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University of Chicago physicists pioneer
method for nanotechnology fabrication
December 12, 2001
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An experiment that University of Chicago physicists conducted
just for fun has unexpectedly led them to a new technique for
producing nanoscale structures. The Chicago physicists have built
simple electronic devices using the new technique, which precisely
controls the growth of metal wires along tiny scaffolds that automatically
assemble themselves, following nature's own tendencies.
"This is perhaps the first time that it has been possible
to assemble large numbers of parallel, continuous wires that are
truly nanometer scale in cross-section," said Heinrich Jaeger,
Professor in Physics at the University of Chicago. Jaeger and
Ward Lopes of Arryx Inc. in Chicago describe the technique in
the Dec. 13 issue of the journal Nature.
Self-assembly is a hot research field today because of the promise
it holds for producing new technology at the nanoscale, the scale
of atoms and molecules. Conventional methods for building smaller,
faster computer components involves chiseling ever-finer structures
out of a larger piece of material. Self-assembly, in contrast,
builds up larger structures from smaller building blocks. The
nanowires that Lopes fabricated during the course of his Ph.D.
research at the University measure 30 nanometers by 10 nanometers
in diameter. A nanometer is a billionth of a meter, or the width
of a double strand of DNA. Lopes also fabricated "nanochains,"
tiny strings of metal beads of similar size that could serve as
switches.
The most perfect wirelike structures are formed with silver,
Jaeger said. "Silver is unique in that it forms the wires.
Essentially all other metals‹gold, copper, tin, lead, bismuth‹form
nanochains under normal conditions.
"We can also form nanochains with silver, but the exciting
advance of Ward's research is that he was able to combine experimental
results with computer simulations to get a feeling of what it
is about a particular metal that makes it behave in a wirelike
fashion or the chainlike fashion."
This productive line of research started on a lark.
"In Heinrich's lab we had a tradition on Friday afternoons
of doing experiments that you couldn't justify spending time on,
that you would only do because you wanted to have fun and try
things out," Lopes said. In his experiment, Lopes attempted
to see if silver would chemically react to certain copolymers‹synthetic
compounds‹the way gold did, as would be expected. But Lopes
noticed that the silver exhibited strange behavior. All other
metals formed balls on the copolymers and, if he added too much
metal the balls would bond to each other and ignore the template.
When he added enough silver he expected the silver to ignore the
copolymer template, but the silver spheres had become long and
thin.
"I just followed my nose and said, how long can I get these
things to be?"
Potential applications for the technique include the production
of high-density computer disks, and to make lenses for X-ray lithography,
a process for transferring ultrasmall patterns to silicon computer
chips.
The Chicago physicists used commonly available copolymers and
simple methods with an eye toward easing the transfer of their
results to potential applications.
"The plastics in the copolymer we used are standard, everyday
plastics," Lopes said. "One was polystyrene, which is
used to make Styrofoam, and the other, polymethylmethacrylate,
is familiar from Plexiglas."
Added Jaeger, "The technology for making these structures
is extremely straightforward. It's not high technology in some
sense. That's the beauty of this."
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