Crystallographically-Oriented Electrochemically-Deposited Bismuth Nanowires
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Crystallographically-Oriented Electrochemically-Deposited Bismuth Nanowires Oded Rabin (a), Gang Chen (b), Mildred S. Dresselhaus (c,d) Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. (a) Dept. of Chemistry. (b) Dept. of Mechanical Engineering. (c) Dept. of Electrical Engineering and Computer Science. (d) Dept. of Physics. ABSTRACT Bismuth nanowires 200 nm in diameter were synthesized via electrochemical deposition into the pores of anodic alumina templates. A near neutral pH solution and a special sample holder were employed. Both polycrystalline and (012) oriented bismuth nanowire arrays were prepared. The electrical resistance of the samples versus temperature was measured in the range 2-300 K, and the results were fitted to a transport model. Despite the crystallographic alignment of the nanowire array, the model calculations suggest the dominance of a temperature independent scattering mechanism (such as grain boundary or nanowire boundary scattering). The results are compared to the electrical resistance of nanowires formed by impregnation techniques. INTRODUCTION Nanowires of thermoelectric materials have been predicted to have superior thermoelectric characteristics over their bulk material counterparts [1]. Both the quantization of the electronic density-of-states and the enhancement of surface scattering are expected to increase the value of the Seebeck coefficient and decrease the value of the thermal conductivity in low-dimensional systems. In this paper, recent advances in the preparation of bismuth nanowires by electrochemical deposition into porous anodic alumina templates are presented. Previously, bismuth nanowires have been synthesized via electrochemical deposition [2-3], pressure injection of the melt [4-5], and vapor phase condensation [6] in the pores of porous anodic alumina templates. While electrochemical deposition is advantageous for the preparation of large quantities of small diameter nanowires, the nanowires obtained by this method thus far are inferior in terms of their crystal quality to samples made by other methods: the nanowires are polycrystalline and show no preferred crystallographic orientation along the main nanowire axis. These materials properties are highly relevant to the thermoelectric performance, particularly with anisotropic materials such as bismuth, since low effective carrier masses, and long mean free paths and phase coherence lengths are desirable. We have found synthetic conditions that lead to improvements in the crystallinity and orientation of 200 nm nanowire arrays. Comparison of the electrical transport properties of these samples to those of samples prepared by other methods is shown to be essential for the evaluation of the structural quality of the arrays. Transport measurements are also necessary to improve theoretical predictions about the influence of structural factors on thermoelectric performance of nanowires.
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EXPERIMENTAL DETAILS
Figure 1. Scheme of the components of the electrodeposition setup. On the right, a
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