Electronic structure and morphology of MgO submonolayers at the Ag(001) surface:: An ab initio model study -: art. no. 155404

Submonolayer structures of MgO epitaxially deposited on Ag(001) have been simulated theoretically in order to gain information on the mode of growth. The model adopted consists in a thin silver slab covered on both sides by MgO "polymers" (narrow ribbons monoatomic in height) sufficiently distant to prevent interactions among them. An ab initio DFT periodic technique has been adopted, whose adequacy has previously been checked for the case of perfect complete MgO/Ag overlayers. Two orientations of the polymers have been considered, corresponding to two modes of growth experimentally observed: a nonpolar ( NP) and a polar (P) one, the latter being limited at the two borders by rows of bicoordinated O and Mg ions. We have considered both the case where the polymers sit (are "supported") on the flat metal surface, and where they are "embedded" in grooves at the surface. The results of the simulations (equilibrium geometries, electronic structure and energy data) can be summarized as follows: (i) Interaction with the metal substrate enormously contributes to reducing the instability of the polar border; nevertheless, both for islands supported or embedded at the silver surface, the role of the metal is not sufficient to stabilize the P- with respect to the NP orientation. (ii) Steps or vacancy depressions at Ag(001) with edges oriented along the silver < 110 > symmetry direction are considerably favored (by almost 25% per unit length) over the < 100 > oriented edges in agreement with observations; the preexistence of the former type of steps may favor kinetically the nucleation of P-oriented islands leaning against them. (iii) The formation of islands embedded in the metal is in all cases thermodynamically favored with respect to supported ones; this means that, in favorable conditions, the growing oxide film might displace silver atoms and penetrate to some extent in the metal. (iv) The electronic and electrostatic features at polar borders suggest that they could be characterized by very high chemical activity.