Epitaxial silver layer at the MgO(100) surface

Transition from monolayer-thick 2D to 3D nano-clusters on α-Al2O3(0001)

This paper reports on the long-standing puzzle of the atomic structure of the Ag/α-Al2O3(0001) interface by combining X-ray absorption spectroscopy, to determine Ag local environment [i.e. average Ag-Ag (dAg-Ag) and Ag-O (dAg-O) interatomic distances and Ag coordination numbers (CN)], and numerical simulations on nanometric-sized particles. The experimental key was the capability of a structural study of clusters involving only a few atoms. The concomitant decrease of dAg-Ag and CN with decreasing cluster size provides unambiguous fingerprints for the dimensionality of the Ag clusters in the subnanometric regime leading to a series of unexpected results regarding the size-dependent interface structures. At low coverage, Ag atoms sit on surface Al sites to form buckled monolayer-thick islands associated with a Ag-Ag distance (2.75 Å) which fits the alumina lattice. Upon increasing Ag coverage, as 3D clusters appear, the Ag interface atoms tend to leave Al sites to sit atop O atoms as dAg-Ag increases. The then highlighted size-dependent evolution, is built on structural models which seemed so far contradictory in a static vision of the interface. Theory generalizes the case as it predicts the existence of alumina-supported 2D clusters of Pd and Pt at small coverage and a similar 2D-3D transition upon increasing the size. The structural transformation from 2D Ag clusters to macroscopic 3D islands is accompanied by a noticeable reduction of adhesion energy at the Ag/α-Al2O3(0001) interface.

Surface and Epitaxial Stresses on Supported Metal Clusters

Surface stress and energy are basic quantities in the Gibbsian formulation of the thermodynamic description of surfaces which is central in the formation and long-term behavior of materials at the nanoscale. However, their size dependence is a puzzling issue. It is even unclear whether they decrease or increase with decreasing particle size. In addition, for a given metal, estimates often span over an order of magnitude, far apart from bulk data, which, in the absence of any explicit size-dependence rule, escapes understanding. Here, we combine X-ray absorption and nanoplasmonics data with atomistic simulation to describe α-Al2O3(0001)-supported silver particles. By comparison to MgO(001)-supported and embedded silver, we distinguish epitaxial and surface stress. The latter is shown to dominate above 3 nm in size. Since the observation mostly relies on surface/bulk ratio, a metal-independent picture emerges that is expected to have far-reaching consequences for the understanding of the energetics of nanoparticles.

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

The interaction of TM atoms like Co with two-dimensional NaCl/Au(111) and MgO/Ag(001) ultrathin films is completely different.

Layer-resolved study of Mg atom incorporation at the MgO/Ag(001) buried interface

By combining x-ray excited Auger electron diffraction experiments and multiple scattering calculations we reveal a layer-resolved shift for the Mg KL23L23 Auger transition in MgO ultrathin films (4-6 Å) on Ag(001). This resolution is exploited to demonstrate the possibility of controlling Mg atom incorporation at the MgO/Ag(001) interface by exposing the MgO films to a Mg flux. A substantial reduction of the MgO/Ag(001) work function is observed during the exposition phase and reflects both band-offset variations at the interface and band bending effects in the oxide film.

High-brightness photocathodes through ultrathin surface layers on metals

We report how ultrathin MgO films on Ag(001) surfaces can be used to control the emittance properties of photocathodes. In addition to substantially reducing the work function of the metal surface, the MgO layers also favorably influence the shape of the surface bands resulting in the generation of high-brightness electron beams. As the number of MgO surface layers varies from 0 to 3, the emitted electron beam becomes gradually brighter, reducing its transverse emittance to 0.06 mm mrad. We suggest the use of such photocathodes for the development of free-electron x-ray lasers and energy-recovery linac x-ray sources.

CO Adsorption on Ag(100) and Ag/MgO(100)

Theoretical studies of CO adsorption on a two-layer Ag(100) film and on a two-layer Ag film on a MgO(100) support are reported. Ab initio calculations are carried at the configuration interaction level of theory using embedding methods to treat the metal-adsorbate region and the extended ionic solid. The metal overlayer is considered in two different structures: where Ag-Ag distances are equal to the value in the bulk solid, and for a slightly expanded lattice in which the Ag-Ag distances are equal to the O-O distance on the MgO(100) surface. The calculated adsorption energy of Ag(100) on MgO(100) is 0.58 eV per Ag interfacial atom; the Ag-O distance is 2.28 A. A small transfer of electrons from MgO to Ag occurs on deposition of the silver overlayer. CO adsorption at an atop Ag site is found to be the most stable for adsorption on the two-layer Ag film and also for adsorption on Ag deposited on the oxide; CO adsorption energies range from 0.12 to 0.19 eV. The CO adsorption energy is reduced for the Ag/MgO system compared to adsorption on the unsupported metal film thereby providing evidence for a direct electronic effect of the oxide support at the metal overlayer surface. Expansion of the Ag-Ag distance in the two-layer system also reduces the adsorption energy.

Iron oxidation, interfacial expansion, and buckling at the Fe/NiO(001) interface

In order to provide a structural basis for a physical understanding of exchange bias in metal/magnetic-oxide interfaces, we have determined the structure of the Fe/NiO(001) interface by means of x-ray absorption spectroscopy and ab initio density functional theory calculations. A Fe-Ni alloyed phase on top of an interfacial FeO planar layer is formed. The FeO layer exhibits a 7% expanded interlayer distance and a 0.3 A buckling; its presence is predicted to increase the spin magnetic moment of the interface Fe atoms by 0.6 mu(B), compared to the ideally abrupt interface.

Metal adsorption and adhesion energies on MgO(100)

The calormetically measured heats of adsorption of Cu, Ag, and Pb on MgO(100), previously measured in our group, are correlated with bulk properties of the metals and their sticking probabilities and film morphologies. The low-coverage heats of adsorption (when the metals are mainly in two-dimensional (2D) islands) are used to estimate metal-MgO(100) bond energies within a pairwise bond additivity model. These values correlate well with the observed initial sticking probabilities and saturation island densities of the metals. This supports a transient mobile precursor model for adsorption. The values also correlate with their bulk sublimation energies, which suggests that covalent metal-Mg bonding dominates the interaction at low coverage, probably due to very strong bonding at defects. The heats of adsorption integrated up to multilayer coverages provide the metal-MgO(100) adhesion energies and metal-MgO(100) bond energies for metals in 3D films. These values correlate with the sum of magnitudes of the metal's bulk sublimation energy plus the heat of formation of the bulk oxide of the metal per mole of metal atoms. This suggests that local chemical bonds, both metal-oxygen and covalent metal-Mg, dominate the interfacial bonding for 3D films.

Surface X-ray absorption spectroscopy: principles and some examples of applications

The application of X-ray spectroscopy to surface problems has started almost at the same time as that in materials science. While the theoretical bases are obviously the same, SEXAFS has some experimental peculiarities which are reviewed here. Some examples of this technique will then be given.