High-efficiency, solid-state conversion of thermal energy to electrical energy using thermoelectric phenomena is an appealing and potentially low-cost way to capture electricity from waste heat, thereby reducing the need to harness additional sources of energy. However, after decades of study, the best thermoelectric figure-of-merit (ZT) remains approximately equal to 2, whereas a ZT of 3 or 4 is needed to open new markets. Recent semiclassical Bloch-Boltzmann calculations predict greatly enhanced ZT for nanowires with a diameter less than the thermal de Broglie wavelength of electrons in the material.[1] As a result, nanowire-based materials provide an innovative route to increasing ZT. Further, practical devices fabricated from large parallel arrays of nanowires embedded within an insulating support are also predicted to have an enhanced ZT.[2]

One promising route is the electrochemical deposition of thermoelectric materials within highly-ordered nanostructured silica films synthesized by evaporation induced self-assembly[3-6] of surfactants with metal oxide precursors on conductive substrates. After removal of the surfactant template, a nanoporous film remains, which is used to template the growth of nanowires during electrochemical deposition. Of the many topologies synthesized over the last decade, few allow accessibility to the substrate for ions in solution.[7] Tricontinuous double-gyroid derived nanoporous films templated by triblock copolymers are the exception.[8] It was recently shown by our research group that electrochemical deposition of metals such as platinum and cobalt are facile within these nanoporous materials.[8, 9]

Here, we report the first electrochemical deposition of semiconductor bismuth telluride with effective diameters less than the thermal de Broglie wavelength (8 nm). Bismuth telluride was deposited under conditions similar to those found on bare substrates[10] and large pore system.[11] After deposition, use of the optical microscope confirmed that the pore filling was in excess of 80%. Confirmation of high microscopic pore filling, as well as atomic composition, was achieved using the electron microscope. Finally, results from detailed electrochemical deposition studies on the effects of potential, potential pulse, and changes in the composition of the electrolyte bath on the structural features of the nanowire networks are also presented.

REFERENCES

1. Hicks, L. D. and M. S. Dresselhaus, Thermoelectric figure of merit of a one-dimensional conductor, Physical Review B, 47, pp. 16631-16634 (1993).

2. Hillhouse, H. W. and M. T. Tuominen, Modeling the thermoelectric transport properties of nanowires embedded in oriented microporous and mesoporous films, Microporous and Mesoporous Materials, 47, pp. 39-50 (2001).

3. Grosso, D., F. Cagnol, G. Soler-Illia, E. L. Crepaldi, H. Amenitsch, A. Brunet-Bruneau, A. Bourgeois and C. Sanchez, Fundamentals of mesostructuring through evaporation-induced self-assembly, Advanced Functional Materials, 14, pp. 309-322 (2004).

4. Brinker, C. J., Y. F. Lu, A. Sellinger and H. Y. Fan, Evaporation-induced self-assembly: Nanostructures made easy, Advanced Materials, 11, pp. 579-585 (1999).

5. Lu, Y. F., R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker, W. L. Gong, Y. X. Guo, H. Soyez, B. Dunn, M. H. Huang and J. I. Zink, Continuous formation of supported cubic and hexagonal mesoporous films by sol gel dip-coating, Nature, 389, pp. 364-368 (1997).

6. Zhao, D. Y., J. L. Feng, Q. S. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores, Science, 279, pp. 548-552 (1998).

7. Wei, T. C. and H. W. Hillhouse, Mass Transport and Electrode Accessibility Through Periodic Self-Assembled Nanoporous Silica Thin Films, Langmuir, 23, pp. 5689-5699 (2007).

8. Urade, V. N., T.-C. Wei, M. P. Tate, J. D. Kowalski and H. W. Hillhouse, Nanofabrication of Double Gyroid Thin Films, Chemistry of Materials, 19, pp. 768-777 (2007).

9. Urade, V. N., L. Bollmann, J. D. Kowalski, M. P. Tate and H. W. Hillhouse, Controlling Interfaction Curvature in Nanoporous Silica Films formed by Evaporation Induced Self-Assembly from Nonionic Surfactants: II Effect of Processing Parameters on Film Structure, Langmuir, 23, pp. 4268-4278 (2007).

10. Martin-Gonzalez, M., A. L. Prieto, R. Gronsky, T. Sands and A. M. Stacy, Insights into the Electrodeposition of Bi2Te3, Journal of The Electrochemical Society, 149, pp. C546-C554 (2002).

11. Martin-Gonzalez, M., A. L. Prieto, R. Gronsky, T. Sands and A. M. Stacy, High-Density 40 nm Diameter Sb-Rich Bi2-xSbxTe3 Nanowire Arrays, Advanced Materials, 15, pp. 1003-1006 (2003).

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