This illustration shows how specific chiral varieties of carbon nanotubes can be selected for production when catalytic particles are drawn away at specific speeds by localized feedstock supply. This proposed growth strategy is similar to the 19th century Lamarckian theory that giraffes evolved long necks by stretching progressively higher for food. Image: Ksenia Bets/Rice University.Like a giraffe stretching for the leaves in a tall tree, making carbon nanotubes reach for food as they grow could lead to a long-sought breakthrough. Materials theorists Boris Yakobson and Ksenia Bets in Rice University’s George R. Brown School of Engineering have shown how putting constraints on growing nanotubes could facilitate a ‘holy grail’ of synthesizing batches with a single desired chirality.
In a paper in Science Advances, they describe a strategy by which constraining the carbon feedstock in a furnace would help control the ‘kite’ growth of nanotubes. In this method, the nanotube begins to form at a metal catalyst on a substrate, but lifts the catalyst as it grows, creating something that resembles a kite on a string.
Carbon nanotube walls are basically graphene, where its hexagonal lattice of atoms has been rolled into a tube. Chirality refers to how the hexagons are angled within that lattice, between 0° and 30°, which determines whether the nanotubes are metallic or semiconducting.
Normally, nanotubes grow in random fashion with single and multiple walls and various chiralities. That’s fine for some applications, but many need ‘purified’ batches that require centrifugation or other costly strategies to separate the nanotubes. The ability to grow long nanotubes of a single chirality could allow, for instance, the manufacture of highly conductive nanotube fibers or semiconducting channels in transistors.
The researchers suggest that passing hot carbon feedstock gas through moving nozzles could effectively lead nanotubes to grow for as long as the catalyst remains active. Because tubes with different chiralities grow at different speeds, they could then be separated by length, and slower-growing types could be completely eliminated. One additional step, which involves etching away some of the nanotubes, could then allow specific chiralities to be harvested, they propose.
The lab’s work to define the mechanisms of nanotube growth led them to consider whether the speed of growth as a function of individual tubes’ chirality could be useful. The angle of the ‘kinks’ in the growing nanotubes’ edges determines how energetically amenable they are to adding new carbon atoms.
“The catalyst particles are moving as the nanotubes grow, and that’s principally important,” said lead author Bets, a researcher in Yakobson’s group. “If your feedstock keeps moving away, you get a moving window where you’re feeding some tubes and not the others.”
According to Bets, the paper’s reference to the 19th century theory of French naturalist Jean-Baptiste Lamarck for how giraffes evolved such long necks isn’t entirely out of left field.
“It works as a metaphor because you move your ‘leaves’ away and the tubes that can reach it continue growing fast, and those that cannot just die out,” she said. “Eventually, all the nanotubes that are just a tiny bit slow will ‘die’.”
Speed is only part of the strategy. In fact, Yakobson and Bets suggest nanotubes that are a little slower should be the target to assure a harvest of single chiralities.
Because nanotubes of different chiralities grow at their own rates, a batch would likely exhibit tiers. Chemically etching the longest nanotubes would degrade them, preserving the next level of tubes. Restoring the feedstock could then allow the second-tier nanotubes to continue growing until they are ready to be culled.
“There are three or four laboratory studies that show nanotube growth can be reversed, and we also know it can be restarted after etching,” Bets said. “So all the parts of our idea already exist, even if some of them are tricky. Close to equilibrium, you will have the same proportionality between growth and etching speeds for the same tubes. If it’s all nice and clean, then you can absolutely, precisely pick the tubes you target.”
The Yakobson lab won’t make the nanotubes, as it focuses on theory rather than experimentation. But other labs have turned past Rice theories into products like boron buckyballs.
“I’m pretty sure every single one of our reviewers were experimentalists, and they didn’t see any contradictions to it working,” Bets said. “Their only complaint, of course, was that they would like experimental results right now, but that’s not what we do.”
She hopes more than a few labs will pick up the challenge. “In terms of science, it’s usually more beneficial to give ideas to the crowd,” Bets said. “That way, those who have interest can do it in 100 different variations and see which one works. One guy trying it might take 100 years.”
“We don’t want to be that ‘guy’,” added Yakobson. “We don’t have that much time.”
This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.