Photoluminescence properties of novel Eu-activated CaSi6N8O oxynitride phosphor for white LED applications

A series of novel Eu²⁺-activated CaSi₆N₈O greenish-yellow phosphors were synthesized via solid state reaction method. The synthesized CaSi₆N₈O:Eu²⁺ phosphors could absorb UV to near UV light. The emission spectra exhibited a broad greenish-yellow emission band when excited with a light of wavelength 405 nm. The emission spectrum shifted towards longer wavelength with increasing Eu²⁺ concentration. The emission intensity increased with the increasing Eu²⁺ concentration and the quenching concentration was found to be 0.1 mol. The observed results were explained by the energy transfer mechanism due to dipole-dipole interactions among Eu²⁺ ions. These novel greenish-yellow CaSi₆N₈O:Eu²⁺ phosphors could be considered as potential candidate for the white LED applications.     

Synthesis and photoluminescence properties of Yb doped Ba5(PO4)3Cl phosphor for white light-emitting diodes

Yb²⁺ ion doped Ba₅(PO₄)₃Cl phosphor was synthesized by solid state reaction. Four distinct absorption bands were observed in the Ultraviolet (UV) light region due to the electronic transitions of Yb²⁺ ion from ¹S₀ ground state to ²F_5/2(t_2g), ²F_5/2(e_g), ²F_7/2(t_2g), and ²F_7/2(e_g) excited states. The main emission wavelength of the phosphor was around 630 nm. The optimized Yb²⁺ ion concentration was 0.2 mol% (λ_exc. = 400 nm). The calculated critical distance was about 8.729 Å and the concentration quenching was observed above 0.2 mol% due to the electric dipole–dipole interaction.     

Color tuning of Y3Al5O12:Ce phosphor and their blend for white LEDs

Gadolinium or lanthanum co-doped (0.5 mole) yttrium aluminum garnet doped with cerium phosphors were synthesized by a citric acid gel method and the effect of co-dopants on the structural and luminescent properties were studied. A significant peak shift in the photoluminescence spectra of yttrium aluminum garnet doped cerium was observed from 535 to 556 and 576 nm for gadolinium or lanthanum co-doped phosphors, respectively. The color tuned phosphor were blended with yttrium aluminum garnet doped cerium which showed a considerable improvement in the Commission International De Eclairage chromaticity co-ordinate values of gallium nitride based blue light emitting diode pumped white light. White light emitted from yttrium aluminum garnet doped cerium shows a Commission International De Eclairage value of (0.229, 0.182) whereas the yttrium aluminum garnet doped cerium phosphor blended with gadolinium or lanthanum co-doped phosphor shows (0.262, 0.243) and (0.295, 0.282), respectively. These results demonstrate the possibility to use these phosphor blends to enhance the white light generation in the field of white-light emitting diode solid-state lighting.     

A novel blue-emitting Sr3Al2O5Cl2:Ce,Li phosphor for near UV-excited white-light-emitting diodes

A novel blue-emitting Sr₃Al₂O₅Cl₂:Ce³⁺,Li⁺ phosphor has been synthesized by solid state reaction. The excitation spectrum shows a broad band extending from 300 to 400 nm, and the emission spectrum shows a broad blue band peaking at 450 nm with a half width of about 100 nm. The emission intensity at 250 °C remains at about 50% of that at room temperature. The decay curve at the emission peak consists of fast and slow components. The Sr₃Al₂O₅Cl₂:Ce³⁺,Li⁺ should be a promising blue phosphor for near ultraviolet-based white-light-emitting diodes.     

Photoluminescence properties of Eu doped Ba2ZnS3 phosphor for white light emitting diodes

The synthesis and photoluminescence properties of novel Eu²⁺ doped Ba₂ZnS₃ phosphors for white light emitting diodes (LEDs) are reported. Diffuse reflection spectra of Ba₂ZnS₃ host and synthesized phosphors have been measured. The excitation spectra of synthesized phosphors consist of three broad bands between 250 nm and 550 nm and are consistent with the diffuse reflectance spectra. The emission spectra show the characteristic 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ion and there exists efficient energy transfer from host to Eu²⁺ ions when excited by 350-nm light. The dependence of emission spectra on temperature is also measured; the possible reasons applied to explain the experimental results are also discussed. The fluorescence lifetime of Eu²⁺ in Ba_1.995ZnS₃:0.005Eu²⁺ is measured and the values are 1.49 and 23.4 μs.     

Ce dopant segregation in Y3Al5O12 optical ceramics

Ce³⁺-doped YAG garnet optical ceramic have been sintered at the Shanghai Institute of Ceramics in China to characterize dopant distribution in optical ceramics by combining optical spectroscopy and two spatially resolved techniques as imaging confocal microscopy and transmission electron microscopy. A strong Ce³⁺ segregation and spatial variations of content between grains and grain boundaries has been confirmed by quantitative data obtained by TEM microscopy. This observation is another evidence of the inhomogeneous Ce³⁺ distribution across grain and grain boundaries in optical ceramics comparable to that of Nd³⁺ ions in YAG ceramics. These results correlate well with low segregation coefficients of Nd³⁺ and Ce³⁺ observed in the garnet crystals grown from the melt and/or flux.     

Energy transfer and enhanced 1532 nm emission in Er and Ce co-doped Y3Al5O12 nano-particles

A series of Y₃Al₅O₁₂:Ce³⁺, Er³⁺ (YAG:Ce, Er) nano-particles were synthesized by polymer-assisted sol–gel method. X-ray diffraction measurements reveal that a pure crystalline phase of YAG is achieved at temperature as low as 800 °C. The energy transfer from Ce³⁺ to Er³⁺ is studied based on photoluminescence spectroscopy and fluorescence decay patterns. It results that the emission intensity of Er³⁺ at near infrared (NIR) 1532 nm under indirect excitation of Ce³⁺ (460 nm) is 10 times stronger than that of direct excitation of Er³⁺ (275 or 380 nm). The energy transfer efficiency is estimated as 95.5% for YAG:Ce_0.03Er_0.09 sample. The very efficient energy transfer path and mechanism are also discussed.     

Dependence of photoluminescence (PL) emission intensity on Eu and ZnO concentrations in Y2O3:Eu and ZnO·Y2O3:Eu nanophosphors

Y₂O₃:Eu³⁺ and ZnO·Y₂O₃:Eu³⁺ nanophosphor powders with different concentrations of Eu³⁺ ions were synthesized by a sol-gel method and their luminescence properties were investigated. The red photoluminescence (PL) from Eu³⁺ ions with the main emission peak at 612 nm was observed to increase with Eu³⁺ concentration from 0.25 to 0.75 mol% and decreased notably when the concentration was increased to 1 mol%. The decrease in the PL intensity at higher Eu³⁺ concentrations can be associated with concentration quenching effects. The red emission at 612 nm was shown to increase considerable when ZnO nanoparticles were incorporated in Y₂O₃:Eu³⁺ while green emission from ZnO was suppressed. The increase is attributed to energy transfer from ZnO to Eu³⁺.     

Translucent Y3Al5O12 ceramics

The sintering of Y₃Al₅O₁₂ to full density and translucency using either SiO₂ or MgO as a dopant is described. The resulting ceramics have a regular microstructures and low optical absorption coefficients.     

Electro-deposition of Y2O3:Eu thin film phosphor and its luminescent properties

Y₂O₃:Eu³⁺ red-emitting thin film phosphor was prepared by a two-step process: the cathodical deposition of thin film of yttrium hydroxide and europium hydroxide followed by an annealing process to achieve Eu³⁺ doped Y₂O₃ film. It is found that the atomic content of Eu³⁺ can be well controlled by simply adjusting the volume ratio of Y(NO₃)₃ to Eu(NO₃)₃ solutions. Dependence of the photoluminescence intensity on the atomic content of Eu³⁺ in Y₂O₃ was also studied. The best photoluminescence performance of Y₂O₃:Eu³⁺ thin film phosphor was achieved as atomic content of Eu³⁺ equal to 1.85 at.%.     

Influence of fuels on the morphology of undoped Y3Al5O12 and photoluminescence of Y3Al5O12:Eu prepared by a combustion method

Undoped and Eu-doped yttrium aluminum garnet nano-powders were prepared by a facile combustion method with citric acid/ethylene diamine tetraacetic acid (EDTA) as fuels and nitrates as oxidizers. The precursors and powders calcined at 1030 °C were investigated using thermogravimetric (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscope (SEM), and Brunauer-Emmett-Teller (BET) surface area measurements. It was found that the powders could be indexed with a garnet structure. The grains were in shape of hemispherical with sizes between 60 nm and 100 nm. With decreasing the citric acid/EDTA ratio, the crystallite size decreased steadily and the specific surface area increased. Investigations of photoluminescence (PL) revealed that as-synthesized YAG:Eu³⁺ phosphor samples exhibited an orange emission band with a main peak at 591 nm under the excitation of 394 nm. As citric acid amounts increased, the quality of crystallinity became higher and the luminescent properties were monotonously enhanced.     

Low-temperature synthesis,phonon and luminescence properties of Eu doped Y3Al5O12 (YAG) nanopowders

This contribution presents two simple and cost-effective routes for the low-temperature and large-scale production of pure and Eu-doped Y₃Al₅O₁₂ (yttrium aluminum garnet YAG) nanopowders. The proposed methodologies combine a mechanically assisted metathesis reaction or coprecipitation from solution followed by crystallization of the obtained precursors from molten sodium nitrate/nitrite. Both procedures allow obtaining pure and/or doped YAG nanopowders at remarkably low temperatures, i.e. already at 350 °C although firing at 500 °C is needed in order to get single phase and fully crystalline materials. As-obtained samples were characterized by XRD, TEM, Raman, IR and luminescence methods. These methods showed that the mean crystallite size is near 23–31 and 51 nm, when crystallization is performed from the amorphous precursor obtained by a mechanically assisted metathesis reaction and coprecipitation, respectively. Raman and IR spectra indicated better crystallinity of the powders prepared at 500 °C. The emission study showed that the intensity ratio between hypersensitive ⁵D₀ → ⁷F₂ and magnetic-dipole ⁵D₀ → ⁷F₁ transitions of Eu³⁺ is significantly larger than expected for well-crystallized YAG. Origin of this behavior is discussed.     

Flame aerosol deposition and annealing of Y3Al5O12:Tb phosphor coatings

Sub-micrometer-sized powders of Y₃Al₅O₁₂:Tb phosphor (d_SEM = 320 nm) were prepared by flame-assisted spray pyrolysis of aqueous precursors in a premixed propane/air flame and in situ deposited onto quartz substrates. Phosphor screens with densities of up to 0.7 mg cm⁻² could be produced within 20 min. As-deposited coatings were amorphous and required a thermal post-treatment. After annealing in an oven for 2 h (T ≥ 900 °C), the yttrium aluminum garnet phase (YAG:Tb) was obtained. Alternatively, the phosphor coatings were treated by an impinging flame in the same setup used for the deposition. Quasi-amorphous Y₃Al₅O₁₂:Tb coatings demonstrated bright green photoluminescence upon flame annealing at T ≈ 1100 °C for just several minutes and could outperform YAG:Tb when excited in the wavelength ranges 205–220 nm and 230–260 nm. For example, brightness of emission from the quasi-amorphous coatings was up to five times higher than that of the fully crystalline YAG:Tb phosphor at a technically important wavelength of 254 nm.     

Photoluminescence properties of Y2O3:Tb and YBO3:Tb green phosphors synthesized by hydrothermal method

In this work, two Tb³⁺ activated green phosphors: Y₂O₃:Tb³⁺ and YBO₃:Tb³⁺ were prepared by hydrothermal method. Photoluminescence properties of both phosphors were studied in details. Both phosphors exhibit similar luminescent characteristics symbolized by the dominant green emission at 545 nm. Concentration quenching occurs at the Tb³⁺ concentration of 1.60 atomic% and 2.57 atomic% for Y₂O₃:Tb³⁺ and YBO₃:Tb³⁺, respectively. Luminescence decay properties were characterized to better understand the mechanism of concentration quenching. Based on the calculation, the concentration quenching in both phosphors was caused by the dipole–dipole interaction between Tb³⁺ ions.     

Synthesis and photoluminescence properties of new NaAlSiO4:Eu phosphors for near-UV white LED applications

A new NaAlSiO₄:0.1Eu²⁺ phosphors were synthesized at different temperatures using a liquid phase precursor (LPP) technique. The XRD patterns indicate the presence of hexagonal nepheline phase for all the samples. The synthesized phosphors can be excited efficiently in the broad near-UV region. The PL emission spectra showed a broad emission peak at around 551 nm corresponding to 5d → 4f transition of Eu²⁺ ions. The synthesized phosphors showed better thermal stability when compared with the standard YAG:Ce³⁺ phosphor.     

Improved optical photoluminescence by charge compensation in the phosphor system CaMoO4:Eu

It has been found that charge compensated CaMoO₄:Eu³⁺ phosphors show greatly enhanced red emission under 393 and 467 nm-excitation, compared with CaMoO₄:Eu³⁺ without charge compensation. Two approaches to charge compensation, (a) 2Ca²⁺ → Eu³⁺ + M⁺, where M⁺ is a monovalent cation like Li⁺, Na⁺ and K⁺ acting as a charge compensator; (b) 3Ca²⁺ → 2Eu³⁺ + vacancy, are investigated. The influence of sintering temperature and Eu³⁺ concentration on the luminescent property of phosphor samples is also discussed.     

Lu2.94Al5O12:0.06Ce3+绿色荧光粉的制备及光致发光

采用高温固相法制备了Lu_2.94Al₅O₁₂:0.06Ce³⁺绿色荧光粉。通过X射线粉末衍射(XRD)、扫描电子显微镜(SEM)和光致发光光谱(PL)对样品的物相、形貌及发光性能进行了表征。结果表明,所合成的Lu_2.94Al₅O₁₂:0.06Ce³⁺绿色荧光粉为立方晶系,表面为类球形。激发光谱中,位于340和450 nm的激发峰分别归属于4f的两个能级到5d能级的跃迁而产生的吸收,340 nm处的激发峰是由于发光是由于²F_5/2到5d的跃迁,而450nm处的激发峰是由于²F_7/2到5d的跃迁。发射光谱中,位于525 nm的发射峰对应Ce³⁺的4f-5d电子跃迁。当Ce₃₊掺杂量为6%,1500℃煅烧5 h时,Lu_2.94Al₅O₁₂:0.06Ce³⁺绿色荧光粉CIE色坐标为(0.3683,0.5959),是一种可以用作白光LED的绿色荧光粉。     

Factors for determining photoluminescence properties of YBO3:Ce phosphor prepared by hydrothermal method

YBO₃:Ce³⁺ blue-emitting phosphors were prepared from boric acid and nitrates of yttrium and cerium(III) by hydrothermal method. An excess amount of boric acid, prolonged aging, high temperature, and a high pH value promote the formation of crystalline YBO₃. The higher crystallinity results in the higher photoluminescence (PL) intensity corresponding to the 5d-4f transition of Ce³⁺ under the irradiation of near-UV light. The PL intensity also depends on the pH value of precursor suspension and the nominal Ce³⁺ concentration, where the sample prepared at pH = 8 and Ce/(Y + Ce) = 0.25-0.5 at% shows the maximum PL intensity. In addition, the hydrothermally prepared sample shows the characteristic photobleaching behavior under the continuous irradiation of near-UV light. These results suggest that the crystallinity of the host YBO₃ crystal and the homogeneity of substituted Ce³⁺ ions play significant roles in the PL properties.     

Effect of metal halide fluxes on the microstructure and luminescence of Y3Al5O12:Ce phosphors

Yellow-emitting Y_2.95Al₅O₁₂:0.05Ce (YAG:Ce) phosphor particles with high luminescence efficiency under 450 nm ultraviolet (UV) excitation were prepared by a heat treatment of submicrometer-sized oxide powder mixture in a reducing atmosphere at 1550 °C. Prior to the heat treatment, the oxide mixture was blended with 5 wt% of metal halides – BaF₂, BaCl₂, NaF, NaCl, and KF – as a flux. It was observed that YAG:Ce particles prepared with fluorine containing flux demonstrated spherical morphology, high photoluminescence properties, and diameters of 5–20 μm, whereas those prepared with chlorine were small-sized particles (≤5 μm) with relatively low intensity. The highest relative photoluminescence (PL) intensity (∼118%) was obtained for the samples prepared with 5% BaF₂. A set of rules was established for the formation of spherical YAG:Ce particles in the presence of this flux.     

A novel green emitting phosphor Ca1.5Y1.5Al3.5Si1.5O12:Tb

Vacuum ultraviolet (VUV) excitation and emission properties of Tb³⁺ ion doped silico-aluminate phosphor Ca_1.5Y_1.5Al_3.5Si_1.5O₁₂:Tb³⁺ was studied. Upon excitation with vacuum ultraviolet (VUV) and near UV light excitation, the phosphor showed strong green-emission peaked at 545 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺, and the highest PL intensity at 545 nm was found at a content of about 14 mol% Tb³⁺. The 4f–5d transition absorption of Tb³⁺ is in the range from 150 nm to 260 nm, and there is an energy transfer from the host to the rare earth ions. Field emission scanning electron microscopy (FE-SEM) images showed the particle size of the phosphor was less than 3 μm.