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 $150\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}480\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, have been carried out over a wide range of temperatures $(4--300\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and magnetic fields $(0--14\phantom{\rule{0.3em}{0ex}}\mathrm{T})$. We find that the thermopower of intrinsic Bi wires in this diameter range is positive (type $p$) below about $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, displaying a peak at around $40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. 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.