Thermoelectric properties of Bi1-xSbx nanowires and lead salt superlattice nanowires

This thesis involves an extensive experimental and theoretical study of the thermoelectric-related transport properties of Bii_2Sb. nanowires, and presents a theoretical framework for predicting the electrical properties of superlattice nanowires. A template-assisted fabrication scheme is employed to synthesize Bi-based nanowires by pressure injecting liquid metal alloys into the hexagonally packed cylindrical pores of anodic alumina. These nanowires possess a very high crystalline quality with a diameter-dependent crystallographic orientation along the wire axis. A theoretical model for Bii_2Sb, nanowires is developed, taking into consideration the effects of cylindrical wire boundary, multiple and anisotropic carrier pockets, and non-parabolic dispersion relations. A unique semimetal-semiconductor (SM-SC) transition is predicted for these nanowires as the wire diameter decreases or as the Sb concentration increases. Also, an unusual physical phenomenon involving a very high hole density of states due to the coalescence of 10 hole carrier pockets, which is especially advantageous for improving the thermoelectric performance of p-type materials, is uncovered for Bii_.2Sb_ nanowires. Various transport measurements are reported for Bi-related nanowire arrays as a function of temperature, wire diameter, Sb content, and magnetic field. R(T) measurements show distinct T dependences for semimetallic and semiconducting nanowires, as predicted by the theory, and the condition for the SM-SC transition can be clearly identified. Enhanced thermopower is observed for Bii_,Sb, nanowires as the diameter decreases or as the Sb content increases, indicating that both quantum confinement effects and Sb alloying effects are important for improving the thermoelectric performance. The theoretical model is further extended to study transport properties of Te-doped Bi nanowires and Sb nanowires, and good agreement between theoretical predictions and experimental results is obtained. A model for superlattice nanowires is presented to evaluate their potential for thermoelectric applications. Thermoelectric properties of superlattice nanowires made of various lead salts (PbS, PbSe, and PbTe) are investigated as a function of segment length, wire diameter, crystal orientation along the wire axis, and length ratio of