Phosphor Particle Size Research Articles

Yellow-white emitting phosphor-in-glass with LuAG:Ce and embedded CdS quantum dots

One-step sintering process was introduced to fabricate phosphor-in-glass (PiG) composite. Commercial cerium doped lutetium aluminum garnet (LuAG:Ce, Lu3Al5O12:Ce3+) phosphor was subjected to high-energy ball milling in a planetary ball mill. By controlling the milling time and velocity, the LuAG:Ce phosphors with average diameters from 0.5 to 10 μm was prepared. The effect of phosphor particle size on densification, final microstructure, thermal and optical properties of PiG was discussed. The results showed that the as-sintered PiG sample had amorphous phase and the LuAG:Ce phosphor particles uniformly distributed in glass matrix. LuAG:Ce particles were well preserved in silica glass matrix without any decomposition reaction. As the ball-milling process proceeded, the luminescent efficiency of phosphor was degraded owing to the low crystallinity of the surface of the LuAG:Ce particles. However, the sintering of LuAG:Ce with the glass and CdS QDs led to healing of the luminescence efficacy up to 23 % and shifting of chromaticity coordinates from green to yellow region. The improved value of correlated color temperature and the color rendering index of prepared PiG was achieved around 3100 K′ and 95, respectively. Moreover, the PiG with milled LuAG:Ce performs good laser output and can resist higher laser power owing to the effective heat dissipation.

Highly efficient blue‐emitting phosphor via charge compensation strategy and its application for white LED lighting

AbstractHighly efficient phosphors are indispensable for near‐ultraviolet (n‐UV) pumped white light emitting diodes (WLEDs). In this work, a novel blue emitting Sr6GdSc(BO3)6:0.08Ce3+ phosphor was synthesized, exhibiting a broadband blue emission (full width at half‐maximum (FWMH) = 133 nm) excited by 375 nm. Furthermore, the blue emission of Sr6GdSc(BO3)6:0.08Ce3+ was enhanced by 3.9 times through codoping Na+ ions due to the influence of charge compensation and micromorphology and size of phosphor particles. At the same time, the internal quantum efficiency was improved from 45.4% to 98.6% and the external quantum efficiency up to 75.0%. Finally, the 365 nm excited two WLED devices were constructed by integrating Sr6GdSc(BO3)6:0.08Ce3+, Sr6GdSc(BO3)6:0.08Ce3+,0.08Na+, commercial green and red phosphors with high color rendering index. These results reveal that the phosphors have prospective application in high‐quality warm white lighting.

Pixelated Phosphor Converter for Laser‐Driven Adaptive Lighting

AbstractWith the advent of artificial intelligence and unmanned driving, it is an inevitable trend to provide laser‐driven adaptive lighting with super‐brightness, high penetration, and high spatial resolution; one of the best solutions is to pump pixelated phosphor converters by laser diodes (LDs). However, achieving high‐performance pixelated phosphor converters remains a great challenge due to the large size of commercial phosphor particles. In this study, promising pixelated phosphor films are proposed to be fabricated by producing arrayed holes in the Ag substrate with the laser direct writing technology and then filling the holes with the mixture of phosphor particles and the heat‐resisting inorganic glue. A strong thermal transfer field grid is built in Y3Al5O12:Ce3+‐pixelated‐film@Ag, which can withstand a high luminance saturation threshold of over 18.08 W mm−2 pumped by blue LDs. The maximum luminous flux density is ≈1300 lm mm−2, and the luminance is 3.36 × 107 cd m−2 of the white pixel light with excellent uniformity. Further, based on this fabrication strategy, the full‐color‐pixelated‐film@Ag is successfully prepared for the first time. The realization of laser‐driven white pixel light and full‐color pixel light shows an interesting application in intelligent adaptive lighting, and this study paves an avenue for designing pixelated phosphor converters.

Nano-Coprecipitation Route Synthesis of Highly-Efficient Submicron (Sr,Ba)2SiO4:Eu2+ Phosphors with Enhanced Thermal Stability for MicroLED Color Conversion.

As the size of MicroLED chips shrinks below 50 μm, the emergence of quantum dots (QDs)-based color conversion with narrow-band emission and nanoscale size properties has become one of the powerful full-color solutions for MicroLED displays. However, the stability and toxicity of quantum dots limit their application in full-color MicroLEDs. The phosphor-based conversion has the prominent features of high thermal and chemical stability relative to those of QD-based conversion. Nevertheless, the particle size of phosphor prepared by a traditional high-temperature solid-state reaction (SSR) is equivalent to or even larger than that of the MicroLED chip. In this work, we propose a strategy to prepare (Sr,Ba)2SiO4:0.03Eu2+ (SBSO:0.03Eu2+) submicron phosphors via a nano-coprecipitation method (NCP) using nanoSi3N4 as the Si source materials, which enables the particle size to be reduced while maintaining the luminescence efficiency. The optimized SBSO:0.03Eu2+ has an average size of less than 2 μm, showing a narrow band green emission centered at 522 nm, with a full width at half-maximum of 60 nm and an external quantum efficiency of 40.2%. At 150 °C, its thermal stability is greatly enhanced to 80.2% of the emission at room temperature. Further, the mechanism for defect compensation thermal stability is investigated. By employing it as a green emitter, we fabricate a high-performance white LED device (WLED) with a wide color gamut of 86.7% NTSC. This work for the preparation of high-brightness and thermal stability enhancement SBSO:0.03Eu2+ phosphor not only provides a facile method but also helps to provide an alternative green fluorescent material for the realization of full color MicroLED.

Strategies for Enhancement Luminance of Sub‐Micrometer Phosphor: A Sub‐Micrometer Narrow‐Band Red Phosphor with Ultrahigh Thermal Stability

AbstractNowadays, phosphors are widely used in high quality white light emitting diodes (W‐LEDs) and display technology. Micro light emitting diode (Micro‐LED) has the advantages of ultrahigh resolution color saturation, while ordinary phosphors are difficult to apply due to their large particle size. As is known to all, once the phosphor particle size is reduced, its luminescence intensity will loser. This paper presents a strategy to improve the luminescence intensity of sub‐micrometer narrow‐band red phosphors. By increasing the doping concentration of rare‐earth ions through total substitution and reducing the nonradiative transition in the cell, ultrahigh concentration doping of sub‐micrometer phosphors is achieved to improve the luminescence intensity. Surprisingly, with increasing Eu3+ up to entirely substitution for Gd3+ site, both the absorption efficiency and the internal quantum efficiency of the sample increased greatly. The external quantum efficiency increased from 5.42% to 19.70%. Sub‐micrometer EuAl3(BO3)4 exhibits ultrahigh thermal stability, with a luminous intensity at 225 °C that is 128% of that at room temperature, meeting the conditions of extreme high‐temperature environments or high‐power LEDs. In summary, this work has great potential for application in W‐LED and presents a solution idea to solve the luminous intensity reduction of sub‐micrometer phosphors.

(Digital Presentation) Fine Nitride Phosphors for Mini-LED Backlights

Mini-LED backlights, combining color conversion materials with blue Mini-LED chips, promise traditional liquid crystal displays (LCDs) with higher luminance, better contrast and wider color gamut. As color conversion materials, Cd- or Pb-based quantum dots are usually toxic and unstable, and commercially available inorganic phosphors are quite stable and cheap, but they have a large size due to their synthesis at high temperature, and their quantum efficiency and reliability are size dependent. To make inorganic phosphors to be well combined with Mini-LED chips (usually 100 x100 μm in size) in the emerging Mini-LED backlights, the particle size of the phosphors needs to be reduced to several micrometers or sub-micrometers while the quantum efficiency remains unchanged. In this work, we prepared fine nitride phosphors by treating commercially available phosphors, including CaAlSiN3:Eu2+ and (Sr, Ba)2Si5N8:Eu2+, with ball milling, centrifuging and acid washing. The particle size of phosphors can be easily controlled by the milling speed, and the phosphors with a size varying from 0.5 to 15 mm were thus obtained. The samples remained the comparable quantum efficiency with the original ones even when their particle size was reduced to 2 μm as they contained less surface defects after acid washing. We demonstrated that red-emitting Mini-LEDs can be fabricated by combining the red phosphors with blue Mini-LED chips, which show higher external quantum efficiency, luminance and stability than those made by using quantum dots. It indicates that fine and high efficiency phosphors can be obtained by the proposed method in this work, and they have great potentials for use in Mini-LED displays.

Luminous LaAlO3 :Dy3+ perovskite nanomaterials: Synthesis, structural, and luminescence characteristics for white light-emitting diodes.

A single-phase perovskite LaAlO3 :Dy3+ phosphor was synthesized using a gel-combustion method at 600°C utilization with hexamethylenetetramine as a fuel. Further calcination of the samples was carried out (800 and 1000°C) to investigate the resultant effect on the crystalline and luminescence behaviour. The crystal structure had a cubic unit cell (space group Pm3̅m) and was examined using the Rietveld refinement and X-ray diffraction data. Additionally, Debye-Scherrer and Williamson-Hall equations were applied to determine other structural features. The particle size and morphology of phosphors were evaluated using transmission electron microscopy. Diffraction measurements were supported by the various metal-oxygen vibration modes studied with the help of Fourier transform infrared spectroscopy. Energy-dispersive X-ray analysis verified the chemical composition of synthesized sample. Luminescence spectra of the LaAlO3 :Dy3+ phosphors exhibited intense bands for 4 F9/2 →6 H15/2 (482 nm, bluish region) and 4 F9/2 →6 H13/2 (574 nm, yellowish region) transitions. Commission Internationale de L'Eclairage and correlated colour temperature data confirmed the cool-white emission of the samples under ultraviolet light excitation. The interesting and advantageous luminescence characteristics of LaAlO3 :Dy3+ phosphors make them potential materials for white light-emitting diodes.

Fine-grained phosphors for red-emitting mini-LEDs with high efficiency and super-luminance

Mini-LED backlights, combining color conversion materials with blue mini-LED chips, promise traditional liquid crystal displays (LCDs) with higher luminance, better contrast, and a wider color gamut. However, as color conversion materials, quantum dots (QDs) are toxic and unstable, whereas commercially available inorganic phosphors are too big in size to combine with small mini-LED chips and also have strong size-dependence of quantum efficiency (QE) and reliability. In this work, we prepare fine-grained Sr2Si5N8:Eu2+-based red phosphors with high efficiency and stability by treating commercially available phosphors with ball milling, centrifuging, and acid washing. The particle size of phosphors can be easily controlled by milling speed, and the phosphors with a size varying from 3.5 to 0.7 μm are thus obtained. The samples remain the same QE as the original ones (∼80%) even when their particle size is reduced to 3.2–3.5 μm, because they contain fewer surface suspension bond defects. More importantly, SrBaSi5N8:Eu2+ phosphors show a size-independent thermal quenching behavior and a zero thermal degradation. We demonstrate that red-emitting mini-LEDs can be fabricated by combining the SrBaSi5N8:Eu2+ red phosphor (3.5 μm in size) with blue mini-LED chips, which show a high external quantum efficiency (EQE) of above 31% and a super-high luminance of 34.3 Mnits. It indicates that fine and high efficiency phosphors can be obtained by the proposed method in this work, and they have great potentials for use in mini-LED displays.

Improving Light Absorption in a Perovskite/Si Tandem Solar Cell via Light Scattering and UV‐Down Shifting by a Mixture of SiO2 Nanoparticles and Phosphors

AbstractThe optical properties of a textured antireflective coating (ARC) polymeric film are engineered by combining the down‐conversion effect of large phosphor particles and the multiple scattering effect of SiO2 nanoparticles. In order to address the parasitic absorption of ultraviolet (UV) light, phosphors are added to convert UV light to visible light. However, the embedded phosphors increase the reflectance of the ARC film, due to the large particle size (>5 µm) and high refractive index (n ≈ 1.9) of phosphors. Such a backward scattering problem of phosphors is compensated by adding spherical SiO2 nanoparticles. Experimental and computational results show that SiO2 nanoparticles in the ARC film decrease the reflectance by increasing the diffuse transmittance. This optically engineered ARC film successfully promotes the light absorption of the perovskite/silicon tandem solar cell, leading to the improvement of power conversion efficiency of the tandem cell from 22.48% to 23.50%.

Influence of Phosphor Particle Size on Visual and Non-Visual Performances of White Light-Emitting Diodes for General Lighting

Influence of Phosphor Particle Size on Visual and Non-Visual Performances of White Light-Emitting Diodes for General Lighting

Detection of cracked teeth using a mechanoluminescence phosphor with a stretchable photodetector array

Cracked tooth syndrome (CTS) is an incomplete fracture of a human tooth that commonly arises from chewing hard foods. Although it is a very common syndrome, CTS is often difficult to diagnose owing to the common small size of tooth cracks. Conventional techniques for the detection of cracks, such as transillumination and radiographic methods, are inaccurate and have poor imaging resolution. In this study, we devise a novel method for the in vivo detection of tooth microcracks by exploiting the mechanoluminescence (ML) phenomenon. ZrO2:Ti4+ (ZRT) phosphor particles are pasted onto suspected regions of tooth cracks and emit cyan-colored light as a result of masticatory forces. Then, a stretchable and self-healable photodetector (PD) array laminated on top of the phosphor particles converts the emitted photons into a photocurrent, which facilitates the two-dimensional mapping of the tooth cracks. Because of the high photosensitivity of the PD, intense ML and small size of ZRT phosphor particles, it is possible to image submicron- to micron-sized cracks with high resolution. Furthermore, the uniqueness of this technique over the conventional techniques stems from the application of a simple optical phenomenon, i.e., ML, for obtaining precise information regarding the locations, depth, and length of tooth cracks.

Evaluation of (GdxY1-x)VO4: Er3+ (x = 0, 0.25, 0.5, 0.75, 1) compounds: Study of crystal structure, microstructure, luminescence and adsorption properties

In this investigation, (GdxY1-x)VO4:Er3+ (x = 0, 0.25, 0.5, 0.75, 1) compounds were synthesized via a facile solid-state method at 1100 °C. The XRD results clarified that the increase of Gd element gives rise to the left shift of diffraction peaks. The employment of SEM revealed that the ratio of Gd/Y significantly influences the microstructure while the smallest particle size of phosphors (∼2 μm) was achieved in GdVO4 luminescent material. The photoluminescence characterization confirmed that the highest excitation intensity is occurred at the wavelength of 310 nm while the corresponded emission spectrum consists of two strong peaks at the approximate wavelengths of 534 and 562.5 nm, which are attributed to 2H11/2-4I15/2 and 4S3/2-4I15/2, respectively. Finally, it was interestingly concluded that the strongest emission intensity belongs to GdVO4 phosphor material. The adsorption spectra proved that the employment of ball milling efficiently enhances the adsorption characteristics.

Improvement of Luminescence Characteristics of ZnGa2O4:Tb3+ Nanophosphors by Ce3+ Codoping

AbstractA series of ZnGa2O4:Tb3+,Ce3+ persistent luminescence nanophosphors were prepared by high temperature solid‐state reaction. The X‐ray diffraction (XRD) patterns of the representative samples are consistent with that of the ZnGa2O4 standard card. Scanning electron microscope (SEM) images show that the particle size of phosphor is enlarged when the Tb3+ ion is doped, however, it becomes smaller again after the Ce3+ ion is codoped. For ZnGa2O4:Tb3+ nanophosphors, there exist intense 492 and 551 nm blue/green emissions under 280 nm excitation, and the optimum Tb3+ doping concentration is 1.7 mol% and the long afterglow time is about 140 min. The PL and long afterglow characteristics of ZnGa2O4:1.7Tb3+ nanophosphor are significantly improved once the Ce3+ ion is codoped. The PL intensity of ZnGa2O4:1.7Tb3+ nanophosphor is enhanced by 1.5 times and the long afterglow time is extended to 210 min when Ce3+ doping concentrations are 0.3 and 0.5 mol%, respectively. The internal mechanism of the long afterglow effect of ZnGa2O4:1.7Tb3+,0.5Ce3+ nanophosphor is discussed in detail with the help of measurement results of thermoluminescence spectra.

Organic solvent-assisted co-precipitation synthesis of red-emitting K2TiF6:Mn phosphors with improved quantum efficiency and optimized morphology.

We report an organic solvent-assisted (OSA) co-precipitation strategy for the production of Mn4+-activated K2TiF6 phosphor. The phosphor particle size was controlled through the selection of organic solvents with an alcohol functional group and different carbon chain lengths used in the synthesis. The synergistic effect of the organic solvent and hydrofluoric acid results in large smoothed hexagonal-shaped crystal sheets of particles that become larger as the carbon chain length of the organic solvent increases. The photoluminescence (PL) properties of K2TiF6:Mn powders strongly depend on the size and thickness of the particles. The addition of n-butanol during the synthesis increases the emission intensity of K2TiF6:Mn by 208%. The PL quantum efficiency of phosphors prepared using the n-butanol-assisted strategy is much higher (98.2%) than that of conventionally prepared phosphors (89.9%). Our findings demonstrate a way to prepare the K2TiF6:Mn phosphor with targeted morphology and very high quantum efficiency and also provide the route for the optimization of all Mn4+-activated fluoride phosphors used in white light-emitting diodes.

Numerical Simulation of Light Extraction from Remote Phosphor LED

In this paper, light extraction from remote phosphor LEDs were calculated in Zemax OpticStudio. The dependence of the optical characteristics of the remote phosphor LED on the parameters of phosphor and its geometrical form was considered. In case of a thin plate as a remote phosphor, phosphor particle size, phosphor mass fraction and phosphor plate thickness were carefully analyzed. Furthermore, a plane-convex lens and Fresnel lens were also considered as geometrical form of remote phosphor. The simulation results show that color coordinates of LED, using remote phosphor plate (thickness 0.25 mm, mass fraction 30% and particle size 3 μm of phosphor), are the closest to D65 point on color space compared to other considered LEDs. The use of plane-convex lens (thickness 1 mm, radius of curvature 7 mm, base diameter 7 mm) as remote phosphor results in the maximum luminous flux compared to other forms.

Emitting area limitation via scattering control in phosphor film realizing high-luminance laser lighting

One major benefit of laser lighting is the possibility to achieve very high luminance. In phosphor-converted laser lighting systems, a blue (pump) laser can be focused into a very small spot. However, after excitation of the phosphor, the white-light-emitting area usually increases considerably, which reduces the luminance parameter substantially. Herein, we design and investigate a highly scattering YAG:Ce/glass composite film with a porous microstructure. Both the glass/phosphor interfaces and the introduced pores act as scattering centers, which can confine the emission area effectively. The relationship between the spot size and the microstructure (porosity, phosphor-particle size, thickness) is elucidated. Under excitation with blue laser, the composite film shows a uniform white-light emission with high luminous efficacy (230 lm/W) and high saturation threshold (> 40 W/mm2), thus achieving a high luminous exitance of ∼1239 lm/mm2. With above excellent properties, the designed composite films show great potential for use in high-luminance laser lighting.

Synthesis and photoluminescence analysis of Y1.50Sc0.50O3:Sm3+ and Y1.50Sc0.50O3:Dy3+ phosphors

In this study, Y1.50Sc0.50O3:Sm3+ and Y1.50Sc0.50O3:Dy3+ have been synthesized by a solid state reaction method and their photoluminescence (PL) properties have been investigated. Single-phase and well-crystallized cubic Y1.50Sc0.50O3:Sm3+ and Y1.50Sc0.50O3:Dy3+ phosphors are obtained after calcinations at 800 ºC and 900 ºC for 6 h. X-ray diffraction patterns confirm that both phosphors have a cubic phase without secondary phase. According to scanning electron microscope results, the particle size of both phosphors exhibit relatively irregular and their particle size are determinated to be between 0.27 µm–0.70 µm and 0.25 µm–2.04 µm for Y1.50Sc0.50O3:Sm3+ and Y1.50Sc0.50O3:Dy3+, respectively. The PL excitation and emission spectra are recorded for both types of the phosphors. Emission spectra of Y1.50Sc0.50O3:Sm3+ and Y1.50Sc0.50O3:Dy3+ are found to be typical for Sm3+and Dy3+.

Synthesis and luminescent properties of Ba3Y3.88Gd0.12O9:Eu3+ phosphors with enhanced red emission

In this work, the Gd3+/Eu3+ activated Ba3Y4O9 (BYO) phosphors were successfully synthesized via coprecipitation method at 1400 °C. The precursor composition, crystal structure stability, microscopic morphology, photoluminescence (PL)/photoluminescence excitation (PLE) spectra and fluorescence attenuation analysis of the phosphors are discussed in detail. The chemical composition of the precursor was determined by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetry (TG) analysis; According to field emission-scanning electron microscopy (FE-SEM) analysis, it is found that the particle size of phosphor is uniform and the agglomeration is few. According to PL/PLE spectra analysis, Ba3Y3.28Eu0.6Gd0.12O9 phosphors has the strongest excitation band at 260 nm and the strongest emission band at 614 nm, and the fluorescence intensity of Ba3Y3.28Eu0.6Gd0.12O9 is higher than that of Ba3Y3.4Eu0.6O9. The quenching concentration of Eu3+ in Ba3Y3.88–4xEu4xGd0.12O9 phosphors is x = 0.15 and the mechanism of quenching concentration of Eu3+ is electric dipole-quadrupole type interactions. The lifetime value of Ba3Y3.88–4xEu4xGd0.12O9 (x = 0.15) phosphors is 0.686 ms and decreases with the increase of Eu3+ content. In addition, the CIE chromaticity diagram of Ba3Y3.28Eu0.6Gd0.12O9 phosphors is (0.66, 0.34). Finally, the lamp beads assembled with Ba3Y3.28Eu0.6Gd0.12O9 phosphors have an ideal luminous effect. Therefore, the Ba3Y3.88–4xEu4xGd0.12O9 phosphors designed in this work may hopefully meet the requirements of various lighting and optical display applications.

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.

Exploring a particle-size-reduction strategy of YAG:Ce phosphor via a chemical breakdown method

Particle size reduction of Y3Al5O12:Ce (YAG:Ce) phosphor is highly needed for micro-LED display applications. In this work, size control of YAG:Ce phosphor particles is achieved via carbon coating and further heat treatment. A thin layer of carbon is deposited on the surface of YAG:Ce by chemical vapor deposition. During the heat treatment, carbon reacts with oxygen element in the phosphor and escapes from phosphor particles. The reaction results in the phosphor breaking into smaller particles. The phosphors were characterized by laser light scattering for particle size distribution, scanning electron microscopy (SEM), transmission electron microscopy for morphologies, X-ray diffraction refinements for crystal structure and electron energy-loss spectroscopy for elemental analysis. It is demonstrated that the median diameter (D50) of the phosphor particle size is significantly reduced from 19.1 to 0.96 μm yet the photoluminescent properties have little changes. The carbon coating and further heat treatment method show potential application in size control of phosphors.