Low-coordinated Surface Sites Research Articles

The role of FA(I):Tl+ and FA(II):Ga+ defects in laser light generation and photographic sensitization at the low-coordination surface sites of AgBr first principles calculations

Abstract FA(I):Tl+ and FA(II):Ga+ color centers at low coordination surface sites of AgBr thin films play important roles in providing tunable laser oscillation and photographic sensitization. Double-well potentials at these sites are investigated using ab initio molecular electronic structure calculations. Clusters of variable sizes were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces, and ions that are the nearest neighbors to the FA defect site were allowed to relax to equilibrium in each case. The calculated Stokes shifted optical transition bands suggest that laser generation is too sensitive to the simultaneous effects of (i) the dopant cations size (ii) the coordination number of the surface ions, and (iii) the choice of the basis set centered on the Br vacancy. An attempt has been made to explain these results in terms of Madelung potentials, electron affinities and optical–optical conversion efficiencies. All relaxed excited states of the defect containing surfaces were deep below the lower edges of the conduction bands of the ground-state defect-free surfaces, indicating that FA(I):Tl+ and FA(II):Ga+ are suitable laser defects. The probability of orientational destruction of the two centers, attributed to the assumed saddle point ion configurations along the 〈1 1 0〉 axis, was found to be directly proportional to the size of the dopant cation, and inversely proportional to the surface coordination number. For optical memories, a high recording sensitivity was assigned to FA(II):Ga+ relative to FA(I):Tl+ and to the flat surface relative to the edge and corner surfaces. The possibility of exciton (energy) transfer between sites of different coordination numbers was clarified. The relaxation and defect formation energies were calculated. The Glasner–Tompkins empirical relation was generalized to include the dopant cations and surface coordination. As far as photographic sensitization is concerned, the highest occupied molecular orbital of only one out of three acyclic developers was found to be high enough for electron transfer to the Fermi level of the silver bromide thin film. The FA defect formation and rotational diffusion of the central silver cluster broaden the energy gaps between the highest occupied molecular orbitals of the developer molecules and the lowest unoccupied molecular orbitals of the defect containing surfaces and make the process of electron injection less facile. About 59% of the reduction of silver ions at the flat surface of silver bromide is attributed to the host anions and FA defect formation, leaving about 41% for the reduction of photoelectrons and developer molecules. The unrelaxed rotational diffusions of the central silver clusters by 30° decrease the latter percentage to about 10%, but are severely hindered by activation energy barriers.

CO adsorption on MgO( [formula omitted]) surface with oxygen vacancy and its low-coordinated surface sites: embedded cluster model density functional study employing charge self-consistent technique

The adsorption of CO on regular and defect sites of MgO(0 0 1) surface has been studied theoretically using embedded cluster models by DFT/B3LYP method. The value of embedded point charges is determined by the charge self-consistent technique. The calculated results indicate that CO adsorption energy on the regular site of MgO(0 0 1) surface can agree well with the recent experimental data. The frequency shifts of CO for regular five-coordinated terrace, low four-coordinated edge and three-coordinated corner sites, via C bound down on cationic centers of MgO(0 0 1) surface are also satisfactorily close to the values of experiment. At the same time, the adsorption of CO on MgO(0 0 1) surface with neutral and charged oxygen vacancies, F s, F s + and F s 2+ centers, has been investigated whose results show that MgO(0 0 1) surface with neutral oxygen vacancy has probably the good catalysis structure for CO adsorptive-decomposition. This is consistent with our previous study using a different method. By analyzing the bond component of Mg–C, it is found that the essential reasons why C–O bond strength is weaken or strengthen are the competitive results of 4σ* lone pair electrons and 5σ electrons of CO transferring from the adsorbate to substrate simultaneously.

F+ tunable laser activity and interaction of atomic halogens (F, Cl and Br) at the low coordinated surface sites of SrOAb initio and DFT calculations.

The twofold potential of F+ color centers at the low coordinated surfaces of SrO for providing tunable laser activity and adsorption properties for atomic halogens (F, Cl and Br) has been investigated using ab initio electronic structure calculations. SrO clusters of variable sizes were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces and the nearest neighbor ions to F+ were allowed to relax to equilibrium. Based on Stokes shifted optical transition bands and horizontal shifts along the configuration coordinate diagrams, the F+ laser activity was found to decrease as the coordination number of the surface ions decreases from 5 (flat) to 4 (edge) to 3 (corner). An attempt has been made to explain this result in terms of Madelung potentials and optical-optical conversion efficiencies. All relaxed excited states are deep below the conduction bands of the perfect ground states, implying that F+ is a laser-suitable defect. The most laser active flat surface is the least probable for relaxed excited state orientational destruction of F+. The excited state at the edge has the highest energy, implying exciton (energy) transfer to the flat and edge sites. F+ relaxation and defect-formation energies increase with increasing surface coordination number. The Glasner-Tompkins relation between the fundamental optical absorption of F+ in solids and the fundamental absorption of the host crystals can be generalized to include the low coordinated surfaces of SrO. The F+ color center changes the nature of halogen-surface interaction (adsorption energies) from physical adsorption to chemical adsorption. The halogen-surface interactions increase with increasing electronegativity of the halogen. The calculated adsorption energies can be explained in terms of surface electrostatic potentials, and the covalent spin pairing mechanism plays a dominant role in determining adsorbate-substrate interactions.

Density-functional theory calculations of the interaction of protons and water with low-coordinated surface sites of calcium oxide

Electronic structure calculations, using the density-functional theory within the generalized-gradient approximation and ultrasoft pseudopotentials, have been used to investigate the interaction of protons and water with five-, four-, and three-coordinated surface sites of calcium oxide, modeled by the {100}, {310}, and microfacetted {111} surface, respectively. The calculated structural parameters of bulk CaO are found to be in good agreement with experiment. The nonhydrated {100} surface shows negligible ionic relaxation from bulk termination due to the minimal distortion of the electron density in the surface layer and electron-density plots show the crystal to be strongly ionic. On the microfacetted {111} surface redistribution of the electron density along the bond between three-coordinated calcium ion and four-coordinated oxygen ions leads to the contraction of the bond lengths and relaxation of the surface calcium into the surface. Absorption of water at the six-coordinated ions in the bulk crystal is calculated to be highly endothermic. On the five-coordinated {100} surface sites, water molecules, which were initially dissociatively adsorbed, recombine to form associatively adsorbed species with an adsorption energy of approximately 65 ${\mathrm{kJmol}}^{\mathrm{\ensuremath{-}}1}$, indicating physisorption. The water molecules are adsorbed by their oxygen ion to surface calcium ions but electron-density plots show additional interactions between surface anions and hydrogen atoms. On four- and three-coordinated calcium sites, adsorbed water molecules dissociate to form hydroxyl groups indicating the higher reactivity of the lower-coordinated species. The adsorption energy at the four-coordinated calcium site is higher (156 ${\mathrm{kJmol}}^{\mathrm{\ensuremath{-}}1}$) than at the three-coordinated site (127 ${\mathrm{kJmol}}^{\mathrm{\ensuremath{-}}1}$), due to bonding of the OH group to two surface calcium ions. When calcium ions are replaced by two protons each we find that the replacement energy decreases by approximately 70 ${\mathrm{kJmol}}^{\mathrm{\ensuremath{-}}1}$ per loss of bond, from +21 ${\mathrm{kJmol}}^{\mathrm{\ensuremath{-}}1}$ for the six-coordinated calcium ions in the bulk to -187 ${\mathrm{kJmol}}^{\mathrm{\ensuremath{-}}1}$ for the three-coordinated calcium ions on the faceted {111} surface.

Atomically resolved edges and kinks of NaCl islands on Cu(111): Experiment and theory

Atomically resolved dynamic force microscopy (DFM) images of step and kink sites of NaCl films grown on the Cu(111) surface are presented. Combining experimental results with an atomistic modeling of DFM imaging, we study the mechanism of contrast formation and extract more information about the tip and NaCl film structure. The experimental results and theoretical modeling systematically demonstrate the enhanced interaction of step and kink sites of one kind with the tip. This is explained by the enhanced gradient of the electrostatic potential at low-coordinated surface sites, and considerable displacements of the step edge and kink atoms from their sites due to the interaction with the tip upon approach. The theoretical analysis predicts that the silicon tip is effectively an insulator, and that the NaCl island cannot be thicker than two monolayers. We discuss the shape and chemical structure of the tip and the mechanism of damping during DFM imaging.

Atomistic modelling of radiation effects: Towards dynamics of exciton relaxation

This brief review is focused on recent results of atomistic modelling and simulation of exciton related processes in ionic materials. We present an analysis of thermal fluctuations of the electrostatic potential in cubic ionic crystals and their relation to formation of a tail in the electron density of states and localisation of electronic states. Then the possible ‘fast’ mechanism of formation of F–H pairs in KBr as a result of decomposition of relaxing excitons is discussed. We briefly describe some ideas related to the possibility of coherent control of exciton decomposition into Frenkel defects in alkali halides. Next we turn to the results of modelling of exciton excitation and localisation at the low-coordinated surface sites of MgO. And finally, we present the results of quantum-mechanical simulation of peroxy linkages and their reaction which creates oxygen molecules in silica.

Atomistic simulation of oxide dislocations and interfaces

Atomistic simulation techniques have been used to study screw dislocations, grain boundaries, thin films and surfaces. The results show that the a/2⟨110⟩ screw dislocations in bulk MgO and NiO are more stable than the a⟨100⟩ although the latter are stabilised by vacancies. Adsorption of MgO units at the a⟨100⟩ spiral dislocation shows a complicated two-layer growth mechanism. Self-diffusion through MgO grain boundaries is shown to be faster than in the bulk crystal, with pipe diffusion the energetically most favourable route. Study of thin iron oxide films on MgO found that the most stable MgO/Fe3O4 {001}/(001) interface is an open structure with closely matching spacing between substrate Mg ions and oxygens of the film. The interaction of water with oxides MgO and SiO2 has been investigated. The dominance of the MgO {100} surface is shown through facetting of the less stable {110} and {111} surfaces. The low-coordinated surface sites hence formed are the most reactive towards adsorption of water and dissolution. Similarly, α-quartz surfaces with dangling bonds are more reactive towards water and NaOH than the fully-coordinated {0001} surface sites.

Electronic structure of excited states at low-coordinated surface sites of MgO

We study a dependence of the optical excitation energies of low-coordinated sites in nano-clusters of MgO on the oxygen coordination. The calculations were performed using the ab initio Hartree–Fock method for a quantum cluster of up to 12 ions embedded in an (MgO) 108 cubic nano-cluster. The optical absorption energies were calculated using the configuration interaction technique for single (CIS) and double electronic excitations (CISD). The low-coordinated sites included the oxygen corner, cluster edge and cluster grain. The results show a consistent reduction of the exciton excitation energies as the oxygen coordination decreases. The hole component of the unrelaxed excited state at the oxygen corner is localised on the corner oxygen ion, whereas the excited electron is delocalised around the hole outside the cluster. The hole and electron components of excited states for other coordinations are delocalised around at least several oxygen ions. We also demonstrate the possibility of energy transfer from the sites with higher coordination to those with lower coordination and finally to the localisation centres.

Spectroscopy of low-coordinated surface sites: Theoretical study of MgO

We demonstrate a dramatic dependence on the oxygen coordination of the calculated optical absorption and luminescence energies of low-coordinated sites at the surfaces and in nanoclusters of MgO. The calculations for $(\mathrm{MgO}{)}_{256}$ cubic nanoclusters were performed using an embedded molecular cluster model and both semiempirical and ab initio Hartree-Fock methods. The optical-absorption energies were calculated using the configuration interaction technique for single (CIS) and double electronic excitations. The luminescence energies were calculated using the CIS method. The low-coordinated sites included corners, kinks, step and cluster edges, and corner vacancy defects. We have also studied the zigzag steps and monatomic steps at infinite surfaces using a periodic density-functional theory method and a plane-wave basis set. For both the nanoclusters and infinite surfaces, the results show a consistent significant reduction of the exciton excitation energies and of the luminescence energies of relaxed excitons as the oxygen coordination decreases. We also demonstrate the possibility of exciton transfer from the sites with higher coordination to those with lower coordination and finally to the localization centers. Selective optical excitation of low-coordinated surface sites could be used to study molecular adsorption at surface sites, photocatalytic surface processes and desorption induced by electronic transitions.

The adsorption and activation of CO molecules on ZrO2 catalysts having low-coordinated surface sites

The adsorption and activation of CO molecules on microcrystalline ZrO2 were investigated by photoluminescence (PL), UV, FT-IR, and ESR measurements. The ZrO2 catalysts degassed at high temperatures exhibited abnormal absorption and photoluminescence associated with the presence of low-coordinated surface sites. The addition of CO molecules onto the pretreated ZrO2 catalysts led to the formation of various CO adsorption species. Moreover, increasing the contact time gave rise to polymeric radical anion species of CO showing that UV-irradiation of the ZrO2 catalysts in the presence of CO promoted the formation of these radical anion species of CO.

H2 chemisorption and consecutive UV stimulated surface reactions on nanostructured MgO

MgO nanoparticles obtained by chemical vapour deposition (CVD) were exposed to H2 and subsequently to UV irradiation and/or molecular oxygen at room temperature. A combined IR/EPR study reveals the role of low coordinated surface sites and anion vacancies in the diverse reaction steps. The hydride groups emerging from the initial H2 chemisorption processes (heterolytic splitting) play an active role in the consecutive reactions. They provide the electrons which are required for the UV induced formation of surface colour centres and for the production of superoxide anions (redox reaction). Both the colour centres and the superoxide anions are EPR active. The hydroxy groups resulting from H2 chemisorption do not actively participate in the consecutive reactions. Together with the OH groups formed in the course of colour centre formation they rather play the role of an observer. They undergo specific electronic interactions with both the colour centre and the superoxide anion which are IR inactive (or IR inaccessible) surface species. They may, however, be observed by IR spectroscopy via the specifically influenced OH stretching vibrations. This proves the intimate interplay between IR and EPR spectroscopy as applied to the surface processes under investigation. As a result, two paths were found for the three consecutive surface reaction steps: H2 chemisorption, colour centre formation and superoxide anion formation. In the first one a single, well defined surface area element is involved, namely a low coordinated ion pair, the cation of which is a constituent of an anion vacancy. In the second path a diffusion controlled intermediate step has to be adopted in which the electron required for the colour centre is transported by an H atom travelling from a hydride group to a remote anion vacancy. In either case there is clear experimental evidence that the finally resulting superoxide anions are complexed by the colour centre cations.

Structure, morphology and surface properties of nanostructured ZrO2 particles

Nanocrystalline zirconium dioxide particles have been produced by means of thermal decomposition of a liquid metal organic precursor in a flow reactor system. X-Ray diffraction patterns show that the powder consists of a mixture of the monoclinic and tetragonal phase and that the phase fractions depend on the experimental conditions during the preparation as well as on the subsequent thermal treatment of the sample. The mean particle diameter of the synthesized powders is ca. 40-50 A as estimated from the line width of Bragg reflections and from transmission electron micrographs. The crystallites are non-porous and exhibit a rough surface incorporating a high concentration of low-coordinated surface sites. In agreement with the small particle size, the powder has a high specific surface area (>200 m 2 g –1 ), which surmounts that of commercially available materials by a factor >3. The structure and morphology of the particles is essentially preserved during extensive thermal treatment up to 500°C. Hydroxyl groups as IR active surface probes clearly indicate that the phase fractions on the surface and in the bulk strongly differ from each other. Low-coordinated OH groups on monoclinic surface domains give rise to H/D exchange reactions with D 2 already at room temperature. They are evidenced and monitored with time by FTIR spectroscopy.

Structure and stability of oxygen vacancies on sub-surface, terraces, and low-coordinated surface sites of MgO:: an ab initio study

We have performed ab initio Hartree–Fock cluster model calculations on the electronic structure and properties of neutral and charged oxygen vacancies, the F color centers, at various sites of the MgO(001) surface. Sub-surface, surface, step, and corner sites have been considered. For each site we have determined the optimal structure, the relative stability of neutral versus charged vacancies, the formation energy, and, for the paramagnetic F + centers, the spin distribution as given by the isotropic hyperfine coupling constants of the unpaired electron with the surrounding Mg 2+ nuclear spin. The barrier for diffusion of oxygen ions in the presence of F 2+ centers has been estimated for the migration from the sub-surface to the surface, from a terrace site to another terrace site, and from a step to a terrace site.

FTIR Spectroscopic and Density Functional Model Cluster Studies of Methane Adsorption on MgO

Methane adsorption on the MgO surface has been studied by combining FTIR spectroscopy and density functional (DF) cluster model calculations. Different configurations for the adsorbed methane molecule have been computationally investigated on both Lewis acid and basic surface centers located at the (100) planar surface and at the low-coordinated surface sites (edge and corner). Methane has been found to be very weakly bound on the MgO surface and mainly at the low-coordinated ions. Upon interaction with the surface, the C−H symmetric stretching mode (IR symmetry forbidden for the free molecule) becomes IR active and shifts toward lower frequencies. To account for the role of Lewis acid sites in methane adsorption, CO coadsorption has been also considered.

Hydroxyl groups on oxide surfaces

The hydroxylation of a-Al2O3(000l) and MgO surfaces is studied in ultra-high vacuum by high-resolution electron energy loss spectroscopy (adsorbate and phonon losses being distinguished via resonance scattering) and X-ray photoemission spectroscopy. Hydroxyl groups which arise from the dissociative adsorption of H2O on a-Al2O3(0001) surfaces and low-coordinated MgO surface sites are characterized by chemical shifts and stretching frequencies.

The Photocatalytic Isomerization of 2-Butenes on ZrO2 Catalyst with Low Coordinated Surface Sites

Abstract The photocatalyzed isomerization of cis-2-butene has been investigated on active ZrO2 catalysts which exhibited a typical photoluminescence associated with low coordinated surface sites. A geometrical isomerization reaction occurred predominantly and with high efficiency under UV-irradiation of the ZrO2 catalyst in the presence of cis-2-butene. A good parallel between the photoluminescence intensity and the rate of photocatalytic activity clearly indicated that the low coordinated surface sites play a significant role in the photocatalytic isomerization of cis-2-butene on the active ZrO2 catalysts.

Correlation between the Madelung field and the reactivity of the MgO low-coordinated surface sites

We study correlations between Madelung constants, charges of surface ions, and chemical activity of low-coordinated sites of the MgO crystal surface with respect to dissociative adsorption of hydrogen. The ab initio Embedded Cluster model [Puchin et al., Phys. Rev. B 47, 6226 (1993)] employed in this study allowed us to reproduce correctly both short range and long range (Madelung) parts of the interaction between ions in a quantum cluster and the rest of the crystal. Our results show that sites having the same coordination numbers may have different properties, depending on values of Madelung constants. Lower Madelung constants correspond to lower ionicity, higher energy of H2 adsorption, stronger O–H and Mg–H bonds, and larger spatial separation of two adsorbed hydrogen atoms.

Surface core-level shift of 4f states for Tb(0001).

For Tb(0001), a surface core-level shift of the 4${\mathit{f}}^{7}$ photoemission multiplet of ${\mathrm{\ensuremath{\delta}}}_{\mathit{s}}$=0.26\ifmmode\pm\else\textpm\fi{}0.03 eV is determined. This value is by more than a factor of 2 smaller than the result of a recent self-consistent SLAB calculation, but agrees with the prediction of a thermochemical model. It is demonstrated that the larger values of ${\mathrm{\ensuremath{\delta}}}_{\mathit{s}}$ reported previously for polycrystalline samples originate from contributions of low-coordinated surface sites. Implications of the present findings for an improved understanding of the surface electronic structure of rare-earth metals are briefly discussed.

Bonding and vibration of CO molecules adsorbed on low-coordinated surface sites of MgO: a LCGTO-LDF cluster investigation

The adsorption of CO molecules on low-coordinated edge and corner cationic centers of a MgO surface is investigated theoretically by the LCGTO-LDF cluster method. Internuclear distances as well as frequencies and absolute IR intensities of the MgO-CO and C-O vibrational modes are calculated for model clusters embedded in large arrays of point charges. The MgO-CO bond is found to be predominantly electrostatic. But there is also a minor redistribution of the surface electronic charge similar to that occurring for the adsorption at Mg 2+ sites of a perfect MgO(001) surface. For the C-end-bound CO the main contribution to the increased blue shift of the stretching frequency at the more open sites comes from a change of the electrostatic field near the center of coordination. For CO molecules bound via the oxygen atom to the same sites a red shift is calculated which should provide a facile way to distinguish O-end bound species from the moiety coordinated via carbon. Surface relaxation is found to have only a negligibly small effect on the MgO/CO vibrational frequencies and intensities, even for the most relaxed corner site. The cluster models predict an approximate doubling of the C-O absolute IR intensity upon the CO adsorption C-end down. The intensity shows only a moderate dependence on the coordination of the adsorption center. This enhancement derives to a large extent from a change of the π component of the CO dynamical dipole moment due to interactions between the adsorbate at the cation-atop position and the nearest-neighbor surface anions. For the CO molecule adsorbed via the oxygen atom the intensity enhancement is mainly of electrostatic origin. The LCGTO-LDF characteristics are compared to the available data obtained at the Hartree-Fock level of theory for non-stoichiometric cluster models of MgO. The effect of the composition and charge of the model cluster on calculated vibrational parameters and on the adsorption energy is also analyzed.