Purifying Nanotubes

Metallic single-walled carbon nanotubes (SWNTs) can be selectively etched and eliminated from a substrate without damaging their semiconducting SWNT neighbors, thanks to a finely tuned methane plasma reaction (Science 2006, 314, 974). The process, developed by scientists at Stanford University, is getting attention from chip makers for its potential in manufacturing nanotube-based electronics.

To date, all SWNT syntheses inevitably produce a mixture of both semiconducting and metallic nanotubes, which differ in their architecture and properties. This mix has been the primary roadblock to using SWNTs in high-performance electronic devices. Metallic SWNTs will short-circuit electrical devices, and so far, there's no simple way to separate the different types of tubes.

Hongjie Dai and coworkers report that a gas-phase methane plasma reaction selectively adds hydrogen and methyl groups to metallic nanotubes, which are slightly more reactive than semiconducting nanotubes. Annealing the material under vacuum leaves behind only pristine semiconducting SWNTs.

"The process is efficient, effective, and compatible with semiconductor fabrication methods," Dai says, although he points out that fine-tuning the conditions is key to the selectivity. Furthermore, the process narrows the semiconducting nanotubes' size distribution because the plasma reacts with both types of SWNTs if their diameters measure less than 1.4 nm. Presumably, the greater curvature makes the semiconducting SWNTs more reactive.

"The technique is important, in part, because it has the potential to advance efforts that seek to use dense, aligned arrays of nanotubes as effective thin-film semiconductors for scalable, ultra-high-performance electronics," comments John Rogers, a chemistry professor at the University of Illinois, Urbana-Champaign.

George Grüner, a physics professor at the University of California, Los Angeles, thinks the process will be important for emerging plastic electronic applications. He does, however, point out that "further innovative steps are needed before single-nanotube devices with adequate reproducibility can be made."