The explosive growth of flat panel displays and other electrochromic devices has been fueled in part by transparent thin-film technology that conducts electricity but does not interfere with image quality.Indium tin oxide is the thin film of choice for this purpose, but the material has some problems, not the least of which is the increasing expense of indium because of a tightening supply.Researchers Alexander A. Green and Mark C. Hersam of Northwestern University in Evanston, Ill., have created a thin film fashioned from single-walled carbon nanotubes — a substance that is plentiful and relatively cheap — that has the potential to be a substitute conductor.Single-walled carbon nanotubes were separated according to their diameters, with each size displaying a different color. Carbon nanotubes of uniform size can be used to create conductive thin films that are efficient and transparent. Courtesy of Mark C. Hersam.Although carbon nanotubes are nothing new, current production methods result in nanotubes that come in a muddle of different diameters and shapes. This, in turn, hampers their performance as a conductive material by affecting their electronic character and optical properties. This problem has limited the use of nanotubes in a variety of applications. What is needed is a way to sort out a batch of mixed nanotubes into subsets that are uniform in size.The researchers achieved this breakdown by using a technique known as density gradient ultracentrifugation to uniformly separate the nanotubes with an angstrom-level resolution in diameter. An assortment of nanotubes fabricated by various methods — and thus with widely different diameters — was mixed together and subjected to the ultracentrifuge at a very high rotational frequency.The method sorted the nanotubes by size, bandgap and electronic type by attaching surfactants — soaplike molecules — to the tube-shaped structures. This changes their surface densities and enables them to be sorted with very little error. After sorting, more than 97 percent of the nanotubes are within a 0.02-nm-diameter range. They are spun at tens of thousands of rotations per minute in the ultracentrifuge until they have joined like-size structures. The technique has the potential to be used on an industrial scale. The investigators found that the process eliminated many of the impurities, thus enhancing conductor electrical performance. In comparison with thin films created with a medley of nanotubes, the design used by the researchers enhanced conductivity by a factor of 5.6 in the visible spectrum and by a factor of 10 in the infrared.Additionally, the uniform diameter of the nanotubes resulted in thin films that exhibited distinct colors. This ability to fine-tune the optical properties of a thin film — something that is limited with current technology — could be used to filter unwanted light that affects device performance.In addition to flat panel displays, thin films created with sorted nanotubes could be used in a variety of other applications, including solar panels and LEDs. The investigators said that as more single-walled carbon nanotube sources become available, they expect that nanotubes sorted by diameter will increasingly challenge indium tin oxide as the conductor of choice.Nano Letters, ASAP Edition, April 8, 2008, doi: 10.1021/nl080302f.