New aspects of the luminescence of magnesiumtitanate part II: Activation with manganese

Measurements of the emission and excitation spectra of powdered Mg₂TiO₄:Mn⁴⁺ phosphors reveal that single Mn⁴⁺ ions are not responsible for the luminescence — as assumed till now — but so-called N centres. These centres are excited either by direct UV absorption or by radiationless energy transfer from the excited ²E state of the Mn⁴⁺ ions to the N centres. The Mn⁴⁺ ions absorb the energy corresponding to the transition ⁴A_2g→ ²E which is used for the excitation of the N centres, but do not emit the corresponding lines. The observed different luminescence spectra (sharp lines or broad bands) depend on the annealing conditions and indicate that the N centres consist of Mn⁴⁺ ions associated with unknown lattice defects. The annealing process does not only form the tetravalent state of the manganese, but creates above all the N₁ centres, which emit sharp lines. Two more lines observed at 697.8 nm and 699.4 nm, called R^m lines, are due to a certain amount of MgTiO₃:Mn⁴⁺ in the phosphor.     

Determination of the charge state of chromium in Cr:Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> single crystals by spectral luminescence methods

The valence state and structural localization of Cr ions in Cr:Bi₁₂GeO₂₀ single crystals grown by the Czochralski method in air and annealed in argon or UV-irradiated have been determined in the spectral luminescence study. It is established that chromium is in the state of tetrahedrally coordinated Cr⁴⁺ ions in the samples studied. Octahedrally coordinated Cr²⁺ and Cr³⁺ ions and tetrahedrally coordinated Cr⁵⁺ ions were not revealed.     

Luminescence of Cr ions associated with surpassing the green-emissive defect centers in β-Ga2O3

Photoluminescence of undoped and Cr³⁺-doped β-Ga₂O₃ was investigated. The transparent, undoped β-Ga₂O₃ film was successfully prepared by thermal conversion from GaOOH. The film exhibited predominant green luminescence in response to ultraviolet light excitation at 250 nm. This luminescence behavior, which was proposed to result from the oxygen defect centers, was used in examining excitation and emission mechanisms for Cr³⁺ ions doped in β-Ga₂O₃. It was found that red luminescence of Cr³⁺ surpasses green luminescence of the host lattice, as evidenced by the dependence of the spectral structure on the Cr³⁺ concentration. The excitation of Cr³⁺ was then suggested to be caused by the energy transfer from Ga³⁺O₆ octahedra present in the monoclinic β-Ga₂O₃ lattice.     

Preparation and luminescent properties of (Ca1-x,Srx)S:Eu red-emitting phosphor for white LED

The (Ca_1−x,Sr_x)S:Eu²⁺ red-emitting phosphors were prepared by using sulfide and sulfate in CO-reductive atmosphere, respectively, and their luminescent properties have been investigated. The XRD data showed that the doping of divalent Europium ion (Eu²⁺) enlarges the lattice parameters, and Sr/Ca ratio not only affects the lattice parameters, but also influences the emission peak. The excitation and emission spectra indicated that this phosphor can be effectively excited by visible light from 430 to 500 nm, and exhibits a satisfactory red performance. The annealing treatment process was also discussed in detail. Compared with the commercial afterglow sulfide phosphor, the present synthesized phosphor has higher emission efficiency and is a favorable choice for white light-emitting diode (LED). The white LED implanted by present phosphor shows desirable luminescence and chromaticity properties.     

Double-electromagnetically induced transparency in a Y-type atomic system

We present secondary phase identification studies on Cr doped ZnO nanoparticles prepared by the sol-gel method. X-ray diffraction analysis confirms the formation of chromium oxides and there is found to be an increase of lattice parameter with thermal annealing. Scanning electron microscopic studies show the increase in the crystalline nature and particle size. Optical absorption measurements of the as prepared sample exhibit a strong band at 356 nm due to the free exciton absorption of the ZnO nanoparticles. An absorption band at 277 nm is due to the ³T₁ → ³T₂ transition in Cr⁴⁺ ions which appears only for the annealed samples. Photoluminescence studies show that deep level emission is completely suppressed after Cr₂O₃ formation/thermal annealing. Raman and FTIR spectra reveal formation of the Cr₂O₃ phase. Thermal annealing leads to the increase of crystalline nature which gives an enhancement to the Raman modes.     

Cr<Superscript>3+</Superscript> spectroscopy study in ZnO-codoped and Zn-in-diffused congruent LiNbO<Subscript>3</Subscript>:Cr crystals

This paper reports on the spectroscopy properties, absorption and luminescence, of Cr³⁺ ions in singly doped, ZnO-codoped, and Zn in-diffused LiNbO₃:Cr crystals. In addition to the broad absorption, inter-ionic transitions ascribed to Cr³⁺ ions located in Li⁺ and Nb⁵⁺ sites; [Cr]_Li and [Cr]_Nb centres two absorption bands at higher energy are reported and ascribed to the charge transfer transitions of the Cr³⁺ ions of the two defect centres. The charge transfer transitions are used as optical probe to study the role of the Zn ions in the Zn in-diffused LiNbO₃:Cr samples. It has been observed that the Zn-in-diffused processes created [Cr]_Nb centres in the diffusion zone. The location of the diffused Zn²⁺ ions is considered to be in Li⁺ site, displacing the Cr³⁺ ions from the Li⁺ sites, [Cr]_Li, to the Nb⁵⁺ positions, [Cr]_Nb.     

Luminescence of Cr<Superscript>3+</Superscript> impurity ions in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> nanocrystals and clusters embedded in lithium germanate glasses

The properties of Cr³⁺-doped Li₂Ge₇O₁₅ (LGO) lithium germanate nanocrystals produced in lithium germanate glasses under isothermal heating were studied. The samples were characterized by x-ray diffraction and small-angle scattering, as well as by transmission electron microscopy. The luminescence spectra of the impurity chromium measured in lithium germanate glasses containing LGO crystals revealed transitions in Cr³⁺ ions residing in the glass phase and in LGO crystallites starting from extremely small clusters. This provided the possibility of following the process of crystallization of the lithium germanate glass from Cr³⁺ luminescence spectra. The effects observed in the Cr³⁺ luminescence spectrum revealed a ferroelectric phase transition in LGO nanocrystals embedded in the glass.     

Temperature-dependence of the structural and afterglow luminance properties of polymer/SrAlxOy:Eu,Dy composites

Although aluminate phosphors have attracted great interest for applications in lamps, cathode ray tubes and plasma display panels, there still remain issues affecting operational parameters such as luminescence efficiency, stability against temperature, high color purity and perfect decay time. In addition, issues involving important aspects of the monoclinic↔hexagonal phase transition temperature still exist. In this work, SrAl_xO_y:Eu²⁺,Dy³⁺ phosphor powders were prepared by the sol–gel method. X-ray diffraction (XRD) has shown that both crystallinity and crystallite sizes increased as the temperature increased. Both SrAl₂O₄ and Sr₂Al₃O₆ phases were observed. Photoluminescence (PL) characterization shows temperature-dependence, which indicates emission at low and high annealing temperatures originating from Eu²⁺ and Eu³⁺ ions. Thermoluminescence glow and decay measurements provided useful insight on the influence of traps on luminescence behavior. Differential scanning calorimetry (DSC) and thermogravimetric studies (TGA) on composites of the phosphor in low density polyethylene (LDPE) demonstrated the varied influence of annealing temperature on some luminescence and thermal properties.     

Optical and structural properties of nanosized ZnGa2O4:Cr phosphor

Nanosized ZnGa₂O₄:Cr³⁺ powder is synthesized through hydrothermal method. The average particle size is 20 nm and they are spherical in shape. The excitation band from the charge transfer between Cr³⁺-O²⁻ shows a blueshift behavior due to quantum confinement effect. X-ray diffraction pattern, Fourier transform-infrared spectrum, and electron paramagnetic resonance signal indicate that nanosized ZnGa₂O₄:Cr³⁺ phosphor shows many defect-related energy states and heavy lattice distortion in comparison with bulk ZnGa₂O₄:Cr³⁺ phosphor. Many defect states result in more nonradiative loss and shorter decay time.     

Luminescence characteristics of Pb centres in undoped and Ce-doped Lu3Al5O12 single-crystalline films and Pb→Ce energy transfer processes

At 4.2-350 K, the steady-state and time-resolved emission and excitation spectra and luminescence decay kinetics were studied under excitation in the 2.5-15 eV energy range for the undoped and Ce³⁺-doped Lu₃Al₅O₁₂ (LuAG) single-crystalline films grown by liquid phase epitaxy method from the PbO-based flux. The spectral bands arising from the single Pb²⁺-based centres were identified. The processes of energy transfer from the host lattice to Pb²⁺ and Ce³⁺ ions and from Pb²⁺ to Ce³⁺ ions were investigated. Competition between Pb²⁺ and Ce³⁺ ions in the processes of energy transfer from the LuAG crystal lattice was evidenced especially in the exciton absorption region. Due to overlap of the 3.61 eV emission band of Pb²⁺ centres with the 3.6 eV absorption band of Ce³⁺ centres, an effective nonradiative energy transfer from Pb²⁺ ions to Ce³⁺ ions takes place, resulting in the appearance of slower component in the luminescence decay kinetics of Ce³⁺ centres and decrease of the Ce³⁺-related luminescence intensity.     

Effect of non-stoichiometry on the occurrence of Er3+ luminescence sites in erbium-implanted zinc selenide

Erbium-implanted ZnSe cubic crystals with a high V_Zn concentration gave luminescence due to Er³⁺ ions at Zn_sub sites (Centre C) but no luminescence due to Er³⁺ ions at Zn_int sites (Centre A) or Se_sub sites (Centre B). On the other hand, Centres A and B were detected in samples with low V_Zn concentrations but Centre C was absent. To detect luminescence due to all three erbium centres moderate V_Zn concentrations were necessary. In such samples post-implantation annealing at 250–325°C caused Er³⁺ ions to occupy Se_sub and Zn_int sites but annealing at 400°C caused Er³⁺ ions to migrate from the Se_sub sites and be detected at Zn_sub sites.     

Luminescence of impurity 3d and 4f metal ions in different crystalline forms of Al2O3

The ² E-⁴ A ₂ luminescence spectra of Cr³⁺ ions in Al₂O₃ are investigated in the course of transitions between the structural forms γ-δ-θ-α. The spectral lines observed are assigned to Cr³⁺ ions in these structural forms, which are identified by an X-ray powder diffraction analysis. The lifetimes of the Cr³⁺ excited states in transient forms of Al₂O₃ are measured. Investigations of the luminescence spectra of Al₂O₃: Eu³⁺ demonstrate that the Eu³⁺ ions can form regular centers only in α-Al₂O₃ and, unlike the Cr³⁺ ions, give no rise to similar centers in moderately ordered θ-Al₂O₃.     

Structural and optical properties of Au-implanted ZnO films

The structural and luminescence related optical behaviours of Au ion implanted ZnO films grown by magnetic sputtering and their post implantation annealing behaviours in the temperature range of 100-700 °C have been investigated. Optical absorption and transmittance spectra of the films indicate that band edge of Au-implanted ZnO has shifted to high energy range and optical band gap has increased, because the sharp difference of thermal expansion induces the lattice mismatch between ZnO and SiO₂. PL spectra reveal that UV and visible luminescence bands of ZnO films can be improved after thermal annealing due to recovery of defects and Au ions incorporation. Importantly, green luminescence band of 530 nm has been only observed in the Au-implanted and subsequently annealed ZnO films and it enhances with the increasing annealing temperature, which can be related to Au atoms or clusters in ZnO films. Furthermore, X-ray photoelectron spectroscopy measurements reveal that the Au⁰ is dominant state in Au implanted and annealed ZnO films. Possible mechanisms, such as optical transitions of Au atoms or clusters and deep level luminescence of ZnO, have been proposed for green emission.     

Combustion synthesized MgAl2O4:Cr phosphors—An EPR and optical study

Magnesium aluminate (MgAl₂O₄) doped with trivalent chromium (Cr³⁺) was synthesized by the combustion method. The prepared sample was characterized by X-ray powder diffraction, Brunauer-Emmet-Teller (BET) adsorption isotherms and diffuse-reflectance UV-vis spectroscopy techniques. Electron paramagnetic resonance (EPR) and photoluminescence (PL) studies have been performed at room temperature and at 110 K. The EPR spectrum exhibit resonance signals at g=5.37, 4.53, 3.82, 2.26 and 1.96 characteristic of Cr³⁺ ions. The luminescence of Cr³⁺-activated MgAl₂O₄ exhibits a red emission peak around 686 nm from the synthesized phosphor particles upon 551 nm excitation. The luminescence is assigned to a transition from the upper ²E_g→⁴A_2g ground state of Cr³⁺ ions. By correlating EPR and optical data the crystal field splitting parameter (D_q), Racah inter-electronic repulsion parameter (B) and the bonding parameters have been evaluated and discussed. The bonding parameters suggests that the ionic nature of Cr³⁺ ions with the ligands and the Cr³⁺ ions are in distorted octrahedral environment.     

Degradation of phosphors under cathode-ray excitation

A systematic analysis of phosphor degradation is presented. An increase of the self-absorption of the luminescence, destruction of luminescence centres, a decreased energy flow to these centres, and increased energy losses in the phosphor host lattice - both in the bulk and at the surface - can be distinguished as causes of the deterioration of the external radiant efficiency of phosphors. Simple measurements are suggested to disentangle the contribution of each of these parameters to the observed degradation effects. As an illustration degradation effects of Zn₂SiO₄ : Mn, Y₂SiO₅ : Ce, Sr₂Al₆O₁₁ : Eu and a Tb³⁺ activated borate glass are analyzed.     

Characterization of luminescence centres in neodymium implanted zinc sulphide

Analysis of the cathodoluminescence emission spectra of neodymium implanted cubic single crystal ZnS as a function of post-implantation annealing temperature has enabled groups of emission lines to be identified as originating from transitions within particular luminescence centres. For each group of lines the wavelength separations yield energy splittings characteristic of the crystal field at the luminescence centre. Comparison of these splittings with values calculated by crystal field theory on the basis of a point charge model, with due regard to possible sites of cubic symmetry in ZnS, has allowed two luminescence centres (A and B) to be identified.Centre A, which exists at low annealing temperatures, consists of a Nd³⁺ ion situated interstitially with 4 tetrahedrally disposed nearest neighbour S^2? ions. At higher annealing temperatures the emissions from centre A were found to decline giving way to those of centre B, which analysis shows to be a Nd³⁺ ion situated on a zinc substitutional site.     

Crystallization of mullite from kaolin according to optical spectroscopy of impurity ions

The luminescence spectra of Cr³⁺ and Eu³⁺ ions embedded in mullite ceramic samples are studied. The samples were annealed at temperatures from 600 to 1200°С. Beginning at a temperature of 890°С, the spectra demonstrated the presence of a newly formed crystal phase. At temperatures from 920 to 1200°С, the crystalline form manifested itself as a stable mullite phase. A temperature of 920°С corresponded to a sharp change in the symmetry of the crystal field around Eu³⁺ (phase transition). The parameters of the spectral lines of ions were used to estimate the mullite crystal-lattice disorder and the residual deformations in ceramic samples. The spread of the mullite crystal-lattice parameter is determined as |Δа/а| ≈ 0.028 and is independent of the annealing temperature.     

Effect of annealing on structural and optical properties of lead tungstate microcrystals

Shuttle-like lead tungstate (PbWO₄) microcrystals are synthesized at room temperature using the precipitation method with the cetyltrimethyl ammonium bromide. Results from both the X-ray diffraction and the scanning electron microscopy show that the lattice distortions of the PbWO₄ microcrystals are reduced significantly when the annealing temperature is increased to 873 K. The result from the ultraviolet-visible diffuse reflectance spectroscopy shows that the exciton absorption appears in the sample annealed at 673 K. The self-trapped exciton luminescence due to the Jahn-Teller effect is also observed in the blue band. The interstitial oxygen ions in the WO₄^2- groups are mainly resposible for the enhancement effect of the green luminescence of the annealed samples. The above results are supported by the spectrum analysis of the as-grown and the post-annealed samples using the X-ray photoelectron spectroscopy.     

Luminescence from Ce in sol–gel SiO2

The sol–gel process provides an attractive low temperature alternative to the melt process for producing Ce-doped silica, but reports of the emission wavelength have not been consistent. In this paper, luminescence measurements using a variety of excitation methods, including cathodoluminescence not yet reported by other researchers, are compared and evaluated in the light of previously published data. Several papers report luminescence around 350 nm but emission near this wavelength was not found from our samples. This luminescence originates from Ce that has not yet been incorporated in the silica and is found in samples that have not undergone high temperature annealing. Our photoluminescence results from samples annealed in a reducing atmosphere suggest that emission from Ce incorporated in the silica lattice occurs near 455 nm, and some indication of the emission from Ce in amorphous clusters at 400 nm is also found. However, our results also confirm earlier indications that intrinsic defects in silica can create photoluminescence near both these wavelengths, which can make identification of the luminescence due to Ce difficult. Finally, it has been found that samples which have been annealed in air, and therefore display poor photoluminescence because most of the Ce occurs in the tetravalent form, are luminescent under electron beam excitation. It is suggested that during cathodoluminescence measurements Ce⁴⁺ ions capture electrons to form excited Ce³⁺ ions from which the luminescence originates.     

Optical spectrum, local lattice structure and EPR g factors for Cr impurity ions in MgTiO3 and LiTaO3

On the basis of the 120×120 complete energy matrices for a d³ configuration ion in a trigonal ligand field, for Cr³⁺ ions doped in MgTiO₃ and LiTaO₃, the local structures and EPR g factors of the octahedral (CrO₆)⁹⁻ clusters have been studied, respectively. By simulating the calculated optical spectra and the EPR spectra data to the experimental results, local structure parameters are obtained. The calculated results show that although the local lattice structures around the M (M=Mg²⁺, Ta⁵⁺) ions are obviously different, after Cr³⁺ replacing the M, the local lattice structures around the Cr³⁺ ions are quite similar and close to those of the Cr₂O₃. This may be ascribed to the fact that the octahedral Cr³⁺ center in MgTiO₃:Cr³⁺ and LiTaO₃:Cr³⁺ systems and that in Cr₂O₃ exhibit similar octahedral (CrO₆)⁹⁻ clusters. Moreover, the corresponding theoretical values of the optical spectra have been reported. It is also found that the orbital reduction factor k is very important to understand the EPR g factors for Cr³⁺ ions doped in MgTiO₃ and LiTaO₃.