  As a two-dimensional material, graphene can be naturally obtained via
epitaxial growth on a suitable substrate. Growth condition optimization usually
requires an atomistic level understanding of the growth mechanism. In this
article, we perform a mechanistic study about graphene growth on Ir(111)
surface by combining first principles calculations and kinetic Monte Carlo
(kMC) simulations. Small carbon clusters on the Ir surface are checked first.
On terraces, arching chain configurations are favorable in energy and they are
also of relatively high mobilities. At steps, some magic two-dimensional
compact structures are identified, which show clear relevance to the nucleation
process. Attachment of carbon species to a graphene edge is then studied. Due
to the effect of substrate, at some edge sites, atomic carbon attachment
becomes thermodynamically unfavorable. Graphene growth at these difficult sites
has to proceed via cluster attachment, which is the growth rate determining
step. Based on such an inhomogeneous growth picture, kMC simulations are made
possible by successfully separating different timescales, and they well
reproduce the experimentally observed nonlinear kinetics. Different growth
rates and nonlinear behaviors are predicted for different graphene
orientations, which is consistent with available experimental results.
Importantly, as a phenomenon originated from lattice mismatch, inhomogeneity
revealed in this case is expected to be quite universal and it should also make
important roles in many other hetero-epitaxial systems.
