A novel method for sorting semiconducting carbon nanotubes based on their diameter has been developed by scientists who said the long-awaited development could form the basis of a nanotube purification system capable of producing the necessary raw materials for use in nanocircuits, therapeutic agents, next-generation power cables and more. Nanotubes, tiny cylinders of carbon no wider than a strand of DNA, possess an array of properties coveted by materials scientists. Nanotubes are stronger than steel, but weigh one sixth as much. Some varieties are excellent semiconductors, while others are metals that conduct electricity as well as copper. But there are dozens of varieties of nanotubes, each slightly different in size and atomic structure and each with very different properties. For many applications, engineers need to use just one type of nanotube, but that's not possible today because all production methods turn out a mishmash of types, the scientists said. New research from the scientists at Rice University describes a method that uses electric fields to sort nanotubes by diameter. "People have developed sorting methods based on both chemical and electrical properties, but ours is the first that's capable of sorting semiconducting nanotubes based upon their dielectric constant, which is determined by their diameter," said corresponding author Howard Schmidt, executive director of Rice's Carbon Nanotechnology Laboratory (CNL). To sort nanotubes, the CNL team built a system that capitalizes on the fact that each type of nanotube has a unique dielectric constant, or 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, according to Schmidt. The experimental work was primarily performed by research scientist Haiqing Peng and first-year graduate student Noe Alvarez. The paper's co-authors include research scientist Carter Kittrell and distinguished faculty fellow Robert Hauge. The research, which will appear in an upcoming issue of the Journal of the American Chemical Society, was supported by NASA, the Department of Energy, the Army Research Laboratory and the Air Force Office of Scientific Research. For more information, visit: www.rice.edu