Luminescent properties of YVO4:Eu/SiO2 core–shell composite particles

We report an efficient process for preparing monodisperse SiO₂@Y_0.95Eu_0.05VO₄ core–shell phosphors using a simple citrate sol–gel method and without the use of surface-coupling silane agents or large stabilizers. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectra were used to characterize the resulting SiO₂@Y_0.95Eu_0.05VO₄ core–shell phosphors. The XRD results demonstrate that the Y_0.95Eu_0.05VO₄ particles crystallization on the surface of SiO₂ annealing at 800 °C is perfectly and the crystallinity increases with raising the annealing temperature. The obtained core–shell phosphors have a near perfect spherical shape with narrow size distribution (average size ca. 500 nm and an average thickness of ~50 nm), are not agglomerated, and have a smooth surface. The thickness of the YVO₄:Eu³⁺ shells on the SiO₂ cores could be easily tailored by changing the mass ratio of shell to core (W = [YVO₄]/[SiO₂]) (~50 nm for W = 30%). The Eu³⁺ shows a strong PL luminescence (dominated by ⁵D₀ − ⁷F₂ red emission at 618 nm) under the excitation of 320 nm UV light. The PL intensity of Eu³⁺ increases with increasing the annealing temperature and the values of W.     

Synthesis and photoluminescence of Eu- or Tb-doped Mg2SiO4 nanoparticles prepared by a combined novel approach

Tb³⁺- or Eu³⁺-doped magnesium silicate phosphors were prepared for the first time by using a novel approach, combined sol-gel-microwave heating. X-ray diffraction (XRD), transmission electron microscope (TEM), thermogravimetric analysis (TGA) and photoluminescence analyses were used to characterize the phosphors. XRD confirmed a forsterite lattice of Mg₂SiO₄ for the phosphors. TEM observation indicated that the phosphors have a spherical-like shape with little aggregation and the particle size is about 50 nm, and the small size is favorable to the potential application in field emission displays. The luminescent colors of Mg₂SiO₄:Tb³⁺ and Mg₂SiO₄:Eu³⁺ phosphors are green and red respectively, furthermore the luminescent intensities of them are relatively higher than the traditional Zn₂SiO₄:Tb³⁺ and Zn₂SiO₄:Eu³⁺ phosphors. In addition, Eu³⁺ ion emissions as a structural probe suggest that the rare earth ions replace the Mg²⁺ ions in the site of M2 (C_s) in the forsterite lattice of Mg₂SiO₄.     

Surface treatment investigation and luminescence properties of SiO2-coated Ca2BO3Cl:0.02Eu2+ phosphors via sol–gel process

SiO₂-coated Ca₂BO₃Cl:Eu²⁺ phosphors were prepared by the sol–gel method in order to enhance the chemical and thermal stabilities of Ca₂BO₃Cl:Eu²⁺ phosphor. The phase structures, microstructures and luminescence properties were studied by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and fluorescence spectrometer, respectively. The emission intensity of SiO₂-coated Ca₂BO₃Cl:Eu²⁺ phosphor decreased a little compared to that of the uncoated phosphor. The moisture resistances of the phosphors were comparatively examined by the aging treatment experiment in the water, and the thermal stability was studied by the temperature dependent photoluminescence spectra. The results indicated that SiO₂ coating on the surface of the phosphor particles improved the moisture-resistance and thermal stability to a large extent.     

Sr<Subscript>4</Subscript>Al<Subscript>14</Subscript>O<Subscript>25</Subscript>: Eu<Superscript>2+</Superscript>, Dy<Superscript>3+</Superscript>/silica core–shell particles synthesized via urea combustion method for carbon dioxide reduction in plants

Strontium Aluminate doped with Europium and Dysprosium is one of the most widely studied phosphors because of its high intensity and long persistence time. In this study, the unique characteristics of strontium aluminate based phosphors, specifically Sr₄Al₁₄O₂₅: Eu²⁺, Dy³⁺, was utilized as light source for plants for enhanced carbon dioxide reduction in dark field conditions. The Sr₄Al₁₄O₂₅: Eu²⁺, Dy³⁺ phosphor was synthesized using the combustion method. Stoichiometric amounts of aqueous precursors were dissolved in water, then placed in a high temperature furnace at 600 °C to obtain a foamy, amorphous precursor powder. The powders were cooled to room temperature and then grinded. After grinding, the powders were calcined for 8 h at 1300 °C. The powders were then encapsulated with silica particles using the Stöber process to prevent the oxidation of Eu²⁺ without a reducing atmosphere during calcination. The obtained coated and uncoated particles were then characterized using SEM, TEM–EDX, XRD and photoluminescence analysis to determine the effect of the core–shell structure on the luminescence properties of the phosphors. Finally, the obtained phosphor-silica core–shell particles will be attached to the surface of four different plant species commonly grown indoors using a mixture of natural oils and waxes as adhesive. The effect of the addition of phosphor as an external light source on the amount of carbon dioxide production of the plants will be monitored and compared to a control specimen without the phosphor as well as with other artificial light sources.     

Effect of nano-sized SiO2 on the optical properties of YVO4:Eu3+ phosphors

In this paper, YVO₄: 1%Eu³⁺ phosphor was synthesized via solid state method at 1100 °C. Then TEOS was used as the source of silica, to coat the phosphors, using sol–gel approach. HRTEM analysis confirmed the formation of adhered and smooth coating layer with the thickness of 40–50 nm. From the experiments and characterizations, we found that although the amounts of added SiO₂ to the phosphors were not remarkable, but it resulted in enhancement of photoluminescence properties. Interestingly, under the excitation wavelength of 310 nm, the efficiency of the phosphors increased by about 20%. Also, a considerable effect of coating layer on decrease in surface oxygen vacancies was studied using ESR technique. Finally it was found that SiO₂ coating of YVO₄:Eu³⁺ phosphors, improves both chemical stability and thermal quenching, effectively.     

A potential red-emitting phosphor CaSrAl2SiO7:Eu for near-ultraviolet light-emitting diodes

A novel red-emitting phosphor CaSrAl₂SiO₇:Eu³⁺ was firstly synthesized through the high temperature solid state reaction at 1300 °C. The structure, diffuse reflection spectra, photoluminescence spectra, color-coordinate parameters and quantum efficiencies (QE) of phosphors were investigated. The obtained CaSrAl₂SiO₇:Eu³⁺ phosphors have the same structure with that of the Ca₂Al₂SiO₇ and Sr₂Al₂SiO₇ phosphor, which have the melilite structure. Optical properties were studied as a function of Eu³⁺ concentration x, when x>0.14, the intensity of absorption of the f–f transitions of Eu³⁺ at 393 nm is stronger than that of the broad charge transfer transition band (CTB) around 254 nm, and which matches well with the output lights of NUV–LEDs, whereas, the concentration of Eu³⁺x≤0.14, the absorption of 393 nm is weaker than that of CTB. The underlying reason of Eu³⁺ concentration on their luminescent properties was investigated and discussed in detail. As a result, comparing with the commercial red phosphor Y₂O₂S:Eu³⁺, the CaSrAl₂SiO₇:xEu³⁺ (x>0.14) phosphor exhibited excellent color purity and much higher brightness and could be considered as promising red phosphors for NUV–LEDs.     

Synthesis and characterization of spherical core-shell particles SiO2@AgEu(MoO4)2

Submicron spherical SiO₂ particles have been coated with AgEu(MoO₄)₂ phosphor layers by a sol-gel process, followed by surface reaction at high temperature, to get core/shell structured SiO₂@AgEu(MoO₄)₂ particles. X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) were used to characterize the structure and morphology of the resulted core-shell phosphors. The luminescent properties of the core-shell structured phosphors have also been measured at room temperature, and their photoluminescence (PL) spectra are similar to the pure AgEu(MoO₄)₂ phosphor prepared by the same sol-gel method exhibiting red emission.     

Synthesis,characterization and photoluminescence of Eu3+, Ce3+ co-doped CaLaAl3O7 phosphors

Powder phosphors of CaLa_{1? x} Eu _x Al₃O₇, CaLa_{1? x} Ce _x Al₃O₇ and CaLa_{0.99? x} Eu _x Ce_0.01Al₃O₇, where x = 0.01, 0.03, 0.05 and 0.07, were prepared by a combustion method. The powders were well characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and photoluminescence techniques. Emission spectra of Eu³⁺-doped powder phosphors showed strong red emission at 613 nm (⁵D₀ → ⁷F₂); no concentration quenching was observed. Generally, Ce³⁺ acts as an efficient sensitizer when doped with other trivalent lanthanide ions. However, interestingly, in the CaLaAl₃O₇ powder phosphors, the addition of Ce³⁺ with Eu³⁺ exhibited an adverse effect–decreased photoluminescence intensity. The reasons for this behavior are discussed.     

Sol-gel growth of Gd2MoO6:Eu nanocrystalline layers on SiO2 spheres (SiO2@Gd2MoO6:Eu) and their luminescent properties

SiO₂@Gd₂MoO₆:Eu³⁺ core-shell phosphors were prepared by the sol-gel process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectra (EDS), transmission electron microscopy (TEM), photoluminescence (PL) spectra as well as kinetic decays were used to characterize the resulting SiO₂@Gd₂MoO₆:Eu³⁺ core-shell phosphors. The XRD results demonstrate that the Gd₂MoO₆:Eu³⁺ layers on the SiO₂ spheres begin to crystallize after annealing at 600 °C and the crystallinity increases with raising the annealing temperature. The obtained core-shell phosphors have a near perfect spherical shape with narrow size distribution (average size ca. 600 nm), are not agglomerated, and have a smooth surface. The thickness of the Gd₂MoO₆:Eu³⁺ shells on the SiO₂ cores could be easily tailored by varying the number of deposition cycles (50 nm for four deposition cycles). The Eu³⁺ shows a strong PL luminescence (dominated by ⁵D₀-⁷F₂ red emission at 613 nm) under the excitation of 307 nm UV light. The PL intensity of Eu³⁺ increases with increasing the annealing temperature and the number of coating cycles.     

Investigation of crystal/electronic structure effects on the photoluminescence properties in the BaO–SiO2:Eu2+ systems

BaO–SiO₂:Eu²⁺ phosphors with different Ba/Si mole ratio were prepared using a solid-state reaction method, and their crystal structure dependent-photoluminescence properties were investigated. The prepared phosphor powders were characterized using X-ray diffraction (XRD), field-emission electron microscopy (FE-SEM) and fluorescence spectroscopy. The emission band of the Eu²⁺ activator varied from orange to blue with varying crystal structure of the host materials, which was related to the crystal field splitting of the Eu 5d orbitals. These emission color changes were examined by calculating the electronic band structure properties such as the density of the state. Moreover, the host material with Ba/Si=1 (BaSiO₃) for Eu²⁺, which exhibited a yellow emission when excited with near UV light, was further characterized for enhancing its emission intensity.     

Synthesis and Optical Characterization of Red-Emitting BaTa<Subscript>2</Subscript>O<Subscript>6</Subscript>:Eu<Superscript>3+</Superscript> Phosphors

Undoped and Eu³⁺ doped BaTa₂O₆ phosphors were synthesized via solid state reaction method and characterized by using XRD, SEM-EDS and photoluminescence (PL) analyses. The XRD results revealed that the crystal structure of BaTa₂O₆ allowed up to 10 mol% levels of Eu³⁺ ions due to the TTB characteristic network of adjacent octahedrals. SEM-EDS analyses confirmed the formation of BaTa₂O₆ structure and EuTaO₄ secondary phase. BaTa₂O_6:Eu³⁺ phosphors exhibited orange and red emissions at 592.2 nm and 615.7 nm in the visible region respectively. The Commission Internationale d’Eclairage (CIE) chromaticity coordinates of the BaTa₂O_6:Eu³⁺ phosphors that excited at λ _ex = 400 nm ranged from orangish-red to pinkish-red depending on increasing Eu³⁺ concentration.     

Effects of non-stoichiometry on crystallinity, photoluminescence and afterglow properties of Sr2MgSi2O7:Eu, Dy phosphors

Eu²⁺, Dy³⁺ co-doped Sr₂MgSi₂O₇ phosphors with deficient, stoichiometric or excess amounts of silicon are prepared by solid-state reaction. XRD and SEM results indicate that all the samples studied are found to be free from impurities and samples with SiO₂ excess possess better crystallinity and larger grain size. Photoluminescence reveals that the position of Eu²⁺ emission is not changed with various compositions. However, both photoluminescence intensity and afterglow properties are increased by an incorporation of excess SiO₂ and are decreased by SiO₂ deficiency. The thermoluminescence results show that the corresponding increase or decrease in afterglow is associated with trap density, but no change in trap depth. The underlying reason of photoluminescence and afterglow enhancement is discussed.     

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.     

Eu3+ emission in SrAl2B2O7 based phosphors

Eu³⁺ doped SrAl₂B₂O₇ phosphors were fabricated by the wet method. The structures of the phosphors were characterized by XRD. The doping content of Eu³⁺ ions in SrAl₂B₂O₇:Eu³⁺ phosphors are 1%, 4%, 6%, 8%, 10% (molar fraction), respectively. Luminescence properties were analyzed by measuring the excitation and photoluminescence spectra. The luminescent properties of SrAl₂B₂O₇:Eu³⁺ phosphors are discussed. It is shown that from 4% to 6% of doping content of Eu³⁺ ions under 392 nm excitation in SrAl₂B₂O₇:Eu³⁺ phosphors is optimum.     

Synthesis and luminescence properties of Eu3+,Bi3+-doped BaWO4 phosphors

BaWO₄:Eu³⁺,Bi³⁺ phosphors have been prepared by the conventional high-temperature solid-state reaction and chemical precipitation. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) technologies. When the phosphors are prepared by the high-temperature solidstate reaction, Bi³⁺ doping into BaWO₄:Eu³⁺ can increase the emission intensity of 613 nm. However, maximum emission at about 595 nm was observed in Eu³⁺,Bi³⁺-doped BaWO₄ phosphors prepared by the chemical precipitation. The decay constants (monitored at 595 and/or 613 nm) are within 45–100 s. The color purity of the Ba_0:865WO₄: Eu_0:11,Bi_0:025 phosphor (prepared by chemical precipitation) was 100%. The emission mechanism of Eu³⁺,Bi³⁺ in the BaWO₄ phosphors is briefly discussed.     

White light-emitting Mg0.1Sr1.9SiO4:Eu phosphors

Divalent europium-activated strontium orthosilicate Sr₂SiO₄:Eu²⁺ and Mg_0.1Sr_1.9SiO₄:Eu²⁺ phosphors were synthesized through the solid-state reaction technique. Their luminescent properties under ultraviolet excitation were investigated. The X-ray diffraction (XRD) results show that these phosphors are of α′-Sr₂SiO₄ phase with a trace of β-Sr₂SiO₄. Doping of Eu²⁺ ion into the crystal lattice results in the lattice constant being expended, while Mg²⁺ makes the lattice constant shrinking. A solid solution with the same crystal structure is formed when Eu²⁺ or Mg²⁺ substitutes part of Sr²⁺ ions and occupies the same lattice sites. The Sr₂SiO₄:Eu²⁺ phosphors show two emission spectra peaked at 535 and 473 nm originated from the 5d-4f transition of Eu²⁺ ion doped in two different Sr²⁺ sites in the host lattice. By substitution of 0.1 mol of Sr²⁺ with Mg²⁺, these two emission bands are tuned to be in the blue and yellow region (459 and 564 nm for Mg_0.1Sr_1.88SiO₄:Eu_0.02), respectively. The tuning effect is discussed. With a combination of the blue and yellow emission bands the phosphors show white color, indicating that these phosphors may become promising phosphor candidates for white light-emitting diodes (LEDs).     

Luminescent properties of Li2 (Ca0.99, Eu0.01) SiO4: B3+ particles as a potential bluish green phosphor for ultraviolet light-emitting diodes

In our study, the 1% mol Eu²⁺ doped Li₂CaSiO₄: B³⁺ phosphors were prepared by the combustion method as fluorescent material for ultraviolet, light-emitting diodes (UV-LEDs) used as a light source. The properties of Li₂ (Ca_0.99, Eu_0.01) SiO₄: B³⁺ phosphors with urea concentration, doping boric acid and a series of initiating combustion temperature were investigated. The crystallization and particle sizes of Li₂ (Ca_0.99, Eu_0.01) SiO₄: B³⁺ has been investigated by using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). Luminescence measurements showed that the phosphors can be efficiently excited by UV to the visible region, and exhibited bluish green light with a peak of 480 nm. The results showed that the boric acid was effective in improving the luminescence intensity of Li₂ (Ca_0.99, Eu_0.01) SiO₄: B³⁺ and the optimum molar ratio of boric acid to calcium nitrate was about 0.06. The optimized phosphors Li₂ (Ca_0.99, Eu_0.01) SiO₄: B _0.06 ³⁺ showed 180% improved emission intensity compared with that of the Li₂ (Ca_0.99, Eu_0.01) SiO₄ phosphors under ultraviolet (λ_ex =287 nm) excitation.      

Tunable bluish green to yellowish green Ca<Subscript>2(1-x)</Subscript>Sr<Subscript>2x</Subscript>Al<Subscript>2</Subscript>SiO<Subscript>7</Subscript>:Eu<Superscript>2+</Superscript> phosphors for potential LED application

A series of solid solutions with a general formula of Ca_2(1-x)Sr_2xAl₂SiO₇:Eu²⁺ were synthesized by a high temperature solid state reaction. The structure, diffuse reflection spectra, photoluminescence spectra, color-coordinate parameters and lifetimes of phosphors were investigated. XRD results show that Ca₂Al₂SiO₇ is totally miscible with Sr₂Al₂SiO₇. These solid solution phosphors show a broad excitation band of 350–450 nm that matches well with the output lights of near-UV LEDs and tunable emission from bluish green to yellowish green. These optical properties originate from the 4f⁷–4f⁶5d transition of Eu²⁺ ions. The crystal field strength was considered to be tailed by controlling the host composition, which leads to the shift of absorption band and emission band, and the varying of color coordinates. PACS  78.55.-m; 42.70.-a; 61.05.C-     

Blue-emitting long-lasting phosphor, Sr3Al10SiO20:Eu,Ho

A novel blue-emitting long-lasting phosphor Sr₃Al₁₀SiO₂₀:Eu²⁺,Ho³⁺ is prepared by the conventional high-temperature solid-state technique and their luminescent properties are investigated. XRD, photoluminescence (PL) and thermoluminescence (TL) are used to characterize the synthesized phosphors. These phosphors are well crystallized by calcinations at 1500-1600 °C for 3 h. The phosphor emits blue light and shows long-lasting phosphorescence after it is excited with 254/365 nm ultraviolet light. TL curves reveal the introduction of Ho³⁺ ions into the Sr₃Al₁₀SiO₂₀:Eu²⁺ host produces a highly dense trap level at appropriate depth, which is the origin of the long-lasting phosphorescence in this kind of material. The long-lasting phosphorescence lasts for nearly 6 h in the light perception of the dark-adapted human eye (0.32 mcd/m²). All the results indicate that this phosphor has promising potential practical applications.     

Luminescent properties of Eu3+ and Sm3+ activated M2SiO4 (M=Ba,Sr and Ca) red-emitting phosphors for WLEDs

Eu³⁺ and Sm³⁺ activated M₂SiO₄ (M=Ba, Sr and Ca) red-emitting phosphors were synthesized by a solid state reaction. The results of XRD and SEM measurements show that the samples are single phase and have irregular shape. The excitation and emission spectra indicate that these phosphors were effectively excited by ultraviolet (395 nm) and blue (466 nm) light and exhibited red performance. The charge compensator R⁺ (R⁺=Li⁺, Na⁺ and K⁺) injecting into the host efficiently enhanced the luminescence intensity of the M₂SiO₄: Eu³⁺ and M₂SiO₄: Sm³⁺ phosphors. The emission intensity of M₂SiO₄: Eu³⁺ and Sm³⁺ doping Li⁺ were higher than that of Na⁺ or K⁺.