Morphology Tunable Self‐Assembled Sr2P2O7:Ce3+, Mn2+ Phosphor and Luminescence Properties

Uniform orange‐to‐red spherical phosphors of Sr₂P₂O₇:Ce³⁺, Mn²⁺ have been synthesized by the co‐precipitation method and characterized by X‐ray powder diffraction, scanning electron microscopy, and photoluminescence spectroscopy. The results indicate that the morphology, size, and photoluminescence properties of Sr₂P₂O₇:Ce³⁺, Mn²⁺ phosphors can be effectively controlled by the reaction and the sintering temperatures. Energy transfer from Ce³⁺ to Mn²⁺ in Sr₂P₂O₇ phosphor was observed from photoluminescence spectra of Sr₂P₂O₇:Ce³⁺, Sr₂P₂O₇:Mn²⁺, and Sr₂P₂O₇:Ce³⁺, Mn²⁺. Moreover, based on a self‐assembly process, a possible formation mechanism for the spherical phosphors is proposed. The uniform phosphor spheres obtained in this work exhibit great potential for high‐resolution display devices such as light emitting diodes.     

Enhancing photoluminescence properties of Mn4+-activated Sr4−xBaxAl14O25 red phosphors for plant cultivation LEDs

Non–rare earth Mn⁴⁺-activated strontium aluminate phosphor is considered to be a promising material for plant cultivation field owing to their advantage of inexpensively, environment friendly, nontoxic, and appropriate spectral range. In this paper, a Sr_4−xBa_xAl_13.99O₂₅:0.01Mn⁴⁺,1.4H₃BO₃ strontium aluminate phosphor is synthesized by a convenient high-temperature solid-state reaction. The photoluminescence spectra located at red region with a peak at 655 nm in the range of 600 to 750 nm that can be excited under the excitation of ultraviolet (~346 nm) or blue light (~450 nm). The shape emission band at 655 nm is attributed to the transition of ²E_g-⁴A_2g. Furthermore, the emission intensity of Sr_4−xBa_xAl_13.99O₂₅: 0.01Mn⁴⁺,1.4H₃BO₃ phosphor is conducted in detail with the variety of Ba²⁺ doping concentration and the intensity can be enhanced to 140.9% than the Sr₄Al_13.99O₂₅:0.01Mn⁴⁺,1.4H₃BO₃ when the value of Ba²⁺ concentration equal to 0.1 mol. In addition, the X-ray diffraction spectra, element mapping, composition modifying, optical properties, FT-IR spectra, diffuse reflectance spectra, thermal stability, and fluorescence lifetime are systematically investigated. According to the electro-luminescent spectra of as-packaged LED, indicating that the Sr_3.9Ba_0.1Al_13.99O₂₅:0.01Mn⁴⁺,1.4H₃BO₃ phosphor will become a great candidate for plant cultivation LEDs due to the emission spectra match well the plant pigment spectrum.     

Deep‐red photoluminescence and long persistent luminescence in double perovstkite‐type La2MgGeO6:Mn4+

A novel non‐rare‐earth doped phosphor La₂MgGeO₆:Mn⁴⁺ (LMG:Mn⁴⁺) with near‐infrared (NIR) long persistent luminescence (LPL) was successfully synthesized by solid‐state reaction. The phosphors can be effectively excited using ultraviolet light, followed by a sharp deep‐red emission peaking at 708 nm, which is originated from ²E_g → ⁴A_2g transition of Mn⁴⁺ ions. The luminescent performance was analyzed by photoluminescence (PL) and photoluminescence excitation (PLE) spectra. The crystal field parameters were calculated to describe the environment of Mn⁴⁺ in LMG host. The LPL behaviors as well as the mechanisms were systematically discussed. This study suggests that the phosphors will broaden new horizons in designing and fabricating novel NIR long phosphorescent materials.     

Synthesis,Structure, and Performance of Efficient Red Phosphor LiNaGe4O9:Mn4+ and Its Application in Warm WLEDs

For phosphor‐converted warm white light‐emitting diodes (WLEDs), it is essential to find highly efficient red oxide phosphors, which are better chemically stable and benign to environment and can be prepared in a much milder condition. Here, we report a red phosphor LiNaGe₄O₉:Mn⁴⁺ with a quantum yield up to 78% after systematic optimization in synthesis temperature, dopant concentration of Mn⁴⁺, and sintering time. Best performance of the phosphor can be reached when it is synthesized in a mild reaction condition, that is, at 850°C for 3 h in air. The integrated emission intensity is more than four times stronger than commercial red phosphor 3.5MgO·0.5MgF₂·GeO₂:Mn⁴⁺ (MFG:Mn⁴⁺) under a blue light excitation at 470 nm. Crystal structural analysis reveals that the high efficiency Mn⁴⁺ exhibits in the compound is mainly due to the well separation of GeO₆ groups from each other by GeO₄ tetrahedra in the neighborhood and the ideal substitution of octahedral Ge⁴⁺ site by Mn⁴⁺ in view of both size and charge matches. The high performance of the phosphor encourages us to apply the blue absorbing red phosphor to WLED, which is based on combination of a blue LED chip and YAG:Ce³⁺, and the warm perception WLED is therefore achieved with a color temperature of 3353 K.     

Thermometry of red nanoflaked SrAl12O19:Mn4+ synthesized with boric acid flux

This paper presents the luminescence properties and potential of red SrAl₁₂O₁₉:Mn⁴⁺ (SAO:Mn⁴⁺) phosphor for optical thermometry application. The SAO crystal consisted of a spinel block along with two mirror-like blocks. The Al³⁺/Sr²⁺ molar ratio of the precursor solution affected the crystalline-phases, morphology, and photoluminescence of the phosphor. The addition of flux H₃BO₃ promoted the growth of hexagonal-nanoflakes and enhanced the external quantum efficiency of phosphor 2.6-fold. The absolute sensitivity S_a and relative sensitivity S_r of SAO:Mn⁴⁺ showed a linear function of the temperature. The value of S_a was 4.17?×?10^?3 K^?1, and the maximum S_r was 2.70?×?10^?3 K^?1 at 393?K. A stable emission color was observed even with a change in temperature and a bright red light was seen in both daylight and a darkroom.     

Host sensitized photoluminescence in Sr2.9-3x/2LnxAlO4F: 0.1Eu3+ (Ln = Gd,Y) for innovative flexible lighting applications

Tetragonal structured Sr₃AlO₄F is highly strained as reported from its global instability index estimation. Moreover, our results of X-ray photoelectron spectroscopy (XPS) also ascertained that the structure of Sr₃AlO₄F is highly strained with oxygen vacancies. Herein, aliovalent substitutions of divalent Sr ions with trivalent Ln (Ln = Gd/Y) ions were carried out to improve the stability of Sr₃AlO₄F lattice, which subsequently enhanced the photoluminescence in a series of Sr_2.9-3x/2Ln_xAlO₄F: 0.1Eu³⁺ phosphors. All the phosphors showed intense red-orange emission (⁵D₀ → ⁷F_1,2) at excitation with UV and near-UV light. The critical concentrations of Gd³⁺ and Y³⁺ up to which the Eu³⁺ emission intensities increased linearly were observed to be x = 0.09 and x = 0.07, respectively. Nevertheless, further enhancement in the Eu³⁺ luminescence of the optimized phosphors was realized by subsequently annealing in low oxygen atmospheres. The enhancement in oxygen deficiency during the post-annealing in Ar or vacuum led the energy transfer (O^2--Eu³⁺) to a greater extent which afterward increased the Eu³⁺ luminescence. The optimized Sr_2.765Gd_0.09AlO₄F: 0.1Eu³⁺ and Sr_2.795Y_0.07AlO₄F: 0.1Eu³⁺ phosphors showed high red color purity (~99%), as well as CIE coordinates of (0.62, 0.38), indicated that these phosphors could be appropriate red-emitting components for making flexible optical films for many lighting devices. Therefore, flexible polydimethylsiloxane based films were also fabricated using optimized Sr_2.765Gd_0.09AlO₄F: 0.1Eu³⁺ phosphor. The electroluminescence of a flexible PDMS-phosphor composite film showed an intense and pure red color with good thermal stability suggesting its suitability in flexible lighting and display devices.     

Luminescence enhancement of Mn4+-activated double-perovskite phosphors for LEDs through a co-replacement strategy

Phosphors doped with Mn⁴⁺ ions have strong emission in the red and far-red light regions and are therefore used as red phosphors for indoor plant cultivation light-emitting diodes (LEDs) and white LEDs (w-LEDs). This paper introduces La₂Mg(Mg_1/3Ta_2/3)O₆: Mn⁴⁺ (Mg₂La₃TaO₉: Mn⁴⁺) red phosphors prepared by conventional high-temperature solid-phase method. The broad excitation band of Mg₂La₃TaO₉: Mn⁴⁺ phosphor is effectively excited by ultraviolet and blue light in the range of 250–600 nm, with the emission of 707 nm centered on far-red light. The phosphor has a high color purity of 99.07% and an internal quantum efficiency of 59.87%. To further enhance the performance of the phosphor, a cation substitution method was adopted in this paper to synthesize La₂Mg(Al_1/2Ta_1/2)O₆: Mn⁴⁺ phosphor by replacing [1/3Mg²⁺–2/3Ta⁵⁺] in La₂Mg(Mg_1/3Ta_2/3)O₆: Mn⁴⁺ with [1/2Al³⁺–1/2Ta⁵⁺]. The luminescence intensity and thermal stability of the samples were enhanced. The emission spectrum of the Mg₂La₃TaO₉: Mn⁴⁺ samples matched well with the phytochrome P_FR (phytochrome that absorbs far-red light) and is suitable for the preparation of LEDs for indoor plant cultivation. The concentration quenching effect of the samples was investigated, the main mechanism of which is the electric dipole–dipole interaction. Red LEDs and w-LEDs devices were prepared with the synthesized phosphors that produce light stably at different currents. The w-LEDs have a correlated color temperature of 5310 K and a color rendering index of 80.1. Therefore, these samples are expected to be used as red components for w-LEDs.     

Luminescence of MgAl2O4 and ZnAl2O4 spinel ceramics containing some 3d ions

The luminescence properties of ceramic phosphors based on two spinel hosts MgAl₂O₄ and ZnAl₂O₄ doped with manganese ions have been studied. It has been found that the spectral properties of these phosphors can be strongly varied by changing synthesis conditions. Both types of doped ceramic spinel can serve as efficient Mn²⁺ green-emitting phosphors having peak emissions at 525 and 510 nm, respectively. Mn-doped MgAl₂O₄ spinel can also be prepared as an efficient Mn⁴⁺ red-emitting phosphor having peak emission at ~651 nm by using specific temperatures of heat treatment in air. It has also been shown that the conversion of Mn²⁺ to Mn⁴⁺ and viсe versa, as well as the coexistence of Mn²⁺ green and Mn⁴⁺ red emissions, can be accomplished by properly chosen annealing conditions of the same initially synthesized MgAl₂O₄:Mn sample. Manganese doped MgAl₂O₄ spinel with an optimal intensity ratio of green and red emissions can be a promising single-phase bicolor phosphor suitable for the development of warm white phosphor-converted LED lamps. On the other hand, it has been determined that perfectly normal ZnAl₂O₄ spinel cannot be doped with Mn⁴⁺ ions in contrast to partially inverse MgAl₂O₄ spinel. However, ZnAl₂O₄ samples unintentionally doped with impurity Cr³⁺ ions show emission spectra in the far-red region with well pronounced R, N and vibronic lines of Cr³⁺ luminescence due to the perfect normal spinel structure of synthesized ZnAl₂O₄ ceramics. Also, by partially substituting Al³⁺ cations for Mg²⁺ in ZnAl₂O₄ there is an opportunity to obtain Mn⁴⁺ doped or Mn⁴⁺/Cr³⁺ codoped far-red emitting phosphors which can be suitable for indoor plant growth lighting sources.     

Effect of Al/Sr ratio on the luminescence properties of SrAl2O4:Eu2+, Dy3+ phosphors

Recent studies have brought out many phosphors like Eu²⁺, Dy³⁺-doped alkaline earth aluminates. The trivalent Dy³⁺ ions as co-dopants greatly enhance the duration and intensity of persistent luminescence. These phosphors show excellent properties, such as high quantum efficiency, long persistence of phosphorescence, good stability and suitable color emission.In this work the effect of Al/Sr ratio on the afterglow and phosphorescence decay properties of Eu²⁺ and Dy³⁺ co-activated strontium aluminates synthesized by a solid-state process has been investigated. The luminescence properties of samples were investigated by means of excitation spectra, emission spectra and X-ray diffraction analysis.A variety of strontium aluminates, such as SrAl₂O₄, Sr₄Al₂O₇, Sr₃Al₂O₆, Sr₃Al₂(Eu, Dy, Y)O_7.5, Al₅(Eu, Dy, Y)O₁₂, Sr₄Al₁₄O₂₅, SrAl₁₂O₁₉ and (Eu, Dy, Y)AlO₃ have been identified in the samples prepared from starting precursors with Al/Sr mole ratios ranging from 0.44 to 5. The afterglow decay rate was found to be the fastest for sample with Al/Sr ratio of 4.18, in which SrAl₄O₇ phase was dominant. The afterglow decay rate for phosphor with Al/Sr ratio of 2, in which SrAl₂O₄ phase was dominant, was detected to be slow. Moreover, the emission spectra of the samples shift to yellow-green long wavelength from bluish-green-ultraviolet short wave with the increase of Al/Sr ratios resulting from the change in the composition.     

Eu2+‐Doped Yellow‐Emitting CsBaB3O6 Glass‐Ceramic Prepared by Melt Quenching and Re‐Crystallization Method

Eu³⁺‐doped cesium barium borate glass with the composition of Cs₂O·2BaO·3B₂O₃ was prepared by the conventional melt quenching method. The glass‐ceramic sample was obtained from the re‐crystallization of the as‐made glass to change the amorphous glass into a crystalline host. This reduces the Eu³⁺ in glass to Eu²⁺ ions resulting in a yellow‐emitting phosphor of Eu²⁺‐activated CsBaB₃O₆. The samples were investigated by the XRD patterns and SEM micrograph, the optical absorption, the photoluminescence spectra, and decay curves. The as‐made glass has only Eu³⁺ centers. Under the excitation of blue or near‐UV light, Eu²⁺‐doped CsBaB₃O₆ presents yellow‐emitting color from the allowed inter‐configurational 4f–5d transition in the Eu²⁺ ions. The maximum absolute luminescence quantum efficiencies of Eu²⁺‐doped CsBaB₃O₆ phosphor was measured to be 47% excited at 430 nm light at 300 K. By taking into account the efficient excitation in blue wavelength region, this new phosphor could be a potential yellow‐emitting phosphor for an application in white light‐emitting diodes fabricated with blue chips.     

Optimized photoluminescence of red phosphor Na2SnF6:Mn4+ as red phosphor in the application in “warm” white LEDs

The Mn⁴⁺ activated fluostannate Na₂SnF₆ red phosphor was synthesized from starting materials metallic tin shots, NaF, and K₂MnF₆ in HF solution at room temperature by a two‐step method. The formation mechanism responsible for preparing Na₂SnF₆:Mn⁴⁺ (NSF:Mn) has been investigated. The influences of synthetic parameters: such as concentrations of HF and K₂MnF₆ in reaction system, reaction time, and temperature on crystallinity, microstructure, and luminescence intensity of NSF:Mn have been investigated based on detailed experimental results. The actual doping concentration of Mn⁴⁺ in the NSF:Mn host lattice is less than 0.12 mol%. The most of K₂MnF₆ is decomposed in HF solution especially in hydrothermal system at elevated temperatures. The color of the as‐prepared NSF:Mn samples changes from orange to white when the temperature is higher than 120°C, which indicates the lower concentration of luminescence centers in the crystals. A series of “warm” white light‐emitting diodes with color rendering index (CRI) higher than 88 and correlated color temperatures between 3146 and 5172 K were obtained by encapsulating the as‐prepared red phosphors NSF:Mn with yellow one Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) on 450 nm blue InGaN chips. The advantage of the synthetic strategy to obtain NSF:Mn can be extended to developing Mn⁴⁺‐doped red phosphors from low‐costing metals at room temperature for large‐scale industrial applications.     

A Single‐Phased Sr1−y‐3x/2Al2Si2O8: xCe3+, yMn2+ with Efficient Energy Transfer as a Potential Phosphor for White‐Light‐Emitting Diodes

A series of Ce³⁺‐ and Mn²⁺‐(co‐)activated SrAl₂Si₂O₈ phosphors have been prepared at 1350°C under a reducing atmosphere and their photoluminescence properties have been studied as a function of the (co‐)dopant ions concentrations. We have discovered that energy transfer (ET) not only from Ce³⁺ to Mn²⁺ but also from “defects” to Mn²⁺ by the facts that there is existing significant overlap between the emission spectrum of Ce³⁺ (“defects”) and the excitation spectrum of Mn²⁺. The source of the “defects” in the host lattice is originated from the different charge substitution between Ce³⁺ and Sr²⁺. By adopting the principle of ET, the material SrAl₂Si₂O₈: Ce, Mn can act as a phosphor for white‐light ultraviolet light‐emitting diodes (UV‐LEDs) by tuning of the dopants contents.     

Accessing deep-red emission using chemical units cosubstituted LaTiSbO6:Mn4+ phosphor

Red light-emitting phosphors are important to the field of plant lighting. Therefore, it is necessary to further optimize phosphor materials. Herein, we synthesized a sequence of phosphor LaTiSbO₆:Mn⁴⁺ (LTS:Mn⁴⁺). Due to the ²E to ⁴A₂ transition, LTS:Mn⁴⁺ phosphors can emit red light in the range of 620–780 nm, with an emission peak at 687 nm. Chemical unit cosubstitution (substituting W⁶⁺ - Al³⁺/W⁶⁺ - Ga³⁺ for Ti⁴⁺ - Sb⁵⁺) was used as a method to enhance the luminescence properties of LTS:Mn⁴⁺. When the substitution ratio of W⁶⁺ - Ga³⁺ and W⁶⁺ - Al³⁺ reached 0.1% and 0.75%, respectively, the luminescence intensity increased to 204% and 182%. Using the LTS:Mn⁴⁺, W⁶⁺, Ga³⁺ phosphor and a 470 nm blue chip to fabricate a pc-LED device, the electroluminescence (EL) spectrum is well matched with the phytochrome absorption range. Therefore, the LTS:Mn⁴⁺ phosphor will be very promising for plant growth.     

Photoluminescence of Sr2P2O7:Sm3+ Phosphor as Reddish Orange Emission for White Light‐Emitting Diodes

A reddish orange emission Sr₂P₂O₇:Sm³⁺ phosphor is prepared by the solid‐state reaction method in air, and the crystal structure and luminescence properties of phosphors are investigated. Sr₂P₂O₇:Sm³⁺ phosphor shows Commission International de I'Eclairage (CIE) chromaticity coordinates (x = 0.5753, y = 0.4147). White light‐emitting diodes (W‐LEDs) fabricated using Sr₂P₂O₇:Sm³⁺ phosphor etc. show CIE chromaticity coordinates (x = 0.3471, y = 0.3124). These results indicate that Sr₂P₂O₇:Sm³⁺ phosphor could be a potential suitable reddish orange emitting phosphor candidate for W‐LEDs with excitation of a ~400 nm n‐UV LED chip.     

Formation mechanism and optimized luminescence of Mn4+‐doped unequal dual‐alkaline hexafluorosilicate Li0.5Na1.5SiF6

A novel red phosphor Li_0.5Na_1.5SiF₆:Mn⁴⁺ (LNSF:Mn) based on the unequal dual‐alkaline hexafluorosilicate with superior optical performances has been synthesized via ion‐exchange between [MnF₆]^2? and [SiF₆]^2? at room temperature. The composition and the crystal structure of the as‐obtained phosphor LNSF:Mn were determined by energy‐dispersive x‐ray spectroscopy (EDS) and x‐ray diffraction (XRD), respectively. The formation mechanism of the red phosphor LNSF:Mn has been discussed in detail. The phosphor LNSF:Mn exhibits good chromaticity properties and a quantum yield (QY) of 96.1%, which are better than the identified fluorosilicate phosphors Na₂SiF₆:Mn⁴⁺ (NSF:Mn) and K₂SiF₆:Mn⁴⁺ (KSF:Mn). A broad and intense absorption in the blue and a bright emission in red‐shifted wavelengths make the phosphor LNSF:Mn a desired candidate for applications in warm white light‐emitting diodes.     

Enhanced luminescence and energy transfer performance of double perovskite structure Gd2MgTiO6:Bi3+, Mn4+ phosphor for indoor plant growth LED lighting

Deep-red light emitting phosphors are widely used in LEDs for indoor plant growth because of the critical role played by red light in plant growth. The luminescence properties of deep-red phosphors are still not well understood at present. An energy transfer strategy is a common and effective method to improve luminescence properties. In principle, the energy transfer process may occur when the sensitizer's emission spectra overlap with the activator's excitation spectra. In this work, Bi³⁺ and Mn⁴⁺ were incorporated into the matrix of Gd₂MgTiO₆ as sensitisers and activators, respectively. Mn⁴⁺ ions tend to occupy the [TiO₆] octahedral site and the Bi³⁺ ions are expected to substituted in the site of Gd³⁺. The energy transfer process from Bi³⁺ to Mn⁴⁺ was realised and the photoluminescence (PL) intensity of Mn⁴⁺ increased with the doping content of Bi³⁺. Upon excitation at 375 nm, the PL intensity of Mn⁴⁺ increased to 116.4% when the doping concentration of Bi³⁺ reached 0.3%. Finally, the pc-LED devices were prepared by a Gd₂MgTiO₆:Bi³⁺, Mn⁴⁺ phosphor. The high red luminescence indicated that this phosphor has potential applications in indoor LED lighting.     

Light‐induced electrons suppressed by Eu3+ ions doped in Ca11.94−xSrxAl14O33 caged phosphors for LED and FEDs

Control of light‐induced electron generation is of vital importance for the application of caged phosphors. For Eu‐doped Ca_11.94?xSr_xAl₁₄O₃₃ caged phosphors, the suppressed effect of strontium doping on the light‐induced electrons is observed compared to the europium‐free Ca_11.94?xSr_xAl₁₄O₃₃ phosphors. In the presence of europium ions, Sr doping will promote the reduction of Eu³⁺ to Eu²⁺. The Rietveld refinement suggests that unit cell volumes of the Ca_11.94?xSr_xAl₁₄O₃₃:Eu_0.06 samples are expanded when Ca²⁺ ions are replaced by Sr²⁺ ions. The absorption and FTIR transmittance spectra confirm that the competitive reaction of encaged O^2? anions with H₂ is suppressed. For the sample (x=0.48), the higher thermal activation energy (~0.40 eV) for luminescence quenching can be attributed to the more rigid framework structure after Sr doping. For Ca_11.94?xSr_xAl₁₄O₃₃:Eu_0.06 phosphors, their emission colours are tuned from red to purple upon 254 nm excitation and from pink to blue under electron beam excitation through Sr substitution. The insight gained from this work may have a significant guiding to design new phosphors for LED and FEDs and novel nanocaged mutifunctional materials.     

A novel Mn4+‐activated red phosphor for use in light emitting diodes,K3SiF7:Mn4+

Phosphors that exhibit a narrow red emission are particularly interesting due to the advantage of providing a more extensive color gamut and better rendering in LED applications such as displays and solid‐state lighting. Although some Eu²⁺‐activated nitridosilicates have been discovered in this regard, K₂SiF₆:Mn⁴⁺ phosphors are the only option in actual LED applications thus far. We discovered a novel phosphor, K₃SiF₇:Mn⁴⁺, with P4/mbm symmetry. The luminescent properties of K₃SiF₇:Mn⁴⁺ are almost identical to those of the K₂SiF₆:Mn⁴⁺ phosphor, but its materials identity is distinct due to a completely different crystallographic structure, which leads to reduced decay time. The fast decay is one of the most serious disadvantages of existing K₂SiF₆:Mn⁴⁺ phosphors. The K₃SiF₇:Mn⁴⁺ phosphor was examined in comparison to the K₂SiF₆:Mn⁴⁺ via density functional theory calculation, Rietveld refinement, X‐ray photoelectron spectroscopy, X‐ray absorption near‐edge structure spectroscopy, and time‐resolved photoluminescence.     

Effect of H3BO3 flux on the morphology and optical properties of Sr2MgAl22O36:Mn4+ red phosphors for agricultural light conversion films

H₃BO₃ was added during the preparation of Sr₂MgAl₂₂O₃₆:Mn⁴⁺ phosphors by a high-temperature solid-state reaction method. The influence of H₃BO₃ flux on the crystal structure, particle morphology and photoluminescence properties of the Sr₂MgAl₂₂O₃₆:Mn⁴⁺ phosphors was investigated by employing X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence spectroscopy, respectively. The results indicate that adding H₃BO₃ flux can improve the luminescence intensity and morphology, and reduce the synthesis temperature of the Sr₂MgAl₂₂O₃₆ phosphor. The formation temperature of pure-phase Sr₂MgAl₂₂O₃₆ was significantly decreased when H₃BO₃ flux as introduced. The excited state lifetime of the Sr₂MgAl₂₂O₃₆:1.2 mol% Mn⁴⁺ phosphor by the addition of 2.0 wt% H₃BO₃ was ~1.02 ms. We demonstrated the potential of these phosphors to enhance sunlight harvesting by agricultural light conversion film testing. We propose that films containing the Sr₂MgAl₂₂O₃₆:1.2 mol% Mn⁴⁺ phosphor can be applied to increase the production of agricultural plants.     

Unusual concentration induced antithermal quenching of the Eu2+ emission at 490 nm in Sr4Al14O25:Eu2+ for near ultraviolet excited white LEDs

Thermal quenching of phosphor is an important challenge for its practical application in phosphor-converted white light-emitting diodes (pc-WLEDs) and it usually becomes aggravated with the increase of activator concentration. Conversely, this work finds the thermal quenching of Eu²⁺ emission at 490 nm in Sr₄Al₁₄O₂₅:Eu²⁺ does not follow this in the temperature range of 300 to 480 K, and the rate of it is even slowed down as the concentration of Eu²⁺ increases. However, at the same time, the experiment on three heating-cooling cycles of Sr₄Al₁₄O₂₅:Eu²⁺ reveals that the thermal degradation of Eu²⁺ emission becomes improved. Once Eu²⁺ ions are doped into Sr₄Al₁₄O₂₅, they will prefer substituting for the 10- and 7-coordinated strontium sites Sr1 and Sr2, respectively. The emission centers Eu1 and Eu2, therefore, appear. The abnormal phenomenon is perhaps partly due to the enhanced energy transfer from the emission center Eu1 at 407 nm to the one Eu2 at 490 nm. It is also found interesting that the introduction of AlN can enhance the emission of Sr₄Al₁₄O₂₅:Eu²⁺ without leading to the deterioration of thermal degradation. In the end, a prototype of pc-WLED was fabricated with Sr₄Al₁₄O₂₅:Eu²⁺ to demonstrate the application of white lighting. This work is not only beneficial to the understanding of the relationship between concentration and thermal quenching, but also conducive to the design of the heavily doped phosphor for WLEDs with better resistance to thermal quenching.