Scanning thermoelectric imaging of topological surface-states in two-d…

Thermoelectric (ThE) energy conversion is a basic process for solid-state coolers and power generators, and its efficiency is principally determined by the thermopower (or Seebeck coefficient, S) of a given ThE material. Therein the S of a bulk homogeneous conductor is intrinsically associated with the energy-dependent carrier conduction processes near the Fermi level. Bulk bismuth/antimony telluride, displaying the highest thermoelectric figure of merit at room temperature, is typically known as a narrow band-gap n/p-type semiconductor, however, in a few monolayers thickness regime their electronic structures are substantially modified by electronic confinements and strong spin-orbit interaction, as also known as topological insulators. Here, by scanning photo-induced thermoelectric current imaging, we show direct evidence that efficient thermoelectric conversion is available at the monolayer step homojunction by hot electrons/holes motions through two-dimensional subbands and topologically protected surface states. Our discovery suggest that the thermoelectric conversion can be interiorly achieved within a homogeneous medium by directly exploiting quantum nature of topological insulator at room temperature, and thus provide a new design rule for the compact thermoelectric circuitry.