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A strong effect of buoyancy-driven convection on the diameter of single-walled carbon nanotubes (SWCNTs) has been demonstrated for the first time. Because of favourable convection, SWCNTs with average diameters of 0.8 nm have been produced using the electric arc technique in the vertical configuration. Up to the present, in the horizontal configuration, the electric arc technique has yielded only nanotubes with diameters ~1.4 nm. Growth of our small-size SWCNTs in the vertical configuration is explained by the relatively low growth temperature caused by free convection, as predicted by the `scooter' mechanism model (Thess A et al 1996 Science 273 483) and the supersaturation-segregation model (Kanzow H and Ding A 1999 Phys. Rev. B 60 11 180).

We report a high-resolution transmission electron microscope image showing a thick bundle composed of around a thousand very thin (∼4 Å) carbon nanotubes. The deposit containing the bundle was synthesized using the vertical electric-arc process. Such thin nanotubes should correspond to (4, 2), (3, 3) or (5, 0) structures which, to date, have only been produced confined in zeolite structures.

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