Teasing Apart Nanotubes: Fast-spun carbon fibers may feed an industry

Nanoscale tubes of carbon could potentially lead to novel technologies, such as electronic circuits that are much faster and more compact than those made today. However, the batches of carbon nanotubes that manufacturers now produce are difficult to use because they contain a hodgepodge of tubes of varying electronic properties and diameters.

FULL SPECTRUM. Colorful bands of purified carbon nanotubes stack up in a test tube (illustrated left) after being processed by a new technique that segregates nanotubes (cylinders on right) of different diameters or electronic properties. Z. Deretsky/National Science Foundation

Now, researchers have devised a way to sort these nanotubes. The technique could clear a major obstacle to industrial-scale application of the tubes in circuits, sensors, computer screens, and other products, say Michael S. Arnold and his colleagues at Northwestern University in Evanston, Ill.

The new approach is “a landmark breakthrough,” comments nanotechnologist and chemist Ray H. Baughman of the University of Texas at Dallas.

“This method is surely going to accelerate the process for developing real applications of nanotubes,” adds chemist Jie Liu of Duke University in Durham, N.C.

The atomic structures of carbon nanotubes enable some of them to serve as semiconductors in nanoscale transistors (SN: 9/10/05, p. 165: Available to subscribers at Electronics Gets Y’s: Nanotubes branch out as novel transistors). Other nanotubes behave like metal wires. Nanotubes’ diameters also affect their properties.

In previous research, other scientists had devised ways to separate carbon nanotubes. However, those methods required costly additives or had other shortcomings that limited their potential for large-scale use, says Arnold.

In the new method, he and his colleagues, led by Mark C. Hersam, mix cheap, soaplike molecules called surfactants with a black powder containing a jumble of carbon nanotubes. The surfactants render the nanotubes water soluble. The researchers then add the blend to a test tube that they’ve filled so that the concentration of an iodine compound—and the density of the solution—increases with depth.

Next, they centrifuge the materials for hours at tens of thousands of revolutions per minute. Buoyancy differences segregate the nanotubes into homogeneous, horizontal bands. The nanotubes’ structures cause each band to appear as a distinct color. “The first time we did this … and saw a rainbow, we knew that it worked,” Hersam recalls.

Exquisitely precise, the new technique separates nanotubes that are only a few hundredths of a nanometer different in diameter, the team reports in the October Nature Nanotechnology. Metallic and semiconductor nanotubes segregate because they attract different loads of surfactant molecules, the scientists speculate.

Although the team’s experiments have produced only micrograms of purified nanotubes per band, a path to much larger yields is straightforward, Hersam says: Use bigger centrifuges, and lots of them.

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