Phosphors For White LED Applications Research Articles

Ge4+, Eu3+‐codoped Y2SiO5 as a novel red phosphor for white LED applications

The phosphors Y2SiO5: Ge4+, Eu3+ were synthesized by solid state reaction method assisted by flux, and have been characterized by X‐ray powder diffraction and fluorescence spectrometer. The results showed that the phosphors can be effectively excited by near‐UV (394 nm), and the major peak is located at 611 nm ascribed to the electric‐dipole 5D0 → 7F2 transition of Eu3+, the critical quenching concentration of Eu3+ in the phosphor is determined to be 15 mol% and the critical transfer distance is calculated as 8.90 Å. Co‐doping Y2SiO5: Eu3+ with Ge4+ helps to improve the luminescence intensity and color purity. The red emission of the phosphor under 394 nm excitation shows a good chromaticity index (0.652, 0.347) compared to commercial red phosphors Y2O2S: Eu3+ (0.631, 0.350). The quantum efficiency of the Y2Si0.97O5: 0.03Ge4+, 0.15Eu3+phosphor under 394 nm excitation is estimated to be 45.24%. It can be concluded that efficient red light emitting diodes were fabricated using Ge4+, Eu3+ co‐doped phosphor based on near ultraviolet(NUV) excited LED lights.

Luminescence properties of Ca2+ and Si4+ co-doped strontium molybdate red phosphors for white LEDs

Sr0.85−xCaxMoO4:Eu0.153+ (0.15 ≤ x ≤ 0.45) and Sr0.6Ca0.25(Mo1−ySiy)O4:Eu0.153+ (0 ≤ y ≤ 0.08) red phosphors materials for white LEDs were synthesized by the high temperature solid state reaction. The properties were characterized by means of X-ray diffraction (XRD) and photoluminescence spectra. The XRD pattern shows that the Sr0.85−xCaxMoO4:Eu0.153+ and Sr0.6Ca0.25(Mo1−ySiy)O4:Eu0.153+ phosphors sintered at 800 °C for 5 h are SrMoO4 phase. And the samples are crystallized well with the scheelite structure. The luminescent properties of Sr0.85−xCax(Mo1−ySiy)O4:Eu0.153+ with various concentration of Ca2+ and Si4+ were investigated in detail. The results indicate that Sr0.85−xCaxMoO4:Eu0.153+ and Sr0.6Ca0.25(Mo1−ySiy)O4:Eu0.153+ phosphors can be excited by 394 and 464 nm lights and their light emitting properties were improved by co-doping Si4+ and Ca2+ ions. The emission spectra exhibit the most intense emission at 611 and 615 nm, and the latter is corresponding to the 5D0 → 7F2 transition of the Eu3+. Compared with Sr0.85−xCaxMoO4:Eu0.153+, the emission intensity of Sr0.6Ca0.25(Mo1−ySiy)O4:Eu0.153+ is increased obviously. It is found that appropriate Ca2+ and Si4+ concentration can enhance the luminescent intensity and improve crystallization, and the highest emission intensity on the 615 nm is exhibited when x = 0.25 and y = 0.05. Moreover, the CIE chromaticity (x, y) of Sr0.85−xCax(Mo1−ySiy)O4:Eu0.153+ excited by 394 nm light are analyzed. When the doping x of Ca2+ is 0.02 and y of Si4+ is 0.05, phosphors of the samples turn from pink to red. These results present that the Sr0.85−xCax(Mo1−ySiy)O4:Eu0.153+ is an efficient red emitting phosphor for white LED applications.

Photoluminescence properties of Eu3+-and Tb3+-doped YAlO3 phosphors for white LED applications

Eu3+-and Tb3+-doped YAlO3 phosphors are prepared by solid-state reaction method and their photoluminescence properties are investigated as functions of the Eu3+and Tb3+concentrations. The phosphors crystallize in the orthorhombic crystal structure. Under near-ultraviolet excitation (398nm), the emission spectra of Y1−xAlO3:xEu3+(0.02≤x≤0.1) phosphors show several strong peaks at 593, 616, 655, and 697nm, which are attributed to the 5D0→7FJ (J=1, 2, 3, and 4) transitions of Eu3+ions. A high value of (5D0→7F2)/(5D0→7F1) is obtained for the Y1−xAlO3:xEu3+phosphors, indicative of high-purity red emission. Furthermore, the emission peaks of Y1−yAlO3:yTb3+(0.04≤y≤0.2) phosphors are detected at 489, 544, 585, and 621nm, which are due to the 5D4→7F6, 5D4→7F5, 5D4→7F4, and 5D4→7F3 transitions of the Tb3+ions, respectively. The emission peak (544nm) corresponding to the 5D4→7F5 transition is the most intense, which is advantageous for high green color purity. White light can be obtained by combining blue GaN LED chips with the red Y1−xAlO3:xEu3+and green Y1−yAlO3:yTb3 phosphors prepared in this study.

Effects of Eu3+ and Dy3+ doping or co-doping on optical and structural properties of BaB2Si2O8 phosphor for white LED applications

AbstractA series of Eu3+ and Dy3+ doped/co-doped as well as un-doped BaB2Si2O8 phosphors were synthesized via solid state reaction method. The PL result showed typical blue and green emission from Dy3+ and red emission from Eu3+. The f-f transitions involving the lanthanide ions along with dopant site occupancy were discussed thoroughly. Phonon assisted energy transfer process was observed from Eu3+ to Dy3+, which enhanced the emissions of Dy3+. Combinations of the emissions from Eu3+ and Dy3+ showed a possible white to red tuneable emission on the CIE diagram. The white warmth emissions of the phosphor were revealed to be adjustable through designing the dopant concentration and excitation wavelengths. An unusual energy transfer that originated from Eu3+ to Dy3+ was also discovered and the energy transfer mechanism was discussed. Proposed energy transfer mechanism was investigated using luminescence decay lifetime. All the phosphor exhibited efficient excitation in the UV range which matched well with the emissions from GaN-based LED chips. This presented the BaB2Si2O8 phosphor as a promising candidate for white LED applications. The effects of doping on the structural properties and the optical band gap of BaB2Si2O8 phosphor were also discussed in this study.

Photoluminescence Properties of (Ba<sub>1-(x+y)</sub>Sr<sub>x</sub>Eu<sub>y</sub>)<sub>2</sub>Si<sub>6</sub>O<sub>12</sub>N<sub>2</sub> Phosphors for White LED Applications

(Ba1-(x+y)SrxEuy)2Si6O12N2 oxynitride phosphors were successfully synthesized by the solid-state reaction method at 1200°C under a H2(5%) + N2(95%) atmosphere. The Sr2+ content (x) was varied in the range 0-0.6 and the Eu2+ content varied in the range 0.05-0.25, with the Si/(Ba+Sr+Eu) ratio fixed at 3. Results showed that the emission characteristics of (Ba1-(x+y)SrxEuy)2Si6O12N2 phosphors under UV or blue-light excitation was strongly dependent on the chemical composition. The phosphor (Ba0.95Eu0.05) Si6O12N2 showed an intense green emission peak at 520 nm, whilst the phosphor (Ba0.45Sr0.5Eu0.05)Si6O12N2 had a weaker emission maximum at 548 nm. Ba2+ substitution with Sr2+ decreased the lattice volume of the (Ba1-(x+y)SrxEuy)2Si6O12N2 phosphors and was responsible for the red-shift in the emission peak. Optimization of the Eu2+ concentration at a fixed Sr2+ content of 0.2 identified the phosphor (Ba0.65Sr0.2Eu0.15)2Si6O12N2 as a potential alternative to YAG:Ce yellow phosphors for white LED applications.

Energy transfer and tunable luminescence of Na2(Y,Eu)Mg2V3O12 phosphors for white LED applications

A series of solid-solution phosphors Na2(Y,Eu)Mg2V3O12 were prepared using solution combustion reaction. X-ray diffraction studies verified the formation of single phase Na2(Y,Eu)Mg2V3O12 with garnet structure. Na2(Y,Eu)Mg2V3O12 phosphors show not only a broad emission band with a maximum at 510nm due to the [VO4]3− group but also several sharp emission lines due to the Eu3+ ions. The energy transfer from [VO4]3− to Eu3+ was discussed on the base of the spectral analysis. The color-tunable emissions of the Na2(Y,Eu)Mg2V3O12 phosphor as a function of Y/Eu ratio are realized by continuously generating the varied hues from yellowish-green to reddish-orange. This indicates that the obtained phosphor may have potential applications in the field of UV-based white LEDs.

Synthesis of K2XF6:Mn4+ (X = Ti, Si and Ge) red phosphors for white LED applications with low-concentration of HF

Red phosphors K2XF6:Mn4+ (X=Ti, Si and Ge) have been successfully synthesized by one-pot chemical co-precipitation method in low HF concentration with the addition of acetic acid glacial. The crystal structure and morphology of these phosphors were characterized by powder X-ray diffraction (XRD), scanning-electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) in detail. Their optical properties were investigated by excitation and emission spectra, diffuse reflectance spectra, and decay curves. These phosphors exhibit intense red emission with high color-purity under blue light excitation. Finally, the applications on the white-light-emitting diodes (WLEDs) of the as-prepared sample were investigated. The WLEDs fabricated with K2XF6:Mn4+ display warm white light with lower color temperature (Tc<4000K) and higher color rendering index (Ra>80).

Enhanced photoluminescence of K2Ba(WO4)2:Eu3+ by Cl− co-doping

K2Ba(WO4)2:Eu3+ phosphors were synthesized using solid state reaction method. The effects of Cl− doping on the crystal structure, morphology, and luminescent properties of K2Ba(WO4)2:Eu3+ were investigated. All phosphors exhibit red emission peaking at 615 nm upon 394 nm excitation. The appropriate addition of F−, Cl− and Br− ions in K2Ba(WO4)2:Eu3+ can increases prominently the emission intensity. Unlike the common view, the charge compensation mechanism cannot account for the enhancement of emission intensity. Based on the detail study of the photoluminescence of Cl− doped samples, it can be deduced that the change of the lattice symmetry of Eu3+ sites by the addition of Cl− ions is responsible for the emission enhancement, which results the increase of 5D0 → 7F2 transition probability. It is also interesting that the change of the symmetry at Eu3+ sites is coincident with the crystallinity of the phosphors. Meanwhile, the addition of Cl− decreases the lifetime of Eu3+ luminescence. Compared with the commercial red-emitting phosphor Y2O3:Eu3+, K2Ba(WO4)2:Eu3+, Cl− phosphors show higher luminescent intensity and best color purity. The results suggest the suitability of K2Ba(WO4)2:Eu3+, Cl− phosphors for white LED applications.

Synthesis and photoluminescence properties of Ba2CaZn2Si6O17:Eu3+ red phosphors for white LED applications

Novel pellyite type Ba2CaZn2Si6O17:Eu3+ red emitting phosphors with different Eu3+ contents were synthesized by the solid state reaction method. The crystal structure, photoluminescence properties and concentration quenching of Ba2CaZn2Si6O17:Eu3+ phosphors were investigated. Powder X-ray diffraction measurements confirmed the structure of the samples. The photoluminescence emission (PL) and excitation (PLE) spectra were measured. The results showed that the dominant hypersensitive red emission peak of the phosphors Ba2CaZn2Si6O17:Eu3+ was located at 613nm attributed to the Eu3+ transition (5D0→7F2) which could be effectively excited by 395nm (near-UV). The latter band matched well with the emission from the near-UV LED chips. The intensity ratio of 5D0→7F2 to 5D0→7F1 transition showed slight variation with Eu3+ concentrations. The Eu3+ emission intensity was maximum for 9mol%. The luminescence quantum efficiency was determined and also the decay profiles were obtained and analyzed. In addition, the Commission International del'Eclairage (CIE) chromaticity coordinates of Ba2CaZn2Si6O17:0.09Eu3+ phosphor were calculated to be 0.637 and 0.362. The experimental results demonstrated that the Ba2CaZn2Si6O17:Eu3+ red emitting phosphor is a potential candidate for white light emitting diodes (WLEDs) pumped by near-UV chip.

Thermostability and photostability of Sr3SiO5:Eu2+ phosphors for white LED applications

Sr3SiO5:Eu2+ phosphors were synthesized by solid-state reactions, and their photoluminescence (PL) stability under the impact of heat or light was studied. The thermal degradation in air was observed for the Sr3SiO5:Eu2+ phosphor, furthermore, this drop in emission intensity could be accelerated by simultaneously illuminating the phosphor with a high-power blue LED during the heating process. This enhanced thermal degradation phenomenon in the blue light can be ascribed to the photo-assisted thermal oxidation of Eu2+ ions. A reversible decrease in emission intensity at room temperature was observed on illuminating the Sr3SiO5:Eu2+ phosphor alone by the blue light, which could be related to trapping centers in the phosphor. The existence of traps is also responsible for the anomalous increase of emission intensity with increasing temperature from 30 to 100°C. Considering the influences of Ba2+ doping on temperature-dependent PL properties of the Sr3SiO5:Eu2+ phosphor, the thermal quenching origin may be ascribed to the thermally assisted photoionization process.

Simultaneous realization of two approaches to white light in single-component phosphors.

A novel single-component warm white light-emitting Sr(2)Ca(0.995)MoO(6): Sm(3+) (0.005) phosphor was synthesized by solid-state reaction. The photoluminescence excitation spectra ranging from 300 to 450 nm and 460 to 500 nm broadly are observed. Direct full-color warm white light [(x, y) = 0.3221, 0.3525] was realized in this single-phase phosphor with exposure to 380 nm UV light. When this phosphor is pumped by 466 nm radiation we obtained yellow emission with an intense red component, suggesting that this material is also competitive as a blue-pumped yellow phosphor. Thus two approaches to white light are realized simultaneously in Sm(3+) doped single-component phosphor for the first time. The quantum yield and the reliability of the as-synthesized phosphors for White LED applications were also investigated.

Luminescence properties of a novel CaLa4Si3O13:Sm3+ phosphor for white light emitting diodes

A new series phosphors of Sm3+ ions activated CaLa4Si3O13 (CLSO) were synthesized by solid state reaction. The crystal structure and photoluminescence properties of CLSO:Sm3+ phosphors were investigated. The excitation and emission spectra indicated that the CLSO:Sm3+ phosphors could be excited by near UV or blue LED chip effectively, giving a strong orange–red emission centered at 602nm which corresponded to 4G5/2→6H7/2 transition of Sm3+ ions under the excitation of 404nm. The results suggested that the CLSO:0.015Sm3+ phosphor was the brightest orange–red emission with CIE coordinates (x=0.59, y=0.41). CaLa4Si3O13:Sm3+ was a promising orange–red emitting phosphor for white LED applications.

Synthesis of Na2SiF6:Mn4+ red phosphors for white LED applications by co-precipitation

A one-step approach to synthesize Na2SiF6:Mn4+and K2SiF6:Mn4+ red phosphors by co-precipitation is reported in this paper.

Liquid Phase Precursor driven approach for the synthesis of Ca–Sr–Ba orthosilicate phosphors for white LED applications

Liquid Phase Precursor driven approach for the synthesis of Ca–Sr–Ba orthosilicate phosphors for white LED applications

Structure and photoluminescence properties of La2Mo(W)O6:Eu3+ as red phosphors for white LED applications

Novel La2Mo(W)O6:Eu3+ phosphors for white light-emitting (W-LEDs) were successfully prepared by the solid state reaction technology at 1250°C for 5h and characterized by powder X-ray diffraction (XRD) and photoluminescence (PL) spectrum. The XRD result suggest that La2MoO6 phosphors have a tetragonal structure. Photoluminescence results indicate that the phosphors series can be excited efficiently by near-UV light (394nm) or blue light (466nm), then exhibit strong red emission, which is due to the characteristic transition of Eu3+. Meanwhile, the 5D0→7F2 transition is the dominant band and is split into two peaks (614nm, 621nm). The concentration quenching takes place at relatively high concentration of Eu3+. Lastly, an enhancement of Eu3+ luminescence is observed when Mo6+ is substituted by isovalent W6+ in La2MoO6:Eu3+ phosphors. Additionally, the composition-optimized La1.70Eu0.30Mo0.8W0.2O6 phosphor presents the best luminescence performance, which can be used as a potential candidate for white light emitting diodes pumped by near-UV or blue chip.

Luminescence and photometric characterization of K+ compensated CaMoO4:Dy3+ nanophosphors

A series of CaMoO4 phosphors doped with trivalent dysprosium (Dy(3+)) and codoped with potassium (K(+)) ions were prepared by hydrothermal method. The nanostructures of the as-synthesized phosphors were investigated by X-ray diffraction (XRD). The results reveal that the obtained powder phosphors are single-phase scheelite structure with tetragonal symmetry and the crystallite size is in the range of 10-60 nm. The emission spectra show a bright yellow emission at 576 nm and blue emission at 487 nm. As a charge compensator, K(+) ions were incorporated into CaMoO4:Dy(3+) phosphors, which enhance the PL intensities depending on the doping concentration of K(+). The CIE parameters such as colour coordinates, colour correlated temperature and luminous efficacy of radiation were calculated using spectral energy distribution functions and McCamy's empirical formula. Photometric characterization indicates the suitability of K(+) compensated CaMoO4:Dy(3+) phosphor for white LED applications.

Luminescence properties of stoichiometric EuM2S4 (M = Ga, Al) conversion phosphors for white LED applications

AbstractEuM2S4 (M = Ga, Al) phosphors were synthesized using the solid‐state reaction method. The structure and photoluminescence (PL) properties of the phosphors were investigated in detail. XRD pattern results indicated that the two phosphors are isostructures to the orthorhombic structure. The PL spectra showed that the EuGa2S4 and EuAl2S4 phosphors can be excited efficiently by UV–visible light from 350 to 480 nm and that they emit intense green and bluish‐green light, with emission bands peaking at 545 and 506 nm, respectively. The fluorescence lifetimes of Eu2+ were calculated to be 0.076 and 0.064 µs for EuGa2S4 and EuAl2S4, respectively. The thermal quenching temperature was 412 K for EuGa2S4. A green LED was fabricated by the combination of a near‐UV InGaN chip and the prepared EuGa2S4 phosphor. The EuGa2S4 phosphor exhibits much better green color purity than EuAl2S4. As a result, EuGa2S4 might be a promising green phosphor candidate for solid‐state lighting LED technology.

On the stability and reliability of Sr_1-xBa_xSi_2O_2N_2:Eu^2+ phosphors for white LED applications

Sr1-xBaxSi2O2N2:Eu2+ phosphors were synthesized using high temperature solid state reactions and the reliability of the as-synthesized phosphors for White LED applications was investigated. The oxidation resistance of the phosphors was investigated by baking the phosphors at various temperatures in air for 2 hours and also at 85 °C, 85% relative humidity for 150 hours. The photo stability of the phosphor was studied by illuminating the phosphor using a high power laser diode (450 nm) at various laser fluxes. Phosphor converted LEDs were fabricated using the as-synthesized Sr1-xBaxSi2O2N2:Eu2+ (x = 0 and 0.40) phosphors. The long term stability of the fabricated LEDs was tested by keeping the LEDs at 85 °C and 85% relative humidity for 800 hrs. Even though phosphors show high thermal and chemical stability, the electroluminescent (EL) intensity of the fabricated LEDs drops by 15% after 800 hrs of operation. The degradation of EL intensity of the device might be the result of thermally assisted photo-ionization of Eu2+ ions in the phosphor.

Photoluminescence properties of a new Eu2+-activated CaLaGa3S7 yellowish-green phosphor for white LED applications

A thiogallate chalcogenide phosphor CaLaGa3S7:Eu2+ was synthesized by a solid-state reaction at 950°C in a H2S atmosphere. The photoluminescence excitation,emission spectra, concentration quenching, fluorescence lifetime, and thermal quenching process of the phosphor were investigated in detail. It was found that the synthesized phosphor emitted intense and broadband yellowish-green light with a peak at 554nm. Thus, the proposed phosphor is suitable for the development of blue or near UV LED. The critical dopant concentration of Eu2+ (Rc=15Å) per unit formula was found to be 0.15mol. At room temperature, the fluorescence lifetime of Eu2+ in CaLaGa3S7 was found to be 0.216μs. The activation energy for thermal quenching was 0.29eV. The chromaticity coordinates of our phosphor is very close in color to Y3Al5O12:Ce3+. Therefore, CaLaGa3S7:Eu2+ can be a good alternative as a yellowish-green phosphor and can be used for white light generation in phosphor-converted LEDs.

Photoluminescence properties of green-emitting Eu 2+-activated Ba 3Si 6O 12N 2 oxynitride phosphor for white LED applications

Eu 2+-activated Ba 3Si 6O 12N 2 green-emitting phosphors were synthesized by a solid-state reaction method. X-ray diffraction patterns showed that the synthesized phosphor sintered at 1200 °C for 12 h was a Ba 3Si 6O 12N 2 pure phase. The synthesized phosphors were excited in UV to blue light. The emission spectra showed a broad green emission band when excited with a light at 465 nm. The highest emission intensity was observed at a Eu 2+ concentration of 0.25 mol and the NH 4Cl concentration of the optimized flux was found to be 9 wt.%. The obtained green-emitting Ba 3Si 6O 12N 2:Eu 2+ phosphors could be applied to white LED applications.