Nanotube ID: New signatures aid nanotech progress

Carbon nanotubes have been on researchers’ A list of promising materials for a decade. However, these tiny tubes–each essentially a rolled-up sheet of graphite about a nanometer wide–are a diverse lot. That’s made it tough for scientists to know what kinds of tubes they have in hand, and those with different diameters and structures can have very different properties.

In a step that could alleviate that problem, researchers now have developed a means for rapidly distinguishing among 33 semiconducting varieties of the tiny cylinders. Means to inventory the nanotubes in a diverse sample provide the first step toward sorting them or producing specific types. Supplies of particular tubes, in turn, could speed development of products ranging from electronic displays to spacecraft shells.

Researchers have long sought efficient methods for identifying and sorting carbon nanotubes, says R. Bruce Weisman of Rice University in Houston. Even a minor difference in a nanotube’s structure, for example, can make the material act like a metal instead of a semiconductor.

Weisman, Rice University chemist Richard E. Smalley, and their colleagues report the nanotube signatures in an upcoming issue of Science.

The work ranks as “landmark research,” says carbon nanotube scientist Ray Baughman of the University of Texas at Dallas. The Rice researchers used methods that “elegantly, efficiently, and reliably characterize the structure of semiconducting nanotubes,” he says.

The work grew out of previous studies at Rice. Last July, groups led by Smalley and Weisman reported that semiconducting carbon nanotubes fluoresce. In other words, when they’re hit with light, they emit light at a different wavelength.

Since nanotubes fluoresce differently depending on their diameter and a structural property called the chiral angle, the scientists suspected they could find a characteristic optical signature for each type of tube.

To detect and assign the signatures, the team used spectrofluorometry and Raman spectroscopy to analyze the light coming from fluorescing nanotubes. The researchers were guided by approximate theoretical models of nanotubes’ electronic structure, says Weisman.

With these optical signatures in hand, researchers ought to be able to identify carbon-nanotube types in minutes or even seconds, says Weisman. Such quick analyses could enable nanotube producers to associate specific manufacturing conditions, such as temperature and gas pressure, with the production of specific types of tubes.

“The Rice research group . . . has presented a substantial body of work that allows fingerprinting of semiconducting nanotubes,” says nanotube researcher Pulickel M. Ajayan of Rensselaer Polytechnic University in Troy, N.Y. The new approach may serve as a powerful tool for designing efficient ways of obtaining uniform nanotubes, he says.

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