New one-dimensional form of carbon may be the strongest material ever

Carbyne nanorods may have uses in electronics and for energy storage

October 11, 2013

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Nanoropes or nanorods of carbyne, a chain of carbon atoms, would be stronger than graphene or even diamond if they can be manufactured, according to new calculations by Rice University. Theoretical physicist Boris Yakobson said the material might also find uses in electronics and for energy storage. (Credit: Vasilii Artyukhov/Rice University)

Rice U. theorists calculate atom-thick carbyne chains may be the strongest material ever, if and when anyone can make it in bulk.
Carbyne is a chain of carbon atoms held together by either double or alternating single and triple atomic bonds. That makes it a true one-dimensional material, unlike atom-thin sheets of graphene, which have a top and a bottom, or hollow nanotubes, which  have an inside and outside.
According to calculations by theoretical physicist Boris Yakobson and his group:

• Carbyne’s tensile strength — the ability to withstand stretching — surpasses “that of any other known material” and is double that of graphene. (Scientists have calculated it would take an elephant on a pencil to break through a sheet of graphene.)
• It has twice the tensile stiffness of graphene and carbon nanotubes and nearly three times that of diamond.
• Stretching carbyne as little as 10 percent alters its electronic band gap significantly.
• If outfitted with molecular handles at the ends, it can also be twisted to alter its band gap. With a 90-degree end-to-end rotation, it becomes a magnetic semiconductor.
• Carbyne chains can take on side molecules that may make the chains suitable for energy storage.
• The material is stable at room temperature, largely resisting crosslinks with nearby chains.

“You could look at it as an ultimately thin graphene ribbon, reduced to just one atom, or an ultimately thin nanotube,” Yakobson said.. It could be useful for nanomechanical systems, in spintronic devices, as sensors, as strong and light materials for mechanical applications, or for energy storage.
Based on the calculations, he said carbyne might be the highest energy state for stable carbon.
Theories about carbyne first appeared in the 19th century, and an approximation of the material was first synthesized in the USSR in 1960. Carbyne has since been seen in compressed graphite, has been detected in interstellar dust, and has been created in small quantities by experimentalists.
Yakobson said the researchers were surprised to find that the band gap in carbyne was so sensitive to twisting. “It will be useful as a sensor for torsion or magnetic fields, if you can find a way to attach it to something that will make it twist,” he said.
Another finding of great interest was the energy barrier that keeps atoms on adjacent carbyne chains from collapsing into each other. “When you’re talking about theoretical material, you always need to be careful to see if it will react with itself,” Artyukhov said. “This has never really been investigated for carbyne.”
The literature seemed to indicate carbyne “was not stable and would form graphite or soot,” he said. Instead, the researchers found carbon atoms on separate strings might overcome the barrier in one spot, but the rods’ stiffness would prevent them from coming together in a second location, at least at room temperature.
“Bundles might stick to each other, but they wouldn’t collapse completely,” Yakobson added. “That could make for a highly porous, random net that may be good for adsorption.”
Rice graduate student Fangbo Xu and former postdoctoral researcher Hoonkyung Lee, now a professor at Konkuk University in South Korea, are co-authors of the paper. Yakobson is Rice’s Karl F. Hasselmann Professor of Mechanical Engineering and Materials Science, a professor of chemistry and a member of the Richard E. Smalley Institute for Nanoscale Science and Technology.
The Air Force Office of Scientific Research and the Welch Foundation supported the research. Calculations were performed on the National Science Foundation-supported DaVinCI supercomputer, administered by Rice’s Ken Kennedy Institute for Information Technology.

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Abstract of ACS Nano paper
We report an extensive study of the properties of carbyne using first-principles calculations. We investigate carbyne’s mechanical response to tension, bending, and torsion deformations. Under tension, carbyne is about twice as stiff as the stiffest known materials and has an unrivaled specific strength of up to 7.5 × 107 N·m/kg, requiring a force of 10 nN to break a single atomic chain. Carbyne has a fairly large room-temperature persistence length of about 14 nm. Surprisingly, the torsional stiffness of carbyne can be zero but can be “switched on” by appropriate functional groups at the ends. Further, under appropriate termination, carbyne can be switched into a magnetic semiconductor state by mechanical twisting. We reconstruct the equivalent continuum elasticity representation, providing the full set of elastic moduli for carbyne, showing its extreme mechanical performance (e.g., a nominal Young’s modulus of 32.7 TPa with an effective mechanical thickness of 0.772 Å). We also find an interesting coupling between strain and band gap of carbyne, which is strongly increased under tension, from 2.6 to 4.7 eV under a 10% strain. Finally, we study the performance of carbyne as a nanoscale electrical cable and estimate its chemical stability against self-aggregation, finding an activation barrier of 0.6 eV for the carbyne–carbyne cross-linking reaction and an equilibrium cross-link density for two parallel carbyne chains of 1 cross-link per 17 C atoms (2.2 nm).


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