Nanotubes cut to ribbons
New techniques open up carbon tubes to create ribbons.
Two groups have unzipped tiny cylindrical structures called carbon nanotubes to make graphene ribbons tens of nanometres wide. Such ribbons have been touted as a promising material for everything from solar cells to computers, but have so far proved harder to produce than the tubes.
Graphene, an atom-thick sheet of honeycombed carbon, is one of the hottest materials around. It conducts electrons well, but is thin, transparent and strong, making it potentially useful in displays and solar panels. Ribbons of graphene could be more useful still. At widths of around 10 nanometres or less, electrons are forced to move lengthwise, and make the graphene behave as a semiconductor. Semiconducting graphene could be a boon to the electronics industry.
"Ribbon structures are very important structures and they're not easy to make," says James Tour, a chemist at Rice University in Houston, Texas. Early techniques used chemicals or ultrasound to chop graphene sheets into ribbons, but could not make ribbons in large amounts or with controlled widths.
As a solution, Tour and his co-workers, and a separate group led by Hongjie Dai of Stanford University in California, decided to try to generate ribbons from carbon nanotubes. Nanotubes are essentially rolled up sheets of graphene, sometimes nested inside each other. Researchers can already synthesize them in large quantities. The trick, says Dai, was figuring out how to open up the tubes to make one or more layers of graphene. "These things don't really have a zipper on them," he says. Both groups report their work today in Nature1,2.
Making the cut
Dai and his colleagues opted to slice the tubes using an etching technique borrowed from the semiconductor industry. They stuck nanotubes onto a polymer film and then used ionized argon gas to etch away a strip of each tube. Once cleaned, the remaining ribbons were just 10–20 nanometres wide.
Tour's group, by contrast, used a combination of potassium permanganate and sulphuric acid to rip the tubes open along a single axis. The resulting ribbons are wider — around 100–500 nanometres — and not semiconducting, but easier to make in large amounts.
"The techniques complement each other," says Mauricio Terrones, a physicist at the Institute for Science and Technology Research of San Luis Potosi in Mexico, who was not involved in the work.
Both techniques are likely to be useful to researchers, and both have a variety of potential applications. Tour believes that his larger ribbons could be used in solar panels and flexible touch displays, where cheap, transparent materials are in demand. They could even be spun into lightweight, conducting fibres that might replace bulky copper wiring on aircraft and spacecraft. Dai's narrower ribbons, meanwhile, might find uses in electronics because of their semiconducting properties
Dai says that his group has already used the ribbons to make basic transistors, but, he adds, it's too early to tell whether they will be commercially competitive. "It's very early in the game," he says.
- Kosynkin, D. V. et al. Nature 458, 872–876 (2009).
- Jiao, L., Zhang, L., Wang, X., Diankov, G. & Dai, H. Nature 458, 877–880 (2009).