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.     

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.     

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.     

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.     

Far‐Red‐Emitting BiOCl:Eu3+ Phosphor with Excellent Broadband NUV‐Excitation for White‐Light‐Emitting Diodes

Rare‐earth ion‐doped semiconducting phosphor has attracted extensive attention due to the ability to achieve efficient luminescence through the host sensitization. Here, we present a new type red‐emitting Eu³⁺ ‐doped BiOCl phosphors possessing a broad excitation band in the near‐ultraviolet (NUV) region. Experimental measurements and theoretical calculations confirm that Eu³⁺ ion dopants result in forming impurity energy level near valence band, and the excellent broadband NUV‐exciting ability of Eu³⁺ ion is due to the electronic transitions of BiOCl band gap. Moreover, the highest emission intensity of the phosphors is from the ⁵D₀ → ⁷F₄ transition of Eu³⁺ around 699 nm (far‐red) through whether host excitation or direct Eu³⁺ ions excitation, which lie in the particular structure of BiOCl crystals. Our results indicate that the Eu³⁺ ‐doped BiOCl crystals show great potential as red phosphors for white‐light‐emitting diodes.     

Layered Ceramic Phosphors Based on CaAlSiN3:Eu and YAG:Ce for White Light‐Emitting Diodes

To develop warm‐white light‐emitting diodes via conversion phosphors, blue light‐emitting diodes are generally combined with mixtures of green and red‐emitting phosphor powders. Generally, the phosphors are provided by resin embedded particle dispersions. Such resin‐based solutions cause several drawbacks with respect to LED lifetime and quality. Therefore, it has been investigated whether the red‐emitting nitride phosphor CaAlSiN₃:Eu and the green‐emitting oxidic phosphor YAG:Ce can be cofired to layered ceramic composites. The shrinkage behavior and the composition of the interface in dependence of sintering temperature and the effect of interdiffusion processes at the interface on the luminescence properties were investigated. The formation of secondary phases at the interface in the cofired structures was found to limit the phosphor functionality for the nitride‐based CaAlSiN₃:Eu in such composite ceramics. To counteract this, sacrificial interlayers were introduced to produce multilayered ceramics comprising CaAlSiN₃:Eu and YAG:Ce for LED lighting applications. It is shown for the first time, that it is possible to sinter layered CaAlSiN₃:Eu and YAG:Ce composite ceramics in a pressureless process at moderate sintering temperatures if one uses thin‐film passivated interfaces to reduce luminescence‐disturbing diffusion phenomena. These results demonstrate that diffusion barriers can be suitable means to obtain layered ceramic composites comprising CaAlSiN₃:Eu and YAG:Ce in a pressureless sintering process with good optical properties.     

Improvement of Luminescence Properties of Blue‐to‐Red Emitting NaCaBO3: Ce3+, Mn2+ Phosphor Prepared by Sol–Gel Method

Color‐tunable phosphors NaCaBO₃: Ce³⁺, Mn²⁺ were synthesized by sol–gel (SG) and solid state (SS) method. SEM observation indicated that the microstructure of phosphor (SG) consisted of regular fine grains with an average size of about 5 μm. NaCaBO₃: Ce³⁺, Mn²⁺ showed two emission bands: one at 425 nm for Ce³⁺ and another at 610 nm for Mn²⁺. NaCaBO₃: Ce³⁺, Mn²⁺ (SG) exhibit higher energy‐transfer efficiency (90%) and higher Mn²⁺ quantum efficiency (80%) than SS samples, due to smooth surface, narrow size distribution, and improved homogeneity of sensitizer/activator ions. NaCaBO₃: Ce³⁺, Mn²⁺ exhibits blue‐to‐red tunable color by changing Ce³⁺/Mn²⁺ ratio.     

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.     

Yellow‐Emitting Y3Si6N11: Ce3+ Phosphors for White Light–Emitting Diodes (LEDs)

A novel Y_3?xSi₆N₁₁: xCe³⁺ yellow phosphor was synthesized using the carbothermal reduction and nitridition method at 1550°C for 16 h in this letter. Photoluminescence spectra indicated that the phosphor showed broad excitation spectrum and had strong absorption in range of 350–450 nm. It also gave a broad emission band (Full width at half maximum = 153 nm) centered at 575 nm under 425‐nm excitation. With increasing Ce³⁺ concentration, the strongest emission intensity was obtained at 5 mol% Ce³⁺ doping amount and a systematic redshift was observed as the Ce³⁺ concentration increased. The results indicate that this novel yellow phosphor is a promising candidate for using in blue‐chip‐excited white light–emitting diodes (LEDs).     

Luminescence Properties and Energy‐Transfer Behavior of a Novel and Color‐Tunable LaMgAl11O19:Tm3+, Dy3+ Phosphor for White Light‐Emitting Diodes

Novel LaMgAl₁₁O₁₉:Tm³⁺, Dy³⁺ phosphors were prepared utilizing a high‐temperature solid‐state reaction method. The phase formation, luminescence properties, energy‐transfer mechanism from the Tm³⁺ to the Dy³⁺ ions, the thermal stability, and CIE coordinates were investigated. When excited at 359 nm, the LaMgAl₁₁O₁₉: xTm³⁺ phosphors exhibit strong blue emission bands at 455 nm. After codoping with Dy³⁺ and excitation at 359 nm, the LaMgAl₁₁O₁₉:0.03Tm³⁺, yDy³⁺ phosphors emitted white light consisting of the characteristic emission peaks of Tm³⁺ and Dy³⁺. The Dy³⁺ emission intensity increased with the Dy³⁺ concentration due to the energy transfer from Tm³⁺ to Dy³⁺, and concentration quenching due to the high Dy³⁺ doping concentration (y = 0.1 mol) did not occur. The calculation of the CIE coordinates of the LaMgAl₁₁O₁₉:Tm³⁺, yDy³⁺ phosphors revealed the tunability of the emission color from blue to bluish‐white and to white by changing the excitation wavelength and the doping concentration. An energy transfer from Tm³⁺ to Dy³⁺ by dipole–dipole interaction was confirmed by the decay curve, lifetime, and energy‐transfer efficiency measurements. When excited at 359 nm, the LaMgAl₁₁O₁₉:Tm³⁺, Dy³⁺ phosphor also showed good thermal stability, suggesting that it can be used in white LEDs excited by a GaN‐based ultraviolet LED.     

Synthesis and Luminescence Properties of Blue‐Emitting Phosphor Eu2+‐Doped Zinc Fluoro‐Phosphate Zn2[PO4]F

Eu²⁺‐doped zinc fluoro‐phosphate Zn₂[PO₄]F was synthesized by the conventional high‐temperature solid‐state reaction. The phase formation was confirmed by X‐ray powder diffraction measurements and the structure refinement. The photoluminescence excitation and emission spectra, and the decay curves were measured. The natures of the Eu²⁺ emission in inorganic hosts, e.g., the emission and excitation properties, the chromaticity coordinates, the Stokes shifts, the absolute quantum efficiency, and the luminescence thermal stability were reported. Under the excitation of near‐UV light, Eu²⁺‐doped Zn₂[PO₄]F presents a narrow blue‐emitting band centered at 423 nm. The thermal stability of the blue luminescence was evaluated by the luminescence intensities as a function of temperature. The phosphor shows an excellent thermal stability on temperature quenching effects.     

Highly Stable Red‐Emitting Sr2Si5N8:Eu2+ Phosphor with a Hydrophobic Surface

One of the biggest problems in white light‐emitting diodes (WLEDs) is the moisture‐induced degradation of phosphors. This paper proposes a simple and feasible surface modification method to solve it, whereby a hydrophobic surface layer is developed on the surface of the phosphors. The particular case of orange‐red‐emitting Sr₂Si₅N₈:Eu²⁺ (SSN) phosphor was investigated. The mechanism to develop the hydrophobic layer involves hydrolysis and polymerization of tetraethylorthosilicate (TEOS) and polydimethylsiloxane (PDMS). The experimental results showed that the surface layer of SSN phosphor was successfully modified to a hydrophobic nanolayer (8 nm) of amorphous silicon dioxide that contains CH₃ groups in the surface. This hydrophobic surface layer gives the modified phosphor superior stability in high‐pressure water steam conditions at 150°C.     

Ethyl vinyl acetate copolymer—SrAl2O4:Eu,Dy and Sr4Al14O25:Eu,Dy phosphor‐based composites: Preparation and material properties

New photoactive composite based on ethyl vinyl acetate (EVA) and SrAl₂O₄:Eu,Dy or Sr₄Al₁₄O₂₅:Eu,Dy were prepared by melt mixing or extrusion methodologies. The phosphorescent behavior and material properties of the polymer‐phosphor composites were studied. The morphology of the polymer and the composites were studied by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X‐ray spectroscopy (EDS). The SEM shows that the physical and chemical behavior of the matrix and the extrusion conditions were primarily responsible for the surface morphology. TEM and EDS show that the phosphor particles were uniformly dispersed in the EVA matrix. A broad band of ultraviolet (UV)‐excited phosphorescence of SrAl₂O₄:Eu,Dy and Sr₄Al₁₄O₂₅:Eu,Dy phosphors and the respective composites were observed at wavelengths of 516 nm and 490 nm, respectively. The photoluminescence (PL) spectra showed less intense phosphorescence but no shift in the wavelength of the emission peak for all the composites. The Hamburg wheel test was done on all the composites and the PL measurements before and after the test showed almost no change in the intensity of the emission. Thermal studies showed that the presence of the phosphors in the matrix slightly increased the crystallinity of EVA, which leads to higher melting enthalpies. Tensile testing shows very little change in the tensile strength and flexibility of the ethylene vinyl acetate copolymer. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

White Light Emission from Sr2LiSiO4F:Sm3+,Tb3+ Phosphors Under Near‐UV Light Excitation

The Tb³⁺/Sm³⁺ codoped Sr₂LiSiO₄F white emitting phosphors were synthesized by a solid‐state reaction technique at high temperature. The X‐ray diffraction patterns, photoluminescence properties, and decay behaviors have been investigated. The Tb³⁺ emissions (blue and green) and Sm³⁺ emissions (orange) are both observed in the codoped samples Sr₂LiSiO₄F: 0.05Sm³⁺, xTb³⁺ by near‐UV light (370 nm) exciting. The white emission has been obtained by adjusting Tb³⁺ doping concentration at 5% (x = 0.05). These luminescent powders are expected to be a potential candidate as white emitting phosphor for near‐ultraviolet InGaN‐based white light‐emitting diodes.     

Photoluminescence Characteristics of a New Thiogallate‐Based Green‐Emitting Phosphor: MgGa2S4:Eu2+

A new thiogallate‐based green‐emitting phosphor, MgGa₂S₄:Eu²⁺, was first synthesized via a high‐temperature solid‐state reaction in a CS₂ atmosphere. We then investigated the structures and luminescent properties of the MgGa₂S₄:Eu²⁺ phosphors. The MgGa₂S₄:Eu²⁺ phosphors can be excited efficiently by UV–visible light in the wavelength range from 350 to 520 nm and they emit an intensely green light with emission bands peaking at 538 nm. The optimal concentration for Eu²⁺ in MgGa₂S₄ was found to be about 6 mol%, and the corresponding concentration quenching mechanism was the electric multipole–multipole interaction. The quenching temperature was calculated to be 402 K, and the Huang–Rhys factor was about 4. The energy barrier for thermal quenching was calculated to be 0.28 and 0.27 eV by the two types of the Arrhenius equations. The small variation in the color coordinates of MgGa₂S₄:Eu²⁺ under high temperatures indicates that the as‐synthesized phosphor has good color stability. Due to their broadband absorption in the 350–520 nm wavelength range, these phosphors may be able to fulfill the requirements for application in the development of Ga(In)N‐based white LEDs.     

不同添加剂对Sr4Al14O25: Eu, Dy长余辉光致发光性能的影响

研究了-H3BO3,NH4H2PO4,CaF2几种添加剂对Sr4Al14O25Eu,Dy发光性能的影响.结果发现这些添加剂的加入,有利于Sr4Al14O25相的生成.发射光谱显示,随着H3BO3,NH4H2PO4,CaF2的加入,Sr4Al14O25Eu,Dy磷光材料的发射光谱主峰出现蓝移现象.而且,添加剂的加入,不同程度上改善了Sr4Al14O25Eu,Dy磷光材料长余辉发光性能.尤其是H3BO3的加入,使Sr4Al14O25Eu,Dy材料的初始亮度大大提高,并使发光持续时间延长.初始亮度从1756mcd@m-2增加到5327mcd@m-2.     

Photoluminescence Mechanism and Thermal Stability of Tb3+‐Doped Y4Si2O7N2 Green‐Emitting Phosphors

With solid‐state reaction method, series of Y₄Si₂O₇N₂:Tb³⁺ phosphors were prepared under the high‐temperature and high‐pressure conditions. The photoluminescence properties at room and high temperature were investigated. Two groups of emission lines have been observed, which are corresponding to Tb^{3+ 5}D₃ → ⁷F_J (J = 6, 5, 4, 3, 2) and ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transitions. The physical mechanisms for excitation, emission, concentration quenching, and thermal quenching were investigated. The cross‐relaxation mechanism between the ⁵D₃ and ⁵D₄ emission was investigated and discussed. The Tb–Tb critical distance for cross‐relaxation was calculated to be ~13 Å. The optimum Tb³⁺ concentration in this phosphor is 15 mol%. The quadrupole–quadrupole interaction dominates the non‐radiative energy transfer between the Tb³⁺ luminescence centers and causes the concentration quenching. This phosphor shows high thermal stabilities that at 150°C the intensity remains 92% compared with that measured at room temperature. The present work suggests that this Tb³⁺‐doped Y₄Si₂O₇N₂ material is a kind of potential green‐emitting phosphor.     

Synthesis and Photoluminescence of a Novel Green‐Emitting La5Si3O12N:Eu2+ Phosphor

A novel green‐emitting La₅Si₃O₁₂N:Eu²⁺ phosphor has been synthesized with the solid‐state reaction method under the high temperature, high pressure, and reducing atmosphere. This phosphor shows stable Eu²⁺ photoluminescence spectra. No Eu³⁺ line spectra have been found. For the emission band, an abnormal blue shift has been found. Concentration quenching occurs at Eu²⁺ concentration of 4 mol%. The photoluminescence property at high temperature was investigated. The physical mechanisms for blue shift, concentration quenching and thermal quenching were investigated and discussed. The proposed synthesis process could be a suitable method to acquire the stable Eu²⁺ photoluminescence in the trivalent metal cation compounds.     

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.     

Coprecipitation synthesis of Ca14Al10Zn6O35:Mn4+ deep-red phosphor and silica-modified waterproofing ability

Ca₁₄Al₁₀Zn₆O₃₅:Mn⁴⁺ (CAZ:Mn) phosphor material, which shows deep-red luminescence, was synthesized by the coprecipitation (COP) method using a Na₂CO₃/NaOH solution as the precipitant. COP–CAZ:Mn phosphor exhibited a 2.1 times higher luminescence intensity than the corresponding phosphor prepared using the conventional solid-state reaction (SSR) method. This substantial increase in luminescence was mainly ascribed to the existence of a greater proportion of tetravalent manganese in COP–CAZ:Mn phosphor. Furthermore, COP–CAZ:Mn phosphor was modified with SiO₂ via hydrolysis of tetraethoxysilane (TEOS) to waterproof the compound because it is easily decomposed through hydrolysis under humid conditions. The SiO₂-modified CAZ:Mn phosphor maintained its crystal structure and high photoluminescence intensity after the water-resistance test. Therefore, waterproof CAZ:Mn phosphor with a high luminescence intensity was successfully discovered by utilizing the coprecipitation method and SiO₂ modification.