Adhesion trends and growth mode of ultra-thin copper films on MgO

Ab initio simulations are performed for Cu atoms adsorbed on the perfect MgO(001) substrate, with an ordered metal coverage varied from 1/4 monolayer (ML), i.e. almost single atoms, up to 1 ML. A strong dependence of the adhesion energy and the sub-monolayer film distance from the substrate on the surface coverage and adsorbate positions (Mg²⁺ or 0^2-) is discussed. The nature of interfacial bonding at all coverages is physisorption. When increasing Cu atomic fraction, a decrease of the substrate-induced polarization of adatoms accompanied by an increase of both in-plane metallic bonding and the interfacial distance has been found. Combining results of ab initio calculations with thermodynamic theory (taking into account the lattice mismatch), we show that the metal cluster formation becomes the predominant growth mode even at low Cu coverages, in agreement with experiment.