Carbon nanotubes -- strong tubular structures formed from a single layer of carbon atoms and only about a billionth of a meter in diameter -- display previously unknown properties with significant technological potential, a new University of North Carolina at Chapel Hill study shows.
University and industrial laboratories around the world have been eagerly investigating such nanotubes for their possible applications, according to UNC-CH physicists Yue Wu and Otto Zhou. Future uses may include flat panel display and telecommunications devices, fuel cells, Li-Ion batteries, high-strength composites and novel molecular electronics.
Part of the interest in the tiny, thread-like cylinders stems from the knowledge that single-walled nanotubes can behave either as metals or as semiconductors, depending on their chirality, the folding direction of the graphene sheet forming them, the scientists said. Before the tubes can be used in electronic equipment, however, industry needs to know how to fabricate materials with controlled structure and properties. New techniques for evaluating such macroscopic materials also are required.
In an article appearing in the current (April 21) issue of the journal Science, a UNC-CH research team reports a major step in that direction. Members work in the university's physics and astronomy department and curriculum in applied and materials sciences.
"We used carbon-13 nuclear magnetic resonance (NMR) to measure the number of current-carrying electrons in single-walled nanotubes and therefore their electronic properties," said Wu, an associate professor. "In a sense you might say we wanted to take the 'pulse' of nanotubes by finding out how the carbon atoms in them feel and behave and thus distinguish the metallic nanotubes from the semiconducting nanotubes."
The NMR group led by Wu also includes Drs. Xiao-Ping Tang and Alfred Kleinhammes, research assistant professors, and undergraduate Kamal Bennoune.
The study involved measuring interactions between the C-13 nuclear spins and the conduction electron spins, he said. Nuclear magnetic resonance is a technique widely employed in science and also in medicine for creating images inside the body.
Physicists found the tubes' unusual properties also can change significantly in response to alterations made in their surroundings, such as exposing them to oxygen or other gases.
"The NMR method we've established could be used for studying various other aspects of carbon nanotubes, including assessing the potential of hydrogen storage," Wu said. "Such study is very important for development of hydrogen fuel cells."
By controlling the conditions under which the carbon nanotubes were synthesized, the materials group at UNC-CH, led by Zhou, fabricated the cylindrical particles with variable diameters.
Zhou, an assistant professor and director of the N.C. Center for Nanoscale Materials, worked with graduate students Leslie Fleming and Chris Bower and postdoctoral fellows Hideo Shimoda and Saion Sinha.
The researchers found that the mass ratios of the metallic and semiconducting tubes were different in the materials. NMR studies showed that a third of the tubes exhibited metallic behavior in samples made with nickel-cobalt catalysts, which was consistent with a random distribution of tube chirality. A much higher fraction of metallic tubes was observed in samples made by rhodium-palladium catalysts.
Results suggest that eventually it may be possible to make either all metallic or all semiconducting carbon nanotubes and with tunable structures, which is essential if the tubes are to be used in electronic devices. The new study was supported by the Office of Naval Research and the National Science Foundation.
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