Dec. 16, 1997
BERKELEY, CA. -- Scientists with the Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab) have confirmed the existence of atom- sized electronic devices on nanotubes, hollow cylinders of pure carbon about 50,000 times more narrow than a human hair in diameter. Nanotube devices have been predicted by theorists but this is the first demonstration that such devices actually exist.
Alex Zettl, a physicist with Berkeley Lab's Materials Sciences Division and a professor of physics on the University of California's Berkeley campus, led a study in which nanotubes of pure carbon were shown to function as a two-terminal electronic device known as a diode.
"What we are seeing is the world's smallest room temperature rectifier, one that is only a handful of atoms in size," says Zettl. "When we grow nanotubes, electronic devices naturally form on them."
Past attempts to identify nanotube devices employed submicron-sized electrode contact pads that could only measure small isolated sections of the tube. Evidently, the experimenters were measuring the wrong sections. Zettl succeeded by measuring nanotubes along their entire length. He accomplished this through the use of the ultrafine tip of a scanning tunneling microscope.
The research was reported in a recent issue of the magazine Science (10/10/97). Co-authoring the paper with Zettl were Phil Collins, of Zettl's research group; Hiroshi Bando, from the Electrotechnical Laboratory in Japan; and Andreas Thess and Richard Smalley of Rice University.
Nanotubes are only a few nanometers (billionths of a meter) in diameter. When made exclusively from carbon molecules, they are chemically inert, about 100 times stronger than steel, and offer a full range of electrical and thermal conductivity possibilities.
Carbon nanotubes were discovered by the Japanese electron microscopist Sumio Iijima. They are created by heating ordinary carbon until it vaporizes, then allowing it to condense in a vacuum or an inert gas. The carbon condenses in a series of hexagons, like sheets of graphite, that curl and connect into hollow tubes.
Depending upon its diameter, a pure carbon nanotube can conduct an electrical current as if it were a metal, or it can act as a semiconductor, meaning it will only conduct a current beyond a critical voltage. According to a theory proposed by Berkeley Lab physicists Marvin Cohen and Steven Louie, both also with UC Berkeley, an electronic device could be created at the interface between two dissimilar nanotubes, one that acts as a metal and one that acts as a semiconductor. This would create a "Schottky barrier," which means the current will only flow in one direction -- from the semiconductor to the metal. Under the scheme envisioned by Cohen and Louie, the two dissimilar tubes would be connected by the introduction of pentagon- heptagon pair defects (rings of five and seven carbon atoms) into the interface region.
Zettl and Collins have been able to confirm that Schottky barriers do exist along carbon nanotubes. The key to their success was the scanning tunneling microscope or STM. An STM features a metallic tip that is the world's smallest pyramid: a few layers of atoms descending in number down to a single atom at the point. The Berkeley researchers would bring the tip of an STM into contact with a tangle of nanotubes on a substrate then slowly withdraw it. Van der Waals forces would induce a single nanotube to stick to the tip of the STM and the researchers would carefully stretch it out from the other nanotubes on the substrate, much like unravelling a single fibre from a nest of thread. Once a single nanotube was extracted, the researchers would then slide the STM tip across its entire surface to measure variations in an electrical current passing through.
"We measured distinct changes in the conductivity as the active length of the nanotube was increased, suggesting that different segments of the nanotube exhibit different electronic properties," says Zettl. "The changes occurred over very short lengths and were suggestive of on- tube nanodevices."
Zettl does not expect nanotubes to replace silicon overnight in the electronics industry but can see this as a possibility down the road. Silicon must be doped with other atoms to make an electronic device. As the size of a device shrinks, the dopant atoms eventually begin to move about, degrading the device's performance. Heat also becomes a problem despite silicon's good thermal conductivity. The use of diamond film, with its exceptionally high thermal conductivity, has been proposed to protect silicon-based devices but this adds further complications to the manufacturing process. Size and heat are no issue for nanotubes because they are covalently bonded (which means their atoms are locked firmly into place) and are predicted to be even better thermal conductors than either silicon or diamond at room temperature.
"Silicon is eventually going to hit a brick wall where devices can't be made any smaller," Zettl says. "Nanotubes are already smaller and don't have a problem with heat. You could not ask for anything better in a material."
Rather than wiring individual devices in nanotubes for specific purposes, as is done with silicon chips, Zettl suggests a better approach might be to make a "tube cube," a block of nanotubes that would be densely packed with billions upon billions of devices. The tube could then be wired to form a random network of "nanocomputers." This random network would be able to train itself to perform tasks, reconfiguring its input/output architecture to improve its performance as it learns and develops. In other words, this random computer would not just get older, it would get better.
"The idea is not as far off as you might think," says Zettl, whose group has already constructed and wired up a tube cube of sorts. The cube cannot yet perform any useful function, but Zettl says it does yield some "interesting" responses to input signals.
"Nanotube technology might be exploited in a conventional manner or we might have to go off in a completely different direction," says Zettl. "The technology simply has too much potential to not figure out how to use it."
The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.
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