Keywords Bond-breaking, charge transfer insulator, finite size effects, gap, interface band alignment, interfacial dislocations, ionicity, Madelung potential, metal substrate, Mott-Hubbard insulator, oxide thin films, polarity, quantum confinement, structural relaxation, work function. Glossary Band alignment: Relative positions of the band structures of two compounds at a heterojunction. Iono-covalent character: Extent of valence electron sharing between anions and cations, from full localization at anions (ionic limit) to delocalization along the anion-cation bonds (covalent limit). Madelung potential: In an ionic or a mixed iono-covalent compound, the Madelung potential is the electrostatic potential value at any atomic site due to all other charges. Metal induced gap states: Electronic gap states at the interface between a semiconductor/insulator and a metal, due to the penetration of the metallic wave functions inside the semiconductor/insulator. Work function: Minimum energy needed to move an electron from inside a solid to a point in the vacuum far from the solid surface.