PHILADELPHIA
-- Physicists at the University of Pennsylvania
have overcome a major hurdle in the race to create
nanotube-based electronics. In an article in
the August issue of the journal Nature Materials,
available online now, the researchers describe their
method of using nanotubes tiny tubes entirely composed
of carbon atoms -- to create a functional electronic
circuit. Their method creates circuits by dipping
semiconductor chips into liquid suspensions of carbon
nanotubes, rather than growing the nanotubes directly
on the circuit.
"Given their amazing electric properties, nanotubes
have been a subject of keen interest for creating
such things as chemical sensors, flexible electronics
and high-speed, high-device-density microprocessors
for computing," said Alan T. Johnson, associate professor
in Penn's Department of Physics and Astronomy. "The
problem is that the properties we like best about
nanotubes their size and physical properties also
make them very difficult to manipulate."
Instead
of growing nanotubes in a pattern on a silicon
chip, as is conventionally done, the Penn researchers
devised a means of "sprinkling" nanotubes onto chips.
"We dip the chips into nanotubes much like dipping
an ice cream cone into candy," said Danvers Johnston,
a graduate student in Johnson's laboratory and lead
author of the study. "Ultimately we can make
it so that the nanotubes only stick where want them
to in order to form a circuit."
Single-walled
nanotubes are formed by turning a single sheet
of carbon atoms into a seamless cylinder approximately
one nanometer a billionth of a meter in diameter.
Nanotubes
can be either semiconducting or "metallic" the
latter is highly conductive to electricity depending
on the exact geometry of the carbon atoms. Semiconducting
nanotubes make for exceptional transistors, which
is why so much attention has been devoted to finding
a way to use them in electronics.
Previously,
most nanotube circuits have been made by growing
each nanotube on the surface of a chip, using a
process known as chemical vapor deposition. Unfortunately,
this method often results in a circuit comprised
of both types of nanotubes, metallic and semiconducting. Furthermore,
the growth direction of the nanotube is arbitrary,
and their diameters are large. Small diameter
carbon nanotubes are more useful for switches.
"Fortunately, other researchers have made it possible
to grow small diameter nanotubes and to separate
metallic from semiconducting nanotubes in solution," said
Arjun Yodh, a professor in Penn's Department of Physics
and Astronomy. "Ultimately our process can
create a large batch of small diameter nanotubes
in solution, can separate out the semiconducting
nanotubes and then can place them in proper position
on a patterned silicon chip."
The
researchers, along with post-doctoral associate
Mohammed F. Islam, found their biggest challenge
in purifying the mass-produced nanotubes. The
process they used to create nanotubes in bulk frequently
adds impurities usually stray bits of carbon and
leftover catalysts that ultimately detract from the
quality of the nanotubes.
The
Penn researchers found a purification scheme for
the nanotubes by heating them in moist air with
a gentle acid treatment and then using magnetic
fields to separate the nanotubes from the impurities. They
deposit the nanotubes by dipping a chip covered with
a glue-like substance into the nanotube solution,
and then they wash off the excess glue and whatever
solvents that remain.
The
resulting circuits take advantage of unique electrical
properties of nanotubes and can be produced in
bulk. Since the researchers can create nanotubes
via processes separate from the chips, this process
allows for a better control of the quality and diameter. The
Penn researchers believe there is a definite role
for nanotechnology in the future of electronics.
"The only way to make faster processors is to cram
more transistors together," Johnson said. "Nanotubes
are just about the smallest transistors that exist
right now. So the more densely they can be
packed on a chip, the faster the chips can become."
Funding for this research was provided by grants
from the National Science Foundation and NASA.
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