Phosphor For White Light-emitting Diodes Research Articles

Synthesis and luminescence properties of blue-emitting phosphor Ca12 Al14 O32 F2 :Eu2+ for white light-emitting diode.

A blue-emitting phosphor Ca12 Al14 O32 F2 :Eu2+ was synthesized using a high-temperature solid-state reaction under a reductive atmosphere. The X-ray diffraction measurements indicate that a pure phase Ca12 Al14 O32 F2 :Eu2+ can be obtained for low doping concentration of Eu2+ . The phosphor has a strong absorption in the range 270-420nm with a maximum at ~340nm and blue emission in the range 400-500nm with chromatic coordination of (0.152, 0.045). The optimal doping concentration is ~0.24. In addition, the luminescence properties of the as-synthesized phosphor were evaluated by comparison with those of Ca12 Al14 O32 Cl2 :Eu2+ and the commercially available phosphor BaMgAl10 O17 :Eu2+ . The emission intensity of Ca12 Al14 O32 F2 :Eu2+ was ~72% that of BaMgAl10 O17 :Eu2+ under excitation at λ=375nm. The results indicate that Ca12 Al14 O32 F2 :Eu2+ has potential application as a near-UV-convertible blue phosphor for white light-emitting diodes.

Energy transfer properties and enhanced color rendering index of chromaticity tunable green-yellow-red-emitting Y_3Al_5O_12: Ce^3+, Cr^3+ phosphors for white light-emitting diodes

White light-emitting diodes using YAG: Ce3+ phosphors suffer from the deficiency of red component, leading to a low color rendering index (CRI) and a high correlated color temperature (CCT). Herein, a yellow-red single-phase YAG: Ce3+, Cr3+ phosphor was synthesized by a traditional solid-state reaction. Compared with Cr3+ ion single-doped YAG phosphor, the emission intensity in the far-red region of the co-doped YAG: Ce3+, Cr3+ sample increases because of the energy transfer from Ce3+ ions to Cr3+ ions. For sample Y3Al5O12: 0.02Ce3+, 0.008Cr3+ phosphor, the internal and external quantum efficiencies are 58.9% and 46.7%, respectively. And, the fabricated white LED shows a CCT of 6085 K at CIE 1931 coordinate (0.3208, 0.3273). Moreover, the CRI is as high as 77.9 while that of the corresponding Ce3+ single-doped YAG phosphor is only 63.2. Thus, the Ce3+ and Cr3+ co-doped YAG phosphors are suitable for white light-emitting diodes (WLEDs).

Broad near-ultraviolet and blue excitation band induced dazzling red emissions in Eu3+-activated Gd2MoO6 phosphors for white light-emitting diodes

A series of Eu3+-activated Gd2MoO6 phosphors were synthesized via a citric acid-assisted sol–gel route.

Synthesis and Luminescence Properties of a Blue-Emitting Sr<sub>3</sub>NaLu(PO<sub>4</sub>)<sub>3</sub>F:Eu<sup>2+</sup> Phosphor

A novel blue-emitting Sr3NaLu (PO4)3F:Eu2+ phosphor for white light-emitting diodes (w-LEDs) have been successfully prepared via the solid-state reaction. The crystal structure and phase of the prepared phosphors were investigated using X-ray diffraction (XRD). The band gap of Sr3NaLu (PO4)3F was estimated to be about 5.79 eV from the diffuse reflection spectrum. The Sr3NaLu (PO4)3F:0.05Eu2+ phosphor exhibits an intense and broad excitation band ranging from 200 to 450 nm in the near ultraviolet region and produces a bright blue emission band with the CIE chromaticity coordinates of x = 0.133 and y = 0.148 under the excitation of 365 nm, which were ascribed to the 4f-5d transitions of Eu2+. Moreover, the fluorescence decay curves were also investigated in detail. All the results suggest that Sr3NaLu (PO4)3F:Eu2+ might be a promising blue-emitting phosphor used in w-LEDs.

The luminescent properties and crystal structure of Sr(1.5-x)-(1.5y) Mg0.5 SiO4 :xEu2+ ,yCe3+ blue phosphor synthesized by co-precipitation method.

In order to improve the luminescent performance of silicate blue phosphors, Sr(1.5-x)-(1.5y) Mg0.5 SiO4 :xEu2+ ,yCe3+ phosphors were synthesized using one-step calcination of a precursor prepared by chemical co-precipitation. The crystal structure and luminescent properties of the phosphors were analyzed using X-ray diffraction and fluorescence spectrophotometry, respectively. Because the activated ions (Eu2+ ) can occupy two different types of sites (Sr1 and Sr2 ), the emission spectrum of Eu2+ excited at 350nm contains two single bands (EM1 and EM2 ) in the wavelength range 400-550nm, centered at 463nm, and the emission intensity first increases and then decreases with increasing concentrations of Eu2+ ions. Co-doping of Ce3+ ions can greatly enhance the emission intensity of Eu2+ by transferring its excitation energy to Eu2+ . Because of concentration quenching, a higher substitution concentration of Ce3+ can lead to a decrease in the intensity. Meanwhile, the quantum efficiency of the phosphor is improved after doping with Ce3+ , and a blue shift phenomenon is observed in the CIE chromaticity diagram. The results indicate that Sr(1.5-x)-(1.5y) Mg0.5 SiO4 :xEu2+ ,yCe3+ can be used as a potential new blue phosphor for white light-emitting diodes.

Synthesis and luminescence properties of ZnMoO4:Eu3+,M+(M+ = Li+, Na+ and K+)phosphors

Zn1−1.5x MoO4:xEu3+ (x = 0.05, 0.10, 0.15, 0.20, 0.25) and Zn0.75MoO4:0.15Eu3+,0.05M+ (M+ = Li+, Na+, K+) phosphors were synthesized by the conventional solid state method in air. The XRD patterns of the phosphors indicate that the Eu3+ and M+ ions have successfully been introduced into the lattice. The luminescence intensity of ZnMoO4:Eu3+ phosphor decreases when the concentration of Eu3+ exceeds x = 0.15, the optimum dopant ion concentration. The charge compensator ions Li+, Na+, and K+ can improve the emission intensity of the ZnMoO4:Eu3+ phosphor with excitation at 394 nm and that of the Li+-doped system is greatest. The observed lifetimes for Eu3+ emission at 614 nm are 2.15, 3.46, 2.70 and 0.91 μs in ZnMoO4:Eu3+ and ZnMoO4:Eu3+,M+ (M = Li, Na, K), respectively. Hence, the Li+ co-doped ZnMoO4:Eu3+ phosphor has the longest lifetime. Under 394 nm excitation, the CIE coordinates of ZnMoO4:Eu3+,Li+ and ZnMoO4:Eu3+,Na+ are closer to those of ideal red light (0.67, 0.33) than those of commercial Y2O3:Eu3+ (0.625, 0.338). These experimental results are helpful in developing novel and high-efficiency red phosphors for white light-emitting diodes.

Cu–In–S/ZnS Quantum Dots Embedded in Polyvinylpyrrolidone (PVP) Solids for White Light-Emitting Diodes (LEDs)

Here, we report a facile and green route to prepare Cu–In–S (CIS)/ZnS core/shell quantum dots (QDs) embedded in polyvinylpyrrolidone (PVP) solids and their application as phosphors for white light-emitting diodes (WLEDs). CIS/ZnS core–shell QDs were easily synthesized in the aqueous phase, assisted by microwave irradiation within 20 min at 95 °C. The emission wavelength of CIS/ZnS QDs could be tunable between a large range of 543–700 nm by adjusting the [Cu]/[In] ratios, and the photoluminescence (PL) quantum yield could be up to 43% by coating a ZnS layer. QD-embedded PVP solids then were facilely fabricated by mixing CIS/ZnS QDs and PVP in water, followed by drying, to show improved PL intensity, higher photostability, and higher thermostability. Finally, we demonstrated their potential application as a white light source by using green- and red-emitting CIS/ZnS/PVP nanocomposite solids as color converters, in combination with a blue light-emitting diode (LED) chip.

Synthesis and luminescence properties of MgO: Sm3+ phosphor for white light-emitting diodes

MgO: xSm3+ (0.02 ≤ x ≤ 0.08) red phosphors are prepared by low temperature solution combustion method. The MgO: XSm3+ phosphor characterized by X-ray diffraction (XRD), FT-IR, field emission scanning electron microscopy (FE-SEM), luminescence and decay time measurements. The results of XRD and FE-SEM show that resultant samples are single phase. The vibration properties MgO phosphor was studied by Fourier transform infrared spectroscopy. The luminescence spectra exhibits four emission bands at 566, 603, 651 and 708 nm corresponding to the various transition from 4G5/2 ground state to lower lying levels 6H5/2, 6H7/2, 6H9/2 and 6H11/2 states, with an excitation wavelength at 404 nm. The chromaticity coordinates have been estimated from the emission spectra and the suitability of title phosphor for white light applications. The lifetime corresponding to the 4G5/2 level of the title phosphor has been found to decrease with the increase in Sm3+ ion concentration.

White-Light Emission From GaN-Based TJ LEDs Coated With Red Phosphor

We report the fabrication of white light-emitting diode (LED) lamps by coating red phosphor onto the green/blue dual-color tunnel-junction (TJ) LED chips. For the dual-color TJ LED chips without phosphor coating, it was found that electroluminescence intensity of the green peak increased faster than that of the blue peak, as we increased the injection current. Under the same injection current, it was found that power consumption and heat generation were larger for the TJ LEDs, as compared with the conventional blue LED chips. By coating 0.1-g red phosphor onto the $660 \times 760~\mu \text{m}^{2}$ dual-color TJ LED chips, we achieved white LED lamps with color temperature, $T_{C}$ , of 4102 K, color-rending index, CRI, of 68, and the Commission Internationale de L’Eclairage color coordinates of (0.32, 0.36). Furthermore, it was found that $T_{C}$ observed from the dual-color TJ LED + red phosphor white LED lamps were smaller than those observed from the blue LED+yellow phosphor white LED lamps.

Understanding the Local and Electronic Structures toward Enhanced Thermal Stable Luminescence of CaAlSiN3:Eu2+

It is a great challenge to maintain thermally stable luminescence of red phosphors in white light-emitting diodes (LEDs), because of the large Stokes shift. For the purpose of overcoming this challenge, this work elucidates the intrinsic mechanism of the thermal quenching luminescence of CaAlSiN3:Eu2+. The empty 5d orbital of Eu2+ is partly filled with electrons upon Eu2+ increasing, as observed using XANES; and the exceptional expansion of the local Eu–N bond length, the ratio of which is far larger than the volume expansion of crystal lattice brought by doping Eu2+, is measured using EXAFS. The shift of Fermi level predicted with the first-principles calculations is confirmed by the valence band spectra. Therefore, the changeable distribution of electrons on the excited substates and then thermal delocalization to the conduction band are the intrinsic mechanisms of thermal quenching luminescence of CaAlSiN3:Eu2+. The results provide a solid basis for exploring the methods to enhance the thermal stable l...

Novel single-phase white-emitting phosphor Sr6Gd2Na2(PO4)6F2:Eu2+,Mn2+: band gap, luminescence, and energy transfer

White-emitting phosphors Sr6Gd2Na2(PO4)6F2:Eu2+,Mn2+ (SGNPF:Eu2+,Mn2+) were synthesized under a weak reductive atmosphere. Phase purity was examined based on XRD patterns, and results showed that the obtained SGNPF:Eu2+,Mn2+ phosphors were well crystallized with no impurity phase. The emission spectra of SGNPF:Eu2+,Mn2+ phosphor exhibited broad blue and broad yellow emission bands with two peaks at approximately 460 and 560 nm under 300 nm excitation. The energy transfer process from Eu2+ to Mn2+ was verified through luminescence spectra and fluorescence decay curves. The energy transfer from Eu2+ to Mn2+ in SGNPF was demonstrated to be a resonant type via a dipole–dipole mechanism. Adjusting the relative contents of Eu2+ and Mn2+ could change the emitting colors of SGNPF:Eu2+,Mn2+ phosphors from blue and azure to white, and the ideal white light-emitting phosphor SGNPF:0.05Eu2+,0.25Mn2+ with color coordinate (0.29, 0.31) was obtained. All investigations indicated that the SGNPF:Eu2+,Mn2+ phosphors exhibit great potential as a single component white-emitting phosphor for white light-emitting diodes.

Facile fabrication and luminescence characteristics of Ce:YAG phosphor glass thick films coated on a glass substrate for white LEDs

A pivotal step in providing a better fluorescent material that has high luminous efficacy and excellent thermal stability is to utilize inexpensive phosphors for white light-emitting diodes (W-LEDs). Herein, we demonstrate a feasible tape-casting technique for creating phosphor thick films that consist of Ce: YAG phosphor embedded in relatively low melting point glass frits on an ultrathin glass substrate with controllable film thickness. The glass matrix has ideal densification and interfaces with the glass substrate at a relatively low temperature of 580°C. Subsequently, the structure and optical properties of the phosphor layer are investigated. In addition, the effect of the phosphor concentration, thick film thickness and location (top or bottom) of the phosphor layer on the photoluminescence properties and chromaticity are also discussed with respect to use in W-LEDs. Significantly, this promising structure has excellent thermal stability and the potential to overcome current limitations of phosphors in high-power W-LEDs. Finally, a high-performance W-LED based on the planar phosphor glass exhibits a luminous efficiency of 108.45lmW−1, a correlated color temperature of 5408K and a color rendering index of 76.

Ce3+-activated Ca8Zn(SiO4)4Cl2: A novel near-ultraviolet converting blue-emitting chlorosilicate phosphor for white light-emitting diodes

A series of Ce3+-activated chlorosilicate phosphors, Ca8-xZn(SiO4)4Cl2: xCe3+, was firstly synthesized by solid-state reactions. The crystal structure and luminescence properties of the phosphors, as well as their thermal luminescence quenching capabilities and CIE chromaticity coordinates were systematically investigated. Studies revealed the incorporation of Ce3+ into 8-coordinated Ca site, thereby favoring the novel blue luminescence corresponding to the electric-dipole allowed 4f°5d1–4f1 transition of the Ce3+ ions. Furthermore, the dipole-dipole interaction was dominantly involved in the mechanism of concentration quenching of Ce3+ in the phosphors, and the activation energy for thermal quenching (ΔE) was calculated to be 0.17eV. Compared with the commercial BaMgAl10O17: Eu2+ phosphor, the composition-optimized Ca7.97Zn(SiO4)4Cl2: 0.03Ce3+ showed a preferable light yield with chromaticity coordinates of (0.156, 0.028) and color temperature of 1896K. Because of the strong absorption in the near-ultraviolet region and the intense blue emission peaking at 411nm, our developed phosphors exhibit great potential for use as the promising blue-emitting phosphors for white light-emitting diodes.

Near-Green-Emitting Tb3+-Doped Transparent Glass Ceramics Containing Ba2LaF7 Nanocrystals for Application in White Light-Emitting Diodes

Tb3+-activated transparent glass ceramics containing Ba2LaF7 were successfully synthesized by a conventional melt–quenching technique, and the glass ceramics displayed a near-green emission under near-ultraviolet excitation. Differential scanning calorimetry and X-ray diffraction were used to monitor the evolution and microstructural changes in the prepared glass specimens. The transparency levels of the glass ceramics remained high with increasing treatment temperatures from 640 to 690°C. Investigations on the luminescence properties of the glass ceramics revealed that Tb3+ ions were gradually incorporated into the precipitated fluoride crystalline phase. After heat treatment at 640°C for 2 h, major emission was observed at 543 nm owing to 5D4 → 7F5 transition of Tb3+ under 373-nm excitation. Current findings indicate that the prepared materials have potential as replacements to commonly employed green phosphors in white light-emitting diodes. Additionally, the decay lifetimes of Tb3+ ions at 543 nm under 373-nm excitation were studied.

Salt-assisted combustion synthesis of Ca-α-SiAlON:Eu2+ phosphors

In this study, Ca-α-SiAlON:Eu2+ yellow phosphors for white light-emitting diodes (LEDs) were synthesized via a facile combustion synthesis method by using CaO, Eu2O3, Si, and Al as raw materials, while NaCl and Si3N4 were utilized as diluents to contain the combustion temperature. The concentration of Eu was varied, and the resulting changes in phase purity, photoluminescence properties, and thermal quenching were investigated. We observed that the increase in lattice constant depended on the amount of Al, while the Ca sites were partially substituted by Eu ions. The synthesized Ca-α-SiAlON:Eu2+ powders exhibited emission wavelength at 556–560 nm and high thermal stability, revealing high potential for application in white LEDs.

Synthesis and photoluminescence properties of NaCaPO4:Eu2+ phosphor for solid state lighting

A novel method to prepare orthophosphate NaCaPO4:Eu2+ phosphor for white light-emitting diodes (w-LEDs) is given in this paper. Phosphor was successfully synthesized by Pechini (citrate gel) method which is efficient than conventional high temperature solid state reaction. X-ray powder diffraction (XRD) analysis confirmed the single phase formation of NaCaPO4:Eu2+ with orthorhombic crystal structure (also called as rhenanite or β-NaCaPO4). Luminescence results showed that the phosphor could be efficiently excited by near UV with intense excitation peak at 394nm and exhibited two bright red emission narrow bands in the wavelength range 575nm to 640nm corresponding to 5D0→7F1and 5D0→7F2 transitions of Eu3+ respectively in NaCaPO4:Eu3+ with maximum intensity peak at 595nm without reduction. When Eu3+ in NaCaPO4:Eu3+ is reduced by carbo-thermal reduction to Eu2+ it gives intense broad band emission at 563nm resulting green emitting phosphor. Hence both of phosphors could be useful for solid state lighting and light-emitting diode (LED) application for fabrication of w-LED’s.

Synthesis and Luminescence Studies of Sm3+ Doped Ca9Y(PO4)7 Orange-Red-Emitting Phosphor.

Ca9Y(PO4)7:Sm3+ phosphor was prepared via solid state reaction. The crystal structure of the phosphor was characterized by X-ray powder diffraction (XRD). The photoluminescence (PL) properties of CagY(PO4)7:Sm3+ were investigated. The emission of the Ca9Y(PO4)7:Sm3+ phosphor consisted of some sharp emission peaks of Sm3+ centered at 563 nm, 600 nm, and 647 nm. The strongest one is located at 600 nm due to the 4G5/2-6H7/2 transition of Sm3+. The critical distance was calculated to be 10.5 A. The chromatic properties of the sample Ca9Y(PO4)7:Sm3+ phosphor were located in the orange reddish region. The CagY(PO4)7:Sm3+ phosphor may be potentially used as red phosphor for white light-emitting diodes (WLED).

Effects of Ti4+ Ions on Fluorescence Properties of Sr2CeO4:Sm3+ Phosphors.

Ti4+-doped Sr2CeO4:Sm3+ phosphors were synthesized with the solid-state reaction method and the effects of doping Ti4+ on the photoluminescence properties were investigated in detail. A broad excitation band ascribed to the O2- -Ce4+ transition was observed in the range of 200 to 400 nm and with doping Ti4+ nto Ce4+ sites, the intensity of charge transfer band of O2- --> Ce4+ (300-370 nm) was significantly broaden and enhanced. As a result, the emission intensity of Sr2Ce1-xTixO4 has improved about 85% by doping 0.01 mol Ti4+. White emission of Sr2-yCe0.99Ti0.01O4:ySm3+ at y < 0.03 is due the co-existence of Ce4+ --> O2- CT emission and 4G5/2-6HJ Sm3+ transitions whereas only the Sm3+ red emission prevails for 0.03 < y ≤ 0.15. The Sr1.99Ce0.99Ti0.01O4:0.01 Sm3+ phosphor exhibited excellent color purity. Its chromaticity coordinate is measured to be (0.326, 0.322), which is close to the pure white (0.33, 0.33). The results showed that Sr1.99Ce0.99Ti0.01O4: 0.01 Sm3+ phosphors could be considered as a potential single-phase white-emitting phosphor for white light-emitting diodes.

Red emission enhancement by strong electronegativity in Na5Y4(SiO4)4F:Eu3+ phosphor for white light-emitting diodes

A red-emitting Eu3+ doped Na5Y4(SiO4)4F fluorosilicate phosphor with tetragonal structure was synthesized by conventional solid state reaction method. The crystal structure, photoluminescence properties and quenching concentration of Na5Y4(SiO4)4F:Eu3+ were investigated in detail. Excited by near-ultraviolet light at 397 nm, the dominant red emission in Na5Y4(SiO4)4F:Eu3+ phosphor is located at 615 nm, which is from the hypersensitive 5D0–7F2 transition of Eu3+ ion. The optimum doping concentration of Eu3+ ion into the host material is up to 50 %, based on which the critical transfer distance is calculated to be 7.72 A. The large transfer distance inhibits the emission quenching. Importantly, through the comparison with NaYSiO4:Eu3+, the intense red emission is attributed to the effect of the F− ion electronegativity on the distorted local crystal environment. All the results show that Na5Y4(SiO4)4F:Eu3+ could be a potential red phosphor for near-ultraviolet white light-emitting diodes.

Photoluminescence of Vanadate Garnet Ca2NaMg(2-x)V3O12:xEu3+ Phosphors Synthesized by Solution Combustion Method.

In this study, a series of nano-sized Ca2NaMg(2-x)V3O12:xEu3+ (0.06 < or = x < or = 0.18) red phosphors is synthesized by solution combustion method. The microstructure and photoluminescence properties of the Ca2NaMg(2-x)V3O12:xEu3+ phosphors are studied in accordance with the Eu3+ content. Annealed Ca2NaMg(2-x)V3O12:xEu3+ phosphors form a single phase with the cubic garnet structure and Ia3d space group. The emission from (VO4)3- in the Ca2NaMg(2-x)V3O12:xEu3+ phosphors is almost completely quenched due to the efficient energy transfer from the (VO4)3- to the Eu3+ ions. The emission intensity increases sharply with the increased Eu3+ content, reaching a maximum value at x = 0.15, and then decreases with further Eu3+ content. Ca2NaMg1.85V3O12:0.15Eu3+ shows great potential as a red phosphor for white light-emitting diodes.