Getting Nanowired

One day, they could be everywhere, powering your computer and keeping its microprocessor cool at the same time. They’re silicon nanowires, narrow devices hundreds to thousands of times thinner than this piece of paper. Two groups of Caltech researchers are discovering the remarkable properties of silicon nanowires, enabling the wires to harness solar power and to act as refrigerators by converting heat to electricity, and vice versa.

The latter group, led by James Heath, the Gilloon Professor and professor of chemistry, found that silicon could be an efficient thermoelectric material when made into wires only 10 nanometers (10 billionths of a meter) wide. “At these tiny dimensions, nature is doing things that were previously not thought possible,” he says.

In a thermoelectric material, a difference in temperature sends electrons scurrying to the cooler end, creating a current. To be efficient, the material must conduct electricity well; but to maintain a temperature difference, it must conduct heat poorly. Most thermoelectric materials efficient enough to be useful are expensive and hard to make, restricting them to niche applications. Silicon, on the other hand, is one of the most abundant elements in the universe. The microprocessor industry has also made processing silicon inexpensive and easy. But because silicon is also an excellent conductor of heat, it didn’t seem promising as a thermoelectric material—until now.

By growing silicon into nanowires, researchers in Heath’s lab improved silicon’s thermoelectric efficiency by a factor of 100. One of the reasons for the enhanced performance might be a phenomenon called phonon drag, according to the team. Phonons are heat-carrying vibrations that travel across the material. Constricted by the small size of the nanowire, the phonons don’t scatter off the sidewalls in the nanowire. Instead, they travel unimpeded down the wire and drag electrons with them, which improves thermoelectric performance.

Although the silicon nanowires are still only about half as efficient as state-of-the-art thermoelectric materials, further improvements—as well as lower manufacturing costs—could make these tiny devices useful in a host of applications. They can make microprocessor chips more efficient by recovering leaked heat. Eventually, they may be able to recover heat from larger systems like car engines, and may also be used in refrigeration devices. The researchers, who include William Goddard (PhD ’65), the Ferkel Professor of Chemistry, Materials Science, and Applied Physics; Jamil Tahir-Kheli (MS ’86, PhD ’92), a senior staff scientist with the Materials and Process Simulation Center; and graduate students Akram Boukai (PhD ’08), Yuri Bunimovich (PhD ’07), and Jen-Kan Yu, reported their findings in the January 10 issue of Nature.

Silicon nanowires may also help solve the energy crisis. Researchers in the labs of Nate Lewis (BS, MS ’77), the Argyros Professor and professor of chemistry, and Harry Atwater, the Hughes Professor and professor of applied physics and materials science, are using the wires to build a new kind of photovoltaic cell.

Most conventional cells are made from silicon wafers. Incoming photons from the sun are absorbed by the silicon and dislodge electrons from their atoms. The electrons are then free to move, producing enough current to power calculators, light bulbs, and even entire homes. The drawback is that these solar cells must use pure, top-quality silicon, which is expensive to process.

Growing silicon nanowires is not only cheaper, but can also be done with lower-quality silicon. The trick to turning nanowires into solar cells is a unique geometry, an idea first developed by graduate student Brendan Kayes (MS ’04) in 2005. Regular solar cells are flat, and absorb photons face-on. The newly freed electrons then move along the same direction, parallel to the incoming photons. They’re collected at the surface of the silicon slab, where they then join the electrical current. Additionally, the cells have to be thick enough to capture all of the photons.

In the new photovoltaic cells, silicon nanowires sit alongside one another like blades of grass. Light is absorbed along the length of the wires, which, at tens of microns, are still long enough to snatch all the photons. The advantage of this configuration, however, is that the electrons move widthwise—perpendicular to the photons’ paths. The nanowires are only several microns in width, so the electrons don’t have to travel as far, allowing them to produce electricity more easily. Once the electrons are collected in the outer shell of the wire, they quickly travel to the top of the wire and enter the current.

The team has made nano-wire arrays one square centimeter in area—orders of magnitude larger than any made before. The researchers have also been able to embed the nanowires in a flexible membrane for added versatility. The membrane is excellent at absorbing light, as is evident from its near-black color.

The best conventional silicon solar cells are about 25 percent efficient at converting sunlight to energy, says postdoc Michael Filler. The researchers’ nanowire cells are just over one percent electrically efficient. But Filler says they are making rapid progress, and are aiming for 20 percent efficiency. “Our group has been pushing the forefront of the field right now,” he says. Other members include postdocs Stephen Maldonado and Kate Plass, and graduate students Michael Kelzenberg (MS ’06), James Maiolo, Leslie O’Leary, Morgan Putnam, and Josh Spurgeon (MS ’06). Once the researchers achieve higher efficiencies, Filler hopes industry will jump in and push the design toward commercial use within the next decade. —KS/MW