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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