Credit: © 2009 AAAS

The energy band gap is a key carbon nanotube characteristic. If it is small or zero, the tube is metallic, otherwise it is semiconducting. The gap is known to depend on geometry, and can be increased by strain, curvature and twist, even in metallic tubes. Now, James Hone, Marc Bockrath and colleagues1 at Caltech, Columbia University and Yale University show that electron interactions also contribute to the gap.

The researchers applied a magnetic field to an ultraclean nanotube to tune the size of its gap. They found that the gap never dropped to zero and the gap minimum followed an inverse-power law dependent on radius. This behaviour can not be explained by non-interacting electron models. In addition, it matches existing theoretical descriptions of nanotubes as Mott insulators: insulators that would be metallic if it were not for electron–electron interactions.

These results may contribute to the understanding of superconductivity, which has been described as depending on an energy gap arising from electron interactions. Furthermore, the dependence of the band-gap size on radius indicates that small nanotubes may be semiconducting regardless of chirality. Control over nanotube geometry might therefore allow for the manufacture of purely semiconducting tubes — a long-sought-after goal in the nanotechnology community.