Narrow Red Emission Band Fluoride Phosphor KNaSiF6:Mn(4+) for Warm White Light-Emitting Diodes.

Red phosphors AMF6:Mn(4+) (A = Na, K, Cs, Ba, Rb; M = Si, Ti, Ge) have been widely studied due to the narrow red emission bands around 630 nm. The different emission of the zero-phonon line (ZPL) may affect the color rendering index of white light-emitting diodes (WLED). The primary reason behind the emergence and intensity of ZPL, taking KNaSiF6:Mn(4+) as an example, was investigated here. The effects of pressure on crystal structure and luminescence were determined experimentally and theoretically. The increase of band gap, red shift of emission spectrum and blue shift of excitation spectrum were observed with higher applied pressure. The angles of ∠FMnF and ∠FMF(M = Si, Ti, Ge) were found clearly distorted from 180° in MF6(2-) octahedron with strong ZPL intensity. The larger distorted SiF6(2-) octahedron, the stronger ZPL intensity. This research provides a new perspective to address the ZPL intensity problem of the hexafluorosilicate phosphors caused by crystal distortion and pressure-dependence of the luminescence. The efficacy of the device featuring from Y3Al5O12:Ce(3+) (YAG) and KNaSiF6:Mn(4+) phosphor was 118 lm/W with the color temperature of 3455 K. These results reveal that KNaSiF6:Mn(4+) presents good luminescent properties and could be a potential candidate material for application in back-lighting systems.

Absorption, fluorescence and second harmonic generation in Cr³âº-doped BiB3O6 glasses.

Synthesis, spectral properties and photoinduced nonlinear optical effects of chromium-doped BiB3O6 glass are studied in the present paper. Absorption, excitation and time resolved luminescence spectra are presented and luminescence decay behavior is discussed. Detailed analysis of the obtained spectra (assignment of the most prominent spectral features in terms of the corresponding Cr(3+) energy levels, crystal field strength Dq, Racah parameters B and C) was performed. A weak photostimulated second harmonic generation signal was found to increase drastically due to poling by proton implantation in the investigated sample.

Specific features of the electronic structure of a novel ternary Tl3PbI5 optoelectronic material.

A novel Tl3PbI5 crystal has been studied both experimentally and theoretically. Complex measurements of the X-ray photoelectron core-level and valence-band spectra for the pristine and Ar(+)-ion irradiated surfaces of a Tl3PbI5 single crystal grown by the Bridgman-Stockbarger method were performed in order to clarify their principal properties (charge carriers mobility, effective inter-band distances, effective absorption etc.) relevant for optoelectronic applications. The principal role of two heavy cations - Tl and Pb - is explored. The X-ray photoelectron spectroscopy results reveal a high chemical stability of the Tl3PbI5 single crystal surface which makes it very promising for technological applications. Theoretical band-structure calculations for the Tl3PbI5 compound reveal that the I 5p states dominate in the top of the valence band and play a crucial role in the formation of the optical features and charge carrier mobility. The bottom of the Tl3PbI5 valence band is formed mainly by the admixture of Tl 6s and Pb 6s states, while the unoccupied Pb 6p and Tl 6p states dominate at the bottom of the conduction band. The band energy dispersion related to effective masses and the charge carrier mobility is studied in detail. Crucially, the theoretical calculations reveal an indirect band gap for Tl3PbI5, which indicates a strong influence of the electron-phonon interaction on the observed optoelectronic features. The temperature measurements of the fundamental absorption have shown that the band energy gap of Tl3PbI5 increases from 2.29 to 2.39 eV when the temperature changes from 300 to 100 K.

Structural, electronic, and optical features of CuAl(S(1-x)Se(x))2 solar cell materials.

Detailed first-principles calculations of the structural, electronic, and optical properties of solid solutions of the promising solar cell material CuAl(S(1-x)Se(x))2 over the whole range of Se concentration from x = 0 to x = 1 were performed. It was established that the calculated lattice parameters, band gap, and anisotropic refractive indices vary linearly with the Se concentration. The obtained linear dependences allow for reliable estimations of all these quantities for any value of x, which determines the solid solution composition. The calculated results were compared with the experimental data available for x = 0, 0.5, and 1.0; very good agreement was demonstrated, which gives confidence in the properties calculated for other Se concentrations (x = 0.25, 0.75). The findings from the present paper can be used in a straightforward way for the successful production of CuAl(S(1-x)Se(x))2 mixed compounds with desired optoelectronic parameters, which are defined by the composition-tuned mobility of the charge carriers in the upper valence band and the conduction band. Extension of the presented approach to other materials is also possible.

Spectral kinetics of fluorescence spectra of fluoroderivatives of pyrazoloquinoline in different polymer matrices.

Three fluoro-substituted pyrazoloquinoline derivatives have been placed in polymer matrices: polycarbonate, poly(methyl methacrylate) and polystyrene. Absorption, excitation and time-resolved fluorescence spectra have been recorded and luminescence lifetime of the optically active composites has been determined. Influence of the dielectric environment the optical properties of the chromophores are discussed. Experimental data and conclusions are supported with (TD)DFT calculations.

Photoinduced features of energy bandgap in quaternary Cu2CdGeS4 crystals.

The quaternary chalcogenide crystal Cu2CdGeS4 was studied both experimentally and theoretically in the present paper. Investigations of polarized fundamental absorption spectra demonstrated a high sensitivity to external light illumination. The photoinduced changes were studied using a cw 532 nm green laser with energy density about 0.4 J cm(-2). The spectral maximum of the photoinduced anisotropy was observed at spectral energies equal to about 1.4 eV (energy gap equal to about 1.85 eV) corresponding to maximal density of the intrinsic defect levels. Spectroscopic measurements were performed for polarized and unpolarized photoinducing laser light to separate the contribution of the intrinsic defect states from that of the pure states of the valence and conduction bands. To understand the origin of the observed photoinduced absorption near the fundamental edge, the benchmark first-principles calculations of the structural, electronic, optical and elastic properties of Cu2CdGeS4 were performed by the general gradient approximation (GGA) and local density approximation (LDA) methods. The calculated dielectric function and optical absorption spectra exhibit some anisotropic behavior (shift of the absorption maxima in different polarizations) within the 0.15-0.20 eV energy range not only near the absorption edge; optical anisotropy was also found for the deeper inter-band transition spectral range. Peculiar features of chemical bonds in Cu2CdGeS4 were revealed by studying the electron density distribution. Possible intrinsic defects are shown to affect the optical absorption spectra considerably. Pressure effects on the structural and electronic properties were modeled by optimizing the crystal structure and calculating all relevant properties at elevated hydrostatic pressure. The first estimations of the bulk modulus (69 GPa (GGA) or 91 GPa (LDA)) and its pressure derivative for Cu2CdGeS4 are also reported.

First-principles calculations of the structural, electronic, optical and elastic properties of the CuYS2 semiconductor.

The ternary semiconductor CuYS2 is studied by using the first-principles methods in the density functional theory (DFT) framework. The structural, electronic, optical and elastic properties were calculated at the ambient and elevated hydrostatic pressures. The compound was shown to have an indirect band gap of about 1.342/1.389 eV (in the generalized gradient and local density approximations, respectively). The anisotropy of the optical properties was studied by calculating the absorption spectra, dielectric function and index of refraction for different polarizations. The anisotropy of the elastic properties was visualized by plotting the three-dimensional dependence of the Young's moduli on the direction in the crystal lattice. The obtained results, which are reported for the first time to the best of the author's knowledge, can facilitate assessment of possible applications of the title material.

Analysis of vacuum ultraviolet electronic spectra of Ce3+ and Pr3+ ions in Ca9Lu(PO4)7: crystal-field calculations and simulation of optical spectra.

The 4f-5d excitation and emission spectra of Ce(3+) and Pr(3+) ions in Ca9Lu(PO4)7 as recently reported (2012 J. Phys.: Condens. Matter 24 385502) were further analyzed and simulated by employing the effective Hamiltonian model for the 4f(N) and 4f(N-1)5d electronic configurations of impurity lanthanide ions and the exchange charge model of crystal-field theory. The multi-site effect on the 4f-5d transition spectra was explicitly discussed from the points of view of the local structure and site occupation ratios of lanthanide ions in Ca9Lu(PO4)7. An excellent agreement between the predicted and measured spectra confirms the validity of the performed calculations. Based on these energy level and intensity calculation results, the radiative lifetimes of the 5d-4f emissions of Ce(3+) and Pr(3+) ions have been modeled to show nearly independent temperature trends. Comparison with the measured lifetimes suggests the nonradiative relaxation process in this host is probably related to the intrinsic defect states. In addition to the studies of the 4f-5d transitions, a general theoretical scheme to calculate the lowest 4f-6s transition energy of the Ce(3+) ion was proposed for the first time on the basis of the ligand polarization model. The predicted 6s energy position of the Ce(3+) ion in Ca9Lu(PO4)7 is solid evidence corroborating our previous spectroscopic assignment.

Electronic structure of ytterbium-implanted GaN at ambient and high pressure: experimental and crystal field studies.

The results of high-pressure low-temperature optical measurements in a diamond-anvil cell of bulk gallium nitride crystals implanted with ytterbium are reported in combination with crystal field calculations of the Yb(3+) energy levels. Crystal field analysis of splitting of the (2)F(7/2) and (2)F(5/2) states has been performed, with the aim of assigning all features of the experimental luminescence spectra. A thorough analysis of the pressure behavior of the Yb(3+) luminescence lines in GaN allowed the determination of the ambient-pressure positions and pressure dependence of the Yb(3+) energy levels in the trigonal crystal field as well as the pressure-induced changes of the spin-orbit coupling coefficient.

Specific features of Tm3+ doped BiB3O6 glasses fluorescence spectra and their kinetics.

Synthesis and spectral fluorescent features of the thulium-doped BiB(3)O(6) glasses are presented. All spectra were recorded using a pulsed (pulse energy ca. 1µJ, pulse duration 10ns) 355nm third harmonic of 10kHz Nd:YAG laser as an excitation source. A laser beam was focused in a backscattering geometry onto about 1mm(2) spot on the surface of a sample. The Andor SR-303i spectrograph equipped with an Andor DH-501 intensified charge coupled device with spectral resolution up to 1nm was used as a spectra recorder. The time-resolution of this system can be as low as 5ns. The decay kinetics was derived from integrated time-resolved spectra. Additionally the absorption and excitation spectra were measured. The main parameters of the Judd-Ofelt analysis were calculated and comparison of the obtained results with corresponding data for other materials was carried out.

Spectral and fluorescent kinetics features of Nd3+ ion in Nb2O5, Ta2O5 and La2O3 mixed lithium zirconium silicate glasses.

Li(2)O-ZrO(2)-SiO(2):Nd(3+) glasses mixed with Nb(2)O(5), Ta(2)O(5) and La(2)O(3) were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature. The Judd-Ofelt theory was successfully applied to characterize Nd(3+) spectra of all the three glasses. From this theory, various radiative properties like transition probability A, branching ratio ß(r), the radiative lifetime τ(r), for (4)F(3/2) emission level in the spectra of these glasses has been evaluated. The radiative life time for (4)F(3/2) level of Nd(3+) ions has also been measured and quantum efficiencies were estimated. Among the three glasses studied, the La(2)O(3) mixed glass has exhibited the highest quantum efficiency. The reasons for such higher value have been discussed based on the relationship between the structural modifications taking place around the Nd(3+) ions.

A luminescence spectroscopy and theoretical study of 4f-5d transitions of Ce3+ ions in SrAlF5 crystals.

This research is focused on the 4f-5d transitions in Ce(3+) centers doped into tetragonal ß-SrAlF(5) single crystals belonging to the I4(1)/a space group. The presence of four non-equivalent Sr(2+) sites in this compound leads to the appearance of three spectroscopically non-equivalent Ce(3+) luminescence centers, which can be well distinguished using a time-resolved laser spectroscopy technique. All 4f-5d transitions have slightly varying excitation and emission energies with characteristic probabilities resulting in several decay times that can be determined experimentally. One of these centers experiences strong perturbation due to a defect nearby, probably the O(2-) impurity ion substituting for the F(-) ion and acting as a charge compensator as well. Identification of these photoluminescence centers is performed using crystal field calculations. The crystal field parameters are calculated for two identified centers using the structural data for SrAlF(5); diagonalization of the crystal field Hamiltonian results in obtaining the splitting of the Ce(3+) 5d states. This method allows 'regular' unperturbed Ce(3+) centers with selected Sr(2+) sites to be assigned.

Spectroscopy of gadolinium gallium garnet crystals doped with Yb(3+) revisited.

The optical spectroscopy measurements of gadolinium gallium garnet (GGG) crystals doped with Yb show evidence of the presence of non-equivalent optical centers with very similar radiative decay rates. The energy level schemes of those centers have been determined on the basis of optical absorption, luminescence and Raman experiments. Crystal field fitting resulted in two sets of slightly different crystal field parameters for two non-equivalent Yb centers. Both sets of parameters describe perfectly the experimentally detected Yb(3+) energy levels. Correlation between systematic trends in the experimental energy level schemes and crystal field parameters is discussed.

Microscopic analysis of the crystal field strength and electron-vibrational interaction in cubic SrTiO(3) doped with Cr(3+), Mn(4+) and Fe(5+) ions.

A detailed microscopic study of the crystal field strength 10Dq for different interionic distances in cubic SrTiO(3) doped with three isoelectronic ions (Cr(3+), Mn(4+) and Fe(5+)) was performed. The exchange charge model of the crystal field was used to calculate the 10Dq values at different distances between impurity ions and ligands. The obtained results were represented by the power laws 1/R(n), with n = 4.9050, 5.7990 and 6.5497 for Cr(3+), Mn(4+) and Fe(5+), respectively. For the first time the role of two different contributions (the point charge and exchange charge) into the total crystal field strength was studied separately. With the obtained 10Dq(R) dependences, a number of important physical quantities describing the optical and dynamical properties of impurity centers (such as the constants of the electron-vibrational interaction, Huang-Rhys parameters, Stokes' shifts, Jahn-Teller stabilization energies, changes of the chemical bond lengths due to the combined effect of the local vibrational normal modes, bulk modulus and Grüneisen constants for the a(1g) normal mode) were calculated. The obtained results are in good agreement with available experimental data and can be readily applied for analysis of the optical spectra, electron-vibrational interaction and pressure effects for these and other similar systems.

First-principles study of the electronic and optical properties of CuXS(2) (X = Al, Ga, In) and AgGaS(2) ternary compounds.

First-principles calculations (using the CASTEP code, as implemented in the Materials Studio package) of electronic and optical properties of several representative ternary semiconductors (CuXS(2), X = Al, Ga, In, and AgGaS(2)) were performed. After geometry optimization of the crystal structures, the band structures and partial and total densities of states were calculated and analyzed for all compounds considered. A scissor operator value of about 1.5 eV was introduced systematically to overcome the intrinsic drawback of the calculation technique-underestimation of the calculated band gaps. From the dielectric functions calculated with this correction, Sellmeyer's approximations for the dependence of the refractive index on the wavelength were obtained for all crystals studied. The values of the refractive indices calculated are in reasonable agreement with the experimental data.

Structural investigations on PbO-Sb(2)O(3)-B(2)O(3):CoO glass ceramics by means of spectroscopic and dielectric studies.

PbO-Sb(2)O(3)-B(2)O(3) glasses mixed with different concentrations of CoO (ranging from 0 to 2.0 mol%) were crystallized. The samples were characterized by x-ray diffraction, scanning electron microscopy and differential scanning calorimetric techniques. The x-ray diffraction and scanning electron microscopic studies have revealed the presence of CoSb(2)O(6), Co(2.33)Sb(0.67)O(4), Pb(5)Sb(2)O(8),Pb(3)(SbO(4))(2), PbB(4)O(7) and Co(3)O(4) crystalline phases in these samples. The DSC studies have indicated the spreading of the crystallization from the inside to the surface of the samples as the concentration of the crystallizing agent is increased. The IR and Raman spectroscopic studies have pointed out the existence of conventional BO(3), BO(4), SbO(4) and also Co(III)-O structural units in the glass ceramic samples. These studies have further indicated the decreasing concentration of symmetrical structural vibrational groups with increase in the concentration of CoO. The results of various studies, namely dielectric properties over a range of frequency and temperature, photo-induced birefringence, optical absorption, fluorescence and magnetic susceptibility at room temperature of PbO-Sb(2)O(3)-B(2)O(3):CoO glass ceramics, have also been reported. The variations observed as a function of the concentration of crystallizing agent in all these properties have been analyzed in the light of different oxidation states and environments of cobalt ions in the glass ceramic network.

Spectroscopic and crystal field studies of YAlO(3) single crystals doped with Mn ions.

Detailed analysis of the spectroscopic properties of the YAlO(3) crystals doped with manganese ions has been performed. The exchange charge model of the crystal field was used to calculate the crystal field parameters and energy levels of the Mn(4+) and Mn(5+) ions in YAlO(3). It was shown that both ions contribute to the formation of the absorption spectra. The calculated energy levels are in good agreement with the main observed absorption peaks. Comparison of the Racah parameters B for both ions in YAlO(3) with those for free ions shows a significant role played by the covalent effects, especially for Mn(5+).

Spectral analysis of Er(3+)-, Er(3+)/Yb(3+)- and Er(3+)/Tm(3+)/Yb(3+)-doped TeO(2)-ZnO-WO(3)-TiO(2)-Na(2)O glasses.

In this paper, we present the spectroscopic properties of Er(3+)-, Er(3+)/Yb(3+)- and Er(3+)/Tm(3+)/Yb(3+)-doped novel tellurite glasses. From the measured absorption spectra, Judd-Ofelt (JO) intensity parameters (Ω(2), Ω(4) and Ω(6)) have been evaluated for the Er(3+)-doped glass. With 980 nm excitation three strong upconversion emission bands centered at 505, 520 and 630 nm were observed for both Er(3+)- and Er(3+)/Yb(3+)-codoped glasses and the characteristic near-infrared emission bands were spectrally centered at 1.535 µm. The near-infrared spectra of Er(3+)- and Er(3+)/Yb(3+)-doped glasses have shown full width at half-maxima (FWHM) around 100 nm and 120 nm for the erbium [Formula: see text] transition, respectively. The measured maximum decay times of the [Formula: see text] transition (at wavelength 1.535 µm) are about 7.24 ms and 7.68 ms for 1.0Er(3+) and 1.0Er(3+)/ 2Yb(3+) (mol%)-codoped glasses, respectively. The maximum stimulated emission cross sections for the [Formula: see text] transition of Er(3+) and Er(3+)/Yb(3+) are 8.64 × 10(-21) and 6.78 × 10(-21) cm(2). From 1Er(3+)/ 1Tm(3+)/ 2Yb(3+) (mol%)-codoped glass, broad near-infrared emission bands centered at 1510 nm ([Formula: see text]) and 1637 nm ([Formula: see text]) with full width at half-maxima (FWHM) around 52 nm and 60 nm, respectively, were observed. These glasses with broad near-infrared emissions should have potential applications in tunable lasers and broadband optical amplification at low-loss telecommunication windows.

Spectral analysis of RE(3+) (RE = Er, Nd, Pr and Ho):GeO(2)-B(2)O(3)-ZnO-LiF glasses.

This paper reports on the absorption, visible and near-infrared luminescence properties of Er(3+), Nd(3+), Pr(3+) and Ho(3+)-doped alkali fluoride zinc boro germanate glasses. The x-ray diffraction (XRD) and differential thermal analysis (DTA) profiles of the host glass have been carried out, to confirm its structure and thermal stability. From the measured absorption spectra, Judd-Ofelt (JO) intensity parameters (Ω(2), Ω(4) and Ω(6)) have been calculated for all the studied ions. For Pr(3+) and Ho(3+)-doped glasses, upconversion emission spectra have also been measured. Decay curves were measured for the visible emissions of Er(3+), Pr(3+) and Ho(3+) ions at 547 nm (Er(3+): green), 604 nm and 641 nm (Pr(3+): orange and red) and 658 nm (Ho(3+): red), respectively. The near-infrared emission spectra of Er(3+)/Yb(3+) and Nd(3+)-doped glasses have shown full width at half maximum (FWHM) around 95 nm (for the [Formula: see text] transition) and 63 nm (for the [Formula: see text] transition), respectively. The measured maximum decay times of [Formula: see text] transition (at wavelength 1.51 µm) are about 5.72 ms and 6.14 ms for 1.0Er(3+) and 1.0Er(3+)/ 2Yb(3+) (mol%)-codoped glasses, respectively. The maximum stimulated emission cross sections for [Formula: see text] transition of Er(3+) and Er(3+)/Yb(3+) are 5.447 × 10(-21) cm(2) and 4.540 × 10(-21) cm(2). These glasses with better thermal stability, bright visible emissions and broad near-infrared emissions should have potential applications in broadly tunable laser sources, interesting optical luminescent materials and broadband optical amplification in low-loss telecommunication windows.

Crystal field analysis of energy level structure of LiAlO2:V3+ and LiGaO2:V3+.

A detailed analysis of the energy level structure of tetrahedrally coordinated V(3+) ion in lithium aluminum oxide LiAlO(2) (gamma-phase) and lithium dioxogallate LiGaO(2) is performed using the exchange charge model of the crystal field theory. The parameters of the crystal field acting on the V(3+) optical electrons are calculated from crystal structure data assuming C(1) point symmetry of the [VO(4)](5-) impurity center in LiAlO(2) and LiGaO(2). Crystal field splitting of all five LS terms of the V(3+) ion ((3)F, (3)P, (1)S, (1)D, (1)G) is calculated. The energy levels obtained are compared with experimental absorption spectra and results of application of other crystal field models (the angular overlap model and Racah theory) to the considered crystals; though only one fitting parameter of the exchange charge model was used, a good agreement with experimental data on the ground and excited state absorption is demonstrated.