Thermopower Measurements of Arrays of Small Diameter (18-60 nm) Bi Nanowires
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1044-U03-05
Thermopower Measurements of Arrays of Small Diameter (18-60 nm) Bi Nanowires T.E. Huber1, A. Adeyeye1, T. Odunfa1, A. Nikolaeva2,3, L. Konopko2,3, R. Johnson4, and M.J. Graf4 1 Howard University, 500 College St. N.W., Washington, DC 20059, Washington, DC, 20059 2 Academy of Sciences Moldova, Kishinev, 3018, Moldova 3 International High Magnetic Fields and Low Temperatures Laboratory, Wroclaw, 0000, Poland 4 Boston College, Chestnut Hill, MA, 02467 ABSTRACT Theoretical work based on one-dimensional (1D) models indicates that Bi wires with diameter smaller than 50 nm can exhibit superior thermoelectric properties since the density of states at the Fermi level of a 1D system can be tuned to very high values. Recently, angleresolved photoemission spectroscopy (ARPES) studies of Bi surfaces have shown that Bi nanowires support Rashba spin-orbit surface states, with high carrier densities of around 5 × 10 12 cm −2 , that have not been considered in current models of Bi nanowires. According to our estimates, the sheath of surface charge on Bi nanowires should contribute substantially to the thermopower of Bi nanowires. We carried out an experimental study of the transport properties and thermopower of bismuth nanowire arrays (NWA) with wire diameters ranging between 60 nm and 13 nm to investigate these phenomena. The Rashba interaction is a spin orbit interaction that is important for surfaces of materials consisting of heavy ion elements; thermoelectric materials frequently consist of these elements (i.e. PbTe) because they scatter phonons strongly and consequently form low-thermal-conductivity materials. I. INTRODUCTION Nanowire systems have become the focus of intense experimental and theoretical investigation. The most exciting prospect involves an ideal quantum wire of a diameter d that is less than the Fermi wavelength, λF, and with the Fermi level chosen such that the nanowire transport is mediated by a single conduction channel. The properties of such a one-dimensional quantum wire have been investigated theoretically for the particular case of Bi nanowires by Hicks and Dresselhaus [1]. In bulk Bi, there is substantial overlap (E0 ~ 40 meV) between the electron and hole bands. In a Bi nanowire, the effect of quantum confinement is to raise the zeropoint energy of electrons and to lower that of the holes by roughly π 2h 2 / m*d 2 , where h is
Planck’s constant and m* is the corresponding carrier in-plane effective mass transverse to the wire axis, thus effectively decreasing E0. Theoretically, this effect induces a semimetal-tosemiconductor (SMSC) transition in Bi nanowires. Detailed calculations [2], which assume that the effective masses do not change upon confinement, show that the SMSC transition occurs at d ~ 50 nm. This theory also shows that the thermopower and the thermoelectric figure of merit of materials based on small diameter (d< 50 nm) Bi nanowires, such as Bi nanowire arrays (NWA), are enhanced by this effect. We have studied NWAs of various diameters using the method based on Shubnikov
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