Physicists show how narrow ‘diet’ can produce monochromatic carbon nanotubes
Like a giraffe stretching out leaves on a tall tree, making carbon nanotubes reach for food as it grows could lead to a long-overdue breakthrough.
Materials theorists Boris Jakobson and Ksenia Bates at the George R. Brown School of Engineering at Rice University show how placing restraints on growing nanotubes can facilitate the “holy grail” of growth groups with a desired pigment.
their paper in science progress Describes a strategy by which carbonate feedstocks can be constrained in a furnace to help control the “kite” growth of nanotubes. In this method, the nanotube begins to form at the metal catalyst on the substrate, but lifts the catalyst as it grows, similar to a kite on a thread.
The walls of carbon nanotubes are mainly graphene, and its hexagonal network of atoms rolls into a tube. Chirality refers to how the hexagons angle within the grid, between 0 and 30 degrees. This determines whether the nanotubes are metallic or semiconductor. The ability to produce long nanotubes in a single symmetry, for example, could enable the fabrication of highly conductive nanotube fibers or semiconducting channels for transistors.
Normally, nanotubes grow in a random fashion with single and multiple walls and different troughs. This is OK for some applications, but many require “purified” batches that require centrifugation or other costly strategies to separate the nanotubes.
The researchers suggested that feeding hot carbon raw gases through moving nozzles could effectively lead to nanotube growth as long as the catalyst remains active. Since tubes with different deviations grow at different speeds, they can then be separated by length, and slow-growing species can be completely eliminated.
They found that an additional step involving etching some of the nanotubes could then allow the harvesting of specific chirals.
Laboratory work to determine the growth mechanisms of nanotubes led us to consider whether growth velocity as a function of individual nanotubes could be useful. The angle of ‘kinks’ in the edges of the growing nanotubes determines their energy potential for adding new carbon atoms.
“The catalyst molecules move as the nanotubes grow, and this is fundamentally important,” said lead author Bates, a researcher in Jakobson’s group. “If the feedstock continues to move away, you get a movable window where it feeds some tubes and not others.”
Bates said the paper’s reference to Lamarck’s giraffes — a 19th-century theory of how such long necks evolved — is not entirely out of left field.
“It works as a metaphor because you move your ‘leaves’ away and the tubes they can reach keep growing rapidly, and those that can’t just die,” she said. Eventually, all the nanotubes that are a little slow will die. “
Speed is only part of the strategy. In fact, they suggest that nanotubes that are slightly slower should be a target to ensure a harvest of single symmetry.
Since nanotubes with different reflectances grow at their own rates, the batch is likely to show layers. Chemically etching longer nanotubes will degrade them, preserving the next level of tubes. The recovery of the feedstock could then allow the Class II nanotubes to continue to grow until they are ready for disposal, Bates said.
“There are three or four lab studies that show that nanotube growth can be reversed, and we also know that it can be restarted after drilling,” she said. “So all parts of our idea are already there, even if some of them are difficult. Near the equilibrium, you will have the same proportion between the growth and drilling speeds of the tubes themselves. If everything is nice and clean, you can certainly, choose precisely which tubes you target.”
Jakobson’s lab won’t make it, because it’s focused on theory, not experiment. But other labs have turned Rice’s theories into products like boron balls.
“I’m sure all of our reviewers were empirical, and didn’t see any inconsistencies caused by this,” Bates said. “Their only complaint, of course, was that they would like experimental results right now, but that’s not what we’re doing.”
She hopes more than a few labs will meet the challenge. “In terms of science, it’s usually better to give ideas to the public,” Bates said. “This way, those with an interest can do it in 100 different species and see which one works. One person trying it could take 100 years.”
“We don’t want to be that ‘the guy,'” Jakobson added. “We don’t have much time.”
Boris Jakobson, Synthesis of monomeric nanotubes through directed evolutionary selection, science progress (2022). DOI: 10.1126/sciadv.add4627. www.science.org/doi/10.1126/sciadv.add4627
Presented by Rice University
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