Search results for "Combination"
showing 10 items of 1379 documents
First-principles modeling of oxygen interaction with SrTiO3(001) surface: Comparative density-functional LCAO and plane-wave study
2011
Large scale first-principles calculations based on density functional theory (DFT) employing two different methods (atomic orbitals and plane wave basis sets) were used to study the energetics, geometry, the electronic charge redistribution and migration for adsorbed atomic and molecular oxygen on defect-free SrTiO3(001) surfaces (both SrO- and TiO2-terminated), which serves as a prototype for many ABO3-type perovskites. Both methods predict substantial binding energies for atomic O adsorption at the bridge position between the oxygen surface ions and an adjacent metal ion. A strong chemisorption is caused by formation of a surface molecular peroxide ion. In contrast, the neutral molecular …
Histological and histomorphometrical analysis of a silica matrix embedded nanocrystalline hydroxyapatite bone substitute using the subcutaneous impla…
2010
The clinical suitability of a bone substitute material is determined by the ability to induce a tissue reaction specific to its composition. The aim of this in vivo study was to analyze the tissue reaction to a silica matrix-embedded, nanocrystalline hydroxyapatite bone substitute. The subcutaneous implantation model in Wistar rats was chosen to assess the effect of silica degradation on the vascularization of the biomaterial and its biodegradation within a time period of 6 months. Already at day 10 after implantation, histomorphometrical analysis showed that the vascularization of the implantation bed reached its peak value compared to all other time points. Both vessel density and vascula…
Millisecond radiative recombination in poly(phenylene vinylene)-based light-emitting diodes from transient electroluminescence
2007
The current and electroluminescence transient responses of standard poly phenylene vinylene -based light-emitting devices have been investigated. The electroluminescence time response is longer milliseconds scale than the current switch-off time by more than one order of magnitude, in the case of small area devices 0.1 cm2 . For large area devices 6 cm2 the electroluminescence decay time decreases from 1.45 ms to 100 s with increasing bias voltage. The fast current decay limits the electroluminescence decay at higher voltages. Several approaches are discussed to interpret the observed slow decrease of electroluminescence after turning off the bias. One relies upon the Langevin-type bimolecu…
Influence of stoichiometry on high temperature intrinsic defects in fused silica
1992
Abstract The set of intrinsic defects, their generation, interactions and recombination reactions are discussed, including the participation of atomic and molecular oxygen, which allows one to take into account the stoichiometry of fused silica. The mathematical solution of the set of equations, which characterizes the equilibrium concentrations of intrinsic defects, gives the specific values of intrinsic defect concentrations at different temperatures and stoichiometries. The influence of such intrinsic defects frozen in at room temperature on the spectroscopic characteristics of fused silica is analyzed.
Angular dependence of recombination luminescence-detected EPR in a ZnO crystal
2015
Angular dependency of electron paramagnetic resonance, optically detected by UV-excited recombination luminescence (RL-EPR), was measured for nominally pure ZnO single crystals. Observed magnetic resonances belong to the broad yellow RL band with slow decay centred at 610 nm, which is characteristic of untreated ZnO crystals. In the sample, irradiated with the 266 nm UV laser, an additional RL band centred at 740 nm appears, which has considerably faster decay time than the yellow one. This RL band is characteristic of the luminescence of Fe3+ ions in the ZnO crystals. It could be observed only after the UV laser treatment. No RL-EPR signal was detected for this RL band. Our spectral simula…
Ab initio calculations of charged point defects in GaN
2005
The Generalized Gradient Approximation (GGA) of a Density Functional Theory (DFT) in LCAO basis is used to determine electronic structure of charged point defects in hexagonal GaN. Impurities of MgGa, ZnGa, SiN and CN (the most common dopants) were considered for charge states 0, ±1, –2. The vacancies of VGa and VN were considered for charge states 0 and 0, ±1, ±2 respectively. The defects in consideration are compared from the point of view of the one-electron states localization. All the calculations were performed for a supercell containing 96 atoms. The divergence due to periodically repeated charges has been eliminated by the use of a uniform background charge of an opposite sign. (© 2…
The Use of Atomic Force Microscopy in Determining the Stiffness and Adhesion Force of Human Dentin After Exposure to Bleaching Agents
2009
Introduction: Oxidant bleaching agents may induce several alterations on mineralized teeth tissues. Our aim is to study,at the ultrastructurallevel,mechanicalmodifications induced on dentin after exposure to different bleaching agents.Methods: Nanoindentation performed with atomic force microscopy was used to measure changes in dentin stiffness as well as the adhesion force between the tip and the tissue both in intertubular and peritubulardentin.Foreachspecimen,dentinlocalization, and bleaching agent, 100 independent nanoindentations were performed. Carbamide peroxide (30%) and hydrogenperoxide(35%) wereusedasbleaching agents. Results: A significant reduction of both stiffness and adhesion…
Doped TiO2 Nanomaterials and Applications
2008
This special issue brieflyreviews some trends and factors that have impacted heterogeneous photocatalysis with next generation TiO2 nanophotocatalysts that could absorband make use of both UV (290–400 nm) and visible (400–700 nm) sunlight to enhance process efficiencies, along with some issues of current debate in the fundamental understanding of the science that underpins the field. Preparative methods and some characteristics features of doped TiO2 as well as its environmental applications are presented and described. The next generation of doped TiO2 photocatalysts should enhance overall process photoefficiencies in many cases, since doped TiO2s absorb a greater quantity of solar radiati…
Charge Transport Layers Limiting the Efficiency of Perovskite Solar Cells: How To Optimize Conductivity, Doping, and Thickness
2019
Perovskite solar cells (PSCs) are one of the main research topics of the photovoltaic community; with efficiencies now reaching up to 24%, PSCs are on the way to catching up with classical inorganic solar cells. However, PSCs have not yet reached their full potential. In fact, their efficiency is still limited by nonradiative recombination, mainly via trap-states and by losses due to the poor transport properties of the commonly used transport layers (TLs). Indeed, state-of-the-art TLs (especially if organic) suffer from rather low mobilities, typically within 10(-5) and 10(-2) cm(-2) V-1 s(-1), when compared to the high mobilities, 1-10 cm(-2) V-1 s(-1), measured for perovskites. This work…
Effect of Pressure and Temperature on the Local Structure and Lattice Dynamics of Copper(II) Oxide
2016
Abstract Microcrystalline and nanocrystalline (6 nm) CuO were studied in situ by the Cu K-edge X-ray absorption spectroscopy as a function of pressure (0-20 GPa) and temperature (10-300 K). Pressure dependence of X-ray absorption near edge structure (XANES) was interpreted within the full-multiple-scattering formalism based on the relaxed atomic structure determined by ab initio linear combination of atomic orbital (LCAO) calculations. Temperature dependence of the mean-square relative displacement (MSRD) for the four shortest Cu–O distances was obtained from the analysis of extended X-ray absorption fine structure (EXAFS) and described by the correlated Einstein model with the characterist…