Tag Archives: Mingjie Liu

Carbyne stretches from theory to reality and reveals its conundrum-type self

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,

[downloaded from http://pubs.acs.org/doi/abs/10.1021/nl5017317]

[downloaded from http://pubs.acs.org/doi/abs/10.1021/nl5017317]

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.

Carbyne vs. graphene

Rice University (Texas, US) researchers are announcing a new carbon-based nanomaterial, carbyne, according an Oct. 9, 2013 news release,

Carbyne will be the strongest of a new class of microscopic materials if and when anyone can make it in bulk.

If they do, they’ll find carbyne nanorods or nanoropes have a host of remarkable and useful properties, as described in a new paper by Rice University theoretical physicist Boris Yakobson and his group. The paper appears this week in the American Chemical Society journal ACS Nano.

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 that have a top and a bottom or hollow nanotubes that have an inside and outside.

According to the portrait drawn from calculations by 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 had already 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.

That’s a remarkable set of qualities for a simple string of carbon atoms, Yakobson said.

“You could look at it as an ultimately thin graphene ribbon, reduced to just one atom, or an ultimately thin nanotube,” he 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.

“Regardless of the applications,” he said, “academically, it’s very exciting to know the strongest possible assembly of atoms.”

Based on the calculations, he said carbyne might be the highest energy state for stable carbon. “People usually look for what is called the ‘ground state,’ the lowest possible energy configuration for atoms,” Yakobson said. “For carbon, that would be graphite, followed by diamond, then nanotubes, then fullerenes. But nobody asks about the highest energy configuration. We think this may be it, a stable structure at the highest energy possible.”

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.

“I have always been interested in the stability of ultimately thin wires of anything and how thin a rod you could make from a given chemical,” Yakobson said. “We had a paper 10 years ago about silicon in which we explored what happens to silicon nanowire as it gets thinner. To me, this was just a part of the same question.”

The Rice researchers, led by Rice graduate student Mingjie Liu and postdoctoral researcher Vasilii Artyukhov, were aware of a number of papers that described one property or another of carbyne. They set out to detail carbyne with computer models using first-principle rules to determine the energetic interactions of atoms, Artyukhov said.

“Our intention was to put it all together, to construct a complete mechanical picture of carbyne as a material,” Artyukhov said. “The fact that it has been observed tells us it’s stable under tension, at least, because otherwise it would just fall apart.”

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. “We didn’t look for this, specifically; it came up as a side product.”

“That’s the good thing about studying things carefully,” Artyukhov 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. “They would look like butterfly wings,” Artyukhov said.

“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.” Artyukhov said the nominal specific area of carbyne is about five times that of graphene.

When the team’s paper became available this summer on arXiv, the scientific press and even some of the popular press were so excited over the calculations that they picked up on the paper and its implications before the team submitted it for peer review. Now that the complete paper is ready for public consumption, the researchers said they’ll carry their investigation in new directions.

They’re taking a more rigorous look at the conductivity of carbyne and are thinking about other elements as well. “We’ve talked about going through different elements in the periodic table to see if some of them can form one-dimensional chains,” Yakobson said.

Given Rice’s prominence in the nanotechnology field and its status as the ‘home’ of the fullerene, aka, buckyballs, this discovery reaffirms the university’s standing.

Here’s a ‘carbyne’ image the researchers have provided,

Rice University researchers have determined from first-principle calculations that carbyne would be the strongest material yet discovered. The carbon-atom chains would be difficult to make but would be twice as strong as two-dimensional graphene sheets. (Credit: Vasilii Artyukhov/Rice University)

Rice University researchers have determined from first-principle calculations that carbyne would be the strongest material yet discovered. The carbon-atom chains would be difficult to make but would be twice as strong as two-dimensional graphene sheets. (Credit: Vasilii Artyukhov/Rice University)

Here’s a citation for and a link to the paper,

Carbyne From First Principles: Chain of C atoms, a Nanorod or a Nanorope by Mingjie Liu , Vasilii I. Artyukhov , Hoonkyung Lee , Fangbo Xu , and Boris I. Yakobson. ACS Nano, Just Accepted Manuscript DOI: 10.1021/nn404177r Publication Date (Web): October 5, 2013
Copyright © 2013 American Chemical Society

This paper is behind a paywall.

ETA Oct. 9, 2013 at 4:40 pm PDT: I forgot to follow through on my headline which refers to graphene, the current ‘wonder’ nanomaterial, and its possible future dethroning by carbyne. I think it unlikely as part of what makes graphene so attractive are the properties that could allow its use in electronics applications, properties which carbyne does not seem to share.