Synthesis of nitride phosphor using entire oxides raw materials and their photoluminescence properties

Sr₂Si₅N₈:Eu²⁺ nitride phosphors application for light emitting diodes were synthesized for the first time using less‐expensive and air‐stable entire oxides raw materials, SrCO₃, SiO₂, and Eu₂O₃, through a carbothermal reduction and nitridation (CTRN) route. The reaction dynamic processes of the CTRN route and the phase components, photoluminescent properties, quantum efficiency as well as thermal stability for the resultant Sr₂Si₅N₈:Eu²⁺ phosphors were investigated in detail. High‐crystalline pure Sr₂Si₅N₈:Eu²⁺ red‐emitting phosphor with acceptant luminescent performances could be obtained through finely controlling addition amount of carbon reductant agent and carefully removing the residual carbon without oxidation damage. An economic route is outlined in this work for the synthesis of nitride phosphors using cost‐effective entire oxides raw materials, thereby driving the development and practical application of nitrides phosphors.     

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.     

Achieving a thermally stable Eu2+-doped Sr2Si5N8 phosphor via adjusting the lattice defects of oxygen atoms and protection by the graphene-like multilayer

Sr₂Si₅N₈:Eu²⁺ (258) phosphor has attracted much attention due to its excellent red emission properties. However, its poor thermal stability hinders its further development in electronic display devices. Therefore, it is important to solve this problem in order to produce stable and long life-time WLED devices. In this work, a 258 phosphor was synthesized by a simple solid-phase reaction method, which was then coated by an ultrathin carbon layer by chemical vapor deposition. The carbon-coated powders were further annealed in N₂ atmosphere at a high temperature to trigger the carbothermal reaction. On the one hand, the carbothermal reaction resulted in the removal of oxygen in the phosphor particle by a part of the carbon on the surface, which reduced the oxidation of both the luminescent centers and the host lattice. On the other hand, the remaining carbon crystallized, forming a graphene-like multilayer that protected the phosphor particle from the penetration of the external oxygen. This eventually resulted in a significant improvement of the thermal stability and oxidation resistance of the phosphor.     

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.     

Facile synthesis and spectroscopic characterization of siliconitride phosphors for white light‐emitting diodes

A modified chemical vapor deposition (CVD) technique is used to synthesize the color‐tunable siliconitride Sr_2‐1.5x‐yCe_xEu_ySi₅N₈ (x = 0.000‐0.016 and y = 0.000‐0.020) phosphors. In comparison with the conventional solid‐state method, the CVD approach successfully improved the crystallinity, particle size distribution, and photoluminescence through the enhanced gas‐solid reaction. Under blue excitation, Sr_1.98Eu_0.02Si₅N₈ exhibited a red emission band at 618 nm. The incorporation of Ce³⁺ ions increased the emission intensity of Eu²⁺ ions by approximately 10% owing to the enhanced absorption and dipole‐dipole energy transfer process from Ce³⁺ to Eu²⁺ ions. It resulted in a shift of the emission colors from yellow to red region. The external and internal quantum efficiencies of Sr_1.906Ce_0.06Eu_0.004Si₅N₈ were calculated as 54% and 70%, respectively. The activation energy of thermal stability for Sr_1.906Ce_0.06Eu_0.004Si₅N₈ was evaluated as 0.31 eV. A white LED with a color rendering index of 80 and a CCT of 4964 K was successfully fabricated with the present phosphors. The current research demonstrated a new series of Sr₂Si₅N₈:Ce³⁺, Eu²⁺ phosphors with color‐tunability for fabricating white LEDs with high color‐rendering index.     

Insight into the evolution mechanism of carbon film and Eu valence in carbon coated BaMgAl10O17: Eu2+ phosphor annealed in air

Performing carbon coating on the surface of phosphors has been proven to be an effective strategy to enhance the oxidation resistance, which is an important factor to achieve stable luminescent devices. Therefore, a good understanding of the protection mechanism favors a continuous improvement of oxidation resistance of phosphors. In present paper, the evolution of the carbon layer, Eu valence (Eu²⁺/Eu³⁺), and luminescent properties for the C coated BaMgAl₁₀O₁₇: Eu²⁺ phosphor when annealed at high temperature is investigated carefully. Decrease of carbon layer promotes the appearance color transition from black to white as the annealing temperature rises to 1000?°C in air. As expected, the decrease of carbon layer will enhance the luminescence intensity, but risk the possible oxidation of Eu²⁺ to Eu³⁺, which inhibits the blue emission ascribed to Eu²⁺. The results indicate that luminescence intensity of phosphor is dependent on the synergistic effect of carbon thickness and Eu²⁺/Eu³⁺ ratio. Additionally, a reduction reaction of Eu³⁺ to Eu²⁺ is observed in C coated BaMgAl₁₀O₁₇: Eu²⁺ phosphor when annealed at high temperature, which also contributes to the higher luminescence intensity.     

Luminescence properties of Eu activated Ba2Si5N8 red phosphors with various Eu contents

This study was carried out to characterize the crystal structure and luminescence properties of Eu²⁺ doped red-emitting Ba₂Si₅N₈ phosphor. In this research, Ba₂Si₅N₈ phosphors with various Eu compositions were prepared by normal pressure sintering (NPS). Ba₃N₂, Si₃N₄ and Eu₂O₃ were sintered at a high temperature in a mixture of N₂ and H₂. The crystal structure was analyzed by X-ray diffraction(XRD), and the photoluminescence(PL) properties of the Eu²⁺ - activated Ba₂Si₅N₈ phosphors were evaluated as a function of the Eu²⁺ activator concentration. The red-emitting Ba₂Si₅N₈ phosphors showed a broad excitation band range as well as high quantum output.     

Formation of Sr2Si5N8:Eu2+ and Its Transformation to SrSi6N8:Eu2+ Controlled by Temperature and Gas Pressure

Firing temperature and gas pressure effect of synthesizing Sr₂Si₅N₈:Eu²⁺ were investigated. The emission intensity is positively correlated with the firing temperature under 0.1 and 0.5 MPa gas pressure. The Sr₂Si₅N₈:Eu²⁺ with the highest emission intensity was found at 1700°C and 1980°C under 0.1 and 0.5 MPa gas pressure, respectively. Although the maximum emission intensity of Sr₂Si₅N₈:Eu²⁺ obtained under 0.5 MPa gas pressure condition is higher than that under 0.1 MPa. The Sr₂Si₅N₈:Eu²⁺ synthesized under 0.5 MPa gas pressure in the temperature range from 1600°C to 1800°C have lower emission intensities than that synthesized under 0.1 MPa indicating that the melting of Sr₃N₂ is an important step for the formation of Sr₂Si₅N₈:Eu²⁺. Moreover, the Sr₂Si₅N₈:Eu²⁺ undergoes phase transition into SrSi₆N₈:Eu²⁺ completely after elongating the heating duration to 6 h at 1980°C under 0.5 MPa gas pressure. The same feature was observed under 0.1 MPa gas pressure after firing 8 h at 1750°C. Different heating durations led to different degrees of phase transition.     

Preparation of Sr1−xCaxLiAl3N4:Eu2+ Solid Solutions and Their Photoluminescence Properties

A series of quaternary nitride solid solutions with a general formula of Sr_1?xCa_xLiAl₃N₄:0.5%Eu²⁺ was synthesized by a solid‐state reaction method. The experimental results showed that a proper amount of Ca‐doping can improve the crystallinity and the photoluminescence properties of the produced phosphors. Rietveld refinement showed that the volume of the unit cell shrank with the increase of Ca substitution for Sr, which resulted in a red shift of the emission spectra from 654 to 665 nm under blue excitation at 475 nm. Rietveld refinement and CASTPE calculations suggested that Ca²⁺ ions prefer to occupy the smaller Sr(I) sites in the crystal lattice, which increases the amount of Eu²⁺ ions in Sr(II) sites and enables the tuning of the chromaticity coordinates of the obtained phosphors. The thermal stability of the produced phosphors is better than that of commercial Sr₂Si₅N₈:Eu²⁺ phosphor. The experimental results qualify the solid‐solution Sr_1?xCa_xLiAl₃N₄:0.5%Eu²⁺ for consideration as a potential candidate for application in white LEDs.     

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.     

A novel Eu2+/Tb3+ co-doped phosphor with pyroxene structure applied for cryogenic thermometric sensing

Pyroxene-type phosphors were widely developed due to the advantages of high chemical stability, luminous efficiency, and low production cost. In this contribution, a series of Eu²⁺/Tb³⁺ co-doped Ca_0.75Sr_0.2Mg_1.05Si₂O₆ (CSMS) phosphors with pyroxene structure were successfully synthesized by the solid-state method. Under the 340 nm excitation, the emission peaks of the phosphor show a redshift with the increase of Eu²⁺ concentration. The emitting color of Eu²⁺/Tb³⁺ co-doped samples shows a redshift attributed to the energy transfer from Eu²⁺ to Tb³⁺. Simultaneously, acquired thermometer exposes superbly temperature-sensitive properties (S_a and S_r having maximum values 4.7% K⁻¹ and 0.6% K⁻¹, respectively) over the cryogenic temperature range (77–280 K). Furthermore, it has good stability and precision at cryogenic temperatures, indicating that CSMS:0.03Eu²⁺/0.03Tb³⁺ phosphor is a very promising fluorescent material suitable for cryogenic temperature sensing.     

Synthesis and luminescent properties of Sr4−xSi3O8Cl4: xEu3+ red phosphor by hydrothermal method

Sr_4‐xSi₃O₈Cl₄:xEu³⁺ (SSOC:Eu³⁺) phosphors were successfully synthesized by hydrothermal method. The crystallization of this phosphor was analyzed by means of X‐ray diffraction patterns. The size and morphology were recorded using SEM patterns of samples. And the PLE and PL spectra were characterized by a PL spectrophotometer. Excited by 394 nm UV light, the intense red emission is recognized in SSOC:Eu³⁺ phosphor and the main emission peak located at 620 nm. The influences of Eu³⁺ concentration, pH value of reaction solution, and charge compensator on PL spectra of SSOC:Eu³⁺ phosphors were investigated. The results revealed that this red phosphor had potential applications for white LEDs.     

Enhanced luminescence and thermal stability of (Sr,Ca)AlSiN3:Eu2+ via superficial organic carbon modification

The Eu²⁺-activated nitride phosphors have been widely used in solid-state lighting, but the applications in high-power white-light-emitting diodes (wLEDs) field require higher thermal stability of luminescent materials. The oxidation of Eu²⁺ and the damage of nitride host in the Eu²⁺-activated nitride phosphors are the two crucial reasons for the luminescence loss while operating. A superficial organic carbon modification is performed on the red-emitting (Sr,Ca)AlSiN₃:Eu²⁺ phosphor via the incorporation of organic carbon by solution mixing and thermal post-treatment under the N₂-H₂ atmosphere. After the superficial organic carbon modification, the oxidation of Eu²⁺ and the formation of impurity phases on the phosphor surface are effectively reduced. When the superficial organic carbon modified sample was treated in the 2 wt.% sucrose solutions, the relative brightness is strengthened by 2.15%, the thermal quenching characteristic is improved by 8.9% at 300℃, and the aging test results show an excellent thermal stability. All above indicate that the superficial organic carbon modification is a promising technique to enhance the thermal stability of analogous Eu²⁺-activated nirtide phosphors.     

Preparation of phosphor coated with a polymer by emulsion polymerization and its application in low‐density polyethylene

The long‐afterglow phosphor SrAl₂O₄ : Eu²⁺, Dy³⁺ is liable to hydrolyze in water with deterioration of its luminescent properties. In this study, in situ emulsion polymerization was first used to prepare phosphor coated with poly(methyl methacrylate‐co‐butyl acrylate) [P(MMA‐co‐BA)] to improve water resistance. Fourier transform infrared spectra suggested that the polymer attached to the phosphor by chemical bonding. Observation by scanning electron microscopy (SEM) showed that a polymer layer formed on the surface of the phosphor. The resistance to water of the phosphor coated with the polymer layer was much better than that of the uncoated phosphor because the transparent polymer layer could suppress its hydrolysis process. Low‐density polyethylene (LDPE) plastics, doped with long‐afterglow phosphors, were manufactured with an extrusion technique. Through coating with P(MMA‐co‐BA), the compatibility of phosphor with the LDPE matrix was improved, as determined by SEM. The luminous LDPE plastics blended with the phosphor coated with polymer showed long and strong phosphorescence with little loss of persistence phosphorescence compared to the uncoated phosphor. The LDPE plastics still retained their mechanical properties through doping with 3% (mass fraction) of the phosphors. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Enhanced Optical Properties of Bredigite‐Structure Ca13.7Eu0.3Mg2[SiO4]8 Phosphor: Effective Eu Reduction by La Co‐Doping

A green phosphor, La_0.4Ca_13.3Eu_0.3Mg₂Si₈O_31.6+δN_0.4?δ (LaCMSN:Eu²⁺), was prepared by a solid‐state reaction and an efficient green emission was observed at 506 nm under near‐ultraviolet (NUV) excitation. The structural and optical properties of LaCMSN:Eu²⁺ phosphors as well as their thermal quenching were investigated. The partial substitution of La³⁺ and N^3? in Ca_13.7Eu_0.3Mg₂Si₈O₃₂ led to a considerable enhancement in the peak emission intensity by as much as 194%. This demonstrates not only that the total number of Eu²⁺ activators increased, but also that the probability of a nonradiative transition between Eu²⁺ and Eu³⁺ could be reduced as the increase in concentration of the former is at the expense of the later. The white light‐emitting diode (LED) was fabricated using phosphor with a NUV LED chip. The LED showed warm white light with an excellent color rendering index of 91. The LaCMSN:Eu²⁺ is thus a potential green‐emitting phosphor for white LEDs.     

Encapsulation of nitride phosphors into sintered phosphate glass by pressureless firing and hot isostatic pressing

The encapsulation conditions of Eu²⁺-doped calcium silicon nitride ((Ca_0.90Eu_0.10)₂Si₅N₈) and strontium silicon nitride ((Sr_0.98Eu_0.02)₂Si₅N₈) phosphors into phosphate glass ((50-x)Na₂O–xZnO–5B₂O₃–45P₂O₅; x = 10-40) were examined by pressureless sintering (normal or infrared firing) and subsequent oxygen-assisted hot isostatic pressing. The evaluation of water resistivity, as well as glass transition temperature (T_g), indicated that the excellent water resistivity and low T_g (313°C) could be achieved at x = 30. The colorless and clear glass body could be fabricated by infrared firing at 370°C for 20 minutes in air, followed by the hot isostatic pressing at 370°C for 24 hours (80%Ar + 20%O₂, pressure: 130 MPa). The encapsulations of 3.0 mass% (Ca_0.90Eu_0.10)₂Si₅N₈ and (Sr_0.98Eu_0.02)₂Si₅N₈ into the glass body produced reddish orange light emission (peak wavelength: 604 nm under excitation wavelength of 423 nm) and red-light emission (617 nm under 383 nm), respectively, showing higher chemical and thermal stability of nitride phosphors in the glass matrix.     

Decay Behavior in Ce3+‐doped La3Si6N11 and Lu3Al5O12 Phosphors

Ce³⁺‐activated light emitting diode (LED) phosphors have been extensively examined for photoluminescence, and have been the focus of many detailed structural studies. However, reports of the decay curves of Ce³⁺‐activated LED phosphors are rare. Although we have reported the decay behaviors of several Eu²⁺‐activated LED phosphors such as Sr₂SiO₄, Sr₂Si₅N₈, and CaAlSiN₃, we have never conducted an in‐depth study into the decay behavior for Ce³⁺‐activated LED phosphors. For this study, we investigated the decay curves of well‐known Ce³⁺‐activated LED phosphors such as La₃Si₆N₁₁ and Lu₃Al₅O₁₂. Similar to Eu²⁺‐activated LED phosphors, the decay behavior of Ce³⁺‐activated LED phosphors was sensitive to the Ce³⁺ concentration and to the detection wavelength. There was active nonradiative energy transfer between the Ce³⁺ activators located at different sites.     

Improved thermal stability and luminescence properties of SrSi2O2N2:Eu2+ green phosphor by a heterogeneous precipitation protocol for solid-state lighting applications

The SrSi₂O₂N₂:Eu²⁺ green-emitting phosphor, as a member of oxynitride phosphors, could greatly enhance the color quality of the white-light conversed by single yellow-emitting phosphor. However, SrSi₂O₂N₂:Eu²⁺ phosphors prepared by traditional solid-state ways normally consist hard agglomerates with unevenly dispersed particles and therefore suppress the luminescence properties. This research proposes a heterogeneous precipitation protocol where precipitation process is introduced, in order to produce a precursor in the first phase. And in the second phase, a high-temperature solid-state process is adopted for obtaining the final product. The main results show that, for an optimized SrSi₂O₂N₂:Eu²⁺ phosphor prepared using our protocol, (1) the SrSi₂O₂N₂:Eu²⁺ particles are faceted and with smooth faces and (2) the phosphor photoluminescence, external quantum efficiency, stability against thermal exposure, and physical stability consistently outperform the current commercially used product. This research essentially provides an economic way for production of solid-state lighting with enhanced color quality.     

Optical Energy Storage Properties of (Ca1−xSrx)2Si5N8: Eu2+, Tm3+ Solid Solutions

While the reddish‐orange emitting phosphors M₂Si₅N₈:Eu²⁺(M = Ca, Sr) have been intensively investigated as potential materials for white‐light‐emitting diodes, in this study, optical energy storage properties of (Ca_1?xSr_x)₂Si₅N₈: Eu²⁺, Tm³⁺ (x = 0–1) solid solutions were tuned by cation substitution, which was commonly used to tune color point for improving w‐LEDs. Partial substitution of either Ca by Sr or Sr by Ca resulted in a redshifted Eu²⁺ emission which had a demarcation point at x = 0.5. Furthermore, the (Ca_1?xSr_x)₂Si₅N₈: Eu²⁺, Tm³⁺ materials exhibited similar persistent‐ and photostimulated luminescence behaviors with a maximum intensity at about x = 0.2. Such optical energy storage characters of the samples were attributed to the more appropriate trap depths (322–333 K) and higher density of energy level traps indicated by the thermoluminescence analysis.     

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.