Molecular decomposition reactions and early nucleation in CVD growth of graphene on Cu and Si substrates from toluene

Graphene growth from chemical vapor deposition (CVD) commonly employs methane (CH4) as carbon precursor but requires temperatures in excess of 900°C. Aromatic hydrocarbons, especially toluene (C7H8), may lower the growth temperatures well below 600°C, while preserving graphene quality; however, molecular decomposition reactions and early nucleation steps of CVD graphene using toluene are not known in details. We investigate the decomposition steps of toluene adsorbed onto Cu(111) and c(4×2)-reconstructed Si(100) surfaces through DFT calculations. The geometry and energy of toluene and most likely decomposition by-products were analyzed for various adsorbate structural configurations. Early decomposition reactions were studied through investigation of minimum energy pathways and transition states. Low activation energies were found for H removal from the methyl group of toluene physisorbed on Cu(111) (1.20 eV) or chemisorbed on Si(100) (1.39 eV), leading to the formation of benzyl radicals; further dehydrogenation reactions of the latter lead to C7H6, C7H5 and C7H4 fragments, their formation being energetically feasible on Cu (energy barriers in the 0.87-1.62 eV range) but not on Si. These radicals may act as active species in the formation of sp2-bonded carbon nuclei during CVD growth of graphene. Anthracene (C14H10) formation from two close-by C7H5 radicals has been studied through meta-dynamics and umbrella sampling methods applied to molecular dynamics simulations. Preliminary results indicate that zig-zag anthracene could easily form onto Cu(111), the energy barriers being below 1.1 eV. A prohibitive energy was instead obtained for (100)Si, hindering anthracene formation on this substrate under practical CVD conditions.