Professor Dr. Kornelius Nielsch, Institue of Applied Physics, University of Hamburg, Hamburg, Germany

Defect and Composition Engineering of Bi₂Te₃-based Thermoelectric Nanowires

Chalcogenide nanowires based on Bi₂Te₃ and related materials are of significant interest for two scientific fields: nanostructured thermoelectrics and topological insulators. In the presentation, we will describe two important chemical synthesis approaches for nanostructured thermoelectric materials on the way towards optimized physical model systems. We will present the thermoelectric properties of nanostructured objects which have been synthesized by the following two different approaches:
1) Growth by the Vapour Liquid Solid (VLS) mode of single-crystalline and binary semiconductor nanowires and nanobelts is a widespread technique. The resulting Bi₂Te₃ nanowires exhibit reduced tellurium content at the nanowire surface. After annealing in a Te atmosphere, single-crystalline Bi₂Te₃ nanowires have been obtained, which show reproducible electronic transport properties (electrical conductivity and Seebeck coefficient) close to those of intrinsic bulk Bi₂Te₃.
2) Millisecond-Pulsed Electrochemical Deposition is a quite flexible approach for achieving nanowires of ternary chalcogenide compounds, which have been grown in nanoscale confined spaces. After annealing in Te, enhanced transport properties close to those of bulk materials have been observed: Single Bi₂(Te_1-xSe_x)₃ and (Bi_x-1Sb_x)₂Te₃ nanowires exhibit power factors of 3100 ?W/K²m and 1600 ?W/K²m, respectively.

Both combined approaches, based on a chemical synthesis technique and subsequent balancing of the stoichiometry by annealing under Te atmosphere, have resulted in bulk-like power-factors for the Bi₂Te₃ based nanowires. Furthermore, we present magneto-resistance measurements on the Bi₂Te₃ and Sb₂Te₃ nanowires, which show clear evidences of topological surface states.
The financial support by the German Priority Program DFG-SPP 1386 on Thermoelectric Nanostructures is gratefully acknowledged: www.spp1386thermoelectrics.de