Self-assembled 3D sphere-like SrMoO4 and SrMoO4:Ln3+ (Ln = Eu,Sm, Tb,Dy) microarchitectures: Facile sonochemical synthesis and optical properties

Three-dimensional (3D) well-defined SrMoO₄ and SrMoO₄:Ln³⁺ (Ln = Eu, Sm, Tb, Dy) hierarchical structures of obvious sphere-like shape have been successfully synthesized using a large-scale and facile sonochemical route without using any catalysts or templates. X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), and photoluminescence (PL) spectra were used to characterize the samples. The intrinsic structural feature of SrMoO₄ and external factor, namely the ultrasonic time and the pH value, are responsible for the ultimate shape evolutions of the product. The possible formation mechanism for the product is presented. Additionally, the PL properties of SrMoO₄ and SrMoO₄:Ln³⁺ (Ln = Eu, Sm, Tb, Dy) hierarchical structures were investigated in detail. The Ln³⁺ ions doped SrMoO₄ samples exhibit respective bright red–orange, yellow, green and white light of Eu³⁺, Sm³⁺, Tb³⁺ and Dy³⁺ under ultraviolet excitation, and have potential application in the field of color display. Simultaneously, this novel and efficient pathway could open new opportunities for further investigating about the properties of molybdate materials.     

Photoluminescence properties of Eu-doped ZnO films grown by sputtering-assisted metalorganic chemical vapor deposition

Photoluminescence (PL) properties of Eu-doped ZnO (ZnO:Eu) grown by a sputtering-assisted metalorganic chemical vapor deposition technique were investigated. In PL measurements at 300 K, the samples annealed at 600 °C for 30 min showed clear red-emission lines due to the intra-4f shell transition of ⁵D₀→⁷F_J (J=0–4) in Eu³⁺. In photoluminescence excitation (PLE) spectra, the PL was observed under the high-energy excitation above the band-gap energy of ZnO (indirect excitation) and the low-energy excitation resonant to the energy levels of ⁷F₀–⁵D₃ and ⁷F₀–⁵D₂ transitions in Eu³⁺ (direct excitation). The PL lifetime under the indirect excitation was shorter than that under the direct excitations. These PL properties revealed that the energy transfer from ZnO host to Eu³⁺ was accompanied under indirect excitation.     

Optical properties,luminescence quenching mechanism and radiation hardness of Eu-doped GaN red powder phosphor

We report on the luminescence quenching mechanism of Eu-doped GaN powder phosphor produced with a low-cost, high yield rapid-ammonothermal method. We have studied as-synthesized and acid rinsed Eu-doped GaN powders with the Eu concentration of ～0.5 at.%. The Eu-doped GaN photoluminescence (PL) was investigated with 325 nm excitation wavelength at hydrostatic pressures up to 7.7 GPa in temperature range between 12 K and 300 K. The room temperature integrated Eu³⁺ ion PL intensity from acid rinsed material is a few times stronger than from the as-synthesized material. The temperature dependent PL studies revealed that the thermal quenching of the dominant Eu³⁺ ion transition (⁵D₀ → ⁷F₂) at 622 nm is stronger in the chemically modified phosphor indicating more efficient coupling between the Eu³⁺ ion and passivated GaN powder grains. Furthermore, it was found that thermal quenching of Eu³⁺ ion emission intensity can be completely suppressed in studied materials by applied pressure. This is due to stronger localization of bound exciton on Eu³⁺ ion trap induced by hydrostatic pressure. Furthermore, the effect of 2 MeV oxygen irradiation on the PL properties has been investigated for highly efficient Eu-doped GaN phosphor embedded in KBr–GaN:Eu³⁺ composite. Fairly good radiation damage resistance was obtained for 1.7 × 10¹² to 5 × 10¹³ cm^?2 oxygen fluence. Preliminary data indicate that Eu-doped GaN powder phosphor can be considered for devices in a radiation environment.     

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).     

Development of zeolite-derived novel aluminosilicate phosphors

In this research, zeolite-derived aluminosilicate phosphors were synthesized through the ion exchange route. Red light-emitting property of Eu³⁺-doped aluminosilicate phosphors were discussed from a view point of the Eu content, heat-treatment condition and the oxidation state of Eu ions. The crystalline phase of the host aluminosilicates could be successfully controlled as designed based on the published NaAlO₂–SiO₂ binary phase diagram. Orange-red emission peaks derived from the ⁵D₀→⁷F_j (j=0, 1, 2, 3, 4) transition of Eu³⁺ were observed around 590–700 nm, and 4f⁶5d→4f⁷ transition of Eu²⁺ was observed at around 400–500 nm. The relative intensity I(⁵D₀→⁷F₂) of the dominant emission peak at 612 nm increased consistently with the Eu content. The results of the XANES spectroscopy analysis revealed that Eu²⁺ ion in the 1400 °C as heat-treated host aluminosilicate were successfully converted to Eu³⁺ by the additional annealing at 1100 °C. The Eu contents and heat-treatment conditions were determined to exhibit the best performance as a red phosphor, which were 10 wt% and 1500 °C, respectively     

Luminescent features of sol–gel derived rare-earth multi-doped oxyfluoride nano-structured phosphors for white LED application

Rare-earth doped oxyfluoride 75SiO₂:25PbF₂ nano-structured phosphors for white-light-emitting diodes were synthesized by thermal treatment of precursor sol–gel derived glasses. Room temperature luminescence features of Eu³⁺, Sm³⁺, Tb³⁺, Eu³⁺/Tb³⁺, and Sm³⁺/Tb³⁺ ions incorporated into low-phonon-energy PbF₂ nanocrystals dispersed in the aluminosilicate glass matrix and excited with UV light emitting diode were investigated. The luminescence spectra exhibited strong emission signals in the red (600, 610, 625, and 646 nm), green (548 and 560 nm), and blue (485 nm) wavelength regions. White-light emission was observed in Sm/Tb and Eu/Tb double-doped activated phosphors employing UV-LED excitation at 395 nm. The dependence of the luminescence emission intensities upon annealing temperature and rare-earth concentration was also examined. The results indicated that there exist optimum annealing temperature and activator ion concentration in order to obtain intense visible emission light with high color rendering index. The study suggests that the nanocomposite phosphor based upon 75SiO₂:25PbF₂ host herein reported is a promising contender for white-light LED applications.     

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.     

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.     

5D0→7F1 and 5D0→7F2 transition in europium doped halosulphates for mercury-free lamps

Polycrystalline Na₃SO₄F:Eu and NaMgSO₄F:Eu halosulphate phosphors prepared by a wet chemical method have been studied for its photoluminescence (PL) and thermoluminescence (TL) characteristics. Two well resolved peaks are observed at 593 nm and 614 nm, which are assigned to due to ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions of Eu³⁺ ions. TL is observed at temperatures between 100 °C and 300 °C. In this paper, we report PL emission spectra of Eu³⁺ and TL glow curves, which are more sensitive than the standard TLD-CaSO₄:Dy. The presented phosphors are applicable for the mercury free lamps and solid state lighting devices.     

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.     

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.     

Electron spin resonance and luminescence in Ga2−xEuxS3 single crystals

In present work electron spin resonance (ESR) and luminescence have been studied in Ga_2−xEu_xS₃ single crystals. The ESR and photoluminescence (PL) spectra of Ga_2−xEu_xS₃ were observed and the intensity of these spectra has increased linearly with the Eu concentrations in the samples. At the low temperature of 3.8 K in Ga_2−xEu_xS₃ single crystals, when the magnetic field has been applied as the perpendicular to the direction of [110], the hyperfine structure of ESR spectra has been observed. The exchange interaction of Eu²⁺ atoms in Ga_2−xEu_xS₃ single crystals have been determined as, g=4.2. The PL spectrum of pure Ga₂S₃ single crystals was recorded at 4.2 K and consisted of a narrow high-energy band at 2.53–2.38 eV and a low-energy band at 2.14–1.59 eV. Ga_2−xEu_xS₃ resulted in complete quenching of the impurity luminescence bands gave rise to a green europium band (2.38–2.14 eV) in the luminescence spectrum. Crystals of Ga_2−xEu_xS₃ emitted bright green electroluminescence (EL) spectrum when they excited with static and alternating electric fields at 77 K. We have also determined the field and frequency dependences of the EL.     

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.     

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.     

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2 eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21 eV) measured at 7 K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5 eV were detected and assigned to charge-transfer absorption from O^2? to Fe³⁺ ions; the intrinsic PL band at 3.28 eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54 eV were separated; the strong PL band at 1.72 eV was revealed and attributed to a radiative transition in Fe³⁺ ion.     

Excited state dynamics and energy transfer rates in Sr3Tb0.90Eu0.10(PO4)3

The emission spectrum of neat Sr₃Tb(PO₄)₃ upon excitation at 337 nm in the levels above ⁵D₃ is dominated by ⁵D₄ emission and no significant emission from ⁵D₃ is observed due to efficient cross relaxation involving the Tb³⁺ levels. On the other hand, the emission spectrum of the same host containing 10 mol% Eu³⁺ upon excitation at the same wavelength (in the Tb³⁺ levels) is dominated by strong emission bands from the ⁵D₀ level of Eu³⁺. This clearly indicates that Tb³⁺→Eu³⁺ energy transfer is present. The excitation spectrum of the Eu^{3+ 5}D₀ emission is dominated by Tb³⁺ bands extending in the UV region.The presence of 10 mol% Eu³⁺ in Sr₃Tb(PO₄)₃ very strongly shortens the ⁵D₄ decay time. The decay curve is not far from exponential, indicating that the energy transfer to Eu³⁺ is accompanied by fast energy migration. The transfer regimes are identified and the donor–donor and donor–acceptor transfer microparameters are quantified under the assumption of electric dipole–electric dipole interactions.     

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.     

Effect of Na+ co-dopant and activator concentration on luminescent properties of CaGa4O7:Eu3+

Two series of calcium gallate phosphors: Ca_1?xEu_xGa₄O₇ and Ca_1?2xEu_xNa_xGa₄O₇ (x=0, 0.002, 0.01, 0.02, 0.03, 0.05) were synthesized by a modified Pechini method and their optical properties at 298 and 77 K were investigated. In undoped CaGa₄O₇ upon 255 nm excitation a bluish white emission (λ_max=500 nm) followed by an afterglow of the same color lasting for 10–20 s was observed. Eu³⁺-doping quenched the host-related luminescence and the characteristic red emission of the dopant with maximum at 613 nm appeared. Its excitation spectrum consisted of a broad band assigned to ligand to metal, O^2?→Eu³⁺, charge transfer absorption and narrow lines arising from intraconfigurational transitions within the 4f⁶ states of Eu³⁺ ion. The effects of Eu³⁺ concentration and Na⁺ co-doping on the luminescence properties and decay kinetics were studied. Low temperature emission spectra showed that Eu³⁺ ions are positioned in environments of different symmetries. Their relative populations changed with the activator content. Co-doping with Na⁺ ions led to a remarkable reduction of the number of Eu³⁺ sites as well as to noticeable improvement of the luminescence brightness though it did not affect the decay time of the emission. The quantum efficiencies of singly doped CaGa₄O₇:Eu³⁺ were very low (in the range of 1–3.7%). Na⁺ co-doping improved this parameter leading to the highest efficiency of 11% for CaGa₄O₇:3%Eu³⁺,3%Na⁺.     

Photoluminescence properties of Eu2+–Mn2+ codoped Ca-α-SiAlON phosphors

Eu²⁺–Mn²⁺ codoped Ca-α-SiAlON phosphors, Ca_0.736?ySi_9.6Al_2.4O_0.8N_15.2:0.064 Eu²⁺, yMn²⁺, were firstly synthesized by the high temperature solid state reaction method. The effects of doped Eu²⁺ and Eu²⁺–Mn²⁺ concentrations on the photoluminescence properties of the as-prepared phosphors were investigated systematically. Powder X-ray diffraction shows that pure Ca-α-SiAlON phase is synthesized after sintering at 1700 °C for 2 h under 0.5 MPa N₂ atmosphere. The excitation spectra of Eu²⁺-doped Ca-α-SiAlON phosphors are characterized by two dominant bands centered at 286 nm and 395 nm, respectively. The photoluminescent spectrum of Eu²⁺-doped Ca-α-SiAlON phosphor exhibits an intense emission band centered at 580 nm due to the allowed 4f ⁶5d→4f ⁷ transition of Eu²⁺, showing that the phosphor is a good candidate for creating white light when coupled to a blue LED chip. The intensities of both excitation and emission spectra monotonously decrease with the increment of codoped Mn²⁺ content (i.e. y value), indicating that energy transfer between Eu²⁺ and Mn²⁺ is inefficient in the case of Eu²⁺–Mn²⁺ codoped Ca-α-SiAlON phosphors.     

Synthesis and photoluminescence properties of YVO4:Eu3+, Al3+ phosphor

The Y_0.95?xAl_xVO₄:5%Eu³⁺ (0≤x≤0.1) phosphors were successfully synthesized by solid state reaction at 900 °C for 6 h, and their luminescence properties were investigated under UV and VUV excitation. Monitoring at 619 nm, a strong broad absorption was enhanced by co-doping of Al³⁺ into the YVO₄:Eu³⁺ lattices at 256 nm under UV excitation. The VUV excitation spectra also showed the enhanced excitation bands at about 156 and 200 nm. Under 254 or 147 nm excitation, it was found that Y_0.95?xAl_xVO₄:Eu³⁺(0≤x≤0.1) phosphors showed strong red emission at about 619 nm corresponding to the electric dipole ⁵D₀^–7F₂ transition of Eu³⁺. The improvement of luminescence intensity of YVO₄:Eu³⁺ was also observed after partial substituting Y³⁺ by Al³⁺ and the optimal luminescence intensity appeared with incorporation of 2.5 mol% Al³⁺.