Ionic Crystal Surfaces Research Articles

Quantum Chemistry of Oxide Surfaces From CO Chemisorption to the Identification of the Structure and Nature of Point Defects on MgO

The electronic structure and chemisorption properties of the surface of ionic crystals are reviewed, with emphasis on two topics: a critical overview of the experimental and theoretical studies of the adsorption of CO on single crystal and polycrystalline MgO, and a discussion on the most important defect centers at the MgO surface — low-coordinated sites, single oxygen and magnesium vacancies, divacancies, and impurity or substitutional atoms. The two subjects are to some extent interconnected. From the detailed theoretical and experimental study of the adsorption of a nonreactive molecule like CO and from the comparison of experiments done on single crystal or thin films and on powder samples, one can learn about the nature and concentration of the defects at the surface. A more precise characterization of defects requires, however, a careful spectroscopic investigation and a direct comparison with quantum-chemical calculations of both geometric structure and observable properties. The combined theoretical–experimental approach offers new opportunities for a better understanding of the complexity of oxide surfaces.

QUANTUM CHEMISTRY OF OXIDE SURFACES: FROM CO CHEMISORPTION TO THE IDENTIFICATION OF THE STRUCTURE AND NATURE OF POINT DEFECTS ON MgO

The electronic structure and chemisorption properties of the surface of ionic crystals are reviewed, with emphasis on two topics: a critical overview of the experimental and theoretical studies of the adsorption of CO on single crystal and polycrystalline MgO, and a discussion on the most important defect centers at the MgO surface — low-coordinated sites, single oxygen and magnesium vacancies, divacancies, and impurity or substitutional atoms. The two subjects are to some extent interconnected. From the detailed theoretical and experimental study of the adsorption of a nonreactive molecule like CO and from the comparison of experiments done on single crystal or thin films and on powder samples, one can learn about the nature and concentration of the defects at the surface. A more precise characterization of defects requires, however, a careful spectroscopic investigation and a direct comparison with quantum-chemical calculations of both geometric structure and observable properties. The combined theoretical–experimental approach offers new opportunities for a better understanding of the complexity of oxide surfaces.

Consequences of combining laser irradiation with other stimuli on laser desorption and ablation from wide bandgap insulators

When a transparent, wide bandgap insulator is exposed to sub-bandgap laser irradiation, defects at and near the surface often dominate the response in terms of particle emission and eventual ablation of material. We explored the consequences of applying a variety of stimuli that can generate defects on single crystal surfaces of inorganic ionic crystals. The stimuli include: electron beam irradiation, a second laser beam, mechanical treatment, and thermal treatment. We found that a common theme evolves where these stimuli generate sites for strong interactions of the probing laser beam, leading to a dramatic decrease in the laser intensities needed for ejection of ions, neutrals, and eventual plume formation.

Transfer of a pollutant molecule through a water film on a single crystal surface

We study the transfer of a CO2 molecule from the gas phase through a thin liquid water film supported on ionic single crystal surfaces NaCl and MgO (001). The free energy profile for the CO2 is calculated at 300 K using constrained molecular-dynamics simulation, and the detailed analysis of the competition between interaction and entropic contributions can help to the understanding of the pollution kinetic process of a water covered solid surface. It is shown that the CO2 solvation and its adsorption directly on the solid surface is easier for a NaCl supported film than for a MgO one.

Theory of charge transfer on MgO [formula omitted] surfaces

We present a theoretical study of the negative ion production from fluorine and hydrogen projectiles during low energy, small incidence angle collisions with a MgO (1 0 0) surface. The calculations are based on the binary encounter model developed to describe the electron capture by a projectile upon collision with an insulating ionic crystal surface [see A.G. Borisov, V. Sidis, Phys. Rev. B 56 (1997) 10628]. Our theoretical results for the negative ion yield explain for both fluorine and hydrogen projectiles the low-velocity behaviour of the available experimental data by S. Ustaze et al. [Phys. Rev. Lett. 79 (1997) 3526].

Formation of negative ions from fluorine projectiles scattered off a MgO(100) surface: Theory

We present a parameter-free theoretical study of the negative-ion production from fluorine projectiles during low-energy (0.5--4 keV), small incidence angle (3.5\ifmmode^\circ\else\textdegree\fi{}) collisions with a MgO(100) surface. Our calculations are based on the binary encounter model developed to describe the electron capture by a projectile upon collision with an insulating ionic crystal surface [see A. G. Borisov and V. Sidis, Phys. Rev. B 56, 10 628 (1997)]. For the low-velocity behavior of the negative-ion yield, good agreement is obtained between our theoretical results and the available experimental data by S. Ustaze et al. [Phys. Rev. Lett. 79, 3526 (1997)].

Apparent contrast of molecularly thin films of water at ionic crystal surfaces

We focus the apparent contrast of water islands formed at ionic crystal surfaces in air at room temperature (RT) using the non-contact monitoring mode of a scanning force microscope (SFM). Their apparent heights of water islands formed on LiF(001), CaF 2(111) and BaF 2(111) surfaces have been measured to be about 2, 1 and 1 nm, respectively. The water growth of a NaF(001) surface has two phases (phase I and phase II). The apparent heights of phase I and phase II have been measured to be about 2 and 1.5 nm, respectively. The apparent height of water films is shown to be strongly modified by the dielectric constant of sample and water, and the true thickness of water islands is discussed.

Interface Madelung constants for isoelectronic impurities: generalized Madelung constants

Generalized Madelung constants are derived in a simple manner, starting from the interface Madelung constants calculated within the framework of the Evjen method. The results obtained in this work can be used to elucidate the surface and interface electronic states of ionic crystals since they are fundamental to evaluating the electronic potentials of an ion or an isoelectronic impurity at surfaces of ionic crystals and interfaces consisting of dielectrics/ionic crystals.

Infrared absorption of Fe ultrathin films on ionic crystals

Ultrathin (<100 Å) metal films as well as island metal films could be considered as systems of reduced dimensionality. The motion of free carriers is confined by film and island boundaries, which affects, for example, the infrared absorption. We have studied the infrared transmission (from 900 to 4100 cm −1) of ultrathin metal films grown in situ on ionic crystal surfaces. Obviously, both the reduced dimensionality and the film morphology strongly influence the infrared spectra. Experimental spectra of ultrathin Fe films (thickness <30 Å) on KBr(100) could not be described in terms of Fe-bulk parameters.

Electron scattering factors of ions and dynamical RHEED from surfaces of ionic crystals

Electron scattering factors of ions are represented in a parametrized form which separates the diverging Coulomb term due to ionic charge from the contribution of the screened atomic field. Using this representation it is shown how dynamical reflection high-energy electron diffraction (RHEED) calculations can be performed for ionic surfaces using conventional numerical techniques. Following the classification of ionic surfaces proposed by Tasker [Surf. Sci. 78, 315 (1979)], we analyze the effect of ionicity on RHEED intensities for three classes of ionic surfaces. In the first case both positive and negative ions are located in the same plane and this leads to the cancellation of long-range Coulomb contribution to the crystal potential. The second class of ionic surfaces consists of layers of ions having net nonzero charge. Ionic layers may be grouped into repeat units of planes of the form (anion)-(cation)-(anion) characteristic of the (111) surface of the fluorite structure so that each repeat unit satisfies the charge neutrality condition and also has zero net dipole moment perpendicular to the surface. For this class of surfaces the Coulomb term leads to unusual features in the distribution of the potential in the crystal. The third type of the termination of an ionic crystal lattice gives rise to a sequence of repeat units characterized by a nonzero net dipole moment resulting in a divergent behavior of the potential. Our calculations show that charge transfer between lattice sites occurring in an ionic crystal affects very substantially the form of RHEED rocking curves. The effect can be interpreted in terms of the change in the effective inner potential. This in turn can be used for quantitative determination of the degree of charge transfer occurring in the surface layer of an ionic crystal.

Theory of negative-ion conversion of neutral atoms in grazing scattering from alkali halide surfaces

The theoretical approach proposed by Borisov et al. [Phys. Rev. Lett. 77, 1893 (1996)] to treat negative-ion conversion of neutral atoms at ionic crystal surfaces is described in detail. Due to the localization of the valence-band electrons at the anionic sites of the crystal, the conversion process is viewed as a result of successive binary collisions between the projectile and the negatively charged sites at the surface. Parameter-free calculations of ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ formation in grazing scattering from LiF(100) and KI(100) are performed using a model in which all sites of the crystal lattice but one, the active site, are represented by eventually polarizable point charges. Parallel velocity thresholds for negative-ion formation, relative efficiency of the negative-ion formation for LiF and KI crystals, and dependences of this efficiency on the scattering angle correspond well to the experimental results.

Transfer-matrix study of the staggered body-centered solid-on-solid model

The phase diagram of the staggered six vertex, or body-centered solid-on-solid model, is investigated by transfer-matrix and finite-size scaling techniques. The phase diagram contains a critical region, bounded by a Kosterlitz-Thouless line, and a second-order line describing a deconstruction transition. In part of the phase diagram the deconstruction line and the Kosterlitz-Thouless line approach each other without merging, while the deconstruction changes its critical behavior from Ising-like to a different universality class. Our model has the same type of symmetries as some other two-dimensional models, such as the fully frustrated XY model, and may be important for understanding their phase behavior. The thermal behavior for weak staggering is intricate. It may be relevant for the description of surfaces of ionic crystals of CsCl structure.

Temperature-programmed desorption of positive ions and neutral molecules from alkali halide layers deposited on a metal surface

To elucidate the process of thermal ionic and neutral desorption from an ionic crystal surface, a thin film (¯θ≈ 10 2–10 3molecular layers) of alkali halide (e.g. NaCl) deposited on a platinum plate was heated to a high temperature ( T) at a constant rate (β ≈ 2–65K s −1), and the desorption rates of both positive ions and neutral molecules were simultaneously measured as functions of T and ¯θusing a dual-ion source system developed in our laboratory. Analysis of the relation between β and the peak appearance temperature ( T p ≈ 700–1300K) of each desorbing particle shows that the activation energies for desorbing Na + and NaCl are3.0 ± 0.2and2.0 ± 0.1eV, respectively, while the respective frequency factors are∼ 4 × 10 11and∼ 5 × 10 10s −1. Desorption parameters evaluated at T psuggest that NaCl and Na + are desorbed only from upper layers (more than¯/θ2) and mainly from high work function sites on Pt at¯θ< 1layer, respectively. The data obtained similarly with other alkali halides also indicate that the desorption process of each particle follows the first-order kinetics with regard to each surface concentration.

Surface phase transitions in two-component systems

The (001) surface of a two-component crystal of CsCl structure is described by a six vertex model with staggered vertex weights. Its rich phase diagram includes a c(2 × 2) reconstructed phase undergoing, when temperature is increased, a second order phase transition into a disordered flat phase. This is followed, in a certain range of the parameters' values, by two Kosterlitz-Thouless transitions, the first one into a rough phase, and the second one back into the disordered flat phase. Possible applications of the model to the surfaces of ionic crystals are briefly discussed.

Atomic processes induced by relaxation of excitons in the bulk and on the surfaces of non-metallic solids

Abstract

Subsurface ionic space charges in silver chloride and silver bromide

The surfaces of ionic crystals are, in general, electrically charged, with a compensating subsurface ionic space charge. Although a wide variety of phenomena are expected to result from the effects of this double layer, the surface charge has been well-characterized only for the silver halides. This paper reviews the origin of the space charge phenomenon, its effects on near-surface properties of ionic crystals (and especially, the photographic process), and the experimental techniques that have been employed in silver halide studies. In particular, some recent work, in which sub-surface radiotracer distributions are measured, have yielded quantitative data on the individual formation enthalpies and entropies of the cation vacancy and the cation interstitial ion.

Electron-stimulated desorption from ionic crystal surfaces

Inelastic interactions of electrons with surfaces of ionic crystals result in emission of various particles such as ions, atoms and molecules. We will review such electron-stimulated desorption processes for the particular class of ionic crystals, namely for alkali halides. In this case, a dominant fraction of the emission is in the form of halogen and alkali atoms characterized by a thermal (Maxwellian) spectrum of translational energies. For several alkali halides (potassium and rubidium chlorides, bromides, and iodides), however, a significant part of the halogen atoms is ejected with nonthermal energies, i.e. energies of the order of 0.1 eV. The results of recent systematic studies of angular-resolved kinetic energy distributions of the emitted particles will be reported and current views on the electronic mechanisms of desorption will be described. In particular, it will be shown that the ESD mechanism can be understood in terms of the model involving a surface localisation of the so called “hot-holes” created by electron bombardment of alkali halides. A role of hot holes in ESD processes will further be discussed in relation to very recent experimental results obtained for the KBr crystals doped with In impurities which act as efficient hole traps.

Oscillatory Dissolution of an Ionic Single Crystal Surface Observed with the Scanning Electrochemical Microscope

Oscillatory Dissolution of an Ionic Single Crystal Surface Observed with the Scanning Electrochemical Microscope

Ion neutralization at an ionic crystal surface

Ion neutralization near the surface of an ionic crystal is studied within the framework of the approximate many-level method. The tight-binding approximation is employed to model the ionic crystal by a one-dimensional semi-infinite chain of alternating s- and p-orbitals. The results obtained show that the substrate's electronic structure and its surface density of states play important roles in the resonance-charge-transfer process.

Friction force microscopy on clean surfaces of NaCl, NaF, and AgBr

With a bidirectional atomic force microscope, measurements are performed on insulating surfaces of ionic crystals, such as NaF, NaCl, and AgBr. Atomic-scale friction is observed on surfaces, and is being prepared and studied in ultrahigh vacuum, where contaminants can be excluded. Comparative measurements show the favorable frictional properties of NaCl.