Carbon nanohorns under cold compression to 40 GPa: Raman scattering and X-ray diffraction experiments

We report a high-pressure behavior of carbon nanohorns (CNHs) to 40GPa at ambient temperature by in situ Raman spectroscopy and synchrotron radiation x-ray diffraction (XRD) in a diamond anvil cell. In Raman measurement, multiple structural transitions are observed. In particular, an additional band at similar to 1540cm(-1) indicative of sp(3) bonding is shown above 35GPa, but it reverses upon releasing pressure, implying the formation of a metastable carbon phase having both sp(2) and sp(3) bonds. Raman frequencies of all bands (G, 2D, D+G, and 2D') are dependent upon pressure with respective pressure coefficients, among which the value for the G band is as small as similar to 2.65 cm(-1) GPa(-1) above 10 GPa, showing a superior high-pressure structural stability. Analysis based on mode Gruneisen parameter demonstrates the similarity of high-pressure behavior between CNHs and single-walled carbon nanotubes. Furthermore, the bulk modulus and Gruuneisen parameter for the G band of CNHs are calculated to be similar to 33.3 GPa and 0.1, respectively. In addition, XRD data demonstrate that the structure of post-graphite phase derives from surface nanohorns. Based on topological defects within conical graphene lattice, a reasonable transformation route from nanohorns to the post-graphite phase is proposed. Published by AIP Publishing.