Jacek Goniakowski

Polarity of oxide surfaces and nanostructures

Whenever a compound crystal is cut normal to a randomly chosen direction, there is an overwhelming probability that the resulting surface corresponds to a polar termination and is highly unstable. Indeed, polar oxide surfaces are subject to complex stabilization processes that ultimately determine their physical and chemical properties. However, owing to recent advances in their preparation under controlled conditions and to improvements in the experimental techniques for their characterization, an impressive variety of structures have been investigated in the last few years. Recent progress in the fabrication of oxide nano-objects, which have been largely stimulated by a growing demand for new materials for applications ranging from micro-electronics to heterogeneous catalysis, also offer interesting examples of exotic polar structures. At odds with polar orientations of macroscopic samples, some smaller size polar nano-structures turn out to be perfectly stable. Others are subject to unusual processes of stabilization, which are absent or not effective in their extended counterparts. In this context, a thorough and comprehensive reflexion on the role that polarity plays at oxide surfaces, interfaces and in nano-objects seems timely.This review includes a first section which presents the theoretical concepts at the root of the polar electrostatic instability and its compensation and introduces a rigorous definition of polar terminations that encompasses previous theoretical treatments; a second section devoted to a summary of all experimental and theoretical results obtained since the first review paper by Noguera (2000 J. Phys.: Condens. Matter 12 R367); and finally a discussion section focusing on the relative strength of the stabilization mechanisms, with special emphasis on ternary compound surfaces and on polarity effects in ultra-thin films.

Acido-basic properties of simple oxide surfaces: III. Systematics of H+ and OH− adsorption

Thanks to a self-consistent tight binding approach, applied to clusters embedded in a Madelung field created by an infinite array of charges at the lattice sites, we have modeled the acido-basic properties of various simple oxide surfaces: BaO(001), SrO(001), CaO(001), MgO(001), rutile-TiO(in2)(110), α-quartz SiO2(0001). We have studied H+, OH− and dissociated H2O adsorption characteristics: adsorption energy, charge transfer and density of states. We have proved that the decreasing basic character of the oxygen sites along the series comes from the larger covalency of the compounds, which opposes the proton adsorption, while the increasing acid character of the surface cations comes from their lower and lower electropositivity and from their smaller and smaller ionic radii. This detailed study completes the model that we had elaborated previously [Surf. Sci. 262 (1992) 245 and 259], and confirms the dominant role of covalent effects, as opposed to electrostatic effects, in the acido-basic properties of these insulating oxides.

Theoretical investigation of hydroxylated oxide surfaces

Abstract A self-consistent electronic structure calculation, in the slab geometry, is performed to model the dissociative adsorption of water on several oxide surfaces in the limit of complete saturation. We discuss the adsorption characteristics along a series of oxides presenting a growing acidity: BaO, SrO, CaO, MgO, TiO 2 , and SiO 2 , and on three MgO surfaces: (100), (110), and (211) on which the atoms have different environments. Special emphasis is borne on the charge transfers, the densities of states and the oxide gap modification upon hydroxylation. We show that these quantities are dependent upon the coverage and that unstable MgO surfaces are more reactive towards water dissociation. Finally, by optimizing the geometry on the three MgO surfaces, we discuss the link between the electronic and structural degrees of freedom on hydroxylated surfaces.

Electronic States and Schottky Barrier Height at Metal/MgO(100) Interfaces

Using an ab initio total energy approach, we study the electronic structure of metal/MgO(100) interfaces. By considering simple and transition metals, different adsorption sites and different interface separations, we analyze the influence of the character of metal and of the detailed interfacial atomic structure. We calculate the interface density of states, electron transfer, electric dipole, and the Schottky barrier height. We characterize three types of electronic states: states due to chemical bonding which appear at well defined energies, conventional metal-induced gap states associated to a smooth density of states in the MgO gap region, and metal band distortions due to polarization by the electrostatic field of the ionic substrate. We point out that, with respect to the extended Schottky limit, the interface formation yields an electric dipole mainly determined by the substrate characteristics. Indeed, the metal-dependent contributions (interfacial states and electron transfer) remain small with respect to the metal polarization induced by the substrate electrostatic field.

Modeling free and supported metallic nanoclusters: structure and dynamics

We compare atomic structure and dynamics of free and supported metallic clusters via molecular dynamics simulations using tight-binding semiempirical potentials for metal–metal interactions and a potential fitted to ab initio calculations for the metal-supported ones, the support being essentially the MgO(100) surface in the case of a nonreactive metal–oxide interface. The structural transition for free Ni, Pd, Pt, Cu, Ag, Au clusters with noncrystalline structures (mainly icosahedral and decahedral) at small sizes to FCC truncated octahedrons for larger sizes is reported as well as the variation of the critical size of transition from 3d to 5d metals. In the case of Pd clusters on the MgO(100) surface, we analyze the substrate-induced modifications in morphology and atomic structure and follow their evolution as a function of cluster size. The mechanism of strain release by misfit interfacial dislocations in 3D clusters is described at the atomic level. Dynamics of growth and melting of free silver clusters are discussed and some effects of the oxide substrate in melting transition are pointed out, notably the delay in melting induced by the epitaxial relation with the support.

A theoretical study of the stability and electronic structure of the polar 111 face of MgO

Abstract The stability of the polar 111 face of MgO is discussed on the basis of the results obtained by a total-energy semi-empirical Hartree-Fock method, and on general theoretical arguments relying on a perturbative approach of covalent effects in mixed iono-covalent solids. The ideal stoichiometric (1 × 1) surface is studied and compared to two (2 × 1) and (2 × 2) reconstructed surfaces, and to a fully hydroxylated (1 × 1) surface. On the ideal surface, a strong electron redistribution takes place in order to cancel the polarity, which is associated with a metallization of the surface layers and with a high surface energy. A spontaneous symmetry breaking of the Peierls type takes place on this surface, with an opening of a gap at the Fermi level which induces a desorption of half of the surface atoms and a (2 × 1) reconstructed surface configuration. On the two reconstructed surfaces the modification of the stoichiometry in the surface layers provides the compensating charges. As a result, they are much more stable and insulating. The (2 × 2) reconstruction which involves nanopyramids with 001 facets is favored, but we predict a trend to form larger facets in (2 n × 2 n ) reconstructions.

Relaxation and rumpling mechanisms on oxide surfaces

Abstract We associate a self-consistent electronic structure calculation with a conjugate gradient technique for geometry optimization, to study structural distortions on stoichiometric oxide surfaces. We discuss the relaxation and rumpling effects and their consequences on the surface electronic structure in a series of three MgO surfaces: (100), (110), (211) characterized by an increasing number of broken bonds, on the (110) faces of rocksalt oxides presenting various ionic characters: MgO, CaO, SrO and BaO, and on two non-polar rutile TiO 2 faces: (110) and (001). The numerical results are interpreted in the framework of an analytical model and the competition between covalent and electrostatic effects is investigated. A new mechanism of rumpling on oxide surfaces is proposed.

Structures of metal nanoparticles adsorbed on MgO"001…. II. Pt and Pd

The structure of metal clusters supported on a MgO(001) substrate is investigated by a computational approach, with the aim to locate stable structural motifs and possible transition sizes between different epitaxies. Metal-metal interactions are modeled by a second-moment approximation tight-binding potential, while metal-oxide interactions are modeled by an analytic function fitted to first-principles calculations. Global optimization techniques are used to search for the most stable structural motifs at small sizes (N < or = 200), while at larger sizes different structural motifs are compared at geometric magic numbers for clusters up to several thousand atoms. Metals studied are Ag, Au, Pd, and Pt. They are grouped according to their mismatch to the oxide substrate (lattice constant of the metal versus oxygen-oxygen distance on the surface). Ag and Au, which have a smaller mismatch with MgO, are studied in Paper I, while Pd and Pt, with a larger mismatch, are investigated in Paper II. For Ag the cube-on-cube (001) epitaxy is favored in the whole size range studied, while for Au a transition from the (001) to the (111) epitaxy is located at N=1200. The reliability of the model is discussed in the light of the available experimental data.

Electronic structure of clean insulating oxide surfaces I. A numerical approach

Abstract Systematic trends in the surface electronic structure of simple insulating oxides are studied thanks to a quantum self-consistent numerical approach. Four oxides of various ionicity have been chosen: SrO, MgO, rutile TiO2 and α-quartz SiO2, and for each of them, several surfaces, involving atoms with different coordination numbers, have been modeled: the (100), (110) and (211) faces of the rocksalt oxides, the (110), (100) and (001) faces of rutile and the (0001) and (1010) faces of quartz. We discuss the competition between electrostatic and covalent processes, through an analysis of the density of states characteristics, the gap width, the ionic charges and the surface energy.

Water on extended and point defects at MgO surfaces

The interaction of water with extended defects such as mono- and diatomic steps at the MgO(100) surface is investigated through first-principles simulations, as a function of water coverage. At variance with flat MgO(100) terraces, water adsorption is always dissociative on mono- and diatomic steps, as well as on MgO(110) surfaces. In most of the equilibrium configurations, the oxygen of the hydroxyl groups is two- or fourfold coordinated, but single-coordinated OH groups can be stabilized at diatomic step edges. The structural properties of the hydroxyl groups are discussed as a function of their coordination numbers and mutual interactions, as well as the surface defect morphology. It is shown that characteristics of water adsorption are primarily driven by the coordination number of the surface acid-base pair where the dissociation occurs. However, the OH groups resulting from water dissociation are also considerably stabilized by the electrostatic interaction with coadsorbed protons. At low coverage such an interaction, considerably stronger than hydrogen bonding, practically hinders any proton diffusion away from its neighboring hydroxyl. The computed adsorption energies allow us to discuss the onset of water desorption from flat MgO(100) terraces, diatomic and monoatomic steps, and from Mg-O divacancy.