nature06458 - Vol 451 | 10 January 2008 |...

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LETTERS Silicon nanowires as efficient thermoelectric materials Akram I. Boukai 1 { , Yuri Bunimovich 1 { , Jamil Tahir-Kheli 1 , Jen-Kan Yu 1 , William A. Goddard III 1 & James R. Heath 1 Thermoelectric materials interconvert thermal gradients and electric fields for power generation or for refrigeration 1,2 . 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 ther- moelectric power, and of the electrical and thermal conductivities. Maximizing ZT is challenging because optimizing one physical parameter often adversely affects another 3 . Several groups have achieved significant improvements in ZT through multi-component nanostructured thermoelectrics 4–6 , such as Bi 2 Te 3 /Sb 2 Te 3 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 3 20 nm and 20 nm 3 20 nm. By varying the nano- wire 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 elec- trical 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. The most efficient thermoelectrics have historically been heavily doped semiconductors because the Pauli principle restricts the heat- carrying electrons to be close to the Fermi energy 1 for metals. The Wiedemann–Franz law, k e / s T 5 p 2 /3( k / e ) 2 5 (156 m VK 2 1 ) 2 , where k e is the electronic contribution to k , constrains ZT 5 S 2 sT / k , where S is the Seebeck coefficient (or thermoelectric power, measured in 2 1 ), and s and k are the electrical and thermal conductivities, respectively. Semiconductors have a lower density of carriers, leading to larger S values and a k value that is dominated by phonons ( k ph ), implying that the electrical and thermal conductivities are somewhat decoupled 1 . k can be reduced by using bulk semiconductors of high atomic weight, which decreases the speed of sound. However, this strategy has not yet produced materials with ZT . 1. For a metal or highly doped semiconductor, S is proportional to the energy derivative of the density of electronic states. In low-dimen- sional (nanostructured) systems the density of electronic states has sharp peaks 7–9 and, theoretically, a high thermopower. Harnessing this electronic effect to produce high- ZT materials has had only limited success 10,11 . However, optimization of the phonon dynamics and heat transport physics in nanostructured systems has yielded results 4–6 . Nanostructures may be prepared with one or more dimen- sions smaller than the mean free path of the phonons and yet larger than that of electrons and holes. This potentially reduces
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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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nature06458 - Vol 451 | 10 January 2008 |...

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