Enhanced luminescent properties of Y3Al5O12:Tb,Ce phosphor prepared by spray pyrolysis

Yttrium aluminum garnet (YAG) particles doped with Tb³⁺ or double doped with Tb³⁺ and Ce³⁺ were prepared by spray pyrolysis and characterized by photo- and cathode-luminescence. It was tried to incorporate a broad band of Ce³⁺ activator into the line peaks of Tb³⁺ in YAG host without the reduction of emission intensity. Ce-codoped YAG:Tb particles showed a broad band emission due to the d-f transition of Ce³⁺ and a reduction in the intensity of emission peaks due to ⁵D₃-⁷F_j (j=3, 4, 5, 6) transition of Tb³⁺ when they were excited by the ultraviolet light of 270 nm. These results supported that an effective energy transfer occurs from Tb³⁺ to Ce³⁺ in YAG host. Codoping Ce³⁺ ions greatly intensified the excitation peak at 270 nm for the emission at 540 nm of Tb³⁺, which means that more lattice defects, involving in the energy absorption and transfer to Tb³⁺, are formed by the Ce³⁺ codoping. The finding gives a promising approach for enhancing the luminescence efficiency.     

Energy transfer process in CaSO4:Tb,Ce phosphor

Thermoluminescence (TL) and photoluminescence studies have been carried out on CaSO₄:Tb, CaSO₄:Ce and CaSO₄:Tb,Ce phosphors with the aim of studying energy transfer process in the CaSO₄:Tb,Ce phosphor. CaSO₄:Tb,Ce shows TL peaks at 150, 220, 320 and 400°C. Changes in Tb and Ce concentrations influence the relative heights of these glow peaks. Co-doping with 0.1 mol% of Ce in CaSO₄:Tb enhances the sensitivity of 320^oC TL peak by a factor of 15. Fluorescence results show that there is energy transfer from Ce to Tb ion. The defect centres formed in CaSO₄:Tb,Ce phosphor are studied using electron spin resonance technique. The 320^oC glow peak correlates with a centre (SO₃⁻radical) with g-values: g_||=2.0061 and g_⊥=2.0026.     

Luminescence enhancement by energy transfer from Ce ions in Ba1.6Ca0.4P2O7:Ce:Tb phosphor

The optical properties of Ba_1.6Ca_0.4P₂O₇ doped with Ce³⁺ and Tb³⁺ are investigated. Under excitation at 280 nm the emission spectrum of Ba_1.6Ca_0.4P₂O₇:Ce³⁺ consists of a peak at 370 nm and a shoulder at the longer wavelength side. The emission spectra of Ba_1.6Ca_0.4P₂O₇:Tb³⁺ shows the well-known emission lines due to ⁵D₄-⁷F_J transitions of Tb³⁺. The green emissions of Tb³⁺ ions are enhanced upon UV excitation through energy transfer from Ce³⁺ to Tb³⁺ ions. The efficiency of such an energy transfer is estimated based on spectroscopic data. The dependence of photoluminescence (PL) intensities of Ce³⁺ and Tb³⁺ emissions on Ce³⁺ or Tb³⁺ concentrations in the systems (Ba_1.6Ca_0.4P₂O₇:0.04Ce³⁺,xTb³⁺ and Ba_1.6Ca_0.4P₂O₇:xCe³⁺,0.04Tb³⁺) and the temperature dependence of PL emission spectra of Ba_1.6Ca_0.4P₂O₇:0.06Ce³⁺,0.04Tb³⁺ is also investigated.     

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.     

Enhanced luminescence of Tb by efficient energy transfer from Ce in Sr2B5O9Cl host

Ce³⁺ and Tb³⁺ co-doped Sr₂B₅O₉Cl phosphors with intense green emission were prepared by the conventional high-temperature solid-state reaction technique. A broad band centered at about 315 nm was found in phosphor Sr₂B₅O₉Cl: Ce³⁺, Tb³⁺ excitation spectrum, which was attributed to the 4f-5d transition of Ce³⁺. The typical sharp line emissions ranging from 450 to 650 nm were originated from the ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transitions of Tb³⁺ ions. The photoluminescence (PL) intensity of green emission from Tb³⁺ was enhanced remarkably by co-doping Ce³⁺ in the Tb³⁺ solely doped Sr₂B₅O₉Cl phosphor because of the dipole-dipole mechanism resonant energy transfer from Ce³⁺ to Tb³⁺ ions. The energy transfer process was investigated in detail. In light of the energy transfer principles, the optimal composition of phosphor with the maximum green light output was established to be Sr_1.64Ce_0.08Tb_0.1Li_0.18B₅O₉Cl by the appropriate adjustment of dopant concentrations. The PL intensity of Tb³⁺ in the phosphor was enhanced about 40 times than that of the Tb³⁺ single doped phosphor under the excitation of their optimal excitation wavelengths.     

Synthesis, structure and luminescence properties of Y(V,P)O4:Eu,Bi phosphors

YVO₄:Eu³⁺-based red-emitting phosphors with the compositions of Y_0.95−xVO₄:0.05Eu³⁺,xBi³⁺ (x=0.01, 0.03, 0.05, 0.07 and 0.09) and Y_0.90(V_1−zP_z)O₄:0.05Eu³⁺,0.05Bi³⁺ (z=0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0) were synthesized by the high temperature solid-state method. The as-prepared phosphors have the similar tetragonal phase structure and their morphologies varied with the relative content ratio of V to P. The photoluminescence spectra for the as-synthesized phosphors show that a dominant red emission line at around 619 nm, which is due to the Eu³⁺ electric dipole transition of ⁵D₀-⁷F₂, is observed under different excitation wavelengths (254 and 365 nm). Further, the emission intensities of ⁵D₀-⁷F₂ transition upon 365 nm excitation increase sharply owing to the Bi³⁺ doping. Energy transfer process, luminescent lifetime and quantum efficiency for the selected Y_0.90(V_1−yP_y)O₄:0.05Eu³⁺,xBi³⁺phosphors were also studied in detail.     

Synthesis and luminescence of SrZnO2 phosphors

SrZnO₂ phosphors have been synthesized by two new methods viz. carbonate decomposition at 1000 °C and combustion synthesis. Phosphors activated with Pb²⁺, Sm³⁺, Tb³⁺, Bi³⁺ and Pr³⁺ could be prepared in one step using the combustion synthesis. Characteristic emission and excitation were observed for Bi³⁺. For the remaining activators excitation spectra always contained a band at 283 nm. Presence of this band for all these different types of activators was interpreted as host sensitization.     

Orange emission enhancement by energy transfer in Sr3Al2O5Cl2:Ce, Eu phosphor for solid-state lighting

Orange-emissive Ce³⁺/Eu²⁺ co-doped Sr₃Al₂O₅Cl₂ phosphors were synthesized by a solid-state reaction. The large overlap between the emission spectrum of blue Sr₃Al₂O₅Cl₂:Ce³⁺ and the excitation spectrum of orange Sr₃Al₂O₅Cl₂:Eu²⁺, and the shortening trend in lifetime of Ce³⁺ donors with increasing Eu²⁺ concentration in Sr₃Al₂O₅Cl₂:Ce³⁺, Eu²⁺ provide the strong evidence of energy transfer from Ce³⁺ to Eu²⁺ ions. It supports that the orange emission intensity of the optimal co-doped phosphor is 1.5 times stronger than that of single Eu²⁺-doped one. The Sr₃Al₂O₅Cl₂:Ce³⁺, Eu²⁺ phosphor is a promising orange-emitting phosphor for warm-white-light-emitting diode because of its effective excitation in the near ultraviolet range.     

Luminescence properties of Ce and Tb co-activated ZnAl2O4 phosphor

In this study, a solution combustion method was used to prepare green emitting Ce³⁺–Tb³⁺ co-activated ZnAl₂O₄ phosphor. The samples were annealed at 700 °C in air or hydrogen atmosphere to improve their crystallinity and optical properties. X-ray diffraction study confirmed that both as-prepared and post-preparation annealed samples crystallized in the well known cubic spinel structure of ZnAl₂O₄. An agglomeration of irregular platelet-like particles whose surfaces were encrusted with smaller spheroidal particles was confirmed by scanning electron microscopy (SEM). The fluorescence data collected from the annealed samples with different concentrations of Ce³⁺ and Tb³⁺ show the enhanced green emission at 543 nm associated with ⁵D₄→⁷F₅ transitions of Tb³⁺. The enhancement was attributed to energy transfer from Ce³⁺ to Tb³⁺. Possible mechanism of energy transfer via a down conversion process is discussed. Furthermore, cathodoluminescence (CL) intensity degradation of this phosphor was also investigated and the degradation data suggest that the material was chemically stable and the CL intensity was also stable after 10 h of irradiation by a beam of high energy electrons.     

Photoluminescence of SrGa2S4:Sn,Re(=Ce, Gd) phosphors

The photoluminescence (PL) spectra, PL excitation spectra, color coordinates, and X-ray diffraction spectra are reported for SrGa₂S₄:Sn,Re(=Ce and Gd, respectively) phosphors. By mixing SrGa₂S₄:Sn,Ce phosphors with different Ce³⁺ concentrations, white emissions can be obtained under the excitation of a 340-nm UV LED. Emissions in the green to yellow color range can be obtained from SrGa₂S₄:Sn,Gd phosphors. The rare earth ions enhance the green emission band, which peaks at 534 nm, instead of the yellow one. The origin of this enhancement is discussed. The resonant energy transfer rates are estimated in the cases from Ce³⁺ to the green and yellow centers of Sn²⁺ and between the yellow centers and the green centers.     

Preparation and luminescence properties of Tb doped ZrO2 and BaZrO3 phosphors

ZrO₂:Tb³⁺ and BaZrO₃:Tb³⁺ powders are prepared by combustion synthesis method and the samples were further heated to 500, 700 and 1000 °C to improve the crystallinity of the materials. The structure and morphology of materials have been examined by X-ray diffraction, Raman spectra and scanning electron microscopy. It is remarkable that all the samples of ZrO₂:Tb³⁺ and BaZrO₃:Tb³⁺ have similar morphology. These images exhibited homogeneous aggregates of varying shapes and sizes, which are composed of a large number of small cuboids and broken cuboids. The cuboids and broken cuboids size of all the samples are less than 0.5 μm. Photoluminescence for both materials increases with increase of temperature and found maximum for the samples heated to 1000 °C with 5 mole% doping of Tb³⁺ ions. Luminescence is almost double for the zirconia compared to that of barium-zirconate.     

Enhanced photoluminescence of Sm/Bi co-doped Gd2O3 phosphors by combustion synthesis

Gd₂O₃:Sm³⁺ and Gd₂O₃:Sm³⁺,Bi³⁺ powders were prepared by a combustion method. Their structures were determined using X-ray diffraction. UV-visible absorption and photoluminescence spectra were investigated for Gd₂O₃:Sm³⁺ and Gd₂O₃:Sm³⁺,Bi³⁺ at different annealing temperatures and different doping concentrations. The emission spectra of all samples presented the characteristic emission narrow lines arising from the ⁴G_5/2→⁶H_J transitions (J=5/2, 7/2, and 9/2) of Sm³⁺ ions upon excitation with UV irradiation. The emission intensity of Sm³⁺ ions was largely enhanced with introducing Bi³⁺ ions into Gd₂O₃:Sm³⁺ and the maximum occurred at a Bi³⁺ concentration of 0.5 mol%. The relevant mechanisms were discussed with the sensitization theory by Dexter and the aggregation behavior of Bi³⁺ ions.     

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.     

Improved moisture resistance of SrS:Eu phosphors with nanoscale SiO2 coating

Orange-emitting SrS:Eu²⁺ phosphors were coated with nanoscale SiO₂ and their photoluminescence (PL) degradation behavior in moist air was investigated. The SiO₂ coating was obtained by sol-gel process using diethoxydimethylsilane (DEDMS) and the coating content was varied from 0.5 to 2 wt%. The coatings were composed of a uniform, continuous, and amorphous SiO₂ layer of 30-50 nm thickness and the coating thickness was not varied significantly with the coating content. No peak shift and no decrease of PL intensity were observed after coating. The PL intensity of the coated phosphors decreased to ∼75% of the original value after 10 h exposure to moist air, while the uncoated phosphor decreased to ∼33%, which indicates the improved moisture resistance of the nanoscale SiO₂ coated SrS:Eu²⁺ phosphors.     

Preparation and luminescence property of Dy-doped YPO4 phosphors

The co-precipitation reaction soft chemistry method was employed to prepare fine YPO₄:Dy³⁺ phosphor particles calcined at 950 °C. Adjusting appropriate pH and introducing lithium may greatly affect the particle size of the product and further affect the luminescence intensity. The as-prepared samples were characterized by X-ray diffraction (XRD), transmission electron micrograph (TEM) and photoluminescence (PL) spectroscopy. The obtained YPO₄ nanocrystals appeared to be spherical with some agglomeration and their sizes ranged from 20 to 40 nm. The characteristic transitions of Dy³⁺ due to ⁴F_9/2→⁶H_15/2 (blue) and ⁴F_9/2→⁶H_13/2 (yellow) were detected in the emission spectra.     

Enhanced luminescence of Ca3B2O6:Dy phosphors by Na-codoping for LED applications

The Ca_2.95−yDy_0.05B₂O₆:yNa⁺ (0≤y≤0.20) phosphors were synthesized at 1100 °C in air by the solid-state reaction route. The as-synthesized phosphors were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), photoluminescence excitation (PLE), photoluminescence (PL) spectra and thermoluminescence (TL) spectra. The PLE spectra show the excitation peaks from 300 to 400 nm due to the 4f-4f transitions of Dy³⁺. This mercury-free excitation is useful for solid-state lighting and light-emitting diodes (LEDs). The emission of Dy³⁺ ions on 350 nm excitation was observed at 480 nm (blue) due to the ⁴F_9/2→⁶H_15/2 transitions, 575 nm (yellow) due to ⁴F_9/2→⁶H_13/2 transitions and 660 nm (red) due to weak ⁴F_9/2→⁶H_11/2 emissions. The PL results from the investigated Ca_2.95−yDy_0.05B₂O₆:yNa⁺ phosphors show that Dy³⁺ emissions increase with the increase of the Na⁺ codoping ions. The integral intensity of yellow to blue (Y/B) can be tuned by controlling Na⁺ content. By the simulation of white light, the optimal CIE value (0.328, 0.334) can be achieved when the content of Na⁺-codoping ions is y=0.2. The results imply that the Ca_2.95−yDy_0.05B₂O₆:yNa⁺ phosphors could be potentially used as white LEDs.     

Photoluminescence properties of nanocrystalline La2TeO6:Eu phosphor prepared by Pechini sol-gel method

La₂TeO₆:Eu³⁺ nanophosphors were prepared by Pechini sol-gel process, using lanthanide nitrates and telluric acid as precursor. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), thermogravimetric analysis (TG), photoluminescence spectra (PL) and fluorescence lifetime were used to characterize the resulting phosphors. The results of XRD indicate that all samples crystallized completely at 1023 K and are isostructural with the orthorhombic La₂TeO₆. SEM study reveals that the samples have a strong tendency to form agglomerates with an average size ranging from 50 to 80 nm. The photoluminescence intensity and chromaticity were improved for excitation at 254 and 395 nm. The optimized phosphor La_1.80Eu_0.10TeO₆ could be considered as an efficient red-emitting phosphor for solid-state lighting devices based on GaN LEDs.     

Characterization, EPR and luminescence studies of ZnAl2O4:Mn phosphors

ZnAl₂O₄:Mn green light emitting powder phosphors have been prepared by urea combustion technique involving furnace temperatures about 500 °C in a short time (<5 min). The prepared powders were characterized by X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectrometry and the surface area measurements by a Brunauer-Emmet-Teller (BET) adsorption isotherms. The EPR spectrum exhibits a resonance signal at g≈2.0, which shows a six-line hyperfine structure (hfs). From the EPR spectra the spin-Hamiltonian parameters have been evaluated at room temperature as well as at 110 K. EPR and photoluminescence (PL) studies revealed that manganese ions were present in divalent state and the site symmetry around Mn²⁺ ions is distorted tetrahedral. The spin concentration (N), the paramagnetic susceptibility (χ) and the zero-field splitting parameter (D) have been evaluated and discussed. The green emission at 511 nm in ZnAl₂O₄:Mn phosphor is assigned to a transition from the upper ⁴T₁→⁶A₁ ground state of Mn²⁺ ions.     

Effects of SnO2 surface coating on the degradation of ZnS thin film phosphor

Pulsed laser deposited ZnS bare and SnO₂ coated ultra thin films were subjected to prolonged electron beam bombardment with 2 keV energy and a steady 44 mA/cm² current density, in 1 × 10⁻⁶ Torr O₂ pressure backfilled from a base pressure of 3 × 10⁻⁹ Torr at room temperature. Auger electron spectroscopy (AES) was used to monitor changes of the surface chemical composition of both the bare and coated phosphor films during electron bombardment. Degradation was manifested by the decrease of sulphur and accumulation of oxygen on the surface of the bare phosphor. However, with the SnO₂ coating this phenomenon was delayed until the protective SnO₂ was depleted on the surface through dissociation and reduction.     

Photoluminescence of cubic and monoclinic Gd2O3:Eu phosphors prepared by flame spray pyrolysis

In this paper, europium-doped gadolinium phosphor, which is a potentially bifunctional material with both fluorescent and magnetic properties, has been prepared in a one-step procedure via flame spray pyrolysis, and its crystal structure, morphology, and PL intensity were investigated. All the prepared phosphors were submicron-sized with spherical shapes and either a pure cubic or pure monoclinic phase. In order to observe the effects of temperature on the crystal phases of the prepared phosphors, we applied a H₂ vs. N₂/O₂ diffusion flame, with the maximum flame temperature ranging from T_max=1375 to 2050 K. The temperature profiles under various flame conditions are also reported herein to further elucidate the rapid synthesis process. The PL intensity in the cubic phase improved linearly with increasing flame temperature until the transition to a monoclinic phase. The peak of the photoluminescence(PL) spectrum from the phosphors prepared at T_max=1733 K in the cubic phase was narrower and twice as strong as the peak of the PL spectrum from the phosphors prepared at T_max=2050 K in the monoclinic phase. This paper provides important data showing the relationship between the synthesis temperature and the phase transition in Gd₂O₃:Eu in the continuous one-step use of flame spray pyrolysis.