Luminescence in trivalent rare earth activated Sr4Al2O7 phosphor

In this paper we report the combustion synthesis of trivalent rare-earth (RE³⁺ = Dy, Eu and Ce) activated Sr₄Al₂O₇ phosphor. The prepared phosphors were characterized by the X-ray powder diffraction (XRD) and photoluminescence (PL) techniques. Photoluminescence emission peaks of Sr₄Al₂O₇:Dy³⁺ phosphor at 474 nm and 578 nm in the blue and yellow region of the spectrum. The prepared Eu³⁺ doped phosphors were excited by 395 nm then we found that the characteristics emission of europium ions at 615 nm (⁵D₀?⁷F₂) and 592 nm (⁵D₀?⁷F₁). Photoluminescence (PL) peaks situated at wavelengths of 363 and 378 nm in the UV region under excitation at around 326 nm in the Sr₄Al₂O₇:Ce³⁺ phosphor.     

Synthesis and luminescent properties of Y6W2O15:Eu phosphors

In this paper, a novel phosphor, Y₆W₂O₁₅:Eu³⁺ was synthesized by thermal decomposition and phase transition of its decatungstate gel precursor. With stepwise increase of temperature to 750 °C, a crystalline phase of Y₆W₂O₁₅:Eu³⁺forms that gives intense red emission when excited at 466 nm, the emission is attributed to the Eu³⁺ ions transitions from ⁵D₀ excited states to ⁷F_J (J=0-4) ground states. The long excitation wavelength proves the Eu³⁺ transition follows the photoexcitation of the oxygen-metal (O→W lmct) charge transfer bands in yttrium tungstate. Some structural information regarding Y₆W₂O₁₅ provided by luminescence is in accord with that characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The long-wavelength excitation properties of this material may find application in the production of red phosphors for white light-emitting diodes (LEDs).     

Synthesis of Y2Si2O7:Eu nanocrystal and its optical properties

Nanocrystalline Y₂Si₂O₇:Eu phosphor with an average size about 60 nm is easily prepared using silica aerogel as raw material under ultrasonic irradiation and annealing temperature at 300-600 °C and this nanocrystalline decomposes into Y₂O₃:Eu and silica by heat treatment at 700-900 °C. The excitation broad band centered at 283 and 254 nm results from Eu³⁺ substituting for Y³⁺ in Y₂Si₂O₇ and Y₂O₃/SiO₂, respectively. Compared with Y₂O₃:Eu/SiO₂ crystalline, the PL excitation and emission peaks of Y₂Si₂O₇:Eu nanocrystalline red-shift and lead to the enhance of its luminescence intensity due to the different chemical surroundings of Eu³⁺ in above nanocrystallines. The decrease of PL intensity may be ascribed to quenching effect resulting from more defects in Y₂O₃:Eu/SiO₂ crystalline.     

NIR to visible upconversion in Er/Yb co-doped CaYAl3O7 phosphor obtained by solution combustion process

Using the combustion synthesis, CaYAl₃O₇:Er³⁺ phosphor powders co-doped with Yb³⁺ have been prepared at low temperatures (550 °C) in a few minutes. Formation of the compound was confirmed by X-ray powder diffraction. Near-infrared to visible upconversion fluorescence emission in the Er³⁺ doped CaYAl₃O₇ phosphor powder has been observed. The effect of co-doping with triply ionized ytterbium in the CaYAl₃O₇:Er³⁺ phosphor has been studied and the process involved is discussed.     

Energy transfer and upconversion of Nd doped RbY2Cl7

Single crystals of Nd³⁺:RbY₂Cl₇ were grown by the Bridgman-Stockbarger method. The host crystal contains two slightly inequivalent Y³⁺ ions, each with an approximate C_2v site symmetry. Anti-Stokes emission from the ⁴G_7/2 and ⁴D_3/2 levels was observed after laser excitation of the ⁴F_3/2 and ⁴F_9/2 multiplets. Laser excitation at 413 cm⁻¹ or 453 cm⁻¹ above the ⁴F_3/2 multiplet resulted in emission from the ²P_1/2 level. Laser site-selective upconverted emission spectra have been measured, as well as their emission transients and power dependence. Possible excited state absorption and energy-transfer upconversion (ETU) mechanisms are proposed and discussed. Due to the smaller crystal field and a somewhat different energy level structure for the Nd³⁺ ions in RbY₂Cl₇ as compared with those observed for Nd³⁺ in fluoride or oxide hosts, the ⁴G_7/2 and ⁴D_3/2 multiplets are populated under ⁴F_3/2 excitation in a three and four step ETU process, respectively, instead of in a two and three step process as observed for the lighter hosts.     

Effect of Eu,Tb codoping on the luminescent properties of Y2O3 nanorods

Red-emitting Y₂O₃:Eu³⁺ and green-emitting Y₂O₃:Tb³⁺ and Y₂O₃:Eu³⁺, Tb³⁺ nanorods were synthesized by hydrothermal method. Their structure and micromorphology have been analyzed by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The photoluminescence (PL) property of Y₂O₃:Eu³⁺,Tb³⁺ phosphor was investigated. In the same host (Y₂O₃), upon excitation with ultraviolet (UV) irradiation, it is shown that there are strong emissions at around 610 and 545 nm corresponding to the forced electric dipole ⁵D₀-⁷F₂ transition of Eu³⁺ and ⁵D₄-⁷F₅ transition of Tb³⁺, respectively. Different qualities of Eu³⁺and Tb³⁺ ions are induced into the Y₂O₃ lattice. From the excitation spectrum, we speculate that there exists energy transfer from Tb³⁺ to Eu³⁺ ions .The emission color of powders reveals regular change in the separation of light emission. These powders can meet with the request of optical display material for different colors or can be potentially used as labels for biological molecules.     

Enhanced luminescence of Ca3B2O6:Dy phosphors by Na-codoping for LED applications

The Ca_2.95−yDy_0.05B₂O₆:yNa⁺ (0≤y≤0.20) phosphors were synthesized at 1100 °C in air by the solid-state reaction route. The as-synthesized phosphors were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), photoluminescence excitation (PLE), photoluminescence (PL) spectra and thermoluminescence (TL) spectra. The PLE spectra show the excitation peaks from 300 to 400 nm due to the 4f-4f transitions of Dy³⁺. This mercury-free excitation is useful for solid-state lighting and light-emitting diodes (LEDs). The emission of Dy³⁺ ions on 350 nm excitation was observed at 480 nm (blue) due to the ⁴F_9/2→⁶H_15/2 transitions, 575 nm (yellow) due to ⁴F_9/2→⁶H_13/2 transitions and 660 nm (red) due to weak ⁴F_9/2→⁶H_11/2 emissions. The PL results from the investigated Ca_2.95−yDy_0.05B₂O₆:yNa⁺ phosphors show that Dy³⁺ emissions increase with the increase of the Na⁺ codoping ions. The integral intensity of yellow to blue (Y/B) can be tuned by controlling Na⁺ content. By the simulation of white light, the optimal CIE value (0.328, 0.334) can be achieved when the content of Na⁺-codoping ions is y=0.2. The results imply that the Ca_2.95−yDy_0.05B₂O₆:yNa⁺ phosphors could be potentially used as white LEDs.     

Simultaneous three-photon absorption induced ultraviolet upconversion in Pr:Y2SiO5 crystal by femtosecond laser irradiation

Near-infrared to ultraviolet upconversion luminescence was observed in the Pr³⁺:Y₂SiO₅ crystal with 120 fs, 800 nm infrared laser irradiation. The observed emissions at around 270 nm and 305 nm could be assigned to 5d → 4f transitions of Pr³⁺ ions. The relationship between the upconversion luminescence intensity and the pump power of the femtosecond laser reveals that the UV emission belongs to simultaneous three-photon absorption induced upconversion luminescence.     

Infrared-to-visible upconversion in Er and Er/Yb activated Sb2O4 nanopowders prepared by sol-gel route

We prepared Er³⁺ doped and Er³⁺/Yb³⁺ codoped Sb₂O₄ nanocrystals by the sol-gel method. The Raman, X-ray diffraction (XRD), transmission electron microscope (TEM), and photoluminescence spectra of the samples were studied. The phonon energy of the Sb₂O₄ nanocrystals is very low (the maximum value being 461 cm⁻¹). The upconversion (UC) red emission of the Er³⁺/Yb³⁺ codoped sample is very strong at 975 nm laser diode excitation. The Sb₂O₄ nanocrystals will be a promising luminous material.     

Luminescence properties of Y2WO6:Eu incorporated with Mo or Bi ions as red phosphors for light-emitting diode applications

In this study, the red phosphors, Y₂W_1−xMo_xO₆:Eu³⁺ and Y₂WO₆:Eu³⁺,Bi³⁺, have been investigated for light-emitting diode (LED) applications. In Y₂WO₆:Eu³⁺, the excitation band edge shifts to longer wavelength with the incorporation of Mo⁶⁺ or Bi³⁺ ions. The emission spectra exhibit ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transition of Eu³⁺ ion at 588, 593, and 610 nm, respectively. Moreover, the bluish-green luminescence of the WO₆⁶⁻ at about 460 nm is observed to decrease with the incorporation of Mo⁶⁺, which results in pure red color. Thus, this study shows that the red phosphor, Y₂WO₆:Eu³⁺, incorporated with Mo⁶⁺ or Bi³⁺ ions is advantageous for LEDs applications.     

Abnormally enhanced Eu emission in Y2O2SO4:Eu inherited from their precursory dodecylsulfate-templated concentric-layered nanostructure

A new nanostructure-mediated approach was demonstrated to synthesize Eu³⁺-doped yttrium oxysulfates Y₂O₂SO₄:Eu³⁺ giving rise to abnormally enhanced Eu³⁺ emission. Yttrium and europium salts, sodium dodecylsulfate (SDS), and urea at various Eu³⁺ concentrations were reacted in aqueous solution at 80, 85, and 87 °C to yield Eu³⁺-doped dodecylsulfate-templated yttrium oxide mesophases with straight-layered (S-type), concentric-layered (C-type) and layer-to-hexagonal transient-layered (T-type) structures, respectively. On calcination at 1000 °C, all of these mesophases were converted into Y₂O₂SO₄:Eu³⁺ to exhibit luminescence bands including the ⁵D₀-⁷F₂ transition with a tendency in intensity to saturate or reach a maximum at 10-12 mol% Eu doping. The Eu³⁺ emissions for Y₂O₂SO₄:Eu³⁺ mediated by the T- and C-type mesophases were enhanced in intensity by a factor of about two and three times, respectively, stronger than those for not only compositionally the same sulfate Y₂O₂SO₄:Eu³⁺ obtained from yttrium-based sulfates but also Y₂O₃:Eu³⁺ obtained in the SDS-free system. In contrast, the emission intensities for the S-type-mesophase-mediated Y₂O₂SO₄:Eu³⁺ were close to those for the latter sulfates. The abnormally enhanced emission is likely based on specific deformation of sulfate groups induced through the conversion of concentric dodecylsulfate-layers to straight sulfate-layers in the oxysulfate framework upon calcination.     

Characterization of Y2O3 gate dielectric on n-GaAs substrates

Physical and electrical properties of sputtered deposited Y₂O₃ films on NH₄OH treated n-GaAs substrate are investigated. The as-deposited films and interfacial layer formation have been analyzed by using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). It is found that directly deposited Y₂O₃ on n-GaAs exhibits excellent electrical properties with low frequency dispersion (<5%), hysteresis voltage (0.24 V), and interface trap density (3 × 10¹² eV⁻¹ cm⁻²). The results show that the deposition of Y₂O₃ on n-GaAs can be an effective way to improve the interface quality by the suppression on native oxides formation, especially arsenic oxide which causes Fermi level pinning at high-k/GaAs interface. The Al/Y₂O₃/n-GaAs stack with an equivalent oxide thickness (EOT) of 2.1 nm shows a leakage current density of 3.6 × 10⁻⁶ A cm⁻² at a V_FB of 1 V. While the low-field leakage current conduction mechanism has been found to be dominated by the Schottky emission, Poole-Frenkel emission takes over at high electric fields. The energy band alignment of Y₂O₃ films on n-GaAs substrate is extracted from detailed XPS measurements. The valence and conduction band offsets at Y₂O₃/n-GaAs interfaces are found to be 2.14 and 2.21 eV, respectively.     

Synthesis and luminescence properties of europium doped YBa3B9O18

Europium (Eu³⁺) doped YBa₃B₉O₁₈ were synthesized by conventional solid state solidification methods. (Y_1−xEu_x)Ba₃B₉O₁₈ formed solid solutions in the range of x=0–1.0. The luminescence property measurements upon excitation in ultraviolet–visible range show well-known Eu³⁺ excitation and emission. The charge transfer excitation band of Eu³⁺ dominates the excitation spectra. The emission spectrum of Eu³⁺ ions consists mainly of several groups of lines in the 550–720 nm region, due to the transitions from the ⁵D₀ level to the levels ⁷F_J (J=0, 1, 2, 3, 4) of Eu³⁺ ions. The dependence of luminescence intensity on Eu³⁺ concentration shows no concentration quenching for fully concentrated EuBa₃B₉O₁₈. Eu³⁺ doped YBa₃B₉O₁₈ are promising phosphors for applications in displays and optical devices.     

Photoluminescence properties and effects of dopant concentration in Bi2ZnB2O7:Tb phosphor

A novel green phosphor, Tb³⁺ doped Bi₂ZnB₂O₇ was synthesized by conventional solid state reaction method. The phase of synthesized materials was determined using the XRD, DTA/TG and FTIR. The photoluminescence characteristics were investigated using spectrofluorometer at room temperature. Bi₂ZnB₂O₇:Tb³⁺ phosphors excited by 270 nm and 485 nm wavelengths. The emission spectra were composed of three bands, in which the dominated emission of green luminescence Bi₂ZnB₂O₇:Tb³⁺ attributed to the transition ⁵D₄ → ⁷F₅ is centered at 546 nm. The dependence of the emission intensity on the Tb³⁺ concentration for the Bi_2−xTb_xZnB₂O₇ (0.01 ≤ x ≤ 0.15) was studied and observed that the optimum concentration of Tb³⁺ in phosphor was 13 mol% for the highest emission intensity at 546 nm.     

Enhancing upconversion emissions of NaTm0.02YbxY0.98−xF4 nanocrystals through increasing Yb doping

By increasing the content of Yb³⁺ ions from 20% to 98% in NaTm_0.02Yb_xY_0.98−xF₄ (x=0.2−0.98) nanocrystals with size about 10 nm, the intensities of near infrared (800 nm) and blue (470 nm) upconversion (UC) luminescence can be enhanced by orders of 45 and 49 times, respectively, under 970 nm diode laser excitation. Pump power dependence illustrated that the 800 and 470 nm radiations are still two- and three-photon processes, respectively. TEM imaging showed that the enhancement is not from the change of the crystal size. Steady-state equation and the measured lifetimes indicated that the enhanced 800 nm radiations can induce the enhancement of the 470 nm emissions, which is in good agreement with the experimental data.     

Effects of Tb doping on the photoluminescence of Y2O3:Tb nanophosphors

The effects of Tb doping on the photoluminescence (PL) of Y₂O₃:Tb nanophosphors have been investigated. Nanophosphors were prepared by the glycine-nitrate solution combustion technique using yttria and terbia powders as precursors. PL excitation spectra at room temperature consist of two overlapping bands centered at 277 and 304 nm, whereas emission spectra comprise several groups of lines corresponding to the ⁵D₄→⁷F_J (J=1-6) 4f electronic transitions of the Tb⁺³ ions. A direct comparison of nanophosphor and bulk concentration-quenching curves was obtained by annealing the nanophosphor powder and converting it to bulk material without altering the Tb concentration. The peak in the nanophosphor concentration-quenching curve occurs at a concentration ∼3 times higher than that of the bulk.     

Luminescence of La3F3[Si3O9]:Ce

The luminescence properties of Ce³⁺ in La₃F₃[Si₃O₉] are reported. Excitation and emission bands corresponding to 4f¹→5d¹ transitions of Ce³⁺ were identified. The center of gravity of the 5d states lies at remarkable high energy (43.2×10³ cm⁻¹) for Ce³⁺ in a silicate compound. This high value is attributed to the combined oxygen/fluoride coordination of the Ce³⁺ ion. Emission from the lowest 4f5d level to the ²F_5/2 and ²F_7/2 levels was found at 32.4×10³ and 30.4×10³ cm⁻¹. These results are compared with literature data on silicates and fluorides. From the values found for Ce³⁺, predictions are made for the positions of the 4f5d bands of Pr³⁺ and Er³⁺ in La₃F₃[Si₃O₉]. For both ions, it is concluded that in this host lattice emission is expected from high lying 4fⁿ energy levels.     

Preparation and photoluminescence characteristics of Eu-doped MgAl1.8Y0.2O4 nanocrystals

A simple combustion route was employed for the preparation of Eu³⁺-doped MgAl_1.8Y_0.2−xO₄ nanocrystals using metal nitrates as precursors and urea as a fuel in a preheated furnace at 500 °C. The powders thus obtained were then fired at 1000 °C for 3 h to get better luminescent properties. The incorporation of Eu³⁺ activator in these nanocrystals was checked by luminescence characteristics. These nanocrystals displayed bright red color on excitation under 254 nm UV source. The main emission peak was assigned to the transition [⁵D₀→⁷F₂] at 615 nm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were carried out to understand surface morphological features and the particle size. Crystal structures of the nanocrystals were investigated by the X-ray diffraction (XRD) technique. The crystallite size of the as-prepared nanocrystals was around 29 nm, which was evaluated from the broad XRD peaks. The crystallite size increased to ∼45 nm on further heat treatment at 1000 °C.     

Low-voltage cathodoluminescence property of Li-doped Gd2−xYxO3:Eu

Cathodoluminescent (CL) spectra of Li-doped Gd_2−xY_xO₃:Eu³⁺ solid-solution (0.0?x?0.8) were investigated at low voltages (300 V-1 kV). The CL intensity is maximum for the composition of x=0.2 and gradually reduces with increasing the amount of substituted Y content. In particular, small (∼100 nm) particles of Li-doped Gd_1.8Y_0.2O₃:Eu³⁺ are obtained by firing the citrate precursors at only 650°C for 18 h. Relative red-emission intensity at 300 V of this phosphor is close to 180% in comparison with that of commercial red phosphor Y₂O₃:Eu³⁺. An increase of firing temperature to 900°C results in 400-600 nm sized spherical particles. At low voltages (300-800 V), the CL emission of 100 nm sized particles is much stronger than that of 400-600 nm sized ones. In contrast, the larger particles exhibit the higher CL emission intensity at high voltages (1-10 kV). Taking into consideration small spherical morphology and effective CL emission, Li-doped Gd_1.8Y_0.2O₃:Eu³⁺ appears to be an efficient phosphor material for low voltage field emission display.     

High heat generation ability in AC magnetic field for nano-sized magnetic Y3Fe5O12 powder prepared by bead milling

Nano-sized magnetic Y₃Fe₅O₁₂ ferrite having a high heat generation ability in an AC magnetic field was prepared by bead milling. A commercial powder sample (non-milled sample) of ca. 2.9 μm in particle size did not show any temperature enhancement in the AC magnetic field. The heat generation ability in the AC magnetic field improved with a decrease in the average crystallite size for the bead-milled Y₃Fe₅O₁₂ ferrites. The highest heat ability in the AC magnetic field was for the fine Y₃Fe₅O₁₂ powder with a 15-nm crystallite size (the samples were milled for 4 h using 0.1 mm? beads). The heat generation ability of the excessively milled Y₃Fe₅O₁₂ samples decreased. The main reason for the high heat generation property of the milled samples was ascribed to an increase in the Néel relaxation of the superparamagnetic material. The heat generation ability was not influenced by the concentration of the ferrite powder. For the samples milled for 4 h using 0.1 mm? beads, the heat generation ability (W g⁻¹) was estimated using a 3.58×10⁻⁴ fH² frequency (f/kHz) and the magnetic field (H/kA m⁻¹), which is the highest reported value of superparamagnetic materials.