WSe2 Research Articles

A strategy of forming sandwich electrodes based on graphene and two-dimensional transition metal dichalcogenides as supercapacitors to avoid gas evolution

Two-dimensional (2D) materials such as transition metal dichalcogenides (TMDC) are well-recognized as excellent electrode materials for energy storage applications due to the ion intercalation characteristic. However, gas evolution is central to the problems that limit the potential window of these materials in the energy storage context. This is because the TMDC can behave as an electrocatalyst, especially in an acid environment. Herein, a novel design of electrode materials for avoiding gas evolution in supercapacitor devices was formed using the sandwich configuration between graphene (GP) and TMDC. A family of transition metal dichalcogenide (TMDC) materials e.g., molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2) including GP was constructed as a layer-by-layer electrode materials, like a “sandwich”. In this work, we provide a straightforward method for creating GP/TMDCs/GP free-standing electrodes that have the necessary properties, such as large surface area and high conductivity for electrolyte diffusion. The GP sandwich on both sides of the electrode can diminish the gas evolution of TMDCs in acidic conditions. This makes the sandwiched electrodes perform a wider electrochemical window up to 1.0 V, in 1.0 M H2SO4. Note that the gas evolution e.g. hydrogen typically occurs at about −0.35 V (onset). Compared with conventional GP/activated carbon, the specific capacitances of “sandwich” GP/MoS2/GP, GP/MoSe2/GP, GP/WS2/GP, and GP/WSe2/GP devices are improved by 7.4, 6.2, 14.8, and 17.3 %, respectively. The findings also indicate that the developed sandwich electrode may be a suitable choice for the creation of future large energy storage systems with a high intercalation capacitance in layered materials.

Toward Scalable Electrochemical Exfoliation of Molybdenum Disulfide Powder through an Accessible Electrode Design.

Cathodic electrochemical intercalation/exfoliation of transition metal dichalcogenides (TMDs) with bulky tetraalkylammonium-based cations is gaining popularity as it avoids the semiconducting (2H) to metallic (1T) phase transformation in TMDs like molybdenum disulfide (MoS2) and, generally, produces sheets with a larger aspect ratio - important for achieving conformal sheet-to-sheet contact in optoelectronic devices. Large single crystals are typically used as the precursor, but these are expensive, often inaccessible, and result in limited quantities of material. In this paper, a 3D-printable electrochemical cell capable of intercalating gram-scale quantities of commercially available TMD powders is presented. By incorporating a reference electrode in the cell and physically restraining the powder with a spring-loaded mechanism, the system can probe the intercalation electrochemistry, for example, determining the onset of intercalation to be near -2.5V versus the ferrocene redox couple. While the extent of intercalation depends on precursor quantity and reaction time, a high yield of exfoliated product can be obtained exhibiting average aspect ratios as high as 49 ± 44 similar to values obtained by crystal intercalation. The intercalation and exfoliation of a wide variety of pelletized commercial powders including molybdenum diselenide (MoSe2), tungsten diselenide (WSe2), tungsten disulfide (WS2), and graphitic carbon nitride (gCN) are also demonstrated.

Chiral absorption enhancement via critically coupled resonances in atomically thin photonic crystal exciton-polaritons.

Atomically thin transition metal dichalcogenides (TMDS) offer a promising route to the scaling down of optoelectronic devices to the ultimate thickness limit. But the weak light-matter interaction caused by their atomically thin nature makes them inevitably rely on external photonic structures to enhance optical absorption. Here, we report chiral absorption enhancement in atomically thin tungsten diselenide (WSe2) using chiral resonances in photonic crystal (PhC) nanostructures patterned directly in WSe2 itself. We show that the quality factors (Q factors) of the resonances grow exponentially as the PhC thickness approaches atomic limit. As such, the strong interaction of high Q factor photonic resonance with the coexisting exciton resonance in WSe2 results into self-coupled exciton-polaritons. By balancing the light coupling and absorption rates, the incident light can critically couple to chiral resonances in WSe2 PhC exciton-polaritons, leading to the theoretically limited 50% optical absorptance with over 84% circular dichroism (CD).

Frictional resistance and delamination mechanisms in 2D tungsten diselenide revealed by multi-scale scratch and in-situ observations

Friction phenomena in two-dimensional (2D) materials are conventionally studied at atomic length scales in a few layers using low-load techniques. However, the advancement of 2D materials for semiconductor and electronic applications requires an understanding of friction and delamination at a few micrometers length scale and hundreds of layers. To bridge this gap, the present study investigates frictional resistance and delamination mechanisms in 2D tungsten diselenide (WSe2) at 10 µm length and 100–500 nm depths using an integrated atomic force microscopy (AFM), high-load nanoscratch, and in-situ scanning electron microscopic (SEM) observations. AFM revealed a heterogenous distribution of frictional resistance in a single WSe2 layer originating from surface ripples, with the mean increasing from 8.7 to 79.1 nN as the imposed force increased from 20 to 80 nN. High-load in-situ nano-scratch tests delineated the role of the individual layers in the mechanism of multi-layer delamination under an SEM. Delamination during scratch consists of stick-slip motion with friction force increasing in each successive slip, manifested as increasing slope of lateral force curves with scratch depth from 10.9 to 13.0 (× 103) Nm−1. Delamination is followed by cyclic fracture of WSe2 layers where the puckering effect results in adherence of layers to the nanoscratch probe, increasing the local maximum of lateral force from 89.3 to 205.6 µN. This establishment of the interconnectedness between friction in single-layer and delamination at hundreds of layers harbors the potential for utilizing these materials in semiconductor devices with reduced energy losses and enhanced performance.

Harnessing Persistent Photocurrent in a 2D Semiconductor-Polymer Hybrid Structure: Electron Trapping and Fermi Level Modulation for Optoelectronic Memory.

Recently, 2D semiconductor-based optoelectronic memory has been explored to overcome the limitations of conventional von Neumann architectures by integrating optical sensing and data storage into one device. Persistent photocurrent (PPC), essential for optoelectronic memory, originates from charge carrier trapping according to the Shockley-Read-Hall (SRH) model in 2D semiconductors. The quasi-Fermi level position influences the activation of charge-trapping sites. However, the correlation between quasi-Fermi level modulations and PPC in 2D semiconductors has not been extensively studied. In this study, we demonstrate optoelectronic memory based on a 2D semiconductor-polymer hybrid structure and confirm that the underlying mechanism is charge trapping, as the SRH model explains. Under light illumination, electrons transfer from polyvinylpyrrolidone to p-type tungsten diselenide, resulting in high-level injection and majority carrier-type transitions. The quasi-Fermi level shifts upward with increasing temperature, improving PPC and enabling optoelectronic memory at 433 K. Our findings offer valuable insights into optimizing 2D semiconductor-based optoelectronic memory.

Interfacial epitaxy of multilayer rhombohedral transition-metal dichalcogenide single crystals.

Rhombohedral-stacked transition-metal dichalcogenides (3R-TMDs), which are distinct from their hexagonal counterparts, exhibit higher carrier mobility, sliding ferroelectricity, and coherently enhanced nonlinear optical responses. However, surface epitaxial growth of large multilayer 3R-TMD single crystals is difficult. We report an interfacial epitaxy methodology for their growth of several compositions, including molybdenum disulfide (MoS2), molybdenum diselenide, tungsten disulfide, tungsten diselenide, niobium disulfide, niobium diselenide, and molybdenum sulfoselenide. Feeding of metals and chalcogens continuously to the interface between a single-crystal Ni substrate and grown layers ensured consistent 3R stacking sequence and controlled thickness from a few to 15,000 layers. Comprehensive characterizations confirmed the large-scale uniformity, high crystallinity, and phase purity of these films. The as-grown 3R-MoS2 exhibited room-temperature mobilities up to 155 and 190 square centimeters per volt second for bi- and trilayers, respectively. Optical difference frequency generation with thick 3R-MoS2 showed markedly enhanced nonlinear response under a quasi-phase matching condition (five orders of magnitude greater than monolayers).

A surface plasmon resonance biosensor for bacteria and virus detection: A Comsol Multiphysics simulation

This study provides a comprehensive simulation-based investigation into the design and performance optimization of a surface plasmon resonance (SPR) biosensor. The main goal of this study is to improve sensitivity and accuracy by combining optical and colorimetric biosensing techniques. The biosensor is studied, examined, and simulated using Comsol Multiphysics. Sensing medium, black phosphorus, tungsten diselenide (WSe2), gold (Au), magnetite (Fe3O4), and N-BK7 glass as prism are the layers that make up the structure of the proposed sensor. The study evaluates various parameters such as electric potential distribution, surface temperatures, conductive heat flux, eigenfrequency, electric field norm, and temperature gradients. The use of WSe2 aims for a higher sensitivity for detecting biomolecules. This paper proves the effect of using Fe3O4 and WSe2 among the six layers of the sensor in increasing the selectivity and sensitivity of the SPR biosensor. The findings reveal intricate interactions between the biosensor layers, which influence its thermal and electromagnetic behavior. The findings of this study contribute to the advancement of SPR biosensor technology, which has the potential for a variety of applications in the biomedical field.

Synergistic Effect of 3D/2D Vanadium Diselenide/Tungsten Diselenide Hybrid Materials: Electrochemical Detection of 5-Nitroquinoline a Hazard to the Aquatic Environment.

The development of multidimensional structured electrode materials with simple synthetic methods and their electrochemical sensing ability against environmental pollution is still a challenge. In this article, we propose a hybrid formed using multidimensional (3D/2D) vanadium diselenide microspheres and tungsten diselenide nanosheets (VSe2/WSe2) for the electrochemical detection of 5-nitroquinoline (5-NQ), a highly toxic and hazardous substance that is polluting aquatic life due to increasing industrial activities. The 3D/2D VSe2/WSe2 hybrids were prepared by a simple solvothermal method and their morphological and structural analysis was confirmed by various spectroscopy and analytical techniques such as powder X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy-energy dispersive X-ray spectroscopy, transmission electron microscopy, cyclic voltammetry, and differential pulse voltammetry. The proposed 3D/2D architecture showed a strong synergistic effect between the two components as well as high electrical conductivity. As a result, an increased peak current for the reduction and detection of 5-NQ was achieved compared to other modified and unmodified disposable screen-printed electrodes (SPE), such as bare SPE, VSe2/SPE, and WSe2/SPE. Under the optimized electrochemical conditions, VSe2/WSe2/SPE showed large linear response ranges (0.012-1053, 1183-3474 μM), a low detection limit (0.002 μM), good sensitivity along with good selectivity, and repeatability for the detection of 5-NQ. With this prominent electrochemical behavior, the VSe2/WSe2 electrode has clear potential to produce high-performance sensor devices.

Simulation of 2D ReS2/WSe2 based complementary field-effect transistors towards 1 nm technology node

Advanced Integrated Circuit technology demands high-performance channel materials and innovative device architectures to sustain the scaling of field-effect transistors. In this study, we simulate the electrical performance of rhenium disulfide and tungsten diselenide nanosheet FETs (NSFETs) with gate lengths ranging from 12 nm to 8 nm using Technology Computer-Aided Design method. The simulated high performance including 393 μA/μm on-state current and over 105 on/off ratio can meet the criteria for integrated circuit applications in the 1 nm technology node. Complementary FET (CFET) simulations are conducted through the vertical stacking of ReS2 and WSe2 NSFETs, exhibiting a small parasitic capacitance of 1.0 fF/μm, and notable noise margin (>235 mV) at different process corners. The construction and performance simulation of Static Random-Access Memory (SRAM) is realized through CFET interconnection, involving the calculation of read current and read/write noise margin. This research offers a forward-looking analysis of performance metrics for future 2D materials-based NSFETs, CFETs, and SRAMs.

High‐β Lasing in Self‐Assembled Photonic‐Defect Microcavities with a Transition Metal Dichalcogenide Monolayer as Active Material

AbstractThe investigation and development of innovative micro‐ and nanolasers using transition metal dichalcogenide (TMDC) monolayers as active materials is attracting considerable attention due to their unique electrical, mechanical, and optical properties. In this report, the fabrication of photonic‐defect microcavities that are self‐assembled and integrated into a dielectric distributed Bragg reflector structure that fully encapsulates a monolayer of tungsten diselenide () is detailed. The encapsulation process of the monolayer with hexagonal boron nitride generates air bubbles that induce parabolic photonic defects in the microcavity. These defects lead to a tight diameter‐dependent three‐dimensional optical confinement, which is confirmed by experimental studies and numerical simulations. In addition, a significant nonlinearity in the input‐output characteristics and excitation‐power‐dependent linewidth narrowing is observed in the resonators, indicating laser operation, which is verified by photon autocorrelation measurements. The photonic‐defect cavities are all formed on a single monolayer sample, suggesting potential advantages for multi‐wavelength emission photonic applications and facilitating TMDC‐based prestructured photonic‐defect microlasers for large‐scale fabrication.

Reduced interface effect of proton beam irradiation on the electrical properties of WSe2/hBN field effect transistors

Two-dimensional transition metal dichalcogenide (TMDC) semiconductors are emerging as strong contenders for electronic devices that can be used in highly radioactive environments such as outer space where conventional silicon-based devices exhibit nonideal characteristics for such applications. To address the radiation-induced interface effects of TMDC-based electronic devices, we studied high-energy proton beam irradiation effects on the electrical properties of field-effect transistors (FETs) made with tungsten diselenide (WSe2) channels and hexagonal boron-nitride (hBN)/SiO2 gate dielectrics. The electrical characteristics of WSe2 FETs were measured before and after the irradiation at various proton beam doses of 1013, 1014, and 1015 cm−2. In particular, we demonstrated the dependence of proton irradiation-induced effects on hBN layer thickness in WSe2 FETs. We observed that the hBN layer reduces the WSe2/dielectric interface effect which would shift the transfer curve of the FET toward the positive direction of the gate voltage. Also, this interface effect was significantly suppressed when a thicker hBN layer was used. This phenomenon can be explained by the fact that the physical separation of the WSe2 channel and SiO2 dielectric by the hBN interlayer prevents the interface effects originating from the irradiation-induced positive trapped charges in SiO2 reaching the interface. This work will help improve our understanding of the interface effect of high-energy irradiation on TMDC-based nanoelectronic devices.

Design of a highly sensitive and precise long-range surface plasmon resonance sensor for early detection of malaria

This manuscript presents an innovative Kretschmann configured for Long Range Surface Plasmon Resonance (LRSPR) sensor for malaria detection. It utilizes tungsten diselenide (WSe2) as a bio+molecular recognition element (BRE) layer to attach target analyte. The optimized sensor design demonstrates exceptional performance in detecting different malaria stages (Schizont with refractive index (RI) = 1.371, Trophozoite with RI = 1.381, and Ring with RI = 1.396) at a 633 nm wavelength. In comparison to conventional surface plasmon resonance (CSPR), the LRSPR biosensor exhibits a substantial improvement, a 6.67 to 10 times increase in detection accuracy (DA), an impressive 40 to 86.55 times enhancement in imaging figure of merit (IFOM), and a 6.0 to 8.66 times boost in imaging sensitivity (Simg.) for the sequential detection of different stages of malaria. COMSOL Multiphysics simulations highlight a deeper penetration depth (PD) of 258.71 nm, 276.85 nm, and 373.04 nm for the detection of Schizont, Trophozoite, and Ring stages, respectively.

Observation of a Chern insulator in crystalline ABCA-tetralayer graphene with spin-orbit coupling.

Degeneracies in multilayer graphene, including spin, valley, and layer degrees of freedom, can be lifted by Coulomb interactions, resulting in rich broken-symmetry states. Here, we report a ferromagnetic state in charge-neutral ABCA-tetralayer graphene driven by proximity-induced spin-orbit coupling from adjacent tungsten diselenide. The ferromagnetic state is identified as a Chern insulator with a Chern number of 4; its maximum Hall resistance reaches 78% quantization at zero magnetic field and is fully quantized at either 0.4 or -1.5 tesla. Three distinct broken-symmetry insulating states, layer-antiferromagnet, Chern insulator, and layer-polarized insulator, along with their transitions, can be continuously tuned by the vertical displacement field. In this system, the magnetic order of the Chern insulator can be switched by three knobs, including magnetic field, electrical doping, and vertical displacement field.

The determination of WSe2/GaN heterojunction band offsets in the semipolar (11–22) and nonpolar (11–20) planes by x-ray photoelectron spectroscopy

Wurtzite structured GaN has a severe polarization effect in the c (0001) plane, compared to which the polarization effect is small in the semipolar (11–22) plane, and there is no polarization effect in the nonpolar (11–20) plane GaN. To investigate the influence of the polarization effect on the band bending at the heterojunction interface, we fabricated tungsten diselenide (WSe2)/(0001) GaN, WSe2/(11–22) GaN, and WSe2/(11–20) GaN heterojunctions. We measured the heterojunction valence band offsets (VBOs) by x-ray photoelectron spectroscopy. The VBOs of the three WSe2/GaN heterojunctions were measured to be 2.43 ± 0.15, 2.51 ± 0.15, and 2.23 ± 0.15 eV, and the conduction band offsets were calculated to be 1.11 ± 0.15, 1.19 ± 0.15, and 0.91 ± 0.15 eV, showing the type II energy band alignment of the three heterojunctions. The results demonstrate that WSe2/(11–22) GaN-faced heterojunction band bending is the largest. This provides theoretical insights for two-dimensional WSe2 and three-dimensional semipolar (11–22) GaN and nonpolar (11–20) GaN heterojunction device preparation.

Low-Temperature Synthesis of WSe2 by the Selenization Process under Ultrahigh Vacuum for BEOL Compatible Reconfigurable Neurons.

Low-temperature large-area growth of two-dimensional (2D) transition-metal dichalcogenides (TMDs) is critical for their integration with silicon chips. Especially, if the growth temperatures can be lowered below the back-end-of-line (BEOL) processing temperatures, the Si transistors can interface with 2D devices (in the back end) to enable high-density heterogeneous circuits. Such configurations are particularly useful for neuromorphic computing applications where a dense network of neurons interacts to compute the output. In this work, we present low-temperature synthesis (400 °C) of 2D tungsten diselenide (WSe2) via the selenization of the W film under ultrahigh vacuum (UHV) conditions. This simple yet effective process yields large-area, homogeneous films of 2D TMDs, as confirmed by several characterization techniques, including reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and different spectroscopy methods. Memristors fabricated using the grown WSe2 film are leveraged to realize a novel compact neuron circuit that can be reconfigured to enable homeostasis.

Knot Architecture for Biocompatible and Semiconducting 2D Electronic Fiber Transistors.

Wearable devices have generally been rigid due to their reliance on silicon-based technologies, while future wearables will utilize flexible components for example transistors within microprocessors to manage data. Two-dimensional (2D) semiconducting flakes have yet to be investigated in fiber transistors but can offer a route toward high-mobility, biocompatible, and flexible fiber-based devices. Here, the electrochemical exfoliation of semiconducting 2D flakes of tungsten diselenide (WSe2) and molybdenum disulfide (MoS2) is shown to achieve homogeneous coatings onto the surface of polyester fibers. The high aspect ratio (>100) of the flake yields aligned and conformal flake-to-flake junctions on polyester fibers enabling transistors with mobilities μ ≈1 cm2 V-1 s-1 and a current on/off ratio, Ion/Ioff ≈102-104. Furthermore, the cytotoxic effects of the MoS2 and WSe2 flakes with human keratinocyte cells are investigated and found to be biocompatible. As an additional step, a unique transistor 'knot' architecture is created by leveraging the fiber diameter to establish the length of the transistor channel, facilitating a route to scale down transistor channel dimensions (≈100 µm) and utilize it to make a MoS2 fiber transistor with a human hair that achieves mobilities as high as μ ≈15 cm2 V-1 s-1.

Room-temperature quantum emission from interface excitons in mixed-dimensional heterostructures

The development of van der Waals heterostructures has introduced unconventional phenomena that emerge at atomically precise interfaces. For example, interlayer excitons in two-dimensional transition metal dichalcogenides show intriguing optical properties at low temperatures. Here we report on room-temperature observation of interface excitons in mixed-dimensional heterostructures consisting of two-dimensional tungsten diselenide and one-dimensional carbon nanotubes. Bright emission peaks originating from the interface are identified, spanning a broad energy range within the telecommunication wavelengths. The effect of band alignment is investigated by systematically varying the nanotube bandgap, and we assign the new peaks to interface excitons as they only appear in type-II heterostructures. Room-temperature localization of low-energy interface excitons is indicated by extended lifetimes as well as small excitation saturation powers, and photon correlation measurements confirm antibunching. With mixed-dimensional van der Waals heterostructures where band alignment can be engineered, new opportunities for quantum photonics are envisioned.

Broadband Artificial Tetrachromatic Synaptic Devices Composed of 2D/3D Integrated WSe2‐GaN‐based Dual‐Channel Floating Gate Transistors

AbstractThe development of artificial tetrachromatic vision holds great potential to enhance human color perception and discrimination, thereby enabling more effective navigation in diverse environments. Herein, an artificial tetrachromatic synaptic device is presented built upon 2D‐3D vertically stacked semiconductors composed of tungsten diselenide (WSe2)‐gallium nitride (GaN) configuration, forming a dual‐channel floating gate transistor (FGT). Under the concerted influence of electrical and optical stimulation, the device successfully mimics fundamental tetrachromatic synaptic behaviors, including short‐term potentiation (STP), weak long‐term potentiation (wLTP), long‐term potentiation (LTP), paired‐pulse facilitation (PPF), spike number‐dependent plasticity (SNDP), and spike rate‐dependent plasticity (SRDP). Notably, the plasticity of the device can be further modulated under ultraviolet (UV) stimulation, providing insights into the modulation of synaptic plasticity through the photogenerated carrier dynamics in the GaN channel. These results imply that WSe2‐GaN‐based FGT architecture with dual‐channel characteristics seamlessly integrates optical sensing and synaptic simulation functionalities, representing a promising avenue for the development of next‐generation artificial visual perception systems (AVPS), with a particular advantage for the pursuit of high‐performance artificial tetrachromatic neuromorphic computing applications of the future.

Van der Waals Decoration of Ultra-High-Q Silica Microcavities for χ(2)-χ(3) Hybrid Nonlinear Photonics.

Optical nonlinear processes are indispensable in a wide range of applications, including ultrafast lasers, microscopy, and quantum information technologies. Among the diverse nonlinear processes, second-order effects usually overwhelm the higher-order ones, except in centrosymmetric systems, where the second-order susceptibility vanishes to allow the use of the third-order nonlinearity. Here we demonstrate a hybrid photonic platform whereby the balance between second- and third-order susceptibilities can be tuned flexibly. By decorating ultra-high-Q silica microcavities with atomically thin tungsten diselenide, we observe cavity-enhanced second-harmonic generation and sum-frequency generation with continuous-wave excitation at a power level of only a few hundred microwatts. We show that the coexistence of second- and third-order nonlinearities in a single device can be achieved by carefully choosing the size and location of the two-dimensional material. Our approach can be generalized to other types of cavities, unlocking the potential of hybrid systems with controlled nonlinear susceptibilities for novel applications.

Tuning the transport polarity of tungsten diselenide-based n-type FET to p-typeby MoO3 contact doping

Atomically thin tungsten diselenide (WSe2) has emerged as a promising material for next-generation electronic and optoelectronic devices. The surface functionalization of molybdenum trioxide (MoO3) improves the performance of WSe2 FET. The strong hole doping from MoO3 to WSe2 was revealed by measuring the electrical properties of the FET and calculating the electron mobility of WSe2 devices. We demonstrated effective P doping of MoO3 on WSe2 FET devices, and the hole mobility of WSe2 FET devices was improved by 3 orders of magnitude after contact doping with MoO3 at 3.2nm.