Can we now unlock the potential of nanotubes?
A new method that produces structurally uniform carbon nanotubes should finally see their exceptional electronic properties put to good use, Daniel Cossins reports.
Single-walled carbon nanotubes (SWCNTs) have been lauded as the new superheroes of materials science. These minute rolled-up sheets of carbon one atom thick boast extraordinary properties. Some, dubbed “metallic”, conduct electricity far faster than copper and could revolutionise the electricity grid. Others are semi-conducting and could dramatically outstrip silicon-based computer chips while others again capture sunlight and are perfect for solar cells. The problem is that the current methods for producing SWCNTs create mixtures of all the types, compromising their usefulness.
Now, after 20 years of foiled attempts, a team of researchers from Switzerland and Germany has found a way to make SWCNTs in single-species batches. “This has taken a long time but it’s now been done very well,” says James Tour, a chemist and nano-engineer at Rice University in Houston, Texas. “Yes, there are still barriers to scaling this up but the science is no longer holding us back. It’s a big step forward.”
SWCNTs are surprisingly diverse. There are more than 100 different species, all with their constituent honeycomb of carbon atoms. But each has a slightly different alignment of the honeycomb. Some look like the nanoscale version of a perfectly rolled sheet of chicken wire while in others it’s as though the wire was rolled up with a twist in it. Up till now researchers have been able to narrow down the mix of species they produce to five. But for many applications that’s still not good enough and trying to separate this mixture is awkward and expensive.
To produce truly uniform nanotubes the Swiss/German team, led by Roman Fasel of the Swiss Federal Laboratories for Materials Science and Technology, decided to use a trick that’s been tried with other carbon structures – synthesise flat molecular templates that fold up in a predetermined fashion to form three-dimensional “cap” structures, much like a flat piece of cardboard folding itself up to make a box, and build up the nanotubes from the caps.
As described last week in Nature, when the researchers place their flat precursors on a platinum surface in a vacuum at 500°C neighbouring carbon atoms in the precursor snap together to form the three-dimensional caps, which appear on the surface like a new suburb of identikit houses. In the next stage, as carbon-loaded gases such as ethanol and ethylene are piped in, additional carbon atoms attach themselves to the open-ended base of each little house. The structures grow upward until the platinum is covered with a city of identical miniature skyscrapers – capped, hollow tubes up to 300 nanometres high. Because the original molecular template defines the structure of the cap and the cap in turn defines the structure of the resulting nanotubes, the researchers end up with a single species.
“We've found a way to make the exact nanotubes you want so you don’t need to separate them,” says Fasel. “That’s an important proof of principle.” The particular kind of nanotubes the team made is known as “armchair” nanotubes. It remains to be seen, however, if the same technique can produce other species. There are also plenty of challenges ahead when it comes to scaling up the process to produce nanotubes in the required quantities.
Given that 50% of the flat precursor molecules produced nanotubes Tour calculates that 1 kg of seeds could produce 5 tonnes of nanotubes. But you would need 30 square kilometres of platinum to sow the seeds. Another problem is that the nanotubes can tangle as they grow and teasing them apart is difficult.
Nevertheless, Tour says, “one of the big hurdles that has held back nanotube chemistry over past 15 years has now been cleared. Engineers can now begin to figure out how we do this on a large scale”.