Enhancement of red emission intensity of Ca9Y(VO4)7:Eu phosphor via Bi co-doping for the application to white LEDs

The present investigation aims at the luminescence properties of Ca₉Y(VO₄)₇:Eu³⁺, Bi³⁺ red phosphor materials. The red emission at 613 nm originating from ⁵D₀–⁷F₂ transition of Eu³⁺ in Ca₉Y(VO₄)₇ is enhanced strongly with Bi³⁺–V⁵⁺ couple as the sensitizer, under excitation either into the ⁵L₆ state or the ⁵D₂ state. The energy transfer from Bi³⁺–V⁵⁺ to Eu³⁺ is discussed. For a fixed Eu³⁺ concentration, there is an optimal Bi³⁺ concentration with 15 mol%, at which the maximum luminescence intensity of Eu³⁺ is achieved. The red emission of Ca₉Y(VO₄)₇:0.8Eu³⁺, 0.15Bi³⁺ (under 395 nm and 465 nm excitations) is stronger than that of commercial Y₂O₃:Eu³⁺ phosphor (under 395 nm and 467 nm excitations). Based on the ratios of the red emission at 613 nm to orange one at 592 nm, it is considered that the symmetry of Eu³⁺ site decreases with doping of Bi³⁺, leading to more opposite parity components. Lifetime and diffuse reflection spectra measurements indicate that the red emission enhancement is due to the enhanced transition probabilities from the ground state to ⁵L₆ and ⁵D₂ states of Eu³⁺ in the distorted crystal field. Therefore the present material is a promising red-emitting phosphor for white-light diodes with near-ultraviolet/blue GaN-based chips.     

Investigation of Na3GdP2O8:Tb as a potential green-emitting phosphor for plasma display panels

The photoluminescent properties of a series of Tb³⁺-doped Na₃GdP₂O₈ phosphors excitable by vacuum ultraviolet and ultraviolet light are reported. The host related absorption, f-f and f-d transitions of Gd³⁺ and Tb³⁺, and charge transfer of O²⁻ → Gd³⁺ and O²⁻ → Tb³⁺ are assigned. Under 147 nm light excitation, Na₃GdP₂O₈:Tb³⁺ phosphors show efficient green emissions with a dominant peak at 545 nm. The optimal sample Na₃Gd_0.4Tb_0.6P₂O₈ shows a shorter decay time and a comparable brightness when compared with the commercial Zn₂SiO₄:Mn²⁺ green phosphor. These results demonstrate that it is a potential candidate for plasma display panels application.     

The UV and VUV luminescence properties of the phosphor Mg2GeO4:Tb

A novel green phosphor Mg₂GeO₄:Tb³⁺ with pure phase was prepared by the solid state reaction. The luminescence properties were investigated in detail. The diffusion reflection spectra of the undoped and Tb³⁺ doped Mg₂GeO₄ phosphors were recorded, the result reveals that there is an absorption band superposition of the host material and Tb³⁺ ion. The study on the excitation and diffusion spectra shows that there is an effective energy transfer from the host material to Tb³⁺ ion. Under 277 and 172 nm excitation, the phosphor presents predominant green emission at 543 and 547 nm respectively. The excitation intensity at 172 nm is about 1.8 times of that at 272 nm. The promising luminescence properties make it a candidate for application in Plasma Display Panel.     

A potential Eu-activated Ca10K(PO4)7 red phosphor for white light-emitting diodes

New red Ca₁₀K(PO₄)₇:Eu³⁺, K⁺ phosphors were synthesized by solid state reaction and their photoluminescence properties as well as those by co-doping Mo⁶⁺ under near ultraviolet excitation were investigated. From the excitation spectra monitored at 611 nm, it can be seen that the strongest excitation peak is situated at 393 nm, well matching with the emission wavelength of near-ultraviolet chips for white LEDs. Upon 393 nm excitation, the brightness of Ca₉K(PO₄)₇:0.5Eu³⁺, 0.5 K⁺ with the optimal Eu³⁺-doping concentration is about 2.3 times stronger than that of the commercial red Y₂O₃:Eu³⁺ phosphor. The introducing of Mo⁶⁺, which results in a possible variety for the excited energy level of the host, can enhance the brightness of Eu³⁺ to be maximized by about 15%. The CIE chromaticity coordinates of Ca₉K(PO₄)₇:0.5Eu³⁺, 0.5 K⁺ are calculated to (0.654, 0.345), which are close to the (0.67, 0.33) standard of the National Television System Committee. All the above results indicate Eu³⁺-activated Ca₁₀K(PO₄)₇ is a potential candidate for white LEDs.     

Rapid synthesis of spherical-shaped green-emitting MgGa2O4:Mn phosphor via spray pyrolysis

Simple, one-step synthesis of spherical-shaped powder phosphors with aqueous precursors via a spray pyrolysis method is reported. Green-emitting MgGa₂O₄:Mn²⁺ phosphor with a controlled shape was successfully obtained by spraying under a reductive atmosphere (N₂ + H₂ carrier gas) without high-temperature post-heat treatment. In addition, the corresponding powder phosphors were well dispersed and showed a clean surface morphology compared to an existing cumbersome process using high-temperature post-annealing. The new method may help to prevent surface residual non-radiative defect sites. The result of highly luminescent and spherical morphology, non-aggregated powder phosphor by this procedure holds promise for a cost-effective and rapid synthesis process for conventional inorganic phosphors.     

White-emitting phosphors Ca6La2Na2(PO4)6F2:Dy and luminescence enhancement through Ce → Dy energy transfer

Ce³⁺ and Dy³⁺ activated fluoro-apatite Ca₆La₂Na₂(PO₄)₆F₂ with chemical formulas Ca₆La_2−xLn_xNa₂(PO₄)₆F₂ (Ln = Ce³⁺, Dy³⁺) were prepared by a solid state reaction technique at high temperature. The vacuum-ultraviolet (VUV) and ultraviolet (UV) spectroscopic properties are investigated. The results indicate that Ce³⁺ ions show the lowest 5d excitation band at ∼305 nm and a broad emission band centered at ∼345 nm. Dy³⁺ ions exhibit intense absorption at VUV and UV range. White-emitting under 172 nm excitation is obtained based on two dominant emissions from Dy³⁺ ions centered at 480 and 577 nm. In addition, the energy transfer from Ce³⁺ to Dy³⁺ in the co-doped samples are observed and discussed.     

Hydrothermal synthesis and photoluminescence of SrWO4:Tb novel green phosphor

Tb³⁺-doped SrWO₄ phosphors with a scheelite structure have been prepared by hydrothermal reaction. X-ray powder diffraction, field-emission scanning electron microscopy, photoluminescence excitation and emission spectra and decay curve were used to characterize the resulting samples. Scanning electron microscopy image showed that the obtained SrWO₄:Tb³⁺ phosphors appeared to be nearly spherical and their sizes ranged from 1 to 3 μm. Photoluminescence spectra indicated the phosphors emitted strong green light centered at 545 nm under ultraviolet light excitation. Because 12 at.% SWO₄:Tb³⁺ phosphor exhibits intensive green emission under 254 nm excitation in comparison with the commercial green fluorescent lamp phosphor (LaPO₄:Ce,Tb), the excellent luminescence properties make it a new promising green phosphor for fluorescent lamps application.     

Correlation between structure and photoluminescence of the europium doped glaserite-type phosphate Na2SrMg(PO4)2

A new phosphate Na₂SrMg(PO₄)₂ has been synthesized and investigated by X-ray diffraction, DTA, ³¹P NMR spectroscopy and photoluminescence measurements. This compound crystallizes in the space group P2₁/a of the monoclinic system with the cell parameters: a = 9.158(1) Å, b = 5.267(1) Å, c = 13.498 (1) Å, β = 90.01(1)° and four formula units per cell. Its structure is closely related to that of the mineral glaserite K₃Na(SO₄)₂ and thus it can be described by the general formula XY₂M(TO₄)₂. The X, Y, M and T sites are fully occupied by Sr²⁺, Na⁺, Mg²⁺ and P⁵⁺ cations, respectively. The anionic framework is consisted by the stacking along the [001] direction of two kinds of alternating [MgP₂O₈⁴⁻]_∞ mixed layers parallel to the (a, b) plane and resulting from a corner-sharing between MgO₆ octahedra and PO₄ tetrahedra. The Sr²⁺ cations are located within the interlayer space, while those Na⁺ are found in large cavities bounded to the layers. The DTA analysis showed a congruent melting of this compound at 1374 K. The ³¹P NMR spectroscopy confirmed the presence of two distinct phosphors sites in the structure. Optical studies were performed on the Na₂SrMg(PO₄)₂ compound doped with Eu³⁺ and Eu²⁺. The trivalent europium was used as a local probe, replacing strontium and sodium, what gives complementary and consistent results to the crystallographic analyses. The divalent europium was used to analyze preliminary the potentiality for this compound to be integrated as phosphor in Light Emitting Diode (LED).     

Vacuum ultraviolet optical properties of (La,Gd)PO4:RE (RE=Eu, Tb)

Vacuum ultraviolet excitation spectra of phosphors (La,Gd)PO₄:RE³⁺ (RE=Eu or Tb) and X-ray photoelectron spectra of LaPO₄ and GdPO₄ are investigated. The vacuum ultraviolet excitation intensity of (La,Gd)PO₄:RE³⁺ is enhanced with the increasing of Gd³⁺ content, which implies that Gd³⁺ plays an intermediate role in energy transfer from host absorption band to RE³⁺. When Gd³⁺ is doped into LaPO₄:Eu³⁺, charge transfer band (CT band) begins to shift to higher energy region and the overlap degree of CT band and the host absorption band gets greater with more Gd³⁺ doped into LaPO₄. These results suggest that the dopant (Gd³⁺) gives an important influence on energy transfer efficiency. The top of LaPO₄ valance band is formed by the 2p level of O²⁻, whereas that of GdPO₄ valance band is formed by the 2p level of O²⁻ and the 4f level of Gd³⁺, showing the differences in band structures between LaPO₄ and GdPO₄.     

A novel red long lasting phosphorescent (LLP) material β-Zn3(PO4)2:Mn, Sm

A novel red long lasting phosphorescent materials β-Zn₃(PO₄)₂:Mn²⁺,Sm³⁺ is firstly synthesized by high-temperature solid-state reaction. The influence of Sm³⁺ ions on luminescence and long lasting phosphorescence properties of Mn²⁺ in phosphor β-Zn₃(PO₄)₂:Mn²⁺,Sm³⁺ are systematically investigated. It is found that the red phosphorescence (λ = 616 nm) performance of Mn²⁺ ion such as brightness and duration is largely improved when Sm³⁺ ion is co-doped into the matrix in which Mn²⁺ ion acts as luminescent center and Sm³⁺ ion plays an important role of electron trap. Thermoluminescence spectrums show that there exists one peak in β-Zn₃(PO₄)₂:Mn²⁺,Sm³⁺, the depth of which is 0.33 eV, and that there are three peaks in β-Zn₃(PO₄)₂:Mn²⁺, among which the depth of the lowest temperature peak in β-Zn₃(PO₄)₂:Mn²⁺ is 0.37 eV. Such differences in the trap depth result in the improvement of red long lasting phosphorescence of Mn²⁺ in present matrix.     

Single crystalline Co3O4: Synthesis and optical properties

Crystals of Co₃O₄ have been prepared from thermal decomposition of molecular precursors derived from salicylic acid and cobalt (II) acetate or chloride at 500 °C. A cubic phase Co₃O₄ micro- and nanocrystals have been obtained. The as-synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The images of electron microscopes showed octahedral crystals of Co₃O₄. The volume and polarizability of the optimized structures of molecular precursors have been calculated and related to the particle size. The optical band gap of the obtained crystals has been measured. The results indicated two optical band gaps with values 2.65 and 2.95 eV for (E_g1) (E_g2), respectively.     

Growth and characterization of (SnS2)x-(SnSe2)1−x mixed crystals

(SnS₂)_x-(SnSe₂)_1−x layered crystals were grown using the vertical Bridgman technique for x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.9, 1.0. IR-spectra of these compounds were obtained using a Bruker-spectrometer. The structural properties were studied using transmission electron microscopy (TEM). The variation of the structural and optical properties was studied as a function of the composition x.     

Persistent luminescence of SrAl2O4: Eu,Dy, Cr phosphors in the tissue transparency window

We report on the persistent luminescence of SrAl₂O₄: Eu²⁺, Cr³⁺ phosphor centered at 760 nm. The phosphor was prepared by sol-gel-combustion method. Persistent luminescence from Cr³⁺ lasted for hundreds of seconds, comparable to the long afterglow from Eu²⁺ ions in the visible region based on the continuous energy transfer from Eu²⁺ ions to Cr³⁺ ions. The introduction of Dy³⁺ ions into the phosphor further prolonged the afterglow time of Eu²⁺ and Cr³⁺ ions through the depth control of the charge traps. The optimum doping concentrations for Eu²⁺, Cr³⁺ and Dy³⁺ were 1%, 2% and 1.5%, respectively.     

Near UV and blue-based LED fabricated with Ca8Zn(SiO4)4Cl2:Eu as green-emitting phosphor

Green-emitting phosphor Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺ has been prepared by the solid state reaction method and there luminescence properties are investigated. The excitation spectrum of Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺ shows an intense excitation band in the blue centered at 450 nm and emits with a maximum at 505 nm. The concentration quenching mechanism is studied and verified to be the energy transfer among the nearest-neighbor ions. Upon 450 nm excitation, the emission intensity of Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺ is much stronger than the green emitting Ca₃SO₄Cl₂:Eu²⁺ phosphor and even higher than YAG:Ce³⁺. This excitation spectrum range matches UV and blue light-emitting diodes (LEDS) chips very well, suggesting Ca₈Zn(SiO₄)₄Cl₂:Eu²⁺ could be a promising green emitting phosphor candidate for LED devices.     

Synthesis and photoluminescence properties of a new red emitting phosphor: Ca3(VO4)2:Eu; Mn

A new red emitting phosphor, Ca₃(VO₄)₂:Eu³⁺; Mn²⁺, was synthesized by a citric acid sol-gel combustion method and characterized by XRD, TEM and photoluminescence (PL) spectra. The red emission located at about 613 nm was ascribed to ⁵D₀-⁷F₂ transition of Eu³⁺. And the red luminescence intensity changed with annealing temperature and concentration of Eu³⁺. The effect of the co-doped Mn²⁺ was also investigated systematically.     

Crystal structure and spectroscopic studies of NaGd(PO3)4

Single crystals of gadolinium–sodium polyphosphate NaGd(PO₃)₄ were grown for the first time using a flux method and characterized by X-ray diffraction. This phosphate crystallizes in a monoclinic system with P2₁/n space group and with the following unit-cell dimensions: a = 9.767(3) Å, b = 13.017(1) Å, c = 7.160(2) Å, β = 90.564(5)°, V = 910.3(4) Å³ and Z = 4. The crystal structure was solved from 3451 X-ray independent reflections with final R(F²) = 0.0219 and R_w(F²) = 0.056 refined with 164 parameters (). The atomic arrangement can be described as a long chain polyphosphate organization. Two infinite (PO₃)∝ chains with a period of eight tetrahedra run along the [0 1 1] direction. The structure of NaGd(PO₃)₄ consists of GdO₈ polyhedra sharing oxygen atoms with phosphoric group PO₄. Each Na⁺ ion is bonded to eight oxygen atoms.     

Optical properties of Ba2SiO4:Eu phosphor for green light-emitting diode (LED)

Eu²⁺ activated Ba₂SiO₄ phosphors were synthesized at 1573 K by solid-state reactions under a weak reductive atmosphere and systematically investigated by photoluminescence excitation and emission spectra, diffuse reflectance spectra, concentration quenching process and lifetime. The intensive green LEDs were fabricated by combining the synthesized phosphors with near-ultraviolet InGaN chips (λ_em = 395 nm). The Commission Internationale de I’Eclairage color coordinate of the fabricated LEDs is calculated to be x = 0.1904, y = 0.4751 under 20 mA forward-bias current. The dependence of the green LEDs on forward-bias currents shows that as the current increases, the relative intensity simultaneously increases and the color coordinate presents excellent stability, falling in the standard area of the Institute of Transportation Engineers for traffic lights and the Society of Automotive Engineers for automotive displays. These results indicate that the fabricated phosphor-converted green LEDs show great potential for traffic lights and automotive display applications.     

Photoluminescence properties of emission-tunable Ca9Y(PO4)7: Tm,Dy phosphor for white light emitting diodes

A white-emitting Ca₉Y(PO₄)₇: Tm³⁺, Dy³⁺ phosphor has been successfully prepared by conventional high-temperature solid-state reaction. X-ray diffraction (XRD) and fluorescence spectrophotometer were used to characterize the as-synthesized phosphors. The excitation and emission spectra show that all the Tm³⁺ and Dy³⁺ co-doped Ca₉Y(PO₄)₇ samples can be effectively excited by UV light and then emit blue and yellow light simultaneously. Furthermore, the emission and color coordinate of as-obtained samples pumped by 365 nm are able to be adjusted around white light by varying the doping concentrations of Tm³⁺ and Dy³⁺. So, the as-fabricated single-composition Ca₉Y(PO₄)₇: Tm³⁺, Dy³⁺ phosphor will have a promising application in the area of white light emitting diodes.     

Near-ultraviolet excitable Ca4Y6(SiO4)6O: Ce,Tb white phosphors for light-emitting diodes

The present investigation aims to demonstrate the potentiality of Tb³⁺ and Ce³⁺ co-doped Ca₄Y₆(SiO₄)₆O phosphors. By incorporation of Ce³⁺ into Ca₄Y₆(SiO₄)₆O: Tb³⁺, the excitation band was extended from short-ultraviolet to near-ultraviolet region. The energy transfer from Ce³⁺ to Tb³⁺ in Ca₄Y₆(SiO₄)₆O host was investigated and demonstrated to be a resonant type via a dipole–dipole mechanism with the critical distance of 10.2 Å. When excited by 352 nm, Ca₄Y₆(SiO₄)₆O: Ce³⁺, Tb³⁺ exhibited a brighter and broader violet-blue emission (421 nm) from the Ce³⁺ and an intense green emission (542 nm) from the Tb³⁺. Combining the two emissions whose intensities were adjusted by changing the doping levels of the co-activator, an optimized white light with chromaticity coordinates of (0.278, 0.353) is generated in Ca₄Y₆(SiO₄)₆O: 2% Ce³⁺, 8% Tb³⁺, and this phosphor could be potentially used in near-ultraviolet light-emitting diodes.     

Fine particle Sr2SiO4:Tb phosphor for the plasma display panel prepared by a combined approach

Sol-gel with microwave heating was employed to prepare fine particles Sr₂SiO₄:Tb phosphor. X-ray diffractometer was used to characterize the structural of the samples. The Scanning Electron Microscope image shows that the particle size is about 300 nm. The phosphor particles have several advantages in the morphology, such as excellent surface quality, spherical shape, and narrow size distribution with no aggregation. The VUV luminescence measurements indicate that the phosphor presents an intense excitation band at 173 nm. Because the wavelength of excitation source in PDP is mainly at 147 and 172 nm, it makes Sr₂SiO₄:Tb a potential candidate for green emitting phosphor for plasma display panel (PDP) application. Photoluminescence (PL) measurements indicate that the Sr₂SiO₄:Tb particles present excellent green emission at 542 and 547 nm excitated at 236 and 172 nm, respectively.