Rice University scientists have developed the first method for sorting semiconducting carbon nanotubes based on their size, a long-awaited development that could form the basis of a nanotube purification system capable of producing the necessary feedstocks for nano-circuits, therapeutic agents, next-generation power cables and more.
There are already sorting methods based on both chemical and electrical properties, but this is the first that's capable of sorting semiconducting nanotubes based upon their dielectric constant, which is determined by their diameter.
To sort nanotubes, the CNL team built a system that capitalizes on the fact that each type of nanotube has a unique dielectric constant – a term that refers to a material's ability to store electrostatic energy. CNL scientists created an electrified chamber and pumped a solution of dissolved nanotubes through it. The chamber traps metallic nanotubes and causes semiconducting varieties to float at different levels in the chamber. The smaller the diameter of the nanotube, the larger the dielectric constant and the lower in the system the tubes float. By varying the speed of flow through the system – with upper-level currents traveling faster than lower-level currents – the scientists were able to collect samples that had three times more small tubes than large and vice versa.