Nudged nested nanotubes may oscillate

By Eric Smalley, Technology Research News

Oscillators are the critical component of many timers and sensors, and, as electronic devices continue to shrink, researchers are looking for ways to make ever smaller oscillators.

Mechanical oscillators that are hundreds of times smaller than the head of a pin and vibrate as fast as several million times a second function as precision timers and sensors in systems ranging from automobiles to satellites. Oscillators are used to trigger automobile airbags by sensing sudden deceleration.

A major goal of nanotechnology is making much smaller oscillators that vibrate billions of times a second and can therefore make more precise instruments.

Researchers from the University of California at Riverside and Tsinghua University in China have proposed a way to do this using multiwalled carbon nanotubes that are thousands of times narrower than a human hair. Multiwalled carbon nanotubes are typically 10 to 50 nanometers in diameter. A nanometer is one millionth of a millimeter.

The atomic forces that hold nested sets of carbon nanotubes together cause inner nanotubes to snap back into place after they are pulled partly out of the outer tubes and released. The researchers calculated that if both ends of a multiwalled nanotube were removed and the inner nanotubes partly pulled out, they should slingshot through to extend partially out the other side, then retract.

The slingshot action should go on for a period of time, allowing the core nanotubes to slide back and forth faster than one billion times per second, said Qing Jiang, a professor of mechanical engineering at the University of California at Riverside.

The oscillator is not a microscopic perpetual motion machine, however. The oscillation produces lateral vibrations in the carbon atoms of both the inner and outer nanotubes, said Jiang's colleague, Quanshui Zheng, a professor of engineering mechanics at Tsinghua University in China. "Such vibration will induce... energy dissipation. Therefore, the oscillating cannot be endless. To maintain a constant oscillation requires energy input," he said.

Imperfections in the nanotubes could interfere with the action of the oscillator, though any effect is probably too small to be detected by even the most sensitive instruments available, said Jiang.

The key to the oscillator is the interplay between the atomic forces that hold the tubes together and the kinetic energy imparted to the inner tubes when they are set in motion. The Van der Waals force is the sum of the attractive and repulsive forces between atoms. It draws atoms together, but only to a point, also keeping them from coming into contact with each other. This makes the sliding action of devices as small as nanotubes friction-free.

According to the researchers' calculations, when the inner nanotubes are fully extended, their kinetic energy is at a minimum and the Van der Waals force is at a maximum, which will cause the inner tubes to slide back inside the outer tubes. When the inner tubes slide back inside, the Van der Waals force is at a minimum and the kinetic energy is at a maximum, causing them to continue through to the other end.

The researchers calculate that a 100-nanometer-long, 4-nanometer-diameter set of inner nanotubes pulled out one quarter of their length should oscillate at about 1.39 gigahertz, or 1,390,000,000 times a second.

The two main challenges to implementing the researchers' proposed oscillator are finding a method to set the nanotubes in motion, and connecting the nanotubes to devices in order to use that motion, said Jiang.

"The [researchers] have proposed an imaginative geometry for a nano-mechanical oscillator," said Vincent Crespi, an associate professor of physics at Pennsylvania State University. However, it will pose "rigorous challenges" for anyone attempting to build and test it experimentally, he said.

The researchers' next steps are to test their proposal experimentally and explore applications for the oscillator. Nanotube oscillators could be made practical in two to five years, said Jiang.

The researchers published their research in the January 28, 2002 issue of the journal Physical Review Letters. The research was funded by the Chinese National Science Foundation, Tsinghua University, the Office of Naval Research and the University of California.

Timeline:   2-5 years
Funding:   Government; University
TRN Categories:   Nanotechnology
Story Type:   News
Related Elements:  Technical paper, "Multiwalled Carbon Nanotubes as Gigahertz Oscillators," Physical Review Letters, January 28, 2002


February 6, 2002

Page One

Tiny chain revs microdevices

Labs-on-a-chip gain micro mixer

Starting over speeds searches

Nudged nested nanotubes may oscillate

Portfolios boost quantum computing


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