Polar and non polar domain borders in MgO ultrathin films on Ag(001)

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 left angle bracket1 1 0right-pointing angle bracket oxide direction, corresponding to polar (P) borders, while on multi-layer domains also non-polar (NP) borders appear, corresponding to the left angle bracket1 0 0right-pointing angle bracket 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.