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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.

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.

Titanium surfaces in implants are covered by titanium oxide passivation layers of 1.5-25 nm thickness. At the surface TiO2 phases form with anatase and rutile structures. To improve the understanding of the initial phases of passivation, we investigated the interfaces between Ti and the most common oxide surfaces rutile (110) and (100) and anatase (101) and (001). Simulations based on a DFT+U approach revealed the presence of metal induced gap states (MIGS) caused by oxygen atoms that have moved toward the metal phase. We discuss the structural disorder around the interface and its effect on the oxide crystal structure. The computed work of separation is 3.4-4.3 J/m(2) which is much higher than the surface energies of Ti metal and TiO2 and results in a negative interface energy. Charge transfer takes place from the metal to the oxide and appears to be less dependent on the oxide phase than on the thickness of the slabs. We computed a charge transfer of 0.07 e per atom, most of which remains located in the first interface layers. The metal work function changes as a consequence of the formation of the passivation layer, but Delta Phi is positive for rutile and negative for anatase.

The ability of Mo (Cr) impurities in a CaO (MgO) matrix to act as charge donors to adsorbed gold has been investigated by means of scanning tunneling microscopy and density functional theory. Whereas CaO(Mo) features a robust donor characteristic, as deduced from a charge-transfer-driven crossover in the Au particles' geometry in the presence of dopants, MgO(Cr) is electrically inactive. The superior performance of the CaO(Mo) system is explained by the ability of the Mo ions to evolve from a +2 oxidation state in ideal CaO to a +5 state by transferring up to three electrons to the Au adislands. Cr ions in MgO, on the other hand, are stable only in the +2 and +3 charge states and can provide a single electron at best. Since this electron is likely to be captured by cationic vacancies or morphological defects in the real oxide, no charge transfer to Au particles takes place in this case. On the basis of our findings, we have developed general rules on how to optimize the electron donor characteristics of doped oxide materials.

The properties of MgO/Mo(100) ultrathin films where the Mg2+ cation is substituted with Li, Al, or Ni impurities have been investigated by means of density functional theory calculations. While on bare MgO the dopants affect the oxide electronic structure by creating holes or excess electrons, for supported ultrathin films the metallic support provides a reservoir to compensate charges via electron transfer through the oxide layer. The charge transfer has direct consequences on the work function (phi) of the combined oxide/metal system. As MgO/Mo(100) films are characterized by a very low work function, doping with monovalent Li atoms results in an increase of phi. On the contrary, a small reduction of phi can be obtained with low concentration of trivalent Al dopants. A more sizable reduction of phi is obtained for Al-doped MgO/Ag(100) films, where the work function of the undoped system is higher. In the case of Ni, the impurity atoms can assume different oxidation states, Ni2+, Ni+, and Ni-0, depending on the position within the film. For all configurations considered, substitutional Ni leads to an increase of the work function.

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.

International audience; We report a first principles study on the properties of Pt(111)-supported FeO(111) monolayer. We confront results issued from PBE+U and HSE06 approximations, and analyze the impact of the more accurate hybrid description of the electronic structure of the metal/oxide interface on a large variety of calculated characteristics of this system. In particular, we analyze the behavior of its work function and its consequences on the spontaneous charging of adsorbed Au adatoms. We also consider the FeO2 nano-oxide phase and its peculiar oxygen storage characteristics, responsible for the unusual catalytic properties of FeOx/Pt system. We show that while the hybrid approximation does indeed substantially improve the electronic characteristics of iron oxide, of individual Au adatoms, or oxygen molecules, its overall impact on the calculated properties of the composed FeO/Pt system is very small. We assign this to the relatively small effect of the hybrid approximation on the band structure alignment. This shows that the less computationally demanding DFT+U approximation remains a fully adequate tool in theoretical studies on this kind of systems. This is particularly important for calculations on realistic systems, with large-size reconstructions induced by the lattice mismatch at the interface between the two materials. (C) 2014 AIP Publishing LLC.

Transition metal atoms incorporated into insulating materials (oxides in particular) can deeply modify their adsorption properties. In particular, charge transfer to adsorbed species can be induced by the presence of substitutional dopants, which introduce new electronic states in the band gap of the host crystal. Here we show, by means of density functional theory calculations, that Nb represents an excellent dopant to turn the rather inactive CaO(100) surface into an electron-rich support. The charge transfer ability of the doped material is shown by comparing the adsorption properties of the electronegative Au atoms on pure and Nb-doped CaO. While in the first case the CaO-Au bonding is relatively weak and the Au atom is essentially neutral, in the Nb-doped system a much stronger adhesion is found due to a net charge transfer from the Nb dopant and to the formation of a gold anion. This mechanism occurs also for Nb in high oxidation states. Nb is thus an excellent modifier of the calcium oxide properties. © 2014 IOP Publishing Ltd.

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.

The use of transition metals (TM) as dopants is a versatile way to change the properties of oxides. One example is the possibility to induce charge transfer from the TM to species adsorbed on the oxide surface. In the case of wide-gap alkaline-earth oxides, such as MgO and CaO, the charge transfer is determined by the respective, position of the energy levels of the impurity and the adsorbate and resides in the capability of the TM to be stabilized in various oxidation states. The structural and electronic properties of Cr- and Mo-doped MgO and CaO are investigated by density functional theory (DFT) calculations. Because of limitations of DFT in the description of band gaps and energy levels alignment, we compare the results obtained with standard GGA, GGA+U, and hybrid exchange-correlation functionals. The ability of the impurity ion to transfer one electron to the adsorbed species is considered for the case of electronegative Au adatoms The study of the thermodynamic stability of the TM ions in combination with the formation of cation vacancies shows that MO impurities are stable in the oxide lattice as Mo3+ or Mo4+ but can assume also higher oxidation states and are therefore able to transfer electrons to adsorbed gold. Conversely, Cr impurities are stabilized as Cr3+ and the high cost of further oxidation results in absence of charge transfer to gold on both Cr-doped MgO and Cr-doped CaO.

Doping is a versatile yet little examined approach to tailor the physical and chemical properties of oxide thin films. By means of scanning tunneling microscopy (STM), we demonstrate how tiny amounts of Mo embedded in a CaO matrix change the growth behavior of gold. Whereas 3D deposits are formed on the pristine oxide surface, strictly 2D growth prevails on the doped material. The crossover in particle shape is driven by charge-transfer processes from the Mo dopants into the Au islands, as elucidated with density-functional theory (DFT) calculations.

The nature of Ag(111)/TiO${}_{2}$ rutile and anatase interfaces, of interest for the design of memristors, has been studied by means of density functional theory calculations using various computational approaches. We have considered interfaces where the lattice mismatch of the oxide crystalline phase and the metal electrode does not result in excessive strain. The bonding at the interface is very weak, and the charge transfer is negligible for stoichiometric oxides. The formation of O vacancies has a lower cost at the interface with Ag than on the bare titiania surface and results in stronger adhesion between the Ag electrode and the reduced TiO${}_{2\ensuremath{-}x}$ oxide. The diffusion of Ag and O atoms or ions across the interface is a thermodynamically unfavorable process which can occur only at high temperatures or under the effect of an external electric field. Once Ag atoms are incorporated in the bulk of TiO${}_{2}$ they can be stabilized in an interstitial site (more favorable) or a position substitutional to Ti. In both cases Ag is ionized and transfers the valence electron to the host crystal with formation of Ti${}^{3+}$ states. The Ag atoms remain positively charged, even when extended Ag chains are formed (nanofilaments). For an Ag filament inside TiO${}_{2}$ to exhibit conductive behavior, a higher density of Ag atoms is required, but this is hardly possible in the regular bulk crystalline lattice of TiO${}_{2}$ without inducing a structural breakdown.

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.

Ultrathin dielectric films deposited on a metal surface induce variations in the work function that can be relevant for the final properties of the metal/oxide interface. In this work we analyze with the help of density-functional theory calculations the effect of depositing three-layer films of insulating materials such as LiF, NaCl, MgO, CaS, and BaO on various (001) metal surfaces. We found that the change in work function $\ensuremath{\Delta}\ensuremath{\Phi}$ is due to three main contributions: an electrostatic ``compression'' effect which dominates for highly ionic films such as LiF, a charge-transfer effect which is largest for less ionic films such as BaO, and the surface relaxation induced by the formation of the interface bond which largely depends on the lattice mismatch between the dielectric film and the metal. Finally, we propose a universal correlation between the work function change and the energy difference between the position of the Fermi level of the metal surface and the top of the valence band of the dielectric film.