Structural, energetic and electronic properties of (100) surfaces for alkaline earth metal oxides as calculated with hybrid density functional theory

We perform a systematic investigation of (100) surfaces for rocksalt-structured group 2 metal oxides, namely MgO, CaO, SrO and BaO, using GGA and Hybrid-DFT exchange-correlation functionals. We examine the structural, energetic and electronic properties of the surfaces, with a specific focus on the surface ionisation potential and band bending; the latter of which we quantify by examining the density of states as a function of depth from the system surface. We report structural and energetic results in-line with previous experimental work when we use the Hybrid-DFT method, and for the electronic structure we find inequivalent band bending for the valence and conduction bands, which results in reduced ionisation potentials and the closure of the band gap at the surface when compared to bulk systems. We also report downward bending of the conduction band for MgO that brings it below the vacuum potential, unlike previous theoretical investigations, and thus indicates an origin of the positive electron affinity found in the experiment