Noncentrosymmetric characteristics of defects on WTe2

Tungsten ditelluride (WTe2) is a transition metal dichalcogenide with novel electronic structures and unique properties for application to next-generation devices. Defects in WTe2 can impact its properties in both positive and negative ways. Therefore, it urgently requires a precise classification to help understand the possible impacts. Here we report on both geometric and electronic characteristics of the defects in WTe2 identified by the combination of scanning tunneling microscopy (STM) and density functional theory (DFT) calculation. We found four types of defects derived from a missing Te atom; two of them are located at the topmost surface while the others are under the surface. The former are imaged topographically and are ascribed to point vacancies of surface Te atoms. The DFT calculations reveal the noncentrosymmetric displacements of atoms around the defects and reasonably reproduce the STM images. Interestingly, the latter defects are hardly observed but they are dressed with the noncentrosymmetric quasiparticle interference (QPI) fringes which enable us to identify them. These findings demonstrate that STM-QPI can be a feasible method to characterize the defects in layered materials.