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.     

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 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.     

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.     

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.     

Synthesis and luminescence properties of color-tunable Ba1−ySryLa4−xTbx(WO4)7 (x = 0.02-1.2, y = 0-0.4) phosphors

Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ (x = 0.02-1.2, y = 0-0.4) phosphors were prepared via a solid-state reaction and their photoluminescence properties were investigated. An analysis of the decay behavior indicates that the energy migration between Tb³⁺ ions is conspicuous in the ⁵D₃ → ⁷F₄ transition due to the cross-relaxation in BaLa₄(WO₄)₇. A partial substitution of Ba²⁺ by Sr²⁺ can not only enhance the emission intensity but also increase the solid solubility of Tb³⁺ in Ba_1−ySr_yLa_4−xTb_x(WO₄)₇. The emission intensity of the ⁵D₄ → ⁷F_J (J = 4, 5, 6) transitions can be enhanced by increasing Sr²⁺ and Tb³⁺ concentrations, with the optimal conditions being x = 1.2, y = 0.4 (Ba_0.6Sr_0.4La_2.8Tb_1.2(WO₄)₇). Under near-UV excitation at 379 nm, the CIE color coordinates of Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ vary from blue (0.212, 0.181) at x = 0.04, y = 0, to green (0.245, 0.607) at x = 1.2, y = 0.4.     

Eu-Mössbauer spectroscopic and X-ray diffraction study on EuyM1−yO2−x (0 ≤ y ≤ 1.0) (M = Th, U)

¹⁵¹Eu-Mössbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the Eu_yM_1−yO_2−x (M = Th and U) systems over the entire composition range of 0 ≤ y ≤ 1.0. The XRD results of the Eu-Th system showed that a very wide defect-fluorite (DF) type phase in which oxygen vacancies (V_O) are disordered (x = y/2) is formed for 0 ≤ y < 0.5 and that two-phase regions sandwitching a narrow C-type (C) single phase around y ≈ 0.8 appear for 0.5 < y < 0.8 (DF + C) and 0.82 < y < 1.0 (C + B-type (monoclinic) Eu₂O₃). The Mössbauer results show that the isomer shifts (ISs) of Eu³⁺ in this system smoothly increase with Eu composition, y. The decrease of average coordination number (CN) of O²⁻ around Eu³⁺ with increasing y (CN = 8 − 2y) (x = y/2) results in the decrease of the average EuO bond length, which is due to the decrease of repulsion force between O²⁻ anions. This result confirms that the IS of Eu³⁺ correlates well with the average EuO bond length in oxide systems. For the Eu-U system, the lattice parameter, a₀, of the system decreases almost linearly with y, in accordance with the calculated a₀ versus y curve for the oxygen-stoichiometric (i.e. x = 0) fluorite-type dioxide (CN = 8). The ISs of Eu³⁺ in this composition range remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu₂Zr₂O₇ and Eu₂Hf₂O₇ (y = 0.5)) with O²⁻-eight-fold coordinated Eu³⁺(CN = 8).     

Tuning the luminescence color and enhancement of afterglow properties of Sr(4−x−y)CaxBayAl14O25:Eu,Dy phosphor by adjusting the composition

Color point tuning is an important challenge for improving the practical applications of various displays, especially there are very limited white color single hosts that emits in the white spectrum. In this paper, the possibility of color tuning by substituting part of host lattice cation (Sr²⁺ ions) by Ca²⁺ or Ba²⁺ ions in an efficient strontium aluminate phosphor, Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺, is reported and found to be very promising for displays. A detail study by replacing part of Sr²⁺ with Ca²⁺ or Ba²⁺ has been investigated. X-ray diffraction study showed that crystal structure of Sr₄Al₁₄O₂₅ is preserved up to 20 mol of Ca²⁺ ion exchange while it is limited to 10 mol of Ba²⁺ ions exchange. Substantial shift in the emission band and color were observed by substitution of Sr²⁺ by Ca²⁺ or Ba²⁺ ions. A bluish-white emission and afterglow was observed at higher Ca²⁺ ions substitution. Further, partial Ca²⁺ substitutions (up to 0.8 mol) resulted in enhanced afterglow of Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphor. However, Ba²⁺ substitution decreased the fluorescence as well afterglow of the Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphor significantly. The enhanced phosphorescence by partial Ca²⁺ substitution is explained on the basis of increased density of shallow traps associated with higher solubility of Dy³⁺ ions in to the host lattice due to equivalent size of Ca²⁺ and Dy³⁺ ions. Thus, Ca²⁺ substitution in the Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphor is a promising method for tuning the emission color and improving the afterglow intensity of the phosphor.     

Luminescent properties of Sr2P2O7:Eu,Mn phosphor under near UV excitation

The phosphors in the system Sr_2−x−yP₂O₇:xEu²⁺,yMn²⁺ were synthesized by solid-state reactions and their photoluminescence properties were investigated. These phosphors have strong absorption in the near UV region, which is suitable for excitation of ultraviolet light emitting diodes (UVLEDs). The orange-reddish emission of Mn²⁺ in these phosphors can be used as a red component in the tri-color system and may be enhanced by adjusting the Mn²⁺/Eu²⁺ ratio. The energy transfer from Eu²⁺ to Mn²⁺ is observed with a transfer efficiency of ∼0.45 and a critical distance of ∼10 Å. The results reveal that Sr_2−x−yP₂O₇:xEu²⁺,yMn²⁺ phosphors could be used in white light UVLEDs.     

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.     

The mechanism of long phosphorescence of SrAl2−xBxO4 (0<x<0.2) and Sr4Al14−xBxO25 (0.1<x<0.4) co-doped with Eu and Dy

The role of B₂O₃ in realizing the long phosphorescence of Eu(II)+Dy(III) doped strontium aluminates has been investigated. IR and solid state MAS NMR spectra show the incorporation of boron as BO₄ in the AlO₄ framework of SrAl₂O₄ and Sr₄Al₁₄O₂₅. Phosphor, made free of glassy phases by washing with hot acetic acid+glycerol, did not show any photoconductivity under UV irradiation, indicating that the mechanism involving hole conduction in valence band is untenable for long phosphorescence. EPR studies confirm the presence of both electron and hole trap centers. Dy³⁺ forms substitutional defect complex with borate; [Dy-BO₄-V_Sr]²⁻, and acts as a hole trap center. The electron centers are formed by the oxygen vacancies associated with BO₃³⁻, i.e. [BO₃-V_O]³⁻. Under indigo light or near UV irradiation, the photoinduced electron centers are formed as [BO₃-V_O(e′)]⁴⁻. The holes are released from [Dy-BO₄-V_Sr(h)]¹⁻ under thermal excitation at room temperature. The recombination of electrons with holes releases energy which is expended to excite Eu²⁺ to induce long phosphorescence.     

The effect of simultaneous addition of Gd and Al on the photoluminescence characteristics of (Y0.94−x−yAlxGdyEu0.06)BO3 phosphors

The (Y_0.94−x−yAl_xGd_yEu_0.06)BO₃ (0 ≤ x ≤ 0.04 and 0 ≤ y ≤0.4) phosphors were single-phase with a hexagonal vaterite crystal structure. The (Y_0.94−x−yAl_xGd_yEu_0.06)BO₃ phosphor powders showed smooth, regular, and spherical morphology. The emission intensity of the Al- and Gd-co-doped (Y_0.74−xAl_xGd_0.2Eu_0.06)BO₃ and (Y_0.925−yAl_0.015Gd_yEu_0.06)BO₃ phosphors was much higher than that of Al-free (Y_0.74Gd_0.2Eu_0.06)BO₃ and Gd-free (Y_0.925Al_0.015Eu_0.06)BO₃ phosphors, respectively. This means that the simultaneous addition of Gd and Al to yttrium borates was desirable for improving their photoluminescent properties.     

Influence of excitation wavelengths on luminescent properties of Sr3Al2O6:Eu, Dy phosphors prepared by sol-gel-combustion processing

Eu²⁺ and Dy³⁺ ion co-doped Sr₃Al₂O₆ red-emitting long afterglow phosphor was synthesized by sol-gel-combustion methods using Sr(NO₃)₂, Al(NO₃)₃·9H₂O, Eu₂O₃, Dy₂O₃, H₃BO₃ and C₆H₈O₇·H₂O as raw materials. The crystalline structure of the phosphors were characterized by X-ray diffraction, luminescent properties of phosphors were analyzed by fluorescence spectrophotometer. The effect of excitation wavelengths on the luminescent properties of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors was discussed. The emission peak of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphor lays at 516 nm under the excitation of 360 nm, and at 612 nm under the excitation of 468 nm. The results reveal that the Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphor will emit a yellow-green light upon UV illumination, and a bright red light upon visible light illumination. The emission mechanism was discussed according to the effect of nephelauxetic and crystal field on the 4f⁶5d¹ → 4f⁷ transition of the Eu²⁺ ions in Sr₃Al₂O₆. The afterglow time of (Sr_0.94Eu_0.03Dy_0.03)₃ Al₂O₆ phosphors lasts for over 600s after the excited source was cut off.     

Preparation and characterization of Sr4Al2O7:Eu, Eu phosphors

A new composition of red strontium aluminate phosphor (Sr₄Al₂O₇:Eu³⁺, Eu²⁺) is synthesized using a solid state reaction method in air and in a reducing atmosphere. The investigation of firing temperature indicates that a single phase of Sr₄Al₂O₇ is formed when the firing temperature is higher than 1300 °C and that a Sr₃Al₂O₆ phase is formed as the main peak below 1300 °C. The effects of firing temperature and doping concentration on luminescent properties are investigated. Sr₄Al₂O₇ phosphors exhibit the typical red luminescent properties of Eu³⁺ and Eu²⁺. A comparison photoluminescence study with Sr₃Al₂O₆ phosphor shows that Sr₄Al₂O₇ has higher emission intensity than Sr₃Al₂O₆ as a result of the higher optimum doping concentration of Sr₄Al₂O₇ phosphor.     

Transport properties and Mössbauer spectra of Fe-substituted La10−x(Si,Al)6O26 apatites

Increasing iron content in apatite-type La_9.83Si_4.5Al_1.5−yFe_yO_26+δ (y=0.5-1.5) leads to increasing unit cell volume, fraction of Fe⁴⁺, partial oxygen ionic and p-type electronic conductivities, and ceramics sinterability. The oxygen ion transference numbers, determined by Faradaic efficiency (FE) measurements at 973-1223 K in air, are in the range 0.986-0.994. Data on total conductivity and Seebeck coefficient as functions of the oxygen partial pressure, varying in the range 10⁻² Pa to 70 kPa, confirm that under oxidizing conditions the ionic conduction in Fe-substituted La_9.83(Si,Al)₆O_26+δ apatites is dominant. Due to stabilization of Fe³⁺, substantially worse transport properties are observed for A-site stoichiometric La₁₀Si₄Fe₂O₂₆, having activation energy for ionic conductivity of 107 kJ/mol and electron transference numbers close to 0.03. The correlation between partial ionic and electron-hole conductivities suggests a significant role of Fe⁴⁺ formation compensated by extra oxygen incorporation into the vacant sites, which are formed due to Frenkel-type disorder induced by La vacancies. The average thermal expansion coefficients of Fe-doped La_10−x(Si,Al)₆O_26+δ ceramics, calculated from dilatometric data in air, are 8.9×10⁻⁶ to 9.9×10⁻⁶ K⁻¹ at 300-1250 K.     

Principal band gaps and bond lengths of the alloy (AlxGa1−x)1−zInzPyAs1−y lattice matched to GaAs

The principal band gaps (E(Γ),E(L), and E(X)) and bond lengths (d(x,y,z))of the alloy (Al_xGa_1−x)_1−zIn_zP_yAs_1−y (where, 0 < x + z < 1, and 0 < y < 1) are calculated over the entire composition range based on the first order correlated function expansion (CFE) scheme. Defining the lattice strain parameter as , it is found that a good quality of alloy (defining ? < ∼ 0.5%) can be obtained in the composition region : 0 < x < ∼ 0.3, 0 < y < ∼ 0.2 and 0 < z < ∼ 0.1. The first order CFE lattice matching relations and corresponding band gaps for the alloy on the GaAs substrate are also determined. It is found that the principal band gaps of the alloy (Al_xGa_1−x)_1−zIn_zP_yAs_1−y lattice matched to GaAs covers band gap ranges: 1.45 eV E < (Γ) 2.69 eV, 1.80 eV < E(L) < 2.38 eV, and 1.97 eV < E(X) < 2.20 eV, while the direct band gap covers from 1.45 eV to 2.05 eV. Our theoretical prediction was compared with the existing experimental data.     

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.     

Structural, magnetic and dielectric properties of Bi1−ySryFe(1−y)(1−x)Sc(1−y)xTiyO3 (x = 0-0.2, y = 0.1-0.3) ceramics

In this study, bulk ceramics with general formula Bi_1−ySr_yFe_{(1−y)(1−x)}Sc_(1−y)xTi_yO₃ (x = 0-0.2, y = 0.1-0.3 mol%) were prepared by traditional solid-state reaction method. As a comparison, bulk BiFeO₃ (BF) was also sintered by rapid sintering method. Their structural, magnetic, dielectric properties were investigated. X-ray diffraction analysis indicated that apart from a small amount of secondary phase detected in BF, all other samples crystallized in pure perovskite structure and maintained original R3c space group. The room temperature M-H curves were obtained. While BF had a coercive magnetic field (H_c) of 150 Oe, Bi_1−ySr_yFe_1−yTi_yO₃ solid solutions had a much larger value (for y = 0.1, 0.2, 0.3, H_c were 4537, 5230 and 3578 Oe, respectively). Sc³⁺ substitution decreased the H_c values of these solid solutions remarkably, and resulted in soft magnetic properties, as well as a decrease of the dielectric loss. At 1 MHz, the tan δ of Bi_0.7Sr_0.3Fe_0.7(1−x)Sc_0.7xTi_0.3O₃ with x = 0.05, 0.1, 0.15, 0.2 were 0.1545, 0.1078, 0.1046 and 0.1701, respectively.