Chinese scientists have created the world's narrowest carbon nanotube.

The tiny cylinder has a diameter of just half a nanometre (billionth of a metre). The previous best was 0.7 nm.

The new record-setter was actually the innermost shell of a mutli-walled nanotube.

Research into carbon nanotubes has become a hugely popular field of study since the structures were first imaged in 1991.

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

The tubes have very interesting electrical and mechanical properties. They are very strong and research teams are already learning how to make tiny wires out of them to create ultra-small electronic circuits.

Atomic arrangement

They are also being used to make completely new composite materials such as plastics that conduct electricity.

The novel properties displayed in what are termed nanophase materials stem from the very precise way in which their atoms are arranged - hence the interest in seeing how small certain objects can be made.

And the smallness of the Chinese tube, reported in the journal Nature, is close to what researchers believe is the theoretical limit of a diameter of just 0.4 nm.

Nanotubes are made by applying a high-voltage electric arc to a graphite rod. The tubes condense out of the plasma and can be detected and measured using a high-resolution electron microscope.

Cobalt catalyst

"We used the normal arc-discharge technique to create the narrowest carbon nanotubes, except the anode was modified," the research team from the Chinese Academy of Sciences in Beijing told BBC News Online.

"We made a 3-mm diameter hole in a 6-mm-diameter graphite rod, and filled it with a mixture of cobalt metal powder, carbon nanotubes and other carbon material."

The team believe that their tiny tubes grew from curved carbon fragments in the rod.

"The curved carbon fragments will require less energy to form carbon nanotubes - especially in the existence of the catalyst. As a result, the narrowest carbon nanotubes with the inner diameter 0.5nm were obtained."

The team now need to see if they can make single-walled tubes of the same diameter. They also plan to refine their technique to see if they can make much larger quantities of the tiny tubes.