No current theory explains why the nanowires' thermal conductivity goes down so drastically. One of the reasons, Yang believes, is that the nearly one-dimensional nanowires and the wires' rough edges block the flow of phonons, which are particles that carry heat. But the complete picture remains unclear.

Ali Shakouri, an electrical-engineering professor at the University of California, Santa Cruz, says that researchers will have to understand how the physics works before they can improve the technology enough to produce commercial devices. Furthermore, using nanowires for energy conversion and power generation has its own limitations, Shakouri says. Such applications require large arrays of nanowires, but in the Nature paper, Yang and his colleagues measured the electrical properties of individual nanowires. The researchers will have to make sure that those properties translate to entire nanowire arrays, says Shakouri: "Variations and interactions between nanowires could take away some advantages."

Still, he says, "this is important work that could have a big impact." Shakouri points not only to the demonstration of silicon's potential as a thermoelectric, but also to the unique engineering that the researchers used to make rough nanowires. "The new way of playing with material properties is very interesting," he says. "It could open up a way to improve thermoelectrics that could be applied to other materials."

Yang and his colleagues, meanwhile, are already thinking about how to improve their nanowires' performance. They plan to reduce the size of the nanowires and make their surfaces rougher than they already are. That should enhance their thermoelectric properties, Yang says. The researchers also plan to make and test an actual thermoelectric device using silicon nanowires.