nature06381 - Vol 451 | 10 January 2008 |...

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LETTERS Enhanced thermoelectric performance of rough silicon nanowires Allon I. Hochbaum 1 * , Renkun Chen 2 * , Raul Diaz Delgado 1 , Wenjie Liang 1 , Erik C. Garnett 1 , Mark Najarian 3 , Arun Majumdar 2,3,4 1,3,4 Approximately 90 per cent of the world’s power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30–40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT . 1, since the parameters of ZT are generally interdependent 1 . While nanostructured thermoelectric materials can increase ZT . 1 (refs 2–4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20–300 nm in diameter. These nano- wires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT 5 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly redu- cing thermal conductivity without much affectingtheSeebeckcoef- ficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials. The most widely used commercial thermoelectric material is bulk Bi 2 Te 3 and its alloys with Sb, Se, and so on, which have ZT 5 S 2 T/ r k < 1, where S , r , k and T are the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature, respectively. It is difficult to scale bulk Bi 2 Te 3 to large-scale energy conversion, but fabricating synthetic nanostructures for this purpose is even more difficult and expensive. Si, on the other hand, is the most abundant and widely used semiconductor, with a large industrial infrastructure for low-cost and high-yield processing. Bulk Si, how- ever, has a high k ( , 150 W m 2 1 K 2 1 at room temperature) 5 , giving ZT < 0.01 at 300 K (ref. 6). The spectral distribution of phonons contributing to the k of Si at room temperature is quite broad. Because the rate of phonon–phonon Umklapp scattering scales
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This note was uploaded on 05/21/2010 for the course MS Thermoelec taught by Professor Snyder during the Spring '10 term at Caltech.

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nature06381 - Vol 451 | 10 January 2008 |...

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