Researchers explore unusual properties of low-resistance ‘nanowire’ systems

Molecular wires are seen by scientists as one key to producing ever-smaller and faster electronic circuits and switches, like those used in computers and complex electronic devices. These “nanowires,” so called because they have dimensions on the order of a nanometer (a billionth of a meter), allow high rates of electron transfer and associated low resistance, or impedance to the flow of current. Now, research focused on finding good candidate materials for these wires is giving scientists a better understanding of how they work.

As described in the online version of the Journal of the American Chemical Society, Brookhaven chemist John Smalley and his colleagues have attempted to solve a mystery uncovered during their initial work in this area ? why the wires’ resistance is not as low as it should be according to certain theoretical expectations.

“We saw something unexpected during our original work with organic molecules called oligophenylenevinylenes, or OPV,” said Smalley. “We wanted to find out if we could see the same behavior in a simpler system.”

Instead of making wires out of OPV, molecules that are essentially “chains’ of repeating links made up of carbon and hydrogen, this time Smalley and his colleagues substituted alkanes, the simplest members of the hydrocarbon family. The researchers’ technique involves employing a laser to heat up a gold electrode and change its electrical potential, then using a very sensitive voltmeter to measure 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.

“Unexpectedly, we saw the same strange behavior — a ‘limit’ in the electron transfer rates and, consequently, in the resistance of the wires”,” said Smalley. “The limiting value of resistance is different from that seen with OPVs, so it appears to be a function of the individual material, but the fact that we’ve now seen it in two different materials tells us it is probably real. We now have to ask ourselves ‘what could be happening within the material to cause this unexpected behavior?'”

Smalley said one possibility is that there is some kind of configurational reorganization going on within the material that is coupled to the electron transfer reaction. “We’ve demonstrated that the limit isn’t tied to our technique, so it must be something else,” he said. “The effect is something we must understand if we want to eventually begin manufacturing nanowires for a variety of applications.”

While the transfer limit is currently seen as an obstacle, it could be turned 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 work was funded by the U.S. Department of Energy, the National Science Foundation and the Fannie and John Hertz Foundation. Other collaborating institutions include Brigham Young University, Case Western Reserve University, Clemson University, Stanford University, the University of Texas at Dallas, and West Virginia University.