Photoluminescence properties of a new Eu2+-activated CaLaGa3S7 yellowish-green phosphor for white LED applications

A thiogallate chalcogenide phosphor CaLaGa₃S₇:Eu²⁺ was synthesized by a solid-state reaction at 950 °C in a H₂S atmosphere. The photoluminescence excitation,emission spectra, concentration quenching, fluorescence lifetime, and thermal quenching process of the phosphor were investigated in detail. It was found that the synthesized phosphor emitted intense and broadband yellowish-green light with a peak at 554 nm. Thus, the proposed phosphor is suitable for the development of blue or near UV LED. The critical dopant concentration of Eu²⁺ (R_c=15 Å) per unit formula was found to be 0.15 mol. At room temperature, the fluorescence lifetime of Eu²⁺ in CaLaGa₃S₇ was found to be 0.216 μs. The activation energy for thermal quenching was 0.29 eV. The chromaticity coordinates of our phosphor is very close in color to Y₃Al₅O₁₂:Ce³⁺. Therefore, CaLaGa₃S₇:Eu²⁺ can be a good alternative as a yellowish-green phosphor and can be used for white light generation in phosphor-converted LEDs.     

Improved photoluminescence of BaMgAl10O17 blue phosphor prepared by spray pyrolysis

BaMgAl₁₀O₁₇:Eu²⁺ (BAM) blue phosphor particles with improved photoluminescence (PL) intensity under vacuum ultraviolet (VUV) excitation were prepared by a spray pyrolysis process. In order to improve the PL intensity, Er³⁺ and Nd³⁺ ions were used as co-doping elements. The VUV characteristics of BAM:Eu²⁺, M⁺ (M=Er, Nd) were monitored with varying the Er³⁺ and Nd³⁺ content in order to find the optimal co-doping concentration when they were prepared by spray pyrolysis. It was found that doping Er³⁺ or Nd³⁺ enhances the PL intensity of BAM:Eu²⁺ blue phosphor particles. In particular, the M³⁺ doping effect on the PL intensity was pronounced when the prepared BAM:Eu²⁺, M³⁺ particles were excited by 172 nm VUV. The maximum intensity was obtained when the M³⁺ content was 1.0 at% with respect to Ba element. The PL intensity of BAM:Eu²⁺, M⁺ (M=Er³⁺, Nd³⁺) particles was also further improved by producing them in a spherical shape, which was successfully achieved by controlling the spray solution. The optimized BAM:Eu²⁺, M⁺ particles had about 10% higher PL intensity than that of the commercial particles, which are made by a conventional solid-state reaction.     

Synthesis,characterization and studies of radiative properties on Eu3+-doped ZnAl2O4

Eu³⁺-doped ZnAl₂O₄ phosphors were successfully synthesized in air atmosphere at 900 °C. The phosphors were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermally stimulated luminescence (TSL) and photoluminescence (PL) techniques. The average particle size of the system as determined from SEM was found to be 100–150 nm (for samples annealed at 900 °C). PL spectra of the doped phosphors showed emission peaks corresponding to Eu³⁺ ions. Lifetime studies revealed Eu³⁺ ions to be in two different sites. The asymmetric ratio (I₆₁₆/I₅₉₂) was observed to be about 3.75. This suggested that Eu³⁺ ion entered the host mainly substituting Al³⁺ site distorting the local environment and is partly located on surface of the phosphors. A prominent glow peak at 430 K was observed in the TSL of γ-irradiated Eu³⁺-doped ZnAl₂O₄ phosphors. Trap parameters for this peak have been determined and the probable mechanism for the glow peak is proposed. CIE chromaticity coordinates for the system were evaluated. It was observed that, the system could be employed as a potential red emitting phosphor. Commercial utility of the phosphor was investigated by comparing it with commercial red phosphor. The PL intensity of the as prepared phosphors was 63% of that of the commercial phosphor. Apart from this, various radiative properties such as the Judd–Ofelt intensity parameters, spontaneous emission probabilities, luminescence branching ratios, radiative lifetimes and quantum efficiency were evaluated for the system.     

Energy transfer induced Eu3+ photoluminescence enhancement in tellurite glass

In this work, structural, thermal and optical properties of Eu³⁺ doped TeO₂–La₂O₃–TiO₂ glass were investigated. The differential scanning calorimetry (DSC) measurements reveal an important stability factor ΔT=143.52 K, which indicates the good thermal and mechanical stabilities of tellurite glass. From the absorption spectrum, the optical band gap was found to be direct with E_g=3.23 eV. The temperature dependences of photoluminescence (PL) properties of Eu-doped and Eu–Tb codoped tellurite glass are investigated. As the temperature increases from 7 to 300 K, both the PL intensity and the PL lifetime relative to the ⁵D₂→⁷F₀ are nearly constant below 230 K and then an enhancement takes place. This anomalous feature is attributed to the thermally activated carrier transfer process from charged intrinsic defects states to Eu³⁺ energy levels.By co-doping tellurite glasses with Eu and Tb, a strong Eu³⁺ PL enhancement is shown due to excitation transfer from Tb³⁺ and intrinsic defects to Eu ions.     

Luminescence enhancement by the reduction–oxidation synthesis in monoclinic RE2O3 (RE=Eu,Gd) phosphors containing Eu3+ activator

A novel synthesis was developed for enhanced luminescence in sesquioxide phosphors containing Eu³⁺ activator. It consisted of two annealing steps: reduction under vacuum with gaseous H₂ at 10 Torr and 1300 °C and re-oxidation at 300–1500 °C in air. The integrated luminescence intensity of the monoclinic Eu₂O₃ phosphor was enhanced ca. 21 times by this method compared with conventional processing. The photoluminescence (PL) intensity was maximized at re-oxidation temperatures of 500–1100 °C. The PL characteristics of monoclinic Eu₂O₃ and Gd₂O₃:0.06Eu samples were compared with a commercial cubic Y₂O₃:Eu phosphor. The evolution of physical characteristics during the two-step annealing was studied by Raman spectroscopy, XPS, XRD, PL decay analysis, and SEM. PL decay lifetime increased proportionally to the PL intensity over the range 0.5–100 μs. Additional vibrational modes appeared at 490, 497, and 512 cm^?1 after the two-step annealing. The increase in PL intensity was ascribed to the formation of excess oxygen vacancies and their redistribution during annealing. Resonance crossovers between the charge transfer state and the emitting ⁵D_J states are discussed in relation to reported luminescence saturation mechanisms for oxysulfides Ln₂O₂S:Eu³⁺ (Ln=Y, La).     

The influence of Eu3+ doping on photoluminescent properties of red emitting phosphor YInGe2O7:Eu3+ by microwave assisted and conventional sintering method

This paper describes an investigation of the crystalline morphology and photoluminescent properties of YInGe₂O₇ powders doped with different Eu³⁺ concentrations using microwave assisted sintering and conventional sintering. X-ray powder diffraction analysis confirmed the formation of monoclinic YInGe₂O₇ structure as YInGe₂O₇:Eu³⁺ powders were sintered at 1200 °C in microwave furnace for 1 h, and the raw material phase of Y₂O₃ was observed when Eu³⁺ concentration was below 30 mol%. Scanning electron microscopy showed microwave assisted sintering results in smaller particle size and more uniform grain size distribution. In the photoluminescent (PL) studies, the concentration quenching effect was observed under the excitation at 393 nm, but not under the excitation at CTS band. The ⁵D₀→⁷F₂ transition (620 nm), exhibits a non-exponential decay behavior as YInGe₂O₇:Eu³⁺ powders were sintered by microwave with the Eu³⁺ concentration higher than 50 mol%.     

Blue emission in Eu2+ activated MgXAl10O17 (X = Sr,Ca) phosphors

Here we reported photoluminescence properties of Eu²⁺ activated in novel and existing MgXAl₁₀O₁₇ (X = Sr, Ca) phosphor which has been prepared by combustion synthesis at 550 °C under UV and near UV excitation wavelength. The PL emission properties of MgSrAl₁₀O₁₇:Eu²⁺ were monitored at 254 nm and 354 nm respectively keeping emission wavelength at 469 nm. Whereas novel MgCaAl₁₀O₁₇:Eu²⁺ exhibit emission band at 452 nm keeping excitation at 378 nm. These blue emission corresponds to 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ions. Further phosphor was analyzed by XRD for the confirmation of desired phase and purity.     

New synthesis of high-quality storage phosphors

We present a single step synthesis method for the photostimulable X-ray storage phosphor BaFBr:Eu²⁺ which results in a highly sensitive powder with a relatively small average grain size of 5.4 μm. The starting chemical reagents are BaCO₃, NH₄F, NH₄Br and EuF₃. The reaction initiated by the decomposition of the ammonium materials leads to highly volatile hydrogen halide gases which react with the BaCO₃ to form BaFBr at temperatures up to 300 °C. Further heating results in the incorporation of Eu²⁺ and the formation of halide vacancies at temperatures in between 390 and 580 °C. The resulting photostimulated luminescence (PSL) efficiency is optimized after sintering at 800 °C. The reaction process is monitored by differential thermal analysis (DTA) and the reaction products are detected by mass-spectroscopy which confirms the proposed chemical reactions. Intermediate and final products are identified using X-ray diffraction. Photoluminescence (PL) and PSL spectra show the incorporation of Eu²⁺ into the lattice, as well as a PL peak at 470 nm which is not present in the PSL spectrum. This peak is shown to originate from O^2? in the lattice and directly affects the PSL sensitivity.     

Preparation and photoluminescence properties of the Sr1.56Ba0.4SiO4:0.04Eu2+ phosphor

The Sr_1.56Ba_0.4SiO₄:0.04Eu²⁺ phosphors were prepared via a combustion reaction and following the calcination method at low temperature. The influences of the amount of the uncommonly used SrCl₂ flux, different calcination temperatures and time on the structure and the photoluminescence (PL) properties of the phosphors were investigated. Under the excitation of 450 nm blue light, the phosphor shows the intense broad emission band from 490 nm to 650 nm, and the emission peak is centered at 553 nm. The luminescence intensity of Sr_1.56Ba_0.4SiO₄:0.04Eu²⁺ was very sensitive to the crystallinity and morphology characteristics of the phosphor. The phosphor calcined at 950 °C for 3 h in 20%H₂/80%Ar atmosphere exhibits improved PL properties due to its high crystallinity and excellent morphology characteristics. The use of the SrCl₂ flux provides a novel way to improve the crystallinity of the silicates phosphors at low preparation temperature.     

Luminescent properties of red-emitting LiSr4B3O(9−3x/2)Nx:Eu2+ phosphor for white-LEDs

An Eu²⁺-activated oxynitride LiSr_(4?y)B₃O_(9?3x/2)N_x:yEu²⁺ red-emitting phosphor was synthesized by solid-state reactions. The synthesized phosphor crystallized in a cubic system with space group Ia–3d. The LiSr₄B₃O_(9?3x/2)N_x:Eu²⁺ phosphors exhibited a broad red emission band with a peak at 610 nm and a full width at half maximum of 106 nm under 410 nm excitation, which is ascribed to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. The optimal doped nitrogen concentration was observed to be x=0.75. The average decay times of two different emission centers were estimated to be 568 and 489 ns in the LiSr_3.99B₃O_8.25N_0.5:0.01Eu²⁺ phosphors, respectively. Concentration quenching of Eu²⁺ ions occurred at y=0.07, and the critical distance was determined as 17.86 Å. The non-radiative transitions via dipole–dipole interactions resulted in the concentration quenching of Eu²⁺-site emission centers in the LiSr₄B₃O₉ host. These results indicate LiSr₄B₃O_(9?3x/2)N_x:Eu²⁺ phosphor is promising for application in white near-UV LEDs.     

Fluorescent properties of a green- to red-emitting Eu3+, Tb3+ codoped amorphous calcium silicate phosphor

A Eu³⁺, Tb³⁺ codoped amorphous calcium silicate phosphor was prepared by heating a Eu³⁺, Tb³⁺ codoped calcium silicate hydrate phosphor formed by liquid-phase reaction for 30 min at 900 °C. The excitation peak wavelength of the resulting phosphor was 379 nm and the emission peak wavelengths were at 542 nm, attributed to the ⁵D₄→⁷F₅ transition of Tb³⁺, and at 613 mm, attributed to the ⁵D₀→⁷F₁ transition of Eu³⁺. The intensity ratio of the two peaks could be freely controlled by varying the Eu/Tb atomic ratio of the Eu³⁺, Tb³⁺ codoped amorphous calcium silicate phosphor, allowing light to be emitted over a wide range from green to red. It was clarified that electron transfer from Tb³⁺ to Eu³⁺ is occurring.     

Novel EGCG assisted ultrasound synthesis of self-assembled Ca2SiO4:Eu3+ hierarchical superstructures: Photometric characteristics and LED applications

This paper reports for the first time ultrasound, EGCG assisted synthesis of pure and Eu³⁺ (1–5 mol%) activated Ca₂SiO₄ nanophosphors having self-assembled superstructures with high purity. The shape, size and morphology of the product were tuned by controlling influential parameters. It was found that morphology was highly dependent on EGCG concentration, sonication time, pH and sonication power. The probable formation mechanism for various hierarchical superstructures was proposed. The PL studies of Ca₂SiO₄:Eu³⁺ phosphors can be effectively excited by the near ultraviolet (UV) (396 nm) light and exhibited strong red emission around 613 nm, which was attributed to the Eu³⁺ (⁵D₀ → ⁷F₂) transition. The concentration quenching phenomenon was explained based on energy transfer between defect and Eu³⁺ ions, electron–phonon coupling and Eu³⁺–Eu³⁺ interaction. The Judd–Ofelt intensity parameters and radiative properties were estimated by using PL emission spectra. The photometric studies indicate that the obtained phosphors could be a promising red component for possible applications in the field of white light emitting diodes.     

Photoluminescence characterization of a novel red-emitting phosphor In2(MoO4)3:Eu3+ for white light emitting diodes

The red-emitting phosphor In₂(MoO₄)₃:Eu³⁺ with cubic crystal structure was synthesized by a conventional solid-state reaction technique and its photoluminescence properties were investigated. The prepared phosphor can be efficiently excited by ultraviolet (395 nm) and blue (466 nm) light. The emission spectra of the phosphor manifest intensive red-emitting lines at 612 nm due to the electric dipole ⁵D₀→⁷F₂ transitions of Eu³⁺. The chromaticity coordinates of x=0.63, y=0.35 (λ_ex=395 nm) and x=0.60, y=0.38 (λ_ex=466 nm) are close to the standard of National Television Standard Committee values (NTSC) values. The concentration quenching of In₂(MoO₄)₃:Eu³⁺ is 40 mol% and the concentration self-quenching mechanism under 466 nm excitation was the dd intereaction. As a result of the strong emission intensity and good excitation, the phosphor In₂(MoO₄)₃:Eu³⁺ is regarded as a promising red-emitting conversion material for white LEDs.     

Structure and site selective luminescence of sol–gel derived Eu:Sr2SiO4

Strontium silicate (Sr₂SiO₄) samples doped with varying Eu³⁺content were prepared via sol–gel route and characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), dynamic light scattering (DLS) and photoluminescence (PL) techniques. The synthesis temperature could be brought down to 600 °C for formation of a singe phase sample. The concentration of the dopant ion and the temperature of annealing were optimized for maximum PL intensity. The critical energy-transfer distance for the Eu³⁺ ions was evaluated based on which the quenching mechanism was verified to be a multipole–multipole interaction. Based on the time-resolved emission data (TRES), it was inferred that, two different types of Eu³⁺ ions were present in the matrix. The first type was a long lived species (τ=4.7 ms) present at 10-coordinated ‘Sr’sites and the other was a short lived species (τ=1.2 ms) present at the 9-coordinated ‘Sr’sites which gets selectively excited at 296 nm. Judd–Ofelt (J–O) intensity parameters for both the species were evaluated. The trend observed for the two species in the J–O parameters, Ω₂ and Ω₄ were different confirming their existence in two different environments. The color coordinates of the system were evaluated and plotted on a CIE index diagram. Commercial utility of the phosphor was investigated by comparing it with commercial red phosphor.     

Synthesis and luminescence properties of green phosphors Ca6Sr4(Si2O7)3Cl2:Eu2+ for white light emitting diodes

Divalent europium-activated chlorosilicate Ca₆Sr₄(Si₂O₇)₃Cl₂:Eu²⁺ phosphors were synthesized by a conventional solid-state reaction under reductive atmosphere. These phosphors can be efficiently excited by UV–visible light from 320 to 420 nm, which matches that of a near UV-emitting InGaN chip. Under the 360 nm excitation, Ca₆Sr_3.97(Si₂O₇)₃Cl₂:0.03Eu²⁺ phosphor shows a strong and broad emission centering at 515 nm, which is attributed to the 5d→4f transition of Eu²⁺ ion. The mechanism of concentration quenching was determined to be the dipole–dipole interaction and the critical energy-transfer distance of Eu²⁺ was calculated as 3.31 nm. The CIE chromaticity coordinates of Ca₆Sr_3.96(Si₂O₇)₃Cl₂:0.03Eu²⁺ phosphor are (0.127, 0.770) according to the emission spectrum. It can be expected that Ca₆Sr₄(Si₂O₇)₃Cl₂:Eu²⁺ phosphor is a promising candidate as the green component for near-ultraviolet InGaN-based white LED.     

Luminescent properties of Mg3Ca3(PO4)4: Eu2+ blue-emitting phosphor for white light emitting diodes

A blue-emitting phosphor, Eu²⁺-activated Mg₃Ca₃(PO₄)₄ phosphor was synthesized by conventional solid-state reaction. X-ray powder diffraction (XRD) analysis confirmed the phase formation. Photoluminescence (PL) results showed that Mg₃Ca₃(PO₄)₄: Eu²⁺ could be efficiently excited by UV–visible light from 250 to 430 nm, which matched well with the emission wavelengths of near-UV and UV LED chips. The effects of the doped-Eu²⁺ concentration in Mg₃Ca₃(PO₄)₄: Eu²⁺ on the PL were also investigated. The result reveals that Mg₃Ca₃(PO₄)₄: Eu²⁺ is a potential blue-emitting phosphor for white LEDs.     

Thermo,Iono and photoluminescence properties of 100 MeV Si7+ ions bombarded CaSiO3:Eu3+ nanophosphor

This paper reports on the thermo (TL), iono (IL) and photoluminescence (PL) properties of nanocrystalline CaSiO₃:Eu³⁺ (1–5 mol %) bombarded with 100 MeV Si⁷⁺ ions for the first time. The effect of different dopant concentrations and influence of ion fluence has been discussed. The characteristic emission peaks ⁵D₀→⁷F_J (J=0, 1, 2, 3, 4) of Eu³⁺ ions was recorded in both PL (1×10¹¹–1×10¹³ ions cm^?2) and IL (4.16×10¹²–6.77×10¹² ions cm^?2) spectra. It is observed that PL intensity increases with ion fluence, whereas in IL the peaks intensity increases up to fluence 5.20×10¹² ions cm^?2, then it decreases. A well resolved TL glow peak at ～304 °C was recorded in all the ion bombarded samples at a warming rate of 5 °C s^?1. The TL intensity is found to be maximum at 5 mol% Eu³⁺ concentration. Further, TL intensity increases sub linearly with shifting of glow peak towards lower temperature with ion fluence.     

Concentration quenching of Eu2+ in a thermal-stable yellow phosphor Ca2BO3Cl:Eu2+ for LED application

A piece-shaped phosphor Ca₂BO₃Cl: Eu²⁺ was synthesized by solid-state reaction method. This phosphor exhibited wide absorption in ultra-violet and visible range, and bright yellow emission band centering at 570 nm. The concentration quenching mechanism was verified to be a dipole–dipole interaction, and its critical transfer distance was about 17 Å by both calculated crystal structural method and experimental spectral method. This phosphor has a good thermal stability with a quenching temperature (T_1/2) of 200 °C. Yellow and white LEDs were fabricated with this phosphor and near UV chips, and the yellow LED has a high color purity of 97.0% and promising current tolerant property, while the white LED shows a luminous efficiency of 11.68 lm/W.     

Luminescent properties of Eu2+and Dy3+ ions in Ba4Al2O7 phosphor for solid state lighting

In this paper we report the combustion synthesis of rare earth (RE=Eu, Dy) doped Ba₄Al₂O₇ phosphors. Prepared phosphors were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), CIE color co-ordinates and their photoluminescence (PL) properties were also investigated. In case of Ba₄Al₂O₇: Eu²⁺, the emission spectra show unique band centered at 495 nm, which corresponds to the 4f⁶5d¹→4f⁷ transition of Eu²⁺, and PL emission spectra of Dy³⁺ ion under 348 nm excitation give two bands centered at 478 nm (blue) and 575 nm (yellow), which originate from the transitions of ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 of Dy³⁺, respectively. The results indicate that the Eu²⁺ and Dy³⁺ activated Ba₄Al₂O₇ phosphor could find application in solid state lighting.     

Optical study of rare earth activated NaBa0.45Sr0.55PO4 phosphor

Dy³⁺ and Eu²⁺ activated triple phosphate NaBa_0.45Sr_0.55PO₄ phosphors were prepared by facile combustion synthesis. Excellent emission observed when NaBa_0.45Sr_0.55PO₄:Dy³⁺ and NaBa_0.45Sr_0.55PO₄:Eu²⁺ excited at 348 nm and 354 nm wavelength respectively. From a powder X-ray diffraction (XRD) analysis, the formation of compound with a trigonal–hexagonal scalenohedral structure was confirmed. In the photoluminescence spectra, the NaBa_0.45Sr_0.55PO₄:Dy³⁺ phosphor emits two distinctive colours: a blue band centred at 482 nm and a yellow band at 576 nm originating from Dy³⁺ whereas NaBa_0.45Sr_0.55PO₄:Eu²⁺ emits blue colour at 470 nm. Also, surface morphology has been studied by scanning electron microscope (SEM). Phosphors exhibit a strong absorption in the range of 340–400 nm and chromatic properties indicated that present phosphor is a hopeful candidate for near ultra violet light emitting diodes (nUV LEDs).