Crystal Phosphors Research Articles

Characterizations of white-light ZnWO4 phosphor prepared by blending complementary phosphor

In the preparation of ZnWO4 phosphor, crystalline ZnWO4 was created, even though the concentration of WO3 was only 10 mol%. ZnWO4 was the dominant crystallization phase when the concentration of WO3 exceeded 40 mol%. The optimal crystallization of ZnWO4 phosphor was obtained when the composition molar ratio of ZnO to WO3 was 1:1, and sintering was carried out at 1,100°C for 3 h. In this condition, a bluish-green emission with a peak at 460 nm was observed. For Y2O3:Eu3+,Li+, the complementary phosphor of ZnWO4, the Li flux improved phosphor crystallization. The red emission peak of the Y2O3:Eu3+,Li+ phosphor was measured at about 612 nm. The optimal photoluminescence intensity of the Y2O3:Eu3+,Li+ phosphor was obtained when it was sintered at 1,200°C for 5 h and was mixed with 11 mol% Eu2O3 and 70 mol% Li2CO3. When the weight ratio of Y2O3:Eu3+,Li+ to ZnWO4 was 1:4, the Y2O3:Eu3+,Li+-blended ZnWO4 phosphor showed white-light emission with Commission Internationale de l’Eclairage coordinates at (0.34, 0.30). The luminance of the white-light phosphor excited by a 6-W UV lamp was around 160 cd/m2.

Green-emissive transparent BaSi 2O 5:Eu 2 + film phosphor on quartz glass created by a sputtering thermal diffusion process

Eu 2+-doped BaSi 2O 5 film phosphors on quartz substrates are fabricated by radio-frequency magnetron sputtering thermal diffusion. The BaSi 2O 5: Eu 2+ phosphor crystals have some preferred orientations that are lattice-spacing matched with the crystallized β - SiO 2 crystals, and they show pore and grain boundary-free morphology with a rod-like shape fused into the crystallized β - SiO 2 crystals. The BaSi 2O 5: Eu 2+ film phosphor has a high transparency, with a transmittance of about 30% in visible light. The BaSi 2O 5: Eu 2+ film phosphor shows 510 nm green emission from the f–d transition of the Eu 2+ ions, and in particular the best sample shows a green photoluminescence brightness of about 5% of a BaSi 2O 5: Eu 2+ powder phosphor screen. These excellences in optical properties can be explained by less optical scattering at pores or grain boundaries, and less reflection at the continuously index-changed interface.

Preparation and luminescent properties of Eu2+‐activated glass ceramic phosphor precipitated with β‐Ca2SiO4 and Ca3Si2O7

AbstractEu2+: calcium silicate glass ceramics (GC) containing β‐Ca2SiO4 and Ca3Si2O7 crystal phosphors were fabricated by cerammization process from a GC that were prepared by a novel GC preparation; frozen sorbet method. The GC obtained (as‐made GC) consisted of glass and spherical β‐Ca2SiO4 crystal phases with size of about 20–40 μm. After post‐ceramization, Ca3Si2O7 crystals were also precipitated. Judging from the cathodoluminescence (CL) mapping images, two emission bands peaked at 515 nm and 600 nm were observed from different parts in the GC. The observed photoluminescence (PL) have two broad emission bands at 515 nm from Eu2+: β‐Ca2SiO4 and at 600 nm from Eu2+:Ca3Si2O7. With increasing heat‐treatment temperature, the color coordinates shifted from green range to center region, which corresponds to pure white in the CIE chromaticity diagram. Eu2+ doped calcium silicate GC containing β‐Ca2SiO4 and Ca3Si2O7 are suitable phosphor as the use of phosphor converting white light emitting diode (pc‐wLED) with high color rendering. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of phase separation on crystallization of glasses in the BaO-TiO2-SiO2 system

The effects of phase separation on the crystallization behavior of glasses were investigated in order to materialize a high-luminous glass-ceramic phosphor, which consists of fine phosphor crystals and a transparent glass matrix prepared due to phase separation. The glass ceramics in BaO-TiO2-SiO2 has the potential to show high photoluminescence intensity, because of multiple light scattering at the boundary between the phosphor crystal and the glass matrix. The crystallization behaviors of the glasses were examined using both the XRD analyses for an isothermal heat treatment process and the DTA measurements as a non-isothermal treatment process. The phase-separation in the glass significantly affected the subsequent crystallization behavior. In the non-phase-separated glasses, surface crystallization occurred first, and crystallization then proceeded inside the glasses. In the phase-separated glasses, the crystallization was considered to start at the boundary of two glass phases inside the glass and the crystals grew in BaO-TiO2-rich phases. This apparent bulk crystallization owing to the phase separation has the potential to materialize a high-performance phosphor composite.

Forming protective nanosize Y_2O_3 coatings on crystal phosphors

This article describes the formation of protective nanosize (10–15nm thick) Y2O3 coatings on the surface of crystal phosphors, using aqueous and water-alcohol solutions of yttrium nitrate. The influence of the concentration and pH of the solutions on the structure of the coatings and the luminescence properties of the crystal phosphors is studied.

Steady-state chemiluminescence of Eu-doped yttrium oxide crystal phosphors in the catalytic reaction of hydrogen oxidation

We report on the luminescence of Eu 3+-doped yttrium oxide phosphors brought in contact with oxyhydrogen gas mixtures at temperatures much below the gas-phase ignition point. Chemical energy released in the catalytic hydrogen oxidation is passed directly to the electron subsystem of the powder and subsequently converted into the visible luminescence radiation at 2 × 10 −6 quantum yield per H 2O molecule produced. Luminescence kinetics at various conditions are investigated to reveal the constituent stages of the catalytic energy conversion process. This observation could have far-reaching consequences including possibilities for the development of original chemiluminescent light sources, and novel electrochemical energy converters.

Luminescence properties of Eu 2+-doped β-Si 6−zAl zO zN 8−z microcrystals fabricated by gas pressured reaction

Eu 2+-doped β-SiAlON microcrystals with compositions of Eu x Si 6− z Al z O z N 8− z ( x=0.016, z=0.1–2.0) were successfully fabricated by gas pressured reaction. The phase purity, microstructure, crystallinity and luminescent properties of Eu 2+-doped β-SiAlON microcrystals were investigated in detail. The prepared β-SiAlON:Eu 2+ phosphor crystals with Al-doping concentration of z<1.0 showed pure single β-SiAlON phase without secondary polytypoid phases. The prepared β-SiAlON:Eu 2+ microcrystals absorbed a UV-visible spectral region, and showed a single intense emission peak in a spectral range from 528 to 536 nm. The optimal conditions of Al 3+-doping concentration ( z value) for emission intensity, peak wavelength and bandwidth for application to light emitting diodes (LEDs) were discussed.

Luminescence and crystallinity enhancement using nano-oxide

The seeded nano-TiO 2 particles in the hydrothermal (HT) solution assist phosphor crystallite nucleation and growth, facilitate formation of cubic grain shape, but limit grain growth during post-annealing so as to strongly enhance the cathodoluminescence (CL) intensity. Nano-crystallites of SrTiO 3:Pr, Al phosphors approximately 80 nm with specific shape are successfully obtained and red cathodoluminescence is easily excited at low accelerating voltage of 1 kV. The obtained phosphors after annealing at 1000 °C 1 h −1 exhibit the highest CL intensity than those obtained under other annealing conditions and those direct-pyrolyzed phosphors as well as mixed-oxide (MO) phosphors having composition similar to hydrothermal ones. The smaller phosphor crystal possesses larger surface and homogeneous activators distribution to facilitate the efficient excitation of low-energy electron bombardment of 1 keV.

Advances in sulfide phosphors for displays and lighting

For traditional applications such as cathode ray tubes and fluorescent lamps, many inorganic phosphors have been optimised during decades. For new applications in display and lighting technology, novel materials are being developed. After giving a short history of sulfide phosphors, the present paper will focus on Ca1−x Sr x S:Eu single crystal particle phosphors. The material is grown by solvothermal synthesis, process not needing toxic gases or high temperature processing steps. This phosphor combines a broad excitation spectrum and a broad—saturated red—emission spectrum, which makes it an ideal material for wavelength conversion in LEDs for general lighting.

Characterization of noise sources for two generations of computed radiography systems using powder and crystalline photostimulable phosphors

The performances of two generations of computed radiography (CR) were tested and compared in terms of resolution and noise characteristics. The main aim was to characterize and quantify the noise sources in the images. The systems tested were (1) Agfa CR 25.0, a flying spot reader with powder phosphor image plates (MD 40.0); and (2) the Agfa DX-S, a line-scanning CR reader with needle crystal phosphor image plates (HD 5.0). For both systems, the standard metrics of presampled modulation transfer function (MTF), normalized noise power spectra (NNPS) and detective quantum efficiency (DQE) were measured using standard radiation quality RQA5 as defined by the International Electrotechnical Commission. The various noise sources contributing to the NNPS were separated by using knowledge of their relationship with air kerma, MTF, absorption efficiency and antialiasing filters. The DX-S MTF was superior compared with the CR 25.0. The maximum difference in MTF between the DX-S scan and CR 25.0 subscan directions was 0.13 at 1.3 mm(-1). For a nominal detector air kerma of 4 microGy, the peak DQE of the DX-S was 43 (+/-3)%, which was over double that of the CR 25.0 of 18 (+/-2)%. The additive electronic noise was negligible on the CR 25.0 but calculated to be constant 3.4 x 10(-7) (+/-0.4 x 10(-7)) mm2 at 3.9 microGy on the DX-S. The DX-S has improved image quality compared with a traditional flying spot reader. The separation of the noise sources indicates that the improvements in DQE of the DX-S are due not only to the higher quantum, efficiency and MTF, but also the lower structure, secondary quantum, and excess noise.

Effect of Substrate Properties on Luminescence of White ZnGa2O4 Phosphor

The variations in the crystallization and luminescence characteristics of ZnGa2O4 phosphor screens deposited on various substrates were investigated. The ZnGa2O4 phosphor deposited on Corning glass, indium tin oxide glass and Si substrates was polycrystalline. However, when a ZnO buffer layer was prepared, it was amorphous; it was polycrystalline after annealing at a temperature higher than 300 °C. The optimal cathodoluminescence was caused by the low backscattering of electrons, a large emission area, and the specific crystallization of the ZnGa2O4 phosphor grown on the ITO glass substrate. When a ZnO buffer layer was prepared, small ZnGa2O4 grains were grown; consequently, the emission area and CL intensity increased. Resistivity measurement revealed that the conductivity and cathodoluminescence are improved by thermal annealing.

Luminescence Enhancement Mechanism of ZnGa2O4 Phosphor Screen with an In2O3 Buffer Layer

The luminescence enhancement mechanism of a white phosphor screen with an buffer layer was studied by growing with optimal sputtering parameters and varying the deposition conditions of . A film with better crystallization and smaller grains was obtained when it was deposited on . These qualities revealed that the resistivity of the phosphor screen was correlated with the surface roughness and resistivity of the buffer layer. The photoluminescence spectra of the phosphor screen revealed that the ligand field of the Ga luminescence center was not modified when the deposition parameters were varied; however, the emission intensities were altered due to the electron-transition probability in the Ga energy levels changing with the crystallization of phosphor. A significantly enhanced emission from the phosphor screen was observed by improving the crystallization, surface morphology, and resistivity of the prepared phosphor film with the buffer layer.

Spectral study on manganese- and silver-doped lithium tetraborate phosphors

Manganese- and silver-doped lithium tetraborate (LTB) single crystal and glassy phosphors with different dopant contents were studied. Optical absorption, steady-state and time-resolved photoluminescence (PhL) techniques were used for spectral characterization of samples. Optical absorption, as well as PhL excitation and emission spectra of LTB:Mn single crystal, revealed that manganese is present only in the form of Mn 2+ions, while in the glassy LTB:Mn simultaneous presence of Mn 2+ and Mn 3+ ions was detected. Optical absorption and PhL spectral characterization of LTB:Ag single crystal showed that only one type of Ag 1+ ions are absorbing and emitting centers while in the LTB:Ag glassy samples the presence of at least two non-equivalent surrounding of Ag 1+ ions was revealed.

Observation of whispering gallery modes in cathode luminescence in TiO2:Eu3+ microspheres

We report cavity enhanced periodic structures in cathode luminescence spectra in TiO2:Eu3+ microspheres ranging from 6.2to12.2μm in diameter. The spectral structures observed at the accelerating voltage below 10kV are attributed to the lower order whispering galley modes, while the additional components appearing at 20kV are to the higher order modes within the microsphere. These observations illustrate that the basic properties of phosphors can be advanced from the point of view of cavity quantum electrodynamics by fabricating a microcavity structure onto a single phosphor crystal.

Trap Levels in Eu‐Doped SrAl2O4 Phosphor Crystals Co‐Doped with Rare‐Earth Elements

SrAl2O4:Eu2+ phosphor crystals co‐doped with auxiliary activators such as La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or Y have been grown by the floating zone (FZ) technique. Photoluminescence spectrum (PL), time‐resolved PL, and thermally stimulated luminescence (TSL) are evaluated to clarify the long‐duration phosphorescence mechanism of SrAl2O4, Eu, and Ln phosphors. TSL spectra were measured in the temperature range from RT to 600 K to evaluate the depth and densities of the traps generated by the doping of auxiliary activators that are responsible for the long‐duration phosphorescence. The peak wavelength of PL does not vary with auxiliary activator elements, while decay curves vary greatly with the auxiliary activators. The trap depth and the densities of the trapped carriers estimated based on the hole trap model also vary with the auxiliary activator elements. The traps generated at around E=0.5 eV by the auxiliary activators, Nd, Dy, and Tm, with sufficient densities are effective for the long‐duration phosphorescence.

Activation of zinc sulfide luminescence by modifying its surface by organic solvents

The influence of the properties of the environment interacting with the surface of crystal phosphor on its radiative recombination has been considered. The influence of treatment with hexane and toluene on the state of the surface of powdery ZnS:Cu:Cl has been investigated. It has been shown that treatment with hexane and toluene leads to an increase in the photoluminescence intensity of the luminophor under investigation. On the basis of the analysis of the luminescence and photoluminescence excitation spectra, a model explaining the change in the concentration of radiationless recombination centers on the surface of powdery ZnS:Cu:Cl is proposed. It has been shown that the interaction between the above organic liquids and the luminophor surface leads to its passivation and improvement of the luminescent properties of the luminophor.

Low temperature fluorescence thermometer application of long afterglow phosphorescent SrAl12O19:Eu2+,Dy3+ crystals

Photoluminescence from long afterglow phosphorescent SrAl12O19:Eu2+,Dy3+ phosphor crystals have been studied for a fluorescence thermometer at a temperature lower than 200K. Broadband emissions peaking at λ=404 and 519nm are observed from the crystals at room temperature. The λ=519nm emission is strongly coupled with hole traps generated by the auxiliary activator, which dominates long afterglow phosphorescence from the crystals. The lifetime of the λ=519nm emission was, therefore, much longer than that at λ=404nm. The intensity of the λ=404nm emission decreases with decreasing temperature and it also decreases with increasing in the intensity of λ=371nm emissions due to P7∕26-S7∕28 transition. The intensity of λ=371nm emissions is found to be extremely sensitive to the temperature below 200K. The peak intensity ratio between λ=371nm emissions and the λ=404nm emission is effective for the highly sensitive measurement of a temperature lower than 200K.

Thermoluminescence characterization of nanocrystalline and single Y3Al5O12 crystal exposed to β-irradiation for dosimetric applications

The thermoluminescence (TL) characterization under β-irradiation of undoped nanocrystalline Y3Al5O12 (YAG) prepared by precipitation process and the thermoluminescence of single crystal are presented. The crystallite size induces changes in the TL glow curves and the kinetic properties modifying the radiative recombination efficiency being different than single crystals. The TL glow curves of nanocrystals present wider thermal band than single crystals and with the advantage that the main TL peaks are located far from room temperature. The TL results suggest that both undoped YAG nanocrystalline phosphor and single YAG crystal presents good potential for β-ray irradiation dosimetric applications.

Needle vs. Powder Imaging Plate for Computer Radiography: Image Quality Measurement and Model Calculation

Computed radiography (CR) is a digital radiography technology in which a storage phosphor plate is used to store a latent X-ray image. The plate is exposed in a light-tight cassette and then read out in a scanner to create the digital image. Conventionally, CR powder imaging plates (PIP) are used based on BaFBr1-xIx:Eu2+ phosphor. The active layer consists of phosphor microcrystals in a polymer binder. A needle imaging plate (NIP), created by of vapor deposition of needle-shaped phosphor crystals, leads to better image quality because thicker phosphor layers, having higher X-ray absorption can be used. BaFBr1-xIx:Eu2+ is an excellent storage phosphor. It decomposes upon vaporization, however. For that reason, it is impossible to vapor deposit BaFBr1-xIx:Eu2+ needle crystals. At Agfa an excellent new storage phosphor, CsBr:Eu2+, was discovered. Since CsBr melts congruently, it allows thermal vapor deposition and production of NIP's. Measurements demonstrate that CsBr:Eu2+ NIP's allow to double CR image quality (DQE). A linear-systems approach is used to model signal and noise transfer in a CR system using PIP or NIP. The transfers are described by cascading transfer relationships for each process. The calculated image quality (DQE) is in good agreement with measurement for the PIP system. The model overestimates the NIP system DQE at high spatial frequencies. An overestimation of the system gain may be the reason.

Characteristics of long afterglow phosphorescent calcium aluminate crystals for fluorescence thermometer application

Temperature dependence of photoluminescence (PL) peaking at wavelength of λ = 440 nm from FZ grown CaAl 2 O 4 ; Eu, Dy phosphor crystal is evaluated for a fiber-optic thermometer application. The PL lifetime from the specimens decreases greatly from 11.24 to 1.17 ms as the temperature increases from 305 to 458 K. The temperature coefficient of PL lifetime, - 0.17 ms / K , in the narrow temperature range from 305 to 345 K is larger than that of ruby (Cr doped Al 2 O 3 ) whose PL lifetime is 4 ms and temperature coefficient is - 0.169 ms / K . The large temperature coefficient of PL lifetime of CaAl 2 O 4 : Eu, Dy phosphor crystal is due to the trap levels generated by Dy doping.