Thermoelectric properties of small diameter Bi nanowires: Evidence for surface charges

Abstract

The thermoelectric properties of quasi-one-dimensional systems attract considerable attention. Semimetallic bismuth, aside from being a good thermoelectric material, has charge carriers with very low effective masses, and, as a result, electronic quantum confinement effects induce a semimetal-to-semiconductor transformation (SMSC) for wires with diameters below roughly 50 nm. Theoretical work based on one-dimensional models indicate that fine Bi wires may exhibit superior thermoelectric properties since the density of states at the Fermi level can be tuned to very high values in this case. However, angle-resolved photoemission spectroscopy (ARPES) studies of Bi surfaces have shown that Bi nanowires support surface states, with high carrier densities of around 5 times 1012 cm-2 and large effective mass, that have not been considered in current models of the SMSC. Our studies of Shubnikov-de Haas oscillations for 30-nm diameter Bi nanowires support this model. According to our estimates, the thermopower of this two dimensional sheet of charge should be observable and we carried out an experimental study of the thermopower of arrays of small diameter Bi nanowires to test these conflicting models of fine Bi nanowires