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International audience; We compare the performances of different DFT functionals applied to ultra-thin MgO(100) films supported on the Ag(100) surface, a prototypical system of a weakly interacting oxide/metal interface, extensively studied in the past. Beyond semi-local DFT-GGA approximation, we also use the hybrid DFT-HSE approach to improve the description of the oxide electronic structure. Moreover, to better account for the interfacial adhesion, we include the van de Waals interactions by means of either the semi-empirical force fields by Grimme (DFT-D2 and DFT-D2*) or the self-consistent density functional optB88-vdW. We compare and discuss the results on the structural, electronic, and adhesion characteristics of the interface as obtained for pristine and oxygen-deficient Ag-supported MgO films in the 1-4 ML thickness range.

A detailed atomic scale description of the interfaces between cerium oxide and metals is necessary for a clear understanding of interfacial interactions, and it may open the way to the optimization of the properties of the combined material in view of its applications. In this study the interface between cerium oxide epitaxial films and the (111) surface of platinum is studied by aberration-corrected scanning transmission electron microscopy and by ab initio density functional theory calculations. Local modifications of the registry between the cerium oxide and platinum lattices and the occurrence of nanometric platinum islands at the interface are detected by high-angle annular dark field imaging. Moreover, the preferential adsorption sites of Ce and O atoms at the interface are determined by a comparison of the images acquired in annular bright-field mode and the results of density functional theory calculations. The presence of a non-negligible concentration of Ce3+ ions at the interface is detected by electron energy loss spectroscopy and it is rationalized in terms of interfacial charge transfer as evidenced by the calculations. This work provides an insight into the atomic and electronic structure of the technologically relevant CeO2/Pt interface and it helps to clarify the properties of the combined material.

Recent experiments have demonstrated that tiny amounts of Mo impurities give rise to drastic changes in the adsorption characteristic of a wide-gap CaO(001) film. In this scanning tunneling microscopy (STM) and density functional theory paper, we elucidate the underlying mechanism by analyzing the energy levels of the Mo dopants as a function of their oxidation state and depth below the surface. We show that Mo2+ ions in CaO subsurface layers can be reversibly charged and discharged by inducing local band-bending effects with the STM tip. A similar charge switching is not possible for Mo species in a higher oxidation state, as their highest-occupied molecular orbitals are located well below the onset of the CaO conduction band. The easiness of charge switching in Mo2+ ions explains the remarkable chemical properties of doped CaO films, as it renders the material a strong electron donor to adsorbates bound to the oxide surface.

By means of scanning tunneling microscopy, Auger spectroscopy, and density-functional theory, we have identified a different strain-relaxation mechanism taking place in ultrathin CaO films grown on Mo(001). Whereas CaO films are amorphous at low growth temperature due to the substantial lattice mismatch with the support, a crystalline phase develops upon annealing to 1000 K. This phase is characterized by a sharp (2 \ifmmode\times\else\texttimes\fi{} 2) pattern in low-energy electron diffraction and displays wide, atomically flat terraces in scanning tunneling microscopy images. The phase transition is initiated by the diffusion of Mo from the support into the film, where it replaces 25% of the Ca ions. The resulting rocksalt-type Ca${}_{3}$MoO${}_{4}$ structure has a negligible lattice mismatch with the Mo(001), enabling the growth of extended, defect-free oxide patches. The oxidation of Mo atoms from the support provides the thermodynamic stimulus for the phase transition. For MgO films grown on the same Mo(001) surface, no relevant intermixing is revealed at the interface, most likely because of the smaller lattice mismatch between both systems.