Single-walled nanotubes (SWNTs) are essentially rolled up sheets of graphite just one atom thick and can be metallic or semiconducting depending on the direction in which the sheet has been rolled. They could be ideal as the building blocks in nanoscale electronics, and might even be the perfect alternatives to silicon thanks to their tiny size and their ability to carry large currents. Semiconducting tubes could make good nanoscale transistors, but only if the samples are free from metallic tubes, which short out electronics devices. Arrays of purely metallic tubes can function as transparent conducting leads.

Although several methods to separate out semiconducting and metallic tubes exist, most of these are chemical-based, such as chromatography or centrifugation, and can only be applied to suspensions of short-length tubes in solution. Such solution processing also often significantly degrades the electronic properties of the tubes.

Pristine tubes

"In contrast, our approach produces chemically pristine tubes grown in perfectly aligned arrays on the surfaces of wafer substrates," explains team leader John Rogers of the University of Illinois at Urbana-Champaign. "The process is essentially 100% efficient in completely removing metallic tubes from a heterogeneous sample of both metallic and semiconducting tubes, without inducing any damage in the latter. Electronic devices, such as transistors, then made from the purified samples are thus also better as a result."

Indeed, the researchers measured switching characteristics, that is on/off ratios, of 10,000; current mobilities exceeding 1000 cm2V–1s–1; and current outputs per tubes in the mA range in transistors made from the purified arrays. These values are similar to those expected for pristine semiconducting tubes themselves.

"In other words, our purified arrays have the same electronic properties as those predicted by previous measurements on test structures built from individual, isolated semiconducting tubes," Rogers told nanotechweb.org. "What is more, their effective mobilities are even higher than those observed in equivalent silicon devices."

Thermocapillary resists

The technique, which could readily be scaled up to treat large numbers of SWNTs, involves placing a mix of metallic and semiconducting nanotubes on thin-film organic coatings that subsequently act as thermocapillary resists. Metallic and dielectric layers patterned at the edges of a coating layer allow the researchers to inject current into the metallic nanotubes only, which then heat up. This selective heating, in turn, produces surface tension gradients that force the metallic tubes to pass through the resists, leaving behind a purified array of semiconducting SWNTs.

The team, which includes researchers from the University of Miami, Northwestern University and Purdue University, is now busy trying to simplify its process by reducing the total number of steps. "We would also like to be able to apply our technique directly to arrays of tubes with densities greater than 10 tubes per micron," revealed Rogers.

The current work is detailed in Nature Nanotechnology.