Photoluminescence and thermoluminescence characterization of Eu- and Dy -activated Ca3(PO4)2 phosphor

Rare-earth-doped polycrystalline Ca₃(PO₄)₂:Eu, Ca₃(PO₄)₂:Dy and Ca₃(PO₄)₂:Eu,Dy phosphors prepared by a modified solid-state synthesis has been studied for its X-ray diffraction, thermoluminescence (TL) and photoluminescence (PL) characteristics. The PL emission spectra of the phosphor suggest the presence of Eu³⁺ ion in Ca₃(PO₄)₂:Eu and Dy³⁺ ion in Ca₃(PO₄)₂:Dy lattice sites. The TL glow curve of the Ca₃(PO₄)₂:Eu compounds has a simple structure with a prominent peak at 228 °C, while Ca₃(PO₄)₂:Dy peaking at 146 and 230 °C. TL sensitivity of phosphors are compared with CaSO₄: Dy and found 1.52 and 1.20 times less in Ca₃(PO₄)₂:Eu and Ca₃(PO₄)₂:Dy phosphors, respectively. The Ca₃(PO₄)₂:Eu,Dy phosphors shows switching behavior under two different excitation wavelengths and enhancement in PL intensity of Dy³⁺ ions were reported. The paper discusses the photoluminescence and thermoluminescence behavior of Eu³⁺ and Dy³⁺ ion in Ca₃(PO₄)₂ hosts, it may be applicable to solid-state lighting as well as thermoluminescence dosimetry applications.     

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.     

Correlated ESR, PL and TL studies on Sr5(PO4)3Cl:Eu thermoluminescence dosimetry phosphor

Electron spin resonance (ESR), thermoluminescence and photoluminescence studies in Eu²⁺ activated Sr₅(PO₄)₃Cl phosphor are reported in this paper. The Sr₅(PO₄)₃Cl:Eu²⁺ phosphor is twice as sensitive as the conventional CaSO₄:Dy phosphor used in thermoluminescence dosimetry of ionizing radiations. It has a linear response, simple glow curve, emission peaking at 456 nm. The defect centers formed in the Sr₅(PO₄)₃Cl:Eu²⁺phosphor are studied by using the technique of ESR. A dominant TL glow peak at 430 K with a smaller shoulder at 410 K is observed in the phosphor. ESR studies indicate the presence at three centers at room temperature. Step annealing measurements show a connection between one of the centers and the dominant glow peak at 430 K. The 430 K TL peak is well correlated with center I, which is tentatively identified as (PO₄)²⁻ radical.     

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.     

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.     

The concentration quenching and crystallographic sites of Eu2+ in Ca2BO3Cl

A yellow phosphor, Ca2BO3CI：Eu2＋, is prepared by the high-temperature solid-state method. Under the condition of excitation sources ranging from ultraviolet to visible light, efficient yellow emission can be observed. The emission spectrum shows an asymmetrical single intensive band centred at 573 nm, which corresponds to the 4f65dl→4f7 transition of Eu2＋. Eu2＋ ions occupy two types of Ca2＋ sites in the Ca2BO3C1 lattice and form two corresponding emission centres, respectively, which lead to the asymmetrical emission of Eu2＋ in Ca2BO3C1. The emission intensity of Eu2＋ in Ca2BO3C1 is influenced by the Eu2＋ doping concentration. Concentration quenching is discovered, and its mechanism is verified to be a dipole-dipole interaction. The value of the critical transfer distance is calculated to be 2.166 nm, which is in good agreement with the 2.120 nm value derived from the experimental data.     

Surface treatment investigation and luminescence properties of SiO2-coated Ca2BO3Cl:0.02Eu2+ phosphors via sol–gel process

SiO₂-coated Ca₂BO₃Cl:Eu²⁺ phosphors were prepared by the sol–gel method in order to enhance the chemical and thermal stabilities of Ca₂BO₃Cl:Eu²⁺ phosphor. The phase structures, microstructures and luminescence properties were studied by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and fluorescence spectrometer, respectively. The emission intensity of SiO₂-coated Ca₂BO₃Cl:Eu²⁺ phosphor decreased a little compared to that of the uncoated phosphor. The moisture resistances of the phosphors were comparatively examined by the aging treatment experiment in the water, and the thermal stability was studied by the temperature dependent photoluminescence spectra. The results indicated that SiO₂ coating on the surface of the phosphor particles improved the moisture-resistance and thermal stability to a large extent.     

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).     

Eu-activated Ba2Mg(BO3)2 yellow-emitting phosphors for near ultraviolet-based light-emitting diodes

A series of Eu²⁺-activated Ba₂Mg(BO₃)₂ yellow phosphors were prepared by a high temperature solid-state reaction. The phosphor emits intense yellow light under near ultraviolet excitation. Large Stokes shift can be attributed to the asymmetric nature of the Eu site and the lack of rigidity in the host. The concentration self-quenching mechanism of Ba₂Mg(BO₃)₂:Eu²⁺ is d-d interaction and the critical transfer distance is calculated to be about 12.29 Å. Prototype light-emitting diodes were fabricated by coating the Ba₂Mg(BO₃)₂:0.07Eu²⁺ phosphor onto ∼370 nm-emitting InGaN chips. The LEDs exhibit intense yellow-emitting under a forward bias of 20 mA. The results indicate that Eu²⁺-activated Ba₂Mg(BO₃)₂ is a candidate as a yellow component for fabrication of near-UV white light-emitting diodes.     

Synthesis and luminescence properties of Eu, Ce and Tb-activated Sr3La2(BO3)4 under UV excitation

The spectroscopic properties in UV-excitable range for the phosphors of Sr₃La₂(BO₃)₄:RE³⁺ (RE³⁺=Eu³⁺, Ce³⁺, Tb³⁺) were investigated. The phosphors were synthesized by conventional solid-state reactions. The photoluminescence (PL) spectra and commission international de I'Eclairage (CIE) coordinates of Sr₃La₂(BO₃)₄:RE³⁺ were investigated. The f-d transitions of Eu³⁺, Ce³⁺ and Tb³⁺ in the host lattices are assumed and corroborated. The PL and PL excitation (PLE) spectra indicate that the main emission wavelength of Sr₃La₂(BO₃)₄:Eu³⁺ is 611 nm, and Sr₃La₂(BO₃)₄:Ce³⁺ shows dominating emission peak at 425 nm, while Sr₃La₂(BO₃)₄:Tb³⁺ displays green emission at 487, 542, 582 and 620 nm. These phosphors were prepared by simple solid-state reaction at 1000 °C. There are lower reactive temperature and more convenient than commercial phosphors. The Sr₃La₂(BO₃)₄:Tb³⁺ applied to cold cathode fluorescent lamp was found to emit green light and have a major peak wavelength at around 542 nm. These phosphors may provide a new kind of luminescent materials under ultraviolet excitation.     

A potential reddish orange emitting phosphor Ca2BO3Cl:Eu for white light-emitting diodes

A series of phosphors Ca₂BO₃Cl:Eu³⁺ were synthesized by using a high-temperature solid-state reaction technique, and their UV–vis luminescence properties were investigated. The f–f transitions of Eu³⁺ in the host lattice were assigned and discussed. The excitation and emission spectra indicate that this phosphor can be effectively excited by ultraviolet (394 nm), and exhibit reddish orange emission corresponding to the ⁵D₀→⁷F_J (J=0, 1, 2) transitions of Eu³⁺. The influence of the doping concentration and charge compensators on the relative emission intensity of Eu³⁺ was investigated, and the optimum doping concentration is 0.04. The critical distance R_c was estimated to be 17.1 Å in terms of the concentration quenching data. The present study suggests that Ca₂BO₃Cl:Eu³⁺ can be a potential candidate as an UV-convertible phosphor for white light-emitting diodes (LEDs).     

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.     

Size-dependent microstructure and europium site preference influence fluorescent properties of Eu-doped Ca10(PO4)6(OH)2 nanocrystal

In this study, Eu³⁺-doped nanocrystalline Ca₁₀(PO₄)₆(OH)₂ (Ca_10−xEu_x(PO₄)₆(OH)₂) with different particle sizes have been prepared by the thermal decomposition of precursors. Size-dependent microstructure could be observed in nanocrystalline Ca_10−xEu_x(PO₄)₆(OH)₂. The lattices of Ca_10−xEu_x(PO₄)₆(OH)₂ nanocrystals were more distorted in comparison with the bulk, and the smaller the particle size, the more distorted the lattices. Room temperature photoluminescence showed europium site preference was also size-dependent, with the majority of Eu³⁺ ions occupying Ca(II) sites in the bulk, but more and more Eu³⁺ ions occupying Ca(I) sites in Ca_10−xEu_x(PO₄)₆(OH)₂ with decreasing particle size. Fluorescent properties of Ca_10−xEu_x(PO₄)₆(OH)₂ were considered to be influenced by both microstructure and site preference of Eu³⁺ ions. An abnormal strong intensity of ⁵D₀-⁷F₀ transition was observed in bulk and larger Ca_10−xEu_x(PO₄)₆(OH)₂ nanocrystals, but the relative intensities of ⁵D₀-⁷F₀ transition to ⁵D₀-⁷F_1,2,3,4 transition of Eu³⁺ became weaker as the particle sizes decreased. As the particle sizes became smaller, the ratios of the red emission transition (⁵D₀-⁷F₂) to the orange emission transition (⁵D₀-⁷F₁) (R/O values) first increased by comparing the bulk sample with 96 nm sample, and then decreased by comparing 96 nm sample to 57 nm sample. The quenching concentrations of Ca_10−xEu_x(PO₄)₆(OH)₂ samples increased with decreasing particle size. Possible mechanisms responsible for these phenomena were proposed. Since nanosized Ca_10−xEu_x(PO₄)₆(OH)₂ showed higher fluorescent intensities, higher R/O values and higher quenching concentrations, this material is considered to be a promising phosphor.     

Luminescent properties of Sr5(PO4)3 Cl:Eu2+, Mn2+ as?a?potential phosphor for UV-LED-based white LEDs

Eu²⁺ and Mn²⁺ co-activated Sr₅(PO₄)₃Cl phosphors with blue and orange color double emission bands, under a broad-band excitation wavelength range of 340–400 nm, were synthesized by the solid-state reaction. It was found that the processing parameters, including the fluxes, annealing time and activators concentrations, affect the emission intensity and other luminescent properties. Energy transfer between Eu²⁺ and Mn²⁺ was discovered and the transfer efficiency was also estimated based on relative intensities of Eu²⁺ and Mn²⁺ emission. Thus the relative strength of blue and orange emission intensities could be tuned by varying the relative concentration of Eu²⁺ and Mn²⁺. Since the photoluminescence excitation spectra of the newly developed Sr₅(PO₄)₃Cl:Eu²⁺, Mn²⁺ phosphors exhibit a strong absorption in the range of 340–400 nm, they are promising for producing UV-LED-based white LEDs.     

Emission-tunable phosphors Ca9MgM′ (PO4)7: Eu2+,Mn2+ (M′=Li,Na, K) for white light-emitting diodes

A series of single-composition phosphors Ca₉MgM′(PO₄)₇:xEu²⁺, yMn²⁺ (CMM′ P:Eu²⁺, Mn²⁺; M′=Li, Na, K; 0.003≤x≤0.03; 0 ≤y≤0.1) were synthesized by solid state reactions. Upon excitation at 337 nm, phosphors Ca₉MgM′ (PO₄)₇: Eu²⁺ exhibit strong blue emissions centered at 417 (Ca₉MgLi(PO₄)₇:Eu²⁺), 457 (Ca₉MgNa(PO₄)₇:Eu²⁺), and 453 (Ca₉MgK(PO₄)₇:Eu²⁺) nm respectively, which correspond to the 4f⁶5d¹→4f⁷ transitions of Eu²⁺ ions, Through an effective resonance-type energy transfer, CMM′P:Eu²⁺,Mn²⁺ phosphors exhibit a series of colors by adjusting the concentration of Mn²⁺. The result indicates that CMM′P:Eu²⁺,Mn²⁺ can be potentially used as a UV excited phosphor for white light-emitting diodes (LEDs).     

Dy3+ emission in M5(PO4)3F (M = Ca, Ba) phosphor

Ultrafine M₅(PO₄)₃F:Dy³⁺ (M = Ca, Ba) phosphors were prepared via combustion process using metal nitrates as precursors. The formation of crystalline phosphate was confirmed by X-ray diffraction pattern. The PL excitation spectra show the excitation peaks observed at 250 to 400 nm due to f → f transition of Dy³⁺ ion, which are useful for solid-state lighting purpose (mercury free excitation). The PL emission of Dy³⁺ ion by 348 nm excitation gave an emission at 489 nm (blue), 582 nm (yellow) and 675 nm (red). All the characteristics of BYR emissions like BGR indicate that Dy doped Ca₅(PO₄)₃F and Ba₅(PO₄)₃F phosphors are good candidates that can be applied in solid-state lighting phosphor (mercury free excited lamp phosphor) and white light LED.      

Luminescence optimization of Eu-doped LnAl3(BO3)4 (Ln=Y, Gd) red phosphor using spray pyrolysis

LnAl₃(BO₃)₄:Eu³⁺ (Ln=Y, Gd) red phosphor particles were prepared by spray pyrolysis and the luminescent intensity under vacuum ultraviolet (VUV) excitation was investigated by changing Eu³⁺ content, Y/Gd molar ratio, and boron content. The concentration quenching for Eu³⁺ activator was observed at 5 at%. The highest luminescent intensity at 615 nm due to the ⁵D₀→⁷F₂ transitions of Eu³⁺ was achieved when the ratio of Gd to Y was 0.55. The R/O ratio (obtained by dividing the red emission intensity at 615 nm with the orange one at 592 nm), however, was not influenced by the G/Y ratio. Using excess boron, up to 135% of the stoichiometric quantity, improved the emission intensity of LnAl₃(BO₃)₄:Eu³⁺ red phosphor. According to XRD analysis, the sample prepared using boron of a stoichiometric quantity had YBO₃ phase as a minor phase. Such YBO₃ phase progressively disappeared with an increase in the excess quantity of boron, which was responsible for the enhancement of emission intensity. In addition, the R/O ratio became larger and larger by increasing the excess content of boron due to a reduction in the symmetry of Y site. Consequently, both the emission intensity and the color coordinate of LnAl₃(BO₃)₄:Eu³⁺ red phosphors were successfully optimized in terms of the Y/Gd ratio and the excess quantity of boron in spray pyrolysis.     

Synthesis and photoluminescence properties of MZr2(PO4)3:Eu3+; Bi3+ (M=Na; K) phosphors

A series of new red phosphors, MZr₂(PO₄)₃:Eu³⁺; Bi³⁺ (M=Na; K), were synthesized using the solidstate reaction method, and their photoluminescence spectra were measured. The MZr₂(PO₄)₃:Eu³⁺; Bi³⁺ (M=Na; K) phosphors were efficiently excited by an ultraviolet (UV; 395 nm) source, and showed intense orange-red emission at 595 nm. Further investigation of the concentration-dependent emission spectra indicated that the MZr₂(PO₄)₃:Eu³⁺; Bi³⁺ (M=Na; K) phosphors exhibit the strongest luminescence intensity when y = 0.01 in NaZr_2(0:95−y)(PO₄)₃:Eu_0.10³⁺, Bi_2y ³⁺ and y = 0.09 in NaZr_2(0.95−y)(PO₄)₃:Eu_0.10³⁺, Bi_2y ³⁺, whereas the relative PL intensity decreases with increasing Bi³⁺ concentration due to concentration quenching. The addition of Bi³⁺ widens the excitation band of NaZr_2(0.95−y)(PO₄)₃:Eu_0.10³⁺, Bi_2y ³⁺ around 320 nm, which provides the useful idea of broadening the excitation band around 300–350 nm to fit the ultraviolet chip.     

Crystal field parameters and energy level structure of the MnO4 tetroxo anion in Li3PO4, Ca2PO4Cl and Sr5(PO4)3Cl crystals

A comparative study concerning the electronic structure of the Mn⁵⁺ ion in the Li₃PO₄, Ca₂PO₄Cl, Sr₅(PO₄)₃Cl host lattices has been carried out in the framework of the exchange charge model. The crystal field parameters have been evaluated using the structural data as the only input information. The 10 K absorption spectra of the investigated compounds have been measured in order to verify the correspondence between experimental and calculated energy levels. A systematic trend of the crystal field splitting of the most intense transitions has been evidenced and discussed by considering the symmetry properties of the coordination polyhedra around Mn⁵⁺.     

New red phosphors BaZr(BO3)2 and SrAl2B2O7 doped with Eu for PDP applications

Vacuum ultraviolet (VUV) excitation and photoluminescence (PL) characteristics of Eu³⁺ ion doped borate phosphors; BaZr(BO₃)₂:Eu³⁺ and SrAl₂B₂O₇:Eu³⁺ are studied. The excitation spectra show strong absorption in the VUV region with the absorption band edge at ca. 200 nm for BaZr(BO₃)₂:Eu³⁺ and 183 nm for SrAl₂B₂O₇:Eu³⁺, respectively, which ensures the efficient absorption of the Xe plasma emission lines. In BaZr(BO₃)₂:Eu³⁺, the charge transfer band of Eu³⁺ does not appear strongly in the excitation spectrum, which can be enhanced by co-doping Al³⁺ ion into the BaZr(BO₃)₂ lattices. The luminescence intensity of BaZr(BO₃)₂:Eu³⁺ is also increased by Al³⁺ incorporation into the lattices. The PL spectra show the strongest emission at 615 nm corresponding to the electric dipole ⁵D₀→⁷F₂ transition of Eu³⁺ in both BaZr(BO₃)₂ and SrAl₂B₂O₇, similar to that in YAl₃(BO₃)₄, which results in a good color purity for display applications.