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The findings will be discussed in a presentation by
Lindsay on February 18 at the American Association for the Advancement
of Science annual meeting in Washington, D.C. in an 8:30 a.m. session
entitled "Frontiers in Bioinspired Materials and Nanosystems." The
findings will also be reported in a forthcoming edition of the
American Chemical Society's journal Nano Letters.
Lindsay's team reports achieving an experimental
result that physicists have been trying to detect for a long time -
negative differential resistance in a single molecule attached to
electrodes.
The specifically designed molecule, a hepta-aniline
oglimer, belongs to a group of molecules that biochemists have long
believed to be capable of being molecular switches, but that have
failed to exhibit those properties in conductance experiments. The
team solved the problem by developing a technique where the molecule
could be tested in an electrolyte solution, a condition that past
experiments have never attempted because of the problem of interaction
between the solution and the electrodes.
"Almost everything we know about charge transfer in
molecules is based on measurements made with the molecules suspended
in solution," Lindsay said. "Chemists have understood for a while that
the solvent itself plays a major part in charge transfer processes -
the ions in the solution are necessary to make the process happen.
"Yet almost every 'molecular electronic'
measurement made to date has been made in a vacuum or other conditions
that suppress solvent-mediated events. It's no wonder that we could
not get reliable results," he said.
Though numerous molecules have been identified as
targets for future use as nano-scale electronic components such as
switches, photoelectric devices and hydrogen generators, some major
technical problems have stymied further research. The first of these
has been the difficulty in making reliable connections with single
molecules in order to test their electronic behavior. Recently, this
problem this problem may have been solved by using the scanning probe
microscope to make and measure single molecule contacts with molecules
designed to bond at their ends with a surface and the probe tip.
The second problem, however, has been that these
connected molecules have failed to exhibit the predicted electrical
properties when tested without a conducting solution. Physicists
attempting these measurements avoided using electrolyte solutions
because the applied current would leak into the surrounding solution.
Lindsay and his team solved this problem by applying an insulating
coating to the entire probe, except its very tip, so there was minimal
electrical contact with the solution.
According to Lindsay, the solution is required to
make the process work because, without it, the initial insulating
property of the molecule prevents the first electron from ever jumping
on to the molecule, a kind of catch-22. Ions in the solution "jiggle"
the molecule enough to bring about an unusual configuration of the
molecule that does allow the electrons from the electrodes to jump on
to the molecule, a process first pointed out by Rudy Marcus of Cal
Tech (for which he was awarded the 1992 Nobel prize in chemistry).
The oligoaniline molecule the team tested has three
electrical states, a neutral state where it is an insulator, a second
state where electrons are removed to oxidize the molecule and make it
a conductor, and a third state where more electrons are removed and
turn it back into an insulator. Measuring the connected molecule in a
sulfuric acid solution, the team was able to make reproducible
measurements showing all three states by measuring the current through
it as electrons were removed by another electrode, turning it from an
insulator to a strong conductor and then back into an insulator again.
Given the measured electrical properties of the
oligoaniline, Lindsay notes that if the molecule is maintained at its
highly conductive state (at low voltage) and then the voltage applied
to the molecule is increased, the connections to the molecule will,
themselves, rip electrons out of the molecule, pushing it back into
its insulating state. This decrease of current with increasing voltage
is called "negative differential resistance" (NDR) and it allows a
useful device to be made with just two electrodes, not the three
originally used.
"NDR is the basis for memories, switches and logic
elements," said Lindsay. "It has been observed in molecules before but
never in controlled conditions, never at low voltages, and not in a
predictable way."
Lindsay stress that the main value of the finding
is not so in having found a molecule that could be developed into a
working electrical switch, as it is in discovering some critical
design parameters that should make possible future successful research
in designing molecular devices.
"We have a working rational roadmap now for how to
do this and we're already hard at work applying it to a wide variety
of potentially exciting applications," he said.
Authors on the Nano Letters paper are Lindsay, who
is also the Nadine and Edward Carson Professor of Physics and
Chemistry and Biochemistry at ASU; ASU physicists Fan Chen, and Jin
He; and Columbia University chemists Colin Nuckolls, Tucker Roberts
and Jennifer E. Klare. |
