The results show that the adsorption energies of Cu adatom on graphene at high and low coverage are both positive, i.e., 0.98 eV and 3.10 eV, respectively, much higher than that on Pt surfaces. Cu adatom would have weak interaction with graphene and would prefer to cluster rather than form uniform films on the graphene surface. To verify that Cu indeed has a higher tendency to cluster on graphene surface, the interaction energy 𝜔 between adatoms is further calculated by: