"Other than low-cost mass production, there's no bigger hurdle to overcome in carbon nanotechnology than finding a reliable, affordable means of sorting single-walled carbon nanotubes," said Richard Smalley, University Professor and director of Rice's Carbon Nanotechnology Laboratory. "If we can develop new technology based on electronic sorting and reliably separate metallic nanotubes from semi-metallic and semi-conducting varieties, we'll have a terrific tool for nanoscience."
James Tour, Chao Professor of Chemistry, said, "The utility of specific carbon nanotubes, based upon their precise electronic characteristics, could be an enormous advance in molecular electronics. Until now, everyone had to use mixtures of nanotubes, and by process of elimination, select the desired device characteristics afforded from a myriad of choices. This could now all change since there is the possibility of generating homogeneous devices."
All single-walled carbon nanotubes are not created equal. There are 56 varieties, which have subtle differences in diameter or physical structure. Slight as they are, these physical differences lead to marked differences in electrical, optical and chemical properties. For example, about one-third are metals, and the rest are semiconductors.
Although carbon nanotubes have been proposed for myriad applications -- from miniature motors and chemical sensors to molecule-size electronic circuits -- their actual uses have been severely limited, in part because scientists have struggled to separate and sort the knotted assortment of nanotubes that result from all methods of production.
As a post-doctoral researcher in Smalley's laboratory, Michael Strano, now a professor of chemical and biomolecular engineering at Illinois, developed a technique for breaking up bundles of nanotubes and dispersing them in soapy water. In the present work, Strano and his graduate students, Monica Usrey and Paul Barone, teamed up with Tour and his postdoctoral researcher Christopher Dyke to apply reaction chemistry to the surfaces of nanotubes in order to select metallic tubes over semiconductors.
To control nanotube chemistry, the researchers added water-soluble diazonium salts to nanotubes suspended in an aqueous solution. The diazonium reagent extracts an electric charge and chemically bonds to the nanotubes under certain controlled conditions.
By adding a functional group to the end of the reagent, the researchers can create a "handle" that they can then use to selectively manipulate the nanotubes. There are different techniques for pulling on the handles, including chemical deposition and capillary electrophoresis.
"The electronic properties of nanotubes are determined by their structure, so we have a way of grabbing hold of different nanotubes by utilizing the differences in this electronic structure," Strano said. "Because metals give up an electron faster than semiconductors, the diazonium reagent can be used to separate metallic nanotubes from semiconducting nanotubes."
The chemistry is reversible. After manipulating the nanotubes, the scientists can remove the chemical handles by applying heat. The thermal treatment also restores the pristine electronic structure of the nanotubes.
"Until now, the consensus has been that the chemistry of a nanotube is dependent only on its diameter, with smaller tubes being less stable and more reactive," Strano said. "But that's clearly not the case here. Our reaction pathways are based on the electronic properties of the nanotube, not strictly on its geometric structure. This represents a new paradigm in the solution phase chemistry of carbon nanotubes."
The research was funded by the National Science Foundation, NASA, the Air Force Office of Scientific Research, and the Office of Naval Research.
The paper, titled, "Selectivity of Electronic Structure in the Functionalization of Single Walled Carbon Nanotubes," appears in this week's issue of Science. Strano and Dyke co-wrote the paper with Smalley, Tour, Usrey, Barone, Rice undergraduate Mathew J. Allen, Research Assistant Hongwei Shan, Research Scientist Carter Kittrell, and Senior Faculty Fellow Robert H. Hauge.