The Key to Creating Smaller Circuits is in Making Tiny Wires


Scientists Investigate "Nanowires" with Very Low Resistance

Source: Brookhaven National Laboratory and Stanford University

February 23, 2001

When it comes to electronic circuits, smaller is better. Smaller circuits can run faster and process more data. The key to creating smaller circuits is in making tiny wires. Scientists at the U.S. Department of Energy's think they've come up with a good candidate. They've developed molecular wires millions of times smaller in diameter than a human hair. The "nanowires" have high rates of electron transfer with very low resistance, according to a paper in the February 23rd issue of Science. "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.

On their quest to build tiny wires, Smalley and his team 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," says Smalley.

To determine whether the molecules would make good wires, the researchers used the chain-like molecules to connect a gold electrode and ferrocene, a substance that can accept and give off electrons. They then used a technique developed at Brookhaven to measure the rate of transfer through the chain. The technique heats the gold electrode with a laser to change its electrical potential. An extremely 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 detected a very high rate of 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."

In addition, although scientists expected the rate of electron transfer to go down when more links were added to the molecular wire chain, this did not happen. The rate remained very fast, and the resistance low, up to lengths of nearly three nanometers, which is 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.

However, he pointed out that the wires aren't perfect. 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 lead to a benefit if the scientists can find out what that factor is and how to control it. That might allow them to make electronic components such as tiny transistors and diodes, which work on the basis of varying the electrical resistance.

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