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 sigma, and thermal conductivity kappa of individual NWs. The NWs studied show near intrinsic transport properties with low electrical conductivities of around sigma = (3.2 +/- 0.9) x 10(4) Omega(-1) m(-1) at room temperature. We observe a transition of the Seebeck coefficient from positive to negative values (S = +133 mu VK-1 to S = -87 mu VK-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 kappa = (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.