Carbon Nanotubes Sorted by Electronic Properties

Carbon nanotubes are highly touted for their exceptional mechanical, thermal, optical and electrical properties, and researchers worldwide are eager to incorporate them into electronics, high-resolution displays, high-strength composites and biosensors. But their widespread use is hindered by current synthesizing methods that produce tubes with differing diameters, twist and electronic properties, making them unsuitable for most proposed applications.

Now, a new method developed at Northwestern University for sorting single-walled carbon nanotubes promises to overcome this problem. The method works by exploiting subtle differences in the buoyant densities of carbon nanotubes as a function of their size and electronic behavior.

Single-walled carbon nanotubes are coated in soap-like molecules called surfactants, then spun at tens of thousands of rotations per minute in an ultracentrifuge. The resulting density gradient sorts the nanotubes according to diameter, twist and electronic structure. (Graphic: Zina Deretsky [adapted from Arnold et al.], National Science Foundation)

"Carbon nanotubes, because of their ultrasmall size and excellent materials properties, have excited the scientific community for the last decade," said Mark Hersam, professor of materials science and engineering at Northwestern's McCormick School of Engineering and Applied Science, who led the research team.

"However, due to their inherent heterogeneity, they have not yet realized their full commercial potential," he said. "A scalable and economical method for producing monodisperse carbon nanotubes will enable the proposed applications for these nanomaterials to be explored at an industrially relevant scale."

Using the Northwestern method, carbon nanotubes first are encapsulated in water by soap-like molecules called surfactants. Next, the surfactant-coated nanotubes are sorted in density gradients which are spun at tens of thousands of rotations per minute in an ultracentrifuge. By carefully choosing the surfactants utilized during ultracentrifugation, the researchers found that carbon nanotubes could be sorted by diameter and electronic structure.

As a part of their study, the researchers demonstrated the fabrication of electrical devices that displayed either semiconducting or metallic behavior, depending on the sorted nanotubes used. The researchers also maintain that their technique can be translated to an industrial scale.

"The technique is especially promising for commercial applications," said Hersam, "because large-scale ultracentrifuges have already been developed and shown to be economically viable in the pharmaceutical industry. We anticipate that this precedent can be straightforwardly translated to the production of monodisperse carbon nanotubes."

The research results were published online Wednesday in the inaugural issue of the journal Nature Nanotechnology. In addition to Hersam, other authors on the paper are Samuel Stupp, Board of Trustees Professor of Materials Science and Engineering and of Chemistry and a professor of medicine; James Hulvat, research assistant professor of materials science and engineering; and graduate students Michael Arnold and Alexander Green, all from Northwestern.

The research was supported by the National Science Foundation, the US Army Telemedicine and Advanced Technology Research Center and the Department of Energy.

For more information, visit: www.northwestern.edu