Visible quantum cutting through downconversion in Eu3+-doped K2GdZr(PO4)3 phosphor

K₂Gd_1?xZr(PO₄)₃:Eu_x³⁺ (0.02 ≤ x ≤ 0.1, x is in mol.%) were prepared by solid-state reaction method and their photoluminescence properties were investigated in ultra-violet (UV) and vacuum ultra-violet (VUV) region. The phenomenon of visible quantum cutting through downconversion was observed for the Gd³⁺–Eu³⁺ couple in this Eu³⁺-doped K₂GdZr(PO₄)₃ system. Visible quantum cutting, the emission of two visible light photons per absorbed VUV photon, occurred upon the 186 nm excitation of Gd³⁺ at the ⁶G_J level via two-step energy transfer from Gd³⁺ to Eu³⁺ by cross-relaxation and sequential transfer of the remaining excitation energy. The results revealed that the efficiency of the energy transfer process from Gd³⁺ to Eu³⁺ in the Eu³⁺-doped K₂GdZr(PO₄)₃ system could reach to 155% and K₂GdZr(PO₄)₃:Eu³⁺ was effective quantum cutting material.     

Synthesis of green-emitting (Gd,La, Tb)2O(WO4)2 phosphors

Green-emitting (Gd_1−x−yLa_xTb_y)₂O(WO₄)₂ (0 ⩽ x ⩽ 0.05, 0.05 ⩽ y ⩽ 0.15) phosphors were synthesized in a single phase form by the conventional solid-state reaction method, and their photoluminescent properties were characterized. The (Gd_1−x−yLa_xTb_y)₂O(WO₄)₂ phosphors showed strong and broad excitation bands from 230 to 350 nm, corresponding to the energy transition from the 4f⁸ to 4f⁷5d configuration of Tb³⁺ and the charge-transfer (CT) transition of O²⁻−W⁶⁺. The oxytungstate phosphors exhibited typical emission peaks assigned to the transition from ⁵D₄ to ⁷F_J (J = 6, 5, 4, and 3) of Tb³⁺, and the luminescence emission intensity was effectively enhanced by the La³⁺ doping into the host Gd₂O(WO₄)₂ lattice. The highest green emission intensity was obtained for (Gd_0.87La_0.03Tb_0.10)₂O(WO₄)₂, where the relative emission intensity was 63% that of a commercial green-emitting (La_0.52Ce_0.31Tb_0.17)PO₄ phosphor.     

Synthesis and photoluminescence properties of a cyan-emitting phosphor Ca3(PO4)2:Eu2+ for white light-emitting diodes

In this paper, a cyan-emitting phosphor Ca₃(PO₄)₂:Eu²⁺ (TCP:Eu²⁺) was synthesized and evaluated as a candidate for white light emitting diodes (WLEDs). This phosphor shows strong and broad absorption in 250–450 nm region, but the emission spectrum is prominent at around 480 nm. The emission intensity of the TCP:Eu²⁺ was found to be 60% and 82% of that of the commercial BaMgAl₁₀O₁₇:Eu²⁺ (BAM) under excitation at 340 nm and 370 nm, respectively. Upon excitation at 370 nm, the absolute internal and external quantum efficiencies of the Ca₃(PO₄)₂:1.5%Eu²⁺ are 60% and 42%, respectively. Moreover, a white LED lamp was fabricated by coating TCP:Eu²⁺ with a blue-emitting BAM and a red-emitting CaAlSiN₃:Eu²⁺ on a near-ultraviolet (375 nm) LED chip, driven by a 350 mA forward bias current, and it produces an intense white light with a color rendering index of 75.     

Luminescence properties of Ba5SiO4(F,Cl)6:Eu2+ phosphor

In this paper, we reported a series of halosilicate photoluminescence (PL) materials, which have the chemical composition Ba₅SiO₄(F_xCl_6−x):0.05Eu²⁺ (x = 0, 1, 3, 4, 6). It is found that, under 365-nm UV light, Ba₅SiO₄Cl₆:Eu²⁺ phosphor exhibits strong blue light with peak wavelength at 442 nm. By adding small amount of F⁻ to the host lattice for the substitution of Cl⁻, the emission wavelength of the Ba₅SiO₄(F, Cl)₆:Eu²⁺ phosphor changes to 503 nm, corresponding to the green light. The possible luminescence mechanism in this phosphor system was also discussed.     

Synthesis and luminescence properties of Ca8NaGd(PO4)6F2: Eu2+, Mn2+ for white LEDs

A series of luminescent emission-tunable phosphors Ca₈NaGd(PO₄)₆F₂: Eu²⁺, Mn²⁺ have been prepared by a combustion-assisted synthesis method. The X-ray diffraction measurement results indicate that the crystal structure of the phosphor is a single phase of Ca₈NaGd(PO₄)₆F₂. The photoluminescence (PL) properties of Eu²⁺ and Mn²⁺-codoped Ca₈NaGd(PO₄)₆F₂ phosphors were also investigated. The phosphors can be efficiently excited by ultraviolet (UV) light and show a blue emission band at about 450 nm and a yellow emission band at about 574 nm, which originated from the Eu²⁺ ions and the Mn²⁺ ions, respectively. The efficient energy transfer from the Eu²⁺ ions to the Mn²⁺ ions was observed and its mechanism should be a resonant type via a nonradiative dipole–quadrupole interaction. A color-tunable emission in Ca₈NaGd(PO₄)₆F₂ phosphors can be realized by Eu²⁺ → Mn²⁺ energy transfer. Our results indicate that the developed phosphor may be used as a potential white emitting phosphor for UV based white LEDs.     

Ribbon-like and rod-like hydroxyapatite crystals deposited on titanium surface with electrochemical method

Homogeneous coatings were attained by electrochemical method in electrolytes containing Ca²⁺ and PO₄³⁻ ions with Ca/P ratio being 1.67. SEM observation showed that the hydroxyapatite (HAp,Ca₁₀(PO₄)₆(OH)₂) crystals prepared with higher concentration electrolyte (4 × 10^− 2 M Ca²⁺) are ribbon-like with thickness of nanometer size, a morphology seldom reported previously. In an electrolyte of lower concentration (6 × 10^− 4 M Ca²⁺), the HAp crystals formed are rod-like with a hexagonal cross section and diameter of about 70–80 nm. XRD patterns and IR spectra confirmed that the coatings consist of HAp crystals. TEM micrographs and SAD indicated that the longitude direction for both ribbon-like and rod-like crystal is [002], and the flat surface of the ribbon is (110). HRTEM showed that the ribbon-like crystal is a mixture of HAp and octacalcium phosphate (OCP, Ca₈H₂(PO₄)₆.5H₂O).     

Luminescence properties of Eu2+-activated Sr5(PO4)2(SiO4) for green-emitting phosphor

A green-emitting phosphor of Eu²⁺-activated Sr₅(PO₄)₂(SiO₄) was synthesized by the conventional solid-state reaction. It was characterized by photoluminescence excitation and emission spectra, and lifetimes. In Sr₅(PO₄)₂(SiO₄):Eu²⁺, there are at least two distinguishable Eu²⁺ sites, which result in one broad emission situating at about 495 nm and 560 nm. The phosphor can be efficiently excited in the wavelength range of 250–440 nm where the near UV (～ 395 nm) Ga(In)N LED is well matched. The dependence of luminescence intensities on temperature was investigated. With the increasing of temperature, the luminescence of the phosphor shows good thermal stability and stable color chromaticity. The luminescence characteristics indicate that this phosphor has a potential application as a white light emitting diode phosphor.     

Multicolor-emitting Ca3 -x-ySry(PO4)2:xEu2 + (0 ≤ x ≤ 0.075, 0 ≤ y ≤ 2.2) phosphors for light-emitting diodes

In this paper, a series of Ca_3 -x-ySr_y(PO₄)₂:xEu^2 +, (0 ≤ x ≤ 0.075, 0 ≤ y ≤ 2.2) phosphors were prepared by flux assisted solid-state reaction method, and their photoluminescence properties were investigated. The β- to β′-phase transition of Ca_3 -ySr_y(PO₄)₂ for high Sr^2 + content was observed from the XRD patterns, and the corresponding optical bandgaps were obtained experimentally. Various Eu^2 + emission centers were found, which generate tunable emission depending on the Sr^2 + concentration. Broad and intense excitation bands exist in Eu^2 + activated Ca₃(PO₄)₂, and the introduction of Sr^2 + further extends and enhances the excitation bands beyond 350 nm, which is beneficial to the applications on near ultraviolet LEDs. The morphology measurement reveals that the average size of particles with smooth surface is about 11.2 μm, which is suitable for the practical applications. These results indicate that the Ca_3 -x-ySr_y(PO₄)₂:xEu^2 + phosphors could be promising candidates for LEDs.     

Eu3+ activated novel alkaline earth metal gallium oxide phosphors: Sr(2.92−x)Ca(x)Ga2O6:Eu3+0.08 for light emitting diodes

Present study deals with Eu³⁺ activated novel alkaline earth metal (Sr and Ca) gallium oxide phosphors, Sr_(2.92?x)Ca_(x)Ga₂O₆:Eu³⁺_0.08 (x = 0 to 2.92). Crystal structure, morphology and luminescence (excitation, emission and CIE coordinate) properties of these phosphors have been studied as a function of Ca concentration. Doping of Ca ions into Sr_2.92Ga₂O₆:Eu³⁺ phosphor gives rise to a significant enhancement in overall fluorescence and the optimum emission is attained for pure Ca_2.92Ga₂O₆:Eu³⁺ phosphor for x = 2.92. The intensity ratio of ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ transitions (monochromaticity) of Eu³⁺ for different doping concentration of Ca suggests that asymmetry around the Eu³⁺ ion increases with increase in Ca ion concentration, which is responsible for enhanced emission. The excellent optical features, such as broad excitation band (230–480 nm) and excellent emission in red region (at 614 nm), conclude that calcium gallet phosphor could be a potential candidate for light emitting diodes and display applications.     

Interplay between random laser performance and self-frequency conversions in NdxY1.00−xAl3(BO3)4 nanocrystals powders

Random Laser emission at 1.06 μm, self-second-harmonic generation at 0.53 μm and self-sum-frequency generation at 0.46 μm were investigated in Nd_xY_1.00−xAl₃(BO₃)₄ nanocrystalline powders, for 0.05 ⩽ x ⩽ 1.00, excited by a pulsed laser operating at 808 nm, focusing on the interplay between the RL performance and the second-order nonlinear processes. The RL performance, characterized by a figure-of-merit relating the laser slope efficiency and the excitation pulse energy threshold, improved as x increased up to 1.00 while the efficiency of the self-frequency conversion processes reduced for increasing x because of distortions introduced in the crystalline structure of the grains. The RL wavelength was also dependent on the Nd³⁺ concentration and presented a redshift from 1061.9 nm to 1063.5 nm for increasing values of x.     

Microstructure,luminescence and thermal stability properties of NaSrPO4:Tb3+ phosphors with various doping concentrations prepared using conventional solid-state sintering

This paper reports the microstructure, luminescence and thermal stability properties of the NaSr_1−xPO₄:xTb³⁺ powders (x = 0.008, 0.01, 0.02, 0.04 and 0.06) via the conventional solid-state sintering at 1200 °C for 5 h. The X-ray diffraction result verifies all diffraction peaks are pure phase of NaSrPO₄. The luminescence results show that the NaSrPO₄:xTb³⁺ powders mainly excited at 370 nm have a series of the emission-states, related to the typical 4f → 4f intra-configuration forbidden transitions of Tb³⁺, and a major emission peak of around 546 nm. The concentration quenching of the NaSr_1−xPO₄:xTb³⁺ phosphors is appeared at x = 0.02. The decay time values of the NaSr_1−xPO₄:xTb³⁺ phosphors for the ⁵D₄ state of the Tb³⁺ are around 3.30 ms to 3.60 ms. It is also found the chromaticity coordinate of NaSrPO₄:Tb³⁺ phosphor varies with the increase of the concentration of Tb³⁺ ions from blue to green. Moreover, the thermal stability of the NaSrPO₄:xTb³⁺ phosphors is slightly better than that of conventional YAG phosphors.     

Spectroscopic investigations on Eu3+ ions in Li–K–Zn fluorotellurite glasses

Eu³⁺ ions incorporated Li–K–Zn fluorotellurite glasses, (70 − x)TeO₂ + 10Li₂O + 10K₂O + 10ZnF₂ + xEu₂O₃, (0 ⩽ x ⩽ 2 mol%) were prepared via melt quenching technique. Optical absorption from ⁷F₀ and ⁷F₁ levels of the Eu³⁺-doped glass has been studied to examine the covalent bonding characteristics, energy band gap and Judd–Ofelt intensity parameters. The emission spectra (⁵D₀ → ⁷F_0,1,2,3,4) of the glasses were used to estimate the luminescence enhancement, asymmetric environment in the vicinity of Eu³⁺ ions, stimulated emission cross section and branching ratios. The phonon side band mechanism of ⁵D₂ level of the Eu³⁺ ions in the prepared glass was examined by considering the excitation and Raman spectra. The radiative lifetime calculated using Judd–Ofelt parameters was compared with the experimental lifetime to estimate the quantum efficiency of ⁵D₀ level of Eu³⁺ ions in Li–K–Zn fluorotellurite glass.     

Photoluminescence properties of Sr3(PO4)2: Eu2+, Dy3+ double-emitting blue phosphor for white LEDs

Double-emitting blue phosphor Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ was synthesized by solid state reaction under H₂ atmosphere. XRD exhibited the pure hexagonal phase of the prepared phosphor. The photoluminescence results showed that all samples had intense broad absorption band between 250 and 450 nm, which matched well with the near-UV (350–420 nm) emission band of InGaN-based chips. The emission spectrum of Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ consisted of two broad bands, peaked at 485 nm and 410 nm, which originated from two luminescent centers, related to 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ in six-coordinated Sr(I) and ten-coordinated Sr(II) sites respectively. The intensity ratio of two emission bands could be easily tuned by adjusting Dy³⁺ co-doping content, which resulted in color-tunable luminescence in bluish green region to purplish blue region.     

Growth,structural and optical characterizations of LiLa(1−x)Eux(WO4)2 single-crystalline fibers by the micro-pulling-down method

Transparent and uniform LiLa_(1?x)Eu_x(WO₄)₂ single-crystalline fibers where x = 0.005, 0.01, 0.03, 0.05, 0.07, 0.1, 0.15, 0.2, 0.25 and 1.0, with an average of 20–40 mm length were obtained by the micro-pulling-down method aiming structural and optical characterization. The optimum pulling rate was found to depend on the difference between alkali and rare earth ionic radii. Rietveld analysis from X-ray diffraction data and excitation spectroscopy at room temperature were applied to investigate the lattice changes due to the Eu³⁺ incorporation in the host. LiLa_(1?x)Eu_x(WO₄)₂ crystal fibers present red emission due to the electric dipole ⁵D₀ → ⁷F₂ transition under 395 nm excitation showing a concentration quenching around 20 mol% of doping. The excitation spectra of the ⁷F₀ → ⁵D₀ transition show small changes in the Eu³⁺ surroundings as function of dopant concentration.     

Synthesis and tunable luminescence of RbCaGd(PO4)2:Ce3+, Mn2+ phosphors

RbCaGd(PO₄)₂ doped with Ce³⁺, Mn²⁺ was synthesized by the sol-gel method. The crystal structure and crystallographic location of Ce³⁺ in RbCaGd(PO₄)₂ were identified by Rietveld refinement. Powder X-ray diffraction (XRD) revealed that the structure of RbCaGd(PO₄)₂:Ce³⁺ compounds is hexagonal structure which is similar to that of hexagonal LnPO₄ with the lattice constant of a = b = 7.005(57) Å, c = 6.352(05) Å, and V (cell volume) = 269.980 Å³. The photoluminescence behavior and emission mechanism were studied systematically by doping activators in the RbCaGd(PO₄)₂ host. The Mn²⁺ incorporated RbCaGd(PO₄)₂:Ce³⁺, Mn²⁺ compounds exhibited blue emission from the parity- and spin-allowed f-d transition of Ce³⁺ and orange-to-red emission from the forbidden ⁴T₁ → ⁶A₁ transition of Mn²⁺. The emission chromaticity coordinates of RbCaGd(PO₄)₂:0.10Ce³⁺, xMn²⁺ (x = 0.16, 0.25) are close to the white region due to an energy transfer process and the energy transfer mechanism from Ce³⁺ to Mn²⁺ in the RbCaGd(PO₄)₂ host was dominated by dipole-dipole interactions.     

Bioactivity and mineralization of hydroxyapatite with bioglass as sintering aid and bioceramics with Na3Ca6(PO4)5 and Ca5(PO4)2SiO4 in a silicate matrix

Hydroxyapatite and Bioglass^®-45S5 were sintered together creating new ceramic compositions that yielded increased apatite deposition and osteoblast differentiation and proliferation in vitro compared to hydroxyapatite. The sintered products characterized by X-ray diffraction, revealed hydroxyapatite as the main phase when small quantities (1, 2.5 and 5 wt.%) of bioglass was added. Bioglass behaved as a sintering aid with β-TCP (Ca₃(PO₄)₂) being the minor phase. The amount of β-TCP increased with the amount of bioglass added. In compositions with larger additions of bioglass (10 and 25 wt.%), new phases with compositions of calcium phosphate silicate (Ca₅(PO₄)₂SiO₄) and sodium calcium phosphate (Na₃Ca₆(PO₄)₅) were formed respectively within amorphous silicate matrices. In vitro cell culture studies of the ceramic compositions were examined using bone marrow stromal cell (BMSC). Cell proliferation and differentiation of bone marrow stromal cells into osteoblasts were determined by Pico Green DNA assays and alkaline phosphatase (ALP) activity, respectively. All hydroxyapatite–bioglass co-sintered ceramics exhibited larger cell proliferation compared to pure hydroxyapatite samples. After 6 days in cell culture, the ceramic with Ca₅(PO₄)₃SiO₄ in a silicate matrix formed by reacting hydroxyapatite with 10 wt.% bioglass exhibited the maximum proliferation of the BMSC's. The ALP activity was found to be largest in the ceramic with Na₃Ca₆(PO₄)₅ embedded in a silicate matrix synthesized by reacting hydroxyapatite with 25 wt.% bioglass.     

Time-dependent PSL studies on CsBrCl:Eu2+: a promising image screen phosphor

The time-resolved PSL studies of CsBr_1−xCl_x:Eu²⁺ system has been studied for different europium concentrations. An efficient PSL material for use in image plate should have very short PSL emission lifetime. PSL emission lifetime for stimulation at 650 and 630 nm (F(Br⁻)- and F(Cl⁻)-centers) for CsBrCl:Eu²⁺ is determined to be 0.69 μs. The lifetime of BaFBr:Eu²⁺, the commercially available image screen phosphor is 0.8 μs. The present observation support the use of CsBrCl:Eu²⁺ as an efficient X-ray image screen phosphor.     

Investigation on the luminescence of RE3+ (RE = Ce,Tb, Eu and Tm) in KMGd(PO4)2 (M = Ca,Sr) phosphates

The VUV excited luminescent properties of Ce³⁺, Tb³⁺, Eu³⁺ and Tm³⁺ in the matrices of KMGd(PO₄)₂ (M = Ca, Sr) were investigated. The bands at about 165 nm and 155 nm in the VUV excitation spectra are attributed to host lattice absorptions of the two matrices. For Ce³⁺-doped samples, the Ce³⁺ 5d levels can be identified. As for Tb³⁺-doped samples, typical 4f–5d absorption bands in the region of 175–250 nm were observed. For Eu³⁺ and Tm³⁺-doped samples, the O²⁻–Eu³⁺ and O²–Tm³⁺ CTBs are observed to be at about 229 nm and 177 nm, respectively. From the standpoints of color purity and luminescent efficiency, KCaGd(PO₄)₂:Tb³⁺ is an attractive candidate of green light PDP phosphor.     

Giant magnetoimpedance in nanocrystalline Fe90.3−xZr7B2.7Cux (0.5 ≤ x ≤ 1.5) as-spun ribbons

The nanocrystalline ribbons Fe_90.3−xZr₇B_2.7Cu_x with low Cu contents can be directly obtained through melt-spinning technique with an appropriate low quenching speed such as 22 m/s. Sizes of bcc-Fe grains precipitated in Fe_90.3−xZr₇B_2.7Cu_x as-spun ribbons were 17 nm for x = 0.75, 15 nm for x = 1 and 12 nm for x = 1.25. The addition of Cu reduces grain size of bcc-Fe in as-spun nanocrystalline Fe_90.3−xZr₇B_2.7Cu_x ribbons. Among the investigated samples (0.5 ≤ x ≤ 1.5), the largest magnetoimpedance can be obtained in the nanocrystalline Fe_80.3Zr₇B_2.7Cu₁ as-spun ribbon with x = 1. The value of magnetoimpedance (Z(H) − Z(0)) / Z(0) under H = 90 Oe for Fe_80.3Zr₇B_2.7Cu₁ as-spun ribbon reaches − 28.2% at a frequency of 1 MHz.     

Photoluminescence of CaAlSiN3:Eu2+-based fine red-emitting phosphors synthesized by carbothermal reduction and nitridation method

In this research, we have presented the synthesis and characterization of the various Ca_1−xEu_xAl_0.76Si_1.18N₃ (x = 0.01 ∼ 0.1) red-emitting phosphors, which were successfully prepared by carbothermal reduction and nitridation (CTRN) method without the strict needs of high pressure. Here, raw materials were CaCO₃, AlN, Si₃N₄, Eu₂O₃, and C. In particular, C was considered as efficient and robust reducing agent. The influences of reaction temperature, holding time, C content, and Eu²⁺ concentration were investigated in the crystal phase compositions and photoluminescence properties of the as-prepared phosphors. Importantly, CaAlSiN₃:Eu²⁺-based red phosphors with interesting properties were obtained with reaction temperature at 1600 °C for 4 h by atmospheric N₂–10%H₂ pressure, and the C/O ratio of 1.5:1, respectively. The emission peak positions of as-prepared phosphors were red-shifted from 607 nm to 654 nm with Eu²⁺ concentration from 1 mol% to 10 mol%. Meanwhile the highest luminescence intensity was achieved with 2 mol% of Eu²⁺ concentration, which showed high external quantum efficiency up to 71%. Combining the phosphor blend of green-emitting β-sialon:Eu²⁺, yellow-emitting Ca-α-sialon:Eu²⁺, and red-emitting Ca_0.98Eu_0.02Al_0.76Si_1.18N₃ with a blue LED (light emitting diodes), warm white LED can be generated, yielding the color rendering index (Ra) of 93 at correlated color temperature (CCT) of 3295 K. These results indicate that CaAlSiN₃:Eu²⁺-based red-emitting phosphors prepared by facile CTRN are highly promising candidates for warm white LEDs.