Direct capture and electrostatic repulsion in the self-assembly of rare-earth atom superlattices on graphene

We show that the moire pattern of graphene on Ir(111) acts as efficient template for the self-assembly of well-ordered superlattices of single rare-earth adatoms. Using Sm and Dy as representative of early and late lanthanides, we observe that the array quality is determined by the deposition temperature and by a large direct capture area, leading to partial dimer formation. These superlattices result from the combination of the inhomogeneous substrate potential for adatom diffusion generated by the moire and the interatomic electrostatic repulsion originating from electron transfer to graphene. Deposition temperature-and coverage-dependent experiments quantify the energy barriers for adatom diffusion between adjacent graphene lattice sites and between moire unit cells, as well as the amount of charge transferred to the substrate by each adatom.