Few-layer WTe2 Research Articles

(Invited) Physical Properties of Novel 2D Semimetal WTe2: From Microscopic Study to Devices

In this study, multiple measurements, including scanning tunneling microscopy (STM), electrical transport, corroborated by DFT calculations are performed on freshly cleaved surfaces of semimetallic tungsten ditelluride (WTe2) crystals. High sample quality with very low density of defects is confirmed by STM observation and in-plane anisotropic conductance (originated from quasi-one dimensional Fermi surfaces) can be conclude based on transport measurement and DFT calculation.We reveal that strong Rashba type spin-orbit effects due to the non-centrosymmetric structure splits the electron and hole bands crossing the Fermi level and lifts the spin degeneracy of these bands. Low temperature STM/S measurements show the umklapp-type interference pattern on WTe2 surface at 4.2 K, proven to be a spectroscopic feature dominated by spin-conserving processes. Furthermore, we investigate electronic properties of devices made from few-layer WTe2 down to a few nanometers. Our results reveal that few-layer WTe2 could be an ideal candidate for future 2D electronic, optoelectronic after solving the ambient sensitivity of device process.

(Invited) Physical Properties of Novel 2D Semimetal WTe2: From Microscopic Study to Devices

An increasing number of alternative two-dimensional (2D) materials are being explored in the “post-graphene age”, such as transition metal dichalcogenides (TMDs), silicene, and germanene. In this study, multiple measurements, including scanning tunneling microscopy (STM), electrical transport and Raman spectroscopy are performed on freshly cleaved surfaces of semimetallic tungsten ditelluride (WTe2) crystals. High sample quality with very low density of defects is confirmed by STM observation and four Fermi pockets can be revealed by Shubnikov–de Haas (SdH) oscillations based on transport measurement. Furthermore, we investigate Raman spectroscopy of thin-layer WTe2. Our results reveal that few-layer WTe2 could be an ideal candidate for future 2D electronic, optoelectronic devices.

Determination of the thickness and orientation of few-layer tungsten ditelluride using polarized Raman spectroscopy

Orthorhombic tungsten ditelluride (WTe2), with a distorted 1T structure, exhibits a large magnetoresistance that depends on the orientation, and its electrical characteristics changes from semimetallic to insulating as the thickness decreases. Through polarized Raman spectroscopy in combination with transmission electron diffraction, we establish a reliable method to determine the thickness and crystallographic orientation of few-layer WTe2. The Raman spectrum shows a pronounced dependence on the polarization of the excitation laser. We found that the separation between two Raman peaks at ∼90 cm−1 and at 80–86 cm−1, depending on thickness, is a reliable fingerprint for determination of the thickness. For determination of the crystallographic orientation, the polarization dependence of the A1 modes, measured with the 632.8 nm excitation, turns out to be the most reliable. We also discovered that the polarization behaviors of some of the Raman peaks depend on the excitation wavelength as well as thickness, indicating a close interplay between the band structure and anisotropic Raman scattering cross section.

The In-Plane Anisotropy of WTe2 Investigated by Angle-Dependent and Polarized Raman Spectroscopy.

Tungsten ditelluride (WTe2) is a semi-metallic layered transition metal dichalcogenide with a stable distorted 1T phase. The reduced symmetry of this system leads to in-plane anisotropy in various materials properties. We have systemically studied the in-plane anisotropy of Raman modes in few-layer and bulk WTe2 by angle-dependent and polarized Raman spectroscopy (ADPRS). Ten Raman modes are clearly resolved. Their intensities show periodic variation with sample rotating. We identify the symmetries of the detected modes by quantitatively analyzing the ADPRS results based on the symmetry selection rules. Material absorption effect on the phonon modes with high vibration frequencies is investigated by considering complex Raman tensor elements. We also provide a rapid and nondestructive method to identify the crystallographic orientation of WTe2. The crystallographic orientation is further confirmed by the quantitative atomic-resolution force image. Finally, we find that the atomic vibrational tendency and complexity of detected modes are also reflected in the shrinkage degree defined based on ADPRS, which is confirmed by corresponding density functional calculation. Our work provides a deep understanding of the interaction between WTe2 and light, which will benefit in future studies about the anisotropic physical properties of WTe2 and other in-plane anisotropic materials.

Single- and few-layer WTe2 and their suspended nanostructures: Raman signatures and nanomechanical resonances.

Single crystal tungsten ditelluride (WTe2) has recently been discovered to exhibit non-saturating extreme magnetoresistance in bulk; it has also emerged as a new layered material from which atomic layer crystals can be extracted. While atomically thin WTe2 is attractive for its unique properties, little research has been conducted on single- and few-layer WTe2. Here we report the isolation of single- and few-layer WTe2, as well as the fabrication and characterization of the first WTe2 suspended nanostructures. We have observed new Raman signatures of single- and few-layer WTe2 that have been theoretically predicted but have not been reported to date, in both on-substrate and suspended WTe2 flakes. We have further probed the nanomechanical properties of suspended WTe2 structures by measuring their flexural resonances, and obtain a Young's modulus of E(Y) ≈ 80 GPa for the suspended WTe2 flakes. This study paves the way for future investigations and utilizations of the multiple new Raman fingerprints of single- and few-layer WTe2, and for explorations of mechanical control of WTe2 atomic layers.

Anomalous Raman scattering and lattice dynamics in mono- and few-layer WTe2.

Tungsten ditelluride (WTe2) is a layered material that exhibits excellent magnetoresistance and thermoelectric behaviors, which are deeply related with its distorted orthorhombic phase that may critically affect the lattice dynamics of this material. Here, we report comprehensive characterization of Raman spectra of WTe2 from bulk to monolayer using experimental and computational methods. We find that mono and bi-layer WTe2 are easily identified by Raman spectroscopy since two or one Raman modes that are observed in higher-layer WTe2 are greatly suppressed below the noise level in the mono- and bi-layer WTe2, respectively. In addition, the frequency of in-plane A1(7) mode of WTe2 remains almost constant as the layer number decreases, while all the other Raman modes consistently blueshift, which is completely different from the vibrational behavior of hexagonal metal dichalcogenides. First-principles calculation validates experimental results and reveals that anomalous lattice vibrations in WTe2 are attributed to the formation of tungsten chains that make WTe2 structurally one-dimensional.