The efficiency of thermoelectric materials 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, note both teams in their papers, it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent. Although nanostructured thermoelectric materials have delivered ZT of greater than 1, the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. The developments with silicon could change that, however.

The two groups measured ZT (thermoelectric figure of merit) values near 1 for silicon nanowires at and below room temperature. Both groups found dramatically lower thermal conductivity values. Intriguingly, the Caltech group found a greatly enhanced Seebeck coefficient due to a 1-D phonon drag effect.

UC Berkeley/LBNL. The Berkeley/LBNL team, led by Arun Majumdar and Peidong Yang, used a unique “electroless etching” method by which arrays of silicon nanowires are synthesized in an aqueous solution on the surfaces of wafers that can measure dozens of square inches in area. The technique involves the galvanic displacement of silicon through the reduction of silver ions on a wafer’s surface. Unlike other synthesis techniques, which yield smooth-surfaced nanowires, this electroless etching method produces arrays of vertically aligned silicon nanowires that feature exceptionally rough surfaces. The roughness is believed to be critical to the surprisingly high thermoelectric efficiency of the silicon nanowires.

The rough surfaces are definitely playing a role in reducing the thermal conductivity of the silicon nanowires by a hundredfold, but at this time we don’t fully understand the physics. While we cannot say exactly why it works, we can say that the technique does work.

—Arun Mahumdar

Bulk silicon is a poor thermoelectric material at room temperature, but by substantially reducing the thermal conductivity of our silicon nanowires without significantly reducing electrical conductivity, we have obtained ZT values of 0.60 at room temperatures in wires that were approximately 50 nanometers in diameter. By reducing the diameter of the wires in combination with optimized doping and roughness control, we should be able to obtain ZT values of 1.0 or higher at room temperature.

—Peidong Yang

The Berkeley Lab researchers will be studying the physics behind this phenomenon to better understand and possibly manipulate it for even further improvements. They will also concentrate on the design and fabrication of thermoelectric modules based on silicon nanowire arrays. Berkeley Lab’s Technology Transfer Department is now seeking industrial partners to further develop and commercialize this technology.

Caltech. The Caltech researchers, led by James Heath, used a method developed in Heath’s labs to construct nanowires with cross-sectional areas of 10 nm x 20 nm and 20 nm x 20 nm. By varying the nanowire size and impurity doping levels, they achieved ZT values representing an approximately 100-fold improvement over bulk Si over a broad temperature range, including ZT of approximately 1 at 200 K (-73° C).

Optimizing materials for cooling or heat recovery applications involves a tricky trade-off of several different parameters, including the electrical conductivity and the thermal conductivity. We find that we can greatly drop the thermal conductivity in these nanowires without affecting the other parameters, and this leads to dramatic improvements in the thermoelectric efficiency.

—James Heath

An additional parameter that the researchers were surprised to see improved in the nanowires is the thermopower, which is the amount of voltage generated in a material for a given thermal gradient. The improvement likely arises from a phenomenon known as phonon drag, which comes when the sound-carrying vibrations in the atomic lattice of the nanowires are not in thermal equilibrium with the current carrying electrons.

We find that for ultrathin nanowires the electrons drag certain sound waves along with them as they move down the nanowire. This extra heat from the sound is enhancing the thermoelectric efficiency.

—Jamil Taher-Kheli, contributing author

Although silicon nanowires are still about a factor of two less efficient than the most efficient known thermoelectric materials, the researchers are optimistic that further improvements in the materials will soon be made.

Our theoretical models indicate that a number of exciting avenues are available to significantly improve the efficiency. However, even at their current efficiencies, these nanowires already outperform many commercially available systems, and so could potentially find near-term applications.

—William A Goddard, director of the Materials and Process Simulation Center, and a contributing author

Resources

• Hochbaum, A. I. et al., “Enhanced thermoelectric performance of rough silicon nanowires.” Nature 451, 163-167 (2008)

• Boukai, A. I. et al., “Silicon nanowires as efficient thermoelectric materials”. Nature 451, 168-171 (2008).

• Vining, C. B., “Desperately seeking silicon”. Nature 451, 132-133 (2008).