Pressure-induced structural transformations on linear carbon chains encapsulated in carbon nanotubes: A potential route for obtaining longer chains and ultra-hard composites

In this paper, we report high-pressure Raman experiments (0 28 GPa) in two different systems, i.e., linear carbon chains encapsulated by multi-walled (C-n@MWCNTs) and double-walled (C-n@DWCNTs) carbon nano tubes. By running high-pressure cycles, it is observed basically two changes in the Raman spectra of chains that are the softening and disappearance of C-n modes. We attributed the irreversible redshift of the Cn band to the coalescence between adjacent chains. On the other hand, the disappearance of the Cn band at the onset of the CNT collapse pressure is assigned to the tube-chain cross-linking. This effect appears to be independent of the C-n length and the number of walls of the tubes, depending only on the innermost CNT diameter. We show that the pressure to coalesce longer chains is higher than 10 GPa. Density functional theory and molecular dynamics calculations were performed in order to support the interpretation of experimental data. The calculations show an irreversible pressure-induced softening of frequency of the confined linear carbon chain. Furthermore, molecular dynamics calculations showed that coalescence between the shorter chains occurs in a region of lower pressure than that of the longer chains, thus supporting the experimental observations. Our findings shed new light on the understanding of the C-n@CNT system stability and pave the way for using high-pressure to obtain ultra-long chains (at lower pressures) and ultra-hard composites (at higher pressures).

Automated summary

In this paper, high-pressure Raman experiments were conducted on linear carbon chains encapsulated by multi-walled and double-walled carbon nanotubes. The experiments revealed the softening and disappearance of carbon chain modes under high pressure, which were attributed to chain coalescence and tube-chain cross-linking. Computational simulations supported the experimental observations and suggested the possibility of obtaining special structures of carbon chains and composites under high pressure.