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Silicon nanowires as efficient thermoelectric materials


Thermoelectric materials interconvert thermal gradients and electric fields for power generation or for refrigeration1,2. Thermoelectrics currently find only niche applications because of their limited efficiency, which is measured by the dimensionless parameter ZT—a function of the Seebeck coefficient or thermoelectric power, and of the electrical and thermal conductivities. Maximizing ZT is challenging because optimizing one physical parameter often adversely affects another3. Several groups have achieved significant improvements in ZT through multi-component nanostructured thermoelectrics4,5,6, such as Bi2Te3/Sb2Te3 thin-film superlattices, or embedded PbSeTe quantum dot superlattices. Here we report efficient thermoelectric performance from the single-component system of silicon nanowires for cross-sectional areas of 10 nm × 20 nm and 20 nm × 20 nm. By varying the nanowire size and impurity doping levels, ZT values representing an approximately 100-fold improvement over bulk Si are achieved over a broad temperature range, including ZT ≈ 1 at 200 K. Independent measurements of the Seebeck coefficient, the electrical conductivity and the thermal conductivity, combined with theory, indicate that the improved efficiency originates from phonon effects. These results are expected to apply to other classes of semiconductor nanomaterials.

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Figure 1: Scanning electron micrographs of the device used to quantitate the thermopower and electrical and thermal conductivity of Si nanowire arrays.
Figure 2: Factors contributing to ZT for various Si nanowires. All nanowires are 20 nm in height.
Figure 3: Temperature dependence of ZT for two different groups of nanowires.
Figure 4: Thermopower calculation plotted along with experimental data (black points) from a 20-nm-wide Si nanowire p-type doped at 3 × 1019 cm-3.

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We thank D. Wang for discussions and J. Dionne, M. Roy, K. Kan and T. Lee for fabrication assistance. This work was supported by the Office of Naval Research, the Department of Energy, the National Science Foundation, the Defense Advanced Research Projects Agency, and a subcontract from the MITRE Corporation.

Author Contributions A.I.B., Y.B., J.-K.Y. and J.R.H. contributed primarily to the design and execution of the experiments. J.T.-K. and W.A.G. contributed primarily to the theory.

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Correspondence to James R. Heath.

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Supplementary Information

The file contains Supplementary Methods and Discussion with Supplementary Figures S1-S8. This file contains information about the materials processing and thermoelectric measurements. Also, a more detailed discussion of the results and theory is included. (PDF 1155 kb)

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Boukai, A., Bunimovich, Y., Tahir-Kheli, J. et al. Silicon nanowires as efficient thermoelectric materials. Nature 451, 168–171 (2008).

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