Mechanically Induced Metal–Insulator Transition in Carbyne

Abstract

First-principles calculations for carbyne under strain predict that the Peierls transition from symmetric cumulene to broken-symmetry polyyne structure is enhanced as the material is stretched. Interpretation within a simple and instructive analytical model suggests that this behavior is valid for arbitrary 1D metals. Further, numerical calculations of the anharmonic quantum vibrational structure of carbyne show that zero-point atomic vibrations alone eliminate the Peierls distortion in a mechanically free chain, preserving the cumulene symmetry. The emergence and increase of Peierls dimerization under tension then implies a qualitative transition between the two forms, which our computations place around 3% strain. Thus, zero-point vibrations and mechanical strain jointly produce a change in symmetry resulting in the transition from metallic to insulating state. In any practical realization, it is important that the effect is also chemically modulated by the choice of terminating groups. Our findings are promising for applications such as electromechanical switching and band gap tuning via strain, and besides carbyne itself, they directly extend to numerous other systems that show Peierls distortion.Carbyne-the linear allotrope of carbon-is perhaps one of the most unusual materials due to its ultimate one-atom thinness. Although carbyne is elusively hard to prepare and has been perceived as an exotic or even completely fictitious material, the development of methods to synthesize carbon chains proceeds at a steady rate, with input from both experiments and theory. 1-7 Among the most notable recent achievements, chains with length of up to 44 atoms 8 and such complex molecular machines as carbyne-based rotaxanes 9,10 have been synthesized. This progress is driven by carbyne's attractive physical properties such as unusual electrical transport 11,12 and intriguing mechanics, 13 or its large specific area. 14 Accordingly, a better theoretical understanding of this material is becoming more and more relevant. 13 It has long ago been established by the quantum chemistry community 15 that carbyne undergoes the Peierls transition [16][17][18][19] that converts it from the cumulene (=C=C=) n to the polyyne (-C≡C-) n form. Later it has been suggested that the zero-point vibrations (ZPV) may substantially affect the Peierls instability 20 and even completely eliminate the distortion in carbyne. 21 As the symmetric and broken-symmetry forms have very distinct electronic properties (metallic and insulating, respectively), this issue becomes crucial from both the fundamental physicochemical perspective and for applications in 1D conducting systems.A whole new dimension is added to the situation by the unusual effects of stretching on carbyne that we have recently found through first-principles calculations 13 (also observed experimentally after the present study had been completed. 22 ) Specifically, stretching increases the bond length alternation (BLA, defined as the difference between the long and short bonds) and the ba...

Linear carbon chains encapsulated in multiwall carbon nanotubes: Resonance Raman spectroscopy and transmission electron microscopy studies

“…As discussed above, carbon chains are not observed when the inner core of the nanotube is large. Recently, the strain induced metal-semiconducting transition in carbon chains has been investigated [19,46]. A temperature-strain phase diagram has been predicted for describing the metal-semiconductor (cumulene-polyyene) transition in a carbon chain [46].…”

“…Recently, the strain induced metal-semiconducting transition in carbon chains has been investigated [19,46]. A temperature-strain phase diagram has been predicted for describing the metal-semiconductor (cumulene-polyyene) transition in a carbon chain [46]. At room temperature, it is expected that a strain of about 3% would stabilized the polyyne strcuture.…”

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