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ITHACA,
N.Y. ---- Using a carbon nanotube, Cornell University
researchers have produced a tiny electromechanical
oscillator that might be capable of weighing a single
atom. The device, perhaps the smallest of its kind
ever produced, can be tuned across a wide range of
radio frequencies, and one day might replace bulky
power-hungry elements in electronic circuits.
Recent
research in nanoelectromechanical systems (NEMS) has
focused on vibrating silicon rods so small that they
oscillate at radio frequencies. By replacing the silicon
rod with a carbon nanotube, the Cornell researchers
have created an oscillator that is even smaller and
very durable. Besides serving as a radio frequency
circuit element, the new device has applications in
mass sensing and basic research.
Paul
McEuen, Cornell professor of physics, Vera Sazonova,
Cornell graduate student in physics and Yuval Yaish,
a visiting scientist in the Laboratory of Atomic and
Solid State Physics (LASSP) at Cornell, report on
the device in the latest issue (Sept. 16, 2004) of
the journal Nature.
Carbon
nanotubes are cylinders of carbon atoms arranged in
a hexagonal pattern similar to that in the geodesic
domes created by architect, inventor and mathematician
Buckminster Fuller. Materials with this structure
are called fullerenes in his honor, and fullerene
spheres are known as buckyballs. A nanotube can be
thought of as an elongated buckyball.
The
Cornell device consists of a carbon nanotube from
one to four nanometers in diameter and about one-and-a-half
micrometers long, suspended between two electrodes
above a conducting silicon plate. (A nanometer is
one-billionth of a meter, the length of three silicon
atoms in a row; a micrometer is one-millionth of a
meter.) The tube is not stretched tight, but hangs
like a chain between two posts in a shallow curve
called a catenary.
The
tube itself is a conductor, and when a voltage is
applied between the tube and the underlying plate,
electrostatic force attracts the tube to the plate.
An alternating voltage sets up vibration as the tube
is alternately attracted and repelled. A static voltage
applied at the same time increases the tension on
the tube, changing its frequency of vibration just
as tightening or loosening a guitar string changes
its pitch. The entire assembly of tube and plate behaves
as a transistor, so the tube's motion can be read
out by measuring the current flow. Experimenting with
various sizes and lengths of tubes, the researchers
have made oscillators that tune over a range from
3 to 200 megaHertz (millions of cycles per second).
Such
a tunable oscillator could be used as a detector in
a radio-frequency device such as a cellular phone,
which must constantly change its operating frequency
to avoid conflicts with other phones.
Like
their larger cousins, nanotube oscillators also could
be used for mass sensing. Since the frequency of vibration
is a function of the mass of the vibrating string,
adding a very small mass can change the frequency.
Silicon rod oscillators have been used to weigh bacteria
and viruses. "This is so much smaller that mass
sensitivity should be that much higher," McEuen
said. "We're pushing the ultimate limit, maybe
weighing individual atoms."
The
researchers conducted their measurements in a vacuum.
If air or any other gas were present, the gas molecules
would adsorb, or collect in a condensed form, on the
surface of the tube, changing its mass. So, McEuen
says, nanotube oscillators could be used as gas detectors.
One
drawback, he points out, is that at present there
is no way to mass-produce carbon nanotubes.
McEuen
looks forward to studying the fundamental physics
of the device. When cooled to cryogenic temperatures,
he says, the nanotube acts like "a skinny quantum
dot," or a sort of box full of electrons.
"We can study the influence of individual electrons
hopping on and off," he says. "What happens
when you have a quantum dot that can wiggle?"
The
Nature paper is titled "A Tunable Carbon Nanotube
Electromechnical Oscillator." Other co-authors
are Hande &Üuml;stünel, a graduate
student in physics, David Roundy, a LASSP postdoctoral
associate and Tomás A. Arias, Cornell associate
professor of physics. The work was funded by the National
Science Foundation (NSF) and the Microelectronics
Advanced Research Program (MARCO) Focus Center on
Materials, Structures and Devices supported by the
Semiconductor Research Corporation. The devices were
fabricated at the NSF-funded Cornell Nanoscale Facility.
Related World Wide Web sites: The following sites
provide additional information on this news release.
Some might not be part of the Cornell University community,
and Cornell has no control over their content or availability.
McEuen
group:
<http://www.lassp.cornell.edu/lassp_data/mceuen/homepage/pubs.html>
--
Cornell
University News Service
Surge 3
Cornell University
Ithaca, NY 14853
607-255-4206
cunews@cornell.edu
http://www.news.cornell.edu
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