Planar versus three-dimensional growth of metal nanostructures at graphene

Employing density functional theory we studied microscopic mechanisms governing initial stages of growth of three selected metals (Li, Ti and Ca) on graphene. Tendency towards planar or three-dimensional (3D) growth is rationalized based on atomic-scale description of the interaction between metal adatoms, as well as adsorption geometries of their trimers and tetramers. Li atoms, featuring a long-ranged electrostatic repulsion, are individually dispersed across the surface, in a sharp contrast with atoms of transition metal Ti which gather into densely-packed 3D clusters due to a strong short-ranged metal-metal attraction. Modest attractive interaction between Ca adsorbates enable formation of monoatomic films with the local coverage of 1/6 monolayer. Since Ca adsorbates induce nearly three-fold increase in adhesion energy between graphene layers, Ca intercalated carbon sheet falls into category of functionalized materials with promising properties for engineering high quality contacts in vertical heterostructures of two-dimensional materials.