"We have proved that there is no need for a confined space for producing small tubes," Takuya Hayashi of Shinshu University told nanotechweb.org. "This is very important if you need to use the small tube itself for an application or to measure some property."
Previous techniques for making small nanotubes have used a template such as a porous material or the centre of a multiwalled nanotube to confine the tubes. Hayashi and colleagues used a nanozeolite floating-reactant chemical vapour deposition method. In this technique, the zeolite powder limits the size of the catalytic iron particles rather than providing a template for the nanotubes. The zeolite also operates as a support for the catalyst and as a floating substrate.
"The present approach is an ideal combination of the substrate method and the floating-reactant method," explained Hayashi.
Following growth, the scientists saw bundles of single-walled carbon nanotubes around 1 nm in diameter, a 0.43 nm diameter single-walled nanotube isolated from the bundles, large hollow particles of carbon and a few metal particles. The researchers say that the diameter of the small tube is consistent with the diameter of a C20 molecule.
The researchers, who reported their work in Nano Letters, believe their technique has potential for the mass production of small single-walled carbon nanotubes. Now they plan to obtain a higher yield of small tubes, and prepare a small nanotube structure "to demonstrate the possibility of the sub-nanometre, or picometre, realm".