Abstracts Engineering

Thermoelectric Properties of Bi1-xSbx Nanowires Electrodeposited in Etched Ion-Track Membranes

by Marco Cassinelli

The unique properties exhibited by bismuth antimony (Bi1-xSbx) nanostructures and the theoretical predictions on their enhanced thermoelectric efficiency two decades ago triggered the development of a wide variety of growth and characterization methods aiming at measuring the thermoelectric properties of Bi1-xSbx nanowires. However, the experimental demonstration of a size-dependent thermoelectric efficiency is still challenged by the difficulties to fabricate well-defined nanowire systems and to achieve reliable and stable electrical and thermal contacts. In this work, Bi1-xSbx nanowire arrays with controlled wire composition (0 ≤ x ≤ 1) and diameter (from ~ 750 to ~ 40 nm) were synthesized by pulsed electrodeposition in etched ion-track membranes. The thermoelectric properties of Bi1-xSbx are strongly influenced by the anisotropic structure of the material. Thus, special consideration was taken in studying the influence of the electrodeposition conditions on the crystallographic orientation and composition of the wires for various diameters. To characterize the wires several methods were applied, including X-ray diffraction and both scanning (SEM) and transmission electron microscopy (TEM). Using such pre-characterized Bi1-xSbx nanowires, measurements of the Seebeck coefficient and electrical resistance were performed in the temperature between 300 and 20 K and for a systematic variation of the composition and diameter of the wires. The Seebeck coefficient values show a non-monotonic behavior when decreasing the wire diameter, which is explained by taking into account the existence of surface states as well as quantum- and finite-size effects. Finally, the chemical and thermal stability of the Bi1-xSbx nanowires was investigated in air both at room and moderate temperatures, revealing the rapid formation of a metal oxide phase. The oxidation process causes an increase of the nanowires surface roughness at low annealing temperatures, and the formation of protuberances at 250°C. Our results on oxidation explain the diverse and high resistance values available in literature and the difficulties to electrically contact single Bi1-xSbx nanowires reported by other group. Advisors/Committee Members: Trautmann, Christina (advisor), Ensinger, Wolfgang (advisor).