UC Berkeley Physicists Create Tiny
Bearings And Springs Out Of Carbon
Nanotubes For Use In Microscopic
Machines
Berkeley - Physicists at the University of California, Berkeley, have
peeled the tips off carbon nanotubes to make seemingly frictionless
bearings so small that some 10,000 would stretch across the diameter
of a human hair.
The minuscule bearings are actually telescoping nanotubes with the
inner tube spinning about its long axis. When sliding in and out,
however, they act as nanosprings.
Both the springs and bearings, which appear to move with no wear
and tear, could be important components of the microscopic and
eventually nanoscale machines under development around the world.
Micromachines, called MEMS devices, for microelectromechanical
systems, are on the scale of a human hair. Nanoelectromechanical
systems (NEMS) are a thousand times smaller, on the scale of a
nanometer or a billionth of a meter. Nanotubes, for example, are
hollow cages of carbon atoms several nanometers thick and up to
several thousand nanometers long, looking on the molecular level
like chicken wire stretched around a baguette.
"Friction is a big problem with MEMS, but these nanoscale bearings
just slide as if there's no friction," said John Cumings, a graduate
student in the Department of Physics at UC Berkeley who created the
bearings. "As a lower limit, friction is a thousand times smaller than
you find in conventional MEMS devices made with silicon or silicon
nitride."
Cumings and advisor Alex Zettl, professor of physics at UC Berkeley,
report on their low-friction bearings in an article in this week's issue
of Science.
Nanotubes were first discovered in the black residue of a carbon arc,
the same place scientists discovered buckyballs - 60 atoms of carbon
arranged in the shape of a soccer ball. Nanotubes are essentially
elongated buckyballs, usually nested within one another with
typically several to several dozen concentric shells.
In order to move these amazingly small structures around, Cumings
first had to build a manipulator. He and Zettl in effect built a scanning
tunneling microscope, typically used to produce atom-by-atom
pictures of the surface of materials, inside a transmission electron
microscope (TEM). TEMs use electron beams to take pictures at
resolutions down to a few nanometers, at a speed of several frames a
second - enough to construct a video. The TEM he used is located at
the Lawrence Berkeley National Laboratory, where Zettl is a member
of the materials science division.
Using the fine-tipped probe of the scanning tunneling microscope
(STM), Cumings was able to manipulate nanotubes and watch what
he was doing in real-time with the TEM.
To make a bearing, he first attached one end of a multi-layer nanotube
to a gold wire. To manipulate this nanotube, he snagged a sturdier
nanotube with the tip of the STM probe. In a report soon to appear in
the British journal Nature, Cumings and Zettl describe how they
wielded the nanotube manipulator to peel off the end of the outer
nanotubes but leave the inner nanotubes intact and protruding. A
typical experiment converted a nine-walled nanotube with an outer
diameter of eight nanometers - the width of about 100 atoms - into
two telescoped tubes, the inner one with four walls and an outer
diameter of four nanometers.
After spot-welding the manipulator to the tip of the inner nanotubes,
he was able to slide the inner tubes in and out of the outer tubes,
telescoping them like a spyglass. Though he was only able to move
the nanotubes in and out as a linear bearing, he said the telescoping
nanotubes would work just as well as a rotating bearing.
Since all this manipulation was performed under the magnification of
a TEM, he was able to look closely at the nanotube structure after
10-20 cycles of pushing and pulling. He saw no change in molecular
structure whatsoever, indicating there is essentially no friction
between the two sliding nanotubes.
"We saw no wear or fatigue, no matter how many times we did it, up
to about 20 times," Cumings said. "Because we're looking at the
molecular level, this means there will be no wear if we did it another
20 times, or a million times. This is like a bearing that doesn't have
any wear."
Once, as Cumings telescoped the nanotubes, the spot-weld broke, and
surprisingly the inner tube automatically retracted into the outer
nanotube. He and Zettl eventually deduced that minuscule
intermolecular forces, called Van der Waals forces, were strong
enough to pull the inner tube completely inside the outer tube. This
means the sliding nanotubes could also serve as nanosprings.
"The transit time for complete nanotube core retraction (on the order
of 1 to 10 nanoseconds) implies the possibility of exceptionally fast
electomechanical switches," the two authors wrote.
The same Van der Waals forces apparently lubricate the nanotube
bearings and are identical to the forces that lubricate the sheets of
carbon in graphite and make graphite break easily along
two-dimensional planes.
Cumings anticipates such nanosprings could prove useful in MEMS
and NEMS devices, not the least because they exert a constant force
throughout their range of motion. He and Zettl plan to improve their
ability to manipulate nanotubes inside a TEM and also develop
microfabrication technology to create more elaborate devices.
"Our results demonstrate that multiwall carbon nanotubes hold great
promise for nanomechanical or nanoelectromechanical systems
(NEMS) applications," they conclude in their paper. "Low-friction,
low-wear nanobearings and nanosprings are essential ingredients in
general NEMS technologies."
The work is supported by the U.S. Department of Energy and the
National Science Foundation.
Editor's Note: The original news release can be found at
http://www.berkeley.edu/news/media/releases/2000/07/27_nano.htm
University Of California, Berkeley:
(http://www.berkeley.edu)
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