"We have shown quite convincingly that by controlling the size of the starting catalyst we can control the diameter of the nanotubes," said Jie Liu of Duke University. "This is the first time that an identical catalyst has been used."

Obtaining nanotubes with a uniform diameter is important because a nanotube's size is strongly linked to properties such as helicity and electrical conduction (i.e. whether the tube exhibits metallic or semiconducting behaviour). Eventually Liu and the team hope to make samples of nanotubes that are either all metallic or all semiconducting, rather than the mixture of types that are produced today.

The nanocluster catalysts contained 84 molybdenum atoms, 30 iron atoms and assorted carbon, hydrogen and oxygen atoms. To prevent the nanoclusters from aggregating the scientists also used 3-aminopropyltriethoxysilane. Chemical-vapour deposition resulted in single-walled carbon nanotubes with diameters of 0.7-1.5 nm, and a standard deviation of 17%.

The researchers claim this is the first time that nanoparticles with a defined formula and a specific structure have been used to catalyze the growth of single-walled nanotubes.

"The ultimate goal of the research is to produce multiple identical nanotubes using the same kind of catalyst particle," added Liu. "We're still pretty far from there but it really represents a step forward to show that we have a collection of identical catalyst particles to start with."

Liu also reckons it should be easy to scale up the process to make large amounts of catalyst and many nanotubes.