Luminescence characterization of (Ca1−xSrx)(S1−ySey):Eu,M (M = Sc and Y) for high color rendering white LED

The highly efficient red phosphors (Ca_1−xSr_x)(S_1−ySe_y):Eu²⁺,M³⁺ (M = Sc and Y) were prepared, starting from CaCO₃, SrCO₃, Eu₂O₃, Sc₂O₃, Y₂O₃, S, and SeO₂ with a flux, by a conventional solid-state reaction. The optimized red phosphors converted 11.8% (Sc³⁺) and 11.7% (Y³⁺) of the absorbed blue light into luminescence. These quantum values are much higher than Q = 3.0% of CaS:Eu²⁺. For the fabrication of light-emitting diodes (LEDs), the prepared phosphors were coated with MgO from non-aqueous solution to overcome their weakness against moisture. White LEDs were fabricated by pasting the prepared red phosphors and the yellow YAG:Ce³⁺ phosphor on an InGaN blue chip (λ_ems = 446.5 nm). The incorporation of the red phosphor to the YAG:Ce³⁺ phosphor resulted in an improved color rendering index (Ra) from 70 to 80.     

Photoluminescence investigation of rare-earth activated GdCa3(GaO)3(BO3)4 phosphors under UV excitation

A borate compound was adopted as a new host material of Eu³⁺ and Tb³⁺ activators to fabricate efficient luminescence materials. The phosphor compositions, Gd_1−xEu_xCa₃(GaO)₃(BO₃)₄ and Gd_1−xTb_xCa₃(GaO)₃(BO₃)₄, were synthesized by conventional solid-state reactions. The crystalline phases of the resulting powders were identified using an X-ray diffraction system. Their photoluminescence properties were investigated under long-wavelength UV excitation. The Eu³⁺-doped and Tb³⁺-doped GdCa₃(GaO)₃(BO₃)₄ phosphors efficiently emitted red and green light, respectively. The temperature dependency of emission intensity was measured in a range from room temperature to 150 °C. The emission intensities of the red and green phosphors at 150 °C are 87% and 91% of those at room temperature, respectively. In addition, the decay times of both the red and green phosphors are shorter than 3 ms.     

One-step synthesis and double emission (Ca1 + x − yEuy)Ga2S4 + x phosphor for white LEDs

(Ca_{1 + x − y}Eu_y)Ga₂S_4 + x phosphors have been synthesized one step by solid state reaction. The photoluminescence excitation and emission spectra of phosphors have been studied; the influence of host composition and Eu²⁺ concentration on emission spectra has also been investigated. The emission spectrum consists of yellow emission at 550 nm and red emission at 650 nm. It also indicates that the excitation spectrum is a broadband and can be well matched with the emission of GaN chip. Combined these phosphors with 460 nm-emitting GaN chips, White LEDs have been fabricated. Their electroluminescence spectra have been measured under 20 mA forward-bias current. Their CIE chromaticity coordinates and color temperature indicate that (Ca_{1 + x − y}Eu_y)Ga₂S_4 + x phosphors are promising phosphors for GaN-based white LEDs.     

Near UV and blue-based LED fabricated with Ca8Zn(SiO4)4Cl2:Eu as green-emitting phosphor

Green-emitting phosphor Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺ has been prepared by the solid state reaction method and there luminescence properties are investigated. The excitation spectrum of Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺ shows an intense excitation band in the blue centered at 450 nm and emits with a maximum at 505 nm. The concentration quenching mechanism is studied and verified to be the energy transfer among the nearest-neighbor ions. Upon 450 nm excitation, the emission intensity of Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺ is much stronger than the green emitting Ca₃SO₄Cl₂:Eu²⁺ phosphor and even higher than YAG:Ce³⁺. This excitation spectrum range matches UV and blue light-emitting diodes (LEDS) chips very well, suggesting Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺ could be a promising green emitting phosphor candidate for LED devices.     

Synthesis and optical properties of (Gd1−x,Eux)2O2SO4 nano-phosphors by a novel co-precipitation method

(Gd_1−x,Eu_x)₂O₂SO₄ nano-phosphors were synthesized by a novel co-precipitation method from commercially available Gd₂O₃, Eu₂O₃, H₂SO₄ and NaOH starting materials. Composition of the precursor is greatly influenced by the molar ratio of NaOH to (Gd_1−x,Eu_x)₂(SO₄)₃ (the m value), and the optimal m value was found to be 4. Fourier transform infrared spectrum (FT-IR) and thermal analysis show that the precursor (m = 4) can be transformed into pure (Gd_1−x,Eu_x)₂O₂SO₄ nano-phosphor by calcining at 900 °C for 2 h in air. Transmission electron microscope (TEM) observation shows that the Gd₂O₂SO₄ phosphor particles (m = 4) are quasi-spherical in shape and well dispersed, with a mean particle size of about 30-50 nm. Photoluminescence (PL) spectroscopy reveals that the strongest emission peak is located at 617 nm under 271 nm light excitation, which corresponds to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions. The quenching concentration of Eu³⁺ ions is 10 mol% and the concentration quenching mechanism is exchange interaction among the Eu³⁺ ions. Decay study reveals that the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions has a single exponential decay behavior.     

Sol-gel composition of multicomponent hybrid precursors to long afterglow of CaxSr1 − xAl2O4: Eu phosphors

Ca_xSr_1 − xAl₂O₄: Eu²⁺ photoluminescent materials with high brightness and long afterglow were in situ synthesized by hybrid precursor assembly sol-gel technology in a reductive atmosphere. The particle size of luminescent materials is in the range of 30-60 nm by the estimation of XRD. And SEM shows that there exists uniform morphology and microstructure owing to the hybrid precursors. The influence of co-doping Ca²⁺ and Sr²⁺ on the luminescence of the phosphor was studied. Their excitation and emission spectra were very similar to that of SrAl₂O₄: Eu²⁺ phosphors and all of them have long afterglow phenomenon. Changing the co-doping concentrations of Ca²⁺ and Sr²⁺ in Ca_xSr_1 − xAl₂O₄: Eu²⁺ phosphors, the luminescent intensities are different. When the proportion of Ca and Sr is 6 to 4, the phosphor reaches the strongest emitting intensity.     

Pechini sol-gel deposition and luminescent properties of SrLa1−xRExGa3O7 (RE = Eu, Tb) phosphor films

SrLa_1−xRE_xGa₃O₇ (RE = Eu³⁺, Tb³⁺) phosphor films were deposited on quartz glass substrates by a simple Pechini sol-gel method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy, field-emission scanning electron microscopy, photoluminescence spectra, and lifetimes were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 700 °C and crystallized fully at 900 °C. The results of FT-IR spectra were in agreement with those of XRD. Uniform and crack-free films annealed at 900 °C were obtained with average grain size of 80 nm, root mean square roughness of 46 nm and thickness of 130 nm. The RE ions showed their characteristic emission in crystalline SrLa_1−xRE_xGa₃O₇ films, i.e., Eu³⁺⁵D₀-⁷F_J (J = 0, 1, 2, 3, 4), Tb³⁺⁵D₄-⁷F_J (J = 6, 5, 4, 3) emissions, respectively. The optimum concentrations (x) of Eu³⁺ and Tb³⁺ were determined to be 50, and 80 mol% in SrLa_1−xRE_xGa₃O₇ films, respectively.     

Luminescent properties of long lasting phosphor Ca2MgSi2O7:Eu

Long afterglow phosphors (Ca_1−xEu_x)₂MgSi₂O₇ (0.002 ≤ x ≤ 0.02) were prepared by solid-state reactions under a weak reductive atmosphere. X-ray diffraction pattern, photoluminescence spectra, decay curve, afterglow spectra and thermoluminescence curves were investigated. The phosphors showed two emission peaks when they were excited by 343 nm, due to two types of Eu²⁺ centers existing in the Ca₂MgSi₂O₇ lattice. However, only one emission peak can be found in their afterglow spectra. Energy transfer between Eu²⁺ ions in inequivalent sites was found. A possible mechanism was presented and discussed. The afterglow decay time of Ca_1.998MgSi₂O₇:Eu_0.002 was nearly 12.5 h which means it was a good long lasting phosphor.     

A novel red phosphor NaLa4(SiO4)3F: Eu

A novel red phosphor NaLa₄(SiO₄)₃F: Eu³⁺ was synthesized by the conventional solid-state reaction at 950 °C for the first time. The luminescence properties of NaLa₄(SiO₄)₃F: Eu³⁺ were investigated, and the critical concentration of the activator concentration (Eu³⁺) was found to be 0.1 mol per formula unit. The phosphor presented red luminescence under the ultraviolet excitation of 254 or 395 nm, attributed to the transitions from ⁵D₀ excited states to ⁷F_J ( J = 0-4) ground states of Eu³⁺ ions. The results indicated that this newly-developed phosphor could find applications in tricolor fluorescent lamp, phosphor-liquid crystal displays and white lighting devices utilizing GaN-based excitation in the near UV.     

Luminescent properties of Eu-activated (Sr1−z, Caz)(Al1−y, By)2O4 phosphors for UV LEDs

A series of (Sr_1−z, Ca_z)(Al_1−y, B_y)₂O₄:xEu²⁺ phosphors were synthesized by the sol–gel process and were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and photoluminescence (PL) excitation and emission spectra. The experiment results revealed that the highest intensity of Sr(Al_1.98, B_0.02)O₄:Eu²⁺ phosphor with pure monoclinic SrAl₂O₄ was achieved by annealing at the temperature of 1200 °C and the Eu²⁺ content of 8 mol%. However, when the post-treatment temperature for Sr(Al_1.98, B_0.02)O₄: Eu²⁺ was over 1200 °C, the Sr₄Al₁₄O₂₅ phase appeared as a minor phase, inducing small blue-shift in the emission peak (520–509 nm). Doping higher content of B³⁺ (y = 0.02–0.40) into SrAl₂O₄:Eu²⁺ at 1200 °C resulted in the transformation of phase from SrAl₂O₄ to Sr₄Al₁₄O₂₅ as well as to SrB₂Al₂O₇, which made the emission intensity enhance and the emission shift to a much shorter wavelength region (λ_p = 467 nm). It was found that, instead of purely using Sr atoms, Ca atoms with content of 20–40% could induce the crystal structure of (Sr_1−z, Ca_z)(Al_1−y, B_y)₂O₄:xEu²⁺, which led to SrAl₂O₄ from monoclinic to hexagonal phase. As a result, SrAl₂O₄ solid solution was obtained and then SrAl₂O₄:Eu²⁺ to emit 518 nm green light. At higher Ca content (z > 40%), a new CaAl₂O₄ solid solution was formed and a blue emission of CaAl₂O₄:Eu²⁺ was obtained.     

A yellow-emitting Ca3Si2O7: Eu phosphor for white LEDs

A series of yellow-emitting phosphors based on a silicate host matrix, Ca_3 − xSi₂O₇: xEu²⁺, was prepared by solid-state reaction method. The structure and photoluminescent properties of the phosphors were investigated. The XRD results show that the Eu²⁺ substitution of Ca²⁺ does not change the structure of Ca₃Si₂O₇ host and there is no impurity phase for x < 0.12. The SEM images display that phosphors aggregate obviously and the shape of the phosphor particle is irregular. The EDX results reveal that the phosphors consist of Ca, Si, O, Eu and the concentration of these elements is close to the stoichiometric composition. The Ca_3 − xSi₂O₇: xEu²⁺ phosphors can be excited at a wavelength of 300-490 nm, which is suitable for the emission band of near ultraviolet or blue light-emitting-diode (LED) chips. The phosphors exhibit a broad emission region from 520 to 650 nm and the emission peak centered at 568 nm. In addition, the shape and the position of the emission peak are not influenced by the Eu²⁺ concentration and excitation wavelength. The phosphor for x = 0.045 has the strongest excitation and emission intensity, and the Ca_3 − xSi₂O₇: xEu²⁺ phosphors can be used as candidates for the white LEDs.     

Luminescence and microstructures of Eu-doped in triple phosphate Ca8MgR(PO4)7 (R = La, Gd, Y) with whitlockite structure

Eu³⁺-doped triple phosphate Ca₈MgR(PO₄)₇ (R = La, Gd, Y) was synthesized by the general high temperature solid-state reaction. This phosphor was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and emission spectra. XRD and FT-IR analysis indicated that Ca₈MgR(PO₄)₇ (R = La, Gd, Y) crystallized in single-phase component with whitlockite-like structure (space group R3c) of β-Ca₃(PO₄)₂. Under the excitation of UV light, the phosphors show bright red emission assigned to the transition (⁵D₀ → ⁷F₂) at 612 nm. The crystallographic sites of Eu³⁺ ions in Ca₈MgR(PO₄)₇ (R = La, Gd, Y) host were discussed on the base of site-selective excitation and emission spectra, luminescence decay and its host crystal structure.     

Dependence of optical properties on the composition of (Ba1−x−ySrxEuy)Si2O2N2 phosphors for white light emitting diodes

BaSi₂O₂N₂: Eu²⁺ is an efficient phosphor because of its high quantum yield and quenching temperature. Partial substitution of Ba²⁺ by Sr²⁺ is the most promising approach to tune the color of phosphors. In this study, a series of (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ (x = 0.0–0.97, y = 0.00–0.10) phosphors are synthesized via high-temperature solid-state reactions. Intense green to yellow phosphors can be obtained by the partial substitution of the host lattice cation Ba²⁺ by either Sr²⁺ or Eu²⁺. The luminescent properties and the relationships among the lowest 5d absorption bands, Stokes shifts, centroid shifts, and the splitting of Eu²⁺ are studied systematically. Then, based on (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ phosphors and near-ultraviolet (∼395 nm)/blue (460 nm) InGaN chips, intense green–yellow light emitting diodes (LEDs) and white LEDs are fabricated. (Ba_0.37Sr_0.60)Si₂O₂N₂: 0.03Eu²⁺ phosphors present the highest efficiency, and the luminous efficiency of white LEDs can reach 17 lm/w. These results indicate that (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ phosphors are promising candidates for solid-state lighting.     

Luminescence of SrGdGa3O7:RE(RE = Eu, Tb) phosphors and energy transfer from Gd to RE

Eu³⁺- and Tb³⁺-activated SrGdGa₃O₇ phosphors were synthesized by the solid-state reaction and their luminescence properties were investigated. Sr(Gd_1 − xEu_x)Ga₃O₇ and Sr(Gd_1 − xTb_x)Ga₃O₇ formed continuous solid solution in the range of x = 0-1.0. Unactivated SrGdGa₃O₇ exhibited a typical characteristic excitation and emission of Gd ion. The SrGdGa₃O₇:xEu³⁺ and SrGdGa₃O₇:xTb³⁺ phosphors also showed the well-known Eu³⁺ and Tb³⁺ excitation and emission. The energy transfer from Gd³⁺ to Eu³⁺ and Tb³⁺ were verified by photoluminescence spectra. The dependence of photoluminescence intensity on Eu³⁺ and Tb³⁺ concentration were also studied in detail and the photoluminescence (PL) intensity of SrGdGa₃O₇:Eu and SrGdGa₃O₇:Tb were compared with commercial phosphors, Y₂O₃:Eu and LaPO₄:Ce,Tb. The luminescence decay measurements showed that the lifetimes of Eu³⁺ and Tb³⁺ were in the range of microsecond. The energy transfer from Gd³⁺ to Tb³⁺ was also observed in decay curve.     

X-ray Rietveld analysis and Fourier transform infrared spectra of the solid solutions [Eu2−xMx][Sn2−xMx]O7−3x/2 (M = Mg or Zn)

Two series of mixed oxides with formula [Eu_2−xM_x][Sn_2−xM_x]O_7−3x/2 (M = Mg or Zn) have been synthesized. The study by X-ray diffraction and Fourier transform infrared spectroscopy shows that the solids obtained are new non-stoichiometric solid solutions with the pyrochlore type structure. For both series a decrease of the cell parameter is observed when the degree of substitution, x, increases. The structural refinements (X-ray studies) were achieved in the space group Fd-3m, no. 227 (origin at center -3m) by using the Fullprof software. The Rietveld refinements show that the divalent cations M²⁺ (Mg²⁺, Zn²⁺) substitute isomorphically for Eu³⁺ and Sn⁴⁺ ions producing vacancies in the anionic sublattice.     

Synthesis and photoluminescence properties of a new red emitting phosphor: Ca3(VO4)2:Eu; Mn

A new red emitting phosphor, Ca₃(VO₄)₂:Eu³⁺; Mn²⁺, was synthesized by a citric acid sol-gel combustion method and characterized by XRD, TEM and photoluminescence (PL) spectra. The red emission located at about 613 nm was ascribed to ⁵D₀-⁷F₂ transition of Eu³⁺. And the red luminescence intensity changed with annealing temperature and concentration of Eu³⁺. The effect of the co-doped Mn²⁺ was also investigated systematically.     

Tartarate precursors to CoxZn1−xGa2O4 spinel oxides

The Co_xZn_1−xGa₂O₄ spinels (x = 0.25, 0.5, 0.75, 1) were synthesized by a wet chemistry method–the precursor route via tartarate decomposition. The complex precursors have been characterized by IR, UV–Vis, magnetic measurements and luminescence spectroscopy. XRD, SEM, IR, UV–Vis and luminescence spectroscopy were used for the structural and morphological investigation of Co_xZn_1−xGa₂O₄ spinels; magnetic susceptibilities of the spinel were also measured. The X-ray diffraction patterns confirmed the formation of gallate spinel phase, Co_xZn_1−xGa₂O₄.     

Novel blue-emitting phosphor NaMg4(PO4)3:Eu, Ce with energy transfer

A blue-emitting phosphor of NaMg₄(PO₄)₃:Eu²⁺, Ce³⁺ was prepared by a combustion-assisted synthesis method. The phase formation was confirmed by X-ray powder diffraction measurement. Photoluminescence excitation spectrum measurements show that the phosphor can be excited by near UV light from 230 to 400 nm and presents a dominant luminescence band centered at 424 nm due to the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ions at room temperature. Effective energy transfer occurs in Ce³⁺/Eu²⁺ co-doped NaMg₄(PO₄)₃ due to large spectral overlap between the emission of Ce³⁺ and excitation of Eu²⁺. Co-doping of Ce³⁺ enhances the emission intensity of Eu²⁺ greatly by transferring its excitation energy to Eu²⁺, and Ce³⁺ plays a role as a sensitizer. Ce³⁺-Eu²⁺ co-doped NaMg₄(PO₄)₃ powders can possibly be applied as blue phosphors in the fields of lighting and display.     

Synthesis,characterization and photoluminescence properties of (Gd1−x,Eux)2O2SO4 sub-microphosphors by homogeneous precipitation method

(Gd_1−x,Eu_x)₂O₂SO₄ sub-microphosphors were synthesized by homogeneous precipitation method from commercially available Gd₂O₃, Eu₂O₃, H₂SO₄ and (NH₂)₂CO (urea) starting materials. Fourier transform infrared spectra show that the precursors with different molar ratios of (NH₂)₂CO to Gd₂(SO₄)₃ (the m value) are mostly composed of gadolinium hydroxyl, carbonate and sulfate groups with some crystal water. X-ray diffraction indicated that the precursor (m = 5) can be transformed into pure Gd₂O₂SO₄ phase after heat treated at 900 °C for 2 h in air. Field emission scanning electron microscope micrographs illustrate that the Gd₂O₂SO₄ phosphor particles (m = 5) are quasi-spherical in shape and well dispersed, with a mean particle size of about 300–500 nm. Photoluminescence spectroscopy reveals that the strongest emission peak for (Gd_1−x,Eu_x)₂O₂SO₄ sub-microphosphors is located at 618 nm under 270 nm light excitation, which corresponds to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions. The quenching concentration of Eu³⁺ ions is 5 mol% and the concentration quenching mechanism is due to the electric dipole–dipole interaction. Decay study reveals that the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions fits with a mono exponential function.     

Novel red phosphors Na2CaSiO4:Eu for light-emitting diodes

Novel red phosphors Na₂CaSiO₄:xEu³⁺ were synthesized using high temperature solid-state reaction and their luminescence characteristics were investigated for the first time. The excitation spectra indicate that the Na₂CaSiO₄:xEu³⁺ phosphors can be effectively excited by ultraviolet (393 nm) light. The emission spectra of Na₂CaSiO₄:xEu³⁺ phosphors invariably exhibit four peaks assigned to the ⁵D₀-⁷F_J (J = 1, 2, 3 and 4) transitions of Eu³⁺ under 393 nm excitation. The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates and quantum efficiency (QE) are (0.66, 0.34) and 58.9%, respectively. The good color saturation and high quantum efficiency indicate that Na₂CaSiO₄:Eu³⁺ phosphors are potential candidate for light-emitting diodes.