Origin of PL intensity increase of CaMgSi2O6:Eu phosphor after baking process for PDPs application

We have synthesized blue-emitting CaMgSi₂O₆:Eu²⁺ (CMS) and evaluated its thermal stability after baking process. To evaluate its thermal stability, CMS was baked in air at 500 and 600 °C for 20 min, respectively, and compared with BaMgAl₁₀O₁₇:Eu²⁺ (BAM) treated in the same condition. After baking process, CMS showed somewhat increased photoluminescence (PL) intensity with baking temperature. To investigate the reasons behind the increase of PL intensity after baking process, vacuum ultraviolet (VUV)/PL, electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS) techniques were applied. From the ESR and the XPS analyses, it is noted that spectral intensity of Eu²⁺ ion somewhat increased. It was believed that due to charge balance Eu³⁺ ions reduced to Eu²⁺ ions during the baking process in air. It is clear that the concentration of Eu²⁺ increased after the baking process in air and it leads to slight increase of the VUV/PL intensity of CMS phosphor.     

Luminescence efficiencies of self- and Tm activated CaWO4 under vacuum ultraviolet radiation excitation

Luminescence efficiency of self-activated CaWO₄ under 147 nm vacuum ultraviolet (VUV) radiation excitation is about 90% of that of BaMgAl₁₀O₁₇:Eu²⁺ (BAM), the commercial blue plasma display panel (PDP) phosphor. However, the color purity and the particle size of the former needs substantial modification before it can be considered for application in PDP. CaWO₄:Tm exhibits Tm³⁺ emission peaks in the blue region due to energy transfer from WO₄ to Tm³⁺ ions but the overall emission intensity under 147 nm excitation is reduced when compared to that of CaWO₄.     

Potential PDP phosphors with strong absorption around 172 nm: Rare earth doped NaLaP2O7 and NaGdP2O7

NaLaP₂O₇ and NaGdP₂O₇ powder samples are prepared by solid-state reactions at 750 and 600 °C, respectively, and the VUV-excited luminescence properties of Ln³⁺ (Ln=Ce, Pr, Tb, Tm, Eu) in both diphosphates are studied. Ln³⁺ ions in both hosts show analogous luminescence. For Ce³⁺-doped samples, the five Ce³⁺ 5d levels can be clearly identified. As for Pr³⁺ and Tb³⁺-doped samples, strong 4f-5d absorption band around 172 nm is observed, which matches well with Xe-He excimer in plasma display panel (PDP) devices. As a result, Pr³⁺ can be utilized as sensitizer to absorb 172 nm VUV photon and transfer energy to appropriate activators, and Tb³⁺-doped NaREP₂O₇(RE=La, Gd) are potential 172 nm excited green PDP phosphors. For Tm³⁺ and Eu³⁺-doped samples, the Tm³⁺-O²⁻ charge transfer band (CTB) is observed to be at 177 nm, but the CTB of Eu³⁺ is observed at abnormally low energy position, which might originate from multi-position of Eu³⁺ ions. The similarity in luminescence properties of Ln³⁺ in both hosts indicates certain structural resemblance of coordination environment of Ln³⁺ in the two sodium rare earth diphosphates.     

Photoluminescence studies of red-emitting NaEu(WO4)2 as a near-UV or blue convertible phosphor

Sodium europium double tungstate [NaEu(WO₄)₂] phosphor was prepared by the solid-state reaction method. Its crystal structure, photoluminescence properties and thermal quenching characteristics were investigated aiming at the potential application in the field of white light-emitting diodes (LEDs). The influences of Sm doping on the photoluminescence properties of this phosphor were also studied. It is found that this phosphor can be effectively excited by 394 or 464 nm light, which nicely match the output wavelengths of near-ultraviolet (UV) or blue LED chips. Under 394 or 464 nm light excitation, this phosphor exhibits stronger emission intensity than the Y₂O₂S:Eu³⁺ or Eu²⁺-activated sulfide phosphor. The introduction of Sm³⁺ ions can broaden the excitation peaks at 394 and 464 nm of the NaEu(WO₄)₂ phosphor and significantly enhance its relative luminance under 400 and 460 nm LEDs excitation. Furthermore, the relative luminance of NaEu(WO₄)₂ phosphor shows a superior thermal stability compared with the commercially used sulfide or oxysulfide phosphor, and make it a promising red phosphor for solid-state lighting devices based on near-UV or blue LED chips.     

Synthesis and luminescent properties of europium-activated Ca2SnO4 phosphors by sol-gel method

In this paper, the Ca₂SnO₄:Eu³⁺ phosphor was prepared by low-temperature sol-gel method. The influence of calcined temperature and time on structure of Ca₂SnO₄:Eu³⁺ was investigated by using X-ray powder diffraction (XRD). The experimental results show that the dried gel was crystallized to the pure orthorhombic phase after calcination at 900 °C in air for 6 h. These phosphors have displayed bright red color under a UV source. The richness of the red color has been verified by determining their color coordination from the CIE standard charts, and this red emission has been assigned to ⁵D₀→⁷F₂ electric dipole transition at 616 and 620 nm. The excellent luminescence properties make it possible as a good candidate for plasma display panel (PDP) application.     

Temperature dependence of persistent luminescence in β-irradiated SrAl2O4:Eu, Dy phosphor

SrAl₂O₄:Eu²⁺, Dy³⁺ is a phosphor characterized by a long persistent luminescence (PLUM) when excited with UV-vis light and ionizing radiation exhibiting intensity variation in the 10-320 K temperature range and maximum intensity around 320 K. In this work, we study the PLUM behavior of SrAl₂O₄:Eu²⁺, Dy³⁺ as a function of temperature from room temperature to 670 K in samples exposed to β irradiation. The room-temperature irradiation followed by PLUM readout revealed an integrated PLUM maximum at 323 K decreasing later. In contrast, irradiation and PLUM readout at temperatures above room temperatures produced integrated PLUM intensities maxima around 425 and 625 K. Successive cycles of preheating followed by irradiation and PLUM readout produced an increasing of the PLUM intensity as a function of cycle number. The observed phenomenon was ascribed to trapped electrons at the multiple trapping states related to the 425 and 625 K defects levels and electron transfer from one trap to another (electron hopping). Eventually, there is a return to the 5d level of Eu³⁺ cations with the characteristic PLUM emission by thermal energy supplied at room temperature (lattice vibrations) or by a preheating-irradiation-readout cycle. This property may allow keeping up the PLUM properties of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors through background radiation self exposure and adequate heating processes.     

Ca2BO3Cl:Ce,Eu: A potential tunable yellow-white-blue-emitting phosphors for white light-emitting diodes

Polycrystalline Ca₂BO₃Cl:Ce³⁺,Eu²⁺ phosphors were synthesized by a solid-state reaction and which could display tunable color emission from blue to yellow under an ultraviolet (UV) source by adjusting the ratio of Ce³⁺ and Eu²⁺ appropriately. The mechanism of resonance-type energy transfer from Ce³⁺ to Eu²⁺ was established to be electric dipole-dipole natured, and the critical distance was estimated to be 31 Å based on the spectral overlap and concentration quenching model. A white light was obtained from Ca₂BO₃Cl:0.06Ce³⁺,0.01Eu²⁺ phosphor with chromaticity coordinates (x=0.31, y=0.29) and relative color temperature of 7330 K upon excitation with 360 nm, which is potentially a good candidate as an UV-convertible phosphor for white light-emitting diodes (LEDs).     

The luminescent properties of CaYBO4:Ln(Ln=Eu, Tb) under UV-VUV range

The luminescent properties of CaYBO₄:Ln(Ln=Eu³⁺, Tb³⁺) were investigated under ultraviolet (UV) and vacuum ultraviolet (VUV) region. The CT band of Eu³⁺ at about 245 nm blue-shifted to 230 nm in VUV excitation spectrum; the band with the maximum at 183 nm was considered as the host lattice absorption. For the sample of CaYBO₄:0.08Tb³⁺, the bands at about 235 and 263 nm were assigned to the f-d transitions of Tb³⁺ and the CT band of Tb³⁺ was calculated according to Jφrgensen's theory. Under UV and VUV excitation, the main emission of Eu³⁺ corresponding to the ⁵D₀-⁷F₂ transition located at about 610 nm and two intense emission of Tb³⁺ from the ⁵D₄-⁷F₅ transition had been observed at about 542 and 552 nm, respectively. With the incorporation of Gd³⁺ into the host lattice of CaYBO₄, the luminescence of Tb³⁺ was enhanced while that of Eu³⁺ was decreased because of their different excitation mechanism.     

Synthesis and luminescence investigation of RE (Eu, Tb and Ce)-doped lithium silicate (Li2SiO3)

Synthesis and photoluminescence (PL) investigations of lithium metasilicate doped with Eu³⁺, Tb³⁺ and Ce³⁺ were carried out. PL spectra of Eu-doped sample showed peaks corresponding to the ⁵D₀→⁷F_j (j=1, 2, 3 and 4) transitions under ultraviolet excitation. Strong red emission coming from the hypersensitive ⁵D₀→⁷F₂ transition of Eu³⁺ ion suggested the presence of the dopant ion in structurally disordered environment. Tb³⁺-doped silicate sample showed blue-green emission corresponding to the ⁵D₄→⁷F_j (j=6, 5 and 4) transitions. Ce-doped sample under excitation from UV, showed a broad emission band in the region 350-370 nm with shoulders around 410 nm. The fluorescence lifetimes of Eu³⁺ and Tb³⁺ ions were found out to be 790 and 600 μs, respectively. For Ce³⁺, the lifetime was of the order of 45 ns. PL spectra of the europium- and terbium-doped samples were compared with commercial red (Y₂O₃:Eu³⁺) and green (LaPO₄:Tb³⁺) phosphors, respectively. It was found that the emission from the doped silicate sample was 37% of the commercial phosphor in case of the Tb-doped sample and 8% of the commercial phosphor in case of the Eu-doped sample.     

Photoluminescence and EPR studies of BaMgAl10O17:Eu phosphor with blue-emission synthesized by the solution combustion method

A blue phosphor, BaMgAl₁₀O₁₇:Eu²⁺, has been synthesized in the furnace at a temperature of 500 °C by solution combustion method. The formation of the as-prepared BaMgAl₁₀O₁₇:Eu²⁺ phosphor was confirmed by the powder X-ray diffraction technique. The EPR spectrum exhibited an intense resonance signal centered at g=4.63 at 150 mT along with a number of resonances in the vicinity of g>2.0 and g<2.0. The number of spins participating in resonance (N) and the susceptibility (c) for the resonance signal at g=4.63 have been calculated as a function of temperature. The excitation spectrum of BaMgAl₁₀O₁₇:Eu²⁺ phosphor showed a strong peak near 336 nm (4f⁷ (⁸S)→5d¹(t_2g) transition) with a staircase like structure in the region 376-400 nm owing to crystal field splitting of the Eu²⁺ d-orbital. The 336 nm excitation produced a broad blue emission at 450 nm corresponding to 4f⁶5d→4f⁷ transition. PL studies reveal the two emission centers one at 450 nm and the other at 490 nm in this phosphor.     

Color tunability of nanophosphors by changing cations for solid-state lighting

X₃MgSi₂O₈: Eu²⁺, Mn²⁺ (X=Ba, Sr, Ca) phosphors with the mean particle size of 200 nm and the spherical shape are synthesized through combustion method. They show three emission colors under near-ultraviolet light: the blue and green colors from Eu²⁺ ions and the red color from Mn²⁺ ions. Three emission bands show the different emission colors with changing X²⁺ cations. These color shifts are discussed in terms of two competing factors of the crystal field strength and the covalency. These phosphors with maximum excitation of around 375 nm can be applied as color-tunable phosphors for white-light-emitting diode based on ultraviolet/phosphor technology.     

Photoluminescence properties of BaMgAl10O17:Eu phosphor prepared by the flux method

BaMgAl₁₀O₁₇:Eu²⁺ phosphors were synthesized by the flux method. When the appropriate amounts of fluxes are added, the synthesis temperature reduced by at least 200 °C compared with the conventional solid-state reaction method. SEM images demonstrated that addition of the flux in the process of phosphor synthesis benefitted the size and morphology of BaMgAl₁₀O₁₇:Eu²⁺ phosphor particles. Photoluminescence measurements under VUV excitation indicated that the luminescent intensity of the phosphor enhanced by adding the flux system (BaF₂+Li₂CO₃). Addition of the flux system can not only enhance the luminescence efficiency and improve the stability, but also control the morphology and grain size of the phosphor. Replacement of Ba²⁺ by Li⁺ could generate traps, which result in slightly longer decay time.     

Photoluminescence of Y2O3:Eu thin film phosphors by sol-gel deposition and rapid thermal annealing

Y₂O₃:Eu³⁺ phosphor films have been developed by using the sol-gel process. Comprehensive characterization methods such as Photoluminescent (PL) spectroscopy, X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy were used to characterize the Y₂O₃:Eu³⁺ phosphor films. In this experiment, the XRD profiles show that the Y₂O₃:Eu³⁺ phosphor films crystallization temperature and optimum annealing temperature occur at about 650 and 750 °C, respectively. The optimum dopant concentration is 12 mol% Eu³⁺ and the critical transfer distance (R_c) among Eu³⁺ ions is calculated to be about 0.84 nm. Vacuum environment is more efficient than oxygen and nitrogen to eliminate the OH content and hence yields higher luminescent phosphor films. The PL emission intensity of Y₂O₃:Eu³⁺ phosphor films is also dependent on the annealing time. It was found that the H₂O impurities were effectively eliminated after annealing time of 25 s at 750 °C in vacuum environment. From the experiment results, the schematic energy band diagram of Y₂O₃:Eu³⁺ phosphor films is constructed.     

Correlation of photoluminescence of (Y, Ln)VO4:Eu (Ln=Gd and La) phosphors with their crystal structures

We have studied the photoluminescence (PL) of (Y, Ln)VO₄:Eu³⁺ (Ln=La and Gd) phosphors and the correlation of the PL of those phosphor with their crystal structure. It is found that (Y, Gd)VO₄:Eu³⁺ phosphors have the same crystal structure as YVO₄:Eu³⁺, which is tetragonal with a little different lattice parameters. In the case of (Y, La)VO₄:Eu³⁺ phosphors, however, the gradual change from tetragonal to monoclinic structure of host lattice was observed as the amount of La ion increased. To investigate the PL property of (Y, Ln)VO₄:Eu³⁺ (Ln=La and Gd) phosphors, vacuum ultraviolet (VUV) and ultraviolet (UV) excitation were used. The favorable crystal structure for the PL intensity of orthovanadate phosphor under 147 and 254 nm excitation was tetragonal containing Gd ion and under 365 nm excitation was monoclinic containing La ion which might have the lowest site symmetry for Eu³⁺ ion.     

A rapid combustion process for the preparation of MgSrAl10O17:Eu phosphor and related luminescence and defect investigations

Blue-emitting europium-ion-doped MgSrAl₁₀O₁₇ phosphor, prepared using the combustion method, is described. An efficient phosphor can be prepared by this method in a muffle furnace maintained at 500 °C in a very short time of few minutes. The phosphor is characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy and BET surface area measurements. Photoluminescence (PL) spectra revealed that europium ions were present in divalent oxidation state. The thermoluminescence (TL) glow curve shows two peaks at around 178 and at 354 °C. The defect centres formed in the phosphor are studied using electron spin resonance (ESR). The ESR spectrum indicates the presence of Fe³⁺ ions in the non-irradiated system. Irradiated MgSrAl₁₀O₁₇:Eu exhibits lines due to radiation-sensitive Fe³⁺ ion and a defect centre. The centre is characterized by an isotropic g-value of 2.0012 and is assigned to a F⁺ centre. The radiation-sensitive Fe³⁺ ion appears to correlate with the main TL peak at 178 °C. During irradiation an electron is released from Fe²⁺ and is trapped at an anion vacancy to form F⁺ centre. During heating, an electron is liberated from the defect centre and recombines with Fe³⁺ emitting light.     

Preparation and luminescent properties of europium-doped yttria fibers by electrospinning

Sub-micrometer-sized fibers of europium-doped yttria (Y₂O₃:Eu³⁺) were prepared by electrospinning followed by high-temperature calcinations for the first time. The fibers were with diameters of 200-400 nm and lengths of several 10 μm and cubic in phase. The spectral properties of the Y₂O₃:Eu³⁺ fibers were studied, in contrast with those of bulk powders. The results indicated that in the present Y₂O₃:Eu³⁺ fibers the excited charge transfer band had slightly blue shift in comparison with that in the bulk due to weaker covalence of Eu-O bonds. In addition, both of the lifetimes of the ⁵D₁ and ⁵D₀ states in the fibers became shorter than that in the bulk due to improved nonradiative transition rates.     

Preparation and characterization of long persistence strontium aluminate phosphor

Long afterglow green phosphor SrAl₂O₄:Eu²⁺,Dy³⁺ is synthesized by a solid-state reaction method at 1350 °C under mild reducing atmosphere of activated carbon. The effects of B₂O₃ flux on the sintering dynamic process and the optimum concentrations of Eu²⁺ and Dy³⁺ for long-lasting bright luminescence property have been investigated. The effect of a small amount of charge compensators like Mg²⁺, Zn²⁺, Na⁺, and K⁺ on long persistence has also been studied. TG/DTA, SEM, and XRD have been used to characterize the synthesized phosphor.     

A study on the luminescence properties of CaAl2B2O7:Eu phosphor

The blue-emitting CaAl₂B₂O₇:Eu²⁺ phosphor was prepared at 800 °C by a modified solid-state reaction. The results of X-ray powder diffraction (XRD) analysis confirmed the formation of CaAl₂B₂O₇:Eu²⁺. Photoluminescence (PL) spectroscopy showed that the phosphor could be excited efficiently by UV-vis light from 250 to 410 nm, and exhibit blue emission (460 nm). The emission intensity of CaAl₂B₂O₇:Eu²⁺ phosphor varies with the increase of Eu²⁺concentration. The mechanism of resonance-type energy transfer from Eu²⁺ to Eu²⁺ was established to be electric multipole-multipole interaction. TEM images showed that the grain size of CaAl₂B₂O₇:Eu²⁺ is about 45 nm, which is in full agreement with the theoretical calculation data from the XRD patterns.     

Low-voltage cathodoluminescence property of Li-doped Gd2−xYxO3:Eu

Cathodoluminescent (CL) spectra of Li-doped Gd_2−xY_xO₃:Eu³⁺ solid-solution (0.0?x?0.8) were investigated at low voltages (300 V-1 kV). The CL intensity is maximum for the composition of x=0.2 and gradually reduces with increasing the amount of substituted Y content. In particular, small (∼100 nm) particles of Li-doped Gd_1.8Y_0.2O₃:Eu³⁺ are obtained by firing the citrate precursors at only 650°C for 18 h. Relative red-emission intensity at 300 V of this phosphor is close to 180% in comparison with that of commercial red phosphor Y₂O₃:Eu³⁺. An increase of firing temperature to 900°C results in 400-600 nm sized spherical particles. At low voltages (300-800 V), the CL emission of 100 nm sized particles is much stronger than that of 400-600 nm sized ones. In contrast, the larger particles exhibit the higher CL emission intensity at high voltages (1-10 kV). Taking into consideration small spherical morphology and effective CL emission, Li-doped Gd_1.8Y_0.2O₃:Eu³⁺ appears to be an efficient phosphor material for low voltage field emission display.     

Enhanced blue emission and EPR study of LaMgAl11O19:Eu phosphors

Europium doped LaMgAl₁₁O₁₉ phosphor was prepared by the combustion method. The as-prepared and post-treated (1350 °C 10 h 5% H₂+95% N₂) phosphors were investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), photoluminescence (PL) and electron paramagnetic resonance (EPR) techniques. XRD patterns show that LaMgAl₁₁O₁₉:Eu phosphors have hexagonal structure. FT-IR spectrum exhibits absorption bands corresponding to the stretching vibration of AlO₄ and AlO₆. Morphological studies reveal that this phosphor has faceted plates of varying sizes and shapes. The as-prepared LaMgAl₁₁O₁₉:Eu phosphor consists of both Eu³⁺ and Eu²⁺ ions. The phosphor exhibits a bright blue emission at 450 nm (4f⁶5d→4f⁷ transition of Eu²⁺). On post-treating the phosphor we are able to enhance the blue emission efficiency by 330%. The process was detected from the evolution of excitation, emission and EPR spectra and the results are discussed.