Phosphor Particles Research Articles

Controlled synthesis and photoluminescence properties of Bi2SiO5:Eu3+ core-shell nanospheres with an intense 5D0→7F4 transition

Core-shell Bi2SiO5 nanosystem with uniform morphology and narrow size distribution has been successfully synthesized via a facile template-assisted route. With the introduction of Eu3+, detailed studies are performed to evaluate its promise as Eu3+-based phosphor host. The yielded Bi2SiO5:Eu3+ nanospheres are proven to be pure tetragonal phase via X-ray diffraction and Rietveld refinement. Moreover, the phosphor particles consist of monodisperse spheres with an average diameter of approximately 285 nm by high-resolution electron microscopy. When excited by near-ultraviolet (NUV) light, the abnormally high-intensity emission at 703 nm arising from the 5D0→7F4 transition of Eu3+ is observed. The temperature-dependent photoluminescence spectra show that the optimized Bi2SiO5:20%Eu3+ have satisfactory thermal stability with 63.7% of emission intensity at 423 K relative to 303 K. The deep-red light-emitting diode (LED) device fabricated by coating NUV chip with the Bi2SiO5:20%Eu3+ phosphors is demonstrated. The newly-developed Bi2SiO5:Eu3+ nanophosphors display commendable photoluminescence properties, demonstrating their promise as deep-red phosphor candidates for use in phosphor-converted LEDs.

Assessment of colloidal NaGdF4:Er3+/Yb3+ upconversion phosphor as contrast enhancer for optical coherence tomography

In-vitro imaging by swept-source optical coherence tomography (OCT) represents a class of advanced tomography that provides comprehensive and accurate clinical diagnostics. Here colloidal NaGdF4:Er3+/Yb3+ particles having an average size of 32 nm were successfully synthesized by a facial thermal decomposition method and then used as a contrast agent in OCT imaging. Prepared phosphor has shown bright green upconversion emission under 976 nm diode laser. The phosphor particles had increased the scattering of OCT scanning radiation, and thus higher image contrast was observed. The results indicate that upconversion phosphors may work as suitable contrast agents for OCT imaging.

Fabrication and luminescent studies of near-spherical phosphor embedded epoxy-resin nanocomposite beads

A facile method for feasible industrial production of long afterglow phosphor nanocomposites, in the form of small beads were developed that continuously emit visible light for >7 h to realize many innovative applications involving hard and soft surfaces. Green emitting SrAl2O4:Eu2+, Dy3+ (SRA) long afterglow phosphor was uniformly dispersed in the epoxy resin (ER) matrix in presence of organic surfactant as hardener to make phosphor-embedded viscous fluid that could then be solidified at room temperature (~25 °C) within few minutes. As ethyl alcohol (solvent) started evaporating, ethylene glycol (surfactant), which is the simplest amongst glycols, started adhering with SRA phosphor particles via van der Waals attraction. The homogeneity of dispersion strongly depends on the shape of SRA phosphor added to ER matrix and confirmed that the near-spherical is the best. The SRA phosphor–ER beads demonstrated an optimum ultraviolet (UV) excitation wavelength at 375 nm and a green-emission peak at 517 nm, where the latter one is due to the transitions from 4f65d1 to 4f7 energy levels of Eu2+ ions. The appearance of SRA phosphor–ER beads in translucent yellow-green color, bright green and green under ambient light, UV (375 nm) irradiation and in dark conditions, respectively was indicated using photoluminescence and colorimetric measurements. The SRA phosphor–ER beads exhibited highly durable, reversible and reproducible afterglow luminescence for longer durations. Photoluminescence, structural, morphological and afterglow properties of the prepared SRA phosphor–ER beads were investigated thoroughly. Moreover, these afterglow beads have several designing and crafting applications such as interior design, glowing toys, ornaments and tracking lights etc.

Controlling Phosphor Particle Distribution for High-Angular-Color-Uniformity and Low-Cost LEDs Based on Thermalcapillary Flow

Low cost and high performance are the pursuing goals of the light-emitting diode (LED) packaging. The phosphor layer configuration of LEDs significantly influences optical performances. In this study, thermalcapillary flow was introduced in the phosphor coating by the blue laser local irradiation. It has been demonstrated to successfully manipulate the phosphor particle distributions. In the phosphor dispensing coating, a centripetally concentrated particle distribution was realized and resulted in high angular color uniformity (ACU) and low-cost manufacture. Its angular correlated color temperature (CCT) deviation was reduced by more than 65%, compared to the traditional phosphor dispensing coating. Besides, the phosphor amount is saved by about 12%. The mechanism of ACU enhancement is analyzed. The realized phosphor particle distribution can inspire further development of the optimized phosphor layer configuration.

Tailoring Particle Distribution for White LEDs With High Color-Uniformity by Selective Curing

The phosphor layer determines optical performances of phosphor-converted white light-emitting diodes (WLEDs). In this letter, a new method to control phosphor particle spatial distributions is proposed. Using blue laser irradiation, phosphor gel was selectively and rapidly cured. In the cured phosphor gel, phosphor particles were fixed. The sedimentation occurred in uncured phosphor gel to form particle patterns. With such a method, a kind of remote-conformal-combined particle distribution was obtained. It has been demonstrated that this phosphor layer configuration can result in the high-angular-color-uniformity LED packaging with different average correlated color temperatures (CCT). Compared to the dispensing coating and settling coating, the presented selective curing coating reduces the angular CCT difference by more than 74% and realizes the minimum CCT difference of 184K in the viewing angles ranging from −70° to 70°. Besides, it has no impact on the light intensity distribution. The Pearson correlation coefficients between the light intensity distributions of the selective curing coating with those of the others are larger than 0.999.

Preparation of Ce-doped yttrium aluminum garnet phosphor particles using spray drying method

ABSTRACT The potential phosphor particles of Ce-doped yttrium aluminum garnet (YAG: Ce) particles were prepared using the spray drying method. This method yields a homogenous composition, regularly shaped particles, and has a short processing time. Meanwhile, acid catalysts affect the yield and particle shape of the spray dried particles. Therefore, in this study, three common acids i.e. formic acid, oxalic acid, and citric acid, were used to prepare the YAG: Ce powders. The X-ray diffraction measurements showed that all acid catalysts formed a single YAG phase. The SEM surface images and FIB cross-sectional images revealed two typical smooth hollow spherical and concaved solid spherical geometries. The statistical analysis showed that the particle size followed the order of citric acid-treated powder > oxalic acid-treated powder > formic acid-treated powder. Meanwhile, the citric acid-treated phosphor had a higher photoluminescence (PL) intensity than the formic acid- and oxalic acid-treated phosphors. The PL intensities of these three phosphors were correlated with their crystallite sizes, which is affected by the heat of combustion of the acid catalysts. Furthermore, the influence of the precursor properties on the corresponding formation mechanisms of the YAG: Ce powders was discussed.

Finite strain elasticity based cohesive zone model for mechanoluminescent composite interface: I. Stiffness of the undamaged interface

Light emission from inorganic crystals due to elastic loading, termed piezoluminescence or EML, is of great practical interest due to repeatability of emission. Two architectures demonstrated for utilization of EML are thin films and particulate composites, with the majority of recent literature focusing on the latter. Specifically, EML of doped zinc sulfide phosphor particles dispersed in polymer matrices has been recently demonstrated for a growing number of applications. This necessitates the need to understand the micromechanics of stress transfer within these composites for building accurate EML models as well as fatigue models to predict functional life estimates of the EML devices. Macroscale stress measurements can be combined with material properties through micromechanical models to estimate stress transfer to the particulate phase. Apart from the material properties of the matrix and particles, the bonding between the two phases also influences stress transfer significantly. The bonding at the interface is modeled through a CZM which provides a deterministic way to track interfacial damage due to fatigue. A bilinear CZM defines the undamaged stiffness of the interface ( and its resistance to damage ( in two piecewise linear stages. In this paper, we utilize established micromechanical models to determine the stiffness of the undamaged interface between EML ZnS:Cu particles and PDMS matrix in the normal and tangential directions ( and ) with the help of experimentally determined mechanical and morphological properties of EML–PDMS composites. and determined are then validated by comparing stress strain behavior of a finite element representative volume with experimental monotonic tensile test results to find good correlation.

Quantum Dots Electrostatically Adsorbed on the Surface of SiO2 Nanoparticle-Decorated Phosphor Particles for White Light-Emitting Diodes with a Stable Optical Performance

White light-emitting diodes (WLEDs) based on phosphor and quantum dots (QDs) have attracted worldwide attention. Phosphor and QDs with different physical characteristics are distributed unevenly in...

Particle based investigation of self-heating effect of phosphor particles in phosphor converted light emitting diodes

Since 2003, self-heating effect of phosphor which reduces the efficacy and reliability of phosphor conversion (Pc) light emitting diodes (LEDs), has been a growing research area of opto-thermal Pc-LED modeling. However, few studies have focused on the particles based nature of the phosphor self-heating. Based on a new approach, accurate Monte Carlo ray tracing simulations are performed in regions of interest over discretized control volumes where only a single phosphor particle is exposed to the light radiation. Governed by the Mie scattering effects, heat generation values of phosphor particles showed strong dependency on their optical and geometrical properties. Additionally, based on the emissive behavior of the LED, space above the LED can be divided into two irradiance levels; near and far scale regions. In near scale region, where irradiance levels are above 0.1 W/mm2, phosphor particles exhibited significant self-heating in milli-Watt scale values. Derived self-heating values are imported to simplified thermal models where phosphor particles showed a temperature rise in excess of 100 °C more than LED chip which can lead to considerable conversion efficiency loss and a reduction in lifetime. Higher temperatures are expected in higher irradiance levels where matrix material carbonization can also occur causing fire hazards.

Micronization of KSrPO4:Eu and KBaPO4:Eu phosphor particles for white light-emitting diodes by pulsed laser ablation in liquid

Eu2+-doped KSrPO4 and KBaPO4 phosphors have attracted much attention for white light-emitting diode (W-LED) applications. In this study, pulsed laser ablation in liquid (PLAL) was carried out in order to micronize fatally aggregated particles of KSrPO4:Eu2+ and KBaPO4:Eu2+ by H2 reduction under a high temperature to a suitable size for the W-LED applications. The diameter of aggregated particles after H2 reduction (approximately 2.0 μm) was successfully decreased to about half by PLAL, which is suitable for the W-LED applications. Furthermore, PLAL was found to maintain the luminescence intensity as high as before PLAL, indicating no contamination of impurities from processing devices. In addition, PLAL was also applied to KSrPO4:Eu3+ and KBaPO4:Eu3+ before H2 reduction and found not only to maintain the luminescence intensity but also to reduce the valence of Eu ions (Eu3+ → Eu2+). These results show not only the effectiveness of the micronization but also the feasibility of laser induced photo reduction of Eu ions in phosphate crystals for the W-LED applications by PLAL.

3D relative dose measurement with a μm thin dosimetric layer

A μm thin dosimetric coating material based on inorganic phosphor particles NaYF4:Yb3+, Er3+ has been developed. A dose dependency of the luminescence decay time is observed in the μs - ms domain which allows relative dose measurement in the kGy range. Dose values are obtained from decay time evaluation which facilitates a measurement independent of luminescence emission intensity. Here, near-IR luminescence after excitation with 5 ms pulses of near-infrared light is analyzed. The dosimetric material can be directly sprayed onto the surface of objects and therefore renders particularly useful for dose distribution measurements on non-planar geometries. Because of the μm thickness and the direct contact of the dosimetric material to the entire surface of an object, the results can be considered as ‘true’ surface doses. Three dimensional dose maps were derived by scanning the entire surfaces of irradiated objects. Proof-of-concept is provided by good agreement in dose results as well as dose pattern with film dosimetry (Risø B3 strips) and the approach therefore qualifies especially for use in 3D dose mapping of complex geometries. This novel approach provides unprecedented insights into dose distributions of irradiated objects of complex geometries, as exemplified by dose results for outer and inner surfaces of e-beam irradiated bottles for two different irradiation set-ups.

Room-temperature densification of MgO bulk ceramics with dispersed nitride phosphor particles

Wave conversion materials with high thermal conductivity are necessary for high-power semiconductor lighting. Ceramics have higher thermal conductivity than existing matrices such as resin or glass in which phosphor particles are dispersed. However, the high densification of ceramics generally requires high-temperature sintering, which degrades and alters the phosphor particles. In this study, we aimed to achieve the high densification of MgO ceramics at room temperature. Applying high hydrostatic pressure with water addition improved the sample packing ratio and promoted the formation of Mg(OH)2. As a result, the relative density was ≥95%. Additionally, various nitride phosphor particles (CaAlSiN3:Eu2+, β-SiAlON:Eu2+, and α-SiAlON:Eu2+) were dispersed in the MgO matrix at room temperature without degrading the luminescence property. The thermal conductivity of the obtained sample was about 8 W m−1K−1, 40 times higher than that of the epoxy matrix.

Improving thermal stability and quantum efficiency through solid solution for Ce3+-activated (Ba1-xSrx)3Y2(BO3)4 phosphors

Thermal stability and quantum efficiency are crucial parameters for phosphors when they act as components in phosphor-converted white light-emitting diodes (pc-wLEDs). Therefore, it is important to explore phosphors with highly thermal stability and quantum efficiency. Blue-emitting Ce3+-activated (Ba1-xSrx)3Y2(BO3)4 phosphors were synthesized through high-temperature solid-state reaction. X-ray powder diffraction suggests the success of complete solid solution in the phosphor series. Scanning electron microscope (SEM) image and elemental mapping suggest uniform distribution of Ba and Sr atoms in the phosphor particles. Rietveld refinement indicates that cell volume decreases with increasing the content of Sr2+, while the related emission peak exhibits a slight blue shift, which implies the existence of remote control effect. Both thermal stability and quantum efficiency increase with the evolution of solid solution from Ba3Y2(BO3)4:Ce3+ to Sr3Y2(BO3)4:Ce3+, which is due to the increase of structural rigidity. The emission intensity of Sr3Y1.94(BO3)4:0.06Ce3+ at 423 K remains ∼56% of the room-temperature value. The internal and external quantum efficiencies of Sr3Y1.94(BO3)4:0.06Ce3+ are ∼93% and ∼67%, respectively. A pc-wLED was fabricated with a 365 nm UV chip, Sr3Y1.94(BO3)4:0.06Ce3+, commercial green phosphor, and commercial red phosphor, which exhibits a color rendering index of 94.5. This work provides an example on the adjustment of luminescent properties through solid solution.

The influences of calcium fluoride and silica particles on improving color homogeneity of WLEDs

The LEDs lighting device with phosphor ingredient (pcLEDs) is among the most common lighting methods in recent years and evaluated by chromatic uniformity and lighting capacity. Therefore, we introduce the phosphor particles that can improve the scattering efficiency (SEPs) to apply in pcLEDs at 8500 K correlated color temperature (CCT) with the expectation to produce better pcLEDs by enhancing both quantity and quality of emitted light. Combining various materials such as CaF 2 and SiO 2 with yellow Y 3 Al 5 O 12 :Ce 3+ phosphor composition in the pcLEDs simulation created by the LightTools program is the mechanism of this research. The simulated pcLEDs are tested and the results will be verified with Mie-scattering theory. The observation of the simulation leads to the conclusion about the scattering coefficients of SEPs at 455 nm and 595 nm wavelengths. The calculation showed that CaF 2 is better for color homogeneity yet suffer from luminous flux deficiency as the concentration gets higher. On the other hand, SiO 2 is the scattering enhancement material that can maintain high luminous flux regardless of its concentration.

Phosphor thermometry at the surface of single reacting large-diameter spherical coke particles to characterise combustion for packed bed furnaces

Packed-bed furnaces fired with large-diameter coke are important in high-temperature material processing industries such as lime and iron production. The combustion conditions are complicated by the presence of an ash layer surrounding the coke particle that remains intact during passage through the furnace and alters oxygen diffusion and heat transfer to the reacting particle core. The objective of this study is to determine the surface temperature of this ash layer using lifetime-based phosphor thermometry during combustion of single spherical 38 mm diameter coke particles in a high temperature tube furnace. Time traces of the coke particle core temperature and mass conversion rate do not significantly differ between experiments performed with and without the phosphor layer, indicating that the presence of the phosphor particles does not alter the overall combustion behaviour. Surface temperatures of up to 950 °C are measured and correlated with the fuel mass conversion rate. When the coke particle starts to react the surface temperature is up to 100 °C higher than that of the core. As the reaction front progresses toward the centre, the core temperature exceeds the surface temperature by 200 °C due to the insulating effect of the ash layer. The surface temperature of the ash layer decreases with time due to the steadily decreasing fuel mass conversion rate. The method and results can be used to provide key validation data for shrinking-core combustion models, for example by constraining the unknown transport properties of the ash layer, thereby assisting the development of complete packed-bed furnace simulations for process optimisation.

Achieving thermally stable and anti-hydrolytic Sr2Si5N8:Eu2+ phosphor via a nanoscale carbon deposition strategy

Chemical stability of phosphors is critical to the efficiency and lifetime of the white light-emitting diodes. Therefore, many strategies have been adopted to improve the stability of phosphors. However, it is still lack of report on the improvement of thermal stability and hydrolysis resistance of phosphors by a single layer coating. Due to the high transmittance and high chemical inertness of graphene, it was coated on the surface of Sr2Si5N8:Eu2+ phosphor by chemical vapor deposition, aiming to improve its thermal stability and hydrolysis resistance. The chemical composition and microstructure of the coating were characterized and analyzed. A nanoscale carbon layer was attached on the surface of Sr2Si5N8:Eu2+ phosphor particles in an amorphous state. In coated Sr2Si5N8:Eu2+ phosphor, the oxidation degree of Eu2+ to Eu3+ was significantly suppressed. At the same time, the surface of Sr2Si5N8:Eu2+ particle turned from hydrophilic to hydrophobic after carbon coating, and consequently the hydrolysis resistance of Sr2Si5N8:Eu2+ phosphor was greatly improved. After tests at 85 °C and 85% humidity for 200 h, the carbon coated Sr2Si5N8:Eu2+ phosphor still maintained about 95% of its initial luminous intensity as compared with 35% of the uncoated. By observing the in-situ microstructure evolution of coated phosphor in air-water vapor environment, remained presence of the carbon layer even at 500 °C explained the excellent chemical stability of carbon coated Sr2Si5N8:Eu2+ phosphor in complex environment. These results indicate that a nanoscale carbon layer can be used to provide superior thermal stability and hydrolysis resistance of (oxy) nitrides phosphors.

Photocatalytic Properties of g-C3N4–Supported on the SrAl2O4:Eu,Dy/SiO2

Graphitic carbon nitride (g-C3N4) was supported on SrAl2O4:Eu,Dy-SiO2 by a colloidal-sol coating method to improve its light absorption property. Transmission electron microscopy (TEM) revealed that the nanoparticles of g-C3N4 were coated on sub-micron phosphor particles and nanoscale surface roughness was imparted by the SiO2-binder. Photoluminescence (PL) spectrum of the g-C3N4 supported on SrAl2O4:Eu,Dy exhibited a broadband emission from 400 to 650 nm. Increasing silica-binder in the g-C3N4/SrAl2O4:Eu,Dy composites suppressed the PL emission peak at 525 nm for SrAl2O4:Eu,Dy. Photocatalytic degradation activity was evaluated with 5 ppm methylene blue (MB) solutions under germicidal ultraviolet (UV) and visible (Vis) solar light illuminations. The UV/Vis photocatalytic efficiency was improved by supporting g-C3N4 on the SrAl2O4:Eu,Dy phosphor and with the addition of SiO2 as a binder. In addition, low silica addition effectively improved the adhesiveness of the g-C3N4 coating on the SrAl2O4:Eu,Dy surface. Recyclability tests of photocatalysis for the SrAl2O4:Eu,Dy-0.01M SiO2/50wt% g-C3N4 composites exhibited a remarkable stability by maintaining the degradation efficiencies above 90% in four cycles. Therefore, the composite of g-C3N4-supported SrAl2O4:Eu,Dy-SiO2 is a prospective photocatalyst activating under UV/Vis light irradiation for the elimination of environmental pollutants.

Localized Bathless Metal-Composite Plating via Electrostamping

Composite plating with particles embedded into the metal matrix can enhance the properties of the metal coating to make it more or less conductive, hard, durable, lubricated or fluorescent. However, it can be more challenging than metal plating, because the composite particles are either 1) not charged so they do not have a strong electrostatic attraction to the cathode, 2) are hygroscopic and are blocked by a hydration shell, or 3) too large to remain stagnate at the cathode while stirring. Here, we describe the details of a bathless plating method that involves anode and cathode nickel plates sandwiching an aqueous concentrated electrolyte paste containing large hygroscopic phosphorescent particles and a hydrophilic membrane. After applying a potential, the nickel metal is deposited around the stagnant phosphor particles, trapping them in the film. The composite coatings are characterized by optical microscopy for film roughness, thickness and composite surface loading. In addition, fluorescence spectroscopy can be used to quantify the illumination brightness of these films to assess the effects of various current densities, coating duration and phosphor loading.

Enhanced Cyan Emission and Optical Tuning of Ca3Ga4O9:Bi3+ for High‐Quality Full‐Spectrum White Light‐Emitting Diodes

AbstractHighly efficient cyan‐emitting phosphor materials are indispensable for closing the cyan gap in spectra of the traditional phosphor‐converted white light‐emitting diodes (WLEDs) to achieve high‐quality full‐spectrum white lighting. In this work, bright cyan‐emitting Ca3Ga4O9 (CGO):0.02Bi3+,0.07Zn2+ phosphor is developed to bridge the cyan gap. Such a Bi3+,Zn2+ codoping enhances the cyan emission of CGO:0.02Bi3+ by 4.1 times due to the influence of morphology and size of phosphor particles, charge compensation and lattice distortion. Interestingly, codoping La3+ ions into the current system can achieve a photoluminescence tuning of CGO:0.02Bi3+ from cyan to yellowish‐green by crystallographic site engineering. Besides, Bi3+–Eu3+ energy transfer is successfully realized in CGO:0.02Bi3+,0.07Zn2+,nEu3+ phosphors and the emission color tuning from cyan to orange is observed. The investigation of thermal quenching behaviors reveals that the incorporation of Zn2+ and La3+ improves the thermal stability of CGO:0.02Bi3+. Finally, CGO:0.02Bi3+,0.07Zn2+,0.10Eu3+ phosphor is employed to obtain a single‐phased warm WLED device. A full‐spectrum WLED device with remarkable color rendering index (Ra) of 97.4 and high luminous efficiency of 69.72 lm W−1 is generated by utilizing CGO:0.02Bi3+,0.07Zn2+ phosphor. This result suggests the important effect of CGO:0.02Bi3+,0.07Zn2+ phosphor on closing the cyan gap, providing new insights of cyan‐emitting phosphors applied in full‐spectrum white lighting.

Synthesis of NaLn(WO4)2 phosphors via a new phase-conversion protocol and investigation of up/down conversion photoluminescence

The two-dimensional (2D) crystallite morphology and low OH−/Ln3+ molar ratio of Ln2(OH)4SO4·2H2O (Ln = lanthanide) make it an ideal precursor for materials synthesis via phase conversion, which was manifested in this work by the direct generation of well-defined NaLn(WO4)2 phosphor particles via hydrothermal reaction with Na2WO4. Kinetics study showed that pure NaLn(WO4)2 can be produced by reaction at 180 °C for ~24 h or at 200 °C for ~6 h under WO42−/Ln3+ = 10 M ratio. Morphology analysis revealed that, though NaLn(WO4)2 evolved via re-precipitation, the layered crystal structure and 2D crystallite morphology of the precursor could have templated the nucleation/growth of NaLn(WO4)2, leading to uniform particles (~4–5 μm) of a unique microdisc-like morphology. Under 394 nm excitation, the Ln = La0.95Eu0.05 phosphor showed down-conversion luminescence having an absolute quantum yield of ~35.4%, a fluorescence lifetime of ~1.13 ms for its 616 nm main emission, and color coordinates of around (0.63, 0.37). Under 978 nm laser excitation, the Ln = La0.97Yb0.02Ho0.01 and Ln = La0.97Yb0.02Er0.01 phosphors exhibited the strongest up-conversion (UC) luminescence at ~660 nm (the 5F5 → 5I8 transition of Ho3+) and 551 nm (the 4S3/2 → 4I15/2 transition of Er3+), average fluorescence lifetimes of ~178.3 and 82.3 µs for the above emissions, and chromaticity coordinates of around (0.62, 0.38) and (0.25, 0.72), respectively. The two UC phosphors were also analyzed to exhibit UC luminescence through a two-photon mechanism.