Claudine Noguera

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.

Polar oxide surfaces

In the light of recent experimental as well as theoretical studies, we summarize our present understanding of polar oxide surfaces and examine fundamental issues regarding their stability. The focus is on the surface atomic configurations (relaxations, reconstructions, non-stoichiometry, etc) obtained under specific preparation conditions and their associated electronic structure. We discuss several mechanisms at work on polar surfaces, such as relaxation effects, change of covalency in the surface layers, partial filling of surface states, and stoichiometry variations, and try to assess their actual efficiency for cancelling the polarity.

Polarity on the SrTiO3 (111) and (110) surfaces

Abstract Relying on the results obtained by a total energy, semi-empirical Hartree–Fock method, we discuss polarity effects at the (111) and (110) surfaces of SrTiO3. For these orientations, we consider some prototypical (1×1) configurations, which differ by their surface composition and the coordination number of the surface atoms. We argue that the compensation for these polar orientations is achieved through anomalous fillings of surface states, which, in principle, should be detectable by surface spectroscopies. The compensation does not imply a partial filling, so that all these surfaces keep their insulating character. The thermodynamic stability of the different configurations is also analyzed, and we conclude that, consistently with the experimental observations, these surfaces are relatively stable, especially when compared with polar faces of more ionic oxides, such as MgO(111). The efficiency of SrTiO3 for screening the strong electrostatic perturbation associated to the polar orientations is invoked to explain such a behavior.

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.

Acido-basic properties of simple oxide surfaces: I. Magnesium oxide

Abstract Thanks to a self-consistent tight-binding approach, applied to clusters of about 80 atoms embedded in a Madelung field created by an infinite array of charges at the lattice sites, we have modelized the acido-basic properties of various sites at the surfaces of MgO. We have studied H + and OH − adsorption, on the perfect (100) and (110) surfaces and on a roughness on the (100) face. The adsorption of a proton was found to belong to the strong coupling limit of the chemisorption process, while the adsorption of an hydroxyl group belongs to the weak coupling limit. The (100) flat surface of MgO thus adsorbs more easily a proton than an hydroxyl group, revealing, by this fact, its basic character, known by the value of 12 pH units of its zero charge point (ZCP). In addition we have shown that this acido-basic character is a function of the surface orientation and of the adsorbing site: e.g., the (110) surface is less basic than the (100) because the adsorption on bridge of an hydroxyl group on (110) is easier. The details of the adsorption processes bring strong indications that covalent effects play an important role.

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.

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.

Density functional study of stoichiometric and O-rich titanium oxygen clusters

We propose a detailed description of the structural and electronic properties of neutral and charged TinO2n+m clusters (n=1–3 and m=0,1), through simulations based on the density functional theory in the local spin density approximation. In all the isomers studied, strongly bound titanyl groups are found. The order of stability of the low-energy stoichiometric clusters may change considerably from that found by the approaches based on classical electrostatics. The most stable isomers of the oxygen-rich neutral clusters show characteristic peroxide groups. All these facts stress the importance of the covalent contribution to the cohesion of the clusters. Large atomic relaxations, accompanying the change from a closed-shell to an open-shell electronic configuration when an electron is added or removed, can often induce reversals of stability among the isomers. A careful discussion of the computed electron affinities and excitation energies as a function of the size and the atomic conformation of the cluster...