Spectral variations of Ca3Sc2Si3O12:Ce phosphors via substitution and energy transfer

The luminescence intensity of emission peak at around 525 nm decreased in the Ce3+ and Er3+ co-doped Ca3Sc2Si3O12 phosphors. Mg2+ ion, which was likely incorporated into the Sc3+ position of the host crystal, was co-doped to adjust the crystal field and compensate for the excess positive charge due to the doping of Ce3+. The green emission belonged to the 5d→4f transition of Ce3+ moved toward longer wavelength by addition of Mg2+ in Ce3+ and Er3+ co-doped Ca3Sc2Si3O12 phosphor, which could increase the brightness of the phosphor. However, the position of weakening of luminescence intensity at around 525 nm remained basically unchanged by increasing the amount of Mg2+. The results showed that the weakening of luminescence intensity at around 525 nm caused by the absorption of Er3+, which had little influence on the environment of the crystal field.     

Luminescence properties of Mn2） ions doped Lu2CaMg2Si3O1） garnet phosphors

Yttrium aluminum garnet structure phosphors Lu2CaMg2Si3O12:Mn2+ were synthesized by conventional high temperature solid-state reaction in reductive atmosphere. The structure and optical properties of samples were characterized by application of powder X-ray diffraction (XRD) and photoluminescence spectroscopy. Results of X-ray diffraction (XRD) analysis showed that the phosphors mainly presented garnet structure with a few weak peaks of impurity phases. Lu2-xCaMg2Si3O12:xMn2+ (x=0.01-0.8) phosphors showed a broad emission band peaking at around 590 nm under ultraviolet (UV) light of 408 nm when Mn2+ concentration was less than 0.08 mol. With an increase in the Mn2+ concentration (above 0.08), another broad emission band peaking at 720 nm besides 590 nm was observed, which may be due to manganese ion having different valence and occupying different host lattice. The critical quenching concentrations of manganese ion in the wavelength of 590 and 720 nm were about 0.06 and 0.2 mol, respectively. With 408 nm excitation wavelength, emission color of the samples had a red shift trend as the Mn2+ concentration increased. All the results indicated that the Lu2CaMg2Si3O12:Mn2+ phosphors could be applicable to n-UV based white LEDs.     

A blue luminescence glass-ceramics of Eu^2＋ ions activated Li2O-BaO-B2O3

New blue luminescence glass-ceramic samples were prepared in air by annealing of the Eu^3＋-doped Li2O-BaO-B2O3 glass. The as-made glass samples only showed the sharp emission peaks assigned to the transitions of 5^D0-7^Fj （J=0, 1, 2, 3, 4） of Eu^3＋ ions. The glass-ceramic samples gave a strong and broad emission band peaking at about 382 nm ascribed to the 5d-4f transition of Eu^2＋ ions. The optical properties such as excitation and emission spectra, and the decay time of the Eu^2＋ ions were investigated in the glasses or the glass-ceramics samples. The X-ray diffraction pattern showed that LiBan9O15 might be demonstrated to be the crystallites in the glass-ceramic, which contributed to the blue luminescence. SEM micrograph was investigated on the glass-ceramic samples obtained by crystallization of the glass matrix resulting in a mixture of poly-crystals.     

Luminescent properties of SrSi2N2O2:Ce3+,Tb3+-a potential phosphor for white light emitting diodes

Ce3+ and Tb3+ co-doped SrSi2N2O2 phosphors were prepared by solid-state reaction. The X-ray diffraction pattern exhibited that the phosphor consisted mainly of oxygen-rich SrSi2N2O2. The optical properties of SrSi2N2O2:Ce3+, SrSi2N2O2:Tb3+ and SrSi2N2O2:Ce3+,Tb3+ were studied, respectively. The emission intensity of Tb3+ at 541 nm was remarkably enhanced by Ce3+ in SrSi2N2O2:Ce3+,Tb3+ phosphor, which was attributed to the energy transfer from Ce3+ to Tb3+. The chromaticity coordinates of phosphors were investigated as a function of Tb3+ concentration. When the Ce3+ and Tb3+ concentrations were 0.02 and 0.18 mol per formula unit, respectively, the chromaticity coordinate was (0.257, 0.337) in the CIE 1931 chromaticity diagram. SrSi2N2O2Ce3+,Tb3+ phosphors could be used for white light emitting diodes.     

Influence of concentration and sintering temperature on luminescence properties of Eu^3＋：SnO2 nanocrystallites

The nanopowders of SnO2 doped with different Eu3+ concentrations were synthesized using the modified Pechini method. The Eu3+ concentrations were high above solubility limit. The average size of crystallites was controlled by the sintering temperatures. The structure and the morphology of obtained powders were examined using the XRD (X-ray diffraction) and TEM (transmission electron microscopy) analyses. The Eu2Sn2O7 phase separation was observed at relatively high concentration of Eu3+ ions. The ZnS:Ag micropowders were mixed with the Eu3+:SnO2 powders and their normalized emission was used to measure a relative efficiency of Eu3+:SnO2. The photoluminescence spectra of mixed powders were measured in function of Eu3+ concentration and average size of nanocrystallites. The reference peak method was used for comparison of intensities of the samples and selection of optimal one. The influence of the average grain size and Eu3+ concen-tration on the phosphor’s efficiency was discussed. The presented results confirmed the rightness of synthesis of the Eu3+:SnO2 in form of nanocrystalites with relatively high Eu3+ concentration.     

Preparation and Characterization of Y_3Sc_2Ga3O（12） Nano-Polycrystalline Powders by Co-Precipitation Method

In order to grow high-quality gallium garnet crystals,polycrystalline materials were used as starting materials.YSGG precursor was synthesized by co-precipitation method using aqueous ammonia as a precipitator,and the precursor was then sintered at different temperatures.The results showed that the feasible pH range was 8.3～9.84 in the process of co-precipitation reaction.The YSGG precursor and the powders sintered at different temperatures were characterized by IR,XRD and TEM methods.It was found that the precursor transformed to pure YSGG polycrystalline phase at 800 ℃.YSGG nano-polycrystalline powders sintered at 800～1000 ℃ were well dispersed and the sizes of the YSGG grains were about 40～100 nm.     

Spectroscopic investigations of Gd3Sc2Ga3O12 garnet doped with Cr^3＋ and Nd^3＋ ions

Absorption, excitation and emission spectra as well as decay time measurements at 10, 77, and 300 K were performed for Ga₃Gd₃Sc₂O₁₂ garnet single-crystal doped with Cr³⁺ and Nd³⁺ ions. Strong reabsorption of Cr³⁺ emission by Nd³⁺ absorption lines was observed. The assignments of f–f and d–d transitions were proposed. The broad emission band of chromium ions is an indication that weak/intermediate crystal field strength is present at the Cr³⁺ site.     

Flux and concentration effect on Eu^3＋ doped Gd2（MoO4）3 phosphor

A novel red emitting phosphor Gd2（MoO4）3：Eu^3＋ was prepared by solid reaction, using Gd2O3, Eu2O3 and WO3 as starting matedals and NH4F as flux. The effects of flux content and Eu^3＋ concentration on the crystal structure, morphology and luminescent properties were investigated using XRD, SEM and fluorescent spectrum measurement. The XRD patterns showed that the resultants had the monoclinic structure. With the increase in flux amount, their crystallization significantly improved. The SEM images indicated that the mean size of the phosphor particles was around 2 μm, and agglomeration of the phosphor particles appeared while introducing higher flux amount. The excitation spectra exhibited more intense f-f transitions originating from ground state 7^F0 to upper states 5^L6 and 5^D2 than the charge transfer band. The concentration quenching of Eu^3＋ emission indicated that energy transfer from Eu^3＋ to molybdate host existed even at lower Eu^3＋ concentration.     

Preparation and Characterization of Gd3Sc2Ga3O12 Polycrystalline Material by Co-Precipitation Method

Gd3Sc2Ga3O12 polycrystalline material for single crystal growth was prepared with Ga, Gd2O3 and Sc2O3 as starting materials and aqueous ammonia as the precipitator by co-precipitation method. The precursors sintered at various temperatures were characterized by infrared spectra （IR）, X-ray diffractometry （XRD） and transmitted electron microscopy （TEM）. The results showed that pure GSGG phase could be obtained at 900 ℃. The sintered powders were well-dispersed and less-aggregated in the sintered temperature range of 900 - 1000 ℃. XRD and TEM show that the polycrystalline particle sizes of the polycrystalline powders were about 20 - 50 nm. Compared with the method that Ga2O3, Gd2O3 and Sc2O3 were mixed directly and sintered to get polycrystalline materials, the synthesized temperature was lower and sintered time was shorter. Thus co-precipitation was a good method to synthesize GSGG polycrystalline material.     

Photoluminescent properties of LaF3：Eu^3＋ and GdF3：Eu^3＋ nanoparticles prepared by co-precipitation method

LaF₃:Eu³⁺ and GdF₃:Eu³⁺ nanoparticles were prepared by a co-precipitation method in the presence of the chelating agent, citric acid. The structural properties of the products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average crystallite size was estimated from the full-width at half-maximum (FWHM) of the diffraction peaks by the Scherrer equation. The sizes of the nanoparticles were 12 nm for LaF₃:Eu³⁺ and 17 nm for GdF₃:Eu³⁺. The luminescent properties of the nanoparticles were investigated by excitation and emission spectra. Energy transfer from Gd³⁺ to Eu³⁺ was observed.     

Enhancement of luminescence and afterglow in Ca0.8Zn0.2TiO3:Pr3+ by Nb5+ substitution for Ti4+

Nb5+ doped Ca0.8Zn0.2TiO3:Pr3+ red long afterglow phosphors were synthesized by solid-state reaction methods. X-ray diffraction, photoluminescence spectroscopy and thermally stimulated spectrometry were used to investigate the effects of Nb5+ content on the crystal characteristics and luminescent properties of Ca0.8Zn0.2Ti1-xNbxO3:Pr3+ phosphors. The results showed that the addition of a small quantity of Nb5+ had negligible effect on the crystal characteristics of Ca0.8Zn0.2Ti1-xNbxO3:Pr3+, but it could change the trapping parameters (the depth of trap, frequency factors and the concentration of trapped charges at t=0) of Ca0.8Zn0.2Ti1-xNbxO3:Pr3+ phosphors, and then led to the enhance-ment of red fluorescence and phosphorescence at 612 nm originating from 1D2→3H4 transition of Pr3+. Both of the red fluorescence intensity and afterglow time reached the largest values in the sample of Ca0.8Zn0.2Ti1-xNbxO3:Pr3+ with x=0.05. The afterglow time of Ca0.8Zn0.2Ti0.95Nb0.05O3:Pr3+ phosphors lasted for over 24 min (≥1 mcd/m2) when the excited source was cut off.     

Luminescent properties of amorphous phosphor 1.4YeO3·2.5Al2O3·0.1Tb2O3 prepared by sol-gel method

Amorphous phosphor 1.4YeO3·2.5Al2O3·0.1Tb2O3 （the same composition as Y2.8Tb0.2Al5O12） was prepared via a sol-gel method at relatively low temperature （i.e., below 650℃）, which is much lower than that for the preparation of polycrystalline Y3Al5O12：Tb^3＋ （above 1400℃）. The amorphous phosphor prepared in the optimized conditions showed a bright green-yellowish luminescence, the intensity of which was comparable with that of polycrystalline sample and the emissions of which were assigned to 5D4 → 7Fj transitions of Tb^3＋. Besides the emissions of Tb^3＋, the amorphous samples prepared at temperatures below 500 ℃ presented a weak blue emission band around 420 nm.     

Electronic structure of the SrAl2O4：Eu^2＋ persistent luminescence material

The electronic structure of the strontium aluminate (SrAl₂O₄:Eu²⁺) materials was studied with a combined experimental and theoretical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structure of the non-optimized and optimized crystal structure were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands as well as the band gap energy of the material together with the position of the Eu²⁺ 4f^{7 8}S_7/2 ground state were calculated. The calculated band gap energy (6.4 eV) agreed well with the experimental value of 6.6 eV. The valence band consisted mainly of oxygen states whereas the bottom of the conduction band of strontium states. In agreement with the experimental results, the calculated 4f^{7 8}S_7/2 ground state of Eu²⁺ lies in the energy gap of the host. The position of the 4f⁷ ground state depended on the Coulomb repulsion strength. The position of the 4f⁷ ground state with respect to the valence and conduction bands was discussed using theoretical and experimental evidence available.     

Ba1-xSrxMgSiO4:Eu2+,Mn2+: A novel tunable single-matrix tricolor phosphor for W-LED

The Ba1-xSrxMgSiO4:Eu2+,Mn2+ phosphors were prepared by solid-state reaction. Their photoluminescence properties were inves-tigated with fluorescence spectrum and CIE chromaticity. The emission color of Eu2+ in Ba0.98-xSrxMgSiO4:0.02Eu2+ could be tuned from green to blue by adjusting the content of Sr2+. The blue emission of Eu2+ overlapped well with the excitation spectra of Mn2+, leading to an ef-ficient energy transfer from Eu2+ to Mn2+ in Ba0.98-xSrxMg1-ySiO4:0.02Eu2+,yMn2+. Ba0.93Sr0.03Mg1-ySiO4:0.02Eu2+,yMn2+ could emit three ef-ficient broad bands at 440, 530 and 640 nm. The emission color of Ba0.93Sr0.03Mg1-ySiO4:0.02Eu2+,yMn2+ could be tuned from greenish blue to yellowish white by increasing the content of Mn2+ from 0 to 0.1. By changing the content of Sr2+/Mn2+, white-light with different hues could be conveniently obtained in the Ba1-xSrxMgSiO4:Eu2+,Mn2+ phosphors. The results showed that Ba1-xSrxMgSiO4:Eu2+,Mn2+ is a promising single-phased tricolor phosphor in the fabrication of W-LED.     

Spectral properties and energy transfer of Tm^3＋/Ho^3＋-codoped Ga2O3-Bi2O3-GeO2-PbO glasses

The 2.0 μm emission originating from Ho^3＋：^5I7→^5I8 were investigated upon excitation with 808 nm laser diode （LD） transition in Ho^3＋/Tm^3＋-codoped gallate-bismuth-germanium-lead glasses Energy transfer （ET） process between Tm^3＋： ^3F4 level and Ho^3＋： ^5I7 level was also discussed. It was noted that the measured peak wavelength and stimulated emission cross-section of Ho^3＋-doped bismuth-germanium-lead glasses were -2.02 μm and 5.1×10^-21 cm^2, respectively. Intense emission of Ho^3＋ in Tm^3＋/Ho^3＋-codoped GBPG glass were observed, which resulted from the ET between Tm^3＋： ^3F4 and Ho^3＋： ^5I7 level upon excitation with 808 nm LD.     

Synchrotron radiation investigations of the Sr2MgSi2OT：Eu^2＋,R^3＋ persistent luminescence materials

The electronic and defect energy level structure of polycrystalline Sr₂MgSi₂O₇:Eu²⁺,R³⁺ persistent luminescence materials were studied with thermoluminescence and different synchrotron radiation spectroscopies (UV-VUV emission and excitation, X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS)). Special attention was paid on the effect of the R³⁺ co-dopants on the persistent luminescence properties of the materials. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy (E_g) value of ca. 7.1 eV agreed very well with the DFT value of 6.7 eV. The variation of the E_g value was attempted to relate with the trap structure as well as with the different properties of the R³⁺ co-dopants. The trap level energy distribution depended strongly on the R³⁺ co-dopant except for the shallowest trap energy above the room temperature remaining practically the same, however. The different processes in the mechanism of persistent luminescence from Sr₂MgSi₂O₇:Eu²⁺,R³⁺ were assembled and their contributions discussed.     

Preparation and Upconversion Luminescence of Y3Al5O12 ： Yb^3＋ , Er^3＋ Trapsparent Ceramics

YAG： 1% （atom fraction） Yb^3＋ , 0.5% （atom fraction） Er3＋ transparent ceramics were fabricated by the solid state reaction method using high-purity Y2O3, Al2O3, Yb2O3, and Er2O3 powders as starting materials. The mixed powder compact was sintered at 1760 ℃ for 6 h in vacuum and annealed at 1500 ℃ for 10 h in an air atmosphere. The ceramics consisted of about 10μm grains and exhibited a pore-free structure. The optical transmittance of the ceramics at 1064 nm was nearly 80%. Upconversion emissions were investigated on the ceramics pumped by a 980 nm continuous wave diode laser, and strong green emission centered at 523 and 559 nm and red emission centered at 669 nm were observed, which originated from the radiative transitions of ^2H11/2→^4I15/2, ^4S3/2→^4I15/2, and ^4F9/2→^4I15/2 of Er^3＋ ions, respectively.     

Citric Gel Synthesis and Luminescent Properties of Ce^3＋-Activated SrGa2O4 Phosphor

Ce^3＋-activated SrGa2O4 phosphor was synthesized by a method of citric gel,wherein citric acid served as a chelate agent,and the as-synthesized powder was calcined in a slightly reduced ambient.The crystallization characteristics of the sample varied with the calcining temperature.Compared with the phosphor prepared by the solid-state reaction,the phosphor synthesized by citric gel was calcined at a relatively lower temperature.Consequently,the volatilization of Ga2O3 during high-temperature calcining process was avoided.The typical double-peak emission of Ce^3＋ originated from 2D（5d）→4F5/2（4f）,and 2D（5d）→4F7/2（4f）was observed,and the intrinsic emission of SrGa2O4 host was much restricted.The emission intensity varied with the calcining temperature because the different crystallinity and the optimal concentration of Ce-dopant was determined at 3%.     

Upconversion luminescence and color tunable properties in Yb-Tb codoped Ca0.15Zr0.85O1.85 inverse opal

The upconversion (UC) luminescence and color tunable properties of Tb3+ ions were investigated by steady spectral under 980 LD excitation in the Ca0.15Zr0.85O1.85:Yb,Tb inverse opals fabricated by the self-assembly technique in combination with a sol-gel method. The in-hibition of UC emission was inspected if the Tb3+ UC emission band was in the regions of the photonic bandgap, while enhancement of the UC emission occurred if the UC emission band located at the edge of the bandgap. Color modification of the UC emission was successfully ob-tained by the suppression or enhancement effect of the photonic band gap on the UC emission.     

Synthesis and luminescence properties of SrAleO4：Eue＋,Dy3＋ hollow microspheres via a solvothermal co-precipitation method

SrAl2O4:Eu2+,Dy3+ hollow microspheres were successfully prepared through a facile and mild solvothermal co-precipitation combining with a postcalcining process.The structure and particle morphology were investigated by X-ray diffraction(XRD),scanning and transmission electron microscopy(SEM and TEM)pictures,respectively.The mechanism for the formation of spherical SrAl2O4:Eu2+,Dy3+ phosphor was preliminary presented.After being irradiated with ultraviolet(UV)light,the spherical phosphor emitted long-lasting green phosphorescence.Both the photoluminescence(PL)spectra and luminance decay,compared with that of commercial bulky powders,revealed that the phosphors had efficient luminescent and long lasting properties.It was considered that the SrAl2O4:Eu2+,Dy3+ hollow microspheres had promising long-lasting phosphorescence with potential scale-dependent applications in photonic devices.