Dramatically decreased magnetoresistance in non-stoichiometric WTe2 crystals

Quantifying the thickness of WTe2 using atomic-resolution STEM simulations and supervised machine learning

For two-dimensional (2D) materials, the exact thickness of the material often dictates its physical and chemical properties. The 2D quantum material WTe2 possesses properties that vary significantly from a single layer to multiple layers, yet it has a complicated crystal structure that makes it difficult to differentiate thicknesses in atomic-resolution images. Furthermore, its air sensitivity and susceptibility to electron beam-induced damage heighten the need for direct ways to determine the thickness and atomic structure without acquiring multiple measurements or transferring samples in ambient atmosphere. Here, we demonstrate a new method to identify the thickness up to ten van der Waals layers in Td-WTe2 using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy image simulation. Our approach is based on analyzing the intensity line profiles of overlapping atomic columns and building a standard neural network model from the line profile features. We observe that it is possible to clearly distinguish between even and odd thicknesses (up to seven layers), without using machine learning, by comparing the deconvoluted peak intensity ratios or the area ratios. The standard neural network model trained on the line profile features allows thicknesses to be distinguished up to ten layers and exhibits an accuracy of up to 94% in the presence of Gaussian and Poisson noise. This method efficiently quantifies thicknesses in Td-WTe2, can be extended to related 2D materials, and provides a pathway to characterize precise atomic structures, including local thickness variations and atomic defects, for few-layer 2D materials with overlapping atomic column positions.

Reversible Iodine Intercalation into Tungsten Ditelluride

The new compound WTe₂I was prepared by a reaction of WTe₂ with iodine in a fused silica ampule at temperatures between 40 and 200 °C. Iodine atoms are intercalated into the van der Waals gap between tungsten ditelluride layers. As a result, the WTe₂ layer separation is significantly increased. Iodine atoms form planar layers between each tungsten ditelluride layer. Due to oxidation by iodine the semimetallic nature of WTe₂ is changed, as shown by comparative band structure calculations for WTe₂ and WTe₂I based on density functional theory. The calculated phonon band structure of WTe₂I indicates the presence of phonon instabilities related to charge density waves, leading to an observed incommensurate modulation of the iodine position within the layers.

Direct Evidence for Charge Compensation-Induced Large Magnetoresistance in Thin WTe2

Since the discovery of extremely large nonsaturating magnetoresistance (MR) in WTe₂, much effort has been devoted to understanding the underlying mechanism, which is still under debate. Here, we explicitly identify the dominant physical origin of the large nonsaturating MR through in situ tuning of the magneto-transport properties in thin WTe₂ film. With an electrostatic doping approach, we observed a nonmonotonic gate dependence of the MR. The MR reaches a maximum (10600%) in thin WTe₂ film at certain gate voltage where electron and hole concentrations are balanced, indicating that the charge compensation is the dominant mechanism of the observed large MR. Besides, we show that the temperature-dependent magnetoresistance exhibits similar tendency with the carrier mobility when the charge compensation is retained, revealing that distinct scattering mechanisms may be at play for the temperature dependence of magneto-transport properties. Our work would be helpful for understanding mechanism of the large MR in other nonmagnetic materials and offers an avenue for achieving large MR in the nonmagnetic materials with electron-hole pockets.

Tuning the electrical transport of type II Weyl semimetal WTe2 nanodevices by Mo doping

We fabricated nanodevices from MoxW1-xTe2 (x = 0, 0.07, 0.35), and conducted a systematic comparative study of their electrical transport. Magnetoresistance measurements show that Mo doping can significantly suppress mobility and magnetoresistance. The results for the analysis of the two band model show that doping with Mo does not break the carrier balance. Through analysis of Shubnikov-de Haas oscillations, we found that Mo doping also has a strong suppressive effect on the quantum oscillation of the sample, and the higher the ratio of Mo, the fewer pockets were observed in our experiments. Furthermore, the effective mass of electron and hole increases gradually with increasing Mo ratio, while the corresponding quantum mobility decreases rapidly.

Enhanced Electrocatalytic Hydrogen Evolution from Large-Scale, Facile-Prepared, Highly Crystalline WTe2 Nanoribbons with Weyl Semimetallic Phase

Tungsten ditellurium (WTe₂) is one of most important layered transition metal dichalcogenides (TMDs) and exhibits various prominent physical properties. All the present methods for WTe₂ preparation need strict conditions such as high temperature or cannot be applied in large scale, which limits its practical applications. In addition, most studies on WTe₂ focus on its physical properties, whereas its electrochemical properties are still illusive with little investigation. Here, we develop a facile and scalable two-step method to synthesize high-quality WTe₂ nanoribbon crystals with 1T' Weyl semimetal phase for the first time. Highly crystalline 1T'-WTe₂ nanoribbons can be obtained on a large scale through this two-step method. In addition, the electrochemical tests show that WTe₂ nanoribbons exhibit smaller overpotential and much better hydrogen evolution reaction catalytic performance than other tungsten-based sulfide and selenide (WS₂, WSe₂) nanoribbons of same morphology and under same preparation conditions. WTe₂ nanoribbons show a Tafel slope of 57 mV/dec, which is one of best values for TMD catalysts and about 2 and 4 times smaller than that for 2H-WS₂ nanoribbons (135 mV/dec) and 2H-WSe₂ nanoribbons (213 mV/dec), respectively. 1T'-WTe₂ nanoribbons also show ultrahigh stability in 5000 cycles and 20 h at 10 mA/cm². The better performance is attributed to high conductivity of semimetallic 1T'-phase-stable WTe₂ nanoribbons with one or two order higher charge-transfer rate than normally semiconducting 2H-stable WS₂ and WSe₂ nanoribbons. These results open the door for electrochemical applications of Weyl semimetallic TMDs.

Composition and temperature-dependent phase transition in miscible Mo1-xWxTe2 single crystals

Transition metal dichalcogenides (TMDs) WTe2 and MoTe2 with orthorhombic Td phase, being potential candidates as type-II Weyl semimetals, are attracted much attention recently. Here we synthesized a series of miscible Mo1-xWxTe2 single crystals by bromine vapor transport method. Composition-dependent X-ray diffraction and Raman spectroscopy, as well as composition and temperature-dependent resistivity prove that the tunable crystal structure (from hexagonal (2H), monoclinic (β) to orthorhombic (Td) phase) can be realized by increasing W content in Mo1-xWxTe2. Simultaneously the electrical property gradually evolves from semiconductor to semimetal behavior. Temperature-dependent Raman spectroscopy proves that temperature also can induce the structural phase transition from β to Td phase in Mo1-xWxTe2 crystals. Based on aforementioned characterizations, we map out the temperature and composition dependent phase diagram of Mo1-xWxTe2 system. In addition, a series of electrical parameters, such as carrier type, carrier concentration and mobility, have also been presented. This work offers a scheme to accurately control structural phase in Mo1-xWxTe2 system, which can be used to explore type-II Weyl semimetal, as well as temperature/composition controlled topological phase transition therein.

Experimental Observation of Anisotropic Adler-Bell-Jackiw Anomaly in Type-II Weyl Semimetal WTe_{1.98} Crystals at the Quasiclassical Regime

The asymmetric electron dispersion in type-II Weyl semimetal theoretically hosts anisotropic transport properties. Here, we observe the significant anisotropic Adler-Bell-Jackiw (ABJ) anomaly in the Fermi-level delicately adjusted WTe_{1.98} crystals. Quantitatively, C_{W}, a coefficient representing the intensity of the ABJ anomaly along the a and b axis of WTe_{1.98} are 0.030 and 0.051  T^{-2} at 2 K, respectively. We found that the temperature-sensitive ABJ anomaly is attributed to a topological phase transition from a type-II Weyl semimetal to a trivial semimetal, which is verified by a first-principles calculation using experimentally determined lattice parameters at different temperatures. Theoretical electrical transport study reveals that the observation of an anisotropic ABJ along both the a and b axes in WTe_{1.98} is attributed to electrical transport in the quasiclassical regime. Our work may suggest that electron-doped WTe_{2} is an ideal playground to explore the novel properties in type-II Weyl semimetals.