Ce-doped Yttrium Aluminum Garnet Research Articles

Atomic-level investigation of structure and formation process of Y3Al5O12 with super-high content of Ce emitting anomalous orange-red emissions

Ce-doped yttrium aluminum garnet (YAG:Ce, Y3Al5O12:Ce) is the most commonly used yellow phosphor in white light-emitting diodes. Recently, we found that (Y1–xCex)3Al5O12:Ce with a super-high content of Ce (x = 0.11–0.21) can be prepared by a polymerized complex method and exhibits anomalous orange–red emission. To understand the nature of this anomalous behavior from the atomic level, we applied high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM-EDS), a state-of-the-art technique that enables analysis of the atomic-level elemental distributions of Al, Y, and Ce. STEM-EDS images on the atomic level were successfully obtained for YAG:Ce. It was found that Ce in Y site has spatially uneven for few nanometers. Ce–K edge extended X-ray absorption fine structure (EXAFS) analysis was also conducted, which revealed a high level of asymmetry around Ce. By combining the STEM-EDS and EXAFS results, a dynamic model for the red emission was proposed. STEM-EDS also revealed that CeO2 nanocrystals were firstly formed with low-temperature annealing and that the reaction of Y with the CeO2 nanocrystals subsequently yielded YAG:Ce, unlike in the conventional one-step high-temperature synthesis.

Effect of down-shifting phosphor particles on crystalline silicon solar cells

Introducing down-shifting phosphor particles onto solar cells can improve their spectral response to short wavelength photons, but it also leads to the rise of reflection in middle and long wavelength range due to the scattering by phosphor particles. In this work, the down-shifting solar cells are fabricated by packaging crystalline silicon solar cells with ethylene vinyl acetate (EVA) films which include different concentration of down-shifting Ce-doped yttrium aluminum garnet (YAG:Ce) phosphor particles. Experimental result shows that the increasing concentration of YAG:Ce phosphors strengthens the down-shifting effect and makes the reflection rise. The competition between the down-shifting effect and the reflection results in the best improvement of the solar cells when the phosphor's concentration is 0.7%. The optical ray-tracing simulations clarify that the packaged solar cell with YAG:Ce absorbs less photons than those without YAG:Ce, but their conversion efficiency is higher than those without YAG:Ce. This indicates that the penetration depth of the short wavelength photons is responsible for the poorer spectral response of the solar cells in the short wavelength range.

Fluorescence-enhanced Si photodiodes for ultraviolet C rays (UVC) measurements

The ultraviolet C rays (UVC, wavelength λ = 100–280 nm) light generated by a Hg lamp (λ = 254 nm) and UVC light-emitting diodes (LEDs, λ = 265 and 275 nm) was detected using a fluorescence-enhanced silicon photodiode (FE-PD). Ce-doped yttrium aluminum garnet (YAG:Ce), YAG:Pr, YAG:Eu, YAG:Tb, YAG:Cr, Al2O3:Ti, Al2O3:Cr, MgAl2O4:Ti, MgAl2O4:Cr, MgAl2O4:Mn, and commercial fluorescent acrylic resins were tested as phosphor sources to enhance the output signal intensity of the FE-PD irradiated with UVC light. The resulting output signal intensity increased linearly with the UVC light strength, which was adjusted by raising the input current of the UVC LEDs from 0 to 40 mA. The sensitivity of the fabricated UVC detectors, assessed based on the calibration curve slope, varied depending on the phosphor materials. The phosphors effectively enhanced the output signal intensity of the FE-PD, which was up to six times greater than that of the visible and near infrared Si-PD without phosphors; the stronger output signal intensity was achieved using YAG:Tb, YAG:Cr, and a red fluorescent acrylic resin. The visible light emitted by phosphors under UVC irradiation is useful for detecting UVC light by the eye when using FE-PD.

Thermochromic properties of some colored oxide materials

The thermochromic properties up to 500 °C of some colored, commercial and non-toxic oxides were studied. It includes Bi2O3, Fe2O3, In2O3, WO3, Er2O3, YAG:Ce (Ce doped yttrium aluminum garnet Y3Al5O12) and YInMn-Blue (Mn doped yttrium indium oxide YInO3). From the CIELAB colorimetric parameters (L*a*b*), evolution of the color contrast ΔE with temperature was calculated. It allowed estimating that most of these oxides change color reversibly with increasing temperature, the strongest thermochromism being found for Bi2O3 (ΔE ∼ 65 at 500 °C). Two exceptions were identified: i) Er2O3 which stayed almost identically pink with ΔE<7 even at 500 °C and ii) WO3 which original color was not recovered upon cooling back to room temperature. While the color change of the other oxides was gradual with increasing temperature, YAG:Ce compound behaved differently since its thermochromicity abruptly started above 300 °C. DSC analyses did not allow detecting any physico-chemical event in relation with these peculiarities. The obtained datasets for all these compounds can be used for anticipating the thermochromic properties of these powders and the blends made out of them.

White light emitting diodes based on green graphene quantum dots and red graphene quantum dots

White light emitting diodes (WLEDs) have attracted the attention of numerous scholars because of its high photoelectric conversion efficiency, low energy consumption and high reliability. They are known as the new generation of lighting energy in the twenty first century. Currently, the most common WLEDs are combined by blue LED chip and ce-doped yttrium aluminum garnet (YAG: Ce3+) yellow phosphors. The advantages are simple production, low cost and high luminous efficiency. However, WLEDs made of blue LED chip and yellow phosphors lacks red spectrum and exhibits luminescence, and its color rendering index (CRI) is low, usually only 70. At the same time, quantum dots (QDs) are considered to be the most potential materials for WLEDs color conversion layers due to their wide excitation spectrum, narrow emission spectrum, adjustable band gap and high quantum yield. In this paper, we fabricated green graphene QDs (GQDs) and red GQDs, measured and analyzed their morphology, structure and photoluminescence. The green GQDs, red GQDs and yellow phosphors were used to fabricate the WLEDs. The produced WLEDs showed excellent optical performance and working performance, among which Commission International de L 'Eclairage (CIE) chromaticity coordinates are (0.3332, 0.3359), and the correlated color temperature (CCT) is 5464 K (driving current is 30 mA). Electroluminescence (EL) spectra of the device at different currents were also measured.

AlN–Ce-doped yttrium aluminum garnet composite ceramic phosphor for high-power laser lighting

Ce-doped yttrium aluminum garnet (YAG:Ce) has been used as a phosphor emitting white light using a blue-light-emitting diode as a light source. Recently, studies on laser lighting as next-generation lighting have been carried out. However, because a high-power blue laser diode is used as the light source, the decrease in phosphor emission due to temperature increase (thermal quenching) has become a serious issue. Therefore, it is desirable to develop a method to suppress the temperature increase of the YAG:Ce phosphor. To increase the thermal conductivity of the YAG:Ce phosphor and suppress the temperature increase, we fabricated a composite ceramic of AlN (nonoxide), which has a high thermal conductivity, and YAG:Ce (oxide) using a modified vapor transport equilibration technique. The oxynitride was not completely removed, but we succeeded in adding up to 80 wt% AlN to YAG:Ce. The ceramic with 50% added AlN had a thermal conductivity up to 19.2 W m−1K−1 and emitted almost the same intensity as that of the nondoped YAG:Ce. This thermal conductivity exceeds the reported values of 12–13 W m−1K−1 for Al2O3–YAG:Ce composite ceramics. Moreover, the luminous efficacy exhibited by the ceramic with a 20 wt% AlN addition was 266.0 lmW−1, which is larger than the reported values obtained from composite ceramics.

Dielectric properties of Ce-doped YAG coatings produced by two techniques of plasma spraying

Yttrium aluminum garnet (YAG) with cerium admixture (Ce:YAG) was plasma sprayed using two different devices – gas-stabilized plasma (GSP) torch and water-stabilized plasma (WSP) torch. Coatings on stainless steel as well as self-standing plates were produced. Besides microstructure and crystallographic characterizations, dielectric tests were performed on these coatings. They included capacitance (i.e. relative permittivity), loss tangent and volume resistivity measurements. After spraying, the YAG crystal phase was preserved without any decomposition, but an amorphous fraction was detected in the as-sprayed coatings deposited by both processes. The dielectric behavior of the coatings was influenced by imperfections like splat boundaries, pores and thin cracks. The Ce:YAG samples were successfully plasma sprayed by both spray techniques. Selected aspects of wear were measured and compared with a single-crystal. The dielectric properties are comparable with the single-crystal and highly promising, particularly the loss tangent with values so low that they were not found in any other as-sprayed ceramic coating.

Luminescence Quenching via Deep Defect States: A Recombination Pathway via Oxygen Vacancies in Ce-Doped YAG

Luminescence quenching via non-radiative recombination channels limits the efficiency of optical materials such as phosphors and scintillators and therefore has implications for conversion efficiency and device lifetimes. In materials such as Ce-doped yttrium aluminum garnet (YAG:Ce), quenching shows a strong dependence on both temperature and activator concentration, limiting the ability to fabricate high-intensity white-light emitting diodes with high operating temperatures. Here, we reveal by means of first-principles calculations an efficient recombination mechanism in YAG:Ce that involves oxygen vacancies and gives rise to thermally activated concentration quenching. We demonstrate that the key requirements for this mechanism to be active are localized states with strong electron-phonon coupling. These conditions are commonly found for intrinsic defects such as anion vacancies in wide band-gap materials. The present findings are therefore relevant to a broad class of optical materials and shine light on thermal quenching mechanisms in general.

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.

High-Particle-Density YAG:Ce Phosphor Coating for High Power Laser Lighting

Abstract Inorganic Ce doped yttrium aluminum garnet (YAG:Ce) fluorescent plates based on various manufacturing methods have been studied in recent years for laser lighting systems. A laser lighting optical system is designed to generate high central intensity beam, and light spreading effect for both YAG single crystal and YAG/Al2O3 composite is investigated. YAG single crystal and YAG/Al2O3 composite lose 50.8% and 25.9% of their light power from central pumping area, respectively, resulting in proportional decrease in central intensity. To solve this problem, a low-cost YAG:Ce high-particle-density phosphor coating (HPD-PC) was prepared for Etendue-limited applications. With only 8.5% of light spreading, under 47.7 W/mm2 pumping power density, YAG HPD-PC emits 1880 lm with the conversion efficiency of 157 lm/W. YAG HPD-PC also could work well under power density more than 100 W/mm2.

Study of the Effect of Injection Currents on White Light Emission of Ce-Doped YAG Phosphor Powder Prepared by Microwave Combustion

The behaviors of yttrium aluminum garnet (YAG) phosphor powder doped by cerium (YAG:Ce3+) was studied. Here, the YAG:Ce3+ nanopowders have been synthesized using microwave combustion (MW) according to the formula, Y(3-0.04) Al5O12: 0.04Ce3+ to produce white light emitting diode (WLEDs) by conversion from blue indium gallium nitride light emitting diode (InGaN LED, 445 nm) chips; the whole process took only 20 min. Pure YAG phase was obtained after annealing at 1050 °C for 5 h with nonaggregated and spherical particles. Thermogravimetry and derivative thermogravimetry (TG/DTG), X-ray powder diffraction (XRPD), transmission electron microscope (TEM), photoluminescence (PL), electroluminescence (EL) emissions and standard CIE 1931 chromaticity diagrams have been used to characterize the samples. The highest WLEDs emission was achieved for the annealed YAG:Ce3+, together with proper color rendering index (CRI), and tunable correlated color temperature (CCT). Finally, we conclude that the decreasing EL intensity at increased injection current is caused by the thermal ionization from the 5d1 level to the conduction band.

Applying InP/ZnS Green-Emitting Quantum Dots and InP/ZnSe/ZnS Red-Emitting Quantum Dots to Prepare WLED With Enhanced Photoluminescence Performances

Based on blue LED chips to excite Ce-doped yttrium aluminum garnet (YAG: Ce3+) yellow phosphors, InP/ZnS green-emitting quantum dots (QDs) and InP/ZnSe/ZnS red-emitting QDs are used as light conversion materials for WLEDs to improve the color rendering index (CRI) of the device and improve its color coordinates. We have proven this in previous work. But in the previous work, we only studied the CRI of the driving current from 5 mA to 100 mA. The CRI and the Commission International de L’Eclairage (CIE) chromaticity coordinates of the WLED device prepared before is not ideal under high current conditions. The emission peak position of InP/ZnSe/ZnS red-emitting QDs we used in previous work is 605nm. In this article, we adjusted the emission peak position of InP/ZnSe/ZnS red-emitting QDs to 611nm, and prepared WLED in the same way, which improved the problem that the CRI of WLED devices decreases rapidly and the chromaticity coordinates change greatly under high current conditions. The CRI of prepared WLED device in this article was increased to 91.0, the CIE chromaticity coordinates is (0.3365, 0.3334), and the correlated color temperature (CCT) is 5313 K. We also tested the dependence of the electroluminescence (EL) spectrum, CRI, CCT and chromaticity coordinates of the prepared WLED device on the current. Compared with the prepared WLED device before, the CRI of the WLED device prepared in this article has always remained above 90, hovering between 90 ~ 91, very stable. The chromaticity coordinate distribution is also very concentrated, the amplitude of the offset black body radiation is small, and it shows excellent color rendering and stable working performance. A 30-day aging test was performed on the prepared WLED device.

Ce: &lt;i&gt;YAG &lt;/i&gt;ceramics: effect of synthesis technology on luminescent and optical properties

The paper studies the influence of the synthesis technology on the luminescent and optical characteristics of transparent Ce doped yttrium aluminum garnet ( YAG ) ceramics synthesized in two ways. The transparency of samples with a size of Ø10×2 mm reaches 60 to 81 % in the 500-900 nm wavelength range. The effect of intermediate synthesis steps on the optical characteristics and final concentration of the Ce3+ activator in the YAG ceramic has been described and discussed

Synthesis, characterization and application of red-emitting CuInS2/ZnS quantum dots for warm white light-emitting diodes

Ce-doped yttrium aluminum garnet (YAG:Ce) is an important phosphor in the phosphor-converted white light-emitting diode (WLED) while YAG:Ce-based WLED is cold and blue so that impair the performance of WLED. In order to achieve warm WLED, in this study, CuInS2/ZnS colloidal quantum dots (CIS/ZnS QDs) has been designed and synthesized. The crystal structure, photoluminescence and thermoluminescence have been measured and analyzed. The lighting film was realized by adding CIS/ZnS QDs into YAG:Ce-silicone, fabricating with InGaN-based blue LED to display warm white light which Commission International de L'Eclairage chromaticity coordinates at (0.3814, 0.3237) and correlated color temperature of 3415K. Our experiments show that film consists of YAG:Ce and red-emitting CIS/ZnS QDs is able to achieve warm WLED with desired results.

Effects of atmosphere and temperature on luminescence property of YAG:Ce synthesized by co-precipitation method

Ce-doped yttrium aluminum garnet (YAG)-based crystal powders were synthesized using a co-precipitation method. The effects of the sintering atmosphere and temperature on the luminescence property were investigated using the photoluminescence (PL) and cathodoluminescence (CL) spectroscopy along with X-ray diffraction (XRD) analysis and X-ray photoelectron spectroscopy (XPS). The PL and CL results showed that YAG:Ce samples sintered in an inert atmosphere at high temperatures had a higher luminescence intensity around 550 nm related to Ce3+ in the YAG host. The XRD results showed that high temperatures and inert atmosphere conditions were favorable to the growth of the YAG crystal. When the YAG:Ce samples were sintered in an inert atmosphere, the conversion of Ce3+ to Ce4+ could be suppressed effectively. Meanwhile, the CL intensity around 370 nm related to the antisite defects (AD) in the YAG lattice decreased significantly. Therefore, the YAG:Ce samples sintered in an inert atmosphere at high temperatures with a good crystal structure, high Ce3+ concentrations, and less AD had better luminescence properties. YAG:Ce samples synthesized using a co-precipitation method with excellent luminescence properties can be used as fast and efficient scintillation materials.

Defect-Controlled Scintillation Process in Undoped Y3Al5O12Crystals

Optical absorption, luminescence spectra, scintillation decay curves, and scintillation light yield were measured in a series of undoped yttrium–aluminum garnet (YAG) crystals of different origins. This paper reveals a correlation between luminescent properties and scintillation efficiency of undoped YAG crystals. All the samples were separated into three groups, Type I, Type II, and Type III, based on their optical properties and scintillation efficiency. The intrinsic ultraviolet luminescence band peaking around 300 nm is found in the samples with high scintillation yield (Type I). Luminescence intensity of this band depends on the crystal purity and on the stoichiometry of the garnets. The UV emission may be related to e–h recombination in vicinity of vacancies. The garnets of Type II contain more impurities which quench the 300-nm emission. The samples of Type III do not exhibit any significant radioluminescence. Light yield of the finest undoped YAG samples in the series, Type I, is similar to the yield of Ce-doped YAG. This Type of crystals can potentially be used in scintillation detectors.

Stable, Heat-Conducting Phosphor Composites for High-Power Laser Lighting.

Solid-state lighting using laser diodes is an exciting new development that requires new phosphor geometries to handle the greater light fluxes involved. The greater flux from the source results in more conversion and therefore more conversion loss in the phosphor, which generates self-heating, surpassing the stability of current encapsulation strategies used for light-emitting diodes, usually based on silicones. Here, we present a rapid method using spark plasma sintering (SPS) for preparing ceramic phosphor composites of the canonical yellow-emitting phosphor Ce-doped yttrium aluminum garnet (Ce:YAG) combined with a chemically compatible and thermally stable oxide, α-Al2O3. SPS allows for compositional modulation, and phase fraction, microstructure, and luminescent properties of ceramic composites with varying compositions are studied here in detail. The relationship between density, thermal conductivity, and temperature rise during laser-driven phosphor conversion is elucidated, showing that only modest densities are required to mitigate thermal quenching in phosphor composites. Additionally, the scattering nature of the ceramic composites makes them ideal candidates for laser-driven white lighting in reflection mode, where Lambertian scattering of blue light offers great color uniformity, and a luminous flux >1000 lm is generated using a single commercial laser diode coupled to a single phosphor element.

Preparation of dispersed submicron YAG:Ce3+ phosphors via the molten salt method

Nano Ce-doped yttrium aluminum garnet (YAG:Ce) phosphor precursor was synthesized using the urea precipitation method, and dispersed submicron YAG:Ce phosphor was prepared using the molten salt method, with fine particle NaCl acting as the salt source. The effects of NaCl particle size on the mixing homogeneity of raw materials, particle morphology, dispersity, and luminescent properties of YAG phosphor were investigated. We found that the mixing homogeneity improved dramatically with a fine (2 µm) NaCl particle. Agglomeration and sintering of the phosphor were significantly reduced, and 200 nm dispersed YAG phosphor can be obtained at 1000 °C. The YAG phosphor obtained with fine NaCl salt also exhibits excellent luminous performance. The YAG phosphor blue light absorption ratio is 38%, and the internal quantum efficiency is 93%. The internal quantum efficiency is superior to YAG phosphors synthesized using commercial large-particle NaCl. These findings provide a new idea for preparing dispersed inorganic powder with fine particle size using the molten salt method.

Physical and optical properties of Ce:YAG nanophosphors and transparent ceramics and observation of novel luminescence phenomenon

Ce doped yttrium aluminum garnet (Ce:YAG) is an important photonic material that is used as a yellow phosphor for white light emitting diodes. In this work, the physical and optical properties of Ce:YAG nanophosphors are investigated and the effects of high-temperature thermal treatments and annealing atmospheres on the particle size and luminescence intensity are discussed. Furthermore, photo-luminescence (PL) was measured as a function of temperature and compared with PL from Ce:YAG single crystals and transparent ceramics to understand the mechanism of luminescence decay with temperature. While the characteristics of PL emission as a function of temperature for single crystals and NPs are similar and follow common decay trends, Ce:YAG transparent ceramics exhibit an interesting unusual increase in PL with temperature. We explained this unique novel behavior by a 4-step mechanism involving localized states in the band gap, and provided evidence from thermo-luminescence measurements to support this interpretation. The work reveals a new luminescence phenomenon arising from the overlap of PL and TL emissions; this phenomenon is most likely characteristic of transparent ceramics.

Fine tuning of emission property of white light-emitting diodes by quantum-dot-coating on YAG:Ce nanophosphors

We report fine tuning of emission color of Ce-doped yttrium aluminum garnet (Y3Al5O12:Ce3+, YAG:Ce) nanophosphor-based white light-emitting diodes (WLED), by coating CdSe/CdS/ZnS quantum dots (QDs) onto the surface of the YAG:Ce nanoparticles via surface functionalization of both the QDs and the YAG:Ce. Mixture of bromo-functionalized QDs and amino-functionalized YAG:Ce nanoparticles results in conformal coating of the QDs onto the YAG:Ce nanoparticles (QD@YAG:Ce). By varying the QD to YAG:Ce weight ratios, the luminescence spectra of the QD@YAG:Ce are tuned. A high-quality warm-white-light emission is achieved by appropriate combination of the yellow and red emissions from the QD@YAG:Ce, and the blue emission from InGaN LED chip. However, without surface functionalization, irregular mixtures of YAG:Ce and QDs are formed, which consequently make it hard to control the emission spectra. This study demonstrates a promising way to prepare uniformly QD-coated nanophosphors and an approach to control the emission spectra the nanophosphors.