Synthesis of Y2O2S:Eu3+, Mg2+, Ti4+ hollow microspheres via homogeneous precipitation route

A phosphorescent material in the form of Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ hollow microspheres was prepared by homogeneous precipitation using monodispersed carbon spheres as hard templates. Y₂O₃:Eu³⁺ hollow microspheres were first synthesized to serve as the precursor. Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ powders were obtained by calcinating the precursor in a CS₂ atmosphere. The crystal structure, morphology and optical properties of the composites were characterized. X-ray diffraction measurements confirmed the purity of the Y₂O₂S phase. Electron microscopy observations revealed that the Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ particles inherited the hollow spherical shape from the precursor after being calcined in a CS₂ atmosphere and that they had a diameter of 350–450 nm and a wall thickness of about 50–80 nm. After ultraviolet radiation at 265 or 325 nm for 5 min, the particles emitted strong red long-lifetime phosphorescence originating from Eu³⁺ ions. This phosphorescence is associated with the trapping of charge carriers by Ti⁴⁺ and Mg²⁺ ions.     

Ca4REO(BO3)3 crystals for green and blue microchip laser generation: from crystal growth to laser and nonlinear optical properties

We have taken advantage of congruent melting behavior of the nonlinear rare-earth oxoborate Ca₄REO(BO₃)₃ family to perfect a process of collective fabrication of self-frequency doubling microchip laser based on Nd:GdCOB (Ca₄Gd_1−xNd_xO(BO₃)₃) crystals. The process goes from Czochralski boule to 1 × 3 mm² chips perfectly oriented (better than 0.1°) to the phase matching direction (θ=90°, φ=46°) in the XY principal plane, with dielectric mirrors directly deposited on both faces of the chips. 20 mW of self-frequency doubling output power at 530 nm was performed under 800 mW of diode laser as incident pump power at 812 nm. In addition, new compositions from the solid solution Ca₄Gd_1−xY_xO(BO₃)₃ (Gd_1−xY_xCOB) (x=0.13, 0.16, 0.44) have been grown by the Czochralski pulling method, in order to achieve noncritical phase matching (NCPM) second harmonic generation of ⁴F_3/2 → ⁴I_9/2 Nd³⁺ doped laser hosts. Three types of laser wavelengths have been chosen: Nd:YAP (YAlO₃) at 930 nm, Nd:YAG (Y₃Al₅O₁₂) at 946 nm, and Nd:ASL (Nd_ySr_1−x La_x−yMg_x Al_12−xO₁₉) at 900 nm. Angular acceptance measurements of these three types of compositions present very large values, compared to pure GdCOB or YCOB oriented in critical phase matching configurations.     

Luminescence properties of Eu ions doped in Y2−xGdxO3 thin films grown by pulsed laser deposition method

Luminescence properties of Y_2−xGd_xO₃:Eu³⁺ (x = 0 to 2.0) thin films are investigated by site-selective laser excitation spectroscopy. The films were grown by pulsed laser deposition method on SiO₂ (100) substrates. Cubic phase Y₂O₃ and Gd₂O₃ and monoclinic phase Gd₂O₃ are identified in the excitation spectrum of the ⁷F₀ → ⁵D₀ transition of Eu³⁺. The emission spectra of the ⁵D₀ → ⁷F_J (J = 1 and 2) transition from individual Eu³⁺ centers were obtained by tuning the laser to resonance with each excitation line. The excitation line at around 580.60 nm corresponds to the line from Eu³⁺ with C₂ site symmetry of cubic phase. New lines at 578.65 and 582.02 nm for the C_S sites of Gd₂O₃ with monoclinic phase are observed by the incorporation of Gd in Y₂O₃ lattice. Energy transfer occurs between Eu³⁺ ions at the C_S sites and from Eu³⁺ ions at the C_S sites to those at the C₂ site in Y_2−xGd_xO₃.     

Crystal, electronic and luminescence properties of Eu-doped Sr2Al2−xSi1+xO7−xNx

The crystal and electronic structures, as well as the luminescence properties of Sr₂Al_2−xSi_1+xO_7−xN_x:Eu²⁺ are reported. First-principles calculations energetically confirm that the Al and Si atoms are in partial ordering in the 2a and 4e sites in Sr₂Al₂SiO₇. In addition, the band structure calculation shows that Sr₂Al₂SiO₇ has an indirect band gap with an energy gap of about 4.07 eV, which is in good agreement with the experimental data (5.3 eV) obtained from the diffuse reflection spectrum. The crystal structure of Sr₂Al₂SiO₇ can be modified by Si–N substitution for Al–O in the lattice with a maximum solubility of about x=0.6. The average bond length of Eu_Sr^-(O,N) slightly increases although the lattice parameters decrease with the incorporation of Si–N in Sr₂Al₂SiO₇:Eu²⁺. Under excitation in the visible spectral region, Sr₂Al_2−xSi_1+xO_7−xN_x:Eu²⁺ emits blue to yellow light with a broad emission band in the range of 480–570 nm, varying with both the Eu concentration and the x value. The red shift of the emission band of Eu²⁺ is associated with an increase in the crystal-field splitting and the covalency, which arise from the incorporation of nitrogen as well as the energy transfer between the Eu ions at high Eu concentrations. Moreover, the Eu ions have a strong effect on both the concentration quenching and the thermal quenching in Sr₂Al_2−xSi_1+xO_7−xN_x. The temperature dependence of photoluminescence indicates that Sr₂Al_2−xSi_1+xO_7−xN_x:Eu²⁺ shows strong thermal quenching due to the dominant nonradiative process at room temperature.     

Synthesis of Y2O2S:Eu3+, Mg2+, Ti4+ hollow microspheres via homogeneous precipitation route

Abstract

A phosphorescent material in the form of Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ hollow microspheres was prepared by homogeneous precipitation using monodispersed carbon spheres as hard templates. Y₂O₃:Eu³⁺ hollow microspheres were first synthesized to serve as the precursor. Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ powders were obtained by calcinating the precursor in a CS₂ atmosphere. The crystal structure, morphology and optical properties of the composites were characterized. X-ray diffraction measurements confirmed the purity of the Y₂O₂S phase. Electron microscopy observations revealed that the Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ particles inherited the hollow spherical shape from the precursor after being calcined in a CS₂ atmosphere and that they had a diameter of 350–450 nm and a wall thickness of about 50–80 nm. After ultraviolet radiation at 265 or 325 nm for 5 min, the particles emitted strong red long-lifetime phosphorescence originating from Eu³⁺ ions. This phosphorescence is associated with the trapping of charge carriers by Ti⁴⁺ and Mg²⁺ ions.     

Up-conversion luminescence properties of Y2O2S:Yb,Er nanophosphors

Up-converting yttrium oxysulfide nanomaterials doped with ytterbium and erbium (Y₂O₂S:Yb³⁺,Er³⁺) were prepared with the flux method. The precursor oxide materials were prepared using the combustion synthesis. The morphology of the oxysulfides was characterized with transmission electron microscopy (TEM). The particle size distribution was 10–110 nm, depending on the heating temperature. According to the X-ray powder diffraction (XPD), the crystal structure was found hexagonal and the particle sizes estimated with the Scherrer equation agreeded with the TEM images. Upon the 970 nm infrared (IR) laser excitation, the materials yield moderate green ((²H_11/2, ⁴S_3/2) → ⁴I_15/2 transition) and strong red (⁴F_9/2 → ⁴I_15/2) luminescence. The green luminescence was enhanced with respect to the red one by an increase in both the crystallite size and erbium concentration due to the cross-relaxation (CR) processes. The most intense up-conversion luminescence was achieved with x_Yb and x_Er equal to 0.10 and 0.005, respectively. Above these concentrations, concentration quenching occurred.     

VUV–UV spectroscopic properties of RE (RE = Ce, Eu and Tb)-doped KMLn(PO4)2 (M = Ca, Sr; Ln = Y, La, Lu)

The VUV excited luminescent properties of Ce³⁺, Eu³⁺ and Tb³⁺ in the matrices of KMLn(PO₄)₂ (M²⁺ = Ca, Sr; Ln³⁺ = Y, La, Lu) were investigated. The bands at about 155 nm in the VUV–UV excitation spectra are attributed to the host lattice absorption, which indicates that the optical band gap of KMLn(PO₄)₂ is about 8.0 eV. Ce³⁺-doped samples show typical Ce³⁺ emission in the range of 300–450 nm, and the energy transfer from host lattice to Ce³⁺ is efficient. For Eu³⁺-doped samples, the O²⁻–Eu³⁺ CTBs are observed to be at about 228 nm except KSrLu(PO₄)₂:Eu³⁺ (247 nm). As for Tb³⁺-doped samples, typical 4f → 5d absorption bands in the region of 175–250 nm were observed.     

Uniform lanthanide-doped Y2O3 hollow microspheres: Controlled synthesis and luminescence properties

Lanthanide-doped uniform pure cubic phase Y₂O₃ hollow microspheres have been successfully synthesized via a facile, high yield urea-based coprecipitation route with assistant of carbon spheres templates. The diameter and shell thickness of the microspheres can be manipulated by adjusting carbon sphere templates. Under a 980 nm excitation, Yb³⁺/Er³⁺, Er³⁺, Yb³⁺/Tm³⁺-doped Y₂O₃ hollow microspheres emit bright upconversion red, green, blue light with high purity, respectively, while Eu³⁺, Eu³⁺/Tb³⁺-doped Y₂O₃ hollow microspheres exhibit intense downconversion red light under the excitation of 254 nm ultraviolet light. Especially, the 610 nm emission intensity of Eu³⁺ in the Eu³⁺/Tb³⁺-codoped Y₂O₃ hollow microspheres is almost 5 times of that in the Y₂O₃:Eu³⁺ hollow microspheres indicating the occurring of the energy transfer from Tb³⁺ to Eu³⁺ ions.     

Splitting and broadening of the emission bands of Y2O3:Eu3+,Nd3+ and its dependence on Nd3+ concentration and annealing temperature

Although Eu³⁺ ion-doped Y₂O₃ has been extensively used as red phosphors, their color rendering needs to be improved for high-quality illumination and displaying. Here, we show that the emission spectra of Y₂O₃:Eu³⁺ red phosphors can be broadened by the doping of Nd³⁺ ion so that the color rendering capability of Y₂O₃:Eu³⁺ was remarkably enhanced. Y₂O₃:Eu³⁺ and Y₂O₃:Eu³⁺,Nd³⁺ colloidal spheres were synthesized by wet chemical procedure and high-temperature treatment. The fluorescence measurement under the 254 and 380 nm ultraviolet excitation indicates that the 612 nm red emission peak of Eu³⁺ can be splitted into two ones by the doping of Nd³⁺ ion, of which the full width at half maximum (FWHM) is broadened from 4.2 nm to 9.6 nm. By varying the concentration of Nd³⁺ ion, it was determined that the optimal doping concentration of Nd³⁺ ion is of 3 mol% for realizing the strongest emission intensity. The further increase of Nd³⁺ ion exceeding 3 mol% would lead to a concentration quenching phenomenon. The analysis based on XRD spectra and the simplified energy diagram suggested that the doped Nd³⁺ ion not only monitored the growth dynamics, but also took an efficient energy transfer and a cross relaxation process to generate intense emission from Eu³⁺ ion in both of C₂ and S₆ sites, instead of preferable one type of Eu³⁺ site (C₂ or S₆) in the Nd³⁺ undoped sample.     

Influence of calcination temperature on luminescent properties of Sr3Al2O6:Eu, Dy phosphors prepared by sol–gel-combustion processing

The detailed preparation process of Eu²⁺ and Dy³⁺ ion co-doped Sr₃Al₂O₆ phosphor powders with red long afterglow by sol–gel-combustion method in the reducing atmosphere is reported. X-ray diffraction, scanning electron microscopy and photoluminescence spectroscopy are used to investigate the effects of synthesis temperature on the crystal characteristics, morphology and luminescent properties of the as-synthesized Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors. The results reveal that Sr₃Al₂O₆ crystallizes completely when the combustion ash is sintered at 1200 °C. The excitation and the emission spectra indicate that the excitation broad-band lies chiefly in visible range and the phosphor powders emit strong light at 618 nm under the excitation of 472 nm. The light intensity and the light-lasting time of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors are increased when increasing the calcination temperatures from 1050 to 1200 °C. The afterglow of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors sintered at 1200 °C lasts for over 600 s when the excited source is cut off. The red emission mechanism is discussed according to the effect of nephelauxetic and crystal field on the 4f⁶5d¹ → 4f⁷ transition of the Eu²⁺ ions.     

The blue-light emission enhancement mechanism of Eu in Eu, Dy: SiO2 matrix

A Eu, Dy co-doped SiO₂ matrix xerogel with blue emission was prepared by the sol–gel method. Strong blue emission located between 425 nm and 525 nm with a peak at 486 nm is observed under UV laser excitation at room temperature, which is related to a 4f → 5d energy transition of Eu²⁺. Such techniques as FT-IR and TGA–DSC were used to measure the microstructure of the luminescent materials. The influence of Dy³⁺ ions on the luminescent property of Eu²⁺ was investigated. The emission intensity of Eu, Dy-codoped samples is stronger than that of Eu doped samples. The emission enhancement mechanism relating to Eu²⁺ is attributed to an energy transfer involving Dy³⁺ → Eu²⁺. Using energy transition theory, we speculate that the mechanism may be one of the resonance transfers via multi-polar interactions, and present a possible energy transfer model. The Eu²⁺ blue emission intensity reaches the maximum when the Dy³⁺ concentration is 0.1 mol%. When the concentration of Dy³⁺ is 0.3 mol%, a fluorescence quenching appears which might be related to the overlap part of Eu²⁺ excitation and emission levels, and also suggests the existence of Eu²⁺ → Eu²⁺ energy transfer.     

Spectral analysis of RE (RE = Sm, Dy, and Tm): P2O5–Al2O3–Na2O glasses

Phosphate glasses in the compositions of 70P₂O₅–15Al₂O₃–14Na₂O–1RE³⁺ (RE = Sm, Dy, and Tm) (mol%) were prepared by melt-quenching technique and characterized optically. The differential thermal analysis (DTA) profile of the host glass was carried out to confirm its thermal stability. For all the glasses absorption, photoluminescence and decay measurements have also been carried out. These glasses have shown strong emission and absorption bands in visible and near-infrared (NIR) region. From the measured absorption spectra, Judd–Ofelt (J–O) intensity parameters (Ω₂, Ω₄ and Ω₆) have been calculated for all the studied ions. For Sm³⁺ doped glass, four emission bands centered at 562 nm (⁴G_5/2 → ⁶H_5/2), 598 nm (⁴G_5/2 → ⁶H_7/2), 644 nm (⁴G_5/2 → ⁶H_9/2), and 704 nm (⁴G_5/2 → ⁶H_11/2) have been observed with 402 nm (⁶H_5/2 → ⁴F_7/2) excitation wavelength. Of them, 598 nm (⁴G_5/2 → ⁶H_7/2) has shown a bright orange emission. With regard to Dy³⁺ doped glass, a blue emission band centered at 486 nm (⁴F_9/2 → ⁶H_15/2) and a bright yellow emission at 575 nm (⁴F_9/2 → ⁶H_13/2) have been observed, apart from 662 nm (⁴F_9/2 → ⁶H_11/2) emission transition with an excitation at 388 nm (⁶H_15/2 → ⁴I_13/2,⁴F_7/2) wavelength. Emission bands of 650 nm (¹G₄ → ³F₄) and 785 nm (¹G₄ → ³H₅) transitions for the Tm³⁺ doped glass, with an excitation wavelength at 466 nm (³H₆ → ¹G₄), have also been observed. The stimulated emission cross-sections of all the emission bands of RE³⁺ glasses (RE = Sm, Dy, and Tm) have been computed based on their measured full-width at half maximum (FWHM, Δλ) and measured lifetimes (τ_m).     

Synthesis and luminescence properties of red long-lasting phosphor Y2O2S:Eu3+, Mg2+, Ti4+ nanoparticles

We report an effective method to synthesize Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ nanoparticles. Tube-like Y(OH)₃ were firstly synthesized by hydrothermal method to serve as the precursor. Nanocrystalline long-lasting phosphor Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ was obtained by calcinating the precursor with co-activators and S powder. XRD investigation shows a pure phase of Y₂O₂S, indicating no other impurity phase appeared. SEM and TEM observation reveals that the precursor synthesized via a hydrothermal routine has tube-like structure and the final phosphor reveals a hexagonal shape. The fine nanoparticles which have the particle size ranging from 30 to 50 nm show uniform size and well-dispersed distribution. From the spectrum, the main emission peaks are ascribed to Eu³⁺ ions transition from ⁵D_J (J = 0, 1, 2) to ⁷F_J (J = 0, 1, 2, 3, 4). After irradiation by 325 nm for 10 min, the Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ long-lasting phosphor shows very bright red afterglow and the longest could last for more than 1 h even after the irradiation source had been removed. It is considered that the long-lasting phosphorescence is due to the contribution from the electron traps with suitable trap depth.     

Sr3.5Mg0.5Si3O8Cl4: Eu bluish-green-emitting phosphor for NUV-based LED

Sr₄Si₃O₈Cl₄:Eu²⁺ and Sr_3.5Mg_0.5Si₃O₈Cl₄:Eu²⁺ phosphors were prepared by a conventional solid state reaction (SS). Excited by 370 nm near-ultraviolet light, the phosphors show an efficient bluish-green wide-band emission centering at 484 nm, which originates from the 4f5d¹ → 4f⁷ transition of Eu²⁺ ion. The excitation spectra of the phosphors are a broad band extending from 250 nm to 400 nm. Mg²⁺-codoping greatly enhances the bluish-green emission of the phosphors. An LED was fabricated by coating the Sr_3.5Mg_0.5Si₃O₈Cl₄:0.08Eu²⁺ phosphor onto an ~ 370 nm-emitting InGaN chip. The LED exhibits bright bluish-green emission under a forward bias of 20 mA. The results indicate that Sr_3.5Mg_0.5Si₃O₈Cl₄:0.08Eu²⁺ is a candidate as a bluish-green component for fabrication of NUV-based white LEDs.     

Fabrication and optical properties of NUV-emitting SiO2–SrB4O7:Eu glass–ceramic thin films

SiO₂–SrB₄O₇:Eu²⁺ glass–ceramic thin films were fabricated for possible application in near ultraviolet (NUV) emitting devices. Nano-sized SrB₄O₇:Eu²⁺ powders were prepared by a Pechini-type sol–gel method and a subsequent ball-milling treatment. The powders showed NUV emissions centered at 367 nm, upon irradiation with UV of shorter wavelengths, due to an allowed 4f⁶5d¹ → 4f⁷ electronic transition of Eu²⁺ ions. The glass–ceramic thin films were prepared by dip-coating of tetraethylorthosilicate (TEOS) solutions dispersed with the nano-sized SrB₄O₇:Eu²⁺ powders and a subsequent heat-treatment. It was found that the glass–ceramic thin films had relatively high thermal stability up to 800 °C in terms of the Eu²⁺ emissions. SiO₂ layers surrounding SrB₄O₇:Eu²⁺ appeared to be effective for the surface passivation of the phosphor particles.     

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.     

A novel green emitting phosphor Ca1.5Y1.5Al3.5Si1.5O12:Tb

Vacuum ultraviolet (VUV) excitation and emission properties of Tb³⁺ ion doped silico-aluminate phosphor Ca_1.5Y_1.5Al_3.5Si_1.5O₁₂:Tb³⁺ was studied. Upon excitation with vacuum ultraviolet (VUV) and near UV light excitation, the phosphor showed strong green-emission peaked at 545 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺, and the highest PL intensity at 545 nm was found at a content of about 14 mol% Tb³⁺. The 4f–5d transition absorption of Tb³⁺ is in the range from 150 nm to 260 nm, and there is an energy transfer from the host to the rare earth ions. Field emission scanning electron microscopy (FE-SEM) images showed the particle size of the phosphor was less than 3 μm.     

Luminescent properties of Na3YSi3O9 doped with Eu under UV-VUV excitation

The luminescent properties of Na₃Y_1−xSi₃O₉:xEu³⁺ (0.05 ≦ x ≦ 0.80) powder crystals were investigated in UV-VUV region. The Eu³⁺-O²⁻ charge transfer band (CTB) was observed to be located at around 233 nm and the environmental parameter (h_e) was estimated to be about 0.730. The excitation spectrum monitoring the 613 nm red emission from Eu³⁺ ions reveals the host absorption band (HAB) to be around 145 nm. The calculated Commission Internationale de l’Eclairage (CIE) chromaticity coordinates indicate the emission by 233 nm rather than by 147 nm excitation has the better color purity and the possible mechanisms have been proposed. The Eu³⁺-emission showed high quenching concentration due to the isolated YO₆ octahedra in the host and the small h_e for the Eu³⁺ ions and the optimum concentration was determined to be as high as x = 0.65 and 0.30 with 233 and 147 nm excitation, respectively.     

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.     

Effects of Ho2O3 addition on defects of BaTiO3

Effects of Ho₂O₃ addition on defects of BaTiO₃ ceramic have been studied in terms of electrical conductivity at 1200 °C as a function of oxygen partial pressure (P_O2°) and oxygen vacancy concentration. The substitution of Ho³⁺ for the Ti site in Ba(Ti_1−xHo_x)O_3−0.5x resulted in a significant shift of conductivity minimum toward lower oxygen pressures and showed an acceptor-doped behavior. The solubility limit of Ho on Ti sites was confirmed less than 3.0 mol% by measuring the electrical conductivity and the lattice constant. Oxygen vacancy concentrations were calculated from the positions of P_O2° in the conductivity minima and were in good agreement with theoretically estimated values within the solubility limit. The Curie point moved to lower temperatures with increasing the oxygen vacancy concentration and Ho contents.