Thermoelectric Transport Properties of Bi Nanowires
An enhanced thermoelectric figure of merit, ZT, has been predicted for Bi nanowires. The enhancement is based on the sharp features in the one-dimensional density of states of the nanowires, the increased boundary scattering of phonons which results in a lowered thermal conductivity, and the semimetal-semiconductor transition. Transport properties are reported for Bi nanowires, which have been prepared by the filling of an alumina template with molten Bi. Lithographic processes are devised to pattern 4-point electrodes on single Bi nanowires that have been removed from the alumina template. High resistance non-ohmic contacts are attributed to a thick oxide layer formed on the surface of the nanowires. The non-linear 2-point i(V) response of these contacts is understood on the basis of a tunneling model. Techniques are developed for making ohmic contacts to single bismuth nanowires through the thick oxide coating using a focused ion beam (FIB) to sputter away the oxide and then deposit contacts. By combining the FIB techniques with electron beam lithography we achieve contacts stable from 300K to 2K for nanowires less than 100nm in diameter. Annealing in H2 and also NH3 environments is found to reduce the oxide completely. However, the high temperatures required for this annealing are not compatible with the lithographic techniques. A lithographic scheme for measuring the Seebeck coefficient of a single Bi nanowire is devised. The electronic band structure of Bi nanowires is modeled theoretically based on the quantum confinement of electrons. 4-point resistivity data on single Bi nanowires are reported and understood on the basis of the theoretical model of the quantized electronic band structure and considering the wire boundary and grain boundary scattering not present in bulk bismuth.