Improved Selectivity In Making Metallic Carbon Nanotubes

A new way of preparing single-walled carbon nanotubes allows chemists to control the tubes’ chirality, which is the property that renders their conductivity behavior either semiconducting or metallike (Science 2009, 326, 116). Standard nanotube synthesis methods usually produce a mixture of semiconducting and metallic tubes, which are difficult to separate. A team led by Avetik R. Harutyunyan of the Honda Research Institute USA, in Columbus, Ohio, used iron nanocatalysts deposited onto a SiO₂/Si framework and varied the environment in which the catalysts were annealed—using a helium or argon environment, different ratios of H₂ and H₂O, and different temperatures. The right combination of these variables produced nanotubes from methane that were up to 91% metallic, compared with the typical 33%. Although numerous re­search groups have had success in devising ways to control carbon nanotube chirality, the mechanisms have remained elusive. The new work by Harutyunyan and coworkers, however, identifies a correlation between catalyst morphology and the resulting nanotube electronic structure. “Our results indicate that, with further optimization, direct control over nanotube structure during growth may well be feasible,” the researchers write.