| Property | Value |
| Material | WTe2 - Tungsten Telluride |
| Bulk Band Gap | Indirect 0.7 eV |
| Monolayer Band Gap | Direct 1.07 eV |
| Crystal Structure | Orthorombic |
| Crystal Group | Pmn2₁ |
Introduction
Tungsten telluride is an inorganic material of the transition metal dichalcogenides series. This material presents a substitute for other 2D materials with a narrow band gap. It has promising thermoelectrical, giant magnetoresistance, superconductive, optical, and mechanical properties with several applications in nanodevices, for example, in tunnelling field effect transistors. WTe2 monolayers aim for promising applications in spintronics, dissipationless transport, and quantum computations due to its unique anisotropic structure and the strain-dependence of its electric properties. This material contains zigzag chains that behave as one-dimension conductors, making it a type II Weyl semimetal. WTe2 monolayers have also been used for energy storage and conversion, with particular applications in electrocatalysis.
Electronic properties of WTe2
In bulk, WTe2 is a semiconductor with an indirect band gap of about 0,7 eV and Orthomobic crystal structure with Pmn2₁ crystal structure symmetry. When exfoliated to a single crystal, the band structure evolves and becomes direct, with a size of 1,07 eV.
Raman spectrum of WTe2
Tungsten telluride is a topological semimetal from the transition metal dichalcogenide series. This material has an orthorhombic crystal structure with two distinct W-Te bonds, thus it shows a distinctive set of vibrational modes compared to other 2D hexagonal materials like WS2 and WSe2. First-principle calculations predict 33 Raman peaks related to modes A1, B1, A2 and B2. Nevertheless, different authors report only between 6 and 10 different peaks, where most of the peaks are related to A1 and A2 modes. In general, the different peaks have been determined as A1n, where n is an integer that corresponds to the order of the peaks. The highest energy mode is associated with a low integer n. Furthermore, there is not a change in symmetry decreasing the thickness of WTe2 down to the bilayer. Monolayer WTe2 has a distinct symmetry group and the peak centred at 110 cm-1 is not present in the 2D limit, which can be useful to determine the presence of monolayers. According to various authors, Raman modes stiffen and soften with decreasing thickness, but these shifts are less than 10 cm-1. For instance, the peaks centred at 208 cm-1 (A12) and 130 cm-1 (A18) show a red shift, while the peaks at 162 cm-1 (A15) and 110 cm-1 (A24) exhibit a blue shift. In summary, for the few-layer WTe2 , Raman spectra can not only be used as reliable fingerprints for determining thickness and crystallographic orientation, but also to detect relatively strong interlayer interactions.
The phonon dispersion of WTe2 is shown above. As published in “Raman-active modes of 1T -WTe2 under tensile strain: A first-principles prediction”, Yang et al, 2019.
References
Origin of the superconductivity of WTe2 under pressure. Lu et al, Arvix, 2015. Tungsten ditelluride (WTe2) undergoes a phase transition under pressure from an ambient orthorhombic structure to a monoclinic structure, which is associated with the emergence of superconductivity in WTe2.
Raman scattering investigation of large positive magnetoresistance material WTe2. Kong et al, Elsevier, 2015. Raman scattering measurements on WTe2 reveal phonon modes consistent with first-principles calculations and C2v point group symmetry, although the unconventional decrease in intensity of the A1 phonon mode at 160.6 cm−1 with decreasing temperature has an unclear origin.
Tungsten dichalcogenides (WS2, WSe2, and WTe2): materials chemistry and applications. Efekhari, Journal of Materials Chemistry A, 2017. This article discusses the potential applications of tungsten dichalcogenides, specifically WS2 and WSe2, which have recently gained attention due to their properties and advantages over other materials, such as being cheaper and less toxic; the review also briefly discusses the unique properties of WTe2.