Electrodeposition of Bi 2 Te 3 Nanowire Composites
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Electrodeposition of Bi2Te3 Nanowire Composites Amy L. Prieto1, Melissa S. Sander1, Angelica M. Stacy1, Ronald Gronsky2, Timothy Sands2 1 Department of Chemistry, University of California, Berkeley 2 Department of Materials Science, University of California, Berkeley Berkeley, CA, 94720 ABSTRACT Widespread applications of thermoelectric materials are limited due to low efficiency. Currently, the most widely used thermoelectric devices consist of alloys based on Bi2Te3. In such devices, the thermoelectric figure-of-merit (ZT) of bulk Bi2Te3 has been increased through doping. It is postulated that further enhancements in ZT may be attained by engineering the microstructure of the material to enhance carrier mobility while suppressing the phonon component of the thermal conductivity. This may be achieved by fabricating Bi2Te3 in the form of one-dimensional (1D) nanowires. We have deposited nanowires of Bi2Te3 with two different diameters (200 nm and 40 nm) by electrodeposition into porous anodic alumina. Characterization of the Bi2Te3/porous Al2O3 composite materials has been accomplished using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Energy dispersive X-ray spectroscopy (EDS) has been used to determine the stoichiometry of the wires. INTRODUCTION There has been much excitement recently over theoretical predictions that quantum confinement of bulk thermoelectric materials could result in a composite material with a ZT significantly above that of bulk values.1-3 An increase in the Seebeck coefficient due to quantum confinement of carriers, and a decrease in the thermal conductivity due to the scattering of phonons off interfaces is predicted to be the result. PbTe/Pb1-xEuxTe superlattices have confirmed these predictions for 2D systems.4-9 The increase in ZT is expected to be more dramatic in 1D (nanowires) as opposed to 2D systems (superlattices). Our goal is to prepare and characterize Bi2Te3 nanowire arrays in order study to the effects of quantum confinement on 1D thermoelectric materials. Arrays of nanowires have been fabricated in a wide variety of template materials.10,11 We have chosen templates of anodic alumina due to the easily tunable pore diameters (from 9-300 nm), the high pore densities (to 7x1010/cm2), and the high aspect ratio pores (~ 100 µm long pores) these templates provide.12 The control of these variables in porous anodic alumina templates is a well-established process. In addition, alumina is an electrical and thermal insulator and is thermally stable, making it a good matrix material for the composite arrays. To fabricate wires within the templates, several different deposition techniques have been developed. We have chosen to employ electrochemical deposition because it offers a high degree of control over the synthesis conditions. The wire stoichiometry and microstructure can be controlled through careful manipulation of a broad range of deposition variables, including concentrations of species in solution, potential, current, elec
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