A "nanobalance" small enough to weigh viruses and other sub-micron scale particles is one application for newly-discovered electronic and micromechanical properties of carbon nanotubes.
A report in the March 5 issue of the journal Science describes how electrical voltage can be used to induce electrostatic deflection and vibrational resonance in individual carbon nanotubes. This ability to selectively deflect or induce resonance in individual nanotubes opens new potential micromechanical applications for the tiny structures, which are smaller than the finest features on modern microcircuits.
Researchers at the Georgia Institute of Technology studied the behavior of multiwalled nanotubes using a transmission electron microscope with a unique sample holder designed and built by Dr. Philippe Poncharal of Georgia Tech. The holder allowed them to rotate specimens, apply electrical voltage and observe many fundamental effects. The work was sponsored by the U.S. National Science Foundation and the U.S. Army Research Laboratory.
"This opens a broad new field of study," said Dr. Walter de Heer, professor in Georgia Tech's School of Physics. "To show that we can manipulate individual carbon nanotubes while examining them with an electron microscope is breaking new ground. This allows us to use the microscope in a much more interactive way with direct visualization and control that enable us to manipulate the nanotubes the way you would manipulate macroscopic objects on a desktop."
By applying a charge to a nanotube placed near an oppositely-charged probe, the researchers were able to severely bend the tiny structures.
"We can bend a nanotube almost 90 degrees, and it will still recover and straighten out," said Dr. Z.L. Wang, professor in Georgia Tech's School of Materials Science and Engineering. "You can keep on bending them and they will not break. This shows that although nanotubes are very rigid, they have an extremely high elastic limit. Very few materials can do this without damage."
The researchers created resonance in the nanotubes by applying an oscillating voltage. By carefully tuning the oscillation frequency, they were able to induce resonant vibration in nanotubes. Resonant nodes appear in the tubes just as they would in a vibrating guitar string. Each nanotube resonates at a specific frequency that depends on its length, diameter, density and elastic properties.
"You can select which one you want to examine and make it resonate," Poncharal explained. "Then you turn up the frequency and another one will resonate."
The resonance occurs in a very narrow range, allowing the researchers to measure the damping properties of the nanotubes. "These resonances were very narrow, so finding them was like tuning for an unknown radio station -- you just keep looking," noted de Heer.
The researchers also studied the mechanism by which the nanotubes bend.
"One of the most important characteristics of nanotubes is that they are extremely rigid and strong," said de Heer. "That's true when they are very thin. But we have found that as you start making them thicker and thicker, their elastic properties become weaker and weaker and they become softer and softer. They enter a new mode of bending."
Using high-resolution transmission electron microscopy, Dr. Daniel Ugarte of the Laboratorio National de Luz Sincotron in Brazil observed a rippling on the surface of thick nanotubes as they deflected. This confirms that bending in these tubes is different.
"The elastic constant is varying as a function of its diameter, which is unexpected for a general material. This elastic constant should be an intrinsic property of the tubes, rather than depending on its geometry or size," explained Wang.
Using the tiny tubes as a "nanobalance" depends on the ability to calculate changes in the resonant frequency that occur with placement of an object onto a nanotube.
"This is comparable to putting an object on the end of a spring and oscillating it," said de Heer. "By knowing the properties of the spring, you can measure the mass of the object. We can use the nanotube like a standard calibrated spring."
Applying this technique, the researchers were able to measure the mass of a 22 femtogram graphite particle attached to the end of a resonating nanotube. "There is no other way to weigh accurately something that small," he noted.
Beyond the small particle measured so far, the researchers believe their nanobalance could be useful for determining the mass of other objects on the femtogram to picogram size range -- such as viruses. Samples would be attached to tubes through condensation or liquid application of suspended particles.
(Note: A femtogram is 10-15 grams.)
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