Nanotubes are dazzlingly strong and can conduct electricity--making them a crucial part of most nanotechnology. Now scientists have an easy way of making them directly on silicon chips. When a semiconductor sheet springs free of its substrate, the sheet curls up into a minuscule tube like a New Year's Eve noisemaker. The discovery of this remarkably simple trick should accelerate the development of tiny molecular pipelines, cables, and girders, scientists report in the 8 March issue of Nature.
The traditional method of making nanotubes is to zap current between two carbon electrodes. That method is efficient, but it is difficult to put the scattered tubes where you want them. An improved technique has emerged, serendipitously, from a problem inherent to semiconductor manufacture: When scientists deposit layers of a germanium semiconductor on a substrate of silicon, the strain exerted by the mismatch between the two crystal types can create tiny defects that degrade performance. "We always thought of misfit strain as a problem," says materials scientist Frank Ernst of Case Western Reserve University in Cleveland, Ohio.
Oliver Schmidt and Karl Eberl, however, have capitalized on misfit strain. While attending a conference in Russia, the two physicists from the Max Planck Institute for Solid State Research in Stuttgart, Germany, realized that the strain would roll up the silicon-germanium layer--if they could peel the layer off the underlying surface. As soon as they got back to their lab, they tried to loosen the layer with various chemicals. The idea worked even better than they had hoped. "We made our first tubes within a week," Schmidt says. "It's so simple that it is strange no one had the idea decades ago." The method is also accurate and flexible; Schmidt and Eberl can tune the tube diameter from a few nanometers up to a micrometer by changing the composition of the layers.
"This is a truly creative, ingenious idea," Ernst says. "It was born simply by changing the way of looking at things." Schmidt and Eberl suggest many potential applications for their semiconductor tubes, including structural elements in molecular machines, pipelines for local medical applications, and who knows what else. "A lot of groups will apply this in all sorts of unexpected ways," Ernst predicts.