Feb. 27, 2001 Upton, NY -- In the world of electronic circuits, smaller is better: Small circuits, such as those used in computers, run faster and process more data. One key to developing smaller circuits is making tiny wires.
Scientists at the U.S. Department of Energy's Brookhaven National Laboratory and Stanford University think they've developed a good candidate, molecular wires millions of times smaller in diameter than a human hair. Described in a paper appearing in the February 23, 2001 issue of the journal Science, these "nanowires," so called because they have dimensions on the order of a nanometer (a billionth of a meter), have high rates of electron transfer with very low resistance. "That means less impedance to the flow of current, with little or no loss of energy," says chemist John Smalley, the lead Brookhaven researcher on the study.
In their search for tiny wires, Smalley and his colleagues were interested in an organic molecule called oligophenylenevinylene (OPV), synthesized at Stanford. "These molecules are essentially 'chains' of repeating links made up of carbon and hydrogen atoms arranged to promote strong, long-range electronic interactions through these molecules," Smalley says.
To learn if these molecules would make good wires, the scientists used the chain-like molecules to connect a gold electrode and ferrocene, a substance capable of accepting and giving off electrons. Then, using a technique developed at Brookhaven, they measured the rate of electron transfer through the chain.
The technique uses a laser to heat up the gold electrode and change its electrical potential. A very sensitive voltmeter then measures the change in electrical potential over time as electrons move back and forth across the connection formed by the molecular wires. The faster the change, the faster the rate of electron transfer, and the lower the resistance in the wire.
The scientists found a very high rate of electron transfer. "We think the electrons are actually popping across through a process called electron tunneling in less than 20 picoseconds (trillionths of a second)," Smalley says. "That means OPV should make pretty good low-resistance molecular wires."
Furthermore, while the scientists expected the rate of electron transfer to decrease as more links were added to the molecular wire chain, making it longer, this didn't happen. The rate remained fast, and the resistance low, up to lengths of nearly three nanometers -- relatively long on a nanometer scale. "That means wiring circuits will be easier because you don’t have to worry so much about the distances," Smalley says.
Smalley points out that the wires aren't perfect, however. The resistance is not as low as it should be according to certain theoretical expectations. "Something else seems to be increasing the resistance," he says. But this drawback could even turn into a benefit if the scientists can figure out what that factor is and how to control it. That might enable them to make electronic components such as tiny transistors and diodes, which work on the basis of varying the electrical resistance.
This research was funded by the U.S. Department of Energy, the National Science Foundation, the National Institute of General Medical Science, and the Stanford University Office of Technology Licensing.
The U.S. Department of Energy's Brookhaven National Laboratory creates and operates major facilities available to university, industrial and government personnel for basic and applied research in the physical, biomedical and environmental sciences and in selected energy technologies. The Laboratory is operated by Brookhaven Science Associates, a not-for-profit research management company, under contract with the U.S. Department of Energy.
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