Dec. 25, 2000 CHAPEL HILL - Electrical resistance between nanotubes -- carbon tubes so thin it would take several million lying side by side to cover an inch -- and graphite surfaces that support them varies according to how the tubes are oriented, a new University of North Carolina at Chapel Hill study shows. The discovery, which could be important to telecommunications and other electronics industries, indicates it's possible to alter the resistance by changing the tubes' position on a flat surface.
Resistance peaks six times as the end of a nanotube is rotated 360 degrees, the scientists found. That makes sense, they say, because atoms in the nanotube and graphite are arranged in hexagons. A report on the research appears in the Dec. 1 issue of the journal Science. Authors include Dr. Scott Paulson, a UNC-CH Ph.D. recipient who recently became a postdoctoral fellow at Duke University, former UNC-CH graduate student Aron Helser now working at 3rd Tech of Chapel Hill and Dr. Marco Buonglorno Nardelli, research professor at N.C. State University. Others are Drs. Russell M. Taylor II, research associate professor of computer science; Mike Falvo, research assistant professor of physics and astronomy; Richard Superfine, associate professor of physics and astronomy; and Sean Washburn, professor of physics and astronomy, all at UNC-CH. The paper is important for several reasons, Superfine said.
"First, it is the most direct measurement that electrons in a material travel in particular directions and that those favored directions need to be matched as you go from one material to another where they touch," he said. "Second, this effect is pronounced in carbon nanotubes, threadlike molecules that conduct electricity and have the potential to be used for ultra-small circuits."
Researchers need to be sure when making such devices that the preferred directions are aligned when the devices are assembled, the scientist said. Conversely, the effect can be used to make sensors that measure the rotation of nanometer-scale objects. A nanometer is a billionth of a meter.
A form of soot, nanotubes are created by arcing electricity between two sticks of carbon. They measure 10 to 30 nanometers in diameter and about one to five millionths of a meter long. Little more than a decade ago, a Japanese scientist discovered the tiny tubes, which are proving to be stiffer and stronger than any other known substance.
"Tunable resistance in nanotubes may be useful in molecular scale machinery where you have moving, sliding and rotating parts," Superfine said. "You need to be able to sense the motion of those parts in an indirect way, such as through the measured current, because in an assembled device you will not be able to look directly at the part."
Earlier research by the UNC-CH team published in Nature last year showed that carbon nanotubes roll across a surface rather than slide when the nanotube is put on graphite. A recent article in Physical Review showed that this rolling occurs because the atoms in the outermost layer of the nanotube interlock with the atoms on the graphite surface. When the atoms interlock, the nanotube rolls, and when the atoms are not enmeshed, the nanotube slides. This means that the atoms are acting like gear teeth. Together with findings on the electrical properties of these atomic scale contacts, the UNC-CH researchers are creating the foundation of the ultra-small scale engineering of machines.
Work they published in 1997 revealed that the structures possess such remarkable flexibility, strength and resiliency that industry should be able to incorporate them into high performance sports and aerospace materials.
Carbon fibers already are used in graphite composite tennis rackets and other products because of their strength and lightness. The research team showed that carbon nanotubes were significantly stronger than carbon fibers and hundreds of times stronger than steel.
The continuing experiments involve recording mechanical and electrical properties of carbon nanotubes with a unique device the UNC-CH researchers invented. Known as the nanoManipulator, the device combines a commercially available atomic force microscope with a force-feedback virtual reality system. The former employs an atomically small, gold-tipped probe capable of bending and otherwise manipulating molecule-sized particles. The latter allows scientists to see and feel a representation of the surface a million times bigger than its actual size. Business Week featured the device in an article on nanotechnology this month.
"People are talking about nanotechnology right now, but if you are going to engineer those kinds of systems, you have to know how they work," Falvo said. "This is one potentially very important piece of that puzzle - how do really small contacts conduct electricity? We've shown that unlike in large contacts, in very small ones their relative orientation can have a profound effect on current flowing through them. Knowing this could be critical to building the tiniest electromechanical switches, for example."
The National Science Foundation, the National Institutes of Health and the Office of Naval Research supported the continuing experiments.
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