Diameter-dependent thermopower of bismuth nanowires

We present a study of electronic transport in individual Bi nanowires of large diameter relative to the Fermi wavelength. Measurements of the resistance and thermopower of intrinsic and Sn-doped Bi wires with various wire diameters, ranging from 150 to 480 nm, have been carried out over a wide range of temperatures (4 - 300 K) and magnetic fields (0 - 14 T). We find that the thermopower of intrinsic Bi wires in this diameter range is positive (type p) below about 150 K, displaying a peak at around 40 K. In comparison, intrinsic bulk Bi is type n. Magnetothermopower effects due to the decrease of surface scattering when the cyclotron diameter is less than the wire diameter are demonstrated. The measurements are interpreted in terms of a model of diffusive thermopower, where the mobility limitations posed by hole-boundary scattering are much less severe than those due to electron-boundary scattering.