Carbon tubes reach their limit

A tube's properties are defined by its size

Scientists have created what they say are the narrowest, stable, carbon nanotubes. The tiny cylinders measure just 0.4 nanometres (0.4 billionths of a metre) in diameter.

The structures are essentially tubular versions of buckminsterfullerene, the closed cages of carbon atoms that look like soccer balls.

Carbon nanotubes have generated huge excitement because of their very interesting electrical and mechanical properties.

They are chemically inert and very strong, and scientists believe the structures can be forged into tiny wires to create ultra-small electronic circuits.

Theoretical limit

Two teams working separately - one from the NEC Corporation of Japan, the other from the Hong Kong University of Science and Technology - report the tubes, which are smaller than a filament of DNA, in the journal Nature.

Buckminsterfullerene is shaped like a soccer ball

Carbon nanotubes are made by applying a high-voltage electric arc to a graphite rod in a chamber filled with a gas such as helium or hydrogen. The tubes condense out of the plasma that is created.

The NEC's tube was the smallest, innermost cylinder in an 18-shell, multi-walled nanotube. The Hong Kong team took a different route: they used the narrow channels in the mineral zeolite as a mould, constraining the upper size of any single molecule that could form out of the plasma.

Researchers believe the 0.4-nanometre-sized tubes mark a theoretical limit.

Curvature problem

"If you increase the curvature further, you increase the energy of the system and it becomes very unstable," Dr Zikang Tang of the university's physics department told BBC News Online.

This has been borne out by research published last week which found that tubes measuring just 0.33 nanometres in diameter break apart.

"The fabrication technology that we have developed is very unique, and the nanotubes that we produced are unprecedented," said Dr Tang.

"Electron microscopic observation shows that our nanotubes are extraordinarily small with uniform diameters. This discovery will provide new samples for further research on nanotubes."

Hydrogen fuel

Scientists expect the carbon cylinders to find wide applications in many hi-tech areas. One possible use currently under investigation is as a storage device for hydrogen fuel.

The gas would normally take up too much space and compressing or liquefying it is expensive.

Scientists have found they can reduce the volume of any given amount of hydrogen by storing it inside nanotubes.

Dr Tang said: "In our case, the diameter of the tubes is similar to the diameter of a hydrogen atom. So the hydrogen can be absorbed inside the nanotubes at a very high density."