Two-Faced Edge Makes Nanotubes Obey

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“We uncovered two things,” Yakobson said. “One is that the carbon atom types at the base of the nanotube separate into armchair and zigzag segments. The second is the tendency for the formation of defects that drive the chirality, or helicity, change. That makes (12,6) a sort of transient attractor, at least during short experiments. If they were able to grow forever, (12,6) nanotubes would eventually switch to armchairs.”

The unusual growth pattern might have been diagnosed much earlier if it weren’t for an age-old typo that required some dogged detective work.

“The trouble was in a standard online database that gives the crystal structure of this cobalt-tungsten alloy,” said Bets, co-lead author of the paper with Penev. “One entry was wrong. That messed up the structure so badly that we couldn’t use it in our density functional theory calculations.”

Once they found the error, Bets and co-author Gupta went back to the 1938 German paper that was first to correctly detail the structure of Co7W6. Even with that in hand, the team’s calculations used every bit of computing power they could find to simulate the energetic connections between each atom in the catalyst and carbon feedstock.

“We figured out that if we had run the calculations in series instead of in parallel, they would have taken the equivalent of at least 2,000 years of computer time,” Bets said.

“This paper is remarkable in many aspects: in the timing, the amount of detail and the surprises we found,” Penev said. “We’ve never had a project like this. We don’t yet know how this will be applicable to other materials, but we’re working on it.”

“There are four or five experimental papers, pretty recent ones, that also show a change of chirality during growth,” Bets said. “In fact, because it’s a probabilistic process, it’s essentially unavoidable. But until now it’s never been considered in the theoretical investigation of growth.”

Yakobson is the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry at Rice. Penev is an assistant research professor and Bets is a research administrator, both in the Department of Materials Science and NanoEngineering.

The National Science Foundation supported the research. Computing resources were provided by the National Energy Research Scientific Computing Center, supported by the Department of Energy Office of Science; the Department of Defense Supercomputing Resource Center; the NSF-supported XSEDE supercomputer; and the NSF-supported DAVinCI cluster at Rice, administered by the Center for Research Computing and procured in partnership with Rice’s Ken Kennedy Institute for Information Technology.

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