A new method developed at Northwestern University in Evanston, Ill., for sorting single-walled carbon nanotubes promises to overcome the problem of sorting carbon nanotubes by size and type. The method works by exploiting subtle differences in the buoyant densities of carbon nanotubes as a function of their size and electronic behavior. Current methods for synthesizing carbon nanotubes produce mixtures of tubes that differ in their diameter and twist. Variations in electronic properties arise from such structural differences, resulting in carbon nanotubes that are unsuitable for most proposed applications.
Single-walled carbon nanotubes are coated in soap-like molecules called surfactants, then spun at tens of thousands of rotations per minute in an ultracentrifuge. The resulting density gradient sorts the nanotubes according to diameter, twist and electronic structure. Credit: Zina Deretsky (adapted from Arnold et al.), NSF