Graphene etching on SiC grains as a path to interstellar polycyclic aromatic hydrocarbons formation


Polycyclic aromatic hydrocarbons appear among the most abundant observed species in interstellar space and are key molecules to understanding the prebiotic roots of life. However, their existence and abundance in space remain a puzzle. We have shown that aromatic species can be efficiently formed on the graphitized surface of the abundant silicon carbide stardust on exposure to atomic hydrogen under pressure and temperature conditions analogous to those of the interstellar medium, mimicked here. 

The steps for the formation of interstellar polyaromatic hydrocarbons (PAH) and aromatic-aliphatic molecules in the envelope of an evolved star can be divided into four different stages. (1) Formation of SiC in the gas phase and condensation into micrometre- and nanometre-sized grains (T = 2 000 K; 1–5 R*). (2) Annealing of the SiC dust grains due to the proximity of the star and the consequent promotion of surface C-rich phases and graphene (T = 2 000–1 500 K, 1–5 R*). (3) Exposure of the surface to atomic hydrogen, promoting graphitization of the C-rich surface and H passivation of the underlying buffer layer (T = 1 500–1 200 K; 5–20 R*). (4) Etching of the graphene by atomic hydrogen and thermally assisted desorption of PAHs (T = 1 200–1 000  K, 5–20 R*).