Luminescence and energy transfer in Eu, Mn co-doped Li4SrCa(SiO4)2 for white light-emitting-diodes

Li₄(Sr_0.96Eu_0.04)(Ca_1 − xMn_x)(SiO₄)₂ phosphors were synthesized by solid-state reactions and photoluminescence (PL) properties were investigated. These phosphors have intense absorption in n-UV region, which is suitable for excitation of UV LEDs. The orange-reddish emission of Mn²⁺ can be adjusted by changing the Mn²⁺/Eu²⁺ ratio. Energy transfer from Eu²⁺ to Mn²⁺ is observed. Li₄(Sr_0.96Eu_0.04)(Ca_1 − xMn_x)(SiO₄)₂ phosphors could be used in white LEDs.     

Luminescent properties of Eu and Ce ions in strontium litho-silicate Li2SrSiO4

The luminescent properties of Eu²⁺ and Ce³⁺ ions in Li₂SrSiO₄ have been studied upon excitation in the 2-20 eV region. Based on the results of luminescent measurements, values of the crystal field splitting and the centroid shift of the Ce³⁺5d configuration in Li₂SrSiO₄ were found and compared with those of Ce³⁺ ions in some other inorganic compounds. The Eu²⁺ ions in Li₂SrSiO₄ exhibit a broad band emission with a maximum at 576 nm, which is due to the 4f⁶5d→4f⁷ transition. It was shown that the long-wavelength position of the Eu²⁺ emission in Li₂SrSiO₄ is caused by the large crystal-field splitting of the Eu²⁺ 4f⁶5d configuration and relatively high degree of covalency of the Eu-O bond. The stabilization of Eu²⁺ ions in Li₂SrSiO₄ during the synthesis process requires a strong reducing agent. Two phenomenological approaches to explain the low stability of Eu²⁺ in Li₂SrSiO₄ are also discussed.     

Luminescence and energy transfer of Eu/Mn co-doped SiO2–Al2O3–ZnO–K2O glass ceramics for white LEDs

Transparent Eu²⁺/Mn²⁺ co-doped new glass ceramics (GC) containing β-Zn₂SiO₄ nanocrystals were prepared under a reduced atmosphere. The optical properties of these samples have been investigated. The emission spectra of Eu²⁺/Mn²⁺ co-doped glass ceramics show two broadband peakings at 458 and 560 nm under ultraviolet radiation, which can be attributed to 4f⁶5d¹→4f⁷ transition of Eu²⁺ and ⁴T₁(⁴G)→⁶A₁(⁶S) transition of Mn²⁺, respectively. Energy transfer (ET) from Eu²⁺ to Mn²⁺ is discovered by directly observing significant overlap of the excitation spectrum of Mn²⁺ and the emission spectrum of Eu²⁺. ET from Eu²⁺ to Mn²⁺ in glass ceramics is further confirmed by fluorescence studies performed on the samples with various activator (Mn²⁺) concentrations. The optimal composition generates white light with chromaticity coordinates (0.291, 0.344). The results indicate that Eu²⁺/Mn²⁺ co-doped glass ceramics is potential material for white light-emitting diodes (LEDs).     

Intense green/yellow emission in Ca8Zn(SiO4)4Cl2:Eu, Mn through energy transfer for blue-LED lighting

Eu²⁺ and Mn²⁺ co-doped Ca₈Zn(SiO₄)₄Cl₂ phosphors have been synthesized by a high temperature solid state reaction. Energy transfer from Eu²⁺ to Mn²⁺ is observed. The emission spectra of the phosphors show a green band at 505 nm of Eu²⁺ and a yellow band at 550 nm of Mn²⁺. The excitation spectra corresponding to 4f⁷-4f⁶5d transition of Eu²⁺ cover the spectral range of 370-470 nm, well matching UV and/or blue LEDs. The shortening of fluorescent lifetimes of Eu²⁺ followed by simultaneous increase of fluorescent intensity of Mn²⁺ with increasing Mn²⁺ concentrations is studied based on energy transfer. Upon blue light excitation the present phosphor can emit intense green/yellow in comparison with other chlorosilicate phosphors such as Eu²⁺ and Mn²⁺ co-doped Ca₈Mg(SiO₄)₄Cl₂ and Ca₃SiO₄Cl₂, demonstrating a potential application in phosphor converted white LEDs.     

Photoluminescence of double-color-emitting phosphor Ca5(PO4)3Cl:Eu, Mn for near-UV LED

Eu²⁺ activated Ca₅(PO₄)₃Cl blue-emitting phosphors were prepared by the conventional solid state method using CaCl₂ as the chlorine source and H₃BO₃ as flux. The structure and luminescent properties of phosphors depend on the concentrations of Eu²⁺, the amount of CaCl₂ and the usage of the H₃BO₃ flux were investigated systematically. Eu²⁺ and Mn²⁺ Co-doped Ca₅(PO₄)₃Cl with blue and orange double-band emissions were also researched based on the optimal composition and synthesis conditions. The energy transfer between Eu²⁺ and Mn²⁺ was found in the phosphor Ca₅(PO₄)₃Cl:Eu²⁺, Mn²⁺, and the Co-doped phosphor can be efficiently excited by near-UV light, indicating that the phoshor is a potentional candidate for n-UV LED used phosphor.     

Fluorescence emission spectrum and energy transfer in Eu and Mn co-doped Ba2Ca(BO3)2 phosphors

Eu²⁺ and Mn²⁺ co-doped Ba₂Ca(BO₃)₂ phosphors yield two emission bands consisting of green and red components under the excitation of 360 nm, which shows a great potential for white LEDs. Effective energy transfer occurs in Eu²⁺/Mn²⁺ co-doped Ba₂Ca(BO₃)₂ host due to the large spectral overlap between the emission of Eu²⁺ and the excitation of Mn²⁺. The energy transfer from Eu²⁺ to Mn²⁺ is thoroughly investigated by their excitation, emission and photoluminescence decay behaviors, and is demonstrated to be via the dipole–quadrupole interaction.     

Eu/Tb ions co-doped white light luminescence Y2O3 phosphors

Y₂O₃:Eu³⁺, Tb³⁺ phosphors with white emission are prepared with different doping concentration of Eu³⁺ and Tb³⁺ ions and synthesizing temperatures from 750 to 950 °C by the co-precipitation method. The resulted phosphors were characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. The results of XRD indicate that the crystallinity of the synthesized samples increases with enhancing the firing temperature. The photoluminescence spectra indicate the Eu³⁺ and Tb³⁺ co-doped Y₂O₃ phosphors show five main emission peaks: three at 590, 611 and 629 nm originate from Eu³⁺ and two at 481 and 541 nm originate from Tb³⁺, under excitation of 250-320 nm irradition. The white light luminescence color could be changed by varying the excitation wavelength. Different concentrations of Eu³⁺ and Tb³⁺ ions were induced into the Y₂O₃ lattice and the energy transfer from Tb³⁺→Eu³⁺ ions in these phosphors was found. The Commission International de l’Eclairage (CIE) chromaticity shows that the Y₂O₃:Eu³⁺, Tb³⁺ phosphors can obtain an intense white emission.     

Electronic energy relaxation and luminescence decay dynamics of Eu in Zn2SiO4:Eu phosphors

Electronic energy relaxation and decay dynamics of Eu³⁺ in Zn₂SiO₄:Eu³⁺ phosphors display evidence of intra-ion energy transfer from the ⁵D₁ to the ⁵D₀ manifold. The energy transfer timescale does not depend on Eu³⁺ concentration, or the addition of Mn²⁺ as a co-dopant and is estimated to be about 11 μs in Zn₂SiO₄. Evidence for intra-ion Eu³⁺ electronic energy transfer has also been observed in Eu-doped MgS as well as Eu³⁺ encapsulated in zeolite-Y. The energy transfer timescale in these other materials is shorter than in Zn₂SiO₄, most likely due to differences in Eu³⁺ surroundings or site symmetry.     

The luminescence properties of the ZnO@ZnWO4:Eu core–shell composites

The ZnO@ZnWO₄:Eu³⁺ core–shell composites were prepared by a two-step hydrothermal method and the photoluminescence properties of the composites were studied in contrast to the corresponding hexagonal ZnO and monoclinic ZnWO₄: Eu³⁺ nanocrystals prepared by the one-step hydrothermal method. The results demonstrate that in the nanocomposites the Eu³⁺ ions in the ZnWO₄ phase occupy two symmetry sites, one a well-crystalline inner site and one a disordered surface site; while in the ZnWO₄: Eu³⁺ nanocrystals, the local environments surrounding Eu³⁺ ions are relatively disordered for both the inner and the surface sites. This indicates that in the composites, the crystallinity of the ZnWO₄ becomes better, which have positive influence on the improvement of photoluminescence. The temperature stabilities for both the emissions of ZnO and Eu³⁺ ions are improved in contrast to the pure ZnO or ZnWO₄:Eu³⁺ nanocrystals.     

Photoluminescence and gamma-ray irradiation of SrAl2O4:Euand Y2O3:Eu phosphors

Y₂O₃:Eu³⁺ phosphor is a very attractive material for use as a red phosphor in many fields. SrAl₂O₄:Eu²⁺ belongs to long lasting phosphor (LLP) and it is a useful bluish-green luminescence material, which can also be a promising candidate as a simple and easy-to-use radiation detection element for visual display of two dimensional radiation distributions. In the present study, both these two kinds of phosphors were synthesized using high temperature solid state reactions. In our work, the influence of gamma-ray irradiation on the properties of these two kinds of phosphors was studied by comparing photoluminescence, brightness and the decay curve of unirradiated and gamma-ray-irradiated samples. Conclusions from the present work can be briefly summarized as follows. In irradiated samples, the brightness is decreased without sensible change in the wavelength distribution of the luminescence spectrum and in the decay kinetic upon gamma exposure. Moreover, the emission due to Eu³⁺→Eu²⁺ conversion in Y₂O₃:Eu³⁺ phosphors was not observed in our sample after irradiation to high exposure. Also the brightness of SrAl₂O₄:Eu²⁺ phosphor turned out to decrease after the exposition to ionizing radiation while the luminescence wavelength distribution remained unchanged. The reason for the effect of gamma-ray irradiation on the properties of phosphors is also discussed in the paper.     

Photoluminescence characterization of Ce and Dy doped Li2CaGeO4 phosphors

Ce³⁺ and Dy³⁺ activated Li₂CaGeO₄ phosphors were prepared by a solid-state reaction method, and characterized by XRD (X-ray diffraction) and photoluminescence techniques. The characteristic emission bands of Dy³⁺ due to ⁴F_9/2→⁶H_15/2 (blue) and ⁴F_9/2→⁶H_13/2 (yellow) transitions were detected in the emission spectra of Li₂CaGeO₄:Dy³⁺. Ce³⁺ broad band emission was observed in Li₂CaGeO₄:Ce³⁺ phosphors at 372 and 400 nm due to 5d→4f transition when excited at 353 nm. Co-doping of Ce³⁺ enhanced the luminescence of Dy³⁺ significantly and concentration quenching occurs when Dy³⁺ is beyond 0.04 mol%. White-light with different hues can be realized by tuning Dy³⁺ concentration in the phosphors.     

Photoluminescence properties of samarium-doped TiO2 semiconductor nanocrystalline powders

In this work, the luminescence properties of samarium ions-doped titanium dioxide prepared by the sol-gel process were studied. A strong orange red emission (⁴G_5/2−⁶H_7/2 (orange) and ⁴G_5/2-⁶H_9/2 (red)) ascribed to the electron transitions in 4f⁵ configuration of Sm³⁺ ions was observed upon excitation into TiO₂ host. The energy transfer from TiO₂ to Sm³⁺ was verified and the relevant mechanism was discussed. In addition, the impacts of metal ion codopants upon the TiO₂:Sm³⁺ luminescence properties were studied. The results indicated that the central excitation band shifted to blue in the bismuth-codoped materials (0.5-4% in molar ratio), while it shifted to red in the zirconium-codoped materials. Such materials may find applications in white light-emission diode (LED) and tunable solid lasers.     

Luminescence of Pr-doped LiCaAlF6 and LiSrAlF6 crystals

The spectral and kinetic parameters of LiCaAlF₆:Pr and LiSrAlF₆:Pr single crystals are studied at LHeT and room temperature (RT). Photon cascade emission (PCE), i.e., ¹S₀→¹I₆ and ³P₀→³H_j, ³F_j transitions after Pr³⁺ excitation via 4f²→4f 5d absorption results in the conversion of the vacuum ultraviolet photon to two visible photons. The excitation and photoemission spectra as well as decay times have been measured and compared with those for CaF₂:Pr and SrF₂:Pr crystals. X-ray luminescence was measured to study the emission origin from both ¹S₀ and ³P₀ states of Pr³⁺. An intense phosphorescence of LiSrAlF₆:Pr was observed in the visible range after X-irradiation at RT, contrary to LiCaAlF₆:Pr.     

Photoluminescence of Eu in SrGa2S4

A photoluminescence (PL) study of the green-emitting SrGa₂S₄:Eu²⁺ phosphor is reported. Diffuse reflectance, excitation, and emission spectra were examined with the aim to enlarge the fundamental knowledge about the emission of the Eu²⁺ ion in this lattice. The thermal dependence of the radiative properties was investigated. In particular, the Stokes shift, the crystal field splitting and the activation energy of the thermal quenching were determined. By combining these results with the information presented in literature, we discussed the location of the Eu²⁺ levels relative to the valence and conduction bands of SrGa₂S₄.     

Synthesis and photoluminescence characteristics of (Sr, Ca)3B2O6:Eu for application in white light-emitting diodes

A series of yellow-green (Sr, Ca)₃B₂O₆:Eu phosphors have been synthesized using precursors prepared via a facile sol-gel route. The solid-solution phases crystallized to materials with the formula of Sr_3−x−yCa_xEu_yB₂O₆ with varied Ca²⁺ and Eu²⁺ contents. The emission peak centered at 540 nm under near-UV excitation exhibited a broad-band distribution in the range of 450-650 nm. The dependences of the luminescence intensity on the contents of Ca²⁺ substitution and Eu²⁺ dopant were also investigated. The composition in the host lattice sensitively affected the chromaticity index. Sr_1.21Ca_1.7Eu_0.09B₂O₆ (SCB:0.09Eu) was shown to possess the highest intensity and broadest emission band. Calcining temperature was shown to greatly influence the luminescent properties of SCB:0.09Eu. It is concluded that SCB:0.09Eu can be used as an efficient yellow-green phosphor for white light-emitting diodes (white LEDs) applications.     

Effects of non-stoichiometry and substitution on photoluminescence and afterglow luminescence of Sr4Al14O25:Eu, Dy phosphor

The effects of nonstoichiometry and cationic substitution on photoluminescence and afterglow characteristics of strontium aluminate phosphor (Sr₄Al₁₄O₂₅:Eu²⁺, Dy³⁺) were investigated. Photoluminescence intensity of both the strontium-deficit and -rich phosphors was enhanced, but no definite correlation was observed between the afterglow intensity and non-stoichiometry. The photoluminescence emission maxima were either blue or green shifted in case of non-stoichiometric phosphors, whereas the afterglow emission maxima were not affected by the non-stoichiometry. Substitution of strontium by calcium resulted in white afterglow emission at higher calcium concentration. The emission centers in case of photoluminescence and afterglow emission appear to be different. Addition of silver significantly enhanced the afterglow intensity due to increased trap density.     

Radiative properties of Eu in BaGa2S4

A photoluminescence study of the blue-green emitting BaGa₂S₄:Eu²⁺ phosphor is reported. Diffuse reflectance, excitation and emission spectra were examined with the aim to enlarge the fundamental knowledge about the emission of the Eu²⁺ rare earth ion in this lattice. The thermal dependence of the radiative properties and the influence of the Eu²⁺ concentration were investigated. The Stokes shift, the crystal field splitting and the activation energy of the thermal quenching were determined. By combining these results with data available in literature, we discussed the radiative properties of the BaAl₂S₄:Eu²⁺ blue phosphor in relation with those determined in this study for the isostructural BaGa₂S₄:Eu²⁺ phosphor.     

Photoluminescence and energy transfer of Tm doped LiIn (WO4)2 blue phosphors

Room temperature steady and time resolved emission spectra of LiIn_1−xTm_x(WO4)2 (where thulium concentration is 0, 0.5, 1, 5 and 10 at%) blue phosphors, under UV excitation energy have been investigated. The concentration quenching effect on the blue emission, due to the (WO₄)⁻² groups and ¹G₄→³H₆ emission transition of Tm³⁺ were studied. Two energy transfer mechanisms are shown. The first takes place between excited (WO₄)⁻² groups and the ¹G₄ energy level of Tm³⁺, and is mainly analyzed by phonon-assisted energy transfer. The second mechanism is due to an energy transfer from the excited Tm³⁺ ions to the surrounding ground state Tm³⁺ ions. The non-exponential decay curves of the ¹G₄ level observed for higher concentrations are analyzed by the Inokuti–Hirayama model. We think that the quenching effect between Tm³⁺ ions is mainly linked to the dipole–dipole interactions.     

White light emitting from single phased K2Ca1−x−yP2O7: xEu, yMn phosphors under UV excitation

A single phased white light emitting phosphors K₂Ca_1−x−yP₂O₇: xEu²⁺, yMn²⁺ were synthesized by solid state reaction method. The Effective energy transfer occurs in this phosphor due to the large spectral overlap between the emission of Eu²⁺ and the excitation of Mn²⁺. The emission hue of K₂Ca_1−x−yP₂O₇: xEu²⁺, yMn²⁺ from blue to white light can be obtained by tuning the Eu²⁺/Mn²⁺ content ratio. The energy transfer mechanism from Eu²⁺ to Mn²⁺ in this phosphor was carefully investigated and demonstrated to be via the dipole–quadrupole interaction.     

A potential green-emitting phosphor Ca8Mg(SiO4)4Cl2:Eu for white light emitting diodes prepared by sol-gel method

A potential green emitting phosphor Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺ was prepared by modified sol-gel method. The factors those affect the photoluminescence intensity including heating temperature, the usage of the chlorine source CaCl₂ and the concentration of dopant Eu²⁺ were also investigated in detail. As comparison, the phosphor prepared by solid-state reaction was also prepared. The phosphors show intense absorption in the range of 375-450 nm, which makes it a potential candidate of green emitting phosphor used for near-UV or blue light excited white LEDs.