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futurity_sci_tech 00237.txt

#Stretching turns carbyne into an insulator Rice university rightoriginal Studyposted by Mike Williams-Rice on July 28 2014 Applying just the right amount of tension to a chain of carbon atoms can turn it from a metallic conductor to an insulator report researchers. 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. 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 Rice university theoretical physicist Boris Yakobson and his coauthors 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 spaced more or less evenly with two bonds between them. But the atoms are never static due to natural quantum uncertainty which Yakobson says keeps them from slipping into a less-stable Peierls distortion. eierls said one-dimensional metals are unstable and must become semiconductors or insulatorsyakobson says. ut it's not that simple because there are two driving factors. ne the Peierls distortion ants to open the gap that makes it a semiconductor. The other called zero-point vibration (ZPV) ants to maintain uniformity and the metal state. akobson explains that ZPV is a manifestation of quantum uncertainty which says atoms are always in motion. t s more a blur than a vibrationhe says. e can say carbyne represents the uncertainty principle in action because when it s relaxed the bonds are confused constantly between 2-2 and 1-3 to the point where they average out and the chain remains metallic. ut 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 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 says. ifferent bond connectivity patterns can affect the metallic/dielectric state balance and shift the transition point potentially to where it may not be accessible anymorehe says. o one has to be extremely careful about making the contacts.?Carbyne s structure is a conundrumhe says. ntil this paper everybody was convinced it was single-triple with a long bond then a short bond caused by Peierls instability. He says 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 didartyukhov says. owever 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. hat 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. akobson notes 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. The Robert Welch Foundation the US Air force Office of Scientific research and the Office of Naval Research Multidisciplinary University Research Initiative supported the research which appears in the journal Nano Letters. The researchers used the Data analysis and Visualization Cyberinfrastructure (DAVINCI) supercomputer supported by the NSF and administered by Rice s Ken Kennedy Institute for Information technology. Source: Rice Universityyou are free to share this article under the Creative Commons Attribution-Noderivs 3. 0 Unported license m

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