Nanotubes With a Semiconducting Filling

Researchers have made miniature electrical cables with three concentric layers--a semiconductor covered with sheaths of an insulator and a metal. These nanowires, described in tomorrow's Science, could potentially be used as tiny components for computer chips and other semiconductor devices.

Excitement about electronic devices just millionths of millimeters in size soared about 7 years ago with the discovery of carbon nanotubes--nanometer-sized structures that are exceedingly strong and good conductors of heat and electricity. To make carbon nanotubes into better electrical wires, researchers learned how to coat them with boron nitride, just like ordinary copper wire is insulated with rubber. Then Yuegang Zhang, a materials scientist at the NEC Corporation in Tsukuba, Japan, wondered if he could make a nanotube that included a semiconducting layer.

Zhang and his colleagues mixed together powders of boron nitride, carbon, and silicon oxide in a crucible, and tried adding a bit of lithium nitride as a catalyst. They seared this hodgepodge to 1200 degrees Celsius and blasted it with lasers. As the vaporized ingredients cooled, they assembled spontaneously into nanotubes with a core of semiconducting silicon carbide, an intermediate layer of insulating silicon oxide, and outer shells of boron nitride and conductive carbon. Exactly how or why this organized structure forms out of the chemical mist is a mystery, says Zhang, but it doesn't happen without lithium nitride.

The self-assembly of these complicated and potentially useful structures bodes well for their future, says Mildred Dresselhaus, a physicist at the Massachusetts Institute of Technology. "If [these structures] were made by some expensive, time consuming technique, there would be no industry applications, but if they self assemble, you have something."

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