Nanotechnology: Two new routes create specific types of single-walled carbon nanotubes with high purity
Scientists have speculated for decades about the potential applications of single-walled carbon nanotubes (SWNTs). With their characteristic strength, flexibility, and conductivity, these nanomaterials that resemble rolled-up chicken wire might one day feature prominently in solar cells and miniaturized electronic circuits. But efforts to make them have always produced a mix of nanotubes with varying diameters and chiralities—the carbon-atom geometry that can make a nanotube behave like a metal or a semiconductor. Two groups provided possible solutions to this long-standing problem this year when they independently reported SWNT syntheses that produce a single type of nanotube with high purity. Yan Li of Peking University and coworkers grew SWNTs that are 92% pure, improving on the previous best of 55% (Nature 2014, DOI: 10.1038/nature13434). The tubes in the pure portion have a single chirality, bestowing the tiny cylinders with metallic properties. The key, Li says, was finding the “right recipe” for making a high-temperature tungsten-cobalt alloy nanocrystal catalyst to seed nanotube growth. Scientists led by Konstantin Amsharov of the Max Planck Institute for Solid State Research, in Germany, and Roman Fasel of the Swiss Federal Laboratories for Materials Science & Technology made one—and only one—type of SWNT by starting from a polycyclic aromatic hydrocarbon seed molecule (Nature 2014, DOI: 10.1038/nature13607). This precursor folds up into a nanotube cap when heated on a platinum surface. The nanotube then elongates as ethanol is added to provide a carbon source. The resulting nanotubes are metallic and free of defects. To capitalize on these discoveries, researchers need to next figure out how to scale up these syntheses and tune them to make pure SWNTs of different sizes and chiralities.
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A polycyclic aromatic hydrocarbon seed folds up into a cap when heated on a platinum surface. This cap dictates the chirality of the nanotube, approximately 2 nm in diameter, that grows from it.
Credit: Juan Ramon Sanchez-Valencia