Monolayer WTe2 Research Articles

Edge conduction in monolayer WTe2

A two-dimensional topological insulator (2DTI) is guaranteed to have a helical 1D edge mode in which spin is locked to momentum, producing the quantum spin Hall effect and prohibiting elastic backscattering at zero magnetic field. No monolayer material has yet been shown to be a 2DTI, but recently the Weyl semimetal WTe2 was predicted to become a 2DTI in monolayer form if a bulk gap opens. Here, we report that at temperatures below about 100 K monolayer WTe2 does become insulating in its interior, while the edges still conduct. The edge conduction is strongly suppressed by in-plane magnetic field and is independent of gate voltage, save for mesoscopic fluctuations that grow on cooling due to a zero-bias anomaly which reduces the linear-response conductance. Bilayer WTe2 also becomes insulating at low temperatures but does not show edge conduction. Many of these observations are consistent with monolayer WTe2 being a 2DTI. However, the low temperature edge conductance, for contacts spacings down to 150 nm, is below the quantized value,

Spin splitting and reemergence of charge compensation in monolayer WTe2 by 3d transition-metal adsorption.

The semimetallic WTe2 has sparked intense interest owing to the non-saturating magnetoresistance, pressure-driven superconductivity and possession of type-II Weyl fermions. The unexpected and fascinating quantum properties are thought to be closely related to its delicate Fermi surface and a special electron-hole-pocket structure. However, in the single-layer limit, the electron-hole-pocket structure is missing owing to the lack of interlayer interaction. Herewith, we demonstrate that 3d transition-metal adsorption is an effective method to modify the electronic properties of monolayer WTe2 by density functional theory. Spin-splitting and spin-degenerate bands are realized in Ti-, V-, Cr-, Mn-, Fe-, and Co- and Sc-, Ni-, Cu-, and Zn-adsorbed systems, respectively. Especially, the reemergence of the electron-hole pockets appears in the Ni-adsorbed system. The calculated results are robust against inclusion of spin-orbit coupling and Coulomb interaction.

Prediction of electronic structure of van der Waals interfaces: Benzene adsorbed monolayer MoS2, WS2 and WTe2

The electronic structure of benzene adsorbed monolayer MoS2, WS2 and WTe2 has been investigated by first-principles calculations with van der Waals forces. The benzene adsorbed monolayer MoS2, WS2 and WTe2 with spin-orbital coupling are found to be direct-band-gap semiconductors. All the benzene adsorbed model show the semiconducting characteristic. The band gap and spin splitting of the benzene adsorbed monolayer MoS2, WS2 and WTe2 are slightly regulated. The calculated results show the potential applications in the optoelectronic devices, spin-filter devices, etc.

Strong anisotropic thermal conductivity of monolayer WTe2

Tungsten ditelluride (WTe2) has attracted increasing attention due to its large magnetoresistance and pressure-induced superconductivity. In this work, we investigate the thermal conductivity (κ) of monolayer WTe2 by performing first-principles calculations, and find strong anisotropic κ with predicted room-temperature values of 9 and 20 W m–1 K−1 along two principal lattice directions, respectively. Such strong anisotropy suggests the importance of orientation when engineering thermal-related applications based on WTe2. The anisotropy of κ is attributed to the in-plane linear acoustic phonon branches, while the out-of-plane quadratic acoustic phonon branch is almost isotropic. The size dependence of κ shows that the size effect can persists up to 10 μm, and the anisotropy decreases with decreasing sample size due to the suppression of low-frequency anisotropic phonons by boundary scattering.

Quantum spin Hall insulator phase in monolayer WTe2 by uniaxial strain

Monolayer WTe2, which is predicted to be large-gap quantum spin Hall (QSH) insulators with distorted 1T (1T’) structure, attracts rapidly growing interests. However, the intrinsic semimetallic nature of the monolayer 1T’-WTe2 limits their direct applications based on QSH effect. By first-principles density functional theoretical calculations, we demonstrate a phase transition from semimetal to QSH insulator under the uniaxial strains along a and b axis in monolayer 1T’-WTe2. The electronic phase transition results from the geometric structure deformation upon the uniaxial strains. This suggests monolayer 1T’-WTe2 as a promising material for application in strain-tunable topological quantum electronics.

Biaxial Strain and Electric Field Dependent Conductivity of Monolayer WTe2 on Top of Fe3O4(111)

The biaxial strain and electric field dependent electronic properties of monolayer WTe2 on top of Fe3O4(111) are investigated by first‐principles calculations. Our results demonstrate that mechanical strain effectively separates the contributions of interface state and valley states to the conductivity of monolayer WTe2. Tensile strain shifts the interface state below Fermi level. The conductivity of WTe2 is dominated by valley states. With compressive strain, the interface state is shifted away from the valley state edge. The conductivity is only subject to the spin‐polarized interface state. Applying perpendicular electric field also yields valley‐state‐dominating conductivity. The valley splitting energy reduces from 139 to −68 meV with an electric field of 0.02 V nm−1. Different from applying strain, valley polarization is robust against wide range of electric field intensity, showing potential applications in spintronic and valleytronic devices.

Charge Mediated Reversible Metal-Insulator Transition in Monolayer MoTe2 and WxMo1-xTe2 Alloy.

Metal-insulator transitions in low-dimensional materials under ambient conditions are rare and worth pursuing due to their intriguing physics and rich device applications. Monolayer MoTe2 and WTe2 are distinguished from other TMDs by the existence of an exceptional semimetallic distorted octahedral structure (T') with a quite small energy difference from the semiconducting H phase. In the process of transition metal alloying, an equal stability point of the H and the T' phase is observed in the formation energy diagram of monolayer WxMo1-xTe2. This thermodynamically driven phase transition enables a controlled synthesis of the desired phase (H or T') of monolayer WxMo1-xTe2 using a growth method such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Furthermore, charge mediation, as a more feasible method, is found to make the T' phase more stable than the H phase and induce a phase transition from the H phase (semiconducting) to the T' phase (semimetallic) in monolayer WxMo1-xTe2 alloy. This suggests that a dynamic metal-insulator phase transition can be induced, which can be exploited for rich phase transition applications in two-dimensional nanoelectronics.

Anisotropic electronic, mechanical, and optical properties of monolayer WTe2

Using first-principles calculations, we investigate the electronic, mechanical, and optical properties of monolayer WTe2. Atomic structure and ground state properties of monolayer WTe2 (Td phase) are anisotropic which are in contrast to similar monolayer crystals of transition metal dichalcogenides, such as MoS2, WS2, MoSe2, WSe2, and MoTe2, which crystallize in the H-phase. We find that the Poisson ratio and the in-plane stiffness is direction dependent due to the symmetry breaking induced by the dimerization of the W atoms along one of the lattice directions of the compound. Since the semimetallic behavior of the Td phase originates from this W-W interaction (along the a crystallographic direction), tensile strain along the dimer direction leads to a semimetal to semiconductor transition after 1% strain. By solving the Bethe-Salpeter equation on top of single shot G0W0 calculations, we predict that the absorption spectrum of Td-WTe2 monolayer is strongly direction dependent and tunable by tensile strain.

Valley polarization and p-/n-type doping of monolayer WTe2 on top of Fe3O4(111).

The electronic properties of monolayer WTe2 on top of Fe3O4(111) are investigated by density functional theory. We find that the substrate termination of Fe3O4(111) can switch the conductivity of monolayer WTe2 from the p- to n-type. However, the stacking pattern can critically influence its electronic structure. For Fe(A)-terminated interfaces, stronger-bonding models show Fermi level pinning. Additionally, the time-reversal symmetry is broken by the proximity that leads to valley polarization. With particular stacking patterns, large valley splittings of 139, -76 and -72 meV are obtained for Fe(A)-, Fe(B)- and O-terminated models, respectively. Moreover, Fe(B)- and O-terminated ones have more applicable significance for valleytronics as no interference of the interface state appears at the valence band maximum. We demonstrate that proximity to a room-temperature ferromagnet is a convenient way to obtain valley polarization and adjust the conductivity of monolayer WTe2.

Perfect charge compensation in WTe2 for the extraordinary magnetoresistance: From bulk to monolayer

The electronic structures of the WTe2 bulk and layers are investigated by using the first-principles calculations. The perfect electron-hole charge compensation and high carrier mobilities are found in the WTe2 bulk, which may result in the large and non-saturating magnetoresistance (MR) observed very recently in the experiment (Ali M. N. et al, Nature, 514 (2014) 205). The monolayer and bilayer of WTe2 preserve the semimetallic property, with equal hole and electron carrier concentrations. Moreover, very high carrier mobilities are also found in WTe2 monolayer, indicating that the WTe2 monolayer would have the same extraordinary MR effect as the bulk, which could have promising applications in nanostructured magnetic devices.

Negative differential resistance in monolayer WTe2 tunneling transistors

We report theoretical investigations of quantum transport in monolayer transition metal dichalcogenide (TMDC) tunneling field effect transistors (TFETs). Due to the specific electronic structure of TMDC , a transmission valley is found in the conduction band (CB). For a proper choice of the doping, gate and supply voltages the TFET can produce a giant negative differential resistance (NDR) with a peak to valley ratio as large as 103. The mechanism of NDR is identified to be due to a transport-mode bottleneck, i.e., the band to band tunneling from the valence band of the source is partially blocked by a transmission valley of the CB of the drain. More generally, our calculations show that electronic structures of at least six TMDC materials possess the transmission valley.

Intrinsic Piezoelectricity in Two-Dimensional Materials

We discovered that many of the commonly studied two-dimensional monolayer transition metal dichalcogenide (TMDC) nanoscale materials are piezoelectric, unlike their bulk parent crystals. On the macroscopic scale, piezoelectricity is widely used to achieve robust electromechanical coupling in a rich variety of sensors and actuators. Remarkably, our density-functional theory calculations of the piezoelectric coefficients of monolayer BN, MoS2, MoSe2, MoTe2, WS2, WSe2, and WTe2 reveal that some of these materials exhibit stronger piezoelectric coupling than traditionally employed bulk wurtzite structures. We find that the piezoelectric coefficients span more than 1 order of magnitude, and exhibit monotonic periodic trends. The discovery of this property in many two-dimensional materials enables active sensing, actuating, and new electronic components for nanoscale devices based on the familiar piezoelectric effect.