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| Computer models of nanotubes illustrate a property called "chirality." When carbon atoms are arranged in straight lines along the tube, the tube is a very good conductor. When they form a spiral pattern, as at right, the tube becomes a semiconductor that can be used to make a transistor. McEuen's research group has also made nanotube diodes. Vin Crespi, Penn State. |
Paul McEuen, Cornell professor of physics, described his research group's work on nanotubes at a session of the American Chemical Society meeting in Chicago today (Aug. 26). His talk,"Carbon Nanostructures," was part of an extended session on molecular electronics.
Carbon nanotubes are tiny cylinders made of carbon atoms interlinked in a hexagonal pattern. Each carbon atom is bonded to its neighbors by three valence electrons, leaving one electron free to roam. (A valence electron takes part in forming chemical bonds.) This is what allows nanotubes to conduct electricity. A typical nanotube is a few nanometers in diameter and several microns long. (A nanometer is one billionth of a meter, defined as three times the diameter of a silicon atom. A micron is one millionth of a meter, or about three times the diameter of a human hair.)
Carbon nanotubes may be extremely good conductors of electricity that do not heat up excessively when current passes through them. Since electrons can only move straight along the path of the tube, they aren't scattered at various angles, as they would be in, say, a copper wire. Instead, they zip along in what is called "ballistic" motion. If the carbon atoms in the chicken-wire lattice of the tube are arranged in a spiral pattern rather than in straight lines, nanotubes become semiconductors that can be used as transistors.
Right now, scientists and engineers are intensively studying the tiny structures to find out how they work. McEuen's group is concentrating on the basic physics of their behavior and using them to study the physics of electrons. Some nanotubes are made up of several concentric tubes. McEuen's group has concentrated on studying single-wall tubes. In 1997, along with a group at the Delft University of Technology in the Netherlands, they were the first to study electron transport in single-wall tubes.
In earlier work, both groups studied the behavior of electrons moving in and out of single-wall tubes by a mechanism known as tunneling. Their conclusion was that it is much harder for an electron to tunnel into a one-dimensional conductor than into a two- or three-dimensional one. "Other electrons have to get out of the way. In a three-dimensional system, that's not so difficult because the electrons can go off every which way," McEuen explained. The experiments confirmed a long-held theory that could not be tested until nanotubes provided a way to create a one-dimensional system.
At the ACS meeting, McEuen described the use of an AFM to study nanotubes. This device suspends a tiny tip above a sample and uses a sensitive cantilever mechanism to measure minute up and down motions of the tip. In McEuen's work, a positive voltage is applied to the tip, and the electrostatic attraction between the tip and the electrons in the nanotube is measured. Turning up the voltage on the tip can cause a single electron to move into the tube; this in turn increases the electrostatic attraction, pulling the tip down. McEuen has found that the addition of a single electron to the tube causes a measurable dip, making it possible to count electrons as they enter or leave the tube. The experiments are done in a cryostat cooled by liquid helium.
"We use an AC voltage at the resonant frequency of the cantilever to amplify the force," McEuen explains. This causes the electron to move rapidly in and out of tube, he says. "[The tip moves] in little hops because the charges are quantized. If I can detect that wiggling I can feel the force of a single electron as it hops on and off the tube. So you can feel the force of single electron motion."
It's possible, by moving the tip along the tube, to image what's happening at various points, he adds.
McEuen also has found that the AFM can be used to introduce defects into a nanotube. "We hold an AFM tip over it and we put a voltage spike on it and we just kind of blow it. We don't really know what we do microscopically, but when we're done, if we zap it hard, we cut it. If we just nick it you won't see anything, but electrically it will have higher resistance." After introducing a few defects, he says, using the AFM to push the tube may cause it to bend sharply.
Earlier work by McEuen's group was done at the University of California at Berkeley, and some of it continues there. McEuen joined the Cornell faculty in January, 2001.
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o The McEuen Group: http://www.lassp.cornell.edu/lassp_data/mceuen/homepage/welcome.html