A topological phase in 2D

2D Mater. 4, 021008 (2017)

The predominantly semiconducting nature of transition metal dichalcogenides (TMDs) favourably contrasts them with gapless graphene for several electronic applications. Contingent upon the atomic arrangement of TMD layers, a number of structural phases with diverse electronic properties may exist. Recently, it has been theoretically suggested that in the case of WTe₂, topological electronic states can be found in the distorted octahedral 1T′ phase. However, this polymorph is not energetically favourable and is therefore hard to study experimentally. Naylor et al. now report the growth and characterization of a monolayer WTe₂, a unique TMD material with a 1T′ ground state.

Due to rapid degradation of 1T′ WTe₂ flakes grown by chemical vapour deposition (CVD), the researchers use a graphene overlayer to protect the material from exposure to air. The results of Raman mapping and X-ray photoelectron spectroscopy characterization confirm the 1T′ phase in monolayer and few-layer WTe₂ flakes. The investigation of their electronic properties suggests metallic conduction at 2 K. Moreover, the magnetoresistance data reveal a weak antilocalization feature, which is commonly observed in topological insulators and could be indicative of the presence of topological states in 1T′ WTe₂ monolayers. Further improvement of the growth protocol is required to increase the surface coverage of the CVD-grown WTe₂ film for extended characterization of the electronic properties of the material.