Scanning tunneling microscopy of the subsurface structures of tungsten ditelluride and molybdenum ditelluride

The surface structure of the van der Waals faces of tungsten ditelluride (${\mathrm{WTe}}_2$) and molybdenum ditelluride (2H-${\mathrm{MoTe}}_2$) have been studied with scanning tunneling microscopy (STM). The hexagonal symmetry observed on the 2H-${\mathrm{MoTe}}_2$ surface is similar to that observed previously on other transition-metal dichalcogenides. On ${\mathrm{WTe}}_2$, which has a distorted layered structure due to the pairing of the metal atoms, the scanning tunneling micrographs distinctly show the dominance of the metal. Buckled, zig-zag chains of paired atomic rows, which are the signature of the tungsten layer, are observed. These results show for the first time that subsurface atoms can be imaged with the STM. The corrugated surface tellurium layer could not be identified unambiguously in two-dimensional scans. These results are surprising because a first-principles pseudofunction calculation of the surface-electronic charge density around the Fermi energy of the ${\mathrm{WTe}}_2$ surface shows that the calculated spatial distribution of the charge density at the surface has the characteristics of the topmost Te atoms. The experimental observations suggest that, unlike the case of graphite images, a direct comparison of the STM image of this surface with calculated surface charge density is not possible. These observations further suggest that the hexagonal symmetry observed in ${\mathrm{MoTe}}_2$ and other transition-metal dichalcogenides is also due to the metal layer rather than the surface chalcogenides.