Nanoporous crystal 12CaO.7Al2O3: a playground for studies of ultraviolet optical absorption of negative ions

A novel nanoporous material 12CaO.7Al2O3 (C12A7) offers a possibility of incorporating large concentrations (>1021 cm-3) of a wide range of extraframework anions inside its nanopores. We have investigated, both experimentally and theoretically, optical absorption associated with several types of such anions, including F-, OH-, O-, O2-, O2-, and O22-, and assigned their optical absorption bands. It is demonstrated that the chemical identity and concentration of extraframework anions can be controlled by an appropriate treatment of "as grown" C12A7. We also show that the position of the adsorption edge is, in turn, determined by the chemical identity of the extraframework species and can be varied in the range of approximately 4-6 eV. We suggest that C12A7 is a unique host material, which can be used as a playground for studying negatively charged species that are unstable in other environments.     

Doubly positively charged oxygen vacancies in silica glass

The geometric and electronic structures of doubly positively charged oxygen vacancies in a-SiO₂ are calculated. We have found four types of relaxed configurations in the amorphous matrix, corresponding to puckered and unpuckerd configurations of Si atoms of the vacancy. The predicted optical absorption strongly depends on the atomic configuration of particular center and transition energies are distributed in the range from 4.5 to 6.5 eV.     

On the analysis of a kind of nonlinear Sobolev equation through locally applied pseudo‐spectral meshfree radial point interpolation

In this study, we develop an approximate formulation for two‐dimensional nonlinear Sobolev problems by focusing on pseudospectral meshless radial point interpolation (PSMRPI) which is a kind of locally applied radial basis function interpolation and truthfully a meshless approach. In the PSMRPI method, the nodal points do not need to be regularly distributed and can even be quite arbitrary. It is easy to have high order derivatives of unknowns in terms of the values at nodal points by constructing operational matrices. The convergence and stability of the technique in some sense are studied via some examples to show the validity and trustworthiness of the PSMRPI technique.     

Interaction of organic molecules with the TiO2 (110) surface: ab inito calculations and classical force fields

We have studied the adsorption of a number of organic molecules consisting of methyl, benzyl, and carboxylic groups on the rutile TiO2 (110) surface using both ab initio and atomistic simulation techniques. We have tested the applicability of a simple embedded cluster model to studying the adsorption of small organic molecules on the perfect rutile TiO2 (110) surface, and used this model to develop a classical force field for the interactions of a wide class of organic molecules consisting of these groups with the rutile TiO2 (110) surface. The force field accounts for physisorption and ionic bonding of organic molecules at the surface. It allows the reproduction of adsorption energies and of geometries of organic molecules on the rutile surface. It should be useful for studying diffusion of these molecules and their manipulation with use of AFM and STM tips.     

Two-dimensional polaronic behavior in the binary oxides m-HfO2 and m-ZrO2

We demonstrate that the three-dimensional (3D) binary monoclinic oxides HfO2 and ZrO2 exhibit quasi-2D polaron localization and conductivity, which results from a small difference in the coordination of two oxygen sublattices in these materials. The transition between a 2D large polaron into a zero-dimensional small polaron state requires overcoming a small energetic barrier. These results demonstrate how a small asymmetry in the lattice structure can determine the qualitative character of polaron localization and significantly broaden the realm of quasi-2D polaron systems.     

Laser control of desorption through selective surface excitation

We review recent developments in controlling photoinduced desorption processes of alkali halides. We focus primarily on hyperthermal desorption of halogen atoms and show that the yield, electronic state, and velocity distributions of desorbed atoms can be selected using tunable laser excitation. We demonstrate that the observed control is due to preferential excitation of surface excitons. This approach takes advantage of energetic differences between surface and bulk exciton states and probes the surface exciton directly. We demonstrate that desorption of these materials leads to controlled modification of their surface geometric and electronic structures. We then extend the exciton mechanism of desorption, developed for alkali halides, to metal oxide surfaces, in particular magnesium oxide. In addition, these results demonstrate that laser desorption can serve as a solid-state source of halogen and oxygen atoms, in well-defined electronic and velocity states, for studying chemical processes in the gas phase and at surfaces.