The interaction of coinage metal clusters with the MgO(100) surface

The results of a systematic study of the interaction of small coinage metal clusters (M-n, n = 1-3) and extended deposition (one and two MLs; ML = monolayer) with the regular and locally defected (F, center and divacancy) neutral MgO(100) surface are presented. The calculations have been performed at the DFT level employing plane waves as a basis set and using a gradient-corrected exchange-correlation functional (PW91). The adhesion energy along the group follows a trend that can be rationalized in terms of the strength and "stickiness" of the metallic bond, electrostatic polarization effects, and chemical interactions. Coinage metal dimers and trimers are absorbed on the regular surface in an upright position with little modification with respect to the gas-phase structure and can easily diffuse from site to site (in the case of trimers, also because of their fluxional character). In the case of extended deposition, the adhesion energy increases when passing from one to two MLs because of a "metal-on-top" stabilization mechanism. Neutral localized defects on the surface such as the F-s center (generated by a missing O atom) and the double vacancy (generated by a missing MgO dimer) act as strong trapping centers for small clusters and remarkably increase the adhesion of metal slabs to the surface in the case of extended deposition. At variance with the Fs center, the double vacancy induces a strong structural and energetic modification of the surrounding oxide lattice, varying also as a function of the metal deposition. A peculiar structural rearrangement consisting of the segregation of the metal slab into "islands" on the surface is observed in the case of one ML Cu and (to a lesser extent) Au.