The influence of a Te-depleted surface on the thermoelectric transport properties of Bi₂Te₃ nanowires.

We report on thermoelectric transport measurements along the basal plane of several individual, single-crystalline Bi₂Te₃ nanowires (NWs) with different cross-sectional areas, grown by a vapor-liquid-solid method. Lithographically defined microdevices allowed us to determine the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ of individual NWs. The NWs studied show near intrinsic transport properties with low electrical conductivities of around σ = (3.2 ± 0.9) × 10⁴ Ω⁻¹ m⁻¹ at room temperature. We observe a transition of the Seebeck coefficient from positive to negative values (S = +133 μVK⁻¹ to S = -87 μVK⁻¹) with increasing surface-to-volume ratio at room temperature, which can be explained by the presence of an approximately 5 nm thick Te-depleted layer at the surface of the NWs. The thermal conductivities of our NWs are in the range of κ = (1.4 ± 0.4) Wm⁻¹ K⁻¹ at room temperature, which is lower than literature values for bulk Bi₂Te₃. We attribute this suppression in thermal conductivity to enhanced scattering of phonons at the surface of the NWs. Despite their reduced thermal conductivities, the NWs investigated only show a moderate figure of merit between 0.02 and 0.18 due to their near intrinsic transport properties.