Tuesday, 25 November 2014
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Stretched carbyne provides mechanical activation

Tension applied to a chain of carbon atoms can turn it from a metallic conductor to an insulator, claim Rice University scientists.

Stretching carbyne - a hard-to-make, one-dimensional chain of carbon atoms - by three per cent can begin to change its properties in ways that engineers might find useful for mechanically activated nanoscale electronics and optics.

The finding by Rice theoretical physicist Boris Yakobson and his colleagues appears in Nano Letters.

What we realised 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

According to the University, carbyne has until recently existed mostly in theory, though experimentalists have made some headway in creating small samples of the material. The carbon chain would theoretically be the strongest material ever if it could be made 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.

In their previous work on carbyne, the researchers believed they saw hints of the transition, but they had to further investigate 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 in a statement. ‘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, said Yakobson

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 three per cent 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 realisation 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 realised 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.’


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