Mn ions-activated Gd3(Al,Ga)5O12 garnet solid-solution ceramics: Cation substitution for dual wavelength red-emission

Red-emitting color-convertors have attracted considerable attention for promising applications in solid-state lighting (SSL) to improve color rendition. However, the current nitride and fluoride phosphor powders have encountered several challenges, such as high cost, narrow emission bands, and insufficient stability during operation, which limit the development of high-power full-spectrum SSL. In this study, thermally robust Gd₃(Al,Ga)₅O₁₂:Mn (GAGG:Mn) solid-solution ceramics (SSCs) with dual wavelength red-emission bands were prepared via an oxygen solid-state sintering reaction. The doped Mn ions occupied octahedral Al³⁺ and Ga³⁺ sites to generate Mn⁴⁺ luminescent centers with pronounced deep-red emissions peaking at 698 nm (²E → ⁴A₂), and Mn²⁺ luminescent centers with broad red emissions at 628 nm (⁴T₁ → ⁶A₁). Because the cationic radius matching effect induced the regulation of valence state of Mn, the photoluminescence of the GAGG:Mn SSCs can be tailored by the substitution of Al³⁺ with Ga³⁺. Moreover, the Mn³⁺ also existed in the GAGG lattice host, and their concentration decreased with increasing Ga³⁺ contents owing to the mismatch of ionic radius between Mn³⁺ and Ga³⁺ ions. With the optimization of Al/Ga ratio and concentration of Mn ions, a broad emission band ranging from 550 to 800 nm (bandwidth = 250 nm) was achieved from Gd₃Al₃Ga₂O₁₂:0.3%Mn SSCs upon 465-nm excitation. Moreover, the GAGG:Mn SSC has over 17-fold enhanced thermal conductivity compared with the corresponding phosphor powder. This paper opens a door of regulating the valence state of luminescence centers with cation substitution and the application of oxide red-emitting color-convertors.     

Production and optical properties of Ce3+-activated and Lu3+-stabilized transparent gadolinium aluminate garnet ceramics

We first report the novel Ce³⁺-activated and Lu³⁺-stabilized gadolinium aluminate garnet (GAG) transparent ceramics derived from their precipitation precursors via a facile co-precipitation strategy using ammonium hydrogen carbonate (AHC) as the precipitant. The resulting precursors in liquid phase were substantially homogeneous solid solutions and could directly convert into sinterable garnet powders via pyrolysis. Substituting 35 at.% of Lu³⁺ for Gd³⁺ was effective to stabilize the cubic GAG garnet structure and transparent (Gd,Lu)₃Al₅O₁₂:Ce ceramics were successfully fabricated by vacuum sintering at 1715°C. The ceramic transparency was improved by optimizing the particle processing conditions and the best sample had an in-line transmittance of ~70% at 580 nm (Ce³⁺ emission center) and over 80% in partial infrared region with a fine average grain size of ~4.5 μm. Transparent (Gd,Lu)₃Al₅O₁₂:Ce ceramics have a short critical wavelength (<200 nm) and a maximal infrared cut-off at ~6.6 μm. Both the (Gd,Lu)₃Al₅O₁₂:Ce phosphor powder and the transparent ceramic exhibited characteristic yellow emission of Ce³⁺ with strong broad emission bands from 490 to 750 nm upon UV excitation into two groups of broad bands around 340 and 470 nm. The photoluminescence and photoluminescence excitation intensities as well as the quantum yield were greatly enhanced via high-temperature densification. Both the phosphor powder and ceramic bulk had short effective fluorescence lifetimes.     

Gd3Al3Ga2O12:Ce,Mg2+ transparent ceramic phosphors for high-power white LEDs/LDs

Gd₃Al₃Ga₂O₁₂:1.5%Ce, xMg²⁺ (GAGG:1.5%Ce, xMg²⁺) transparent ceramic phosphors (TCPs) were prepared via a two-step sintering method. The effects of MgO on microstructures and luminescent properties of GAGG:Ce TCPs are investigated for the first time. For the optimized Mg²⁺ of x = 0.5%, the in-line transmittance of the obtained TCP reaches 78.6%. Performances of the titled TCPs in high-power light-emitting diodes (LEDs) and laser diodes (LDs) lighting are illustrated. The optimized TCP shows the luminous efficacy of 84.0 lm W^?1 in LD lighting. This work provides a strategy to modify TCPs for the next-generation LD lighting.     

Optimization of Ce3+ concentration and Y4MgSi3O13 phase in Mg2+-Si4+ Co-doped Ce: YAG ceramic phosphors

As a promising replacement for nitride red phosphors, Ce: Y₃(Mg_1.8Al_1.4Si_1.8)O₁₂ (Ce: YMASG) ceramic phosphors have attracted significant attention recently for their advantages in inorganic encapsulation and massive red-shifting of Ce³⁺ emission. In this work, Ce: YMASG with different doping concentrations of Ce³⁺ and Al₂O₃, was fabricated by vacuum sintering to investigate its effects on the elimination of the impurity phase and the enhancement of the luminescent properties of white light-emitting diodes (w-LEDs). It was discovered that the emission wavelength redshifts from 592 to 606 nm as the Ce³⁺ concentration increases, while at 450 K, the emission intensity deteriorates from 0.47 to 0.36 of its initial value. The Rietveld analysis revealed the presence of an impurity phase of Y₄MgSi₃O₁₃ with a concentration of 17.021 wt% in Ce: YMASG. With the introduction of Al₂O₃, the impurity phase was eliminated from the matrix completely, the emission peak shifted to a shorter wavelength, and the thermal stability was greatly improved. When the correlated color temperature was controlled at around 3000 K in the packaged w-LEDs, the commission international de l'éclairage (CIE) chromaticity coordinates shifted toward the bottom left corner of the diagram with increasing concentration of Ce³⁺. Conversely, the luminous efficiency (LE) increased from 36 lm/W to 58.6 lm/W as the concentration of Al₂O₃ increased from 0 to 10 wt%, which demonstrated the application prospect of the fabricated phosphor in warm w-LEDs.     

The co-optimization of efficiency and emission bandwidth in GSAGG:Cr3+ NIR ceramic phosphors

The NIR phosphor-converted light-emitting diode (NIR pc-LED) is a new near-infrared light source that has been widely studied. Among various NIR phosphors, Cr³⁺ doped gadolinium aluminum gallium garnet (GAGG:Cr³⁺) ceramic phosphor has shown great potential due to its ultra-high efficiency and thermal stability. Despite its capabilities, its detection range may be limited due to a relatively narrow emission bandwidth. To make the GAGG:Cr³⁺ ceramic phosphors achieve both high efficiency and broadband emission, a series of Gd₃Al_2-x-ySc_xGa₃O₁₂:yCr³⁺ (GASGG:Cr³⁺) ceramic phosphors were prepared. Thanks to the decrease of crystal field strength with the doping of Sc³⁺, the full width at half maximum (FWHM) of GASGG:Cr³⁺ ceramic phosphors were extended from 84 nm to 117 nm, and the emission peak exhibited a red-shift of 46 nm. Meanwhile, it still retained extremely high external quantum efficiency (EQE = 47%) and excellent thermal stability (90.7%@150 °C). Then, a NIR pc-LED prototype device was fabricated by combining GASGG:Cr³⁺ ceramic phosphor with a blue LED chip. The NIR light output power and the photoelectric conversion efficiency of this device achieved 646 mW and 19.2%, respectively. Finally, the application effect in night vision and venography of this prototype device was demonstrated.     

Ce3+ doped Lu3Al5O12 ceramics prepared by spark plasma sintering technology using micrometre powders: Microstructure,luminescence, and scintillation properties

Ce³⁺ doped Lu₃Al₅O₁₂ (Ce:LuAG) ceramics were fabricated by the solid-state reaction method through spark plasma sintering (SPS) from 1350 °C to 1700 °C for 5 min at a pressure of 50 MPa using micro powders. The average grain size of the SPSed ceramics gradually grew from 0.42 µm (1400 °C) to 1.55 µm (1700 °C), which is nearly one order of magnitude lower than that of vacuum sintered (VSed) Ce:LuAG ceramics (~24.6 µm). Characteristic Ce³⁺ emission peaking at around 510 nm appeared and 92% photoluminescence intensity of room temperature can be reserved at 200 °C revealing excellent thermal stability. The maximum radioluminescence intensity reached around 3 times of VSed Ce:LuAG ceramics and 7.8 times of BGO crystals. The maximum scintillation light yield under γ-ray (¹³⁷Cs) excitation reached 9634 pho/MeV @ 2 μs. It is concluded that SPS technology is a feasible way to develop Ce:LuAG ceramics and further optical enhancement can be expected.     

Wide-band blue-emitting in Ce3+ doped Ca2YZr2Al3O12 garnet-type phosphor designed via local structural lattice distortion and synthesized in nonreducing atmosphere

It is of great significance to explore blue photoluminescent phosphor for white light-emitting diodes excited by near-ultraviolet chip. However, it is very challenging to prepare efficient blue luminescence in phosphor by an unsophisticated synthesis process. In this work, a series of blue-emitting Ca₂Y_1-xZr₂Al₃O₁₂: xCe³⁺ phosphors are designed via local lattice distortion and synthesized in nonreducing atmosphere. The crystal structure of samples and the coordination environment of Ce³⁺ have been investigated in detail by X-ray diffraction and Rietveld refinement method. The wide-band blue emission peaking at 460 nm is attributed to a relaxed crystal field strength. Based on dodecahedron distortion caused by unequal increase of Ce–O bond length, the wavelength of blue color luminescence tuning can be realized from 459 nm to 472 nm. In addition to the concentration quenching effect, the fluorescent lifetimes, thermal quenching effect, the internal quantum efficiency, CIE chromaticity coordinates and related mechanisms of samples have also been studied systematically. Using the representative sample with other tricolor phosphors on a 365 nm chip, a prototype LED device with chromaticity point of (0.394, 0.384) and high CRI (93.5) at CCT of 3755 K is fabricated. All the results suggest that Ca₂Y_1-xZr₂Al₃O₁₂: xCe³⁺ phosphors can be conducted as potential alternative of blue-emitting phosphor for near-ultraviolet pumped white light-emitting diodes.     

Sintering of Yb3+:Y2O3 transparent ceramics in hydrogen atmosphere

Highly transparent Yb³⁺:Y₂O₃ ceramics with doping concentration up to 40.0 at.% had been fabricated successfully via hydrogen atmosphere sintering, where the raw powders were synthesized by co-precipitation method. The sintering temperature is about 600 °C lower than its melting temperature. SEM investigation revealed the average grain size of Yb³⁺:Y₂O₃ ceramics sintered at 1850 °C for 9 h was about 7 μm. The highest transmittance of as-prepared 1 mm thickness samples around wavelength of 1050 nm reached 80%, which is close to the theoretical value of Y₂O₃. The optical spectroscopic properties of Yb³⁺:Y₂O₃ transparent ceramics have also been investigated, which shows that it is a very good laser material for diode laser pumping and short pulse mode-locked laser.     

Structural and optical studies on Dy3+ doped Gd2Ti2O7 pyrochlore as white light emission

A series of Dy³⁺ trivalent ion embedded into the GTO pyrochlore (Gd_2-xDy_xTi₂O₇ (x = 0.5, 1.0, 1.5, 2.0, and 2.5 mol%)) have been synthesized through standard solid state reaction technique. The single phasic nature with Fd3m space group was determined through XRD, Raman spectroscopy, and photoluminescence spectroscopy. Rietveld refinement analysis results confirm that the substitution is not going to disturb the crystallographic structure as well unit cell dimensions. From Raman deconvoluted spectrums, it was found that all the vibrational bands show a very small blue shifting on Dy³⁺ substitution. The photoluminescent EM-spectra for all the samples were examined under the excitation wavelength 347 nm gives two high intense emission bands centered around 484 nm (blue) and 581 nm (yellow) due to the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions respectively, and less intense band appeared at wavelength of 456 nm arising due to the ⁴I_13/2 → ⁶H_13/2 transition of Dy³⁺ ions. For the optimum Dy³⁺ concentration (2.0 mol%), the Y/B ratio is found to be 0.8753 which is close to unity hence combination of both yellow and blue emission bands emits white light as validated by CIE chromaticity coordinates calculated values are (x = 0.34959, y = 0.35381). Therefore, the results reveal that Dy³⁺ substituted GTO pyrochlore samples could be used as luminescent material for wLEDs application.     

Anomalous photovoltaic effect in Bi(Ni2/3Ta1/3)O3-PbTiO3 ferroelectric solid solutions

Dense Bi(Ni_2/3Ta_1/3)O₃-PbTiO₃ (BNT-PT) ceramics were prepared by solid-state reaction method. Morphotropic phase boundary between tetragonal and rhombohedral phase was observed around the composition 0.38BNT-0.62PT, at which large photovoltages of 13.2V were obtained under 405 nm laser illumination with power density of 200 mW/cm². By B-site Ni²⁺ ions doping, the bandgap values of BNT-PT solid solutions were reduced to 2.25~1.85 eV, and the anomalous photovoltaic response was extended from the ultraviolet region to a wavelength of 550 nm at the visible light region.     

Enhanced afterglow behavior of a new Eu2+: NaBa4(BO3)3 yellow phosphor co-doped with different cations Dy3+, Ho3+and Nd3+

A new Eu²⁺: NaBa₄(BO₃)₃ phosphor, synthesized under conventional solid state approach, exhibits bright yellow persistent luminescence with the co-doping of Eu²⁺, Ln³⁺ (Ln = Dy³⁺, Ho³⁺, Nd³⁺). XRD investigation reveals that all samples are pure phase phosphors with cubic structure, indicating the doped Eu²⁺and Ln³⁺ cations were incorporated into the lattice that did not form inclusive phase in the material. Since there are two coordination forms ([EuO6] and [EuO8]) of Eu²⁺ cations in the matrix, the above yellow luminescence can be divided into two different emissions centering at 558 nm and 627 nm, confirming the crystallographic environment on the photoluminescence from Eu²⁺ center. Optimal doping concentrations for each cations were further determined as 2% atm for Eu²⁺, 2% atm for Dy³⁺, 3% atm for Ho³⁺ and 2% atm for Nd³⁺. Afterglow behaviors are also observed for samples with Ln³⁺ co-doping, which reveal typical double exponential decay model. Among the phosphors synthesized, the sample 2 at%Eu²⁺/3 at%Ho³⁺: NaBa₄(BO₃)₃ displays the best performance with higher initial brightness and longer lifetime, which can be attributed to the formation of electron traps with high concentration and suitable energy level, as confirmed by the result from thermal stimulated luminescence (TSL) analysis.     

Synthesis of GAGG:Ce3+ powder for ceramics using mechanochemical and solution combustion methods

Gadolinium aluminum gallium garnet doped with cerium (Gd_2.99Al_2.00Ga_3.00O_12.00:Ce_0.01, or GAGG:Ce) powders has been synthesized with high‐energy ball milling (HEBM) and solution combustion synthesis (SCS) techniques. The structures and morphologies of the powders were characterized through use of powder X‐ray diffraction (PXRD), photoluminescence (PL) measurements, dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV‐decay time measurements. Changes in the mechanochemically produced powders were monitored over time, while the solution combustion synthesis of GAGG:Ce was conducted using various ratios and types of fuels. In both production methods, particle sizes on the order of microns gave evidence of agglomeration.     

Fabrication,Microstructure, and Luminescent Properties of Ce3+ ‐Doped Lu3Al5O12 (Ce:LuAG) Transparent Ceramics by Low‐Temperature Vacuum Sintering

The fabrication of 0.5 mol% Ce:LuAG transparent ceramics starting from synthetic nanosized Ce:LuAG powders was investigated by low temperature vacuum sintering. It was found that high quality optical Ce:LuAG ceramics could be densified successfully by vacuum sintering (<10^–3 pa) at 1750°C for 10 h. The in‐line optical transmittance of as‐sintered Ce:LuAG ceramics with thickness of 0.7 mm could reach 73.48% at the wavelength of 550 nm. The microstructure observations revealed that transparent Ce:LuAG ceramics were composed of uniform LuAG grains with average size of 9 μm and HRTEM morphology indicated that no impurity segregation existed at grain boundaries or within Ce:LuAG grains. It was also demonstrated that the annealing treatment (at 1450°C for 20 h in air) could greatly enhance the luminescent intensity of as‐sintered Ce:LuAG ceramics under excitation of X‐ray radiation (75 kV, 25 mA), which makes it a potential candidate to be applied in radiation detector.     

Microanalysis of Calcium Codoped LaAl(Si6−zAlz)(N10−zOz) (z~1): Ce3+ Blue Phosphor

Cathodoluminescence spectra of Ca codoped (La,Ce)Al(Si_6?zAl_z)(N_10?zO_z) (z~1) (termed JEM phase) phosphors were analyzed microscopically. The as‐prepared samples consisted of a major JEM phase and a small amount of β‐sialon and/or α‐sialon depending on the Ca concentration. A microscale distribution of three different emission spectra was clearly observed on one phosphor particle, which was originated from the blue‐emitting JEM:Ce (430 nm), β‐sialon:Ce (435 nm) and Ca‐α‐sialon:Ce (475 nm), respectively. The volume ratio of the three phases thus led to changes in the bandwidth and the maximum position of the emission spectrum, and the redshift of CL spectra with increasing Ca concentration was dominantly attributed to the enhanced emission from Ca‐α‐sialon. The microanalysis of phosphor particles with the aid of cathodoluminescence spectroscopy indicated a similar result of the phase assembly that was identified by the powder X‐ray diffraction.     

Fabrication and phase transition of uranium-doped LaxGd2−xZr2O7 transparent ceramics: A prospective neutron detection material

Via vacuum sintering, 2 mol% uranium-doped La_xGd_2−xZr₂O₇ (x = 2, 1.6, 1, and 0.4) transparent ceramics with Ca²⁺ as charge compensator was first fabricated by solid-state reaction. X-ray diffraction results of as-prepared powders and ceramic samples demonstrate that the phase transition from defective fluorite to pyrochlore occurs with the increase of x. Optical in-line transmittance spectrum shows that four ceramic samples have good in-line transmittance (nearly 80% from 700 to 2200 nm), especially the U:La_1.6Gd_0.4Zr₂O₇ ceramics. The cut-off wavelength of four U-doped transparent ceramics shifted from 250 to near 460 nm, and it is believed that such phenomenon is related to the stable existence form of uranium in ceramics lattices. Observing the excitation emission spectra, the main excitation peaks of four ceramic samples are located at 458 nm, and the main emission peaks are located around 513 nm. In addition, there are low-intensity emission peaks around 520, 537, and 566 nm, and the related explanation is given in combination with the U⁶⁺ ion energy level diagram. Thus, uranium-doped La_xGd_2−xZr₂O₇ transparent ceramics have potential for novel neutron detection materials.     

Effect of Ce dopant on upconversion and temperature sensing performances in homogeneous ultrasmall Y2O3:Yb3+/Ho3+ nanoparticles through flame aerosol synthesis

Flame aerosol synthesis (FAS) is an excellent strategy for continuous, fast, and mass production of small-size upconversion nanoparticles (UCNPs), which have high potential applications in fields like biological imaging, colour display and optical temperature sensing. However, flame-made UCNPs have received less attention, and relevant studies are limited. Herein, for the first time, we successfully fabricated cerium (Ce)-doped homogeneous ultrasmall Y₂O₃:Yb³⁺/Ho³⁺ UCNPs using a liquid-fed FAS method. Ce was doped to improve the upconversion luminescence (UCL) of the Y₂O₃:Yb³⁺/Ho³⁺ UCNPs. The overall UCL intensity was enhanced ～77.9-fold for an optimal concentration of 20 mol% Ce-doped UCNPs, compared with the UCNPs without Ce doping with a relatively homogeneous ultrasmall size of 8–10 nm. Further studies confirmed that both trivalent (Ce³⁺) and tetravalent (Ce⁴⁺) simultaneously exist in the Y₂O₃ hosts and are critical in enhancing the UCL properties. In addition, the fluorescence intensity ratio (FIR) method was used to evaluate the thermal properties of the fabricated UCNPs. Ce doping significantly improved the thermal sensitivity of Y₂O₃:Yb³⁺/Ho³⁺ UCNPs. An excellent relative sensitivity (S_R) of 0.622% K^?1 at 598 K was obtained for flame-made UCNPs doped with 20 mol% Ce.     

Enhanced transmittance of Y2O3 transparent ceramics from near-UV to near-infrared through reduced atmosphere annealing

Y₂O₃ transparent ceramics were annealed under different atmospheric conditions. The samples annealed in H₂ containing atmosphere were colorless and had high in-line transmittances from the near-UV to the mid-infrared wavelength range. This is due to the elimination of carbon contamination and preventing the formation of high concentration oxygen interstitial defects in the sintered samples. Annealing in oxygen containing atmosphere resulted in stronger optical absorption in the visible wavelength region. High temperature annealing in O₂ or hot isostatic pressing under high partial pressure of O₂ (O₂ HIP) led to obviously declining of transparency in a broader wavelength range of 230–800 nm. The Er:Y₂O₃ ceramics annealed in H₂ containing atmosphere had high in-line transmittance of about 80% at 400 nm as well. Room temperature laser oscillation at 2.7 µm was also obtained on the 5%H₂/95%Ar atmosphere annealed Er:Y₂O₃ ceramics.     

Synthesis,morphology, and upconversion luminescence of Tm3 +/Yb3 + codoped bulk and submicro-rod CaSc2O4 phosphors

CaSc₂O₄: Tm^3 +/Yb^3 + submicro-rods were synthesized using the mild solvothermal and annealing technique. The temperature of formed pure phase is as low as 600 °C. The synthesized submicro-rods are uniform in size and shape. The size of rod is only small with 35 nm in width and 200 nm in length, which favors the application in biological assays and medical image. Furthermore, samples prepared exhibit the stronger upconversion luminescence than that of obtained by conventional solid state reaction method under 980 nm excitation. The near infrared emission around 800 nm is enhanced twofold closely. CaSc₂O₄ phosphor synthesized through the solvothermal and annealing method is a promising submicron upconversion material. The solvothermal and annealing technique is a suitable method for preparing CaSc₂O₄ sample with rod-like morphology.     

Spark Plasma Sintering of Hexagonal Structure Yb3+‐Doped Sr5(PO4)3F Transparent Ceramics

Ytterbium‐doped Sr₅(PO₄)₃F transparent ceramics have been developed through spark plasma sintering (SPS) with a low sintering temperature and short dwelling time. The XRD patterns show a polycrystalline hexagonal phase, and the TEM microstructure characterization indicates that the ceramics have a narrow grain size distribution which ranges from 40 to 200 nm, with an average grain size around 150 nm. The transmittance of a 2 mm thick ceramic sample is measured to be 74% at 1000 nm by a UV–Vis–NIR spectrophotometer. Furthermore, there is a strong emission peak around 1040 nm which has a lifetime of 1.06 ms and is exhibited by a PL spectrometer with the 980 nm laser diode excitation.     

BaCl2:Er3+—A High Efficient Upconversion Phosphor for Broadband Near‐Infrared Photoresponsive Devices

Utilization of photons with subband‐gap energy, mostly near‐infrared (NIR) photons, is highly desirable for photovoltaic cells; which can be achieved by adding an upconversion layer at the rear face of photovoltaic cells. Here, we study the upconversion luminescence properties of BaCl₂:Er³⁺ phosphors and hexagonal NaYF₄:Er³⁺ phosphors upon excitation of incoherent NIR sunlight with wavelength λ > 800 nm. Higher efficacious upconversion emissions of BaCl₂:Er³⁺ phosphors have been observed in comparison with the well‐known hexagonal NaYF₄:Er³⁺ phosphors. We demonstrate that the photocurrent response from the thin‐film‐hydrogenated amorphous silicon solar cell attached with the BaCl₂:Er³⁺ phosphor is notably enhanced under irradiation of incoherent NIR sunlight with wavelength λ > 800 nm. This judicious design may be envisioned to shorten the distance for the remarkable improvement of the power conversion efficiency of the next‐generation photovoltaic cells and suggests a promising application for other NIR photoresponsive devices.