WTe2 Research Articles

Adsorption and gas sensing properties of CH4, C2H2, and C2H4 dissolved gases in transformer oil on Pdn(n = 1–4)-doped WTe2 monolayers: A DFT study

This paper investigates the potential of palladium (Pd) and its clusters-doped tungsten ditelluride (WTe2) monolayer materials for the detection of dissolved gases (such as CH4, C2H2, and C2H4) in transformer oil. Density functional theory (DFT) was used to study the effect of Pdn (n = 1–4) doping on the gas sensing and adsorption properties of WTe2 monolayer materials. The results show that Pdn doping can significantly improve the adsorption capacity and gas sensing performance of WTe2 to these gas molecules, especially showing excellent performance in detecting acetylene and ethylene. Studies have shown that Pdn doping improves the electrical conductivity of WTe2, making it more suitable for the development of gas sensors. Through the calculation of state density, molecular orbital and charge density difference, the adsorption mechanism of Pdn-doped WTe2 monolayer to different gas molecules was revealed. This study provides a theoretical basis and guidance for the development of efficient sensor materials for fault gas detection in transformer oil. In conclusion, the proposed Pd-doped WTe2 monolayer demonstrates promising potential for detecting dissolved gases in transformer oil. Future work will focus on experimental validation and further optimization of the material’s sensitivity and selectivity towards other gas species.

Role of Chalcogenides in Sensitive Therapeutic Drug Monitoring Using Laser Desorption and Ionization.

This study investigates the applicability of six transition metal dichalcogenides to efficient therapeutic drug monitoring of ten antiepileptic drugs using laser desorption/ionization-mass spectrometry. We found that molybdenum ditelluride and tungsten ditelluride are suitable for the sensitive quantification of therapeutic drugs. The contribution of tellurium to the enhanced efficiency of laser desorption ionization was validated through theoretical calculations utilizing an integrated model that incorporates transition-metal dichalcogenides and antiepileptic drugs. The results of our theoretical calculations suggest that the relatively low surface electron density for the tellurium-containing transition metal dichalcogenides induces stronger Coulombic interactions, which results in enhanced laser desorption and ionization efficiency. To demonstrate applicability, up to 120 patient samples were analyzed to determine drug concentrations, and the results were compared with those of immunoassay and liquid chromatography-tandem mass spectrometry. Agreements among these methods were statistically evaluated using the Passing-Bablok regression and Bland-Altman analysis. Furthermore, our method has been shown to be applicable to the simultaneous detection and multiplexed quantification of antiepileptic drugs.

Chronocoulometric Quantum Dot Aptasensor for SARS‐CoV‐2 Nucleocapsid Protein Biomarker

AbstractFor the first time, a novel mercaptosuccinic acid‐capped tungsten telluride quantum dot (MSA‐WTe3‐QD)‐based electrochemical aptasensor was designed for the determination of SARS‐CoV‐2 N‐protein in synthetic human serum, plasma and urine. The MSA‐WTe3‐QD which exhibited spherical morphology and an average diameter of 3.56 nm were synthesized through a bottom‐up approach. The aptasensor was prepared by firstly immobilizing MSA‐WTe3‐QD on a screen‐printed carbon electrode (SPCE) by using the 1‐ethyl‐3‐(3‐dimethylaminopropyl)carbodiimide/N‐hydroxysuccinimide (EDC/NHS) coupling chemistry. Thereafter, amine‐terminated 90‐mer ssDNA aptamer was deposited on the modified SPCE and left to cure to form the aptasensor. Using the chronocoulometry signal transduction technique, the aptasensor's calibration with N‐protein gave a linear range value of 1–9 pg.mL−1 and a limit of detection (LOD) value of 0.08 pg.mL−1. The LOD value of the aptasensor was 8 times lower than the LOD value of 0.71 pg.mL−1 obtained with a commercial N‐protein ELISA kit. When tested in simulated real samples the aptasensor recovery rates of 95–103.33 % were obtained. In the presence of various interferents, the aptasensor displayed good selectivity to N‐protein.

Pristine and aurum-decorated tungsten ditellurides as sensing materials for VOCs detection in exhaled human breath: DFT analysis.

In this research, we employed density functional theory (DFT) to evaluate the sensing capabilities of transition metal-decorated two-dimensional WTe2 TMDs nanosheets toward VOCs such as (acetone, ethanol, methanol, toluene, and formaldehyde) that are exhaled in human breath and can serve as potential biomarkers for detecting specific physiological disorders and also gases interfering in exhaled breath (CO2 and H2O) detection. Au can be physically decorated onto the surface of WTe2. We analyzed the density of states (DOS), adsorption energy, charge transfer, and sensing behavior. The pristine WTe2 monolayer, exhibiting a semiconductor characteristic with a band gap of 0.63 eV, transitions to a metallic state upon Au-decoration, due to its actively stable nature and promising negative adsorption energy value, it triggers the emergence of novel states within the DOS. Computed adsorption energies of VOCs range from -0.08 to -0.57 eV, with greater interaction distances confirming the physisorption behavior of these VOCs biomarkers on Au-WTe2. Ethanol displays greater sensitivity compared to other considered VOCs. Au-WTe2 exhibits promising potential as a viable option for detecting VOCs in breath analysis applications at room temperature, owing to its excellent adsorption capabilities and sensitivity. Overall, our results highlight aurum-decorated tungsten ditelluride's potential as an efficient nano-sensor for detecting VOCs associated with early-stage lung cancer diagnoses.

A Gate-Tunable Ambipolar Quantum Phase Transition in a Topological Excitonic Insulator.

Coulomb interactions among electrons and holes in 2D semimetals with overlapping valence and conduction bands can give rise to a correlated insulating ground state via exciton formation and condensation. One candidate material in which such excitonic state uniquely combines with non-trivial band topology are atomic monolayers of tungsten ditelluride (WTe2 ), in which a 2D topological excitonic insulator (2D TEI) forms. However, the detailed mechanism of the 2D bulk gap formation in WTe2 , in particular with regard to the role of Coulomb interactions, has remained a subject of ongoing debate. Here, it shows that WTe2 is susceptible to a gate-tunable quantum phase transition, evident from an abrupt collapse of its 2D bulk energy gap upon ambipolar field-effect doping. Such gate tunability of a 2D TEI, into either n- and p-type semimetals, promises novel handles of control over non-trivial 2D superconductivity with excitonicpairing.

Femtosecond laser thinning for resistivity control of tungsten ditelluride thin-films synthesized from sol-gel deposited tungsten oxide

In this work we present a route for fabricating WTe2 thin-films together with femtosecond laser post processing, enabling to finely control the conductivity. First, we produce amorphous films of WO3 on Si by spin-coating a sol-gel precursor followed by a consolidating annealing and a reduction process in partial H2 atmosphere, leading to porous metallic tungsten cluster layers. To achieve WTe2, the films were exposed to the chalcogen vapours by isothermal closed space vapor transport. The formation of a tungsten ditelluride film composed of piled crystals could be confirmed and a gradient of surface rich Te identified through hard X-ray photoelectron spectroscopy. Finally, it is demonstrated that resistivity can be changed from 0.2 mΩ.m to 1 mΩ.m, while keeping the material characteristics. An anisotropic conductivity can be induced by direct selective thinning with fs laser writing (350 fs pulse duration, 515 nm laser wavelength) of 1D stripes. The obtained results, demonstrate that laser processing is a promising thin-film post-processing technique that can be applied to 2D transition metal dichalcogenide thin films.

Structural, Electronic, Dynamic, and Optical Properties of 2D Monolayer Tungsten Telluride (2H-WTe2) under Small Biaxial Strain Using Density Functional Theory (DFT and DFT + U)

The structural, electronic, vibrational, and optical properties of 2D- 2H-WTe2 monolayer are investigated using density functional theory with respect to a plane wave ultrasoft pseudopotentials (PW-USPPs) in a generalized gradient approximation (GGA) and with the Hubbard potential (GGA + U). The equilibrium state properties such as lattice parameters, unit cell volume, bulk modulus, and its derivative are determined. The band gap values of monolayer 2H-WTe2 are investigated for unstrained, 2% biaxial compression, and biaxial tensile stress using GGA, respectively. The obtained band gap values of 2H-WTe2 with respect to GGA are 1.043, 1.1487, and 0.9439 eV for unstrained, biaxial compression, and tensile strain, respectively. Moreover, the band gap values determined using Hubbard correction (GGA + U) are 1.1089 eV (unstrained), 1.2332 eV (2% biaxial compression), and 0.9945 eV (2% biaxial tensile stress), respectively. The band gap value obtained using Hubbard correction predicts the experimental value more precisely. The projected density of state shows W (3d) orbital is more dominant both in the valence band maximum and conduction band minimum. Moreover, a small amount of tensile or compressive strain is used to tune the band gap of the monolayer without affecting its direct band gap nature. In addition to this, the phonon dispersion and optical properties are discussed for tensile strain, unstrained, and compressive strain, respectively.

Evidence for two dimensional anisotropic Luttinger liquids at millikelvin temperatures

Interacting electrons in one dimension (1D) are governed by the Luttinger liquid (LL) theory in which excitations are fractionalized. Can a LL-like state emerge in a 2D system as a stable zero-temperature phase? This question is crucial in the study of non-Fermi liquids. A recent experiment identified twisted bilayer tungsten ditelluride (tWTe2) as a 2D host of LL-like physics at a few kelvins. Here we report evidence for a 2D anisotropic LL state down to 50 mK, spontaneously formed in tWTe2 with a twist angle of ~ 3o. While the system is metallic-like and nearly isotropic above 2 K, a dramatically enhanced electronic anisotropy develops in the millikelvin regime. In the anisotropic phase, we observe characteristics of a 2D LL phase including a power-law across-wire conductance and a zero-bias dip in the along-wire differential resistance. Our results represent a step forward in the search for stable LL physics beyond 1D.

MOCVD Growth of Tungsten Ditelluride Thin Films

The growth of tungsten ditelluride (WTe2) thin films on c-plane sapphire substrates was demonstrated by metalorganic chemical vapor deposition (MOCVD) using tungsten hexacarbonyl (W(CO)6) and diethyltelluride (DETe) as precursors for W and Te, respectively, in a H2 carrier gas. The effects of substrate temperature, reactor pressure, precursor and carrier gas flow rates, and chalcogen-to-metal ratio on the WTe2 growth rate and film properties were determined. Substrate temperatures less than 500 °C were required to deposit WTe2 which was found to be beneficial to suppress simultaneous carbon deposition from the DETe precursor. The films exhibited a fine-grained morphology independent of growth temperature. The layered crystalline 1 T’ phase of WTe2 was confirmed using high resolution X-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (TEM) which revealed that the films were predominantly c-axis oriented with in-plane twist. X-ray photoelectron spectroscopy (XPS) demonstrated that the WTe2 films were stable in air for a few days but eventually exhibited oxidation.

Gate-Defined Josephson weak-Links in Monolayer WTe2.

Systems combining superconductors with topological insulators offer a platform for the study of Majorana bound states and a possible route to realize fault tolerant topological quantum computation. Among the systems being considered in this field, monolayers of tungsten ditelluride (WTe2 ) have a rare combination of properties. Notably, it has been demonstrated to be a Quantum Spin Hall Insulator (QSHI) and can easily be gated into a superconducting state. Wereport measurements on gate-defined Josephson weak-link devices fabricated using monolayer WTe2 . It is found that consideration of the two dimensional superconducting leads are critical in the interpretation of magnetic interference in the resulting junctions. The reported fabrication procedures suggest a facile way to produce further devices from this technically challenging material and the results mark the first step toward realizing versatile all-in-one topological Josephson weak-links using monolayer WTe2 . This article is protected by copyright. All rights reserved.

Low-Potential Overall Water Splitting Induced by Engineered CoTe2–WTe2 Heterointerfaces

Heterointerfaces in the form of nanostructures can drastically improve electrochemical water splitting. This study is geared toward the design and development of cobalt telluride (CoTe2)-tungsten telluride (WTe2) nanostructures with good porosity and a high specific surface area, leading to faster overall water splitting kinetics. This is attributed to the enhanced synergistic effects of Co and W atoms, which regulate electron transfer and give rise to optimum binding energy for reaction intermediates. CoTe2–WTe2 nanostructures have demonstrated a low oxygen evolution reaction overpotential of 184 mV @ 10 mA cm–2 and a low hydrogen evolution reaction overpotential of 178 mV @ 10 mA cm–2 in alkaline solution. A two-electrode cell with CoTe2–WTe2 nanostructures as both the anode and the cathode exhibits a low cell potential of 1.52 V at a current density of 10 mA cm–2 with good stability for 50 h (7.8% reduction in current density). This study emphasizes the significance of heterointerface design in transition metal telluride electrocatalysts.

Pseudo-Hydrodynamic Flow of Quasiparticles in Semimetal WTe2 at Room Temperature.

Recently, much interest has emerged in fluid-like electric charge transport in various solid-state systems. The hydrodynamic behavior of the electronic fluid reveals itself as a decrease of the electrical resistance with increasing temperature (the Gurzhi effect) in narrow channels, polynomial scaling of the resistance as a function of the channel width, violation of the Wiedemann-Franz law supported by the emergence of the Poiseuille flow. Similar to whirlpools in flowing water, the viscous electronic flow generates vortices, resulting in abnormal sign-changing electrical response driven by backflow. However, the question of whether the long-ranged sign-changing electrical response can be produced by a mechanism other than hydrodynamics has not been addressed so far. Here polarization-sensitive laser microscopy is used to demonstrate the emergence of visually similar abnormal sign-alternating patterns in semi-metallic tungsten ditelluride at room temperature where this material does not exhibit true hydrodynamics. It is found that the neutral quasiparticle current consisting of electrons and holes obeys an equation remarkably similar to the Navier-Stokes equation. In particular, the momentum relaxation is replaced by the much slower process of quasiparticle recombination. This pseudo-hydrodynamic flow of quasiparticles leads to a sign-changing charge accumulation pattern via different diffusivities of electrons and holes.

Adaptive half-metallicity via magnetic proximity in an electrically sensitive 1T’-WTe2/CrBr3 vdW heterostructure

The magnetic proximity effect (MPE) together with electric-field tunability en-routes new physical paradigm in developing nanoscale devices by modulating the functionalities of the materials. By employing first-principles calculation, we investigate MPE in a van der Waals (vdW) heterostructure constituted by a monolayer Weyl semimetal 1T’-WTe2 (tungsten di-telluride), coupled with a monolayer ferromagnet CrBr3 (chromium tribromide) at an interplanar distance of 3.68 Å. The proximity effect infers that the heterostructure system is 100 % spin-polarized leading to half-metallic nature, with a spin-splitting of 25 and 10 meV for two spin configurations. This robustness of MPE arises due to orbital hybridization and charge transfer at the interface of heterostructure system. Moreover, half-metallic nature is tuned and transformed to semiconducting and overlapping states in presence of external bias. The spin-splitting manifests orbital hybridization due to d-orbitals of W and Cr with a notable enhancement of 4 % in the magnetic moment value (12.04 µB per cell). We also observe that proximitized interface is highly sensitive towards an application of external electric field. With external bias, the Fermi level across charge neutrality point is easily tuned and controls charge transport at interface. The consistent nature of conductivity enables the interfacial polarization and coherent electron drift is realized due to seamless proximity interaction across the transport channel of the heterostructure system. This electric field-mediated MPE in vdW heterostructure can strongly recommend for next-generation spin filtering and field effect transistor (FET) devices.

Polarization-resolved ultrafast all-optical terahertz micro-grating array modulator based on Weyl semimetallic microfilm towards 6G technology

The 6th generation (6G) of communication technology, defines 0.1–1 THz as the communication band. As a result, ultrafast modulation and multi-dimensional resolution coding technologies in the terahertz band pose a big challenge.In this work, a polarization-resolved ultrafast optical controlled terahertz modulator has been realized by combining tungsten ditelluride (WTe2) thin film with subwavelength metallic gratings.In the frequency range of 0.1–1.2 THz, which covers the 6G frequency band, the modulation depth of the device reaches up to 36.8% with a centimeter-scale effective area. An ultrafast response time of 0.95 ps is obtained due to the limitations of the grating for carrier degrees of freedom. In addition, the polarization sensitivity of terahertz waves polarized in the x- and y-direction is up to 0.77 due to the sub-wavelength grating structure. The results give a chance to develop the large-area, high-performance polarization-resolved ultrafast THz modulators based on Weyl semimetal, which could provide a reliable alternative for 6G communications equipment.

Vertical Nonvolatile Schottky‐Barrier‐Field‐Effect Transistor with Self‐Gating Semimetal Contact

AbstractEmerging 2D nonvolatile Schottky‐barrier‐field‐effect transistors (NSBFETs) are envisaged to build a promising reconfigurable in‐memory architecture to mimic the brain. Herein, a vertically stacked multilayered graphene (MGr)‐molybdenum disufide (MoS2)‐tungsten ditelluride (WTe2) NSBFET is reported. The semimetal WTe2 with the charge‐trapping effect enables the simultaneous integration of the electrode and the self‐gating function. The effective Schottky barrier height offset ΔΦB is programed from ΔΦB‐p = 132.6 meV to ΔΦB‐n = 109.4 meV, inducing the reversed built‐in electric field to make the NSBFET, so as to provide one with a multifunctional platform to integrate the nonvolatility and the reconfigurable self‐powered photo response. The reversible open‐circuit voltages of NSBFET synapse are programmed from −0.1 to 0.25 V and the self‐powered responsivity with reversed signs is tuned from 290 to −50 mA W−1, which enables the representation of a signed weight in a single device to enrich multiple optical sensing and computing capabilities.

Magnetic Studies of Iron-Doped Probable Weyl Semimetal WTe2

The non-trivial topology of electronic bands in Weyl semimetals originates from band inversion due to strong spin–orbit coupling. The Weyl semimetals have pairs of Weyl gap-less nodes in the bulk Brillouin zone. The tungsten ditelluride WTe2 likely belongs to type II Weyl semimetals. Doping WTe2 with magnetic ions could induce magnetic ordering in this crystal, which provides prospects for practical applications. We studied the magnetic properties of the iron-doped single crystals Fe0.03W0.97Te2, annealed and unannealed, in comparison with the undoped WTe2. Measurements of the dc magnetization were carried out from 1.8 to 400 K. We revealed pronounced ferromagnetic ordering that was affected by annealing. Anomalies associated with antiferromagnetism and paramagnetism were also found. The magnetic order was suppressed by a field of 60 kOe. The rise in susceptibility with increasing temperature was observed at high temperatures in all samples and was treated using a model developed for Weyl semimetals. The Curie–Weiss law fit at 60 kOe showed that the effective magnetic moment was close to that of Fe2+. Metamagnetism was demonstrated for the unannealed doped WTe2 crystal. The data for the heat capacity of the iron-doped sample agreed with results for the undoped WTe2.

Understanding bulk photovoltaic effect in type-II Weyl semimetal Td-WTe2 using polarization dependent photocurrent measurement

Topological effects in a Weyl semimetal are explored in developing self-powered photodetectors at room temperature. The observed photocurrent is attributed to a combined effect of photothermoelectric effect and bulk photovoltaic phenomenon and is found to be a non-linear optical effect that converts light into electrical current. The self-powered photoresponse at 640 nm excitation wavelength reveals the presence of a diverging Berry curvature of tungsten ditelluride (Td-WTe2) at room temperature. The different perspective of polarization dependent photocurrent spectroscopy is used to separate the photothermal current from the shift current and the circular photo galvanic response from the linear photo galvanic response.

Synthesis and electromagnetic transport of large-area 2D WTe2 thin film

Tungsten telluride thin films were successfully prepared on monocrystal sapphire substrates by using atomic layer deposition and chemical vapor deposition technology, and the effects of different tellurization temperatures on the properties of tungsten telluride films were investigated. The growth rate, crystal structure and composition of the film samples were characterized and analyzed by using scanning electron microscope, Raman spectroscopy and X-ray photoelectron spectroscopy. The results showed that tungsten telluride thin films with good crystal orientation in (001) were obtained at telluride temperature of 550 °C. When the telluride temperature reached 570 °C, the tungsten telluride began to decompose and unsaturated magnetoresistance was found.

Boosting Electrocatalytic Reduction of CO2 to HCOOH on Ni Single Atom Anchored WTe2 Monolayer.

Achieving efficient conversion of carbon dioxide (CO2 ) to formic acid (HCOOH) at mild conditions is a promising means to reduce greenhouse gas emission and mitigate the energy crisis. Herein, spin-polarized density functional theory calculations with van der Waals corrections (DFT+D3) are performed to analyze the catalytic activity of seven metals (Ti, Fe, Ni, Cu, Zn, In, and Sn) anchored on a tungsten ditelluride monolayer (M@WTe2 ) and screen favorable CO2 reduction pathways. These results demonstrate that Ni single atoms strongly bind to the WTe2 monolayer and exist in isolated form due to the high diffusion barriers. Also, Ni-anchored WTe2 monolayer (Ni@WTe2 ) possesses a considerably low limiting-potential (-0.11V vs reversible hydrogen electrode) to convert CO2 to HCOOH due to moderate OCHO adsorption energy and a suppressed competing hydrogen evolution reaction (HER). Therefore, Ni@WTe2 monolayer is a promising electrocatalytic material for the CO2 reduction reaction (CO2 RR). This study sheds light on strategies of designing single metal atom anchored WTe2 catalysts for improved CO2 RR performances.

Noncentrosymmetric characteristics of defects on WTe2

Tungsten ditelluride $({\mathrm{WTe}}_{2})$ is a transition metal dichalcogenide with novel electronic structures and unique properties for application to next-generation devices. Defects in ${\mathrm{WTe}}_{2}$ can impact its properties in both positive and negative ways. Therefore, it urgently requires a precise classification to help understand the possible impacts. Here we report on both geometric and electronic characteristics of the defects in ${\mathrm{WTe}}_{2}$ identified by the combination of scanning tunneling microscopy (STM) and density functional theory (DFT) calculation. We found four types of defects derived from a missing Te atom; two of them are located at the topmost surface while the others are under the surface. The former are imaged topographically and are ascribed to point vacancies of surface Te atoms. The DFT calculations reveal the noncentrosymmetric displacements of atoms around the defects and reasonably reproduce the STM images. Interestingly, the latter defects are hardly observed but they are dressed with the noncentrosymmetric quasiparticle interference (QPI) fringes which enable us to identify them. These findings demonstrate that STM-QPI can be a feasible method to characterize the defects in layered materials.