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.
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