Nonlayered tellurene as an elemental 2D topological insulator: experimental evidence from scanning tunneling spectroscopy.

We report the formation of a nonlayered tellurene monolayer in its alpha-phase through an anisotropic ultrasonication method. The nonlayered tellurene has so far been predicted to exhibit a topologically insulating state of matter in two-dimensional (2D) form with an insulating interior and metallic edge states propagating along the perimeter of the 2D objects. In this work, we report direct evidence of elemental topological insulator behavior in the material through a localized mode of measurement, that is, scanning tunneling spectroscopic studies. We moreover deliberate on the length scale the time-reversal symmetry-protected edge states extend towards the interior. The metallic edge, which has been found to span over a 3 nm region, opens and widens monotonically into gapped states. The appearance of the elemental 2D topological insulator phase has been explained in terms of built-in strains in the systems as viewed through a shift in the Raman modes.