Synthesis and luminescence properties of clew-like CaMoO4:Sm, Eu

Eu³⁺ and Sm³⁺ co-doped CaMoO₄ microclews have been successfully synthesized via a facile hydrothermal method directly in surfactant-free environment. The as-prepared phosphor present clew-like agglomerates composed of 40 nm nanosheets under the moderated reaction temperature. The red phosphor CaMoO₄:Eu³⁺, Sm³⁺ can generate a strong absorption line at 405 nm, originating from ⁶H_5/2 → ⁶P_5/2 transition of Sm³⁺, which is suitable for the emission of the near-ultraviolet light-emitting diodes (∼400 nm). Energy transfer between Sm³⁺ and Eu³⁺ is detected from the varied photoluminescence spectra with different Eu³⁺ concentrations and the energy transfer mechanism is clarified via the photoluminescence spectra. When Sm³⁺ is excited (405 nm), the electron is excited from ⁶H_5/2 to ⁶P_5/2, and then relaxed to ⁴G_5/2. It jumps from ⁴G_5/2 to the lower levels corresponding to the emissions of Sm³⁺; meanwhile, the transfers from ⁴G_5/2 state of Sm³⁺ ion to ⁵D₀ state of Eu³⁺ ion come out. The transition of ⁵D₁ → ⁷F_J (J = 0, 1, 2) does not appear indicating that the transfer from ⁴G_5/2 state of Sm³⁺ to ⁵D₀ state rather than ⁵D₁ state of Eu³⁺ is the energy transfer pathway.     

Luminescent and magnetic properties of YVO4:Ln@Fe3O4 (Ln = Eu or Dy) nanocomposites

A series of core-shell bifunctional magnetic-optical YVO₄:Ln³⁺@Fe₃O₄ (Ln³⁺ = Eu³⁺ or Dy³⁺) nanocomposites have been successfully synthesized via two-step method. The nanocomposites have the advantage of high magnetic responsive and unique luminescence properties. The structure, luminescent and magnetic properties of the nanocomposites were investigated by XRD, TEM, PL and VSM. The maximum emission peaks of the nanocomposites are at 618 nm (doping Eu³⁺), 574 nm (doping Dy³⁺). The special saturation magnetization of the nanocomposites is 54 emu/g. The diameter of the nanocomposites is 400-900 nm.     

Effect of surfactants on morphology and luminescent properties of CaMoO4: Eu red phosphors

Using polyethylene glycol (PEG), Tween-80, sodium dodecyl sulphonate (SAS) and cetyltrimethylammonium bromide (CTAB) as surfactants, CaMoO₄: Eu³⁺ red phosphors were prepared by co-precipitation method and their morphology, structure and luminescent properties were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD) and fluorescence spectrometer. The results showed that the introduction of surfactants did not change the crystal structure of CaMoO₄: Eu³⁺ phosphors, but greatly influenced their morphology. CaMoO₄: Eu³⁺ red phosphors were prepared with 5 wt% PEG20000 having small-sized and regularly spherical morphology, and there were greater improvement in the luminescent intensity than phosphors prepared with other surfactants.     

Hydrothermal synthesis of YBO3:Tb microflowers and their luminescence properties

Three-dimensional flowerlike YBO₃:Tb³⁺ phosphors have been successfully prepared by an efficient surfactant-free hydrothermal process directly without further sintering treatment. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDS) spectrometry, selected area electron diffraction (SAED), photoluminescence (PL) spectra were used to characterize the samples. The as-obtained samples present flowerlike agglomerates composed of nanoflakes with thickness of 20 nm and high crystallinity in spite of the moderate reaction temperature of 180 °C. The reaction mechanism has been considered as a dissolution/precipitation mechanism; the self-assembly evolution process has been proposed on homocentric layer-by-layer growth style. The different luminescent intensity with different molar ratio of Y-Tb [Y:Tb = 8:2; 7:3; 6:4; 5:5; 4:6], YBO₃:Tb³⁺ phosphors exhibit different light (white, red, green) under ultraviolet excitation, which might find potential applications in the fields such as light display systems and optoelectronic devices.     

Preparation of Dy-activated strontium orthosilicate (Sr2SiO4:Dy) phosphors and its photoluminescent properties

Dy³⁺-activated β/α′-Sr₂SiO₄ phosphors were successfully prepared by solid-state reaction method with ammonium chloride (NH₄Cl) as the flux. The influences of calcination temperatures, amounts of NH₄Cl and the concentrations of Dy³⁺ on phase composition, morphology and the photoluminescent properties of as-prepared powders were investigated in detail. The β and α′ phases of Sr₂SiO₄ were obtained with 1 wt% and 2-5 wt% NH₄Cl, respectively, as the sintered condition was at 1000 °C for 4 h. With increasing the amount of NH₄Cl, the morphology of phosphors changed from needlelike to regular polyhedron shape and the colors of the Sr₂SiO₄:Dy³⁺ phosphors changed from blue-green to white. The luminescence intensity of ⁴F_9/2 → ⁶H_15/2 transition was slightly higher than that of ⁴F_9/2 → ⁶H_13/2 (ΔL = 2, ΔJ = 2) transition owing to the low-symmetry around Dy³⁺ ions. The optimum concentration of Dy³⁺ was 2.0 mol% and the concentration quenching were caused by the d-d interaction and a cross relaxation. The yellow-to-blue intensity ratio (Y/B) of Dy³⁺ emission did not to change with varying the Dy³⁺ concentration using Li⁺ ions for charge compensation. These indicate that this phosphor can be used as a potential candidate for the phosphor-converted white LEDs with a UV chip.     

Preparation and characteristics of Fe3O4@YVO4:Eu bifunctional magnetic-luminescent nanocomposites

A facile direct precipitation method has been developed for the synthesis of bifunctional magnetic-luminescent nanocomposites with Fe₃O₄ nanoparticles as the core and YVO₄:Eu³⁺ as the shell. Transmission electron microscopy (TEM) images revealed that the obtained bifunctional nanocomposites had a core-shell structure and a spherical morphology. The average size was ∼150 nm, and the thickness of the shell was ∼15 nm. The X-ray diffraction (XRD) patterns showed that a cubic spinel structure of Fe₃O₄ core and a tetragonal phase of YVO₄ shell were obtained. Fourier transform infrared (FT-IR) spectra confirmed that the YVO₄:Eu³⁺ had been successfully deposited on the surface of Fe₃O₄ nanoparticles. Photoluminescence (PL) spectra indicated that the nanocomposites displayed a strong red characteristic emission of Eu³⁺. Magnetic measurements showed that the obtained bifunctional nanocomposites exhibited superparamagnetic behavior at room temperature. Therefore, the bifunctional nanocomposites are expected to develop many potential applications in biomedical fields.     

Sol-gel preparation and characterization of uniform core-shell structured LaInO3:Sm/Tb@SiO2 phosphors

The core-shell structured LaInO₃:Ln³⁺@SiO₂ (Ln³⁺ = Sm³⁺, Tb³⁺) phosphors were realized by coating LaInO₃:Ln³⁺ phosphors on the surface of silica microspheres via a modified Pechini sol-gel process. The phase, structure, morphology, and fluorescent properties of the materials were well characterized by means of X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Fourier transform IR spectroscopy (FT-IR), photoluminescence (PL) spectra, cathodoluminescence (CL) spectra, and the kinetic decays, respectively. The results reveal that the obtained core-shell structured phosphors consist of amorphous silica core and crystalline LaInO₃:Ln³⁺ shell, which keep the uniform spherical morphology of pure silica spheres with narrow size distribution. Upon excitation by ultraviolet (UV) irradiation or electron beam, the phosphors show the characteristic emission lines of Sm³⁺ (⁴G_5/2-⁶H_5/2,7/2,9/2, orange) in LaInO₃:Sm³⁺@SiO₂ and characteristic emissions of Tb³⁺ (⁵D₄-⁷F_6,5,4,3, green) in LaInO₃:Tb³⁺@SiO₂, respectively. This kind of phosphors may have potential applications in field emission displays (FEDs) based on their uniform shape, low-cost synthetic route, and diverse luminescent properties.     

Vacuum ultraviolet spectroscopic properties of KSrPO4:Tb

KSrPO₄:Tb³⁺ phosphors were prepared by a solid-state method and their photoluminescence properties were investigated under vacuum ultraviolet excitation. In the excitation spectrum monitoring at 544 nm, the band in the region of 120-162 nm can be attributed to be the overlap of host absorption and charge transfer transition of O²⁻ → Tb³⁺, and the band ranging from 162 to 300 nm was assigned to the f-d transition of Tb³⁺. The photoluminescence spectrum shows that the phosphors exhibited a strong green emission around 544 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺ under the excitation of 147 nm. Optimal emission intensity was obtained when x = 7% in KSr_1-xPO₄:xTb³⁺ and the luminescent chromaticity coordinates were calculated to be (x = 0.317, y = 0.522) for KSr_0.93PO₄:7%Tb³⁺.     

Luminescence properties of SrIn2O4:Eu incorporated with Gd or Sm ions

The photoluminescence (PL) properties of SrIn₂O₄:Eu³⁺,Gd³⁺ and SrIn₂O₄:Eu³⁺,Sm³⁺ are investigated in this work. When the Gd³⁺ ions are introduced in this compound, the average distance metal-oxygen is increased, and then the vibration of lattice is decreased. It results in that the nonradiation of Eu³⁺ is decreased. Therefore, the emissions of SrIn₂O₄:Eu³⁺ are increased. However, little of energy transfer occurs from Gd³⁺ to Eu³⁺ ions. When the Sm³⁺ ions are introduced into SrIn₂O₄:Eu³⁺, the energy transfers occur from the CTS of O²⁻-Sm³⁺ to Sm³⁺ and Eu³⁺ ions, from the host absorption to Eu³⁺ ions, and from Sm³⁺ to Eu³⁺ ions, but not from the host absorption to Sm³⁺ ions.     

Preparation and luminescent properties of Eu doped Sr3La(PO4)3 phosphor

Eu²⁺-doped Sr₃La(PO₄)₃ phosphors were synthesized by solid-state reaction method. Their luminescent properties were investigated. The phosphor could be excited by ultraviolet light effectively. The emission spectra exhibit two emission peaks located at 418 nm and 500 nm, respectively. These two peaks originated from two different luminescent centers, respectively. One is nine-coordinated Eu(I) center, other is six-coordinated Eu(II) center. It was found that the doping concentration of Eu²⁺ ions affected the shape of emission spectra. As the doping concentration increasing, Eu²⁺ ions are more likely to form Eu(I) luminescent centers and emit purple light.     

Synthesis and phosphorescence properties of Mn, La and Ho in MgAl2Si2O8

Mn⁴⁺, La³⁺ and Ho³⁺ doped MgAl₂Si₂O₈-based phosphors were first synthesized by solid state reaction. They were characterized by thermogravimetry (TG), differential thermal analysis (DTA), X-ray powder diffraction (XRD), photoluminescence (PL) and scanning electron microscopy (SEM). The phosphors were obtained at about 1300 °C. They showed broad red and fuchsia-pink emission bands in the range of 610-715 nm and had a different maximum intensity when activated by UV illumination. Such a fuchsia-pink emission can be attributed to the intrinsic d-d transitions of Mn⁴⁺.     

Infrared spectroscopic properties of Tm, Ho:NaY(WO4)2 single crystals

Singly doped and Tm³⁺/Ho³⁺ co-doped NaY(WO₄)₂ single crystals were grown successfully by Czochralski method. The room temperature polarized absorption and fluorescence spectra as well as the decay curves were measured. Spectroscopic parameters related to the laser operation around 2.0 μm via the ³F₄ → ³H₆ (Tm³⁺) and ⁵I₇ → ⁵I₈ (Ho³⁺) transitions have been evaluated. The energy level scheme and energy transfer processes of Tm³⁺ and Ho³⁺ were analyzed.     

Synthesis and characterization of YNbTiO6:Dy phosphor

A novel yellow phosphor of Dy³⁺ activated YNbTiO₆ has been prepared by high temperature solid-state reaction, and its luminescence properties have been investigated. The excitation spectra monitored at 575 nm have several strong peaks from 350 to 480 nm. Under 365 nm excitation, the emission spectra of composition-optimized (Y_0.9Dy_0.1)NbTiO₆ phosphor exhibit a dominant peak located at about 575 nm with the Commission Internationale de l’Eclairage (CIE) chromaticity coordinates of (0.385, 0.411). The energy transfer between Dy³⁺ is found to be through exchange interaction.     

Concentration-dependent luminescence and energy transfer of flower-like Y2(MoO4)3:Dy phosphor

Flower-like Y₂(MoO₄)₃:Dy³⁺ phosphors have been synthesized via a co-precipitation approach with the aid of β-cyclodextrin. The crystal structure and morphology of the phosphors were characterized by XRD (X-ray diffraction) and FE-SEM (field emission scanning electron microscopy), respectively. The excitation and emission properties of the phosphors were examined by fluorescence spectroscopy. The dependence of color coordinates on the Dy³⁺ doping concentration was analyzed. The energy transfer mechanism between Dy³⁺ ions was studied based on the Huang's theory, I-H and Van Uitert's models. It was concluded simultaneously from these three routes that the electric dipole-dipole interaction between Dy³⁺ ions is the main physical mechanism for the energy transfers between Dy³⁺.     

Low-voltage cathodoluminescence properties of green-emitting ZnAl2O4:Mn nanophosphors for field emission display

A low-cost ZnAl₂O₄:Mn²⁺ green nanophosphor for field emission display (FED) was successfully synthesized by the coprecipitation method and a two-step firing, firstly calcining at 1200 °C for 2 h in air and then annealing at 900 °C for 3 h in flowing NH₃ gas. The effects of the preparation process and the Mn²⁺ concentration on optical properties of ZnAl₂O₄:Mn²⁺ were investigated. The phase composition, particle morphology, photoluminescence (PL) spectra of the ZnAl₂O₄:Mn²⁺ phosphor as well as low-voltage field emission properties of the FED device prepared by using the synthesized ZnAl₂O₄:Mn²⁺ phosphor were examined. Similar to ZnGa₂O₄:Mn²⁺, Mn²⁺-doped ZnAl₂O₄ showed two green emission bands centered at 508 and 517 nm, respectively, which originate from ⁴T₁(⁴G)→⁶A₁(⁶S) transitions of Mn²⁺ on T_d and O_h sites. The PL intensity reached the maximum at 0.5 at.% Mn²⁺. Under the low-voltage excitation, the FED device exhibited bright green emission, high voltage brightness saturation, and high color purity.     

Tunable red-green-blue multicolor luminescence in Yb/Tm/Ho:Gd3Ga5O12 nano-crystals

The Yb³⁺/Tm³⁺/Ho³⁺: Gd₃Ga₅O₁₂ nano-crystals have been successfully prepared via a citric acid complex procedure. The luminescence spectra were measured and the up-conversion processes were discussed. By means of adjusting the doping concentrations of Yb³⁺/Tm³⁺/Ho³⁺, the red-green-blue up-conversion luminescence changed obviously. Results indicated that the ratio of red-green-blue up-conversion emissions enhanced heavily with the increasing concentrations of Tm³⁺ doped in the Yb³⁺/Tm³⁺/Ho³⁺:Gd₃Ga₅O₁₂ nano-crystals, which was rooted in the three-photon resonant cross relaxation processes(¹G₄ (Tm) + ⁵I₇ (Ho) → ³H₅ (Tm) + ⁵S₂ (Ho)). The tunable red-green-blue luminescence could be used in the fields of display, illumination, and photonics such as the white light generation.     

Synthesis and characterization of spherical and rod like nanocrystalline Nd2O3 phosphors

Spherical and rod like nanocrystalline Nd₂O₃ phosphors have been prepared by solution combustion and hydrothermal methods respectively. The Powder X-ray diffraction (PXRD) results confirm that hexagonal A-type Nd₂O₃ has been obtained with calcination at 900 °C for 3 h and the lattice parameters have been evaluated by Rietveld refinement. Surface morphology of Nd₂O₃ phosphors show the formation of nanorods in hydrothermal synthesis whereas spherical particles in combustion method. TEM results also confirm the same. Raman studies show major peaks, which are assigned, to F_g and combination of A_g + E_g modes. The PL spectrum shows a series of emission bands at ∼326-373 nm (UV), 421-485 nm (blue), 529-542 nm (green) and 622 nm (red). The UV, blue, green and red emission in the PL spectrum indicates that Nd₂O₃ nanocrystals are promising for high performance materials and white light emitting diodes (LEDs).     

Preparation and luminescent characteristics of Sr3RE2(BO3)4:Dy (RE = Y, La, Gd) phosphors for white LED

Dysprosium-activated Sr₃RE₂(BO₃)₄ (RE = Y, La, Gd) phosphors were synthesized by a high temperature solid-state reaction method. The phase uniformity of the phosphors was characterized by X-ray powder diffraction (XRD) and the luminescence characteristics were investigated. The excitation spectra at 575 nm emission show strong spectral bands in the region of 300-500 nm. The emission spectra of the phosphors with 365 nm excitation show three bands centered at 484 nm, 575 nm and 680 nm, which originate from the transitions of ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 of Dy³⁺, respectively. The effect of Dy³⁺ concentration on the emission intensity of the phosphors was investigated. The fluorescence decay curves for ⁴F_9/2 → ⁶H_13/2 excited at 365 nm and monitored at λ_em of 575 nm were measured. The decay times decreased slowly with increasing Dy³⁺ doping concentration due to a trap capturing to resonance fluorescence transfer of the activated ions and due to the exchange interactions between activated ion pairs. In order to determine the type of interaction between activated ions, the concentration dependence curves (lg(I/x) versus lg x) of Sr₃RE₂(BO₃)₄:Dy³⁺ (RE = Y, La, Gd) were plotted. The concentration quenching mechanism of the ⁴F_9/2 → ⁶H_13/2 (575 nm) transition of Dy³⁺ is the d-d interaction. All results indicate these phosphors are promising white-color luminescent materials.     

Structural and luminescent properties of nano-sized NaGdF4:Eu synthesised by wet-chemistry route

Hexagonal Eu³⁺:NaGdF₄ fluoride with average grains size of 20 nm was obtained from solution by a co-precipitation method. Morphology of the obtained powder was examined by XRD and TEM methods. Absence of the Eu³⁺-O²⁻ charge-transfer band, expected in excitation spectrum at 260 nm indicates, that oxygen ions are not incorporated into a fluoride lattice. As-received fluoride contains considerable amounts of the water molecules, adsorbed at the surface of the material, which may be relatively easily removed by heating the powder at 300 °C. Thermal treatment at 650 °C is sufficient for removing of the OH⁻ groups built into fluoride lattice. Influence of method of synthesis as well as oxygen, water molecules and OH⁻ groups content on optical properties of the obtained phosphors is investigated and discussed by comparison with optical properties of the Eu³⁺:NaGdF₄ fluoride synthesised by a solid-state reaction.     

Photoluminescence and thermoluminescence studies of Mg2SiO4:Eu nano phosphor

Nanoparticles of Eu³⁺ doped Mg₂SiO₄ are prepared using low temperature solution combustion technique with metal nitrate as precursor and urea as fuel. The synthesized samples are calcined at 800 °C for 3 h. The Powder X-ray diffraction (PXRD) patterns of the sample reveled orthorhombic structure with α-phase. The crystallite size using Scherer's formula is found to be in the range 50-60 nm. The effect of Eu³⁺ on the luminescence characteristics of Mg₂SiO₄ is studied and the results are presented here. These phosphors exhibit bright red color upon excitation by 256 nm light and showed the characteristic emission of the Eu³⁺ ions. The electronic transition corresponding to ⁵D₀ → ⁷F₂ of Eu³⁺ ions (612 nm) is stronger than the magnetic dipole transition corresponding to ⁵D₀ → ⁷F₁ of Eu³⁺ ions (590 nm). Thermoluminescence (TL) characteristics of γ-rayed Mg₂SiO₄:Eu³⁺ phosphors are studied. Two prominent and well-resolved TL glows with peaks at 202 °C and 345 °C besides a shoulder with peak at ∼240 °C are observed. The trapping parameters-activation energy (E), order of kinetics (b) and frequency factor (s) are calculated using glow curve shape method and the results obtained are discussed.