Rice University, Houston, TX, USA, researchers, Junichiro Kono and Bruce Weisman, report using ultracentrifugation (UCF) to create highly purified samples of carbon nanotube species.

The process involves suspending mixtures of single-walled carbon nanotubes in combinations of surfactant-based liquids of different densities. Each nanotube species has its own density and electronic and optical characteristics and when spun by a centrifuge at up to 250,000 g the nanotubes migrate to the liquids that match their densities.

Metallic nanotubes rose to the top of the vial while the semiconducting nanotubes sank to the bottom. It was shown that nearly all of the metallic tubes were armchair single-walled nanotubes, the most desirable species for the manufacture of quantum nanowire.

• Enrichment of Armchair Carbon Nanotubes via Density Gradient Ultracentrifugation: Raman Spectroscopy Evidence
  E. Hároz, W. Rice, B. Lu, S. Ghosh, R. Hauge, R. B. Weisman, S. Doorn, J. Kono
  ACS Nano. 2010, 4, 1955 – 1962.
   

• Advanced sorting of single-walled carbon nanotubes by nonlinear density-gradient ultracentrifugation
  S. Ghosh, S. Bachilo R. B. Weisman
  Nat. Nanotechnol. 2010, Advance Online Publication 9 May 2010