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Scientists at the U.S. Department of Energy's Brookhaven National Laboratory and their collaborators have discovered that a short, organic chain molecule with dimensions on the order of a nanometer conducts electrons in a surprising way.
A Nanowire with a Surprise
New research may advance the nanoelectronics field
Upton, NY. October 18, 2004
Scientists at the U.S. Department of Energy's Brookhaven
National Laboratory and their collaborators have discovered that a
short, organic chain molecule with dimensions on the order of a
nanometer (a billionth of a meter) conducts electrons in a surprising
way: It regulates the electrons' speed erratically, without a
predictable dependence on the length of the wire. This information
may help scientists learn how to use nanowires to create components
for a new class of tiny electronic circuits.
"This is a very unexpected and unique result," said John Smalley, a
guest scientist in Brookhaven's Chemistry Department and the lead
researcher of the study, described in the October 16, 2004, online
edition of the Journal of the American Chemical Society.
The conducting chain molecule, or "nanowire," that Smalley and his
collaborators studied is composed of units of phenyleneethynylene
(PE), which consists of hydrogen and carbon atoms. Like the links
that make up a chain, PE units join together to form a nanowire known
as oligophenyleneethynylene (OPE). PE, and therefore OPE, contains
single, double, and triple carbon-carbon bonds.
The double and triple carbon-carbon bonds promote strong electronic
interactions along OPE such that it conducts an electric current with
low electrical resistance. This property makes OPE nanowires good
candidates for components in nanoelectronic circuits, very small,
fast circuits expected to replace those currently used in computers
and other electronics.
Smalley and his collaborators found that as they increased the length
of the OPE wire from one to four PE units, the electrons moved across
the wire faster, slower, then faster again, and so on. In this way,
OPE does not behave like a similar nanowire the group has also
studied, called oligophenylenevinylene (OPV), which contains single
and double carbon-carbon bonds. When they made OPV wires longer, the
electrons' speed remained the same. They observed the same result
when they studied short wires made of alkanes, another group of
hydrocarbon molecules that contains only single carbon-carbon bonds.
The researchers think that the unusual behavior of OPE may be due to
its tendency to slightly change its three-dimensional shape.
Increasing the wire's length may trigger new shapes, which may slow
down or speed up the electrons as they cross the wire.
This variable resistance could be a benefit. "If the odd behavior is
due to the conformational variability of the OPE wires, figuring out
a way to control the tendency of OPE to change its shape could be
useful," said Smalley. "For example, diodes and transistors are two
types of devices based on variable electrical resistance."
The scientists made another significant finding: They dramatically
increased the rate at which the electrons moved across the wire by
substituting a methyl hydrocarbon group onto the middle unit of a
three-unit OPE wire.
"Because OPE seems sensitive to this substitution, we hope to find
another hydrocarbon group that may further increase the electrons'
speed, and therefore OPE's ability to conduct electrons," said
In the experiment, Smalley and his group created an OPE wire "bridge"
between a gold electrode and a "donor-acceptor" molecule. To measure
the electron transfer rate across the bridge, they used a technique
they developed in which a laser rapidly heats up the electrode. This
causes a change in the electrical potential (voltage) between the
electrode and the donor-acceptor, which disrupts the motion of
electrons crossing the bridge. The group used a very sensitive
voltmeter to measure how quickly the voltage changed in response to
the altered electron movement. From these measurements, they
determined how fast the electrons were moving through the wire.
This research, performed in collaboration with Marshall Newton of the
Brookhaven Chemistry Department and researchers at Stanford
University, Clemson University, and Motorola, is funded by the Office
of Basic Energy Sciences within the U.S. Department of Energy's
Office of Science and the National Science Foundation.
One of the ten national laboratories overseen and primarily funded by
the Office of Science of the U.S. Department of Energy (DOE),
Brookhaven National Laboratory conducts research in the physical,
biomedical, and environmental sciences, as well as in energy
technologies and national security. Brookhaven Lab also builds and
operates major scientific facilities available to university,
industry and government researchers. Brookhaven is operated and
managed for DOE's Office of Science by Brookhaven Science Associates,
a limited-liability company founded by Stony Brook University, the
largest academic user of Laboratory facilities, and Battelle, a
nonprofit, applied science and technology organization.
Visit Brookhaven Lab's electronic newsroom for links, news archives,
graphics, and more: www.bnl.gov/newsroom.
Mona S. Rowe
Copyright © BNL
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