One of the factors that has limited the application of single-walled carbon nanotubes thus far is the inability to isolate and manipulate nanotubes with varying characteristics.
This problem has been overcome by researchers at The University of Illinois at Urbana-Champaign and Rice University who have devised a method to separate carbon nanotubes based on their electronic structure. The technique is all the more remarkable since nanotubes tend to stick together and form bundles.
The technology was developed by Strano and co-workers at Rice University, who had previously developed the technology to break up the carbon nanotube bundles several years ago.
The technology to break up bundles revolved around dispersing them in water doped with surfactant. The new technology applies reaction chemistry to the surfaces of the nanotubes in order to select metallic tubes over the semiconductors.
The latest technology uses water-soluble diazonium salts to suspend carbon nanotubes in aqueous solutions. The diazonium reagent extracts an electric charge and chemically bonds to the nanotubes under certain controlled conditions.
The addition of a functional group to the end of the reagent allows the researchers to be able to selectively manipulate the nanotubes using processes such as chemical deposition and capillary electrophoresis.
The manipulation is made possible by the fact that the electronic properties of nanotubes are dictated by their structure. In this case metallic nanotubes give up electrons more readily than semiconducting nanotubes, a factor that the diazonium reagent can respond to.
After separation, the reagent can be removed by heating. This has the added benefit of returning the nanotubes to their pristine electronic structure.
This work also proves that the belief that nanotube chemistry was controlled by solely by their diameter (with smaller diameter nanotubes being less stable) is in fact not true.