HF-free molten salt route for synthesis of highly efficient and water-resistant K2SiF6:Mn4+ for warm white LED

It has been one of the hot issues to prepare the red-emitting Mn⁴⁺-doped fluoride phosphors with highly efficient and waterproofness for warm white-light-emitting diodes (WLEDs) by the green and environmentally friendly method. Herein, we design a novel green molten salt route to synthesize K₂SiF₆:Mn⁴⁺ red powder using molten NH₄HF₂ salt instead of HF liquor as the reaction medium. The results show that KMnO₄ and MnF₂ could produce Mn⁴⁺ in NH₄HF₂ molten salt through a reduction reaction, and the resulting Mn⁴⁺-doped K₂SiF₆ exhibited a bright red emission peaked at 632 nm under blue light excitation. The luminescence intensity of the as-obtained product after immersing into water for 24 hours maintain nearly 100% of that before soaking and emission peak shape remains unchanged. The thermal stability of the sample was evaluated by temperature-dependent luminescence spectral intensity during heating and cooling. Furthermore, a warm white-light-emitting diodes (WLEDs) with an excellent color rendering index (Ra = 87.1), lower correlated color temperature (CCT = 3536K), and high luminous efficacy (116.99 lm·W⁻¹) was fabricated based on blue chip and K₂SiF₆:Mn⁴⁺ and commercial yellow phosphor (Y₃Al₅O₁₂:Ce³⁺).     

Deep blue,cyan, orange-red,and white multicolor emissions generated by Bi3+/Eu3+ activated KBaYSi2O7 luminescent materials for white light-emitting diodes

A variety of Bi³⁺ and/or Eu³⁺ doped KBaYSi₂O₇ phosphors with deep blue, cyan, orange-red, and white light multicolor emissions have been fabricated by a Pechini sol-gel (PSG) method. The KBaYSi₂O₇:Bi³⁺ phosphors exhibit an intense cyan emission or a unique narrow deep blue emission when excited by different wavelengths, which may bridge the cyan gap or act as a promising deep blue phosphor for white light-emitting diodes (WLEDs). The tunable multicolor emissions can be achieved by changing the Bi³⁺ doping concentrations. The Bi³⁺/Eu³⁺ co-doped KBaYSi₂O₇ phosphors display intrinsic emissions of Bi³⁺ and Eu³⁺ and an energy transfer process between Bi³⁺ and Eu³⁺ can be detected. The luminescence colors of KBaYSi₂O₇:Bi³⁺,Eu³⁺ regularly shift from blue, through cold and warm white, finally toward orange-red by adjusting the relative doping concentrations of Bi³⁺ and Eu³⁺. The single-phase white light-emitting material can be generated in both cold and warm white regions by simply varying the Eu³⁺ doping concentrations. Furthermore, three kinds of WLEDs devices are fabricated by KBaYSi₂O₇:Bi³⁺ or KBaYSi₂O₇:Bi³⁺,Eu³⁺ phosphors, which can exhibit dazzling white light emissions with eminent CIE coordinates, correlated color temperature, and color rendering index. The result offers direct evidence that the as-synthesized phosphors may be potentially applied in WLEDs and solid-state lighting.     

Red-shift and improved afterglow of Sr3Al2-xBxO5Cl2:Eu2+, Dy3+ phosphors via a cation substitution strategy

Sr₃Al_2-xB_xO₅Cl₂:Eu²⁺, Dy³⁺ (x = 0, 0.2, 0.4) long persistent phosphors were prepared via solid-state process. The pristine Sr₃Al₂O₅Cl₂:Eu²⁺, Dy³⁺ phosphor exhibits orange/red broad band emission around 609 nm, which can be attributed to the electric radiation transitions 4f⁶5 d¹→4f⁷ of Eu²⁺. Upon the same excitation, the B³⁺-doped Sr₃Al_2-xB_xO₅Cl₂:Eu²⁺, Dy³⁺ phosphors display red-shift from 609 nm to 625 nm with increasing B³⁺ concentrations. The XRD patterns show that Al³⁺ can be replaced by B³⁺ in the host lattice at the tetrahedral site, which causes lattice contraction and crystal field enhancement, and thereafter achieves the red-shift on the emission spectrum. The XPS investigation provides direct evidence of the dominant 2-valent europium in the phosphor, which can be ascribed for the broad band emission of the prepared phosphors. The afterglow of all phosphors show standard double exponential decay behavior, and the afterglow of Sr₃Al₂O₅Cl₂:Eu²⁺, Dy³⁺is rather weak, while the sample co-doped with B³⁺shows longer and stronger afterglow, as confirmed after the curve simulation. The analysis of thermally stimulated luminescence showed that, when B³⁺ is introduced, a much deeper trap is created, and the density of the electron trap is also significantly increased. As a result, B³⁺ ions caused redshift and enhanced afterglow for the Sr₃Al_2-xB_xO₅Cl₂:Eu²⁺, Dy³⁺ phosphor.     

Novel cyan-emitting KBaScSi2O7:Eu2+ phosphors with ultrahigh quantum efficiency and excellent thermal stability for WLEDs

Owing to the conventional phosphor-converted white LEDs (pc-WLEDs) generally suffer from blue-green cavity, thus, developing an appropriate phosphors covering both the blue and green regions in their emission spectra are very urgent. Herein, a novel Sc silicate phosphor, KBaScSi₂O₇:Eu²⁺ (KBSS:Eu²⁺), has been successfully designed and prepared via a solid-state reaction. The crystal structure, luminescent properties, thermal quenching, quantum efficiency as well as its application in UV-pumped WLEDs have been investigated systematically. The KBSS:Eu²⁺ phosphor exhibits a strong and broad excitation band ranging from 290 to 450 nm, and gives a sufficient cyan emission of 488 nm with a full-width half-maximum (FWHM) of 70 nm, which filled the blue-green cavity. Importantly, the optimized KBSS:Eu²⁺ phosphor possesses an ultrahigh quantum efficiency (QE) up to 91.3% and an excellent thermal stability retaining 90% at 423 K with respect to that measured at room temperature. Finally, the as-fabricated UV-based WLEDs device, with only coupled the mixture of KBSS:Eu²⁺ cyan phosphor and CaAlSiN₃:Eu²⁺ red ones to a commercial 365nm UV chip, exhibits a satisfactory color-rendering index (Ra = 88.6), correlated color temperature (CCT = 3770K), and luminous efficiency (LE = 21 lm/W).     

Tunable Color of Eu2+/Mn2+ Coactivated Na5Ca2Al(PO4)4 via Energy Transfer

Eu²⁺ and Eu²⁺/Mn²⁺‐activated Na₅Ca₂Al(PO₄)₄ phosphors have been synthesized by the combustion method. X‐ray powder diffraction profiles, luminescence spectra, chromaticity variation, and energy transfer of Na₅Ca₂Al(PO₄)₄:Eu²⁺, Mn²⁺ were investigated as a function of the Eu²⁺ and Mn²⁺ concentrations in Na₅Ca₂Al(PO₄)₄. The Na₅Ca₂Al(PO₄)₄:Eu²⁺,Mn²⁺ phosphors can be effectively excited at wavelength ranging from 300 to 430 nm, which matches well with that for near‐ultraviolet (UV) light‐emitting diode (LED) chips. Under excitation at 354 nm, Na₅Ca₂Al(PO₄)₄:Eu²⁺,Mn²⁺ not only exhibits blue‐green emission band attributed to 4f⁶5d¹→4f⁷ of Eu²⁺ but also gives an orange emission band attributed to ⁴T₁→⁶A₁ of Mn²⁺. The emission color of the phosphor can be systematically tuned from blue‐green through white and eventually to orange by adjusting the relative content of Eu²⁺ and Mn²⁺ through the principle of energy transfer. The results indicated that Na₅Ca₂Al(PO₄)₄:Eu²⁺, Mn²⁺ may serve as a potential color‐tunable phosphor for near UV white‐light LED.     

A novel narrow band blue-emitting phosphor Rb2ZrSi3O9: Eu2+ with low thermal quenching and high quantum efficiency

Developing phosphors with narrow band emission and excellent performance is the main goal of current leading research. In this study, a novel narrow band blue-emitting phosphor Rb₂ZrSi₃O₉: Eu²⁺ was prepared that can be fully excited by near ultraviolet (NUV) light, emits a bright blue light peak at 470 nm, and has a full width at half maximum (FWHM) of 60 nm. The synthesized Rb₂ZrSi₃O₉: Eu²⁺ had excellent photoluminescence properties that were reflected in its good thermal stability (up to 82%) and high internal quantum efficiency (up to 75%). These remarkable luminescence properties were mainly ascribed to the highly symmetric and dense crystal structure composed of [Si₃O₉]^6- ring pairs. Multiple emission centers in the Rb₂ZrSi₃O₉: Eu²⁺ phosphor were confirmed through the photoluminescence (PL) spectra and time-resolved (PL) spectra. A bright warm WLED device with a low correlated color temperature (3386 K) and high color-rendering index (CRI~89.3) was produced by combining the blue phosphor with (Sr, Ba)₂SiO₄: Eu²⁺ and CaAlSiN₃: Eu²⁺ and NUV (380 nm). The results indicate that Rb₂ZrSi₃O₉: Eu²⁺ could be a promising phosphor for use in WLEDs.     

Design of a superb Eu2+–Mn2+ co-doped narrow-band green phosphor via nearly 100% energy transfer efficiency

The excellent narrow-band emitters, especially the green ones, are regarded as a pivotal research direction for light-emitting diodes (LED) backlights in liquid-crystal displays (LCDs). A nearly single-peak green emission centered at 513 nm with a full width at half maximum of 28 nm is reached in KAl₁₁O₁₇:0.1Eu²⁺, 0.15Mn²⁺ phosphor via nearly 100% energy transfer (ET) efficiency, and the extended X-ray absorption fine structure analysis elucidates its mechanism, which is that Eu²⁺ and Mn²⁺ are constrained to form Eu²⁺–Mn²⁺ pairs with a small distance 3.7 Å caused by the local environment relaxation inducement. Meanwhile, by creating an unhindered energy flow between Eu²⁺, Mn²⁺ and K⁺/O²⁻ defect levels through ET and multilevel electron trapped and recombination process, the KAO:Eu²⁺, Mn²⁺ phosphors perform superb photoluminescence property with a high color purity of 83%, an excellent thermal stability (94%@200°C), and unexceptionable internal and external quantum efficiencies of 91.7% and 66.4%, which all are superior to characteristics of commercial β-SiAlON:Eu²⁺ phosphor. Moreover, the white LED fabricated using KAO:Eu²⁺, Mn²⁺ to provide green component shows a wide color gamut of 105% National Television System Committee. These results indicate a potential for an application of our material in LCD–LED backlights, and the design of such local relaxation-induced structure provides a significative reference to develop the new narrow-band emitters.     

Tunable luminescence properties and efficient energy transfer in Eu,Mn co-doped Ba2MgP4O13

A series of Ba₂Mg_1−xMn_xP₄O₁₃ (x = 0-1.0) and Ba_1.94Eu_0.06Mg_1−xMn_xP₄O₁₃ (x = 0-0.15) phosphors were prepared by conventional solid-state reaction. X-ray powder diffraction (XRD), the photoluminescence spectra, and the decay curves are investigated. XRD analysis shows that the maximum tolerable substitution of Mn²⁺ for Mg is about 50 mol% in Ba₂MgP₄O₁₃. Mn²⁺-singly doped Ba₂MgP₄O₁₃ shows weak red-luminescence peaked at about 615 nm. The Eu²⁺/Mn²⁺ co-doped phosphor emits two distinctive luminescence bands: a blue one centered at 430 nm originating from Eu²⁺ and a broad red-emitting one peaked at 615 nm from Mn²⁺ ions. The luminescence of Mn²⁺ ions can be greatly enhanced with the co-doping of Eu²⁺ in Ba₂MgP₄O₁₃. The efficient energy transfer from Eu²⁺ to Mn²⁺ is verified by the excitation and emission spectra together with the luminescence decay curves. The emission colors could be tuned from the blue to the red-purple and eventually to the deep red. The resonance-type energy transfer via a dipole-quadrupole interaction mechanism is supported by the decay lifetime data. The energy transfer efficiency and the critical distance are calculated and discussed. The temperature dependent luminescence spectra of the Eu²⁺/Mn²⁺ co-doped phosphor show a good thermal stability on quenching effect.     

Sr-induced color-tunable and thermal stability enhancing in the phosphor (Ba1-xSrx)9Lu2Si6O24:Eu2+ for solid-state lighting

Rare earth ions’ site occupation is significant for studying luminescence properties by changing the host composition. The (Ba_1-xSr_x)₉Lu₂Si₆O₂₄:Eu²⁺ (x = 0-0.4) tunable-color phosphors were synthesized via a high temperature solid-state reaction. With the Sr²⁺ ions concentration increase, the luminescent color could be tuned from blue to green. This phenomenon is discussed in detail through the ions occupation in the host lattice. More importantly, the temperature-dependent luminescence of (Ba_1-xSr_x)₉Lu₂Si₆O₂₄:Eu²⁺ phosphors was investigated and exhibited excellent thermal stability. Furthermore, white LED device has been fabricated using (Ba_1-xSr_x)₉Lu₂Si₆O₂₄:Eu²⁺ phosphor mixed with commercial red phosphor Sr₂Si₅N₈:Eu³⁺ combined with a 370 nm UV-chip. This device showed correlated color temperature (CCT) of 5125 K and high color render index (CRI) of 91. This phosphor will be a promising candidate as a tunable-color phosphor for UV-based white LEDs.     

Eu2+, Dy3+: Sr2B5O9Cl,a new blue-emitting phosphor with long persistence

A new Eu²⁺, Dy³⁺: Sr₂B₅O₉Cl phosphor with long persistence was synthesized in a reducing atmosphere by a solid-state reaction process. The pure-phase phosphor was obtained by calcination at 900 °C. The introduction of Eu²⁺ into the lattice of the matrix resulted in a broad blue emission centered at 423 nm, which was due to the characteristic 4f6⁵d¹ to 4f⁷ energy transfer of Eu²⁺ ions. Both Eu-doped and Dy/Eu-codoped phosphors displayed afterglow behaviors due to the electron traps generated by the incorporation of tri-valanced rare earth cations into the original Sr lattice sites. The afterglow of Eu²⁺: Sr₂B₅O₉Cl and Eu²⁺, Dy³⁺: Sr₂B₅O₉Cl phosphors showed standard double exponential decay behaviors, and the Eu²⁺/Dy³⁺ co-doped sample demonstrated better afterglow properties than Eu²⁺-doped one. A longer lifetime for the electrons was confirmed after the afterglow decay curve simulation. Based on the analysis of thermally stimulated luminescence (TSL), the difference in afterglow was attributed to the different trap concentrations induced by the Dy³⁺ (Eu³⁺) doping in the Sr₂B₅O₉Cl matrix.     

Achieving highly efficient far-red emission from luminescence-ignorable Ca2MgTeO6:Mn4+ to Ca2-zLnzMgTe1-yWyO6:Mn4+ (Ln = La,Y, Gd) phosphors via solid solution and impurity doping strategies

The Mn⁴⁺-doped Ca2MgTeO6 (CMTO) far-red emitting phosphors with double perovskite-type structure were successfully synthesized. Upon near-ultraviolet (n-UV, 300 nm) light excitation, the as-prepared phosphors showed far-red light at 700 nm attributed to the ²E_g→⁴A_2g transition of Mn⁴⁺ ion. The doping concentration of the CMTO:xMn⁴⁺ samples was optimized to be 0.8 mol%. The relevant mechanism of concentration quenching was demonstrated as the dipole-dipole interaction. Furthermore, solid solution and impurity doping strategies were adopted to improve the far-red emission of the luminescence-ignorable CMTO:Mn⁴⁺ phosphor. Series of Ca₂MgTe_(1−y)W_yO₆:0.8 mol%Mn⁴⁺ (y = 0–100 mol%) solid solution and Ca_2−zLn_zMgTe_0.6W_0.4O₆:Mn⁴⁺ (Ln = La, Y, and Gd, z = 10 mol%) phosphors were synthesized through the above two strategies. The luminescence intensity of the optimal Ca_1.9Gd_0.1MgTe_0.6W_0.4O₆:Mn⁴⁺ phosphor was 13.7 times that of the CMTO:Mn⁴⁺ phosphor and 2.51 times that of red commercial phosphor K₂SiF₆:Mn⁴⁺. Notably, both CMTO:Mn⁴⁺ and Ca_1.9Gd_0.1MgTe_0.6W_0.4O₆:Mn⁴⁺ phosphors exhibited remarkable thermal stability compared with most Mn⁴⁺-doped phosphors. Finally, the highly efficient Ca_1.9Gd_0.1MgTe_0.6W_0.4O₆:Mn⁴⁺ phosphor was successfully applied in fabricating the warm white light diode (w-LED). This working along both lines strategy exhibited great potential for luminescence optimization of Mn⁴⁺-doped oxide phosphors.     

Preparation and optical properties of a novel double-perovskite phosphor,Ba2GdNbO6:Mn4+, for light-emitting diodes

Red phosphors serve an important function as red components of warm white light-emitting diodes (WLEDs). Given their remarkable luminescent properties and low cost, Mn⁴⁺-doped phosphors are attracting significant attention. In this study, the novel red phosphor Ba₂GdNbO₆:Mn⁴⁺ was synthesized through high-temperature solid-state reaction. The host Ba₂GdNbO₆ with a double-perovskite structure was investigated. Scanning electron microscopy and thermogravimetric analysis were performed to evaluate the structure and thermal stability of the phosphor, respectively. PLE and photoluminescence spectra were further used to study the luminescence properties of the phosphor. Moreover, crystal field strength and Racah parameters were calculated to estimate the nephelauxetic effect of Mn⁴⁺ on the Ba₂GdNbO₆ host lattice. Thermal quenching characteristics were also analyzed. The fabricated red-emitting LED revealed its potential application in WLEDs.     

Synthesis,Crystal Structure,and Luminescence Properties of Y4Si2O7N2: Eu2+ Oxynitride Phosphors

Y₄Si₂O₇N₂: Eu²⁺ phosphor has been prepared by a pretreatment method. Reduction in Eu³⁺ ions into Eu²⁺ by the use of hydrogen iodide (HI) is verified by X‐ray absorption near‐edge structure (XANES) and electrode potential analysis. Y₄Si₂O₇N₂: Eu²⁺ phosphor has a broad emission band in the range of 400–500 nm. Furthermore, the effect of Zr doping on the structure and luminescence properties of Y₄Si₂O₇N₂: Eu²⁺ phosphor is researched. It found that the Zr doping leads to an emission blueshift, and improves the luminescence intensity and thermal quenching behavior of Y₄Si₂O₇N₂: Eu²⁺ phosphors. Prospectively, the pretreatment approach could be extended to develop other Eu²⁺‐doped compounds.     

Luminescence properties of Eu2+ and Mn2+ doped Sr1.7Mg0.3SiO4 phosphors

Eu²⁺, Mn²⁺ doped Sr_1.7Mg_0.3SiO₄ phosphors were prepared by high temperature solid-state reaction method. Their luminescence properties were studied. The emission spectra of Eu²⁺ singly doped Sr_1.7Mg_0.3SiO₄ consist of a blue band (455 nm) and a green band (550 nm). The relative intensities of two emissions varied with Eu²⁺ concentration. Eu²⁺ and Mn²⁺ co-doped Sr_1.7Mg_0.3SiO₄ phosphors emit three color lights and present whitish color. The blue (455 nm) and green (550 nm) emissions are attributed to the transitions of Eu²⁺, while the red (670 nm) emission is originated from the transition of Mn²⁺ ion. The results indicate the energy transfer from Eu²⁺ to Mn²⁺. The mechanism of the energy transfer is resonance-type energy transfer due to the spectral overlap between the emission of Eu²⁺and the absorption of Mn²⁺.     

Photoluminescence Properties of Heavily Eu3+‐Doped BaCa2In6O12 Phosphor for White‐Light‐Emitting Diodes

Heavily Eu³⁺‐doped BaCa₂In₆O₁₂ phosphors were prepared by conventional solid‐state reaction, and its structural properties were investigated by means of Rietveld refinement method using an X‐ray source. XRD patterns confirm the hexagonal phase of BaCa₂In₆O₁₂: Eu³⁺ phosphors. The obtained spectrum data indicate that the emission spectra of Ba_1?xEu_xCa₂In₆O₁₂ samples excited at 393 nm exhibit a series of shaped peaks assigned to the ⁵D_0,1,2,3 →⁷F_J (J = 0,1,2,3,4) transitions. Luminescence from the higher excited states, such as ⁵D₁, ⁵D₂, and ⁵D₃, were also observed even though the Eu³⁺ concentration was up to x = 0.4. More importantly, the Ba_1?xEu_xCa₂In₆O₁₂ phosphor still emits white luminescence, when the Eu³⁺ ion concentration is up to x = 0.07 before concentration quenching is observed, which shows that the phosphor is a promising single‐phase phosphor for near ultraviolet (NUV) light‐emitting diodes (LED). Furthermore, the temperature's impact on white luminescent properties was studied. Finally, a white‐light‐emitting diodes (W‐LEDs) fabricated with the Ba_0.95Eu_0.05Ca₂In₆O₁₂ phosphor incorporated with an encapsulant in ultraviolet LEDs (λ_max = 395 nm) is discussed.     

Structure and Luminescence Characteristics of Eu3+‐Activated Trigonal Layered Perovskite Ba2La2ZnW2O12

Eu³⁺‐doped tungstate Ba₂La₂ZnW₂O₁₂ phosphors with perovskite‐structure were prepared by the high temperature solid‐state reaction. The X‐ray powder diffraction (XRD) patterns and structure refinements indicate that the phosphors crystalized in the trigonal layer‐perovskite. The luminescence properties of the phosphors were investigated such as photoluminescence (PL) excitation and emission spectra, decay lifetimes, and color coordinates. It was found that the pure host shows self‐activated emission excited by the UV light. Moreover, Ba₂La₂ZnW₂O₁₂ also shows scintillation characteristics under the X‐ray irradiation. The near‐UV and blue light can efficiently excite Eu³⁺‐doped Ba₂La₂ZnW₂O₁₂ phosphors inducing the strong orange–red luminescence. The optimal Eu³⁺ doping concentration in this host is 40 mol%. The luminescence spectra and the luminescence color of the phosphors strongly depend on the doping levels and excitation wavelength. The different luminescence features were discussed on the base of crystal structure. Eu³⁺ ions have two possible substitutions on A or B sites in this trigonal layered perovskite. The phosphor could act as a candidate for the potential application in near‐UV excited white‐LEDs lighting.     

Design of Eu2+-activated broadband orange-emitting phosphors with excellent luminescent performance for warm WLEDs

We report orange-emitting Sr₈La_0.5Na_0.5Mg_1.5(PO₄)₇:Eu²⁺ (SLNMPO-0.5:Eu²⁺) and Sr₇LaNaMg_1.5(PO₄)₇:Eu²⁺ (SLNMPO-1:Eu²⁺) phosphors with broad emission bands covering from 450 to 800 nm. The phosphors can be excited by n-ultraviolet and blue light efficiently. Their crystal structure, diffuse reflection spectra, photoluminescence (PL) spectra, fluorescence decay curves and thermal stability were investigated systematically. Under the excitation of 365 and 400 nm, SLNMPO-0.5:Eu²⁺ and SLNMPO-1:Eu²⁺ both exhibit better PL properties and contain more red emissions than SMPO:Eu²⁺. CIE coordinates of SLNMPO-0.5:Eu²⁺ and SLNMPO-1:Eu²⁺ under 365 nm excitation are (0.460, 0.497) and (0.457, 0.494), respectively. Furthermore, high-quality warm white light can be generated by fabricating warm white light-emitting diode (WLED) devices with 370 nm LED chips, BaMgAl₁₀O₁₇:Eu²⁺ commercial blue phosphor and orange-emitting SLNMPO-0.5:Eu²⁺ (or SLNMPO-1:Eu²⁺) phosphor. The correlated color temperature, R_a and color coordinates are 3880 K, 94.05, (0.3895, 0.3922) and 3736 K, 91.73, (0.4005, 0.4078) for the fabricated WLED devices with SLNMPO-0.5:Eu²⁺ and SLNMPO-1:Eu²⁺, respectively. The excellent performances indicate that SLNMPO-0.5:Eu²⁺ and SLNMPO-1:Eu²⁺ have great potential to be attractive candidates in the application of warm WLEDs.     

Electrospinning,optical properties and white LED applications of one-dimensional CaAl12O19:Mn4+ nanofiber phosphors

Non-rare-earth, red-emitting CaAl₁₂O₁₉:Mn⁴⁺ nanofiber phosphors have been successfully prepared by an electrospinning technique followed by an annealing process. The as-prepared precursor fibers have smooth surfaces with an average diameter of 5 µm. After annealing at high temperature, the diameter of the fibers gradually reduces due to the decomposition of the organic polymers. The photoluminescence and crystalline properties of the fibers were investigated as a function of Mn⁴⁺ concentration and the annealing temperature. Under ultraviolet and blue light excitation, CaAl₁₂O₁₉:Mn⁴⁺ exhibits a characteristic red emission at 655 nm with three satellite peaks due to the ²E→⁴A₂ transition of Mn⁴⁺. The highest PL intensity is achieved at a 0.5% Mn⁴⁺ concentration and a firing temperature of 1400 °C. In comparison to CaAl₁₂O₁₉:Mn⁴⁺ prepared by a usual solid-state reaction, the luminescence of the as-prepared nanofiber phosphors in the present work has been strongly enhanced by optimizing the morphology and improving the crystallinity and phase purity. The absorption band in the blue region and a bright emission in the red region make the CaAl₁₂O₁₉:Mn⁴⁺ nanofiber phosphor a candidate for achieving high color rendering in YAG:Ce-based WLEDs. A warm WLED with a high CRI of 88.5 at a CCT of 4553 K has been successfully achieved by coating YAG:Ce with CaAl₁₂O₁₉:Mn⁴⁺ nanofiber phosphors on blue InGaN chips.     

Achieving thermally stable and anti-hydrolytic Sr2Si5N8:Eu2+ phosphor via a nanoscale carbon deposition strategy

Chemical stability of phosphors is critical to the efficiency and lifetime of the white light-emitting diodes. Therefore, many strategies have been adopted to improve the stability of phosphors. However, it is still lack of report on the improvement of thermal stability and hydrolysis resistance of phosphors by a single layer coating. Due to the high transmittance and high chemical inertness of graphene, it was coated on the surface of Sr₂Si₅N₈:Eu²⁺ phosphor by chemical vapor deposition, aiming to improve its thermal stability and hydrolysis resistance. The chemical composition and microstructure of the coating were characterized and analyzed. A nanoscale carbon layer was attached on the surface of Sr₂Si₅N₈:Eu²⁺ phosphor particles in an amorphous state. In coated Sr₂Si₅N₈:Eu²⁺ phosphor, the oxidation degree of Eu²⁺ to Eu³⁺ was significantly suppressed. At the same time, the surface of Sr₂Si₅N₈:Eu²⁺ particle turned from hydrophilic to hydrophobic after carbon coating, and consequently the hydrolysis resistance of Sr₂Si₅N₈:Eu²⁺ phosphor was greatly improved. After tests at 85 °C and 85% humidity for 200 h, the carbon coated Sr₂Si₅N₈:Eu²⁺ phosphor still maintained about 95% of its initial luminous intensity as compared with 35% of the uncoated. By observing the in-situ microstructure evolution of coated phosphor in air-water vapor environment, remained presence of the carbon layer even at 500 °C explained the excellent chemical stability of carbon coated Sr₂Si₅N₈:Eu²⁺ phosphor in complex environment. These results indicate that a nanoscale carbon layer can be used to provide superior thermal stability and hydrolysis resistance of (oxy) nitrides phosphors.     

Combustion synthesis of some compounds in the Li<Subscript>2</Subscript>O-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system

At least four compounds, viz. LiAlO₂, LiAl₅O₈, Li₅AlO₄ and Li₂Al₄O₇, are known in the Li₂O-Al₂O₃ system. These compounds are important for several technological applications. Combustion synthesis of these compounds using urea as a fuel was attempted. LiAlO₂ and LiAl₅O₈ could be successfully prepared by choosing the starting materials in required stoichiometric ratios. Li₂Al₄O₇ was not obtained as a pure phase; γ-LiAlO₂ was formed as an impurity phase. Li₅AlO₄ could not be prepared by combustion process. Some phosphors based on these aluminates could also be prepared. Activation of these aluminates with Fe³⁺, Mn⁴⁺, Cu⁺, etc. was successfully achieved. Excitation and emission spectra for LiAl₅O₈: Fe³⁺, LiAl₅O₈: Mn²⁺, and Li₂Al₄O₇: Cu⁺ are reported.