The path of least resistance

The world of electronic circuit design is one where small is definitely better. Small circuits, such as those used in computers, run faster and process more data.

Tiny wires are integral to smaller circuits and scientists at the US Department of Energy’s Brookhaven National Laboratory and Stanford University have developed nanowires that they say have extremely low resistance,

The nanowires are said to 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,’ said 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), synthesised 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,’ said Smalley.

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 tunnelling in less than 20 picoseconds (trillionths of a second),’ Smalley said. ‘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.

The rate, however, remained fast, and the resistance low, up to lengths of nearly three nanometers. ‘That means wiring circuits will be easier because you don’t have to worry so much about the distances,’ said Smalley.

Smalley noted, however, that the wires are far from 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 said.

But this drawback could prove beneficial if the scientists can discover what that factor is and how to control it. Resolution of this issue may enable the Brookhaven team to make electronic components such as tiny transistors and diodes, which work on the basis of varying the electrical resistance.