WSe2 Research Articles

Assembled Reduced Graphene Oxide/Tungsten Diselenide/Pd Heterojunction with Matching Energy Bands for Quick Banana Ripeness Detection.

The monitoring of ethylene is of great importance to fruit and vegetable quality, yet routine techniques rely on manual and complex operation. Herein, a chemiresistive ethylene sensor based on reduced graphene oxide (rGO)/tungsten diselenide (WSe2)/Pd heterojunctions was designed for room-temperature (RT) ethylene detection. The sensor exhibited high sensitivity and quick p-type response/recovery (33/13 s) to 10–100 ppm ethylene at RT, and full reversibility and excellent selectivity to ethylene were also achieved. Such excellent ethylene sensing behaviors could be attributed to the synergistic effects of ethylene adsorption abilities derived from the negative adsorption energy and the promoted electron transfer across the WSe2/Pd and rGO/WSe2 interfaces through band energy alignment. Furthermore, its application feasibility to banana ripeness detection was verified by comparison with routine technique through simulation experiments. This work provides a feasible methodology toward designing and fabricating RT ethylene sensors, and may greatly push forward the development of modernized intelligent agriculture.

Femtomolar‐Level Molecular Sensing of Monolayer Tungsten Diselenide Induced by Heteroatom Doping with Long‐Term Stability

AbstractSurface‐enhanced Raman scattering (SERS) is a sensitive, fast, and nondestructive technology to detect trace amounts of molecules. The development of ultrasensitive and environmentally stable noble‐metal‐free SERS substrates is crucial for practical applications but still very challenging. In this contribution, an in situ substitutional doping strategy to synthesize Re‐doped WSe2 (Re‐WSe2) with different doping levels is reported. By increasing the Re content to ≈50 at%, the Re‐WSe2 alloy inherits the 1T″ phase of the ReSe2 lattice. Furthermore, Nb atoms are doped into the 1T″ Re‐WSe2 alloy to further modulate its electronic structure. The as synthesized 1T″ Nb, Re‐WSe2 demonstrates a femtomolar‐level molecular sensing capability with a detectable concentration of 5 × 10–15 m and the corresponding enhancement factor is 2.0 × 109, which is superior to that of most non‐noble‐metal SERS substrates and comparable or even superior to that of noble‐metal substrates to the best of the authors’ knowledge. More importantly, the as‐synthesized 1T″ Nb, Re‐WSe2 exhibits excellent air‐stability over a long term (≈6 months) and selective detection capability in the mixed molecular solution, which are essential for their practical applications. The work provides a new strategy for the rational design of noble‐metal‐free SERS substrates to achieve ultrasensitive molecular sensing.

Role of spin-resolved anti-bonding states filling for enhanced HER performance in 3d transition metals doped monolayer WSe2

Transition metal dichalcogenide monolayers are an emerging family of electrocatalysts, in which Tungsten diselenide (WSe2) has attracted attention for Hydrogen evolution reaction (HER). Here, the magnetism and HER performance of 3d transition metal atoms doping WSe2 (TM-WSe2) are comprehensively studied using density functional theory. The hybridization between the localized TM-3d and the delocalized Se-4p or W-5d states leads to the ferromagnetic/antiferromagnetic coupling between the TM spins and Se or W spins, which induces magnetism in WSe2. With the analysis of the integrated-crystal orbital Hamilton population (ICOHP), the ICOHPspin up values of TM-WSe2 are decreased compared to the pure WSe2, which improves the interaction of Se-H, thereby the H atom is easier to be adsorbed on the Se1 site. While all the ICOHPspin down values of TM-WSe2 are increased, this indicates the weak interaction of Se-H, which promotes the desorption of H2 molecule. Thus, the synergistic effect of spin up and spin down anti-bonding states filling makes TM-WSe2 possess an enhanced HER performance. Moreover, the Ni/Cu-WSe2 possesses optimal HER performance with lower ΔGH* (-0.20 and 0.06 eV). Therefore, our results provide a primary understanding for the relationship between the spin up/down anti-bonding states filling and HER electrocatalytic performance.

Defect‐Engineered Magnetic Field Dependent Optoelectronics of Vanadium Doped Tungsten Diselenide Monolayers

AbstractThe ability to dope transition metal dichalcogenides such as tungsten diselenide (WSe2) with magnetic transition metal atoms in a controlled manner has motivated intense research with the aim of generating dilute magnetic semiconductors. In this work, semiconducting WSe2 monolayers, substitutionally doped with vanadium atoms, are investigated using low‐temperature luminescence and optoelectronic spectroscopy. V‐dopants lead to a p‐type doping character and an impurity‐related emission ≈160 meV below the neutral exciton, both of which scale with the nominal percentage of V‐dopants. Measurements using field‐effect devices of 0.3% V‐doped WSe2 demonstrate bipolar carrier tunability. The doped monolayers display a clear magnetic hysteresis in transport measurements both under illumination and without illumination, whereas the valley polarization of the excitons reveals a nonlinear g‐factor without a magnetic hysteresis within the experimental uncertainty. Hence, this work on V‐doped WSe2 provides crucial insights concerning suitable characterization methods on magnetic properties of doped 2D materials.

Photonic bandgap engineering in (VO2) n /(WSe2) n photonic superlattice for versatile near- and mid-infrared phase transition applications

The application of the photonic superlattice in advanced photonics has become a demanding field, especially for two-dimensional and strongly correlated oxides. Because it experiences an abrupt metal–insulator transition near ambient temperature, where the electrical resistivity varies by orders of magnitude, vanadium oxide (VO2) shows potential as a building block for infrared switching and sensing devices. We reported a first principle study of superlattice structures of VO2 as a strongly correlated phase transition material and tungsten diselenide (WSe2) as a two-dimensional transition metal dichalcogenide layer. Based on first-principles calculations, we exploit the effect of semiconductor monoclinic and metallic tetragonal state of VO2 with WSe2 in a photonic superlattices structure through the near and mid-infrared (NIR–MIR) thermochromic phase transition regions. By increasing the thickness of the VO2 layer, the photonic bandgap (PhB) gets red-shifted. We observed linear dependence of the PhB width on the VO2 thickness. For the monoclinic case of VO2, the number of the forbidden bands increase with the number of layers of WSe2. New forbidden gaps are preferred to appear at a slight angle of incidence, and the wider one can predominate at larger angles. We presented an efficient way to control the flow of the NIR–MIR in both summer and winter environments for phase transition and photonic thermochromic applications. This study’s findings may help understand vanadium oxide’s role in tunable photonic superlattice for infrared switchable devices and optical filters.

Distinctive Photo‐Induced Memory Effect in Heterostructure of 2D Van Der Waals Materials and Lanthanum Aluminate

The role of interface in atomically thin two-dimensional (2D) van der Waals materials is crucial in their novel optoelectronic properties. This study reports a mixed-dimensional optoelectronic memory device based on a heterostructure comprising 2D monolayer molybdenum disulfide and bulk lanthanum aluminate. The reversible photo-induced doping process accompanying persistent photocurrent phenomena is controlled using a gate voltage which is applied across the lanthanum aluminate dielectric substrate under light illumination. The extremely low gate electric field (<2 × 103 V cm−1) and the opposite gate voltage polarity compared with the general photo-doping cases indicate that the conventional band bending mechanism cannot be applied to this optoelectronic device. This distinctive photo-induced memory concept is validated in lanthanum aluminate-based heterostructures with other 2D materials such as graphene and tungsten diselenide. It is postulated that the heterostructure of atomically thin van der Waals materials that are in contact with various functional oxides provides a novel platform for next-generation optoelectronic devices.

Performance analysis of WSe2-based bifacial solar cells with different electron transport and hole transport materials by SCAPS-1D

In recent years, solar cells made of tungsten diselenide (WSe2) have received comprehensive consideration because of their good photoelectric properties. The planar WSe2-based heterojunction solar cell with a preliminary device structure of Au/WSe2/electron transport layer (ETL)/FTO/Al was designed and investigated numerically by SCAPS-1D. CdS ETL is widely used in thin film solar cells (TFSCs). Due to environmental issues and the low band gap (2.42 eV) of CdS ETL, an alternative to CdS ETL was being explored for WSe2 solar cells. In this work, the photovoltaic (PV) performance of the WSe2-based TFSCs with different ETLs were simulated, analyzed and compared, in an attempt to track down a suitable substitute for the CdS ETL. In addition to CdS ETL, ZnO, TiO2 and SnO2 ETLs were independently used to simulate the PV performance of WSe2-based TCSCs. In the wake of analyzing the J-V curves of different cell configurations, SnO2 ETL yielded the best results with PCE of 27.14 % for the single-junction WSe2/SnO2 TFSC. Then, our simulation predicted that the PV performance of the WSe2 device can be improved significantly by using N doped Cu2O as a hole transport layer (HTL). The optimized WSe2 device with SnO2 ETL and Cu2O:N HTL showed an improved PCE of 33.84 % with very good performance stability at higher temperature. Furthermore, this article proposes to use the Au/Cu2O:N/WSe2/SnO2/FTO/Al heterojunction solar cell in bifacial mode and PV performance of the proposed bifacial device have been also studied using SCAPS-1D. Bifacial WSe2 device leads to enhanced PV performance with bifaciality factor for PCE is 83.64 %. Bifacial gain of the proposed device under simultaneous irradiation of 1 sun from the front and 20 % of 1 sun from back side is found to be 13.95 %. Our simulation predicts that the proposed WSe2 bifacial solar cell is capable of converting solar energy into electricity with an efficiency of about 38.38 %.

Ti3C2@WSe2 as photoelectractive materials coupling with recombinase polymerase amplification for nucleic acid detection

The photoelectrochemical biosensor based on double redox cycle amplification technology coupled with Tungsten diselenide and MXene-modified electrode was developed. Signal amplification technology is a commonly used method to improve the sensitivity. In this article, in the double redox cycle amplification method, p-aminophenol is used as the signal molecule, and squaric acid acts as a redox indicator. Tris(2-carboxyethyl) phosphine is an excellent reducing agent, which greatly improves the photoelectric response. Tungsten diselenide and MXene are used as photosensitive materials. In the presence of model target, respiratory syncytial virus RNA, the recombinase polymerase amplification process occurred because there is amplification template, so that the signal molecule p-aminophenol will be produced, leading to redox cycle was carried out, and the photocurrent signal is improved. In the absence of syncytial virus RNA, the photocurrent signal is low. The detection range of the biosensor is from 0.2 fM to 80 fM, and the detection limit reaches 30 aM for respiratory syncytial virus RNA. The method of introducing redox cycle amplification and Tungsten diselenide, MXene into photoelectrochemical biosensing provides a new idea for future biological analysis and has application potential.

Observation of in-plane exciton–polaritons in monolayer WSe2 driven by plasmonic nanofingers

Abstract The transition metal dichalcogenides (TMDs) have drawn great research attention, motivated by the derived remarkable optoelectronic properties and the potentials for high-efficient excitonic devices. The plasmonic nanocavity, integrating deep-sub wavelength confinement of optical mode with dramatic localized field enhancement, provides a practical platform to manipulate light–matter interaction. In order to obtain strong exciton–plasmon coupling effects, it’s crucial to match the vibration direction of exciton to the available strong localized in-plane electric field. Herein, we demonstrate the coupling effect of in-plane exciton in monolayer tungsten diselenide (WSe2) to deterministic gap-plasmon field which is produced by nanometrically gapped collapsed nanofingers. The gap-plasmon field which is completely parallel to the in-plane excitons in WSe2 will drive a strong exciton–plasmon coupling at room temperature. More interestingly, it is experimentally observed that the luminescence of exciton–polariton cannot be influenced by the temperature in the range from 77 K to 300 K due to the presence of nanofingers. According to the theoretical analysis results, we attribute this finding to the dielectric screening effect arising from the extremely strong localized electric field of plasmonic nanofingers. This work proposes a feasible way to harness and manipulate the exciton of low-dimensional semiconductor, which might be potential for quantum optoelectronics.

Recessed-Channel WSe2 Field-Effect Transistor via Self-Terminated Doping and Layer-by-Layer Etching.

Effective channel control with low contact resistance can be accomplished through selective ion implantation in Si and III-V semiconductor technologies; however, this approach cannot be adopted for ultrathin van der Waals materials. Herein, we demonstrate a self-aligned fabrication process based on self-terminated p-doping and layer-by-layer chemical etching to achieve low contact resistance as well as a high on/off current ratio in ultrathin tungsten diselenide (WSe2) field-effect transistors (FETs). Damage-free layer-by-layer thinning of the WSe2 channel is repeated up to a thickness of approximately 1.4 nm, while maintaining the selectively p-doped source/drain regions. The device characteristics of the recessed-channel WSe2 FET are systematically monitored during this layer-by-layer recess-channel process. The WSe2 etching rate is estimated to be 2-3 layers per cycle of oxidation and subsequent chemical etching. The self-terminated tungsten oxide (WOX) layer grown through ultraviolet-ozone treatment induces robust p-doping in the neighboring (or underlying) WSe2 through the electron withdrawal mechanism, which remains in the source/drain regions after channel oxide removal. The adopted self-terminated and self-aligned recess-channel process for ultrathin WSe2 FETs enables the realization of a high on/off output current ratio (>108) and field-effect mobility (∼190 cm2/V·s), while maintaining low contact resistance (0.9-6.1 kΩ·μm) without a postannealing process. The proposed facile and reproducible doping and atomic-layer-etching method for the fabrication of a recessed-channel FET with an ultrathin body can be helpful for high-performance two-dimensional semiconductor devices and is applicable to post-Si complementary metal-oxide semiconductor devices.

Heterostructures of tin and tungsten selenides for robust overall water splitting

Layered transition metal selenides have garnered increased attention in recent times as non-noble metal bifunctional electrocatalysts for electrochemical water splitting. Tungsten diselenide @ tin diselenide heterostructures in the present study significantly increase the electrochemical performance of oxygen evolution reaction with a low overpotential of 250 mV at 10 mA cm−2 and high stability for 16 h (8.9 % loss), hydrogen evolution reaction with a low overpotential of 180 mV at 10 mA cm−2 with a 21.9% loss in 16 h. The overall water splitting using a lab-size electrolyzer shows a low cell voltage (1.52 V @ 10 mA cm−2) and high durability for 50 h (15.2% loss @ 10 mA cm−2 and 4.4% loss @ 50 mA cm−2). As a result, the heterostructures have demonstrated their ability to handle multiple challenges in energy conversion systems due to their unique properties.

ELECTRONIC TRANSPORT PROPERTIES (LOW TEMPERATURE HALL EFFECT MEASUREMENT) OF WSe2 SINGLE CRYSTAL

WSe2 crystals grown by Direct Vapour Transport (DVT) technique. The structural properties of as grown crystals have been measured by X-ray diffraction analysis. The temperature dependence Hall coefficient study revealed that the grown crystal shows mixed conduction behavior in the temperature region 300K to 10K. The variation of mobility with temperature, also show that the WSe2 possesses a mixed scattering mechanism. The Hall scattering factor calculated from the temperature dependent Hall coefficient and carrier density. Detailed analysis of temperature dependent Hall effect data thus yields information concerning the electronic transport properties of tungsten diselenide semiconducting layered materials.

Frequency Scaling, Elastic Transition, and Broad-Range Frequency Tuning in WSe2Nanomechanical Resonators

Nanomechanical resonators based on atomic layers of tungsten diselenide (WSe2) offer intriguing prospects for enabling novel sensing and signal processing functions. The frequency scaling law of such resonant devices is critical for designing and realizing these high-frequency circuit components. Here, we elucidate the frequency scaling law for WSe2 nanomechanical resonators by studying devices of one-, two-, three-, to more than 100-layer thicknesses and different diameters. We observe resonant responses in both mechanical limits and clear elastic transition in between, revealing intrinsic material properties and devices parameters such as Young's modulus and pretension. We further demonstrate a broad frequency tuning range (up to 230%) with a high tuning efficiency (up to 23% V-1). Such tuning efficiency is among the highest in resonators based on two-dimensional (2D) layered materials. Our findings can offer important guidelines for designing high-frequency WSe2 resonant devices.

Gold nanoparticles decorated 2D-WSe2 as a SERS substrate

Developing well-defined surface enhanced Raman scattering (SERS)-active substrates with superior performance is potentially attractive for many areas of research such as new generation sensing and analysis. Here, a nanohybrid SERS platform has been developed by decorating two-dimensional (2D) tungsten diselenide (WSe2) flakes with zero-dimensional (0D) plasmonic gold nanoparticles (Au NPs). The morphology studies showed that under optimal conditions densely populated Au NPs were formed on the WSe2 surface. Here, we report the utility of the Au NPs/WSe2 nanohybrid structure as an efficient SERS substrate by performing concentration-dependent SERS measurements of a highly fluorescent molecule (Rhodamine 6G, R6G). The hybrid substrate displays promising SERS activity with the detection limit as low as 1 × 10–9 M, which is several orders of magnitude higher than the bare WSe2 on its own (10-3 M). The substrate also exhibits reliable signal stability and reproducibility. These results indicate that the nanohybrid structure has great potential in SERS applications.

Self-standing polyvinyl chloride film as flexible substrate for WSe2 based photodetector

Self-standing polymer materials have gained recognition as a flexible substrate for various optoelectronic devices. Here, we incorporated polyvinyl chloride (PVC) with tungsten diselenide (WSe2) microcrystals. The WSe2 microcrystals are added to PVC by solution mixing process. The WSe2-PVC composite film possess excellent flexibility, electrical conductivity of 20 kΩsq−1 at room temperature, high photoresponsivity of 0.3310 mAW−1, detectivity of 1.527 x 109 Jones and external quantum efficiency up to 8.7%. The free-standing WSe2-PVC photodetectors show durability and reproducibility even after 7 months and show stable photoresponse in the bent state. The visible light WSe2-PVC photodetector is sensitive to different monochromatic radiations (470 nm, 540 nm and 670 nm) and demonstrates potential applications in next-generation wearable optoelectronic devices.

Directional Exciton-Energy Transport in a Lateral Heteromonolayer of WSe2-MoSe2.

Controlling the direction of exciton-energy flow in two-dimensional (2D) semiconductors is crucial for developing future high-speed optoelectronic devices using excitons as the information carriers. However, intrinsic exciton diffusion in conventional 2D semiconductors is omnidirectional, and efficient exciton-energy transport in a specific direction is difficult to achieve. Here we demonstrate directional exciton-energy transport across the interface in tungsten diselenide (WSe2)-molybdenum diselenide (MoSe2) lateral heterostructures. Unidirectional transport is spontaneously driven by the built-in asymmetry of the exciton-energy landscape with respect to the heterojunction interface. At excitation positions close to the interface, the exciton photoluminescence (PL) intensity was substantially decreased in the WSe2 region and enhanced in the MoSe2 region. In PL excitation spectroscopy, it was confirmed that the observed phenomenon arises from lateral exciton-energy transport from WSe2 to MoSe2. This directional exciton-energy flow in lateral 2D heterostructures can be exploited in future optoelectronic devices.

Near Unity Absorbance and Photovoltaic Properties of TMDC/Gold Heterojunction for Solar Cell Application

In this paper, near unity broadband absorption of Van der Waals semiconductors on a metallic substrate, and their photovoltaic performances in the visible spectrum are simulated. Ultrathin layered semiconductors such as Molybdenum disulfide (MoS2), Tungsten disulfide (WS2), Molybdenum di-selenide (MoSe2), Tungsten di-selenide (WSe2), Molybdenum ditelluride (MoTe2), and Tungsten ditelluride (WTe2) can create strong interference by damping optical mode in their multilayer form and increase light absorption at their heterojunctions with noble metals. From our simulation, it is observed that this absorbance can reach up to 94% when the semiconductors are placed on a gold substrate. The optimum thickness of these semiconductors in their heterostructures with gold is analyzed to create resonant absorption to generate the maximum amount of current density. The power conversion efficiency of the designed Schottky junction solar cells is calculated from their current density vs bias voltage characteristics that ranges from 1.57% to 6.80%. Moreover, the absorption coefficient, dark current characteristic, electric field intensity distribution in the device, and carrier generation rate during light illumination are presented with a view to characterizing and comparing among the parameters of TMDC based nanoscale solar cell.

Strong reduction of thermal conductivity of WSe2 with introduction of atomic defects

The thermal conductivities of pristine and defective single-layer tungsten diselenide (WSe2) are investigated by using equilibrium molecular dynamics method. The thermal conductivity of WSe2 increases dramatically with size below a characteristic of ~5 nm and levels off for broader samples and reaches a constant value of ~2 W/mK. By introducing atomic vacancies, we discovered that the thermal conductivity of WSe2 is significantly reduced. In particular, the W vacancy has a greater impact on thermal conductivity reduction than Se vacancies: the thermal conductivity of pristine WSe2 is reduced by ~60% and ~70% with the adding of ~1% of Se and W vacancies, respectively. The reduction of thermal conductivity is found to be related to the decrease of mean free path (MFP) of phonons in the defective WSe2. The MFP of WSe2 decreases from ~4.2 nm for perfect WSe2 to ~2.2 nm with the addition of 0.9% Se vacancies. More sophisticated types of point defects, such as vacancy clusters and anti-site defects, are explored in addition to single vacancies and are found to dramatically renormalize the phonons. The reconstruction of the bonds leads to localized phonons in the forbidden gap in the phonon density of states which leads to a drop in thermal conduction. This work demonstrates the influence of different defects on the thermal conductivity of single-layer WSe2, providing insight into the process of defect-induced phonon transport as well as ways to improve heat dissipation in WSe2-based electronic devices.

Thermal release tape assisted mechanical exfoliation of pristine TMD and the performance of the exfoliated TMD saturable absorbers for Q-switched laser generation

This research applied an enhanced material exfoliation and transfer mechanism to process various pristine transition metal dichalcogenide (TMD) materials into a few-layer structure. The mechanical exfoliation of the TMD materials was done using a thermal release tape (TRT) with a different adhesion property compared to the scotch tape (ST) commonly used in the conventional scotch tape assisted mechanical exfoliation (ST-ME) technique. The thermal release tape assisted mechanical exfoliation (TRT-ME) performs better than the ST-ME. Four pristine TMD saturable absorbers, including molybdenum disulfide (MoS2), tungsten disulfide (WS2), molybdenum diselenide (MoSe2), and tungsten diselenide (WSe2), have been fabricated using the TRT-ME technique. Subsequently, the performances of these materials were investigated using the same experimental systems and settings. These TMD materials were tested as saturable absorbers (SAs) to induce pulsed lasing in an Erbium-doped fiber laser (EDFL) cavity. The TRT-exfoliated TMD saturable absorbers were incorporated into the laser cavity by sandwiching in between two fiber ferrules using a fiber connector. Incorporating these saturable absorbers into the laser cavity has successfully induced promising Q-switching operations at the C-band region. Besides the pump power-dependent output performance, the stability and wavelength tunability of the Q-switched lasers were also studied at the maximum pump power. The findings from the studies show that the Q-switched lasers induced by the TRT-exfoliated pristine TMD saturable absorbers were stable and wavelength-tunable. In the context of Q-switched laser, the TRT-exfoliated WS2 has the best performance among other pristine saturable absorbers due to its ability to generate the highest pulse energy.

Structural Deformation of Tungsten Diselenide Nanostructures Induced by Ozone Oxidation and Investigation of Electronic Properties Change

Structural Deformation of Tungsten Diselenide Nanostructures Induced by Ozone Oxidation and Investigation of Electronic Properties Change