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.     

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.     

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.     

Effects of Zn doping on the structural and luminescent properties of Sr4Si3O8Cl4:Eu phosphors

Sr₄Si₃O₈Cl₄: Eu²⁺ phosphors were synthesized by the solid-reaction at high temperature. The emission intensity reaches a maximum at 0.08 mol% of Eu²⁺ concentration. The present paper mainly focused on the effects of Zn²⁺ on the crystallization behavior and photoluminescence (PL) properties of Sr₄Si₃O₈Cl₄:0.08Eu²⁺. Results suggested that no new phase is introduced by co-doping with a small amount of Zn²⁺ ions, but when co-doped with excessive amount of Zn²⁺ ions, Sr₂ZnSi₂O₇ appears. We find that the co-doping of a small amount of Zn²⁺ could remarkably improve the PL intensity of Sr₄Si₃O₈Cl₄:0.08Eu²⁺. When x = 0.05, the intensity of Sr₄Si₃O₈Cl₄:0.08Eu²⁺,xZn²⁺ was increased up to 2.3 times that of pure Sr₄Si₃O₈Cl₄:0.08Eu²⁺, which could be attributed to the flux effect of Zn²⁺ ions, and the Zn²⁺ doping reduces the opportunities of the energy transfer between Eu²⁺.     

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.     

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.     

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.     

Sol-gel composition of multicomponent hybrid precursors to long afterglow of CaxSr1 − xAl2O4: Eu phosphors

Ca_xSr_1 − xAl₂O₄: Eu²⁺ photoluminescent materials with high brightness and long afterglow were in situ synthesized by hybrid precursor assembly sol-gel technology in a reductive atmosphere. The particle size of luminescent materials is in the range of 30-60 nm by the estimation of XRD. And SEM shows that there exists uniform morphology and microstructure owing to the hybrid precursors. The influence of co-doping Ca²⁺ and Sr²⁺ on the luminescence of the phosphor was studied. Their excitation and emission spectra were very similar to that of SrAl₂O₄: Eu²⁺ phosphors and all of them have long afterglow phenomenon. Changing the co-doping concentrations of Ca²⁺ and Sr²⁺ in Ca_xSr_1 − xAl₂O₄: Eu²⁺ phosphors, the luminescent intensities are different. When the proportion of Ca and Sr is 6 to 4, the phosphor reaches the strongest emitting intensity.     

Novel blue-emitting Sr3Ga2O5Cl2:Eu phosphor for UV-pumped white LEDs

A novel blue-emitting Sr₃Ga₂O₅Cl₂:Eu²⁺ phosphor has been synthesized by a two-step solid-state reaction. The luminescence properties have been investigated by photoluminescence (PL) spectra, and temperature-dependent PL spectra. It shows an efficient broad absorption band around 400 nm, which matches well with the commercial near-ultraviolet light-emitting chips, and an efficient blue emission. It shows a higher thermal quenching temperature than that of Sr₃Al₂O₅Cl₂:Eu²⁺ phosphor. Sr₃Ga₂O₅Cl₂:Eu²⁺ phosphor is a promising blue-emitting component for UV chip excited white light-emitting-diodes.     

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.     

Synthesis of CaAl2O4:Eu,Nd long persistent phosphor by combustion processes and its optical properties

Eu²⁺,Nd³⁺ co-doped calcium aluminate with high brightness and long persistent luminescence was prepared by the combustion method. The luminescent properties of CaAl₂O₄-based luminescent materials have been studied systematically. The phosphor powders were further investigated by X-ray diffractometer (XRD), photoluminescence excitation and emission spectra (PL) and brightness meter. The analytical results indicated that the phase of CaAl₂O₄ was formed when the initiating combustion temperature was 400 °C. The broad band UV excited luminescence of the CaAl₂O₄:Eu²⁺,Nd³⁺ was observed at the blue region (λ_max = 440 nm) due to transitions from the 4f⁶5d¹ to the 4f⁷ configuration of the Eu²⁺ ion. The decay time of the persistence indicated that the persistent luminescence phosphor has bright phosphorescence and maintains a long duration.     

A novel red phosphor NaLa4(SiO4)3F: Eu

A novel red phosphor NaLa₄(SiO₄)₃F: Eu³⁺ was synthesized by the conventional solid-state reaction at 950 °C for the first time. The luminescence properties of NaLa₄(SiO₄)₃F: Eu³⁺ were investigated, and the critical concentration of the activator concentration (Eu³⁺) was found to be 0.1 mol per formula unit. The phosphor presented red luminescence under the ultraviolet excitation of 254 or 395 nm, attributed to the transitions from ⁵D₀ excited states to ⁷F_J ( J = 0-4) ground states of Eu³⁺ ions. The results indicated that this newly-developed phosphor could find applications in tricolor fluorescent lamp, phosphor-liquid crystal displays and white lighting devices utilizing GaN-based excitation in the near UV.     

Mechanism of energy transfer in Sr2CeO4:Eu phosphor

Blue–white phosphor Sr₂CeO₄ belongs to a particular class of optical materials whose luminescence is governed by optical transitions associated with the electron charge transfer. The originality of its crystallographic structure, a chain-like sequence of luminescent centers, permits an effective transfer of the electronic excitation energy from the host to doped centers. Sr₂CeO₄, рure and doped with Eu³⁺-ions of different concentrations, was synthesized by the Pechini citrate-gel method. The luminescence spectra and luminescence decay curves of Sr₂CeO₄ and Sr₂CeO₄:Eu³⁺ at 300 and 80 K were investigated. The performed experiments revealed the Förster nonradiative energy transfer under the energy migration condition from the crystal host to the doped europium ions.     

Synthesis and luminescent properties of SrAl2O4:Eu green-emitting phosphor for white LEDs

SrAl₂O₄:Eu²⁺ phosphor was prepared by a solid-state reaction in CO-reductive atmosphere. X-ray powder diffraction (XRD) analysis confirmed the formation of SrAl₂O₄:Eu²⁺. Field-emission scanning electron-microscopy (FE-SEM) observation indicated that the microstructure of the phosphor consisted of irregular fine grains with an average size of about 7–8 μm. Photoluminescence measurements showed that the phosphor can be efficiently excited by UV–visible light from 350 to 430 nm, and exhibited bright green emission peaked at about 516 nm. Bright green LEDs were fabricated by incorporating the phosphor with an InGaN-based UV chip. All the characteristics indicated that SrAl₂O₄:Eu²⁺ is a good candidate phosphor applied in white LEDs.     

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.     

A novel red emitting phosphor CaIn2O4:Eu, Sm with a broadened near-ultraviolet absorption band for solid-state lighting

Red phosphor of CaIn₂O₄:Eu³⁺, Sm³⁺ is synthesized by solid state reaction. The ⁵D₀ → ⁷F₂ transition of Eu³⁺ is dominantly observed in the photoluminescence spectrum, leading to a red emission of the phosphor. The doped Sm³⁺ is found to be efficient to sensitize the emission of Eu³⁺ and be effective to extend and strengthen the absorption of near-UV light with wavelength of 400-405 nm, and the energy transfer from Sm³⁺ to Eu³⁺ occurs and is discussed. The effect of the molar concentration of Sm³⁺ on the emission intensities of the phosphor CaIn₂O₄:Eu³⁺, Sm³⁺ is investigated. The temperature quenching effect is also measured from room temperature to 425 K, and the emission intensity of the phosphor at 425 K shows about 85% of that at room temperature. Furthermore, the chromaticity coordinates, the emission intensities and the conversion efficiencies of CaIn₂O₄:Eu³⁺, Sm³⁺ are compared to those of the conventional red phosphor of Y₂O₂S:Eu³⁺.     

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.     

Luminescence properties of Eu-activated Ca12Al10.6Si3.4O32Cl5.4: A promising phosphor for solid state lighting

In this article, we synthesized and characterized a novel bluish green phosphor for white light-emitting diodes, Eu²⁺-activated Ca₁₂Al_10.6Si_3.4O₃₂Cl_5.4. The phosphor shows broad and strong absorption in the region (320-450 nm), which is essential for improving the efficiency and quality of white light-emitting diodes. When excited at 380 nm, the phosphor shows two emission bands at around 425 and 500 nm. The main emission peak of Eu²⁺-activated Ca₁₂Al_10.6Si_3.4O₃₂Cl_5.4 exhibits red shift in comparison with that of Eu²⁺-activated Ca₁₂Al₁₄O₃₃, which is due to the introduction of Si and Cl ions. The results show Ca₁₂Al_10.6Si_3.4O₃₂Cl_5.4 is a promising host candidate for the phosphors.     

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.     

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.