Enhanced Photoluminescence Emission and Thermal Stability from Introduced Cation Disorder in Phosphors

Design of Eu3+-Doped Fluoride Phosphor with Zero Thermal Quenching Property Based on Density Functional Theory

Although being applied in various fields, white light emitting diodes (WLEDs) still have drawbacks that urgently need to be conquered: the luminescent intensity of commercial phosphors sharply decreases at working temperature. In this study, we calculated the forming energy of defects and confirmed that the VNa defect state can stably exist in β-NaGdF4, by density functional theory (DFT) calculation. Furthermore, we predicted that the VNa vacancies would provide a zero thermal quenching (ZTQ) property for the β-NaGdF4-based red-light phosphor. Then, a series of β-NaGdF4:xEu3+ and β-NaGdF4:0.25Eu3+,yYb3+ red-light phosphors were synthesized by the hydrothermal method. We found that β-NaGdF4:0.25Eu3+ and β-NaGdF4:0.25Eu3+,0.005Yb3+ phosphors possess ZTQ properties at a temperature range between 303-483 K and 303-523 K, respectively. The thermoluminescence (TL) spectra were employed to calculate the depth and density of the VNa vacancies in β-NaGdF4:0.25Eu3+ and β-NaGdF4:0.25Eu3+,0.005Yb3+. Combining the DFT calculation with characterization results of TL spectra, it is concluded that electrons stored in VNa vacancies are excited to the exited state of Eu3+ to compensate for the loss of Eu3+ luminescent intensity. This will lead to an increase of luminescent intensity at high temperatures and facilitate the samples to improve ZTQ properties. WLEDs were obtained with CRI = 83.0, 81.6 and CCT = 5393, 5149 K, respectively, when phosphors of β-NaGdF4:0.25Eu3+ and β-NaGdF4:0.25Eu3+,0.005Yb3+ were utilized as the red-light source. These results indicate that these two phosphors may become reliable red-light sources with high antithermal quenching properties for WLEDs.

Bright Circularly Polarized Mechanoluminescence from 0D Hybrid Manganese Halides

Hybrid metal halides (HMHs) with efficient circularly polarized luminescence (CPL) have application prospects in many fields, due to their abundant host-guest structures and high photoluminescence quantum yield (PLQY). However, CPLs in HMHs are predominantly excited by light or electricity, limiting their use in multivariate environments. It is necessary to explore a novel excitation method to extend the application of chiral HMHs as smart stimuli-responsive optical materials. In this work, an enantiomeric pair of 0D hybrid manganese bromides, [H2(2R,4R)-(+)/(2S,4S)-(-)-2,4-bis(diphenylphosphino)pentane]MnBr4 [(R/S)-1] is presented, which exhibits efficient CPL emissions with near-unity PLQYs and high dissymmetry factors of ± 2.0 × 10-3. Notably, (R/S)-1 compounds exhibit unprecedented and bright circularly polarized mechanoluminescence (CPML) emissions under mechanical stimulation. Moreover, (R/S)-1 possess high mechanical force sensitivities with mechanoluminescence (ML) emissions detectable under 0.1 N force stimulation. Furthermore, this ML emission exhibits an extraordinary antithermal quenching effect in the temperature range of 300-380 K, which is revealed to originate from a thermal activation energy compensation mechanism from trap levels to Mn(II) 4T1 level. Based on their intriguing optical properties, these compounds as chiral force-responsive materials are demonstrated in multilevel confidential information encryption.

A Robust Anti-Thermal-Quenching Phosphor Based on Zero-Dimensional Metal Halide Rb3InCl6:xSb3

High-power phosphor-converted white light-emitting diodes (hp-WLEDs) have been widely involved in modern society as outdoor lighting sources. In these devices, due to the Joule effect, the high applied currents cause high operation temperatures (>500 K). Under these conditions, most phosphors lose their emission, an effect known as thermal quenching (TQ). Here, we introduce a zero-dimensional (0D) metal halide, Rb3InCl6:xSb3+, as a suitable anti-TQ phosphor offering robust anti-TQ behavior up to 500 K. We ascribe this behavior of the metal halide to two factors: (1) a compensation process via thermally activated energy transfer from structural defects to emissive centers and (2) an intrinsic structural rigidity of the isolated octahedra in the 0D structure. The anti-TQ phosphor-based WLEDs can stably work at a current of 2000 mA. The low synthesis cost and nontoxic composition reported here can herald a new generation of anti-TQ phosphors for hp-WLED.

Regulating luminescence thermal quenching based on the synergistic effect of energy transfer and energy gap modulation

Thermal quenching is the core challenge that hinders the application of luminescent materials. Herein, a synergistic mechanism involving energy transfer and energy gap modulation is proposed based on the local crystal field regulation around sensitizers. The substitution of coordination cation V5+/P5+ weakens the crystal field strength of the sensitizer Bi3+, and the weakening of crystal field splitting causes an increase in the 3P1 energy level, thus increasing its energy gap. Compared with the YVO4:Bi3+,Eu3+ phosphor, the thermal stability of the YV0.25P0.75O4:Bi3+,Eu3+ phosphor is significantly improved, and the relative emission intensity of Eu3+ continuously increases with heating and reaches 1.24 times the original intensity at 523 K and does not show a decreasing trend in the studied temperature range. The anti-thermal quenching performance is mainly attributed to the increasing thermal quenching activation energy (ΔE) of the sensitizer by energy gap modulation, which enhances the energy transfer to compensate thermal quenching. Based on the thermal quenching characteristics of the materials, an optical thermometer is designed. The maximum relative sensitivity (Sr) and absolute sensitivity (Sa) are as high as 1.74% K-1 and 0.59 K-1, respectively, and the minimum temperature resolution reaches 0.288 K. The synergistic effect between energy gap modulation of the sensitizer and energy transfer enables the regulation of thermal quenching. Thus, this study provides a new strategy for exploiting high-performance luminescent materials.

Emission enhancement in the luminescence performance of warm red light-emitting LiSrVO4 :Pr3+ phosphors

Warm red-emitting praseodymium-doped LiSrVO4 phosphors were synthesized via solid-state reaction. The phase formation was verified using an X-ray diffraction study and the morphology was investigated using a scanning electron microscope study. The LiSrVO4 :Pr3+ phosphors emitted red light when exposed to ultraviolet light, indicating their possibility for use in warm white light-emitting diodes (WLEDs). Furthermore, the effect of charge compensators on the luminescence characteristics was addressed. The decay time was investigated using time-resolved photoluminescence. Furthermore, thermal quenching was analyzed through temperature-dependent photoluminescence spectra. Their sensitivity was calculated using temperature-dependent decay time analysis. The colour purity of the emitted light could be measured by photometric analysis. This comprehensive investigation provides a thorough understanding of the luminescence properties of phosphors for WLED applications.

Limited energy migration and circumscribed multiphonon relaxation produced non-concentration quenching in a novel dazzling red-emitting phosphor

White LED applications are still constrained by extremely efficient narrow band red emitting phosphors. Meanwhile, the concentration quenching induced by energy migration is the main reason that limits the emission intensity of a red emitting phosphor. Therefore, developing a novel red emitting material with energy migration limitations seems necessary. Here, we proposed and realized the non-concentration quenching doping of Eu3+ ions in a Sr9Y2-2xW4O24:xEu3+ (0 ≤ x ≤ 1.0) phosphor for the first time by means of host preferential selection. By clearly investigating the crystal structure and luminescence kinetics, the long-distance between the nearby Eu3+ ions and the low phonon energy are the main reasons that suppress the energy migration and the cross-relaxation among Eu3+ ions. These advantages result in a high internal (90.47%) and external quantum efficiency (42.1%) of Sr9Eu2W4O24. With the help of the Judd-Ofelt theory and the large value of oscillator strength Ω2, Eu3+ ions are verified to occupy the non-symmetric lattice site with high color purity (94.4%). In addition, only 5.2% emission intensity loss at 140 °C can guarantee its application in LED devices. Moreover, the SYWO:Eu3+ phosphor has high thermal tolerance, high color stability, excellent moisture resistance and superior physical/chemical stability, and thus has broad practical spectral application prospects. The prepared WLED shows superior performance, and the calculated NTSC values are as high as 101.8% and 104.7%, respectively. For comparison, the optical performances of the Sr9Y2W4O24:Eu3+ phosphor outperform those of the standard commercial red phosphors, Y2O3:Eu3+ and Y2O2S:Eu3+, and almost match that of K2MnF6. These results may pave the way for fresh approaches to the study of high-performance Eu3+-activated phosphors.

Oxynitride K2Ba6.72Si16O40-1.5yNy:0.28Eu2+ phosphor with high thermal stability realized by crystal field engineering

Extensive research has gone into modifying the chemical composition of phosphors to achieve desirable optical properties. Here, oxynitride phosphors K2Ba6.72Si16O40-1.5yNy:0.28Eu2+ were synthesized by introducing N3- (y) into a K2Ba6.72Si16O40:0.28Eu2+ lattice. An uneven shrinking of the cell parameters a, b, and c was observed through a combination of X-ray diffraction studies and Rietveld refinements. This shrinking caused a large centroid shift (εc) and splitting of the 5d energy level (εcfs), thus inducing the broadening of the excitation spectra (104 → 127 nm, y = 0 → y = 12) and the red shift of the emission spectra (501 → 543 nm, y = 0 → y = 12). The modified series of samples have a broad excitation spectrum, suitable of use in UV, near-UV, and blue light-emitting LEDs. In addition, the optimal sample, K2Ba6.72Si16O31N6:0.28Eu2+, benefits from an increased activation energy and thermal stability.

The developments of cyan emitting phosphors to fulfill the cyan emission gap of white-LEDs

Future generations of solid-state lighting (SSL) will prioritize the development of innovative luminescent materials with superior characteristics. The phosphors converted into white light-emitting diodes (white LEDs) often have a blue-green cavity. Cyan-emitting phosphor fills the spectral gap and produces "full-visible-spectrum lighting." Full-visible spectrum lighting is beneficial for several purposes, such as light therapy, plant growth, and promoting an active and healthy lifestyle. The design of cyan garnet-type phosphors, like Ca2LuHf2Al3O12 (CLHAO), has recently been the subject of interest. This review study reports a useful cyan-emitting phosphor based on CLHAO composition with a garnet structure to have a cyan-to-green emitting color with good energy transfer. It could be employed as cyan filler in warm-white LED manufacturing. Due to its stability, ability to dope with various ions suitable for their desired qualities, and ease of synthesis, this garnet-like compound is a great host material for rare-earth ions. The development of CLHAO cyan-emitting phosphors has exceptionally high luminescence, resulting in high CRI and warm-white LEDs, making them a viable desire for LED manufacturing. The development of CLHAO cyan-emitting phosphors with diverse synthesis techniques, along with their properties and applications in white LEDs, are extensively covered in this review paper.

Excited-State Dopant-Host Energy-Level Alignment: Toward a Better Understanding of the Photoluminescence Behaviors of Doped Phosphors

Luminescent materials, also known as phosphors, have been widely used for applications such as emissive displays, fluorescent lamps, light-emitting diodes, and X-ray scintillation detectors. The energy-level diagram of a phosphor is extremely important for understanding its photoluminescence behavior. Here, we demonstrate through a combined density functional theory and experimental study that excited-state energy-level alignment accounts for the photoluminescence behaviors much better than ground-state energy-level alignment. An efficient doped phosphor should exhibit a type I excited-state dopant-host energy-level alignment, regardless of whether its ground-state alignment is type I. A type II excited-state dopant-host energy-level alignment implies that exciton dissociation, resulting in photoluminescence quenching. Our results provide not only a better understanding of the photoluminescence behaviors of the reported phosphors but also critical guidance for designing prospective luminescent materials.

Switching of radical and nonradical pathways through the surface defects of Fe3O4/MoOxSy in a Fenton-like reaction

Coexistence of radical and nonradical reaction pathways during advanced oxidation processes (AOPs) makes it challenging to obtain flexible regulation of high efficiency and selectivity for the requirement of diverse degradation. Herein, a series of Fe₃O₄/MoO_xS_y samples coupling peroxymonosulfate (PMS) systems enabled the switching of radical and nonradical pathways through the inclusion of defects and adjustment of Mo⁴⁺/Mo⁶⁺ ratios. The silicon cladding operation introduced defects by disrupting the original lattice of Fe₃O₄ and MoO_xS. Meanwhile, the abundance of defective electrons increased the amount of Mo⁴⁺ on the catalyst surface, promoting PMS decomposition with a maximum k value up to 1.530 min^-1 and a maximum free radical contribution of 81.33%. The Mo⁴⁺/Mo⁶⁺ ratio in the catalyst was similarly altered by different Fe contents, and Mo⁶⁺ contributed to the production of ¹O₂, allowing the whole system to attain a nonradical species-dominated (68.26%) pathway. The radical species-dominated system has a high chemical oxygen demand (COD) removal rate for actual wastewater treatment. Conversely, the nonradical species-dominated system can considerably improve the biodegradability of wastewater (biochemical oxygen demand (BOD)/COD = 0.997). The tunable hybrid reaction pathways will expand the targeted applications of AOPs.

Thermodynamics and Kinetics Accounting for Antithermal Quenching of Luminescence in Sc2(MoO4)3: Yb/Er: Perspective beyond Negative Thermal Expansion

Defects are common in inorganic materials and not static upon annealing of the heat effect. Antithermal quenching of luminescence in phosphors may be ascribed to the migration of defects and/or ions, which has not been well-studied. Herein, we investigate the antithermal quenching mechanism of upconversion luminescence in Sc₂(MoO₄)₃: 9%Yb1%Er with negative thermal expansion via a fresh perspective on thermodynamics and kinetics, concerning the thermally activated movement of defects and/or ions. Our results reveal a second-order phase transition taking place at ∼573 K induced by oxide-ion migration. The resulting variation of the thermodynamics and kinetics of the host lattice owing to the thermally induced oxide-ion movement contributes to a more suppressed nonradiative decay rate. The dynamic defects no longer act as quenching centers with regard to the time scale during which they stay nearby the Yb³⁺/Er³⁺ site in our proposed model. This research opens an avenue for understanding the antithermal quenching mechanism of luminescence via thermodynamics and kinetics.

Organo-Metal Halide Scintillator with Weak Thermal Quenching Up to 200 °C

The prominent thermal quenching (TQ) effect of organic-inorganic metal halides limits their applications for lighting and imaging. Herein, we report an organo-metal halide scintillator (TTPhP)₂MnCl₄ (TTPhP⁺ = tetraphenylphosphonium cation), which exhibits a weak TQ effect up to 200 °C under ultraviolet-visible light (efficiency loss of 5.5%) and X-ray radiation (efficiency loss of 37%). The light yield of the (TTPhP)₂MnCl₄ scintillator (37 000 photons MeV^-1 at 200 °C) under X-ray radiation is >2 times that of the commercial scintillator LuAG:Ce (15 000 photons MeV^-1 at 200 °C). The microscopic mechanism of the weak TQ effect is demonstrated to be the scintillator having the ability to compensate for the emission losses from trapped charges and the large Mn-Mn distance (10.233 Å) suppressing nonradiative recombination at high temperatures. We further demonstrate the applications of (TTPhP)₂MnCl₄ as high-power white-light-emitting diodes operated at currents of ≤300 mA and X-ray imaging at 200 °C with a high spatial resolution.

Thermal Quenching Mechanism of Metal-Metal Charge Transfer State Transition Luminescence Based on Double-Band-Gap Modulation

Bi³⁺-related metal-to-metal charge transfer (MMCT) transition phosphors are expected to become a new class of solid-state luminescent materials due to their unique broadband long-wavelength emission; however, the main obstacle to their application is the thermal quenching effect. In this study, one novel thermal quenching mechanism of Bi³⁺-MMCT transition luminescence is proposed by introducing electron-transfer potential energy (ΔE_T). Y_0.99V_1-xP_xO₄:0.01Bi³⁺ (YV_1-xP_xO₄:Bi³⁺) is used as the model; when the band gap of the activator Bi³⁺ increases from 3.44 to 3.76 eV and the band gap of the host YV_1-xP_xO₄ widens from 2.75 to 3.16 eV, the electron-transfer potential energy (ΔE_T) decreases and the thermal quenching activation energy (ΔE) increases, which result in the relative emission intensity increasing from 0.06 to 0.64 at 303-523 K. Guided by density functional calculations, the thermal quenching mechanism of the Bi³⁺-MMCT state transition luminescence is revealed by the double-band-gap modulation model of the activator ion and the matrix. This study improves the thermal quenching theory of different types of Bi³⁺ transition luminescence and offers one neo-theory guidance for the contriving and researching of high-quality luminescence materials.

Sensitive and Reliable Fluorescent Thermometer Based on a Red-Emitting Li2MgHfO4:Mn4+ Phosphor

Contactless fluorescent thermometers are rapidly gaining popularity due to their sensitivity and flexibility. However, the development of sensitive and reliable non-rare-earth-containing fluorescent thermometers remains a significant challenge. Here, a new rare-earth-free, red-emitting phosphor, Li₂MgHfO₄:Mn⁴⁺, was developed for temperature sensing. An experimental analysis combined with density functional theory and crystal field calculations reveals that the sensitive temperature-dependent luminescence arises from nonradiative transitions induced by lattice vibration. Li₂MgHfO₄:Mn⁴⁺ also exhibits reliable recovery performance after 100 heating-cooling cycles due to the elimination of surface defects, which is rare but vital for practical application. This study puts forward a new design strategy for fluorescent thermometers and sheds light on the fundamental structure-property relationships that guide sensitive temperature-dependent luminescence. These considerations are crucial for developing next-generation fluorescence-based thermometers.

Investigation of the Solid-Solution Limit, Crystal Structure, and Thermal Quenching Mitigation of Sr-Substituted Rb2CaP2O7:Eu2+ Phosphors for White LED Applications

Rb₂CaP₂O₇:Eu²⁺ is a bright reddish-orange-emitting phosphor, but its luminescence thermal stability is poor. In this study, we investigated the solid-solution limit and thermal quenching mitigation of Rb₂CaP₂O₇:Eu²⁺ phosphors by cation substitution with Sr²⁺ and revisited their crystal structure. First, we carefully investigated the solid solution limit of Sr in the structure of Rb₂CaP₂O₇. The results show that up to 80% of Ca can be substituted by Sr, whereas Ca hardly resides in the structure of Rb₂SrP₂O₇. Consequently, the photoluminescence was fine-tuned from reddish-orange (612 nm) to yellow (580 nm) light emission by increasing the Sr²⁺ concentration in the solid-solution phosphors Rb₂Sr_1-xCa_xP₂O₇:Eu²⁺ under excitation at 342 nm. The mechanism for the blue shift of the emission spectrum was discussed. With the associated modification of the local environment of the activator (as reflected by the changes in the effective coordination number, average bond length, distortion index, and quadratic elongation), the luminescence thermal quenching issue of Rb₂CaP₂O₇:Eu²⁺ was mitigated by substituting 20% Sr into the Ca site (Rb₂Ca_0.8Sr_0.2P₂O₇:Eu²⁺). The integrated intensity of bright orange-emitting Rb₂Ca_0.8Sr_0.2P₂O₇:Eu²⁺ (603 nm) at 150 °C retained 53% of its initial value, 1.64 times that of Rb₂CaP₂O₇:Eu²⁺ (32.3%). Such an enhancement could be attributed to the improved rigidity of the crystal structure due to the local structure modification as evidenced by Rietveld refinement. The cation substitution is an effective approach for mitigating the thermal quenching issue of phosphors.

Enhancement in the water resistance and thermal stability of Na3HTiF8:Mn4+ by co-doping with organic amine cations

Organic amine ions change the structural rigidity and improve the thermal stability and water resistance of phosphors.

Achieving highly thermostable red emission in singly Mn2+-doped BaXP2O7 (X = Mg/Zn) via self-reduction

The non-rare earth doped red phosphors are attracting wide attention for warm-white lighting and indoor plant cultivation applications. The Mn2+-doped phosphors have well spectral tunability and great potential to generate...

Variation from Zero to Negative Thermal Quenching of Phosphor with Assistance of Defect States

Proper defect states are demonstrated to be beneficial to overcome thermal quenching of the corresponding phosphors. In this work, a cyan-emitting KGaGeO₄/Bi³⁺ phosphor with abundant defect states is reported, the emission intensity of which exhibits an abnormal thermal quenching performance under excitation with different photon energies. A 100% emission intensity is achieved at 393 K under 325 nm excitation compared with that at room temperature, while significantly enhanced intensities of 207% at 393 K and even 351% at 513 K under 365 nm excitation are recorded. The excellent thermal stability performance is confirmed to be not only related to the direct energy transfer from the defect states but also depended on the efficiency of capturing carriers for the trap centers, which is clarified in this work. In addition, the mechanism of the double tunneling process of carriers from trap centers to luminescence centers and luminescence centers to trap centers is studied. These results are believed to provide new insights into the thermal stability of the corresponding fluorescent materials and could inspire studies to further explore novel fluorescent materials with high thermal stability based on defect state engineering.

Effect of the Concentration of SrAl2O4: Eu2+and Dy3+ (SAO) on Characteristics and Properties of Environment-Friendly Long-Persistent Luminescence Composites from Polylactic Acid and SAO

We report luminous polylactic acid (PLA) composite prepared via a solvent casting method using different amounts of phosphor strontium aluminate (SrAl2O4: Eu2+ and Dy3+) (SAO). The reason for doing this is that the changes of fluorescence and mechanical properties in the composites with different SAO contents can be directly evaluated. The SAO particles should have a variety of excellent characteristics in the PLA matrix, among which dispersibility and compatibility are particularly important; so, they can be modified by 3-aminopropyltriethoxysilane (APS) to achieve the target characteristics. The results showed that the fluorescence and mechanical properties were affected by SAO addition. The mechanical properties significantly improved with 5 wt% SAO; further, addition had no impact. And the emission band of fluorescence and phosphorescence is just at the peak of 524 nm. The composites with 15 wt% SAO have the best fluorescence properties. The fluorescence decreased with further doping. Fluorescence decay curves with various amounts of SAO particles show a similar tendency as pure SAO particles; the speed of decrease in afterglow intensity was higher for the first 30 min. In addition, the detailed morphological scanning and study by scanning electron microscope (SEM) showed that the particles had good adhesion to the matrix. In conclusion, the concentration of SAO into the PLA matrix impacts the fluorescence and mechanical properties of a SAO/PLA composite material.

Phase-formations of Mg2P2O7-Mn2P2O7 mixed pyrophosphates and their desired luminescence abilities

In this work, a series of mixed pyrophosphates (Mg1-xMnx)2P2O7 (x = 0-1.0) were prepared for the first time using a solid-state reaction method, which exhibits two kinds of structural variants, that is, α-(low temperature) and β-(high temperature) phases. The detailed phase-formations were determined via structural Rietveld refinements and the luminescence transitions of 4T1 → 6A1 in Mn2+. The phase-formation of (Mg1-xMnx)2P2O7 (x = 0-1.0) shows a strict dependence on Mn2+ doping contents in the lattices. (Mg1-xMnx)2P2O7 has an α-Mg2P2O7 phase when the Mn2+-doping concentration is lower than x ≤ 0.1. With an increase of Mn2+ substitution above 20 mol% (x = 0.2-1.0), (Mg1-xMnx)2P2O7 presents only the β-phase even at room temperature, while (Mg1-xMnx)2P2O7 (x = 0.15) shows a mixed formation of α- and β-Mg2P2O7 phases. The crystallographic surrounding of the Mn2+ activators in different structures had a strict influence on the spectral profile, luminescence efficiency, and color centers. Interestingly, in this series of phosphors, (Mg1-xMnx)2P2O7 (x = 0.15) with the mixed phases of α- and β-type has overwhelming luminescence abilities such as the best luminescence intensity and high thermal stability. The pyrophosphates were confirmed to qualify for red-emitting LED lamps.

Broadband emission Gd3Sc2Al3O12:Ce3+ transparent ceramics with a high color rendering index for high-power white LEDs/LDs

The discovery of single structure Ce³⁺ doped garnet transparent ceramics (TCs) with a broad full width at half maximum (FWHM) is essential to realize a high CRI for high-power white light emitting diodes (LEDs) and laser diodes (LDs). In this work, by utilizing the ion substitution engineering strategy, pure phase Gd₃Sc₂Al₃O₁₂:Ce³⁺ (GSAG:Ce) TC with a broad FWHM of 132.4 nm and a high CRI value of 80.7 was fabricated through the vacuum sintering technique for the first time. The optimized in-line transmittance of TCs was 58.4% @ 800 nm. Notably, the GSAG:Ce TCs exhibited a remarkable red shift from 546 nm to 582 nm, with a high internal quantum efficiency (IQE) of 46.91%. The degraded thermal stability in Ce:GSAG TCs was observed compared with that of Ce:YAG TC, owing to the narrowed band gap of GSAG. Additionally, remote excitation white LEDs/LDs were constructed by combining GSAG:Ce TCs with blue LED chips or laser sources. A tunable color hue from yellow to shinning white was achieved in white LEDs, whereas the acquired CRI and CCT of the white LDs were 69.5 and 7766 K, respectively. This work provides a new perspective to develop TCs with high CRI for their real applications in high-power white LEDs/LDs.

Thermally stable and highly efficient red-emitting Eu3+-doped Cs3GdGe3O9 phosphors for WLEDs: non-concentration quenching and negative thermal expansion

Red phosphor materials play a key role in improving the lighting and backlit display quality of phosphor-converted white light-emitting diodes (pc-WLEDs). However, the development of a red phosphor with simultaneous high efficiency, excellent thermal stability and high colour purity is still a challenge. In this work, unique non-concentration quenching in solid-solution Cs₃Gd_1 - xGe₃O₉:xEu³⁺ (CGGO:xEu³⁺) (x = 0.1-1.0) phosphors is successfully developed to achieve a highly efficient red-emitting Cs₃EuGe₃O₉ (CEGO) phosphor. Under the optimal 464 nm blue light excitation, CEGO shows a strong red emission at 611 nm with a high colour purity of 95.07% and a high internal quantum efficiency of 94%. Impressively, this red-emitting CEGO phosphor exhibits a better thermal stability at higher temperatures (175-250 °C, >90%) than typical red K₂SiF₆:Mn⁴⁺ and Y₂O₃:Eu³⁺ phosphors, and has a remarkable volumetric negative thermal expansion (coefficient of thermal expansion, α = -5.06 × 10^-5/°C, 25-250 °C). By employing this red CEGO phosphor, a fabricated pc-WLED emits warm white light with colour coordinates (0.364, 0.383), a high colour rendering index (CRI = 89.7), and a low colour coordinate temperature (CCT = 4508 K). These results indicate that this highly efficient red-emitting phosphor has great potential as a red component for pc-WLEDs, opening a new perspective for developing new phosphor materials.

Realizing cool and warm white-LEDs based on color controllable (Sr,Ba)2Al3O6F:Eu2+ phosphors obtained via a microwave-assisted diffusion method

Globally, phosphor converted white-LEDs (W-LEDs) are among the most suitable sources to reduce energy consumption. Nevertheless, modernization of efficient broadband emitting phosphors is most crucial to improve the W-LED performance. Herein, we synthesized a series of novel broadband emitting Sr2-xAl3O6F:xEu2+ phosphors via a new microwave-assisted diffusion method. Rietveld refinement of the obtained X-ray diffraction results was performed to recognize the exact crystal phase and the various cationic sites. Oxygen vacancies (VO) formed under synthetic reducing conditions enabled Sr2Al3O6F to demonstrate bright self-activated bluish emission. Doping of Eu2+ ions unlocked the energy transfer process from the host to the activator ions, owing to which, the self-activated emission diminished and the Eu2+-doped sample showed amplified bluish-green emission. The gradual increase in Eu2+ concentrations regulated the controllable emissions from the bluish (0.34, 0.42) to the greenish (0.38, 0.43) zone under UV excitation. Because of the different absorption preferences of Eu2+ ions located at the different Sr2+ sites, Sr2-xAl3O6F:xEu2+ exhibited bluish-white emission under blue irradiation. A further enhancement in PL intensity had been observed by the cation substitution of Ba2+ for Sr2+ sites in the optimum Sr1.95Al3O6F:0.05Eu2+ phosphor. The as-fabricated W-LEDs utilizing the optimized Sr1.75Ba0.2Al3O6F:0.05Eu2+ phosphor exhibited a cool-white light emission along with a 372 nm NUV-LED and a 420 nm blue-LED with a moderate CRI of 70 and a CCT above 6000 K. Such cool white emission was controlled to natural white with the CCT close to 5000 K, and the CRI above 80 via utilizing a suitable red emitting phosphor. The W-LED performances of the optimized phosphor justified its applicability to produce white light for lighting applications.

Thermal stability of nitride phosphors for light-emitting diodes

Improving thermal stability of nitride phosphors has become an important material challenge. Our review describes three thermal phenomena, lists strategies for enhancing thermal stability of nitride phosphors, and discusses prospects in the future.

Why Do Relative Intensities of Charge Transfer and Intra-4f Transitions of Eu3+ Ion Invert in Yttrium Germanate Hosts? Unravelling the Underlying Intricacies from Experimental and Theoretical Investigations

The dominant intensity of parity-forbidden intra-4f transitions of europium(III) over O → Eu charge-transfer band (CTB) intensity is against common perceptions, yet this trend is observed in many germanate hosts and has not been rationalized so far. In search of a plausible explanation for this unusual trend, present work reports an experimental and theoretical investigations in conjunction on two sibling germanate host, namely, Y₂GeO₅ and Y₂Ge₂O₇ having dopant Eu³⁺ in their respective YO₇ polyhedra. Whereas for Y₂GeO₅:Eu³⁺, the CTB is more intense than the intra-4f transitions in the excitation spectrum, in the case of Y₂Ge₂O₇:Eu³⁺, the relative intensities of CTB and intra-4f transitions are reversed. Comparative structural analysis reveals that Eu³⁺ present in YO₇ of Y₂GeO₅ has a greater number of tetra-coordinated oxygen (O_tetra) and yttrium atom as first and second neighbors, respectively (Eu³⁺-O_tetra-Y³⁺ linkages). Conversely, in Y₂Ge₂O₇ host, the Eu³⁺ ion mostly has tricoordinated oxygen (O_tri) as its nearest neighbor and germanium ions next to O_tri (Eu³⁺-O_tri-Ge⁴⁺ linkage). Theoretical calculations reveal that while Y₂GeO₅:Eu has O_tetra(4Y) dominating at the Fermi level and the 4f state of Eu³⁺ remains inert toward mixing, in Y₂Ge₂O₇:Eu, the Fermi level has major contribution from O_tri(2Y + 1Ge) with significant mixing with 4f states of Eu. The dominant control of Eu³⁺-O_tri-Ge⁴⁺ linkages in geometrical and electronic structure of Y₂Ge₂O₇:Eu owing to the GeO₄ surrounding has been attributed to relative poor intensity of O → Eu CTB. Siege of Eu³⁺ by GeO₄ and subsequent occurrence of Eu³⁺-O_tri-Ge⁴⁺ linkages play a dual role: First, it induces electronic rigidity to hinder excitation of electron at bridging (O_tri) oxygen by highly charged small Ge⁴⁺ cation; second, the covalent character in Eu-O bond is achieved by intermixing of Eu's 4f and O_tri 2p orbital which facilitates relaxing of the parity-selection rule thus enhancing the probability of intra-4f transitions. The inferences drawn remain valid when extrapolated to other inorganic oxides having EuO_x polyhedra surrounded by covalent units like PO₄, SiO₄, etc. and have a prevailing number of low-coordinated oxygen atoms and highly charged small cation in the first and second coordination shells, respectively. The optical basicity concept is also found to endorse our explanation. These remarkable generic inferences will pave the rational way for designing efficient phosphors for solid-state lighting.

Pyrophosphate Phosphor Solid Solution with High Quantum Efficiency and Thermal Stability for Efficient LED Lighting

Phosphors with high quantum efficiency and thermal stability are greatly desired for lighting industries. Based on the design strategy of solid solution, a series of deep-blue-emitting phosphors (Sr0.99-xBax)2P2O7:0.02Eu2+ (SBxPE x = 0-0.5) are developed. Upon excitation at 350 nm, the optimized SB0.3PE phosphor shows a relatively narrow full width at half maximum (FWHM = 32.7 nm) peaking at 420 nm, which matches well with the plant absorption in blue region. Moreover, this phosphor exhibits obvious enhancement of internal quantum efficiency (IQE) (from 74% to 100%) and thermal stability (from 88% to 108% of peak intensity and from 99% to 124% of integrated area intensity at 150°C) compared with the pristine one. The white LED devices using SB0.3PE as deep-blue-emitting component show good electronic properties, indicating that SB0.3PE is promising to be used in plant growth lighting, white LEDs, and other photoelectric applications.

Novel narrow-band blue-emitting Cs3Zn6B9O21:Bi3+ phosphor with superior thermal stability

A novel Bi³⁺-doped CsZn₆B₉O₂₁ blue phosphor shows an extraordinary narrow-band emission with a FWHM of only 50 nm and excellent thermal stability.

Photoluminescence control and abnormal Eu3+ orange emission in Ln3+ (Ln3+ = Ce3+, Eu3+)-doped oxyapatite-type phosphors

Controllable photoluminescence adjustment of Ce³⁺/Eu³⁺-doped phosphors.

The structures and luminescence properties of Sr4Gd3Na3(PO4)6F2:Ce3+,Tb3+ green phosphors with zero-thermal quenching of Tb3+ for WLEDs

UV excited, green-emitting Sr₄Gd₃Na₃(PO₄)₆F₂:Ce³⁺,Tb³⁺ phosphors with zero-thermal quenching of Tb³⁺ emission have been obtained and investigated in detail.

Valence Engineering via Dual-Cation and Boron Doping in Pyrite Selenide for Highly Efficient Oxygen Evolution

Valence engineering has been proved an effective approach to modify the electronic property of a catalyst and boost its oxygen evolution reaction (OER) activity, while the limited number of elements restricts the structural diversity and the active sites. Also, the catalyst performance and stability are greatly limited by cationic dissolution, ripening, or crystal migration in a catalytic system. Here we employed a widely used technique to fabricate heteroepitaxial pyrite selenide through dual-cation substitution and a boron dopant to achieve better activity and stability. The overpotential of Ni-pyrite selenide catalyst is decreased from 543 mV to 279.8 mV at 10 mA cm^-2 with a Tafel slope from 161 to 59.5 mV dec^-1. Our theoretical calculations suggest both cation and boron doping can effectively optimize adsorption energy of OER intermediates, promote the charge transfer among the heteroatoms, and improve their OER property. This work underscores the importance of modulating surface electronic structure with the use of multiple elements and provides a general guidance on the minimization of activity loss with valence engineering.

Mixing the valence control of Eu2+/Eu3+ and energy transfer construction of Eu2+/Mn2+ in the solid solution (1 − x)Ca3(PO4)2–xCa9Y(PO4)7 for multichannel photoluminescence tuning

Multichannel photoluminescence control from blue-to-green to red across the white region was achieved by solid solution evolution, valence mixing of Eu²⁺/³⁺ and Eu²⁺ → Mn²⁺ energy transfer.

Enhanced narrow-band green-emission and thermal stability via the introduction of Mg2+ in ZnB2O4:Mn2+ phosphor

The introduction of Mg²⁺ in ZnB₂O₄:Mn²⁺ may reduce the impurity phase, and increase the emission intensity and thermal stability by 100% and 9%, respectively. The mechanism is considered to be the ion-size compensation effect of Mg²⁺ to Mn²⁺.

Li substituent tuning of LED phosphors with enhanced efficiency, tunable photoluminescence, and improved thermal stability

Solid-state phosphor-converted white light-emitting diodes (pc-WLEDs) are currently revolutionizing the lighting industry. To advance the technology, phosphors with high efficiency, tunable photoluminescence, and high thermal stability are required. Here, we demonstrate that a simple lithium incorporation in NaAlSiO₄:Eu system enables the simultaneous fulfillment of the three criteria. The Li substitution at Al sites beside Na sites in NaAlSiO₄:Eu leads to an enhanced emission intensity/efficiency owing to an effective Eu³⁺ to Eu²⁺ reduction, an emission color tuning from yellow to green by tuning the occupation of different Eu sites, and an improvement of luminescence thermal stability as a result of the interplay with Li-related defects. A pc-WLED using the Li-codoped NaAlSiO₄:Eu as a green component exhibits improved performance. The phosphors with multiple activator sites can facilitate the positive synergistic effect on luminescence properties.

Synthesis, luminescence and application of novel europium, cerium and terbium-doped apatite phosphors

Novel La₈Ba₂(Si₄P₂O₂₂N₂)O₂:Eu²⁺/Ce³⁺/Tb³⁺ apatite phosphors with tuneable light-emission have been developed.

Enhanced photoluminescence and thermal stability in solid solution Ca1−xSrxSc2O4:Ce3+ (x = 0–1) via crystal field regulation and site-preferential occupation

Enhanced photoluminescence efficiency and thermal stability as well as controllable blue-green tuning of solid solution Ca_1−xSr_xSc₂O₄:Ce³⁺ phosphors were realized based on crystal field regulation and site-preferential occupation.

Pressure-Induced Large Emission Enhancements of Cadmium Selenide Nanocrystals

Pressure quenching of optical emission largely limits the potential application of many materials in optical pressure-sensing devices, since emission intensity is crucially connected to performance. Boosting visible-light emission at high pressure is, therefore, an important goal. Here, we demonstrate that the emission of CdSe nanocrystals (NCs) can be enhanced by more than an order of magnitude by compression. The brightest emission can be achieved at pressures corresponding to the phase transitions in different sized CdSe NCs. Very bright blue emission can be obtained by exploiting the increase in band gap with increasing pressure. First-principles calculations indicate that the interaction between the capping oleic acid (OA) layer and the CdSe core is strengthened with increased Hirshfeld charge at high pressure. The effective surface reconstruction associated with the removal of surface-related trap states is highly responsible for the pressure-induced emission enhancement of these CdSe NCs. These findings pave the way for designing a stress nanogauge with easy optical readout and provide a route for tuning bright-fluorescence imaging in response to an externally applied pressure.

Preferential Neighboring Substitution-Triggered Full Visible Spectrum Emission in Single-Phased Ca10.5- xMg x(PO4)7:Eu2+ Phosphors for High Color-Rendering White LEDs

Manipulating the distribution of rare-earth activators in multiple cation lattices can achieve versatile color output for single-phased phosphor-converted white light-emitting diodes (LEDs). However, successful cases are barely reported, owing to the uncertain distribution of rare-earth activators and the special combination of three primary colors for white LEDs. Herein, we took whitlockite β-Ca₃(PO₄)₂ as a multiple cation lattice host to manipulate the redistribution of Eu²⁺ activators, and the surprising Mg²⁺-guided redistribution of Eu²⁺ activators among different Ca sites is reported for the first time to regulate the photoluminescence (PL) behavior in series Ca_{10.5- x}Mg _x(PO₄)₇:Eu²⁺ phosphors. The preferential neighboring substitution of smaller Mg²⁺ cations in Ca(5) and Ca(4) sites triggers a discontinuous evolution of local structure along c axis and induces covalent variable Ca(1), Ca(2), and Ca(3) cation sites for the accommodation of Eu²⁺ activators. The unique optical feature enables the single-phased Ca_9.75Mg_0.75(PO₄)₇:Eu²⁺ phosphor-converted white LED to exhibit quite high color-rendering index R_a (85) and R₉ (91) values. The preferential neighboring-cation substitution reported here can not only manipulate the migration of Eu²⁺ activators among different cation sites for tunable PL properties, but also carve out a new way for next-generation high-quality solid-state lighting.

Atomic-Scale Determination of Cation Inversion in Spinel-Based Oxide Nanoparticles

The atomic structure of nanoparticles can be easily determined by transmission electron microscopy. However, obtaining atomic-resolution chemical information about the individual atomic columns is a rather challenging endeavor. Here, crystalline monodispersed spinel Fe₃O₄/Mn₃O₄ core-shell nanoparticles have been thoroughly characterized in a high-resolution scanning transmission electron microscope. Electron energy-loss spectroscopy (EELS) measurements performed with atomic resolution allow the direct mapping of the Mn²⁺/Mn³⁺ ions in the shell and the Fe²⁺/Fe³⁺ in the core structure. This enables a precise understanding of the core-shell interface and of the cation distribution in the crystalline lattice of the nanoparticles. Considering how the different oxidation states of transition metals are reflected in EELS, two methods of performing a local evaluation of the cation inversion in spinel lattices are introduced. Both methods allow the determination of the inversion parameter in the iron oxide core and manganese oxide shell, as well as detecting spatial variations in this parameter, with atomic resolution. X-ray absorption measurements on the whole sample confirm the presence of cation inversion. These results present a significant advance toward a better correlation of the structural and functional properties of nanostructured spinel oxides.