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.     

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.     

A potential Eu-activated Ca10K(PO4)7 red phosphor for white light-emitting diodes

New red Ca₁₀K(PO₄)₇:Eu³⁺, K⁺ phosphors were synthesized by solid state reaction and their photoluminescence properties as well as those by co-doping Mo⁶⁺ under near ultraviolet excitation were investigated. From the excitation spectra monitored at 611 nm, it can be seen that the strongest excitation peak is situated at 393 nm, well matching with the emission wavelength of near-ultraviolet chips for white LEDs. Upon 393 nm excitation, the brightness of Ca₉K(PO₄)₇:0.5Eu³⁺, 0.5 K⁺ with the optimal Eu³⁺-doping concentration is about 2.3 times stronger than that of the commercial red Y₂O₃:Eu³⁺ phosphor. The introducing of Mo⁶⁺, which results in a possible variety for the excited energy level of the host, can enhance the brightness of Eu³⁺ to be maximized by about 15%. The CIE chromaticity coordinates of Ca₉K(PO₄)₇:0.5Eu³⁺, 0.5 K⁺ are calculated to (0.654, 0.345), which are close to the (0.67, 0.33) standard of the National Television System Committee. All the above results indicate Eu³⁺-activated Ca₁₀K(PO₄)₇ is a potential candidate for white LEDs.     

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).     

The photoluminescence properties of Y2O3:Eu prepared by surfactant assisted co-precipitation-molten salt synthesis

Y₂O₃:Eu³⁺ red phosphors were prepared by surfactant assisted co-precipitation-molten salt synthesis method. The effects of surfactant content and annealing temperature on the structure and luminescence were investigated by X-ray diffraction and fluorescence spectrophotometer. The use of surfactant reduces the impurities on the surface of particles and promotes the reaction. The color purity of as-prepared Y₂O₃:Eu³⁺ red phosphors is improved with the presence of surfactant. In the excitation spectra, two strong bands at 394 and 466 nm are attributed to ⁷F_0,1-⁵L₆, ⁷F_0,1-⁵D₂ transitions of Eu³⁺ ions respectively. With the excitation of 394 or 466 nm, the as-fabricated samples reveal excellent red emission as high as that of samples monitored by 254 nm. Thus, the Y₂O₃:Eu³⁺ is a promising red phosphor for ultraviolet-visible light-emitting diodes.     

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.     

Photoluminescence properties of NaGd(MoO4)2:Eu nanophosphors prepared by sol-gel method

NaGd(MoO₄)₂:Eu³⁺ (hereafter NGM:Eu) phosphors have been prepared by sol-gel method. The properties of the resulting phosphors are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curve. The excitation spectra of NGM:Eu phosphors are mainly attributed to O → Mo charge-transfer (CT) band at about 282 nm and some sharp lines of Eu³⁺ f-f transitions in near-UV and visible regions with two strong peaks at 395 and 465 nm, respectively. Under the 395 and 465 nm excitation, intense red emission peaked at 616 nm corresponding to ⁵D₀ → ⁷F₂ transition of Eu³⁺ are observed for 35 at.% NGM:Eu phosphors as the optimal doping concentration. The luminescence properties suggest that NGM:Eu phosphor may be regarded as a potential red phosphor candidate for near-UV and blue light-emitting diodes (LEDs).     

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.     

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.     

Vacuum ultraviolet excited photoluminescence properties of novel Na3Y9O3(BO3)8:Eu phosphor

The novel vacuum ultraviolet (VUV) excited Na₃Y₉O₃(BO₃)₈:Eu³⁺ red phosphor was synthesized and the photoluminescence (PL) properties were investigated. The phosphor showed strong VUV PL intensity, large quenching concentration (40 mol%) and good chromaticity (0.649, 0.351). The Eu³⁺-O²⁻ charge transition (CT) was observed to be at a higher energy (232 nm, 5.35 eV). The host absorption at 127-166 nm was broad and strong when monitoring the Eu³⁺ emission, which indicated that energy transfer from the host-lattice to the Eu³⁺ ions was efficient in Na₃Y₉O₃(BO₃)₈:Eu³⁺. These excellent VUV PL properties were revealed to be correlated with the unique isolated layer-type structure of Na₃Y₉O₃(BO₃)₈ host. The results showed that the Na₃Y₉O₃(BO₃)₈:Eu³⁺ would be a good candidate for VUV-excited red phosphor.     

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.     

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.     

Luminescence characterization of (Ca1−xZnx)Ga2S4:Eu phosphors for a white light-emitting diode

We investigated the luminescence properties of (Ca_1−xZn_x)Ga₂S₄:Eu²⁺ phosphor as a function of Zn²⁺ and Eu²⁺ concentrations. The luminescence intensity was markedly enhanced by increasing the mole fraction of Zn²⁺ at Ca²⁺ sites. Lacking any Zn²⁺ ions, CaGa₂S₄:0.01Eu²⁺ converted only 18.1% of the absorbed blue light into luminescence. As the Zn²⁺ concentration increased, the quantum yield increased and reached a maximum of 24.4% at x = 0.1. Furthermore, to fabricate the device, the optimized green-yellow (Ca_0.9Zn_0.1)Ga₂S₄:Eu²⁺ phosphor was coated with MgO. White light was generated by combining the MgO-coated phosphor and the blue emission from a GaN chip.     

The photoluminescence properties of Eu,Bi co-doped yttrium oxysulfide phosphor under vacuum ultraviolet excitation

Y₂O₂S co-doped with Bi³⁺,Eu³⁺ phosphors were prepared and their luminescence properties under vacuum ultraviolet (VUV) excitation were investigated. Much stronger red emission for Y₂O₂S:Eu³⁺,Bi³⁺ were observed under 147 nm excitation than that for Y₂O₂S:Eu³⁺. Investigation on photoluminescence and calculation of electronic structure of Y₂O₂S:Eu³⁺,Bi³⁺ revealed that the Bi³⁺ acts as a medium in energy transfer process and the great VUV luminescence enhancement of Y₂O₂S:Eu³⁺ by doping Bi³⁺ is due to effective energy transfer process: charge transfer (CT) transition of Y³⁺-O²⁻ → Bi³⁺ → Eu³⁺. The Y₂O₂S:Eu³⁺,Bi³⁺ showed excellent optical properties when compared with the commercial (Y,Gd)BO₃:Eu³⁺. Thus, the Y₂O₂S:Eu³⁺,Bi³⁺ would be a promising VUV-excited red phosphor.     

A promising red phosphor MgMoO4:Eu for white light emitting diodes

Motivated by the need for new red phosphors for solid-state lighting applications Eu³⁺-doped MgMoO₄ was prepared by solid-state reaction and its excitation and emission spectra were measured at room temperature. In addition, the effects of firing temperature and Eu³⁺ doping concentration on the PL intensities were also investigated. Compared with Y₂O₂S:0.05Eu³⁺, the obtained Mg_0.80MoO₄:Eu³⁺_0.20 phosphor shows a stronger excitation band near 400 nm and intensely red-emission lines at 616 nm correspond to the forced electric dipole ⁵D₀ → ⁷F₂ transitions on Eu³⁺ under 394 nm light excitation. The CIE chromaticity coordinates (x = 0.651, y = 0.348) of Mg_0.80MoO₄:Eu³⁺_0.20 close to the NTSC (National Television Standard Committee) standard values, and therefore may find application on near UV InGaN chip-based white light emitting diodes.     

Luminescence and afterglow in Sr2SiO4:Eu, RE [RE = Ce, Nd, Sm and Dy] phosphors—Role of co-dopants in search for afterglow

Luminescence of Eu²⁺ in Sr₂SiO₄:Eu²⁺, RE³⁺ [RE = Ce, Nd, Sm and Dy] phosphors was studied with a view to obtain an afterglow phosphor. The synthesized phosphors were characterized by powder X-ray diffraction (XRD), diffuse reflectance, photo- and thermoluminescence spectroscopic techniques. Afterglow was observed only with Dy³⁺ co-doped phosphor. The observed afterglow with Dy³⁺ co-doping originated from the formation of suitable traps which was supported by thermoluminescence results.     

A novel red emitting phosphor CaIn2O4:Eu, Sm with a broadened near-ultraviolet absorption band for solid-state lighting

Red phosphor of CaIn₂O₄:Eu³⁺, Sm³⁺ is synthesized by solid state reaction. The ⁵D₀ → ⁷F₂ transition of Eu³⁺ is dominantly observed in the photoluminescence spectrum, leading to a red emission of the phosphor. The doped Sm³⁺ is found to be efficient to sensitize the emission of Eu³⁺ and be effective to extend and strengthen the absorption of near-UV light with wavelength of 400-405 nm, and the energy transfer from Sm³⁺ to Eu³⁺ occurs and is discussed. The effect of the molar concentration of Sm³⁺ on the emission intensities of the phosphor CaIn₂O₄:Eu³⁺, Sm³⁺ is investigated. The temperature quenching effect is also measured from room temperature to 425 K, and the emission intensity of the phosphor at 425 K shows about 85% of that at room temperature. Furthermore, the chromaticity coordinates, the emission intensities and the conversion efficiencies of CaIn₂O₄:Eu³⁺, Sm³⁺ are compared to those of the conventional red phosphor of Y₂O₂S:Eu³⁺.     

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.     

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.     

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.