Improving Bi2Te3-based thermoelectric nanowire microstructure via thermal processing

Achieving control of crystalline quality is a key barrier to developing thermoelectric (TE) nanowires. We show that the structural properties of free-standing Bi2(Te.97Se.03)3 nanowire arrays on substrates can be improved by postdeposition annealing. Nanowires were electrochemically deposited into anodized aluminum oxide nanopore templates formed directly on metallized Si(100). The templates were chemically removed prior to annealing in a 3% H2/Ar environment to prevent microcrack formation that results from thermal stresses. Grain sizes grew exponentially with annealing temperature until reaching the full 75-nm diameter of the nanowires at 300 °C; growth was linear above this temperature since grains could grow further only in the axial directions. Crystalline quality, along with the development of the preferred (110) orientation for optimal TE properties, improved with increasing annealing temperature between 200 and 400 °C. However, continued loss of Te composition with annealing led to a mixed phase of Bi2Te3 and Bi4Te3 at 500 °C.