A combined experimental and theoretical study of the structural, electronic and vibrational properties of bulk and few-layer Td-WTe₂

Abstract

The recent discovery of non-saturating giant positive magnetoresistance has aroused much interest in Td-WTe(2). We have investigated structural, electronic and vibrational properties of bulk and few-layer Td-WTe(2) experimentally and theoretically. Spin-orbit coupling is found to govern the semi-metallic character of Td-WTe(2) and its structural link with the metallic 1 T form provides an understanding of its structural stability. There is a metal-to-insulator switch-over in the electrical conductivity and a change in the sign of the Seebeck coefficient around 373 K. Lattice vibrations of Td-WTe(2) have been analyzed using first-principles calculations. Out of the 33 possible zone-center Raman active modes, five distinct Raman bands are observed around 112, 118, 134, 165 and 212 cm(-1) in bulk Td-WTe(2). Based on symmetry analysis and calculated Raman tensors, we assign the intense bands at 165 cm(-1) and 212 cm(-1) to the A'(1)and A''(1) modes, respectively. Most of the Raman bands stiffen with decreasing thickness, and the ratio of the integrated intensities of the A''(1) to A'(1) bands decreases in the few-layer sample, while all the bands soften in both the bulk and few-layer samples with increasing temperature.

Two-dimensional inorganic analogues of graphene: transition metal dichalcogenides

“…First-principles calculations reveal that electronic band structures for bulk and monolayer of Td-WTe 2 are rather similar, both being semimetallic in nature [75]. This is in contrast with other TMDCs which exhibit a strong dependence of the electronic structure on the number of layers.…”

“…The value of α for the A 1g mode in monolayer MoSe 2 is much smaller (−0.0045 × 10 −2 cm −1 K −1 ) than that of monolayer MoS 2 due to difference in the strain-phonon modes as revealed by first-principles calculations [77]. When exposed to shockwaves, few-layer MoS 2 and other TMDCs undergo morphological changes with [73], (c) adapted with permission from Tongay et al [74], (d) adapted with permission from Gupta et al [31] and (e, f ) adapted with permission from Jana et al [75]. (Online version in colour.)…”

Transition Metal Dichalcogenides and Other Layered Materials

“…In contrast with other TMDs, such as MoS 2 which exhibit a strong dependence of the electronic structure on the number of layers, first-principles calculations reveal that bulk as well as monolayer Td−WTe 2 have similar electronic band structures, both being semimetallic [140]. Td−WTe 2 has 33 zone-centre Raman active modes of which five distinct Raman bands are experimentally observed around 112 cm −1 , 118 cm −1 , 134 cm −1 , 165 cm −1 and 212 cm −1 in bulk Td−WTe 2 (Figure 12(a)).…”

“…Td−WTe 2 has 33 zone-centre Raman active modes of which five distinct Raman bands are experimentally observed around 112 cm −1 , 118 cm −1 , 134 cm −1 , 165 cm −1 and 212 cm −1 in bulk Td−WTe 2 (Figure 12(a)). Based on the symmetry analysis and the Raman tensor, the intense bands at 165 cm −1 and 212 cm −1 have been assigned to the A 1 and the A 1 modes, respectively [140]. In contrast with the 2H-polytypes of group VI TMDs such as MoS 2 , the low crystal-symmetry of Td−WTe 2 results in mixing of the out-of-plane and in-plane components of atomic displacements.…”

“…The Te atoms of the same plane vibrate in-phase in the A 1 vibration, whereas their motion is out-of-phase in the A 1 vibration. The A 1 mode is, therefore, more localised to a layer of Td−WTe 2 and exhibits weaker or no dependence on the number of layers [140]. Bulk ReS 2 , a group VII TMD, occurs in a distorted 1T (1T ) structure with dimerised zigzag Re−Re chains and behaves as electronically and vibrationally decoupled monolayers stacked together [141].…”