Linear Carbon Chains under High-Pressure Conditions N. F. Andrade, † A. L. Aguiar, ‡ Y. A. Kim, ¶ M. Endo, § P. T. C. Freire, † G. Brunetto, ∥ D. S. Galva ̃ o, ∥ M. S. Dresselhaus, ⊥ and A. G. Souza Filho* ,† † Departamento de Física, Universidade Federal do Ceara ́ , 60455-900 Fortaleza, Ceara ́ , Brazil ‡ Departamento de Física, Universidade Federal do Piauí, 64049-550 Teresina, Piauí, Brazil ¶ School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 500-757, Korea § Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano-shi 380-8553, Japan ∥ Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas, 13083-859 Campinas, Sao Paulo, Brazil ⊥ Departments of Physics and Electrical Engineering & Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ABSTRACT: A high-pressure resonance Raman spectroscopy study of linear carbon chains encapsulated inside multiwalled carbon nanotubes (MWCNTs) is reported. While the frequencies of the tangential modes of carbon nanotubes (G band) harden as the pressure increases, the vibrational frequencies of the chain modes (around 1850 cm −1 ) decrease, thus indicating a softening of the carbon−carbon bonds in this 1D solid. Pressure-induced irreversible structural changes in the linear carbon chains are unveiled by the red shift in the vibrational modes when pressure is released. These results have been interpreted as being due to a coalescence of carbon chains, and this hypothesis is supported by state-of-the-art atomistic reactive molecular dynamics simulations. ■ INTRODUCTION The study of linear carbon chains originated from interstellar space research, and this early work is closely related to carbon nanoscience because these early investigations by Kroto historically led to the discovery of fullerenes. 1 The discovery of these fullerene carbon cages inspired conjectures about the possible existence of tubular carbon structures, 2 which were proven true by the report of multiwall carbon nanotubes (MWCNTs) by Iijima in 1991. 3 Single-walled carbon nano- tubes (SWCNTs) became known independently in 1993 after Iijima and Ichihashi 4 and Bethune et al. 5 attempted to produce MWCNTs filled with transition metals. Some years later, Smith et al. found that the core of SWCNTs could host fullerenes, thus leading to the discovery of the new hybrid carbon structure called peapods. 6 Due to their capability of encapsulating different species, the hollow cores of carbon nanotubes (CNTs) have been used as templates for confining different nanowires and molecule arrays within nanotube cores. 7,8 In this scenario, the inner space of CNTs is found to be ideal for encapsulating and stabilizing 1D solids, such as long linear carbon chains, which are not stable under ambient conditions. 9−14 We should mention that small-length carbon chains have been investigated from both the experimental and the theoretical points of view, including looking at different phenomena and properties (electronic states, vibrational modes, transport properties, and mechanical properties) and proposing alternatives for designing and measuring a variety of chain-based systems such as carbon chains terminated with Pt atoms 15 and with different aromatic end groups, 16,17 bridging graphene domains, 18 and N-doped carbon chains. 19 All these studies pointed out relevant properties for these chains that are very promising for nanoelectronics and spintronic devices. MWCNTs are the category of carbon nanotubes with the greatest potential for technological applications, such as energy storage, electronic devices, and nanocomposites, among others, due to their remarkable mechanical, thermal, and electrical Received: January 28, 2015 Revised: April 10, 2015 Article pubs.acs.org/JPCC © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.5b00902 J. Phys. Chem. C XXXX, XXX, XXX−XXX