transfer step by enabling direct growth of the MoSe2 flakes on the Si/SiO2 substrate, eliminating the risk of flake damage arising from the transfer process. Among the catalyst solutions, NaCl yielded the most homogeneous MoSe2 monolayer growth with relatively high coverage.

The Defects Genome of Janus Transition Metal Dichalcogenides.

2D Janus Transition Metal Dichalcogenides (TMDs) have attracted much interest due to their exciting quantum properties arising from their unique two-faced structure, broken-mirror symmetry, and consequent colossal polarization field within the monolayer. While efforts are made to achieve high-quality Janus monolayers, the existing methods rely on highly energetic processes that introduce unwanted grain-boundary and point defects with still unexplored effects on the material's structural and excitonic properties Through high-resolution scanning transmission electron microscopy (HRSTEM), density functional theory (DFT), and optical spectroscopy measurements; this work introduces the most encountered and energetically stable point defects. It establishes their impact on the material's optical properties. HRSTEM studies show that the most energetically stable point defects are single (VS and VSe) and double chalcogen vacancy (VS -VSe), interstitial defects (Mi), and metal impurities (MW) and establish their structural characteristics. DFT further establishes their formation energies and related localized bands within the forbidden band. Cryogenic excitonic studies on h-BN-encapsulated Janus monolayers offer a clear correlation between these structural defects and observed emission features, which closely align with the results of the theory. The overall results introduce the defect genome of Janus TMDs as an essential guideline for assessing their structural quality and device properties.

Rapid and large-scale synthesis of MoS2 via ultraviolet laser-assisted technology for photodetector applications

Two-dimensional transition metal dichalcogenide (TMDC) thin films have been extensively employed in microelectronics research. Molybdenum disulfide (MoS2), as one of prominent candidates of this class, has been applied in photodetectors, integrated electronic devices, gas sensing, and electrochemical catalysis, owing to its extraordinary optoelectronic, chemical, and mechanical properties. Synthesis of MoS2 crystal film is the key to its application. However, the reported technology revealed several drawbacks, containing limited surface area, prolonged high-temperature environment, and unsatisfying crystallinity. In order to enhance the convenience of MoS2 applications, there is a pressing need for optimized fabrication technology, which could be quicker, with a large area, with adequate crystallinity and heat-saving. In this work, we presented an ultraviolet laser-assisted synthesis technology, accomplishing rapid growth (with the growth rate of about 40 μm s−1) of centimeter-scale MoS2 films at room temperature. To achieve this, we self-assembled a displaceable reaction chamber system, coupled with krypton fluoride ultraviolet pulse laser. The laser motion speed and trajectory could be customized in the software, allowing the maskless patterning of crystal films. As application, we exhibited a photodetector with the integration of synthesized MoS2 and lead sulfide colloidal quantum dots (PbS CQDs), displaying broadband photodetection from ultraviolet, visible to near-infrared spectrum (365–1550 nm), with the detectivity of 109–1010 Jones, and the rising time of 0.2–0.3 s. This work not only demonstrated a high-process-efficiency synthesis of TMDC materials, but also has opened up new opportunities for ultraviolet laser used in optoelectronics.

Synthesis and characterization of uniform 3R phase bilayer MoS2 on sapphire

Bilayer (BL) two-dimensional transition metal dichalcogenides (TMDs) exhibit excellent properties in carrier mobility, state density, and room temperature stability, which attract considerable attention. However, previous efforts on synthesis BL TMDs have lacked homogeneity and control over the number of layers. Here, we demonstrate an effective and controllable approach to synthesize highly uniform 3R phase BL MoS2 on a c-plane sapphire substrate. The number of MoS2 layer can be controlled by adjusting the confinement distance, sapphire step height (annealing temperature), and growth temperature. These influencing factors have been thoroughly investigated. Raman and PL analyses demonstrate bilayer properties and remarkable uniformity. SHG and HAADF-STEM measurements reveal 3R stacking structure of BL MoS2. Furthermore, field-effect-transistor (FET) devices fabricated using BL MoS2 domains exhibit high carrier mobility and on/off ratio. This work provides an efficient method for layer-controlled MoS2 growth and offers new insights into the growth mechanism of BL MoS2.

Direct Selective Epitaxy of 2D Sb2Te3 onto Monolayer WS2 for Vertical p-n Heterojunction Photodetectors.

Two-dimensional transition metal dichalcogenides (2D-TMDs) possess appropriate bandgaps and interact via van der Waals (vdW) forces between layers, effectively overcoming lattice compatibility challenges inherent in traditional heterojunctions. This property facilitates the creation of heterojunctions with customizable bandgap alignments. However, the prevailing method for creating heterojunctions with 2D-TMDs relies on the low-efficiency technique of mechanical exfoliation. Sb2Te3, recognized as a notable p-type semiconductor, emerges as a versatile component for constructing diverse vertical p-n heterostructures with 2D-TMDs. This study presents the successful large-scale deposition of 2D Sb2Te3 onto inert mica substrates, providing valuable insights into the integration of Sb2Te3 with 2D-TMDs to form heterostructures. Building upon this initial advancement, a precise epitaxial growth method for Sb2Te3 on pre-existing WS2 surfaces on SiO2/Si substrates is achieved through a two-step chemical vapor deposition process, resulting in the formation of Sb2Te3/WS2 heterojunctions. Finally, the development of 2D Sb2Te3/WS2 optoelectronic devices is accomplished, showing rapid response times, with a rise/decay time of 305 μs/503 μs, respectively.

Spatially Confined Microcells: A Path toward TMD Catalyst Design.

With the ability to maximize the exposure of nearly all active sites to reactions, two-dimensional transition metal dichalcogenide (TMD) has become a fascinating new class of materials for electrocatalysis. Recently, electrochemical microcells have been developed, and their unique spatial-confined capability enables understanding of catalytic behaviors at a single material level, significantly promoting this field. This Review provides an overview of the recent progress in microcell-based TMD electrocatalyst studies. We first introduced the structural characteristics of TMD materials and discussed their site engineering strategies for electrocatalysis. Later, we comprehensively described two distinct types of microcells: the window-confined on-chip electrochemical microcell (OCEM) and the droplet-confined scanning electrochemical cell microscopy (SECCM). Their setups, working principles, and instrumentation were elucidated in detail, respectively. Furthermore, we summarized recent advances of OCEM and SECCM obtained in TMD catalysts, such as active site identification and imaging, site monitoring, modulation of charge injection and transport, and electrostatic field gating. Finally, we discussed the current challenges and provided personal perspectives on electrochemical microcell research.