Five-Membered Loop-Initiated Nucleation of Unlayered Graphene Sheets in a Carbon Melt, with Diffusion Barrier Possibilities

A subset of isotopically “presolar” carbon particles extracted from the Murchison meteorite contain signatures expected of particles formed in the atmosphere of red giant stars. Some of these micron-size particles have spherical cores that show diffraction rings from atom-thick graphene, possibly formed by solidification of liquid carbon at low pressure. Electron phase-contrast transmission electron microscopy (TEM) images suggest that these cores originate from supercooled carbon droplets that formed graphene sheets on randomly oriented 5-membered loops. In addition to presolar data, laboratory synthesis in an “evaporating carbon oven” creates similar core-rim structures by slow cooling of carbon vapor. In research studies, it was shown that 5-membered loops are essential to the initiation of carbon nanotubes on catalyst particles. In addition to offering this experimental context, we present density functional theory (VASP) computer simulations suggesting that 5-member loops are more likely than 6-member loops in a solidifying carbon melt. These things suggest that 5-member loops compete effectively as nucleation seeds for explaining the faceted pentacones inferred from TEM images of presolar particle cores. In that context, pent-first nucleation (along with the crowding of growing sheets by nearby liquid atoms) may reduce the chances of graphite layer formation and lead to unprecedented diffusion barrier properties for this composite material.