Polar and non-polar domain borders in MgO ultrathin films on Ag(0 0 1)

The morphology of ultrathin epitaxial MgO layers reactively grown at 470 K and at 1 ML min−1 on Ag(0 0 1) was investigated by scanning tunnelling microscopy (STM). At deposition of 1.3 and 2.6 ML, square MgO domains of about (10 × 10) nm2 form. On mono- and bi-layer domains, edges are mainly oriented along the 〈1 1 0〉 oxide direction, corresponding to polar (P) borders, while on multi-layer domains also non-polar (NP) borders appear, corresponding to the 〈1 0 0〉 orientation. By using periodic models and ab initio DFT calculations, the energetics of the two orientations was studied. Interaction with the metal substrate was found to strongly reduce the instability of P borders, although not enough to make the two orientations equally stable. The results are discussed, also with reference to conflicting experimental evidence from other laboratories.