Rice University (Texas, US) scientists have taken a rather difficult material, carbyne, and twisted it to reveal new properties according to a July 21, 2014 news item on ScienceDaily,
Applying just the right amount of tension to a chain of carbon atoms can turn it from a metallic conductor to an insulator, according to Rice University scientists.
Stretching the material known as carbyne — a hard-to-make, one-dimensional chain of carbon atoms — by just 3 percent can begin to change its properties in ways that engineers might find useful for mechanically activated nanoscale electronics and optics.
A July 21, 2014 Rice University news release (also on EurekAlert), which originated the news item, describes carbyne and some of the difficulties the scientists addressed in their research on the material,
Until recently, carbyne has existed mostly in theory, though experimentalists have made some headway in creating small samples of the finicky material. The carbon chain would theoretically be the strongest material ever, if only someone could make it reliably.
The first-principle calculations by Yakobson and his co-authors, Rice postdoctoral researcher Vasilii Artyukhov and graduate student Mingjie Liu, show that stretching carbon chains activates the transition from conductor to insulator by widening the material’s band gap. Band gaps, which free electrons must overcome to complete a circuit, give materials the semiconducting properties that make modern electronics possible.
In their previous work on carbyne, the researchers believed they saw hints of the transition, but they had to dig deeper to find that stretching would effectively turn the material into a switch.
Each carbon atom has four electrons available to form covalent bonds. In their relaxed state, the atoms in a carbyne chain would be more or less evenly spaced, with two bonds between them. But the atoms are never static, due to natural quantum uncertainty, which Yakobson said keeps them from slipping into a less-stable Peierls distortion.
“Peierls said one-dimensional metals are unstable and must become semiconductors or insulators,” Yakobson said. “But it’s not that simple, because there are two driving factors.”
One, the Peierls distortion, “wants to open the gap that makes it a semiconductor.” The other, called zero-point vibration (ZPV), “wants to maintain uniformity and the metal state.”
Yakobson explained that ZPV is a manifestation of quantum uncertainty, which says atoms are always in motion. “It’s more a blur than a vibration,” he said. “We can say carbyne represents the uncertainty principle in action, because when it’s relaxed, the bonds are constantly confused between 2-2 and 1-3, to the point where they average out and the chain remains metallic.”
But stretching the chain shifts the balance toward alternating long and short (1-3) bonds. That progressively opens a band gap beginning at about 3 percent tension, according to the computations. The Rice team created a phase diagram to illustrate the relationship of the band gap to strain and temperature.
How carbyne is attached to electrodes also matters, Artyukhov said. “Different bond connectivity patterns can affect the metallic/dielectric state balance and shift the transition point, potentially to where it may not be accessible anymore,” he said. “So one has to be extremely careful about making the contacts.”
“Carbyne’s structure is a conundrum,” he said. “Until this paper, everybody was convinced it was single-triple, with a long bond then a short bond, caused by Peierls instability.” He said the realization that quantum vibrations may quench Peierls, together with the team’s earlier finding that tension can increase the band gap and make carbyne more insulating, prompted the new study.
“Other researchers considered the role of ZPV in Peierls-active systems, even carbyne itself, before we did,” Artyukhov said. “However, in all previous studies only two possible answers were being considered: either ‘carbyne is semiconducting’ or ‘carbyne is metallic,’ and the conclusion, whichever one, was viewed as sort of a timeless mathematical truth, a static ‘ultimate verdict.’ What we realized here is that you can use tension to dynamically go from one regime to the other, which makes it useful on a completely different level.”
Yakobson noted the findings should encourage more research into the formation of stable carbyne chains and may apply equally to other one-dimensional chains subject to Peierls distortions, including conducting polymers and charge/spin density-wave materials.
According to the news release the research was funded by the U.S. Air Force Office of Scientific Research, the Office of Naval Research Multidisciplinary University Research Initiative, and the Robert Welch Foundation. (I can’t recall another instance of the air force and the navy funding the same research.) In any event, here’s a link to and a citation for the paper,
Mechanically Induced Metal–Insulator Transition in Carbyne by Vasilii I. Artyukhov, Mingjie Liu, and Boris I. Yakobson. Nano Lett., Article ASAP DOI: 10.1021/nl5017317 Publication Date (Web): July 3, 2014
Copyright © 2014 American Chemical Society
This paper is behind a paywall.
The researchers have provided an image to illustrate their work,I’m not sure what the bird is doing in the image but it caught my fancy. There is another less whimsical illustration (you can see it in the July 21, 2014 news item on ScienceDaily) and I believe the same caption can be used for the one I’ve chosen from the journal’s abstract page, “Carbyne chains of carbon atoms can be either metallic or semiconducting, according to first-principle calculations by scientists at Rice University. Stretching the chain dimerizes the atoms, opening a band gap between the pairs. Credit: Vasilii Artyukhov/Rice University.”
I last wrote about carbyne in an Oct. 9, 2013 posting where I noted that the material was unlikely to dethrone graphene as it didn’t appear to have properties useful in electronic applications. It seems the scientists have proved otherwise, at least in the laboratory.