Synthesis and photoluminescence properties of new NaAlSiO4:Eu phosphors for near-UV white LED applications

A new NaAlSiO₄:0.1Eu²⁺ phosphors were synthesized at different temperatures using a liquid phase precursor (LPP) technique. The XRD patterns indicate the presence of hexagonal nepheline phase for all the samples. The synthesized phosphors can be excited efficiently in the broad near-UV region. The PL emission spectra showed a broad emission peak at around 551 nm corresponding to 5d → 4f transition of Eu²⁺ ions. The synthesized phosphors showed better thermal stability when compared with the standard YAG:Ce³⁺ phosphor.     

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.     

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.     

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.     

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.     

Enhanced red emission in ZnB2O4:Eu by charge compensation

Eu³⁺ doped ZnB₂O₄ without or with different charge compensation (CC) approaches (co-doping Li⁺, Na⁺, K, decreasing the content of Zn²⁺) were prepared by solid state reactions. The phosphors can strongly absorb 393 nm ultraviolet (UV) light which is coupled well with the emission of currently used InGaN-based near UV light emitting diodes (LEDs) and emit red light with a good color purity. The luminescent intensity of phosphors can be remarkably enhanced with any of CC methods. However, the shape and position of excitation and emission spectra keep unchanged. The introduction of Li⁺ can enhance the red emission intensity of Eu³⁺ by ∼4 times with the optimal effect. Red emission of Eu³⁺ can also be enhanced with the other three CC approaches but the effects are not as good as Li⁺ because the volume unbalance in Li⁺ compensation approach is the smallest while net positive charge was offset. The results of this work suggest that volume compensation and equilibrium of mole number should also be taken into account when a CC approaches is selected.     

Luminescent properties of Eu doped α-Gd2(MoO4)3 phosphor for white light emitting diodes

A novel red emitting phosphor α-Gd₂(MoO₄)₃:Eu³⁺ was developed for white light emitting diodes (LEDs). The phosphor was prepared by solid-state reaction. The effects of the flux content and the activator concentration on the crystal structure, morphology and luminescent properties were investigated by using XRD, SEM, and fluorescent spectra. These results showed that this phosphor can be effectively excited by ultraviolet (UV) (395 nm) and blue (465 nm) light, matching the output wavelengths of ultraviolet or blue LED chips. The α-Gd₂ (MoO₄)₃ phosphor may be a better candidate for solid state lighting application.     

Synthesis and luminescent properties of LaPO4:Eu microspheres

LaPO₄:Eu³⁺ microspheres were synthesized, using LaCl₃, EuCl₃ and (NH₄)₂HPO₄ as starting materials. The morphology, formation mechanism, and luminescent property of samples were systemically studied. X-ray diffraction (XRD) and infrared spectroscopy (IR) show that LaPO₄:Eu³⁺ microspheres have a pure monoclinic phase. Cetyltrimethyl ammonium bromide (CTAB) usually forms spherical micelles above a critical micelle concentration, which plays an important role in the formation of LaPO₄:Eu³⁺ microspheres. The excitation spectrum of LaPO₄:Eu³⁺ microspheres consists of several sharp lines due to the direct excitation of the Eu³⁺ cations from the ground state to higher levels of the 4f-manifold. The emission intensity of microspheres is higher than irregular particles because of the lowlier surface area. The lifetimes of Eu³⁺ ions in the LaPO₄:Eu³⁺ microspheres are determined to be 2.41 ms.     

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.     

Novel blue-emitting phosphor NaMg4(PO4)3:Eu, Ce with energy transfer

A blue-emitting phosphor of NaMg₄(PO₄)₃:Eu²⁺, Ce³⁺ was prepared by a combustion-assisted synthesis method. The phase formation was confirmed by X-ray powder diffraction measurement. Photoluminescence excitation spectrum measurements show that the phosphor can be excited by near UV light from 230 to 400 nm and presents a dominant luminescence band centered at 424 nm due to the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ions at room temperature. Effective energy transfer occurs in Ce³⁺/Eu²⁺ co-doped NaMg₄(PO₄)₃ due to large spectral overlap between the emission of Ce³⁺ and excitation of Eu²⁺. Co-doping of Ce³⁺ enhances the emission intensity of Eu²⁺ greatly by transferring its excitation energy to Eu²⁺, and Ce³⁺ plays a role as a sensitizer. Ce³⁺-Eu²⁺ co-doped NaMg₄(PO₄)₃ powders can possibly be applied as blue phosphors in the fields of lighting and display.     

Dynamical processes of energy transfer in red emitting phosphor CaMoO4:Sm, Eu

Upon ⁴K_11/2 excitation of Sm³⁺ at 405 nm, the performance of energy transfer from Sm³⁺ to Eu³⁺ in the red emitting phosphor CaMoO₄:Eu³⁺, Sm³⁺ significantly extends its excitation region for better matching the near-UV LED. Photoluminescence spectra indicate that the energy transfer pathway concerns the relaxation from ⁴K_11/2 to ⁴G_5/2 of Sm³⁺ and subsequent transfer to ⁵D₀ of Eu³⁺ rather than ⁵D₁ of Eu³⁺. The fluorescent decay pattern of Sm³⁺⁴G_5/2 level in CaMoO₄:0.5% Sm³⁺, 2% Eu³⁺ is studied at 77 K based on the Inokuti-Hirayama formula, revealing an electronic dipole-dipole interaction between Sm³⁺ and Eu³⁺. The coefficient for the energy transfer is obtained to be 8.5 × 10⁻⁴⁰ s⁻¹ cm⁶. The fluorescence rise and decay pattern of Eu³⁺⁵D₀ level as Sm³⁺ is only excited at 77 K is well described by the dynamical processes of the energy transfer.     

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.     

A red-emitting heavy doped phosphor Li6Y(BO3)3:Eu for white light-emitting diodes

A series of Eu³⁺ activated Li₆Y_1−xEu_x(BO₃)₃ (0.05 ? x ? 1) phosphors were synthesized by solid-state reaction method. The structures and photoluminescent properties of the phosphors were investigated at room temperature. The results of XRD patterns indicate that these phosphors are isotypic to the monoclinic Li₆Gd(BO₃)₃. The excitation spectra indicate that these phosphors can be effectively excited by near UV (370-410 nm) light. The red emission from transition ⁵D₀→⁷F₂ is dominant. The emission spectra exhibit strong red performance (CIE chromaticity coordinates: x = 0.65, y = 0.35), which is due to the ⁵D₀−⁷F_J transitions of Eu³⁺ ions. The relationship between the structure and the photoluminescent properties of the phosphors was studied. The concentration quenching occurs at x ≈ 0.85 under near UV excitation. Li₆Y(BO₃)₃:Eu³⁺ has potential application as a phosphor for white light-emitting diodes.     

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.     

Electro-deposition of Y2O3:Eu thin film phosphor and its luminescent properties

Y₂O₃:Eu³⁺ red-emitting thin film phosphor was prepared by a two-step process: the cathodical deposition of thin film of yttrium hydroxide and europium hydroxide followed by an annealing process to achieve Eu³⁺ doped Y₂O₃ film. It is found that the atomic content of Eu³⁺ can be well controlled by simply adjusting the volume ratio of Y(NO₃)₃ to Eu(NO₃)₃ solutions. Dependence of the photoluminescence intensity on the atomic content of Eu³⁺ in Y₂O₃ was also studied. The best photoluminescence performance of Y₂O₃:Eu³⁺ thin film phosphor was achieved as atomic content of Eu³⁺ equal to 1.85 at.%.     

Luminescence properties of Dy-doped Li2SrSiO4 for NUV-excited white LEDs

A series of single-phase full color phosphors, Dy³⁺-doped Li₂SrSiO₄ was synthesized by a solid-state reaction method. The phase of the as-prepared powders was measured by X-ray diffraction pattern (XRD) and the chemical composition was characterized using energy dispersive spectroscopy (EDS). The luminescent properties of Li₂SrSiO₄:Dy³⁺ were systematically investigated by concentration quenching, decay behavior and thermal stability measurements. The results suggested that the emission intensity of the Li₂SrSiO₄:Dy³⁺ was much stronger than that of Li₂SrSiO₄:Eu²⁺. It was worth to mention that Li₂SrSiO₄:Dy³⁺ phosphor possessed excellent thermal stability for use in light-emitting diodes (LEDs) and the emission intensity measured at 300 °C was only decreased 8% comparing with that measured at room temperature. Furthermore, the Commission International del’Eclairage (CIE) chromaticity coordinates of Li₂SrSiO₄:Dy³⁺ moved toward the ideal white light coordinates (0.33, 0.33). All results demonstrated that Li₂SrSiO₄:Dy³⁺ might be a potential phosphor for NUV-based white light-emitting diodes.     

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.     

A new yellowish green luminescent material SrMoO4:Tb

A novel yellowish green phosphor tervalent terbium (Tb³⁺) doped strontium molybdate (SrMoO₄) was synthesized by conventional solid-state reaction method and its crystal structure and luminescent properties are investigated in this paper. The X-ray diffraction patterns (XRD) showed that the phosphor sintered at 750 °C for 3 h was a pure SrMoO₄ phase. The excitation spectrum consisted of two bands and the two excitation peaks located at 375 nm and 488 nm respectively. The emission spectrum was composed of four narrow bands, in which the strongest emission was located at 548 nm. The particle size analysis indicated that the median particle size D₅₀ = 2.89 μm and range of particle size distribution was narrow. These results showed that the SrMoO₄:Tb³⁺ phosphor was a promising yellowish green phosphor for ultraviolet light emitting diode (UVLED) and blue LED based white LED. The appropriate concentration of Tb³⁺ was 5 mol% for the highest emission intensity at 548 nm. Natrium ion (Na⁺) was found to be a promising charge compensator for SrMoO₄:Tb³⁺ phosphor.     

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.