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| A digital rendering of two
carbon nanotubes linked by a single organic
molecule. Such arrangements are much smaller
than the traditional integrated
circuit. |
Researchers at Columbia University's
Nanoscience Center are on the verge of solving one
of the most vexing barriers facing advances in
molecular electronics: incorporating individual
molecules into functional nanoscale devices and
exploiting their electrical and chemical
properties.
Scientists have long been intrigued by carbon
nanotubes, tiny straws of pure carbon measuring
less than a hair's width across. Successfully
linking them in stable arrangements would allow
for an impressive increase in both the speed and
power of a variety of electronics. This new
research at Columbia sets the stage for advances
in real-time diagnosis and disease treatment,
surgical robotics, and information storage and
retrieval, potentially rendering room-sized
supercomputers obsolete.
In the Jan. 20, 2006, issue of Science,
Columbia scientists explain how they have
developed a unique way to connect the ends of
carbon nanotubes by forming robust molecular
bridges between them. The Columbia team was able
to combine the best qualities of carbon nanotubes
and organic molecules in a single electronic
switch, the journal reported.
Previously, researchers working in this area of
nanotechnology have made transistors out of carbon
nanotubes with switches connecting molecules to
metal wire leads. This Columbia research
illustrates a more elegant way of making molecular
transistors, since the nanotube leads are already
the same size as the molecules, and they are made
of carbon, making it easier to connect them
chemically.
This new method of wiring molecules into the
gaps of single-walled carbon nanotubes employs
oxidative cutting -- a lithographic technique that
makes each cut-end of the nanotube more prone to
molecular bonding. These new methods of
constructing molecular bridges could one day
revolutionize the size and scale of computer
hardware by allowing engineers to design circuits
at the nanoscale limits. The Columbia research,
involving the ability to link nanotubes with an
incredibly small diameter, brings scientists
closer to creating miniature devices that also
process information with molecules.
"Molecular electronics has real-world
relevance," says Colin Nuckolls, an associate
professor of chemistry, and a co-author of the
Science paper. "It opens the door to new
types of ultrasmall switches and sensors. We are
able to form a bridge, both literally and
figuratively, by combining reaction chemistry with
ultrafine lithography."
The nanotubes themselves are long, thin
cylinders of carbon unique for their size, shape
and physical structure. They can be thought of as
a sheet of graphite forming a hexagonal lattice of
carbon, (see image) rolled into a tube, explains
Columbia senior research scientist Shalom Wind,
another co-author of the paper. They have been
shown to possess remarkable mechanical and
electronic properties, he added.
Attaching molecular wires to single-walled
nanotubes involves cutting a tube using
nanolithography combined with a localized
oxidation process that leaves a nanotube with two
ends that are capped with carbon-based acid groups
and separated by a molecule-sized gap. In that
tiny space, a molecule can be chemically joined
with each end to form a robust nanotube/molecule
complex, which operates as a nanoscale transistor.
The nature of this work is also expected to
keep alive "Moore's Law," the prediction made in
1965 by Intel co-founder Gordon Moore who
predicted the number of transistors per square
inch on integrated circuits would double every
year. Moore said the trend would continue for the
foreseeable future; but without an
order-of-magnitude shift in the scale of computer
circuitry -- a promise represented in Columbia's
latest work–that prediction could hit a wall in
the next decade, experts say.
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