DFT investigation of active carbon species and early nucleation of graphene on Cu(111) using toluene as CVD carbon precursor

Liquid aromatic hydrocarbons may be used as alternative precursors in the chemical vapor deposition of graphene, leading to reduced growth temperatures with respect to those required by methane. We study by density functional theory the dehydrogenation steps of toluene precursor adsorbed on Cu(111). Dehydrogenations of toluene - at the methyl or ortho positions - are characterized by low activation energies and result in the formation of multiple active species, leading to the abundant production (among others) of adsorbed C7H5 fragments. We extend our analysis to provide a molecular-level description of anthracene (C14H10) formation - as early graphene nuclei - by recombination reactions between two C7H5 fragments. In particular, the formation of zig-zag or armchair anthracene is facilitated by the almost inexpensive energetic cost for the creation of a C–C double bond between the C7H5 fragments; however, its formation could be hindered for specific relative orientations of the two adsorbed species, for which steric hindrance effects strongly impede internal rotations of the reaction intermediate. Finally, the dehydrogenation of anthracene elucidates that vertically chemisorbed configurations make them more prone to lose their hydrogens. This may lead to further condensation reactions and the formation of larger graphene nuclei. These results open the way to a better understanding of surface adsorption, decomposition and recombination phenomena for clear control of the nucleation of high-quality graphene layers by CVD.