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.
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.
UPTON, NY --