Rice University scientists show how ‘solid food’ can form carbon nanotubes (Growing pure nanotubes is a stretch, but possible) with the same chirality.
Like giraffes drawing tall leaves, getting carbon nanotubes to reach food as they grow could lead to much-needed progress.
Property managers Boris Yakobson and Ksenia Bets of George R. The Brown School of Engineering is showing how the limitations in nanotube growth can support the “holy grail” of growing batches with the desired chirality.
Their paper in Science Advances describes a strategy of limiting the amount of carbon in the fire to help control the growth of the “kite” of nanotubes. In this process, the nanotube begins to grow in the material that makes the iron in the seeds, and raises the material that makes it grow, making it in the fiber.
The walls of carbon nanotubes are actually graphene, a hexagonal lattice of atoms packed into a tube. Chirality refers to the way the hexagons are aligned, between 0 and 30 degrees. This determines whether the nanotubes are metallic or semiconducting. The ability to grow long nanotubes in the same chirality can, for example, enable the creation of highly conductive nanotube fibers or semiconductor transistor channels.
Normally, nanotubes grow randomly in single and multiple walls with different chirality. This is fine for some applications, but many require “pure” batches that require centrifugation or other expensive techniques to separate the nanotubes.
The researchers suggested that hot carbon air fed through moving nozzles could stimulate nanotube growth as long as the catalyst worked. Because tubes with different chirality grow in different ways, they can be divided in length, and the types that grow slowly can be completely eliminated.
Another step that involves assembling some nanotubes may allow specific chirality to be introduced, they determined. Laboratory work to define the growth process of nanotubes led them to wonder whether the growth rate as a function of the chirality of individual tubes could be useful. The degree of “curvature” at the ends of the growing nanotubes determines their ability to absorb new carbon.
“The excited particles move as the nanotubes grow, and that’s especially important,” said lead author Bets, a researcher in Yakobson’s group. “If your equipment is continuous, you’ll get a sliding window where you’re feeding some tubes and not others.”
The article’s reference to Lamarck’s giraffes — a 19th-century theory of how they evolved with long necks — isn’t entirely out of left field, Bets said. “It works like a metaphor because you remove your ‘life’ from the tubes that can reach it and grow faster, and those that can’t die,” he said. “Eventually, any nanotubes that are too small will ‘die’.”
Speed is only part of the strategy. In fact, they suggest that slow nanotubes should be the target to ensure a unique harvest.
Since nanotubes of different chirality grow at their own pace, a single batch can show levels. Chemical etching of long nanotubes will degrade them, preserving another layer of tubes. Restoring nutrients can allow the second-stage nanotubes to continue growing until they’re ready for disposal, Bets said. “There are three or four laboratory studies that show that nanotube growth can be reversed, and we know that it can be regenerated after etching,” he said. “So all aspects of our thinking are already there, even if some of them are subtle. In the near-equivalent range, you will have a balance between growth and etching rate for the same tubes. If everything is good and clean, then you can select the tubes you are targeting in a nutshell.
Yakobson’s laboratory will not do them, as they focus on theory, not experiments. But other labs have turned Rice’s old ideas into products like boron buckyballs.
“I’m sure that every one of our inspectors is an inspector, and they don’t see any contradictions in the way it works,” Betts said. “Their only complaint, of course, is that they would like test results now, but that’s not what we’re doing.”
He hopes more than a few labs will take up the challenge. “Scientifically, it’s always useful to give feedback to the crowd,” Betts said. “That way anyone who is interested can do it with 100 different variations and see which one works. Someone who tries it can take 100 years.
Yakobson added: “We don’t want to be that ‘guy’. We don’t have a lot of time.”
