Atomic and Electronic Structures of WTe₂ Probed by High Resolution Electron Microscopy and ab Initio Calculations

Transition metal dichalcogenides (TMDs) are a class of two-dimensional (2D) materials that have attracted growing interest because of their unique electronic and optical properties. Under ambient conditions, most TMDs generally exhibit 2H or 1T structures. Unlike other group VIb TMDs, bulk crystals and powders of WTe₂ exist in a distorted 1T structure (Td) at room temperature and have semimetallic properties. There is so far a lack of direct atom-by-atom visualization, limiting our understanding of this distorted 2D layered material system. We present herein atomic resolution images of Td structured WTe₂. The Td structure can be distinguished in the three major orientations along the [100], [010], and [001] zone axes. Subtle structural distortions are detected by atomic resolution imaging, which match well with the optimized structure relaxed by ab initio calculations. The calculations also showed that both crystal field splitting and charge density wave (CDW) interactions contribute to the stabilization of WTe₂. However, the CDW interaction dominates and leads the Td-WTe₂ to be the most stable structure. The combined atomic resolution STEM and ab initio study on WTe₂ provided the basis for understanding the correlations between atomic structure and electronic properties in Td structured TMD materials.