Particle size influence of starting batches on phosphorescence behaviour of Sr4Al14O25 based bluish green phosphors

Abstract

Photoluminescent material with long afterglow is a kind of energy storage material that can absorb both ultraviolet (UV) and visible lights from sunlight, and gradually releases the energy in the dark at a certain wavelength. These sorts of materials have great potential for various device applications and have been widely studied by many researchers. In recent years, it has also been reported that 2SrO.3Al₂O₃/Eu²⁺ and 4SrO.7Al₂O₃/Eu²⁺ phosphors as green and blue emitters have even higher quantum efficiencies. To determine the initial particle size effect on the phosphorescence behaviour, Eu²⁺/Dy³⁺ doped Sr₄Al₁₄O₂₅ phosphors were synthesised by mixing 4SrO and 7Al₂O₃ with a flux (H₃BO₃) through high temperature solid state reaction method under weak reducing atmosphere. Such an influence on the crystalline structure and emission colour of phosphorescent pigments was studied by means of X-ray diffraction, scanning electron microscope (SEM), particle size analysis, excitation and emission spectroscopy. The results showed that the emission wavelength of the phosphorescence pigments shifted from green to blue region due to the decrease in average particle sizes of the phosphor batches, forming different types of strontium aluminate crystals.     

Alkali metal ion substitution induced luminescence enhancement of NaLaMgWO6:Eu3+ red phosphor for white light-emitting diodes

Alkali metal ion substitution is an effective strategy to improve the luminescence properties of phosphors. In this work, a series of red-emitting phosphors Na_1-xLi_x/2K_x/2La_0.6Eu_0.4MgWO₆ were prepared by a traditional high-temperature solid-state reaction. Their phase structure, microstructure, luminescence properties and potential application in phosphor-converted white light-emitting diodes (pc-WLEDs) were investigated in detail. X-ray diffraction (XRD) result revealed the formation of a solid solution when x?≤?0.3, which kept monoclinic structure of NaLaMgWO₆. Photoluminescence investigation indicated that the partial substitution of Li⁺/K⁺ ions for Na⁺ ions improved largely the red emission of Eu³⁺. Based on the optimized Na_0.7Li_0.15K_0.15La_0.6Eu_0.4MgWO₆ sample with relatively good thermal stability, a WLED device was fabricated by combining a near-ultraviolet (NUV) chip (~400?nm) with the phosphor mixture of commercial green/blue phosphors and the optimized red phosphor. The results indicated that the optimized red phosphor in this work could be a potential candidate for WLEDs pumped by NUV chips.     

Warm white light-emitting diodes using organic–inorganic halide perovskite materials coated YAG:Ce3+ phosphors

This work reports warm white light-emitting diodes (WLEDs) using organic–inorganic halide perovskite materials coated YAG:Ce³⁺ phosphors by a liquid phase synthesis method. The perovskite MAPbBr_3?xI_x-coated YAG:Ce³⁺ phosphors have more red light contribution than YAG:Ce³⁺ phosphors without the perovskite coating. The chromaticity coordinate of white LED with YAG:Ce³⁺ remote phosphor is (x = 0.3134, y = 0.3497) 6359. However, those of MAPbBr_2.5I_0.5 and MAPbBr_2.0I_1.0-coated YAG:Ce³⁺ remote phosphor shift to (x = 0.4220, y = 0.3725) 2908 and (x = 0.4067, y = 0.4028) 3525, respectively. The colors of perovskite-coated samples are more red and warm. Therefore, the perovskite-coated YAG:Ce³⁺ method is useful for warm WLED.     

Phosphor‐in‐fluorescent‐glasses for high color rendering white light emitting diodes

Fluorescent glass frits were prepared and used to synthesize phosphor‐in‐fluorescent glass composites (PiFGs) to realize stable white light emitting diodes with high color‐rendering properties. Commercial red, green, and blue phosphors were co‐sintered and red phosphors were partially replaced by Eu³⁺ in glass frits. Phosphor‐in‐glass composites were placed on UV‐light emitting diodes (UV‐LEDs) to generate white light. Pure white light with a luminous efficacy=58.4 lm/W, general color rendering index R_a=87 and special color rendering index for strong red R₉=73 was realized with glass frits containing 7 mol% Eu₂O₃ and RGB ratio of 35:20:15. Luminous efficacy, R_a and R₉ increased as red phosphors were replaced by red‐fluorescent glass frits.     

Photoluminescence properties of NUV light excited Ba(Mg1/3Nb2/3)O3:Eu3+ red phosphor with high color purity

In this work, a new red phosphor with high color purity, Eu³⁺ ions doped Ba(Mg_1/3Nb_2/3)O₃ phosphor has been prepared by wet chemical method. The structure analysis suggests BMN:x%Eu phosphors have a hexagonal phase and Ba²⁺ ions are replaced by Eu³⁺ ions in BMN. Upon excitation of NUV light, the BMN:x%Eu phosphors emit strong red light around 615?nm, derived from the ⁵D₀-⁷F₂ transition of Eu³⁺ ions. The relationship between luminescent properties and structure of BMN:x%Eu was discussed. The Judd-Ofelt intensity parameters (Ω₂, Ω₄) were calculated to analyze the asymmetry of the Eu³⁺ ions site occupancy further, and the quantum efficiency of BMN:3%Eu was found to be 77.26%. In addition, the decay curve indicates the decay time(τ) of BMN:3%Eu is determined to be 1.34?ms and Eu³⁺ ions occupy only one type of site. The CIE chromaticity coordinate (0.656,0.344) of BMN:3%Eu is quite close to the red phosphors standard value (0.670, 0.330), which indicates BMN:x%Eu can be a suitable red phosphor used in NUV-based white LEDs.     

Highly efficient double perovskite (Bi,Gd)-codoped Cs2Ag0.4Na0.6InCl6 phosphors for warm white LEDs

Bi-doped lead-free double perovskite Cs₂Ag_0.6Na_0.4InCl₆ phosphors have been proved to be effective for use in white LEDs. To further improve the luminescence effect of this promising material, Bi³⁺/Gd³⁺ co-doped lead-free perovskite Cs₂Ag_0.6Na_0.4InCl₆ phosphors were successfully synthesized using the oil bath method. The quantum efficiency of orange-emitting Cs₂Ag_0.6Na_0.4InCl₆:1% Bi³⁺ was effectively improved from 79.72% to 87.57% by co-doping Gd³⁺ ions. White LEDs were prepared by mixing the synthesized phosphor Cs₂Ag_0.40Na_0.60InCl₆: 1%Bi³⁺,10%Gd³⁺ and commercial blue phosphor BaMgAl₁₀O₁₇:Eu²⁺. Excellent warm white LEDs with colour coordinates (0.3464, 0.3224), colour rendering index of 93.9, and colour temperature of 4818 K were produced. The results of this study provide a meaningful reference for new lead-free halide perovskite luminescent material systems.     

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.     

pH dependent hydrothermal synthesis of Ca14Al10Zn6O35:0.15Mn4+ phosphor with enhanced photoluminescence performance and high thermal resistance for indoor plant growth lighting

Artificial light source for indoor cultivation has been vastly impeded by the lack of high far red emitting phosphors. Recently, Mn⁴⁺ activated phosphors were reported to be promising luminescent materials to solve above matter. In this study, controllable design of Ca₁₄Al₁₀Zn₆O₃₅:0.15Mn⁴⁺ (CAZO:0.15Mn⁴⁺) far red emitting phosphors was realized via pH assisted hydrothermal approach. The pure CAZO:0.15Mn⁴⁺ phosphors were obtained merely when the reaction pH was 1 or 2. Meanwhile, by adjusting the pH value of the reaction solution, far red emission CAZO:0.15Mn⁴⁺ phosphors with grains, sphere-like as well as aggregated bulk particles can be achieved at pH =?4, pH =?6 and pH =?10, respectively. Furthermore, the structures and morphologies depended photoluminescence (PL) performances of CAZO:0.15Mn⁴⁺ were checked. The best PL performance was found for the phosphor produced at pH =?6, while over acidic or alkaline conditions would lower the emission intensity. In addition, this phosphor also exhibit good thermal resistance which can maintain 78% initial intensity at 150?°C. The practical indoor tobacco cultivation demonstrated that CAZO:0.15Mn⁴⁺ obtained through this pH adjusted hydrothermal route is a promising phosphor for indoor plant growth lighting.     

Dual-mode dichromatic SrBi4Ti4O15: Er3+ emitting phosphor for anti-counterfeiting application

Fluorescent materials have been widely used for anti-counterfeiting of important documents and currencies, wherein their anti-counterfeit abilities could be improved through multi-mode excitation. Herein, dual-mode-excited double-colour-emitting Er³⁺doped SrBi₄Ti₄O₁₅ up-conversion (UC) phosphors (SBTO: Er³⁺) were synthesised, and their UC spectra included green (2H11/2/4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) emissions from Er3+ ions under 980 or 1550 nm excitation. However, the green emission colour of phosphors was independent of dopant concentration under 980 nm laser irradiation; whereas the final emission colour was dominated by red emission and significantly affected by contents of Er3+ under 1550 nm excitation. These observations demonstrated potential application in dual-mode double-colour anti-counterfeiting. The possible UC mechanisms and emission characteristics of the phosphors using different 980 and 1550 nm irradiation source were contrastively investigated, and some fluorescent security patterns were also designed to demonstrate the potential applications in anti-counterfeiting and concealing important information.     

Tunable full-color emission phosphors: Enhanced security application via a patterned 3-dimensions code

Developing tunable full-color emission photoluminescent materials is always desired in color-on-demand applications and still confronts challenges. Theoretically, full color including white emission can be achieved by the combination of three primary colors (red, green, and blue) based on the additive color theory. Herein, a strategy for the preparation of tunable full-color luminescence is realized by mixing the inorganic rare-earth-doped SrAl₂O₄: Eu²⁺, Dy³⁺ (green emission), Y₂O₂S: Eu³⁺, Mg²⁺, Ti⁴⁺, Ti⁴⁺_0.05 (red emission), and Sr₂MgSi₂O₇: Eu²⁺, Dy³⁺ (blue emission) phosphors with different ratios. By adjusting individual phosphors at certain specific ratios, white light (0.332, 0.332) and full-spectra emission are achieved under a single low excitation energy (λ_ex = 365 nm) using a portable ultraviolet (UV) lamp. Based on the facile preparation and effective tunable full-color emission features of the phosphors, a novel encryption way of the luminescent unit as information storage 3 dimensions (3D) codes is developed. The multiplexed encrypting information capacity of the codes is enhanced in a 3D maneuver strategy by simply adjusting the number of light-emitting units with infinite emission colors. The proposed strategy makes the tunable full-color emission phosphors useful in promising applications including full-color display, high-level information encryption and anti-fake.     

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.     

Rare‐Earth Free Phosphors with Ultra‐Broad Band Emission Application for High Color Rendering Index Lighting Source

Rare‐earth containing phosphors have been widely applied in lighting and display fields in the past century. Lower cost rare‐earth free phosphors with high performance are highly desired driven by the exhaustion of rare earth resources and the requirement of cheaper production. Herein, Cu⁺ ions doped tetracalcium phosphate (TTCP) yellow emitting phosphors with quantum yield of 21% are exploited. Particularly, ultra‐broad band emission with a full width at half maximum (FWHM) about 200 nm throughout almost entire visible light region is observed for TTCP: Cu⁺ phosphors, evidencing its promising application in high color rendering index (CRI) lighting source. White light emission with CRI value about 94.3 is generated by combining this TTCP: Cu⁺ phosphor with commercial BaMgAl₁₀O₁₇: Eu²⁺ blue phosphor, exhibiting superiority over the traditional trichromatic phosphors. Therefore, we predict great potential application for this cheaper rare‐earth free TTCP: Cu⁺ phosphor in high CRI lighting sources.     

Enhanced luminescence properties of Li2MgTiO4: Mn4+, Ge4+ phosphor via single cation substitution for indoor plant cultivation

Red and far-red emitting phosphors have been widely used in phosphor-converted light emitting diode (pc-LED) devices to provide lighting for indoor plant growth, thus achieving desired product qualities. Among the many ways to optimize phosphors’ optical performance, cationic substitution is one of the most effective methods. In this study, red phosphors (Li₂MgTi_1-x-yO₄: xMn⁴⁺, yGe⁴⁺) were synthesized by high temperature solid state method and the optical performance of phosphors were improved with increasing Ge⁴⁺ constituents. In particular, luminescence intensity of Li₂MgTiO₄: 0.002Mn⁴⁺, 0.1Ge⁴⁺ increased by 152% under 468 nm excitation, and the thermostability of emission intensity increases from 22% (y = 0) to 43% (y = 0.1), which is about twice as much. Finally, pc-LED device was fabricated via the red phosphor Li₂MgTiO₄: 0.002Mn⁴⁺,0.1Ge⁴⁺ coated on a 470 nm ultraviolet chip. By changing the proportion of the phosphor, the electroluminescence spectra of pc-LED device could match well with the absorption regions of plant pigments. Therefore, Li₂MgTiO₄: 0.002Mn⁴⁺, 0.1Ge⁴⁺ phosphor has potential application in plant lighting. Furthermore, this work can offer some helpful references for improving luminescent efficiency by simply modulating the chemical composition.     

Preparation and luminescent modulation of KGaSiO4:Eu3+ phosphors using for multiple anti-counterfeiting

Anti-counterfeiting technology is of great significance to information security. To obtain high-quality anti-counterfeiting materials, the developments of inorganic materials are crucial. In this paper, a series KGaSiO₄:xEu³⁺ phosphors with persistent luminescence, photoluminescence, and thermochromic have been successfully prepared and the application of quadruple anti-counterfeiting is realized. The X-ray diffraction and Rietveld refinement indicate that the phosphors are pure phase. With Eu³⁺ ions doping, the structure change, site occupancies, and color-tunable phenomenon are carefully investigated. Different from another Eu³⁺ doping phosphor, the emission of KGaSiO₄:0.2% Eu³⁺ phosphor changes with the excitation light in the region of 240 nm–306 nm. The emission color can be modulated with the surrounding temperature. Surprisingly, this phosphor can emit green afterglow light, which is attributed to the different luminescent properties of the matrix and doping of Eu³⁺ ions. The series of phosphors exhibit abundant luminescent properties. Based on their wavelength dependence, concentration quenching, long afterglow, and thermochromic properties, the KGaSiO₄:xEu³⁺ phosphors can be effective materials for quadruple-modal anti-counterfeiting devices.     

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.     

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.     

Effects of NH3·H2O pretreatment on the fabrication of uniform titania nanocoating in an aqueous solution

An investigation is reported of the TiO₂ nanocoating on the ZnS-based phosphor via a sol-gel method in an aqueous solution using titanium diethanolamine complex as the precursor. The pretreatment of ZnS phosphors with NH₃·H₂O is important to the coating process. When NH₃·H₂O was added into the ZnS phosphor suspension, OH⁻ promoted the hydrolysis of Zn²⁺ on the surface of ZnS phosphors, which resulted in the formation of Zn-OH. Zn-OH reacted with the hydrolysis product of titanium diethanolamine complex, thus the titania coatings were obtained. The coating morphology was strongly dependant on the pH of the pretreated ZnS phosphor suspension and the weight ratio of ZnS to TiO₂.     

Up-conversion luminescence and temperature-sensing properties of Li+, K+ doped Na2Zn3Si2O8: Er3+ phosphors

In this work we detail the preparation of new luminescent Li⁺ and K⁺ doped Na₂Zn₃Si₂O₈: Er³⁺ up-conversion phosphors using the high-temperature solid-phase method. We investigate the phosphors phase structure, elemental distribution, up-conversion luminescence characteristics and temperature sensing properties. Our fabricated samples were found to be homogeneous and when excited using 980 nm light, they emitted wavelengths in the green and red visible wavelength bands, which correspond to two major emission bands of Er³⁺. Doping with Li⁺ and K⁺ increased the luminescence intensity of the Na₂Zn₃Si₂O₈: Er³⁺ phosphor at 661 nm by 36 and 21 times respectively. The highest relative temperature sensitivity (Sa) of the fabricated phosphor reached a value of 19.69% K^?1 and the highest absolute temperature sensitivity (Sr) reached 1.20% K^?1. These values are superior to other materials which utilize up-conversion by Er³⁺ ions as a tool for temperature sensing. We anticipate that these new phosphors will find significant application as components in optical temperature measurement systems.     

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.     

Effects of HfO2 dopant on characteristics of Li2MgTiO4-based red phosphors: Thermal stability,photoluminescence intensity and quantum efficiency improvement

To produce natural and vivid color, the color rendering index of white light-emitting diodes (WLEDs) with single phosphors is usually lower than 70, which is problematic for LED applications. A commonly used method to resolve this issue is to enhance the red component of WLEDs. In the present study, Hf⁴⁺ and Mn⁴⁺ co-doped Li₂MgTiO₄ red phosphors are synthesized using a solid-state reaction method. When this red phosphor is excited at 397 and 468?nm, it exhibits weak reabsorption in the blue region and emits a broad and deep red emission band in the range of 640–750?nm, which is attributed to the ²E_g → ⁴A_2?g transition. With 5?mol% HfO₂ dopant, the photoluminescence intensity is enhanced by 1.45-fold and thermal stability is increased by 7.7%. Moreover, this red phosphor was applied to a red phosphor-in-glass (RPiG) optical device with a low-melting TeO₂-B₂O₃-ZnO-Na₂O-WO₃ glass system. In the RPiG melting process, Li₂MgTiO₄:Mn⁴⁺, Hf⁴⁺ red phosphor triggered neither a chemical reaction nor severe degradation, indicating good thermal stability. Li₂MgTiO₄:Mn⁴⁺, Hf⁴⁺ has potential as a red emission material for warm WLED applications.