Nano World: Sorting apart nanotubes

By CHARLES Q. CHOI  |  June 30, 2006 at 1:27 PM
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NEW YORK, June 30 (UPI) -- Scientists have developed the first method that can tease apart jumbles of carbon nanotubes with different coveted semiconductor properties, experts told UPI's Nano World.

The capability to sort apart mishmashes of nanotubes with different electrical properties created by current production techniques could help supply key ingredients for devices from advanced microchips and next-generation power cables to clinical therapeutics.

Carbon nanotubes are stronger than steel but weigh just a sixth as much. A wide variety of them exist, with very different properties. Some are excellent semiconductors, while others are metallic, conducting electricity as well as copper wires, if not better. They also come in single-walled and multi-walled kinds, which differ in size and strength.

The problem when it comes to nanotube diversity is that all current production methods for nanotubes turn out a hodgepodge of types, where engineers oftentimes need just one kind. The process researchers at the Carbon Nanotechnology Laboratory at Rice University in Houston use to manufacture carbon nanotubes, for instance, creates more than 50 different kinds, explained Howard Schmidt, executive director of the Carbon Nanotechnology Laboratory.

The new method Schmidt and his colleagues developed after three months of work uses electric fields to sort nanotubes with different electrical properties. Each kind of nanotube has a unique dielectric constant, a term that refers to a material's ability to store electrical energy.

The researchers built an electrified chamber they pumped a solution of dissolved nanotubes in. The chamber traps metallic nanotubes and causes semiconducting varieties to float at different levels in the chamber. The larger its dielectric constant, the lower in the system it floats.

By varying the speed of flow inside the chamber, with upper-level currents traveling faster than lower ones, the scientists were able to collect samples that had three times more small tubes than large or vice versa. Schmidt and his colleagues reported their results in the Journal of the American Chemical Society.

"It's an important advance by researchers who have a history of making pioneering advances," said Ray Baughman, director of the University of Texas at Dallas NanoTech Institute. "It's critically important in the carbon nanotube area, where carbon nanotubes are synthesized with different properties, and applications where you need one specific type of nanotube have been held back."

Prior experiments had used electric fields to separate metallic from semiconducting nanotubes, but this is the first research to sort out different kinds of semiconducting nanotubes, Schmidt explained. Purifying nanotubes with precise electrical properties "is key for work in transistors and other electronics," he said.

Future research will aim to optimize the electrode design of the chamber and other aspects of the separation process over the next year. They hope to scale up to separate apart grams of material, enough for electronic and biological applications, Schmidt explained.

Ultimately separation techniques should get combined with methods that create fewer kinds of nanotubes for sorting, Baughman said. "It's a small victory in a long campaign," Schmidt added.

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