Electrical transport properties of single bismuth nanowires

Due to its unique electronic properties bismuth is of enormous interest for studies on the nanoscale. In particular, when the object size is comparable to the mean free path of charge carriers le (~ 100 nm and ~ 400 μm at 300 and 4 K, respectively [1]), mesoscopic effects such as electron scattering at both surface [2] and grain boundaries [3] become significant. For investigating the influence of these two effects on the wire resistivity ρ, we fabricated and individually contacted single Bi nanowires. For an examination of the influence of grain boundary scattering, single wires of different crystallinity are created by varying the deposition parameters in a controlled manner [4]. In order to fabricate single Bi nanowires, polycarbonate foils are irradiated by single swift heavy ions at the UNILAC and subsequently etched in aqueous NaOH solution. After covering one side of a single-pore membrane by a conductive layer, a wire is electrochemically deposited in the nanochannel. The process is continued until a cap is formed on top of the wire, which is then covered with an additional Au layer for electrical contact [5]. The electrical resistance of single wires is recorded at room temperature and as a function of temperature T. Fig. 1 displays the specific wire resistivity versus diameter. The dashed lines represent the mean values of three different types of wire, that were created under three different deposition conditions. The solid line illustrates the resistivity of bulk Bi without taking into account any mesoscopic effects, while the dotted line represents the resistivity including totally diffuse electron scattering at the wire surface. The two approaches can neither explain the high resistivity values nor the difference of ρ for various kinds of wire. ρ can only be explained by incorporating electron scattering at grain boundaries. Since the grain size is larger for wires fabricated at higher T and smaller voltage U, wires created at these conditions exhibit smaller ρ values due to a lower contribution of scattering processes at grain boundaries. Fig. 2 displays the resistance R of single Bi nanowires of diverse diameters, created at 60 °C and -17 mV as a function of temperature. All wires show a nonmonotonic R vs. T behaviour, the resistance maximum shifting to higher temperatures with decreasing wire diameter. In Bi the charge carrier density decreases while the mobility increases with diminishing temperature. In bulk Bi, the influence of growing mobility surpasses the effects of decreasing carrier concentration, leading to a reduction of ρ with lowering T. In contrast, in nanowires le is limited by the electron scattering at grain boundaries. Since thinner wires are expected to possess less grain boundaries, the influence of electron scattering is less pronounced. 0 200 400 600 80