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Friday, March 5, 1999 Published at 14:29 GMT


Sci/Tech

World's smallest scales weigh in

The speck of soot vibrates on the three micron-long nanotube, revealing its weight

The smallest weighing machine ever made has sized up a speck of soot to the nearest million billionth of a gram.

It was created using tiny cylinders of carbon called nanotubes.

It weighed a 22 femtogram speck of graphite, by far the smallest mass ever measured. One femtogram is a million billionth of a gram.

The development marks another advance in the field of nanotechnology, which aims to build machines thousands of times smaller than current microelectronics.

In future, the "nanobalance" could be used to weigh individual viruses.


[ image: The nanotube in the left picture has been twanged in the right picture]
The nanotube in the left picture has been twanged in the right picture
It was developed after new discoveries about the electronic and micromechanical properties of carbon nanotubes.

"This opens a broad new field of study," said Dr Walter de Heer, from the Georgia Institute of Technology. His group has shown they can manipulate individual carbon nanotubes at the same time as examining them with an electron microscope for the first time, he says.

"The weighing method 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."

To bend the spring, the researchers applied an electric charge to a nanotube placed near a probe with the opposite charge.

"We can bend a nanotube almost 90 degrees, and it will still recover and straighten out," said Dr. Zhong Wang, also at Georgia Tech. "You can keep on bending them and they will not break. Very few materials can do this without damage."

Thick, strong and very long

Carbon nanotubes are long, thin cylinders of interlinked carbon atoms. Much research has focused on nanotubes with a single-layered wall of carbon but the Georgia Tech scientists were looking at nanotubes with multiple-layered walls.

"One of the most important characteristics of nanotubes is that they are extremely rigid and strong," said Professor de Heer.

"That's true when they are very thin. But we have found that as you start making the walls thicker and thicker, their elastic properties become weaker and weaker and they become softer and softer. They enter a new mode of bending."

High-resolution transmission electron microscopy performed in Brazil confirms this by showing a rippling on the surface of thick nanotubes as they deflected.

The nanobalance research is published in Science.



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