Diffusion Thermopower Of Bismuth Nanowires And The Role Of Carrier's Boundary Scattering. Doping, Pressure and Magnetic Field Studies

Bulk Bi and Bi-Sb are of interest for solid state cooling applications. Composites of these materials may show to be superior to bulk materials in these applications, because of quantum confinement and phonon scattering. Also, there is an interested in miniature devices and nanoscale coolers that interface with them. We have studied the thermopower of single Bi nanowires of diameters in the range 50-500 nm. The nanowires are fabricated as single strands of thermoelectric material, that are monocrystalline, in a glass envelope (a fiber). We observe that the thermopower peaks of around +90 muV/K at around 50 K. These values are the largest for any electrical-conductor in this temperature range. We interpret these effects in terms of a phenomenological model where boundary scattering is more effective for electrons than for holes. The temperature and the value of thermopower maximum depend sensitively with magnetic fields and Te doping. Also, stretching the fibers cause uniaxial stresses similar to that of "negative pressure" [Hicks, LD, et. al., 1993] that drives an electron topological transition, similarly to the case of Bi-Te [Lin, Y-M, et. al., 2000]. Near the ETT point we recorded very large oscillations of the thermopower, that are associated with the Landau levels in the nanowires. Our work focuses in the development a mathematical model to optimize the thermoelectric figure of merit considering magnetic field, doping, and pressure