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.