Carbon nanotubes require considerably more energy to roll across some surfaces than they do to slide across them, according to a new study from the University of North Carolina (Chapel Hill; 919-962-3526). The study shows for the first time that atomic-scale carbon nanotubes behave differently from almost every other large, solid material. Interactions between electrons on the two surfaces are thought to be responsible for the discrepancy. The researchers believe that other atomic-scale particles behave in a similar fashion.
The research will appear in the Jan. 21, 1999, issue of Nature. Richard Superfine, associate professor of physics and astronomy at the University of North Carolina, led the research team. Other authors of the paper include graduate students Michael Falvo and Aron Helser; Russell Taylor II, research assistant professor of computer science; Vernon Chi, director of the microelectronics systems laboratory; Frederick Brooks Jr., Kenan professor of computer science; and Sean Washburn, professor of physics and astronomy. The National Science Foundation, the National Institutes of Health, the Office of Naval Research, and Topometrix Inc. supported the experiments.
To perform their research, the UNC team created the nanoManipulatora device that combines a commercially available atomic force microscope (AFM) with a force-feedback virtual reality system. The AFM uses a probe that can bend and manipulate molecule-sized particles. Teamed with the virtual system, the device allows scientists to see and feel a representation of the nanotube surface that is one million times bigger than its actual size.
Using the nanoManipulator, researchers could roll and slide nanotubes on a graphite plate while measuring the amount of energy each movement required. The carbon molecules constitute an excellent model for study because their surfaces are almost perfectly smooth.
Instead of acting as lubricants as some had hoped, the nanotubes clung to the surfaces on which they were rolled. The effect is comparable to rolling a ball bearing across Scotch tape, Superfine says. He attributes this to interactions between electrons on the two surfaces.
The researchers hope to translate their work into the world of MEMS, using this new knowledge to find more effective ways of creating motion on a nanometer scale, Superfine says. Future research will focus on learning ways to move surfaces by applying electrical forces to them.
"We're considering other types of objects of this size to move around and study," Superfine says, "but the challenge is to find other nanometer-sized particles with atomically smooth surfaces."
In a report published in Nature in late 1997, Falvo, Superfine, and colleagues discovered that carbon nanotubes possess remarkable flexibility, strength, and resiliency, and suggested that industry should incorporate them into high-performance sports and aerospace materials. For this project, the team bent carbon nanotubes and recorded their properties using the nanoManipulator
Carbon fibers already are used in graphite composite tennis rackets and other products because of their strength and lightness. The UNC research indicated that carbon nanotubes are significantly stronger than carbon fibers and hundreds of times stronger than steel.
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