Superstrong Carbon Chains | Chemical & Engineering News
Volume 91 Issue 43 | pp. 28-29 | Concentrates
Issue Date: October 28, 2013

Superstrong Carbon Chains

Computational analysis predicts carbon chains with alternating single and triple bonds could have novel properties
Department: Science & Technology
News Channels: Materials SCENE, Nano SCENE
Keywords: allotropes, computational chemistry, nanomaterials, carbyne
Carbyne could be a stronger material than graphene and carbon nanotubes.
Credit: Vasilii I. Artyukhov/Rice University
Carbyne—consisting of a chain of single- and triple-bonded carbon atoms—could be the strongest material known, according to theoretical calculations.
Carbyne could be a stronger material than graphene and carbon nanotubes.
Credit: Vasilii I. Artyukhov/Rice University

One-dimensional strings of carbon atoms should be stronger than any known material, according to a theoretical study (ACS Nano 2013, DOI: 10.1021/nn404177r). The carbon allotrope, called carbyne, consists of chains of carbon atoms linked by alternating single and triple bonds. Researchers have previously synthesized polyyne chains of up to 44 atoms in length. Longer chains, as yet unsynthesized, would enable experimental verification of these predictions. Rice University chemist Boris I. Yakobson and colleagues calculated carbyne’s tensile strength by examining what happens when the distance between two carbon atoms increases, as would happen if a chain were stretched. The chemists also calculated how much force it could endure before becoming unstable. From those calculations, they predict that the tensile stiffness of carbyne should be twice that of graphene and carbon nanotubes. They also predict that carbyne could have novel electrical and magnetic properties. For example, twisting the carbon chain 90º from its normal state would transform it into a magnetic semiconductor, which could be useful in digital memory devices.

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Sergey Evsyukov (November 11, 2013 7:44 AM)
The recent frenzy in media about theoretical findings by Yakobson et al. [1] on predicted mechanical properties of carbyne is really both amazing and amusing. As it frequently happens, new things are well-forgotten old ones. As far back as 1975, Perepelkin published theoretical calculations predicting defect-free filamentous carbyne crystals to be strongest of all known and even feasible materials [2]. On the other hand, the reason for euphoria was, still is, and will for unforeseeable time be rather phantasmal. In fact, Sir Harold Kroto said it all in his short essay, having argued that "The existence of carbyne is myth based on bad science and perhaps even wishful thinking" [3]. And Sir Harold is certainly right, particularly if we think of carbyne as a wisp of 'endless', one-dimensionally ordered polyyine and/or cumulene carbon chains ('pencils in a box'). Unfortunately, the most of carbon and carbonaceous materials reported so far and interpreted to be carbyne have in my opinion nothing to do with this structural model, but can rather be assigned to a group of carbynoid materials, a comfortable term coined to combine all that poorly defined stuff, routinely coming from the labs. Carbynoids are generally defined as carbon-rich chain-like poly(or oligo)mers with both polyyne- and cumulene-type moieties, extended interchain cross-linking as well as end-capping and/or pendant side groups. Naturally, carbonaceous materials containing short polyyne or cumulene sequences embedded in whatever as a matrix cannot be considered as an individual allotropic form of carbon.
From mid 80s until late 90s I was closely involved in the carbyne story for some fifteen years, although I had never been a rabid fan of the 'pencils in a box' concept. In an attempt of critically summing up all what was known at that time and to understand the ways to go, we published a review [4] followed by a book [5] on carbyne and carbynoid structures. Having analyzed a lot of publications, I have to admit that wishful thinking did and probably still does play a special role, even in our own early studies. In the preface to the book we wrote in 1998: "Despite a host of papers dealing with different aspects of the physics and chemistry of carbyne, many questions remain to be solved. We believe that a critical assessment of these papers is required but we also emphasize that continuing efforts will eventually result in carbyne single crystals large enough to permit in-depth structural analysis. It is thus not the intention of this text to reveal completely new and stunning research results on carbyne but to provide a rather complete, ordered and concise summary of what is known with certainty today and what is still ambiguous or downright wrong. Hence this book should be considered an important stepping stone towards an understanding of carbyne, an aspect many researchers around the world have attempted hitherto. It will show the research community what has been confirmed, what is still doubtful, what is or was wrong, and most importantly, where do we go from here." Now I have to admit that today, fifteen years later, we could write the same once again. In spite of significant advances made recently in both physical [6] and chemical syntheses [7], no real breakthrough has happened in these years, while other scientists kept baking the same stuff, obstinately calling it carbyne. On the other hand, the notorious reactivity of sp-carbon chains has unambiguously been confirmed [8].
The main problem with carbyne (whatsoever we mean by using this term, excluding a homonymous herbicide) was and still is the embarrassing lack of reasonably large single crystals for a clear-cut X-ray structural analysis. Various structural models of carbyne have been proposed to date, and some of them are based on a layered lattice, wherein the layers are oriented transversely to the chain axis [6a-e]. Such a structure seems to be incompatible with extreme strength predicted by theoreticians. So, summing up, we have to wait for arrival of carbyne-anchored space elevators for quite a while [9].
1. M. Liu, et al.: Carbyne from first principles: Chain of C atoms, a nanorod or a nanorope? // a). ACS Nano, 2013, Ahead of print, Publication date (Web) October 5, 2013. DOI: 10.1021/nn404177r; b)., e-Print Archive, Condensed Matter, (2013), 19 pp., arXiv:1308.2258 [cond-mat.mtrl-sci].
2. a). K.E. Perepelkin, et al.: Evaluation of the ultimate mechanical properties for carbyne, a carbon chain polymer // Dokl. Akad. Nauk SSSR, 1975, 220 (6), 1376-1379 (Russ.); b). Fibres and fibrous materials for reinforcing composites with extreme characteristics // Mechanics of Composite Materials, 1992, 28 (3), 195-208 [Transl. from Mekhanika Kompozitnykh Materialov, 1992, (3), 291-306 (Russ.)]; c). Theory of extremal mechanical and thermal properties of fibres and needle crystals. Comparison with experimental data // Fibre Chemistry, 2004, 36 (4), 237-248 [Transl. from Khim. Volokna, 2004, (4), 3-11 (Russ.)].
3. H. Kroto: Carbyne and other myths about carbon // RSC Chemistry World, 2010, November.
4. Yu.P. Kudryavtsev, et al.: Carbynes: Advances in the field of linear carbon chain compounds // J. Mater. Sci., 1996, 31 (21), 5557-5571.
5. Carbyne and Carbynoid Structures, ed. by R.B. Heimann, S.E. Evsyukov, and L. Kavan, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1999, 452 pp.
6. a). J.G. Korobova, et al.: The structural properties of the sp1-carbon based materials: Linear carbon chains, carbyne crystals and a new carbon material - two dimentional ordered linear-chain carbon // in: Carbon Nanomaterials in Clean Energy Hydrogen Systems - II, NATO Science for Peace and Security Series C: Environmental Security, 2011 (©2008), Volume 2, Springer, Dordrecht, pp. 469-485; b). V.G. Babaev, et al.: Laser-assisted synthesis of carbyne from graphite and amorphous carbon // Nanotekhnologii: Razrabotka, Primenenie [Nanotechnologies: Development, Applications], 2010, (1), 88-94 (Russ.); c). Films of linear chain-like carbon - ordered assemblies of quantum wires - material for nanoelectronics // Ibid., 53-68; d). Yu.P. Kudryavtsev, et al.: Carbyne - the third allotropic form of nanocarbon // Ibid., 37-52; e). V.G. Babaev, et al.: Carbon material with a highly ordered linear-chain structure // in: Polyynes: Synthesis, Properties, and Applications, ed. by F. Cataldo, CRC Press, Boca Raton, 2006, pp. 219-252; f). L. Ravagnan, et al.: Synthesis and characterization of carbynoid structures in cluster-assembled carbon films // Ibid., pp. 15-35; g). G. Bongiorno, et al.: Electronic properties and applications of cluster-assembled carbon films // J. Mater. Sci.: Materials in Electronics, 2006, 17 (6), 427-441.
7. a). W.A. Chalifoux and R.R. Tykwinski: Synthesis of extended polyynes: Toward carbyne // Comptes Rendus Chimie, 2009, 12 (3-4), 341-358; b). Synthesis of polyynes to model the sp-carbon allotrope carbyne // Nature Chemistry, 2010, 2 (11), 967-971.
8. a). L. Ravagnan, et al.: sp Hybridization in free carbon nanoparticles - presence and stability observed by near edge X-ray absorption fine structure spectroscopy // Chem. Commun., 2011, 47 (10), 2952–2954; b). C.S. Casari, et al.: Gas exposure and thermal stability of linear carbon chains in nanostructured carbon films investigated by in situ Raman spectroscopy // Carbon, 2004, 42 (5-6), 1103-1106.
9. T. Shelley: Fifth element makes the ultimate material // Eureka, 1999, 19 (8), 24-25.

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