Reviewed by Alex SmithNov 10 2022
Similar to a giraffe reaching upward for leaves on a tall tree, causing carbon nanotubes to stretch for food as they grow may result in a long-sought revolution.
There are dozens of varieties of nanotubes, each with a characteristic diameter and structural twist, or chiral angle. Carbon nanotubes are grown on catalytic particles using batch production methods that produce the entire gamut of chiral varieties, but Rice scientists have come up with a new strategy for making batches with a single, desired chirality. Their theory shows chiral varieties can be selected for production when catalytic particles are drawn away at specific speeds by localized feedstock supply. The illustration depicts this and an analogous process 19th-century scientists used to describe the evolution of giraffes’ long necks due to the gradual selection of abilities to reach progressively higher for food. Image Credit: Ksenia Bets.
Boris Yakobson and Ksenia Bets, materials theorists at Rice University’s George R. Brown School of Engineering, demonstrate how placing limitations on growing nanotubes could enable a “holy grail” of growing groups with a single chosen chirality.
Their article, published in the journal Science Advances, illustrates how limiting the carbon feedstock in a furnace would help regulate the “kite” growth of nanotubes. In this technique, the nanotube starts to form at the metal catalyst on a substrate but elevates the catalyst as it grows, similar to a kite on a string.
Carbon nanotube walls are typically graphene, with a hexagonal lattice of atoms rolled to form a tube. Chirality can be described as how the hexagons are angled inside the lattice, between 0 and 30 degrees. That establishes whether the nanotubes are semiconductors or metallic.
The capability to grow long nanotubes in a single chirality could, for example, allow the production of semiconductor channels of transistors or extremely conductive nanotube fibers.
Typically, nanotubes grow arbitrarily with multiple and single walls and different chiralities. That is suitable for certain applications, but many require “purified” batches that need centrifugation or other expensive strategies to divide the nanotubes.
The scientists proposed that feeding hot carbon feedstock gas through moving nozzles could successfully lead nanotubes to grow for as long as the catalyst stays active. As tubes with various chiralities grow at various speeds, they could then be divided by length, and slower-growing varieties could be eliminated.
They established one extra step that requires etching away some of the nanotubes that could permit precise chiralities to be harvested.
The laboratory’s work to describe the mechanisms of nanotube growth made them consider whether the speed of growth as a function of individual tubes’ chirality could be beneficial. The angle of “kinks” in the growing nanotubes’ edges controls how energetically pliable they are in incorporating new carbon atoms.
The catalyst particles are moving as the nanotubes grow, and that’s principally important. If your feedstock keeps moving away, you get a moving window where you’re feeding some tubes and not the others.
Ksenia Bets, Study Lead Author and Materials Theorist, Yakobson’s Group, George R. Brown School of Engineering, Rice University
Their study illustrates Lamarck giraffes—a 19th-century notion of how they grew such long necks—is not completely out of the left field, Bets explains.
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. Eventually, all the nanotubes that are just a tiny bit slow will ‘die’.
Ksenia Bets, Study Lead Author and Materials Theorist, Yakobson’s Group, George R. Brown School of Engineering, Rice University
Speed is just part of the approach. In fact, they propose nanotubes that are a bit slower should be the target to guarantee a yield of single chiralities.
Since nanotubes of various chiralities grow at their own speeds, a batch would probably display tiers. Chemical etching would degrade the longest nanotubes, thereby maintaining the subsequent level of tubes. Reestablishing the feedstock could then permit the second-tier nanotubes to continue growing until they are set to be removed, Bets said.
“There are three or four laboratory studies that show nanotube growth can be reversed, and we also know it can be restarted after etching,” she 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.
Ksenia Bets, Study Lead Author and Materials Theorist, Yakobson’s Group, George R. Brown School of Engineering, Rice University
The Yakobson lab will not make them, as it concentrates on theory, not experimentation. However, other labs have transformed earlier Rice theories into products such as boron buckyballs.
I’m pretty sure every single one of our reviewers were experimentalists, and they didn’t see any contradictions to it working. Their only complaint, of course, was that they would like experimental results right now, but that’s not what we do.
Ksenia Bets, Study Lead Author and Materials Theorist, Yakobson’s Group, George R. Brown School of Engineering, Rice University
She is optimistic more than a few labs will take 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.”
Yakobson added, “We don’t want to be that ‘guy.’ We don’t have that much time.”
Yakobson is the Karl F. Hasselmann Professor of Engineering and a professor of materials science and nanoengineering, and of chemistry.
The National Science Foundation (1605848) and the Robert Welch Foundation (C-1490) supported the study.
Journal Reference:
Yakobson, B.I. & Bets, K.V. (2022) Single-chirality nanotube synthesis by guided evolutionary selection. Science Advances. doi.org/10.1126/sciadv.add4627.