Meter-long nanotubes on the cards

Posted by Kate Taylor

Researchers have calculated that it should be perfectly possible to grow a perfect, meter-long single-walled carbon nanotube 50,000 times thinner than a human hair.

Scientists from Rice, Hong Kong Polytechnic and Tsinghua universities say that their research opens up the possibility of weaving nanotubes into long power distribution cables.

Key is a mechanism for healing the topological defects - rings with too many or too few atoms - that inevitably bubble up during the formation of nanotubes and affect their valuable electronic and physical properties.

Iron, they say, is the best and quickest among common catalysts for this. Along with the right temperature, it can lead to kinetic healing in which stray carbon atoms are redirected to form the energetically favorable hexagons that make up nanotubes and graphene.

The team employed density functional theory to analyze the energies necessary for the transformation.

"It is surprising that the healing of all potential defects - pentagons, heptagons and their pairs - during carbon nanotube growth is quite easy," says Feng Ding, who was a research scientist in Yakobson's Rice lab from 2005 to 2009.

"Only less than one-10 billionth may survive an optimum condition of growth. The rate of defect healing is amazing. If we take hexagons as good guys and others as bad guys, there would be only one bad guy on Earth."

The energies associated with each carbon atom determine how it finds its place in the chicken-wire-like form of a nanotube, says Rice's professor Boris Yakobson. But there's been much debate over what actually happens at the interface between the catalyst and a growing tube.

"There have been two hypotheses," he says. "A popular one was that defects are being created quite frequently and get into the wall of the tube, but then later they anneal. There's some kind of fixing process. Another hypothesis is that they basically don't form at all, which sounds quite unreasonable."

The new study helps answer this question by evaluating quantitatively how fast this annealing can take place, depending on location.

Errors occur easily; and, in theory, if one ring has five or seven atoms instead of six, it would skew the way all subsequent atoms in the chain orient themselves.

But the team's calculations show that such isolated defects cannot exist in a nanotube wall; they would always appear in 5/7 pairs. That makes a quick fix easier: If one atom can be prompted to move from the heptagon to the pentagon, both rings come up sixes.

And the researchers found that this transition happens best when carbon nanotubes are grown at temperatures around 930 kelvins (1,214 degrees Fahrenheit). That's the optimum for healing with an iron catalyst, which the researchers found has a lower energy barrier and reaction energy than nickel or cobalt.

The researchers also determined through simulations that the slower the growth, the longer a perfect nanotube could be. And a nanotube growing about one micrometer a second at 700 kelvins could potentially reach the meter milestone, they say.