Technology Research News
One of the challenges of making machines
out of small numbers of molecules is figuring out how to connect them
individually in order to form electrical circuits.
The trouble is, soldering isn't an option on the molecular scale. Instead,
researchers from Arizona State University and Motorola have found a way
to chemically bond each end of a molecule to a metal conductor.
They began with a flat gold surface and covered it with a single layer
of electrically insulating octanethiol molecules, which are a string of
hydrogen and carbon atoms with a sulfur atom on one end. The sulfur bound
chemically to the gold surface.
The researchers removed a few of the molecules, leaving gaps, then filled
the gaps with related octanedithiol molecules, which have sulfur atoms
on both ends. One end of these molecules chemically bonded to the bottom
layer of gold. Then the researchers sprinkled gold nanoparticles on the
surface, and the opposite ends of the octanedithiol molecules bonded to
When the researchers touched a single nanoparticle with the electrified
gold tip of an atomic force microscope, it completed a circuit through
the molecule to the gold surface. "In essence, we have a single octanedithiol
molecule chemically bonded to gold contacts at each end and surrounded
by an insulator. This is like a wire soldered into a circuit," said Devens
Gust, a professor of chemistry at Arizona State University.
The researchers took 4,000 separate measurements of molecules this way.
The connected molecules conducted current more quickly than ordinary molecules,
offering four times less resistance, according to Gust.
The length of each molecular wire is a little over one nanometer, which
is 1,000 times smaller than the circumference of an E. coli bacterium.
A nanometer is one millionth of a millimeter.
There were two main hurdles to connecting single molecules, said Gust.
The first difficulty was designing the chemical layer so that one or only
a few molecules were connected to each gold nanoparticle, said Gust. Then
they had to figure out how to measure the results, he said. "The second
[challenge] was designing and building an atomic force microscope capable
of making the... precise current voltage measurements," he said.
The key to attaching a wire to a molecule in a usable way is making a
chemical rather than a mechanical bond, said Gust. "We found that when
chemical bonds are used at both ends, the conductivity of the molecule
increases by a factor of at least 10,000" over methods that mechanically
attach a molecule to an electrode, he said. The chemical bond is also
not as sensitive to force as a mechanical contact would be, making it
a sturdier connection, he said.
Bonds like these can eventually be used to form single-molecule wires,
transistors and logic elements that can be incorporated into tiny electronic
circuits. It will be at least a few years before even simple circuits
that use single molecules become possible, said Gust.
The work is one more step in the progression of molecular-scale electronics,
said Vincent Crespi, an associate professor of physics at Pennsylvania
State University. The important contribution is the use of bonds to gold
on both sides of the molecule, he said.
The work also allows researchers to measure the behavior of single molecules
under the influence of electrical current, said Gust. It "shows unambiguously
that we are measuring only one molecule, rather than an assembly of some
unknown number of molecules."
This is important because one of the puzzles in studying how electricity
flows through individual molecules has been untangling the influence of
the contact from the influence of the molecule, said Crespi. "In something
this small the contact is just as big as a molecule itself, so an understanding
of the electron transport depends critically on understanding of the molecule/metal
contact," he said.
Gust's research colleagues were Xiaodong Cui, Xristo Zarate, John Tomfohr,
Otto Sankey, Ana Moore, Thomas Moore and Stuart Lindsay of Arizona State,
and Gari Harris and Alex Primak of Motorola. They published the research
in the October 19, 2001 issue of the journal Science. The research was
funded by the National Science Foundation (NSF).
Timeline: > 3 years
TRN Categories: Nanotechnology
Story Type: News
Related Elements: Technical paper, "Soldering Molecules
for Nano-electronics," Science, October 19, 2001.
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