Nucleation of graphene precursors on transition metal surfaces: Insights from theoretical simulations

We present quantum chemical simulations demonstrating graphene precursor formation on bcc (111) transition metal surfaces during the chemical vapor deposition process. We observe that the experimentally reported positive curvature of graphene precursors is a consequence of the natural tendency toward pentagon formation during the precursor self-assembly process. Density functional theory calculations reveal that the stability of these precursors is driven by the dominance of metal-carbon σ bonding over metal-carbon π bonding at the precursor edge. These simulations show that Fe(111) catalysts facilitate precursor formation at lower carbon densities and increase precursor stabilities. However, the stronger catalyst-carbon interaction strength in the case of Fe(111) significantly promotes catalyst surface degradation. The use of more weakly interacting catalysts, such as Ni(111) and Cu(111), circumvents this issue. However, QM/MD simulations of Ni(111)-catalyzed chemical-vapor deposition (CVD) show that graphene nucleation requires a significantly higher carbon density, compared to the case of Fe(111). We propose that the performance of different transition metals with respect to catalyzing graphene growth, akin to carbon nanotube growth, correlates with the catalyst-carbon interaction strength.