FOR RELEASE UNTIL 2 P.M. EDT (11 A.M. PDT) THURSDAY, JULY
27, 2000, TO COINCIDE WITH PUBLICATION IN THE JOURNAL SCIENCE
- 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.
color drawing of the low-friction bearings and telescoping
bearings are actually telescoping nanotubes with the inner
tube spinning about its long axis. When sliding in and out,
however, they act as nanosprings.
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
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.
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."
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.
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
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
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.
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
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
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.
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."
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
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.
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
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.
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."
is supported by the U.S. Department of Energy and the National
image available for download
Research Group Web site