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 K͒ and magnetic fields ͑0-14 T͒. We find that the thermopower of intrinsic Bi wires in this diameter range is positive ͑type p͒ 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.

We present measurements of Shubnikov-de Haas oscillations in arrays of bismuth nanowires. For 80-nm wires, the hole concentration is less than 30% of that for bulk Bi, a finding that is consistent with current models of quantum confinement effects. However, 30-nm-diameter nanowires, which are predicted to be semiconductors, show a nearly isotropic short period of 0.025 T -1 , consistent with a heavy carrier concentration five times that of bulk Bi. These results are discussed in terms of surface-induced charge carriers in a spherical Fermi surface pocket that are uniformly distributed in the 30-nm nanowire volume and that inhibit the semimetal-tosemiconductor transition.

Many thermoelectrics like Bi exhibit Rashba spin-orbit surface bands for which topological insulator behavior consisting of ultrahigh mobilities and enhanced thermopower has been predicted. Bi nanowires realize surface-only electronic transport since they become bulk insulators when they undergo the bulk semimetal-semiconductor transition as a result of quantum confinement for diameters close to 50 nm. We studied 20-, 30-, 50-and 200-nm trigonal Bi wires. Shubnikov-de Haas magnetoresistance oscillations caused by surface electrons and bulklike holes enable the determination of their densities and mobilities. Surface electrons have high mobilities exceeding 2 m 2 sec -1 V -1 and contribute strongly to the thermopower, dominating for temperatures T< 100 K. The surface thermopower is −1.2 T µV/K 2 , a value that is consistent with theory, raising the prospect of developing nanoscale thermoelectrics based on surface bands.2