Tuning the crystallinity of thermoelectric Bi2Te3 nanowire arrays grown by pulsed electrodeposition

Abstract

Arrays of thermoelectric bismuth telluride (Bi2Te3) nanowires were grown into porous anodic alumina (PAA) membranes prepared by a two-step anodization. Bi2Te3 nanowire arrays were deposited by galvanostatic, potentiostatic and pulsed electrodeposition from aqueous solution at room temperature. Depending on the electrodeposition method and as a consequence of different growth mechanisms, Bi2Te3 nanowires exhibit different types of crystalline microstructure. Bi2Te3 nanowire arrays, especially those grown by pulsed electrodeposition, have a highly oriented crystalline structure and were grown uniformly as compared to those grown by other electrodeposition techniques used. X-ray diffraction (XRD) analyses are indicative of the existence of a preferred growth orientation. High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) confirm the formation of a preferred orientation and highly crystalline structure of the grown nanowires. The nanowires were further analyzed by scanning electron microscopy (SEM). Energy dispersive x-ray spectrometry (EDX) indicates that the composition of Bi-Te nanowires can be controlled by the electrodeposition method and the relaxation time in the pulsed electrodeposition approach. The samples fabricated by pulsed electrodeposition were electrically characterized within the temperature range 240 K <= T <= 470 K. Below T approximate to 440 K, the nanowire arrays exhibited a semiconducting behavior. Depending on the relaxation time in the pulsed electrodeposition, the semiconductor energy gaps were estimated to be 210-290 meV. At higher temperatures, as a consequence of the enhanced carrier-phonon scattering, the measured electrical resistances increased slightly. The Seebeck coefficient was measured for every Bi2Te3 sample at room temperature by a very simple method. All samples showed a positive value (12-33 mu V K-1), indicating a p-type semiconductor behavior.