Preparation of SrAl2O4: Eu, Dy nanometer phosphors by detonation method

SrAl₂O₄: Eu²⁺, Dy³⁺ nanometer phosphors were synthesized by detonation method. The particle morphology and optical properties of detonation soot that was heated at different temperatures (600–1100 °C) had been studied systematically by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results indicated SrAl₂O₄: Eu²⁺, Dy³⁺ nanometer powders in monoclinic system (a = 8.442, b = 8.822, c = 5.160, β = 93.415) can be synthesized by detonation method, when detonation soot was heated at 600–800 °C. The particle size of SrAl₂O₄: Eu²⁺, Dy³⁺ is 35 ± 15 nm. Compared with the solid-state reaction and sol-gel method, synthesis temperature of the detonation method is lower about 500 and 200 °C respectively. After being excited under UN lights, detonation soot and that heated at 600–1100 °C can emit a green light.     

Luminescence and energy transfer of Mn co-doped SrSi2O2N2:Eu green-emitting phosphors

Eu²⁺ and Mn²⁺ co-doped SrSi₂O₂N₂ green-phosphors, with promising luminescent properties (examined by their powder diffuse reflection, photoluminescence excitation and emission spectra) suitable for UV converted white LEDs, were produced by high temperature solid-state reaction method. The produced materials exhibited intense broad absorption bands at 220–500 nm and a broad emission band centered at ca. 530 nm, attributed to 4f–5d transitions of Eu²⁺. The emission intensity of Eu²⁺ ions was greatly enhanced by introducing Mn²⁺ ions into SrSi₂O₂N₂:Eu²⁺ due to the energy transfer from Mn²⁺ to Eu²⁺. The energy transfer probability from Mn²⁺ to Eu²⁺ depends strongly on the Mn²⁺ concentration, which is maximized at a Mn²⁺ concentration of 3 mol%. It drastically decreases for higher concentrations. The results indicated that SrSi₂O₂N₂:Eu²⁺, Mn²⁺ is a promising green-emitting phosphor for white-light emitting diodes with near-UV LED chips.     

Synthesis of monodisperse spherical core-shell SiO2-SrAl2Si2O8: Eu2+ phosphors by hydrothermal homogeneous precipitation method

Nanocrystalline SrAl₂Si₂O₈ :Eu²⁺ phosphor layers were coated on nonaggregated, monodisperse and spherical SiO₂ particles using a hydrothermal homogeneous precipitation. After annealing at 1100 °C, core-shell SiO₂@SrAl₂Si₂O₈ :Eu²⁺ particles were obtained. They were characterized with x-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and photoluminescence techniques. XRD analysis confirmed the formation of SiO₂ @SrAl₂Si₂O₈ :Eu²⁺ particles; it indicated that the SrAl₂Si₂O₈ :Eu²⁺ shells on SiO₂ particles consisted of hexagonal crystallites. The core-shell phosphors obtained are well-dispersed submicron spherical particles with a narrow size distribution. The thickness of the coated layer is approximately 20–40 nm. Under ultraviolet excitation (361 nm), the particles emit blue light at about 440 nm due to the Eu²⁺ ions in their shells.     

Near UV-based LED fabricated with Ba5SiO4(F,Cl)6:Eu as blue- and green-emitting phosphor

A series of halosilicate phosphor, Ba₅SiO₄(F,Cl)₆:Eu²⁺, were synthesized by a solid state reaction. Excited by 370-nm light, Ba₅SiO₄Cl₆:Eu²⁺ exhibits a broad emission band peaking at 440 nm. Partial substitution of Cl⁻ with F⁻ in the host lattice leads to red-shift in the emission band with centering wavelength from 440 nm to 503 nm. The possible mechanism for the luminescence change was discussed based on the XRD patterns. Blue and green LEDs were fabricated by combination of a 370 nm-emitting near UV chip and the optimal Ba₅SiO₄Cl₆:Eu²⁺ and Ba₅SiO₄(F₃Cl₃):Eu²⁺, respectively. This series of phosphors is considered as a promising blue and green component used in fabrication of near UV-based white LEDs.     

Yellow-emitting phosphor of Sr3B2O6:Eu for application to white light-emitting diodes

This work focuses on the development of Eu²⁺-doped strontium (Sr)-borate as a yellow-emitting phosphor and its application to the fabrication of white light-emitting diodes (LEDs). Synthesis of Eu²⁺-doped Sr-borate phosphors was finely tuned for obtaining the efficient yellow luminescence through varying host composition, Eu concentration, and firing temperature. The 1300 °C-fired Eu²⁺-doped Sr₃B₂O₆, which was found to be the most efficient candidate to date, was used for white LED fabrication. Their optical properties were evaluated, resulting in warm white lights with CIE chromaticity coordinates of (0.340–0.372, 0.287–0.314) and color rendering indices of 75–77 under the forward currents of 5–40 mA.     

Sr3.5Mg0.5Si3O8Cl4: Eu bluish-green-emitting phosphor for NUV-based LED

Sr₄Si₃O₈Cl₄:Eu²⁺ and Sr_3.5Mg_0.5Si₃O₈Cl₄:Eu²⁺ phosphors were prepared by a conventional solid state reaction (SS). Excited by 370 nm near-ultraviolet light, the phosphors show an efficient bluish-green wide-band emission centering at 484 nm, which originates from the 4f5d¹ → 4f⁷ transition of Eu²⁺ ion. The excitation spectra of the phosphors are a broad band extending from 250 nm to 400 nm. Mg²⁺-codoping greatly enhances the bluish-green emission of the phosphors. An LED was fabricated by coating the Sr_3.5Mg_0.5Si₃O₈Cl₄:0.08Eu²⁺ phosphor onto an ~ 370 nm-emitting InGaN chip. The LED exhibits bright bluish-green emission under a forward bias of 20 mA. The results indicate that Sr_3.5Mg_0.5Si₃O₈Cl₄:0.08Eu²⁺ is a candidate as a bluish-green component for fabrication of NUV-based white LEDs.     

Influence of calcination temperature on luminescent properties of Sr3Al2O6:Eu, Dy phosphors prepared by sol–gel-combustion processing

The detailed preparation process of Eu²⁺ and Dy³⁺ ion co-doped Sr₃Al₂O₆ phosphor powders with red long afterglow by sol–gel-combustion method in the reducing atmosphere is reported. X-ray diffraction, scanning electron microscopy and photoluminescence spectroscopy are used to investigate the effects of synthesis temperature on the crystal characteristics, morphology and luminescent properties of the as-synthesized Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors. The results reveal that Sr₃Al₂O₆ crystallizes completely when the combustion ash is sintered at 1200 °C. The excitation and the emission spectra indicate that the excitation broad-band lies chiefly in visible range and the phosphor powders emit strong light at 618 nm under the excitation of 472 nm. The light intensity and the light-lasting time of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors are increased when increasing the calcination temperatures from 1050 to 1200 °C. The afterglow of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors sintered at 1200 °C lasts for over 600 s when the excited source is cut off. The red emission mechanism is discussed according to the effect of nephelauxetic and crystal field on the 4f⁶5d¹ → 4f⁷ transition of the Eu²⁺ ions.     

A yellow-emitting Ca3Si2O7: Eu phosphor for white LEDs

A series of yellow-emitting phosphors based on a silicate host matrix, Ca_3 − xSi₂O₇: xEu²⁺, was prepared by solid-state reaction method. The structure and photoluminescent properties of the phosphors were investigated. The XRD results show that the Eu²⁺ substitution of Ca²⁺ does not change the structure of Ca₃Si₂O₇ host and there is no impurity phase for x < 0.12. The SEM images display that phosphors aggregate obviously and the shape of the phosphor particle is irregular. The EDX results reveal that the phosphors consist of Ca, Si, O, Eu and the concentration of these elements is close to the stoichiometric composition. The Ca_3 − xSi₂O₇: xEu²⁺ phosphors can be excited at a wavelength of 300-490 nm, which is suitable for the emission band of near ultraviolet or blue light-emitting-diode (LED) chips. The phosphors exhibit a broad emission region from 520 to 650 nm and the emission peak centered at 568 nm. In addition, the shape and the position of the emission peak are not influenced by the Eu²⁺ concentration and excitation wavelength. The phosphor for x = 0.045 has the strongest excitation and emission intensity, and the Ca_3 − xSi₂O₇: xEu²⁺ phosphors can be used as candidates for the white LEDs.     

Long wavelength Ce emission in Y6Si3O9N4 phosphors for white-emitting diodes

Y₆Si₃O₉N₄:Ce³⁺ phosphor was prepared by a solid-state reaction in reductive atmosphere. X-ray powder diffraction (XRD) analysis confirmed the formation of Y₆Si₃O₉N₄:Ce³⁺. Scanning electron microscopy (SEM) observation indicated that the microstructure of the phosphor consisted of irregular fine grains with an average size of about 5 μm. Photoluminescence (PL) measurements showed that the phosphor can be efficiently excited by near ultraviolet (UV) or blue light excitation, and exhibited bright green emission peaked at about 525 nm. Compared with Ce³⁺-doped Y₄Si₂O₇N₂ phosphors, Ce³⁺-doped Y₆Si₃O₉N₄ phosphors showed longer wavelengths of both excitation and emission. The Y₆Si₃O₉N₄:Ce³⁺ is a potential green-emitting phosphor for white LEDs.     

Influence of excitation wavelengths on luminescent properties of Sr3Al2O6:Eu, Dy phosphors prepared by sol-gel-combustion processing

Eu²⁺ and Dy³⁺ ion co-doped Sr₃Al₂O₆ red-emitting long afterglow phosphor was synthesized by sol-gel-combustion methods using Sr(NO₃)₂, Al(NO₃)₃·9H₂O, Eu₂O₃, Dy₂O₃, H₃BO₃ and C₆H₈O₇·H₂O as raw materials. The crystalline structure of the phosphors were characterized by X-ray diffraction, luminescent properties of phosphors were analyzed by fluorescence spectrophotometer. The effect of excitation wavelengths on the luminescent properties of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors was discussed. The emission peak of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphor lays at 516 nm under the excitation of 360 nm, and at 612 nm under the excitation of 468 nm. The results reveal that the Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphor will emit a yellow-green light upon UV illumination, and a bright red light upon visible light illumination. The emission mechanism was discussed according to the effect of nephelauxetic and crystal field on the 4f⁶5d¹ → 4f⁷ transition of the Eu²⁺ ions in Sr₃Al₂O₆. The afterglow time of (Sr_0.94Eu_0.03Dy_0.03)₃ Al₂O₆ phosphors lasts for over 600s after the excited source was cut off.     

A novel blue-emitting NaSrPO4:Eu phosphor for near UV based white light-emitting-diodes

A novel blue-emitting phosphor based on a phosphate host matrix, NaSrPO₄:Eu²⁺, was prepared by a conventional solid-state reaction method. The NaSrPO₄:Eu²⁺ phosphor was efficiently excited at wavelengths of 250-450 nm, which is suitable for the emission band of near ultraviolet (n-UV) light-emitting-diode (LED) chips (350-430 nm). The NaSrPO₄:Eu²⁺ phosphor exhibits a strong blue emission peaking at 453 nm and broadly weak green and red emission bands up to 700 nm. The effect of the activated Eu²⁺ concentration on the emission intensity of the NaSrPO₄:Eu²⁺ was also investigated. Here, a phosphor-converted LED (pc-LED) was fabricated and exhibits bright blue emission under a forward bias of 20 mA. All of these characteristics suggest that the NaSrPO₄:Eu²⁺ phosphors could be applicable to n-UV based white LEDs.     

Luminescent properties of Ca2BO3Cl:Eu yellow-emitting phosphor for white light-emitting diodes

The Ca₂BO₃Cl:Eu²⁺ phosphor was synthesized by the general high temperature solid-state reaction and an efficient yellow emission under near-ultraviolet and blue excitation was observed. The emission spectrum shows a single intense broad emission band centered at 573 nm, which corresponds to the allowed f-d transition of Eu²⁺. The excitation spectrum is very broad extending from 350 to 500 nm, which is coupled well with the emission of UV LED (350-410 nm) and blue LED (450-470 nm). The measured emission of In-GaN-based Ca₂BO₃Cl:Eu²⁺ LED shows white light to the naked eye with a chromatic coordinate of (0.33, 0.36). The Ca₂BO₃Cl:Eu²⁺ is a very appropriate yellow-emitting phosphor for white LEDs.     

Synthesis of monodisperse spherical core-shell SiO2-SrAl2Si2O8: Eu2+ phosphors by hydrothermal homogeneous precipitation method

Abstract

Nanocrystalline SrAl₂Si₂O₈ :Eu²⁺ phosphor layers were coated on nonaggregated, monodisperse and spherical SiO₂ particles using a hydrothermal homogeneous precipitation. After annealing at 1100 °C, core-shell SiO₂@SrAl₂Si₂O₈ :Eu²⁺ particles were obtained. They were characterized with x-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and photoluminescence techniques. XRD analysis confirmed the formation of SiO₂ @SrAl₂Si₂O₈ :Eu²⁺ particles; it indicated that the SrAl₂Si₂O₈ :Eu²⁺ shells on SiO₂ particles consisted of hexagonal crystallites. The core-shell phosphors obtained are well-dispersed submicron spherical particles with a narrow size distribution. The thickness of the coated layer is approximately 20–40 nm. Under ultraviolet excitation (361 nm), the particles emit blue light at about 440 nm due to the Eu²⁺ ions in their shells.     

Tuning the luminescence color and enhancement of afterglow properties of Sr(4−x−y)CaxBayAl14O25:Eu,Dy phosphor by adjusting the composition

Color point tuning is an important challenge for improving the practical applications of various displays, especially there are very limited white color single hosts that emits in the white spectrum. In this paper, the possibility of color tuning by substituting part of host lattice cation (Sr²⁺ ions) by Ca²⁺ or Ba²⁺ ions in an efficient strontium aluminate phosphor, Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺, is reported and found to be very promising for displays. A detail study by replacing part of Sr²⁺ with Ca²⁺ or Ba²⁺ has been investigated. X-ray diffraction study showed that crystal structure of Sr₄Al₁₄O₂₅ is preserved up to 20 mol of Ca²⁺ ion exchange while it is limited to 10 mol of Ba²⁺ ions exchange. Substantial shift in the emission band and color were observed by substitution of Sr²⁺ by Ca²⁺ or Ba²⁺ ions. A bluish-white emission and afterglow was observed at higher Ca²⁺ ions substitution. Further, partial Ca²⁺ substitutions (up to 0.8 mol) resulted in enhanced afterglow of Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphor. However, Ba²⁺ substitution decreased the fluorescence as well afterglow of the Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphor significantly. The enhanced phosphorescence by partial Ca²⁺ substitution is explained on the basis of increased density of shallow traps associated with higher solubility of Dy³⁺ ions in to the host lattice due to equivalent size of Ca²⁺ and Dy³⁺ ions. Thus, Ca²⁺ substitution in the Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphor is a promising method for tuning the emission color and improving the afterglow intensity of the phosphor.     

Synthesis and luminescent characteristic of Eu-doped (Gd,Lu)2O3 nanopowders

Eu³⁺ doped (Gd,Lu)₂O₃ nanopowders with particle sizes ranging from 20 to 70 nm were synthesized by the co-precipitant method using mixed precipitants, namely the mixture of ammonium hydroxide (NH₃⋅H₂O) and ammonium hydrogen carbonate (NH₄HCO₃). The precipitate precursor prepared by this method was believed to possess a basic carbonate composition and its thermal decomposition of the (Gd,Lu)₂O₃:Eu³⁺ powders were investigated by Thermogravimetric analysis and differential thermal analysis (TG-DTA). This preparation was followed by a calcination process at 800-1100 °C and corresponding phosphor structure were examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Photoluminescence measurement of the (Gd,Lu)₂O₃:Eu³⁺ particles show typical red emission at the 612 nm corresponding to the ⁵D₀ → ⁷F₂ transition. We found that the optimal Eu³⁺ molar doping concentration, calcined temperature and reaction time were 7 mol%, 1000 °C, and 2 h, respectively, which is helpful to obtain the final transparent ceramics with excellent properties.     

CaSc2O4:Eu: A tunable full-color emitting phosphor for white light emitting diodes

We report an intense full-color emission originating from ⁵D_0,1,2,3 to ⁷F_0,1,2,3,4 transitions of Eu³⁺ in CaSc₂O₄ upon 395 nm excitation. The emission spectra vary with increasing Eu³⁺ concentration, demonstrating tunable color coordinates from white to red region in the CIE chromaticity diagram. Considering the relaxation from ⁵D_J to ⁵D_J−1 through cross energy transfer, the Eu³⁺ concentration dependent emission spectra are well simulated based on the analysis of steady state rate equations and the measured lifetimes of the ⁵D_J levels. It is suggested that CaSc₂O₄:Eu³⁺ could be a potential single-phased full-color emitting phosphor for near-ultraviolet InGaN chip pumped white light emitting diodes.     

Luminescence properties of Ca4Y6(SiO4)6O:RE (RE = Eu, Tb, Dy, Sm and Tm) under vacuum ultraviolet excitation

The vacuum ultraviolet excited luminescent properties of Eu³⁺, Tb³⁺, Dy³⁺, Sm³⁺ and Tm³⁺ in the matrices of Ca₄Y₆(SiO₄)₆O were investigated. The bands at about 173 nm in the vacuum ultraviolet excited spectra were attributed to host lattice absorption of the matrix Ca₄Y₆(SiO₄)₆O. For Eu³⁺-doped samples, the O²⁻ → Eu³⁺ CTB was identified at 258 nm. Typical 4f-5d absorption bands in the region of 195-300 nm were observed in Tb³⁺-doped samples. For Dy³⁺-doped and Sm³⁺-doped samples, the broad excitation bands consisted of host absorptions, CTB and f-d transition. For Tm³⁺-doped samples, the O²⁻ → Tm³⁺ CTB was located at 191 nm. About the color purity and emission intensity, Ca₄Y₆(SiO₄)₆O:Tb³⁺ is an attractive candidate of green light PDP phosphor, and Ca₄Y₆(SiO₄)₆O:Dy³⁺ has potential application in the field of mercury-free lamps.     

Preparation and characterization of Sr4Al2O7:Eu, Eu phosphors

A new composition of red strontium aluminate phosphor (Sr₄Al₂O₇:Eu³⁺, Eu²⁺) is synthesized using a solid state reaction method in air and in a reducing atmosphere. The investigation of firing temperature indicates that a single phase of Sr₄Al₂O₇ is formed when the firing temperature is higher than 1300 °C and that a Sr₃Al₂O₆ phase is formed as the main peak below 1300 °C. The effects of firing temperature and doping concentration on luminescent properties are investigated. Sr₄Al₂O₇ phosphors exhibit the typical red luminescent properties of Eu³⁺ and Eu²⁺. A comparison photoluminescence study with Sr₃Al₂O₆ phosphor shows that Sr₄Al₂O₇ has higher emission intensity than Sr₃Al₂O₆ as a result of the higher optimum doping concentration of Sr₄Al₂O₇ phosphor.     

Syntheses of Mn3O4 and LiMn2O4 nanoparticles by a simple sonochemical method

Mn₃O₄ and LiMn₂O₄ nanoparticles were prepared by a simple sonochemical method which is environmentally benign. First, Mn₃O₄ nanoparticles were prepared by reacting MnCl₂ and NaOH in water at room temperature through a sonochemical method, operated at 20 kHz and 220 W for 20 min. Second, LiOH was coated onto the resulting Mn₃O₄ under the same sonochemical conditions as above. The thickness of coated LiOH on Mn₃O₄ obtained from the reaction ratio of 3:1 between LiOH and Mn₃O₄ was about 4.5–5.5 nm range. Then, by heating those LiOH-coated Mn₃O₄ particles at the relatively low temperature of 300–500 °C for 1 h, they were transformed into phase-pure LiMn₂O₄ nanoparticles of about 50 to 70 nm size in diameter.     

GeO2 dopant induced enhancement of red emission in CaAl12O19:Mn phosphor

In order to search efficient red-emitting phosphors for white LEDs application, CaAl₁₂O₁₉:Mn⁴⁺ phosphors have been prepared by a combustion method assisted with GeO₂ flux. The influence of GeO₂ concentration and annealing temperature on the structure and luminescence intensity for the phosphors has been investigated. The mechanism for luminescence enhancement has been discussed. At GeO₂ doping concentration of 1.5 mol%, the red emission intensity increases by 81% under 330 nm UVA excitation. More isolated luminescence center Mn⁴⁺ ions rather than pairs of Mn⁴⁺-Mn²⁺ ions are formed in the lattice with the introduction of GeO₂ at high temperature oxidation, leading to the enhancement of the red emission. A feasible new way to enhance the red emission in CaAl₁₂O₁₉:Mn⁴⁺ phosphor is obtained.