Pv Sushko

Mg clusters on MgO surfaces : study of the nucleation mechanism with MIES and ab initio calculations

We combined experimental studies using ultraviolet photoelectron spectroscopy (UPS), metastable impact electron spectroscopy (MIES) and temperature programmed desorption (TPD) with abinitio calculations of metal adsorption on the perfect MgO surface and at defect sites in order to elucidate the role of surface defects in the initial stages of nucleation and growth of metal clusters at oxide surfaces. MgO films (2 nm thick) grown on Mo and W substrates were used as a prototype system. The MIES and UPS (HeI) spectra were collected insitu, and the growth of Mg clusters was observed by monitoring the dynamics of additional MIES peaks during Mg deposition. TPD experiments were made in order to monitor the surface coverage by Mg clusters and to determine the Mg desorption energies. Interpretation of the results was made on the basis of theoretical modelling using density functional theory (DFT) calculations in both periodic and embedded cluster models. The geometric and electronic structures of the surface terrace, F-centre, positively charged anion vacancy, and step edge at the MgO(001) surface were calculated, and their role in adsorption and clustering of Mg atoms on this surface was studied. The absolute position of the top of the surface valence band of MgO with respect to the vacuum was calculated and compared with the MIES results. The MIES spectra were modelled on the basis of surface density of states (SDOS). The calculated SDOS predicted the location of additional peaks in the band gap and their shift as a function of Mg concentration on the surface in agreement with the MIES data. The desorption energies of Mg atoms from small Mg clusters formed at step edges are found to be about 1.3 eV atom-1. Comparison between the theoretical results and the experimental data suggests preferential initial adsorption of Mg atoms at steps and kinks, rather than at charged and neutral vacancies. At larger exposures these Mg atoms serve as the nucleation sites.

Optical Properties of Nanocrystal Interfaces in Compressed MgO Nanopowders

The optical properties and charge trapping phenomena observed on oxide nanocrystal ensembles can be strongly influenced by the presence of nanocrystal interfaces. MgO powders represent a convenient system to study these effects due to the well-defined shape and controllable size distributions of MgO nanocrystals. The spectroscopic properties of nanocrystal interfaces are investigated by monitoring the dependence of absorption characteristics on the concentration of the interfaces in the nanopowders. The presence of interfaces is found to affect the absorption spectra of nanopowders more significantly than changing the size of the constituent nanocrystals and, thus, leading to the variation of the relative abundance of light-absorbing surface structures. We find a strong absorption band in the 4.0−5.5 eV energy range, which was previously attributed to surface features of individual nanocrystals, such as corners and edges. These findings are supported by complementary first-principles calculations. The possibility to directly address such interfaces by tuning the energy of excitation may provide new means for functionalization and chemical activation of nanostructures and can help improve performance and reliability for many nanopowder applications.

Modelling of electron and hole trapping in oxides

We critically review several examples of successful modelling of electron and hole trapping in metal oxides, which demonstrate a breadth of polaronic behaviour. The examples range from self-trapping in the perfect lattice to trapping by structural defects and impurities and illustrate the important phenomenon of charge localization. We present recent results in four different systems: nanoporous mayenite, amorphous SiO2, crystalline hafnia and MgO surfaces and interfaces. The complex nature of charge trapping and polaronic behaviour in these systems can go beyond traditional cases and illustrate the different challenges involved.

The prediction of metastable impact electronic spectra (MIES): perfect and defective MgO(001) surfaces by state-of-the-art methods

We re-examine the theory of metastable impact electron spectroscopy (MIES) in its application to insulating surfaces. This suggests a quantitative approach which takes advantage of recent developments in highly eYcient many-electron computational techniques. It gives a basis to the interpretation of experimental MIES spectra for perfect and defective surfaces. Our method is based on a static approach to predicting Auger de-excitation (AD) rates of He1(1s2s) projectiles. A key quantity is the surface density of states (DOS ) projected on the 1s orbital of the He1 atom, which is calculated along its trajectory. We use density functional theory within both supercell geometry and embedded cluster models to calculate MIES spectra for the perfect MgO surface and for an MgO surface with diVerent concentrations of adsorbed oxygen atoms. First we calculate the Auger de-excitation rates at various positions of the projectile above the surface. To predict MIES spectra, we integrate over projectile trajectories, with a subsequent weighted averaging with respect to various lateral positions of He1 above the MgO surface unit cell. It is important to examine final-state eVects for a correct comparison between theory and experiment, especially when there are localised defect states.

AN EFFECT OF CHARGED AND EXCITED STATES ON DECOMPOSITION OF 1,1‐DIAMINO‐2,2‐DINITROETHYLENE

The electronic structure of 1,1‐diamino‐2,2‐dinitroethylene molecules in the gas phase and its possible dissociation pathways were simulated by Density Functional Theory. It was shown that charging and excitation may not only reduce the activation barriers for decomposition reactions, but also dramatically change the dominating chemistry. Two competing primary initiation mechanisms of FOX‐7 decomposition were found: C–NO2 bond fission and C–NO2 to CONO isomerization; this reconciles earlier seemingly contradictory predictions.

'Atomistic' mesh generation for the simulation of semiconductor devices

A methodology for mesh generation with nodes placed on the atomic positions of the structure is presented. The meshing strategy is based on the use of patterns to decompose a unit cell of the target crystal into tetrahedra. The mesh generation procedure has been applied to crystalline Si and SiO2 (α-quartz) as well as to their interface. The constructed meshes have been consequently randomly populated by dopants using Monte Carlo approach. The dopants are replacing silicon atom in nodes of the crystal. The ‘atomistic’ mesh populated with random discrete dopants has been used to simulate an ensemble of microscopically different double gate MOSFETs in order to demonstrate the functionality of the meshing methodology.