Band energy structure calculations and spin effect in zinc-blende semiconductors

Band energy structure of the n-doped zinc-blende semiconductor was calculated using norm-conserving pseudopotential and Green function methods. The calculations for the semiconductor with different dopant contents were performed in the presence of external circularly polarized axial and polar dc-field potentials. Changes in the spin density distribution for the clusters with different dopant concentrations and at different lattice temperatures and an enhancement of the spin-polarized delocalization states in the presence of the dc-field potential were observed. The observed photo-induced spin effects were explained within a framework of the band energy model of spin physics of taking into account of the photo-induced electron–phonon anharmonicity. The results are consistent with those obtained in experiments of photospintronics.     

Size and nonlinear optical effects of ferroic organic nanocomposites

The third harmonic generation of ferroic (NH₂(C₂H₅)₂)₂CuCl₄ diethylammonium cuprate chlorate (DEACC) single crystals and nanocrystals (NCs) incorporated into polymethyl methacrylate (PMMA) polymer matrices was investigated. It was established that the crystal field spectra of Cu²⁺ ion determines the observed size dependence effect. The role of nanocrystallite size and content and the symmetry of the THG at ambient and nitrogen temperatures were studied.     

Spatial anisotropy of linear electro-optic effect in crystal materials: II. Indicative surfaces as efficient tool for electro-optic coupling optimization in LiNbO3

The electro-optic (EO) properties of pure and MgO-doped LiNbO₃ crystals are characterized by indicative surfaces (IS) describing the spatial anisotropy of the longitudinal and transverse linear EO effects. By analyzing the IS and their stereographic projections, we have determined the geometries with maximum EO coupling. The maximum magnitudes of the linear EO effect are revealed in the longitudinal and transverse geometries along the directions that substantially deviate from the principal crystallographic axes of both crystals. Spatial analysis of the IS thus yields a set of the optimized sample geometries recommended for designers who develop EO modulators or deflectors. Just by switching from the standard cell geometry to the optimized cell geometry, as determined in the present work, one may improve almost three times the modulation efficiency of EO devices based on lithium niobate crystals. This results in a corresponding reduction of their driving voltages being evidently of great practical importance for many applications.     

Eu-doped glass materials for red luminescence

Red luminescence (at wavelength about 622 nm) from Eu³⁺ ions embedded in PbO–Bi₂O₃–Ga₂O₃–BaO glass hosts is reported for room and liquid helium temperatures. The substantial influence of energy transfer processes between the host and Eu³⁺ ions is shown experimentally through the dependences of photoluminescence on light polarization and excitation wavelength. Only polarized, excited pulsed XeII laser light (λ=714 nm) gives substantial luminescence with efficiency up to 14.3%. The role of phonon-relaxation subsystem in the observed luminescence is discussed.     

Single-layered light-emitting diodes possessing methoxy-modified pyrazoloquinoline dyes in poly-N-vinylcarbazole matrix

A principal opportunity to operate by current-voltage and electroluminescent-current dependences for the single-layered light-emitting diodes (LED) on the basis of the polyvinylcarbazole (PVK) polymers doped by methoxy-substituted pyrazoloquinoline (PQ) emitting dye chromphore is demonstrated. The principal light-emitting parameters in the architecture ITO/PQ: PVK/Ca (Al) were investigated. The maximally achieved quantum efficiency of the investigated LED was equal to about 0.87% and corresponded to the brightness about the 44 Cd/m². The absence of blue shift for the electroluminescence (EL) compared to PL may indicate on an absence of near-the-surface exciton diffusion for the methoxy-PQ contrary to the phenyl-methyl-substituted PQ. The quantum chemical calculations have shown principal role of the dye chromophore state dipole moments in the observed carrier kinetics determining the EL.     

Electronic Structure and X‐Ray Photoelectron Spectroscopy of Wurtzite GaxAl1‐xN Solid Alloy

Band energy structure of Ga_xAl_1‐xN solid alloy (x=0.15; 0.65) is calculated. All one‐electron methods of band energy calculations (self‐consistent norm‐conserving pseudo‐potential (NCPP), linear muffin tin orbital method (LMTO) and linear combination atomic orbital (LCAO)) methods give considerable disagreement with the experimental data. We demonstrate that better agreement with experiment is achieved when the NCPP wavefunctions are orthogonalised to the core‐like states. Simultaneously additional geometry optimization improve the agreement with the optical data. We have carried out theoretical and experimental investigation of the band density of states for two representatives of the investigated wurtzite Ga_xAl_1‐xN solid alloys (with x=0.15 and 0.65).     

Molecular dynamics modeling in quantum chemical calculations. Optical absorption spectra of 1,3-Dimethyl-1H-Pyrazolo[3,4-b]quinoline derivatives

Paper presents the quantum chemical modeling of the optical absorption spectra of 6-fluoro, 6-bromo, 7-trifluoromethyl, 6-cyano and 6-carboethoxy derivatives of 1,3-Dimethyl-1H-Pyrazolo[3,4-b]quinoline. The calculations are performed by means of the semiempirical quantum chemical methods (AM1 or PM3) in combination with molecular dynamics (MD) simulations at T=300 K. It is shown that a particular rotational dynamics of the methyl, trifluoromethyl or ethyl groups practically does not influence the optical absorption in the spectral range 200-500 nm whereas broadening of absorption bands may be well reproduced within MD simulations including all types of nuclei vibrations. The results of calculations are compared with the measured spectra of optical absorption. The quantum chemical method AM1 in combination with MD simulations gives for all dyes the best agreement between the calculated and measured spectral positions of the first absorption band (absorption threshold).