DFT studies of graphene in carbon droplets condensed in stellar atmospheres

Elemental carbon at low (ambient) pressure sublimates to vapor near 4000K, but liquid carbon is reported after laser ablation. Some meteoritic carbon particles, formed in red giant atmospheres, show a “graphene-core”/graphite-rim structure likely from super-cooled carbon droplets that nucleated graphene sheets on randomly-oriented 5-member rings. Similar core-rim particles form by slow cooling of carbon vapor in the lab (HAL-02238804). Our computations target growth of carbon rings & graphene sheets at the experimental 1.8 g/cc density estimate, by relaxing random liquid-like configurations of 13/20-atom clusters in a supercell. Inter-atom distances characteristic of covalent vs. metallic interactions (with a gap in 1.7-2 Å range) allow us to identify covalent “bonds” with small separation. Local energy minima at T = 0K show sp² & sp coordination numbers, as in the literature. Ring sizes vary from triangle to heptagon, but pentagons are more abundant than hexagons, also consistent with previous reports. Work remains to see if pent-loops can nucleate the growth of faceted pentacones, as suggested by HRTEM imaging. Unlayered graphene sheets in a frozen matrix may be an effective diffusion barrier.