No one debates that researchers at Sun Yat-sen University have created interesting carbonaceous crystals. It’s the name that they’re using to describe those crystals that’s controversial: carbyne.
The controversy stems from the formal definition of the word itself, or the lack thereof.
Researchers agree that carbyne is a one-dimensional string of carbon atoms, or a crystallized collection of these strings. But that definition permits enough semantic uncertainty and scientific ambiguity to allow for contention.
Some carbon chemists argue that truly pure carbyne cannot exist. Other researchers believe it has already been synthesized. The recent report out of Sun Yat-sen highlights the disagreement among those trying to synthesize this potentially dangerous substance and study its unique properties.
“You could ask 10 different people to define carbyne and get 10 different answers,” says Rik R. Tykwinski, a physical organic chemist at Friedrich-Alexander University, Erlangen-Nuremberg. Tykwinski’s group made news several years ago when it reported synthesizing 1-D chains of up to 44 carbon atoms, setting a new record. These chains are polyynes, however, not carbyne, Tykwinski says.
To Tykwinski, carbyne is a material whereas polyyne is a molecule. The distinction comes down to the chain length. A polyyne chain is short enough that adding carbon atoms to it would change its properties, thermally, spectroscopically, or otherwise. Adding carbons to the chains in carbyne would change nothing, he explains. Tykwinski thinks one of the alluring mysteries of carbyne is unraveling how many carbons it takes to hit that threshold.
Carbyne is also attractive from a materials science standpoint. Computational studies suggest that the material would be incredibly strong and that its electronic band gap could be tuned by bending or compressing it.
But carbyne also represents a void in carbon chemistry. Carbon atoms hybridize into sp3 orbitals to form tetrahedral bonds with four partners in three dimensions to create diamond. The atoms assemble into the familiar, flat hexagons of graphite and graphene by bonding with three neighbors in the same plane using sp2 orbitals. But pure carbon has yet to line up into stable, sp-hybridized 1-D structures.
“Long polyynes are, in principle, not stable,” Tykwinski explains. Polyyne chains want to cross link, and when they do, they form something that is not carbyne, he says. Furthermore, that cross-linking is extremely energetic. A report from the early 1950s documents polyyne-like molecules causing explosions in the absence of air, Tykwinksi says. “These molecules can be dangerous as hell.”
Harold W. Kroto agrees. Kroto, who earned a share of the 1996 Nobel Prize in Chemistry for the discovery of carbon fullerenes, tells C&EN he does not believe the carbyne claim from the Sun Yat-sen researchers, in part, because the substance didn’t explode. “I know that they can’t have made carbyne because they are still alive,” he says.
Researchers like Tykwinski are building toward carbyne by making longer and longer polyyne chains and capping the ends of the molecules with large, stabilizing groups to prevent cross-linking. But some chemists would argue that chains with large end caps cannot even be considered carbyne.
Yet researchers led by Guowei Yang at Sun Yat-sen have reported synthesizing carbyne crystals without building up increasingly long polyyne chains. Instead, the team has directly produced crystals using pulsed laser light, Yang says (Sci. Adv. 2015, DOI: 10.1126/sciadv.1500857).
The team fired nanosecond laser pulses at a gold target submerged in ethanol. This creates a plasma in the liquid immediately above the target, where gold ions can dehydrogenate the alcohol and allow carbon atoms to bond into linear chains. These chains condense into nanoscopic crystals adorned with gold nanoparticles. The researchers synthesized enough material to obtain a white powder, which Yang says is stable, much like graphite and diamond.
The researchers characterized this powder with various spectroscopic techniques and say the results are consistent with the expected spectroscopic signatures of 1-D carbon chains held together by alternating single and triple bonds.
Yang stresses that the crystalline substance his group has created is the solid-state phase of carbyne, not polyyne. Carbyne condenses into inorganic crystals, Yang argues, whereas polyyne usually becomes an organic polymer.
The synthesis method is also safe, Yang says. The laser-growth method creates extreme thermodynamic conditions capable of creating carbyne only in the immediate vicinity of where the laser strikes the target, he explains. The experimental setup itself is kept in air at standard temperature and pressure. Yang adds that this marks the first time carbyne has been created in a lab in ambient conditions.
“What’s neat about this study, from my perspective, is it confirms that laser heating provides a mechanism for forming these structures,” says Nir Goldman, a physical chemist at Lawrence Livermore National Laboratory, who was not involved in the study.
Goldman recently performed computer simulations that demonstrated bombarding graphite with laser light could also create carbyne fibers. The thermodynamic conditions he observed in his study are likely similar to what Yang’s group created in the lab, Goldman says.
Although Goldman states he’s not an expert on carbon material nomenclature, he thinks “carbyne-like material” is a fair label for the structures made by Yang’s team. He is curious to see whether these newly synthesized crystals exhibit the electrical properties expected of carbyne. Still, he understands the reluctance other scientists might have to christening the substance as such.
“I think some skepticism is warranted,” he explains. “But this is still a very neat result that could point the way to synthesizing carbyne in a highly controlled way.”
“The existence of carbyne is myth based on bad science and perhaps even wishful thinking,” Kroto wrote five years ago in Chemistry World. The new report, which he calls “total nonsense,” has done nothing to change his mind.
It cites an old phase diagram that purportedly represents carbyne, Kroto says, but that claim was shown to be erroneous decades ago. The report also includes a nuclear magnetic resonance spectrum showing that the crystalline material created by Yang’s team is not a 1-D allotrope of carbon, according to Kroto.
The spectrum basically contains one chemical shift that appears as a spike on the plot, he says. “We know where the carbon shifts should be. They should be all over the place.” He adds that this study should not have been published.
Yang disagrees, arguing that “we have provided substantial and definitive evidence” of carbyne. Myth has become a reality in his lab, he says.
The journal that published the study, Science Advances, likewise stands behind the new report. The report from Yang’s team was rigorously assessed and deemed to fit the journal’s goal of enhancing the amount of outstanding research that reaches the scientific community, says Marcia McNutt, editor-in-chief for the Science family of journals.
For his part, Tykwinski doesn’t call the new substance carbyne. Current spectroscopic techniques can’t resolve whether there are cross-links that make the difference between sexy carbyne and more pedestrian polyyne material, he says.
“We’re probably talking semantics,” he concedes. “But if you want to say you have crystalline carbyne, then you need to have crystalline carbyne.”
Whether or not carbon's 1-D allotrope has been synthesized—or even can be synthesized—depends on how researchers define carbyne. C&EN asked four carbon researchers to tell us what carbyne is (or should be).