  An operando investigation of graphene growth on (100) grains of
polycrystalline nickel (Ni) surfaces was performed by means of
variable-temperature scanning tunneling microscopy complemented by density
functional theory simulations. A clear description of the atomistic mechanisms
ruling the graphene expansion process at the stepped regions of the substrate
is provided, showing that different routes can be followed, depending on the
height of the steps to be crossed. When a growing graphene flake reaches a
monoatomic step, it extends jointly with the underlying Ni layer; for higher Ni
edges, a different process, involving step retraction and graphene landing,
becomes active. At step bunches, the latter mechanism leads to a peculiar
'staircase formation' behavior, where terraces of equal width form under the
overgrowing graphene, driven by a balance in the energy cost between C-Ni bond
formation and stress accumulation in the carbon layer. Our results represent a
step towards bridging the material gap in searching new strategies and methods
for the optimization of chemical vapor deposition graphene production on
polycrystalline metal surfaces.
