Great Expectations for Tiny Tubes
It looks just like soot. But unlike the mess caked on your barbecue grill, this substance consists of billions of tiny tubes of pure carbon resembling rolled-up chicken wire. And these striking structures, called nanotubes, are poised to have an impact on fields as diverse as computing, materials engineering, and medical devices.
Discovered in 1991 when Sumio Iijima, a physicist at Japan's NEC Corp., zapped carbon with laser beams, nanotubes set off an international race among researchers. So far, laboratories have revealed a steadily lengthening list of their remarkable properties. "There will be applications to virtually every field," says Rodney S. Ruoff, a professor of physics who heads a nanotechnology-research group at Washington University in St. Louis. "The payoff will be very high."
NANO FOREST: Ordered arrays of nanotubes, grown on plates of glass, may be the key to future video displays and acoustical sensors able to hear bacteria moving
Photo: Flavio Noca, Jet Propulsion Laboratory
Why? For starters, nanotubes are 100 times stronger than steel. They also can withstand temperatures of up to 6,500F and are conductors of heat par excellence. They can also act as either metallic conductors of electricity, capable of handling far more current than comparably sized metal wires, or semiconductors, essential to making computer chips. Cooled to extremely low temperatures, they may even function as superconductors for carrying electrical currents without any losses to resistance.
Nanotubes are a descendant of buckyballs, which became a scientific sensation in 1985. Until then, the only known forms of pure carbon were the rigid crystal structure of diamond and slippery sheets of graphite. The arrangement of 60 carbon atoms into round balls -- dubbed buckyballs, or fullerenes, because they resembled the dome structures designed by architect R. Buckminster Fuller -- seemed to presage a new carbon chemistry.
And indeed it did -- but buckyballs weren't the beneficiaries. The research effort soon led to a new form of carbon, now known as nanotubes. Meanwhile, nanotubes' spherical cousins remain a laboratory curiosity. One writer in The Economist quipped that "the only industry the buckyball has really revolutionized is the generation of scientific papers."
Nanotubes haven't transformed any industries yet, either. But while practical applications still are years away, a steady stream of recent discoveries point to real commercial potential. "There's a revolution going on, on the nanoscale," insists David Tomanek, a physicist at Michigan State University. The most promise lies in computing and electronics.
The holy grail of many research groups is building compact and more efficient computers. Their ability to suck heat away clearly equips nanotubes for a role in computing and electronics, where keeping chips cool is becoming an ever greater problem as circuits are increasingly crowded more tightly together. Tomanek, for one, reported at a March meeting of the American Physical Society (APS) in Seattle that his measurements showed that nanotubes were nearly as conductive of heat as diamond, one of the best heat conductors known. He predicted nanotubes ultimately might prove to be twice as conductive as diamond.
CHICKEN WIRE: Rendering shows arrangement of carbon atoms in a single-walled carbon nanotube
Another problem facing computer designers is reducing circuit size. Many researchers predict that the relentless doubling of computing capacity every 18 months or so on today's silicon chips -- a phenomenon known as Moore's Law -- will soon run up against a physical barrier, if it hasn't already. Then, radical molecular technologies will have to be employed. "Silicon is eventually going to hit a brick wall where devices can't be made any smaller," says Alex Zettl, a physicist at the Lawrence Berkeley National Laboratory in Berkeley, Calif.
If smaller is better, nanotubes certainly fill the bill. With walls that can be just one atom thick, these hollow cylinders of carbon atoms arranged in hexagons have a diameter that is 10,000 times smaller than a human hair (or one nanometer, a billionth of a meter). And nanotubes can be made thousands of times as long as they are thick, increasing their versatility. "Nanotubes are already smaller [than silicon] and don't have a problem with heat. You could not ask for anything better," Zettl says.
A milestone in developing nanotubes for computing was coaxing them into becoming the "on-off" switches, or transistors, that computers run on. That was the goal of a team headed by Phaedon Avouris at IBM's T.J. Watson Research Center in Yorktown Heights, N.Y. In 1998, Avouris and colleagues at the Delft University of Technology in the Netherlands demonstrated that a single nanotube could act as a transistor. At the APS meeting in March, the IBM group reported they had vastly improved the current-carrying performance of their transistors.
But computing is only one of the arenas where nanotubes could become competitive. The tiny tubes can be made to stick together, forming fibers or ropes that could be used as superconducting cables or superstrong reinforcements for plastics and other advanced materials.
Given nanotubes' remarkable flexibility, strength, and resilience, they could also be incorporated into high-performance sports and aerospace materials. "Under large strains, they have the extraordinary property of being able to bend without breaking and then be bent back into their original shape," says Michael R. Falvo, a researcher at the University of North Carolina at Chapel Hill. "This is unique."
In addition, nanotubes might be used in applications where efficient heat-conducting is paramount. For example, nanotubes used as heat sinks in electric motors might permit the use of plastic parts that would otherwise melt under the motors' intense heat. The tiny structures also could be embedded in materials regularly called upon to withstand extreme heat, such as the exterior panels of airplanes and rockets. The National Aeronautics & Space Administration (NASA) hopes to see nanotubes embedded in everything from heat shields for rockets to space suits.
Energy companies also are paying attention. Nanotubes could be used in making smaller and lighter batteries and more efficient fuel cells, as well as in storing hydrogen for use as a source of energy.
By arraying billions of nanotubes on flat sheets of glass or other materials, so they resemble a neatly mown wheat field, researchers have discovered still more potential uses. Already, a handful of companies, including NEC and Korea's Samsung, have active programs to turn these nanotube fields into flat-panel screens for TVs. Individual nanotubes could be used to activate a single pixel in a flat-panel display, replacing the conventional cathode-ray tube used in TVs now.
The tubes also may extend human senses into the world of the supertiny. Flavio Noca, a researcher at Jet Propulsion Laboratory in Pasadena, Calif., is using arrays of nanotubes to make an artificial ear so sensitive that it can detect the sound of a swimming bacterium or hear the gurgling of fluids inside living cells (see BW Online, 1/2/01, "The Sound of One Cell Growing"). Harvard University chemist Jason H. Hafner and his colleagues told the APS meeting that they've devised a way to use nanotubes to create an ultrapowerful microscope that yields the most detailed images yet of biomolecules.
As more and more developments pour out of laboratories, the possibilities for putting carbon to work are multiplying. Zettl recently reported that his team had made virtually frictionless bearings and springs by nesting one nanotube inside another.
They believe the structures will be essential in making minuscule mechanical devices and electromechanical switches. And NEC scientists also have found a way to create "nano-peapods" by encasing buckyballs inside nanotubes, where they exhibit enormous pressure and could be used as tiny pistons.
Right now, you can't go out and buy a pound of nanotubes, and no commercial applications are on the immediate horizon. But the research has already shown the incredible potential locked even in a sooty smear about the size of a postage stamp.
By Alan Hall in New York
Edited by Karen Angel
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