A novel red phosphor NaLa4(SiO4)3F: Eu

A novel red phosphor NaLa₄(SiO₄)₃F: Eu³⁺ was synthesized by the conventional solid-state reaction at 950 °C for the first time. The luminescence properties of NaLa₄(SiO₄)₃F: Eu³⁺ were investigated, and the critical concentration of the activator concentration (Eu³⁺) was found to be 0.1 mol per formula unit. The phosphor presented red luminescence under the ultraviolet excitation of 254 or 395 nm, attributed to the transitions from ⁵D₀ excited states to ⁷F_J ( J = 0-4) ground states of Eu³⁺ ions. The results indicated that this newly-developed phosphor could find applications in tricolor fluorescent lamp, phosphor-liquid crystal displays and white lighting devices utilizing GaN-based excitation in the near UV.     

The photoluminescence/excitation (PL/E) spectroscopy of Eu-implanted GaN

Several distinct luminescent centres form in GaN samples doped with Eu. One centre, Eu2, recently identified as the isolated, substitutional Eu impurity, Eu_Ga, is dominant in ion-implanted samples annealed under very high pressures (1 GPa) of N₂. According to structural determinations, such samples exhibit an essentially complete removal of lattice damage caused by the implantation process. A second centre, Eu1, probably comprising Eu_Ga in association with an intrinsic lattice defect, produces a more complex emission spectrum. In addition there are several unidentified features in the ⁵D₀ to ⁷F₂ spectral region near 620 nm. We can readily distinguish Eu1 and Eu2 by their excitation spectra, in particular through their different sensitivities to above-gap and below-gap excitation. The present study extends recent work on photoluminescence/excitation (PL/E) spectroscopy of Eu1 and Eu2 to arrive at an understanding of these mechanisms in terms of residual optically active defect concentrations. We also report further on the ‘host-independent’ excitation mechanism that is active in the case of a prominent minority centre. The relevance of this work to the operation of the red GaN:Eu light-emitting diode is discussed.     

Eu-doped LiF-SrF2 eutectic scintillators for neutron detection

Eu²⁺ 0.05%, 0.1%, and 0.2% activated LiF-SrF₂ eutectic scintillators were prepared by the Bridgman method using ⁶Li enriched (95%) raw material. The α-ray-induced radio luminescence spectra showed intense emission peak at 430 nm due to an emission from Eu²⁺ 5d-4f transition in the Eu:SrF₂ layers. When excited by ²⁵²Cf neutrons, all the samples exhibited almost the same light yields of 5000-7000 ph/n with a typical decay times of several hundreds ns.     

Photoluminescence properties of NaGd(MoO4)2:Eu nanophosphors prepared by sol-gel method

NaGd(MoO₄)₂:Eu³⁺ (hereafter NGM:Eu) phosphors have been prepared by sol-gel method. The properties of the resulting phosphors are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curve. The excitation spectra of NGM:Eu phosphors are mainly attributed to O → Mo charge-transfer (CT) band at about 282 nm and some sharp lines of Eu³⁺ f-f transitions in near-UV and visible regions with two strong peaks at 395 and 465 nm, respectively. Under the 395 and 465 nm excitation, intense red emission peaked at 616 nm corresponding to ⁵D₀ → ⁷F₂ transition of Eu³⁺ are observed for 35 at.% NGM:Eu phosphors as the optimal doping concentration. The luminescence properties suggest that NGM:Eu phosphor may be regarded as a potential red phosphor candidate for near-UV and blue light-emitting diodes (LEDs).     

Scintillation properties of CsBa2Br5:Eu

In this study, the scintillation properties of Eu²⁺ activated CsBa₂Br₅ are reported. It is an analog of Eu²⁺ doped CsBa₂I₅ scintillator that exhibits excellent scintillation properties. Microcrystalline powder samples of this compound were synthesized by reaction in the molten state of cesium bromide, barium bromide and europium bromide. The concentration of an Eu was varied from 0% to 10%. The luminescence was studied under UV and X-ray excitation. The light output is strongly Eu²⁺ concentration dependent, reaching an estimated maximum of about 92,000 photons/MeV at 2% Eu under X-ray excitation. About 50% of the scintillation light decays in less than 1 μs. CsBa₂Br₅:Eu²⁺ exhibits a complex concentration dependent emission centered around 435 nm.     

Characteristics of nanocrystalline AlN:Er films co-deposited by using AlN, Er, and SiO2 targets

We report the characteristics of AlN:Er films that were co-deposited by using AlN, Er, and SiO₂ targets. The PL emission spectra show strong green emissions of Er³⁺ ions in AlN:Er films annealed at an optimal temperature of 750 °C, which is attributed to the intra-4f Er³⁺ transitions of ²H_11/2 → ⁴I_15/2 and ⁴F_7/2 → ⁴I_15/2. This optimal temperature can activate Er species as an efficient visible luminescence center. High-resolution transmission electron microscopy (HREM) observations showed that the AlN:Er film annealed at 750 °C exhibits the microstructure of AlN nanocrystallites embedded in the amorphous matrix. The occurrence of strong Er³⁺ emissions in the amorphous-nanocrystalline AlN:Er films by thermal annealing might contribute to an increased number of excitation Er³⁺ centers and the presence of oxygen related to Er³⁺ excitation and recombination processes. A distinct visible bluish green emission is also confirmed from the EL device with an amorphous-nanocrystalline AlN:Er active layer.     

Novel blue-emitting phosphor NaMg4(PO4)3:Eu, Ce with energy transfer

A blue-emitting phosphor of NaMg₄(PO₄)₃:Eu²⁺, Ce³⁺ was prepared by a combustion-assisted synthesis method. The phase formation was confirmed by X-ray powder diffraction measurement. Photoluminescence excitation spectrum measurements show that the phosphor can be excited by near UV light from 230 to 400 nm and presents a dominant luminescence band centered at 424 nm due to the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ions at room temperature. Effective energy transfer occurs in Ce³⁺/Eu²⁺ co-doped NaMg₄(PO₄)₃ due to large spectral overlap between the emission of Ce³⁺ and excitation of Eu²⁺. Co-doping of Ce³⁺ enhances the emission intensity of Eu²⁺ greatly by transferring its excitation energy to Eu²⁺, and Ce³⁺ plays a role as a sensitizer. Ce³⁺-Eu²⁺ co-doped NaMg₄(PO₄)₃ powders can possibly be applied as blue phosphors in the fields of lighting and display.     

Structural and optical properties of Er implanted AlN thin films: Green and infrared photoluminescence at room temperature

In this work erbium ions were implanted into AlN films grown on sapphire with fluence range: (0.5-2) × 10¹⁵ at/cm⁻², ion energy range: 150-350 keV and tilt angle: 0°, 10°, 20°, 30°. The optical and structural properties of the films are studied by means of photoluminescence and Raman spectroscopy in combination with Rutherford backscattering/channeling (RBS/C) measurements. The photoluminescence spectra of the Er³⁺ were recorded in the visible and infrared region between 9 and 300 K after thermal annealing treatments of the samples. The emission spectrum of the AlN:Er films consists of two series of green lines centered at 538 and 558 nm with typical Er³⁺ emission in the infrared at 1.54 μm. The green lines have been identified as Er³⁺ transitions from the ²H_11/2 and ⁴S_3/2 levels to the ⁴I_15/2 ground state. Different erbium centers in the matrix are suggested by the change of infrared photoluminescence relative intensity of some of the emission lines when different excitation wavelengths are used. The relative abundances of these centers can be varied by using different implantation parameters. The Raman and RBS/C measurements show good crystalline quality for all the studied films.     

Luminescence of SrGdGa3O7:RE(RE = Eu, Tb) phosphors and energy transfer from Gd to RE

Eu³⁺- and Tb³⁺-activated SrGdGa₃O₇ phosphors were synthesized by the solid-state reaction and their luminescence properties were investigated. Sr(Gd_1 − xEu_x)Ga₃O₇ and Sr(Gd_1 − xTb_x)Ga₃O₇ formed continuous solid solution in the range of x = 0-1.0. Unactivated SrGdGa₃O₇ exhibited a typical characteristic excitation and emission of Gd ion. The SrGdGa₃O₇:xEu³⁺ and SrGdGa₃O₇:xTb³⁺ phosphors also showed the well-known Eu³⁺ and Tb³⁺ excitation and emission. The energy transfer from Gd³⁺ to Eu³⁺ and Tb³⁺ were verified by photoluminescence spectra. The dependence of photoluminescence intensity on Eu³⁺ and Tb³⁺ concentration were also studied in detail and the photoluminescence (PL) intensity of SrGdGa₃O₇:Eu and SrGdGa₃O₇:Tb were compared with commercial phosphors, Y₂O₃:Eu and LaPO₄:Ce,Tb. The luminescence decay measurements showed that the lifetimes of Eu³⁺ and Tb³⁺ were in the range of microsecond. The energy transfer from Gd³⁺ to Tb³⁺ was also observed in decay curve.     

Investigation of red-emission phosphors (Ca,Sr)(Mo,W)O:Eu crystal structure, luminous characteristics and calculation of EuD0 quantum efficiency

By sol-gel method, (Ca_0.54, Sr_0.34)(Mo_0.2, W_0.8)O₄: Eu³⁺_0.08 luminescent materials can be synthesized. X-ray diffraction analysis indicates that it owns tetragonal scheelite structure, belonging to I-center with space group I4₁/a [88], Z = 4. The emission spectra, excitation spectra and fluorescence decay curves are measured, and the partial Judd-Ofelt parameters and, under 395 nm excitation, quantum efficiency of Eu^{3+ 5}D₀ energy level are calculated. The results indicate that Eu^{3+ 5}D₀-⁷F₂ red luminescence can be excited by 395 nm, 465 nm and 535 nm in the hosts, respectively, and its quantum efficiency can be improved in space and it has potential application for white light emitting diodes as the red luminescent materials.     

Preparation and fluorescence property of red-emitting Eu-activated amorphous calcium silicate phosphor

This paper describes the energy efficient synthesis of a red-emitting Eu³⁺-activated amorphous calcium silicate phosphor produced by heating a Eu³⁺-activated calcium silicate hydrate phosphor. Concentration quenching of the Eu³⁺-activated calcium silicate hydrate phosphor was not observed and the emission intensity did not decrease up to a Eu/(Ca+Eu) atomic ratio of 0.46. Heating of the Eu³⁺-activated calcium silicate hydrate (Eu/(Ca+Eu) atomic ratio = 0.32) phosphor produced an amorphous Eu³⁺-activated calcium silicate phosphor, which had a maximum emission intensity at 870 °C and emitted in the red under near-ultraviolet irradiation (395 nm). The emission intensity of the Eu³⁺-activated amorphous calcium silicate phosphor was about half that of a commercial BaMgAl₁₀O₁₇:Eu²⁺ phosphor, and shows great potential for application in white light-emitting diodes.     

A red phosphor for nUV LED based on (Y,Gd)BO3:Eu

(Y,Gd)BO₃:Eu³⁺ based phosphors were prepared by combustion technique followed by solid state sintering in air. The 4f-4f excitation at 394 nm of (Y_0.54Gd_0.46)_x (BO₃)_y:Eu³⁺ exhibits red emission at 593 nm. Its photoluminescence (PL) efficiency depends critically on the BO₃ to (Y,Gd) molar ratio. Optimal luminescence was obtained at the y/x molar ratio of 1.38. A 12-fold increase in its luminescence efficiency was seen with an increase in Eu concentration from 0.5 to 8.4 mol% at this y/x ratio. At 8.4 mol% of Eu concentration, the PL sensitivity of (Y_0.54Gd_0.46)_x (BO₃)_y:Eu³⁺ is 40% higher than that of the commercial (Y,Gd)BO₃:Eu³⁺ phosphor. In view of the high luminescence efficiency, this phosphor could serve as a potential candidate for application as a red phosphor for nUV LED apart from its known application in plasma display panels.     

Low temperature luminescence of KMgF3:Eu crystal

Paper presents luminescence spectra and time resolved spectra of KMgF₃:Eu²⁺ system obtained at different temperatures and pressures, under excitation with 325 nm. At temperatures between 200 K and 292 K the spectra consist of sharp line peaked at 27,830 cm⁻¹ related to ⁶P_7/2 → ⁸S_7/2 transition in Eu²⁺ accompanied by the phonon sideband. Under pressure the red spectral shift with the rate equal to −0.6 cm⁻¹/kbar is observed. Luminescence decay is single-exponential with the lifetime equal to 5.2 ms independent of pressure and temperature. The emission spectra obtained at temperatures lower than 125 K consist of 5 sharp lines peaked at 27,590 cm⁻¹, and 27,670 cm⁻¹, 27,722 cm⁻¹, 27,766 cm⁻¹ and 27,809 cm⁻¹, that relative intensity depends on temperature. Pressure shift of these lines was found to be equal to −0.6 cm⁻¹/kbar; the same as ⁶P_7/2 → ⁸S_7/2 transition in Eu²⁺, whereas their lifetime is shorter and is equal to 0.7 ms at 100 K. These new lines disappear at temperature greater than 200 K. We tentatively related them to the luminescence of Eu²⁺-F center (fluorine vacancy with electron) complex.     

Novel red phosphors Na2CaSiO4:Eu for light-emitting diodes

Novel red phosphors Na₂CaSiO₄:xEu³⁺ were synthesized using high temperature solid-state reaction and their luminescence characteristics were investigated for the first time. The excitation spectra indicate that the Na₂CaSiO₄:xEu³⁺ phosphors can be effectively excited by ultraviolet (393 nm) light. The emission spectra of Na₂CaSiO₄:xEu³⁺ phosphors invariably exhibit four peaks assigned to the ⁵D₀-⁷F_J (J = 1, 2, 3 and 4) transitions of Eu³⁺ under 393 nm excitation. The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates and quantum efficiency (QE) are (0.66, 0.34) and 58.9%, respectively. The good color saturation and high quantum efficiency indicate that Na₂CaSiO₄:Eu³⁺ phosphors are potential candidate for light-emitting diodes.     

Synthesis and luminescence properties of YVO4:Eu nanocrystals grown in nanoporous glass

We report on a feasible method to synthesize luminescence nanocrystals in porous glass in this paper. Well dispersed YVO₄:Eu nanocrystals were proved being grown in nanoporous glass by XRD, micro-Raman spectra and HRTEM equipped with EDS. The YVO₄:Eu³⁺ nanocrystal grown in porous glass herein shows very different luminescence properties compared with single Eu-doped sample. By this method, intense red emission from high silica glass due to energy transfers VO₄³⁻ → Eu³⁺ was obtained. The results show that the reduction from Eu³⁺ to Eu²⁺ in porous glass impregnated with Eu³⁺ ions was avoided effectively.     

Reduction of Eu to Eu in aluminoborosilicate glasses under ionizing radiation

Eu₂O₃-doped aluminoborosilicate glasses were prepared by melting in air at high temperature (∼1500 °C). It was shown by luminescence and Electron Paramagnetic Resonance (EPR) measurements that both Eu³⁺ and Eu²⁺ ions can exist simultaneously in the glass matrix studied after glass synthesis as well as after exposure to ionizing radiation. Increase of total Eu₂O₃ concentration leads to the increase of Eu³⁺ luminescence intensity while the luminescence intensity of Eu²⁺ ions tends to decrease. In contrast the EPR indicates that the amount of Eu²⁺ ions in the glass increases with total Eu₂O₃ concentration. The difference in the results of the two spectroscopies is explained in terms of energy transfer from Eu²⁺ to Eu³⁺ leading to an Eu²⁺ luminescence quenching. Irradiation results in the increase of reduced Eu²⁺ quantity detected by EPR measurements. It was shown that Eu²⁺ ions are located in both high (g ∼ 4.6) and low symmetry (“U’ spectrum) sites in the structure of aluminoborosilicate glasses. The increase of Eu²⁺ content by the increase of the irradiation dose manifests the strong reduction process Eu³⁺ → Eu²⁺.     

Tuning the Eu luminescence in glass materials synthesized in air by adjusting glass compositions

The doped Eu³⁺ ions can be partly reduced to Eu²⁺ in a series of MO-B₂O₃: Eu (M = Ba, Sr, Ca) glasses synthesized in air atmosphere, but not in the 12CaO-7Al₂O₃: Eu glass. The different redox-behavior of Eu ions in these two glass systems is attributed to the different host optical basicity. It is found that a lower valence state of Eu²⁺ is more favorable in acidic glasses, which have lower optical basicities. A notion of the critical value of optical basicity is introduced empirically, which can be used as a guide for the selection of glass composition suitable to incorporate Eu²⁺ for luminescence.     

Luminescence characterization of (Ca1−xZnx)Ga2S4:Eu phosphors for a white light-emitting diode

We investigated the luminescence properties of (Ca_1−xZn_x)Ga₂S₄:Eu²⁺ phosphor as a function of Zn²⁺ and Eu²⁺ concentrations. The luminescence intensity was markedly enhanced by increasing the mole fraction of Zn²⁺ at Ca²⁺ sites. Lacking any Zn²⁺ ions, CaGa₂S₄:0.01Eu²⁺ converted only 18.1% of the absorbed blue light into luminescence. As the Zn²⁺ concentration increased, the quantum yield increased and reached a maximum of 24.4% at x = 0.1. Furthermore, to fabricate the device, the optimized green-yellow (Ca_0.9Zn_0.1)Ga₂S₄:Eu²⁺ phosphor was coated with MgO. White light was generated by combining the MgO-coated phosphor and the blue emission from a GaN chip.     

Synthesis and optical properties of KCaPO4:Eu phosphor

A series of Eu²⁺ doped KCaPO₄ phosphors were prepared by high temperature solid state reaction and an efficient blue-green emission was observed. The photoluminescence (PL) spectrum of the phosphor appeared one asymmetric peak under near-ultraviolet (n-UV) excitation and two emission bands at 480 nm and 540 nm were obtained using Gaussian fit, which was because Eu²⁺ ions inhabited two different Ca²⁺ sites: Eu(I) and Eu(II) in the host lattice, respectively. The excitation spectrum was a broadband extending from 250 to 450 nm, which matched well with the emission of ultraviolet light-emitting diodes (UV LEDs). The effect of Eu²⁺ concentration on the emission intensity of KCaPO₄:Eu²⁺ phosphor was investigated in detail.     

Rare-earth doped sol-gel derived oxyfluoride glass-ceramics: Structural and optical characterization

Glass-ceramics containing RE³⁺-doped BaF₂ nanocrystals (RE = Eu, Sm, Dy, Ho and Pr) with the size below 10 nm size have been made by using the controlled crystallization at higher temperatures of the RE³⁺-doped SiO₂-BaF₂ xerogels. Photoluminescence measurements have indicated the incorporation of the RE³⁺ ions in both silica network and in the BaF₂ nanocrystals. Thermoluminescence measurements have shown a peak whose position depends on the nature of RE³⁺-dopant as it follows: 140 °C (for Ho³⁺, Dy³⁺), 340 °C (for Sm³⁺) and 370 °C (for Eu³⁺); in Eu³⁺-doped SiO₂ glass the TL peak is shifted to 383 °C. The peaks in glass-ceramics were assigned to the recombination of the electrons thermal released from the RE³⁺-electron traps located in both glass-matrix and BaF₂ nanocrystals. Within the series the trivalent lanthanide ions act as increasingly deeper electron trapping centres.