(Invited) Optimizations of p and n-Type Bi2Te3-Based Ternary Compounds By Ms-Pulsed Plating and Annealing Under Telluride Atmospheres

In this work, a comprehensive study of thermoelectric chalcogenide materials is presented and the systematic optimization of n-type Bi₂Te₃, p-type Sb₂Te₃ and their ternary compounds is performed. Thermoelectric materials are synthesized by potentiostatic electrodeposition on Au/Pt and stainless steel substrates. The influence of the preparative parameters such as the composition of the electrolyte bath and the deposition potential is investigated in a nitric acid solution.
As a post-deposition step, the influence of annealing of the films is investigated. The use of a potential-controlled millisecond-pulsed deposition method improves both the morphology and the composition of the films.

For n-doped films, approximately -100 µV K⁻¹ (Bi₂Te₃) and -130 µV K⁻¹ (Bi₂(Te_xSe_1−x)₃) are achieved. Power factors and ZT values for p-type and n-type ternary alloys of up to 1325 µW m⁻¹K⁻² (ZT=0.4) and 825 µW m⁻¹K⁻² (ZT=0.3) are realized, respectively. In conclusions, the thermoelectric perform have been improved by one to two orders of magnitude in comparison to previously electrochemical synthesized thermoelectric layers. Based on the electrochemical synthesis and subsequent balancing of the stoichiometry by annealing under Te atmosphere, have resulted in bulk-like power-factors.

Millisecond pulsed electrochemical deposition is also a quite flexible approach for achieving nanowires of ternary chalcogenide compounds, which are grown in nanoscale confined spaces. After the annealing in Te enhanced the transport properties, on single Bi₂(Te_1-xSe_x)₃ and (Bi_x-1Sb_x)₂Te₃ nanowires power factors of 3100 mW K^-2 m^-1 and of 1600 mW K^-2 m^-1 with nearly bulk-like properties are achieved, respectively. 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 Ministry of Science and Educations (BMBF) and by the German Priority Program DFG-SPP 1386 on Thermoelectric Nanostructures is gratefully acknowledged: www.spp1386thermoelectrics.de