Properties of two-dimensional insulators: a DFT study of Co adsorption on NaCl and MgO ultrathin films

Recent experimental and theoretical results have shown that Co atoms deposited on ultrathin NaCl films grown on Au(111) result in spontaneous substitutional doping of the two-layer insulating material (Li et al., Phys. Rev. Lett., 2014, 112, 026102). This result opens the general question of the reactivity of transition metal (TM) atoms with ultrathin films consisting of few atomic layers. In this article, density functional theory with and without dispersion corrections has been used to compare the adsorption of Co atoms on various sites of unsupported and supported NaCl and MgO two-layer (2L) films. We found that Co interacts strongly with NaCl/Au(111) 2L films, and that Co incorporation in interstitial positions between the first and second NaCl layers is thermodynamically preferred compared to adsorption on the surface sites. Differently from NaCl, Co adsorbs preferentially on top of O in both unsupported and supported MgO 2L films. Co incorporation into the interstitial sites of MgO is highly unfavorable. These results show that the reactivity of TM atoms like Co is completely different on NaCl or MgO ultrathin films. The reasons for this difference, the role of dispersion, and the peculiar properties of two-dimensional insulators are discussed.