A novel long lasting phosphor Sr5 （PO4）3FxCll_x：Eu2＋,Gd3＋ prepared in air condition

A series of blue long afterglow mixed halide-phosphate phosphors Sr5 （PO4）3 FxCll-x：Eu2＋,Gd3＋ were synthesized in air by traditional solid-state reaction routte. The crystal structures, photoluminescence, thermolurninescenee properties and afterglow proper- ties of the phosphors were characterized systematically using X-ray diffraction （XRD）, luminescence spectrophotometer, microcom- puter thermoluminescence dosimeter and single photon counter, respectively. Under 280 nm excitation, the broadband emissions of Eu2＋ ions were observed at 445 nm （blue） due to the 4f7→4f65d transition. It was demonstrated that there existed the self-reduction of the Eu3＋ to Eu2＋ ions in this special halide-phosphate matrix in air condition. The addition of Gd3＋ ions obviously enhanced the after- glow properties of the single doped Eu2＋ ions in the halide-phosphate phosphors. And the content of the fluoride anions also had sig- nificant influence on the afterglow properties. All results indicated that Srs （PO4）3 FxCI1-x：Eu2＋,Gd3＋ might be potential phosphors for long lasting phosphorescence （LLP） materials.     

Blue-emitting Sr_(1-x)Ca_xLu_2O_4:Ce~(3+) phosphors for high CRI white LEDs

A broadband blue-emitting Sr_(1-x)Ca_xLu_2 O_4:Ce~(3+)(x=0-0.2) phospho rs were synthesized,which can be used for near-UV pumped white light-emitting diodes(w-LEDs).The crystal structures,photoluminescence pro perties,external quantum efficiency,the rmal stability and application perfo rmance of Sr_(1-x)Ca_xLu_2 O_4:Ce~(3+),by partially substituting Sr~(2+) with Ca~(2+)(x=0-0.2),were studied by various analytical techniques.When the Ca/Sr ratio of Sr_(1-x)Ca_xLu_2 O_4:Ce~(3+) gradually increases,the emission peak of Sr_(1-x)Ca_xLu_2 O_4:Ce~(3+) red-shiftes from 459 to 465 nm,corrected external quantum efficiency increases from 31.8% to 42.9%,and the thermal stability is also improved.The mechanism of the changes of the photoluminescence emission and excitation spectra,external quantum efficiency and thermal stability properties was also investigated in detail.In addition,a w-LED was fabricated by using SrLu_2 O_4:Ce~(3+)(blue),β-sialon:Eu~(2+)(green) and(Sr,Ca)AlSiN_3:Eu~(2+)(red) phosphors combined with a 405 nm near-UV LED chip,and its color rendering index(CRI) reaches 96.0.When Sr_(0.8)Ca_(0.2)Lu_2 O_4:Ce~(3+)is applied as blue phosphor to substitute SrLu_2 O_4:Ce~(3+),the obtained w-LED devices have high luminous efficiency,and CRI greater than 95.0.These re sults show that the Sr_(1-x)Ca_xLu_2 O_4:Ce~(3+) can be potential blue phosphors for n-UV pumped high CRI w-LEDs application.     

Synthesis and luminescent properties of Ba3Y3.88Gd0.12O9:Eu3+ phosphors with enhanced red emission

In this work, the Gd³⁺/Eu³⁺ activated Ba₃Y₄O₉ (BYO) phosphors were successfully synthesized via coprecipitation method at 1400 °C. The precursor composition, crystal structure stability, microscopic morphology, photoluminescence (PL)/photoluminescence excitation (PLE) spectra and fluorescence attenuation analysis of the phosphors are discussed in detail. The chemical composition of the precursor was determined by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetry (TG) analysis; According to field emission-scanning electron microscopy (FE-SEM) analysis, it is found that the particle size of phosphor is uniform and the agglomeration is few. According to PL/PLE spectra analysis, Ba₃Y_3.28Eu_0.6Gd_0.12O₉ phosphors has the strongest excitation band at 260 nm and the strongest emission band at 614 nm, and the fluorescence intensity of Ba₃Y_3.28Eu_0.6Gd_0.12O₉ is higher than that of Ba₃Y_3.4Eu_0.6O₉. The quenching concentration of Eu³⁺ in Ba₃Y_3.88–4xEu_4xGd_0.12O₉ phosphors is x = 0.15 and the mechanism of quenching concentration of Eu³⁺ is electric dipole-quadrupole type interactions. The lifetime value of Ba₃Y_3.88–4xEu_4xGd_0.12O₉ (x = 0.15) phosphors is 0.686 ms and decreases with the increase of Eu³⁺ content. In addition, the CIE chromaticity diagram of Ba₃Y_3.28Eu_0.6Gd_0.12O₉ phosphors is (0.66, 0.34). Finally, the lamp beads assembled with Ba₃Y_3.28Eu_0.6Gd_0.12O₉ phosphors have an ideal luminous effect. Therefore, the Ba₃Y_3.88–4xEu_4xGd_0.12O₉ phosphors designed in this work may hopefully meet the requirements of various lighting and optical display applications.     

Synthesis and luminescent properties of BaGd2O4:Eu3+ phosphors with highly efficient red-emitting

The BaGd_(2-2 x)Eu_(2 x)O_4(BG, x = 0.01-0.09) phosphors were successfully synthesized via the sol-gel method,and BaY_(2-2 y)Eu_(2 y)O_4(BY, y = 0.005-0.07) phosphors were included for comparison. The pure phase BG phosphors with the ordered CaFe_2 O_4-type structure are obtained by annealing at 1300℃ for5 h. The phosphors with uniform particle size of 120 nm and good dispersion display typical Eu~(3+)emission with the strongest peak at 613 nm(~5 D_0→~7 F_2 transition of Eu3+) under optimal excitation band at 262 nm(CTB band). The presence of Gd~(3+) excitation bands on the PLE spectra monitoring the Eu3+emission directly proves an evidence of Gd~(3+)-Eu~(3+) energy transfer. Owing to the concentration quenching, the optimum content of Eu3+ addition is 5 at%(x = 0.05), and the quenching mechanism is determined to be the exchange reaction between Eu3+. All the BG samples have similar color coordinates and temperature of(0.64 ± 0.02, 0.36 ± 0.01) and 2000 ± 100 K,respectively. The lifetime value of BaGd_(1.9)Eu_(0.1)O_4 for 613 nm is fitted to be 2.19 ± 0.01 ms, and the Eu~(3+) concentration does not change the lifetime significantly. Owing to the Gd~(3+)-Eu~(3+) energy transfer, the luminescent intensity of the BaGd_(1.9)Eu_(0.1)O_4 phosphor is better than BY system. The BG system served as a new type of phosphor is expected to be widely used in lighting and display areas.     

Synthesis and luminescence properties of Tb3+/Eu3+ co-doped GdAlO3 phosphors with enhanced red emission

The(Gd_(0.97-x)Eu_xTb_(0.03))AIO_3(x= 0.005-0.07) phosphors were synthesized by the co-precipitation method,using ammonium bicarbonate as a precipitant.The combined technologies of FT-IR,XRD,FESEM,PLE/PL and photo luminescence decay analysis were used to study the phase evolution,morphologies and luminescent properties.The phosphors with good dispersion exhibit strong vivid red emission located at 617 nm(~5 D_0-~7 F_2 transition of Eu~(3+)) under the optimal excitation wavelength of 275 nm(~4 f~8-4 f~75 d~1 transition of Tb~(3+),~8 S_(7/2)→6~I_J transition of Gd~(3+)).The presence of Gd~(3+) and Tb~(3+) excitation bands on the PLE spectra monitoring the Eu~(3+) emission directly gives an evidence of Tb~(3+) → Eu~(3+) and Gd~(3+) → Eu~(~(3+)) energy transfer,The emission intensity varies with the Eu~(3+) amount,and the quenching concentration is ～5 at% which is close to the calculated value.The quenching mechanism is determined to be the exchange reaction between Eu~(3+).The temperature-dependent PL analysis indicates that the best(Gd_(0.92)Eu_(0.05)Tb_(0.03))AlO_3 sample possesses good thermally stable properties.All the(Gd_(0.97-x)Eu_xTb_(0.03))AIO_3 phosphors in this work have similar CIE chromaticity coordinates and color temperatures,which are(0.65 ± 0.02,0.35 ± 0.02) and ～2558 K,respectively.Fluorescence decay analysis shows that the lifetime for～617 nm emission decreases with the content of Eu~(3+) and temperature increasing.Owing to the Tb~(3+)→ Eu~(3+) energy transfer,the luminescent properties of the(Gd_(0.92)Eu_(0.05)Tb_(0.03))AlO_3 phosphors are superior to the single Eu~(~(3+)) doped sample(Gd_(0.95)Eu_(0.05))AlO_3.As a result,the prepared phosphors may be widely used in solid-state display and light emitting devices.     

Investigation on luminescence properties of Ln^3＋ doped Sr3EuMgSi2O8 （Ln=Dy, Er, Ho）

The luminescent properties of Sr2.97MgSi2O8:Eu2+0.01 phosphors were investigated with different Ln3+0.02(Ln3+:Dy3+,Er3+,Ho3+) co-dopants. The co-dopants had no influence on both the structure of the lattice and the position of the emission peak. However, the afterglow properties of samples were enhanced with different co-dopants. The afterglow duration of the Dy3+ co-doped sample was longer than that of the others. Furthermore, the co-doping samples had stronger thermoluminescence (TL) intensity and therefore longer afterglow duration. At last, the self-reduction of Eu3+→Eu2+ was observed in an silicate compound of Sr3-xMgSi2O8:xEu phosphor in air condition. This is the first time to show a blue long afterglow phosphor synthesized avoiding reducing atmosphere.     

Effect of excitation wavelength (blue vs near UV) and dopant concentrations on afterglow and fast decay of persistent phosphor SrAl2O4:Eu2+,Dy3+

The persistent phosphor SrAl₂O₄:Eu²⁺,Dy³⁺ is the subject of numerous investigations. One often neglected aspect is that in this phosphor, as well as in Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺, there are two different Sr²⁺ sites which can be occupied by the dopant Eu²⁺ ions. We first introduce a general scheme of possible energy transfers in these persistent phosphor materials including explicitly both europium ions. This scheme is used as a generic starting point to study experimentally specific pathways. We illustrate this application with the study of the effect of excitation wavelength (444 and 382 nm) on the afterglow of differently doped SrAl₂O₄:Eu²⁺,Dy³⁺ samples, as well as on the emission decay curves. With the same excitation intensity under 444 nm excitation, the resulting afterglow intensity is stronger than under near UV excitation. At 382 nm, Eu²⁺ ions on both Sr²⁺ sites in SrAl₂O₄ are excited, but at room temperature the blue emission is quenched, leading to a loss of photons. The observed effects can further be associated with the ratio of Eu²⁺ ions and trap states which are modulated by the concentrations of Eu²⁺ and Dy³⁺ in SrAl₂O₄, as well as by temperature. Increasing the nominal Dy³⁺ content from 0.1 mol% to 0.5 mol% with respect to Sr results in the doubling of the integrated afterglow intensity and confirms thus that Dy³⁺ ions are indeed involved in the trapping process. The concentration of trap states is much lower than the concentration of Eu²⁺ ions, as even with low excitation densities, a plateau of integrated afterglow intensity (corresponding to the total number of accessible traps) is reached. We postulate that an important fraction of excited Eu²⁺ ions can potentially transfer their energy to trap states. Once that all traps are filled or in a dynamical filling-depletion process under illumination (with thermal and/or optical depletion processes), for the remaining Eu²⁺ a “normal” steady-state emission is observed. The luminescence decay curves at 520 nm measured at 77 K show a mono-exponential decay with a common lifetime of about 1140 ns for all 5 samples under 437 nm excitation, while under 375 nm excitation, a feed process originating from the energy transfer between Eu²⁺ ions is demonstrated. Under 375 nm excitation, the non-exponential decay observed at 440 nm can be quantitatively associated to a Förster energy transfer process with R₀ = 1.58 (8) nm. For the overall understanding of the afterglow processes, it appears that one has to consider the individual contributions of all active ions on different lattice sites.     

Spectral properties and Judd–Ofelt analysis of novel red phosphors Gd2InSbO7:Eu3+ with high color purity for white LEDs

Gd₂InSbO₇:Eu³⁺ red phosphors were successfully synthesized via high-temperature solid–state reaction. The phase purity, particle size, and luminescence properties of obtained phosphors were measured and analyzed in detail. The Gd₂InSbO₇ lattice possesses cubic structure with F_d-3m (227) space group. The phosphors emit bright red emission at 628 nm under 393 nm excitation, and this phenomenon is attributed to the ⁵D₀–⁷F₂ transition. The Judd–Ofelt parameters (Ω₂, Ω₄), transition ratio, and branching ratios (β) of Eu³⁺-doped Gd₂InSbO₇ phosphor were calculated on the basis of the emission spectra and decay lifetimes. The optimal content in Gd₂InSbO₇:xEu³⁺ is identified to be 15 mol%. The thermal quenching of Gd₂InSbO₇:Eu³⁺ is found to be over 500 K, and its activation energy is 0.26 eV. The Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of Gd₂InSbO₇:15%Eu³⁺ are (0.629, 0.371), which are close to ideal red chromaticity coordinates (0.670, 0.330). The fabricated w-LED exhibits good color rendering index (R_a) (86), correlated color temperature (CCT) (6997 K), and CIE chromaticity coordinates (0.302, 0.330). The obtained results demonstrate that Gd₂InSbO₇:Eu³⁺ phosphors have potential applications in white LEDs.     

Synthesis of Long Afterglow Phosphors Doped B SrAl2O4:Eu2+, Dy3+ and Its Luminescent Properties

The long afterglow luminescent material SrAl₂O₄: Eu²⁺, Dy³⁺ was prepared by high temperature solid-state method. Effects of doped B on the luminescent properties of phosphors SrAl₂O₄: Eu²⁺, Dy³⁺ were investigated by means of excitation spectra, emission spectra and X-ray diffraction analysis. As the result, the addition of H₃BO₃ as flux promotes the growth of crystalline and reduces the synthesizing temperature, but the wavelength of emission peak of photoluminescent material did not change with the variation of H₃BO₃ content. The effect of Dy³⁺ concentration on the luminescent properties of material was investigated. It was found that the luminescence of phosphors prepared under the condition of the amount of H₃BO₃ 5% and the mole ratio of Eu/Dy = 1/7(Eu = 0.02 mole) had better luminescent property and longer afterglow time.     

Enhanced luminescence efficiency and thermal stability via introduction of non-rare earth Bi3+ in Gd5Si2BO13:Eu3+

A series of Gd₅Si₂BO₁₃:Eu³⁺ and non-rare earth Bi³⁺ ions doped Gd₅Si₂BO₁₃:Eu³⁺ phosphors was successfully synthesized via high-temperature solid-state method,and the as-obtained phosphors were studied on their phase structures,luminescence characteristics,thermal stability and luminescence lifetime.Transient fluorescence spectroscopy data show that the addition of Bi³⁺ can obviously enha...     

Energy transfer,superior thermal stability and multi-color emitting properties of langbeinite-type solid-solution phosphor K_2Dy_(1.5-x)Eu_xTa_(0.5)(PO_4)_3

Langbeinite type compounds are a large kind of oxometallate with good flexibility structure.Herein,we synthesized a new langbeinite type compound K_2 Dy_(1.5)Ta_(0.5)(PO_4)_3,in which the Dy~(3+) and Ta~(5+) were blended to occupy the same crystallographic sites.Simultaneously,solid solutions of K_2 Dy_(1.5)_(-x)Eu_xTa_(0.5)(PO_4)_3(x=0-1.5) were prepared and their photoluminescence properties were investigated.Due to energy transfer from Dy~(3+) to Eu~(3+),both Dy~(3+) and Eu~(3+) characteristic emissions are observed under 393 nm light excitation.The emitting color of K_2 Dy_(1.5-x)Eu_xTa_(0.5)(PO_4)_3 turns from green through yellow to red by simply adjusting the Eu~(3+) concentration from 0 to 0.4.Moreover,K_2 Dy_(1.48)Eu_(0.02)Ta_(0.5)(PO_4)_3 phosphor possesses excellent fluorescence thermal stability and exhibits zero thermal quenching at 150 ℃.These results manifest that K_2 Dy_(1.5-x)Eu_xTa_(0.5)(PO_4)_3 solutions are promising multi-color emitting phosphors candidate for near-UV LED.     

Effect of Gd~(3+)-,Pr~(3+)-or Sm~(3+)-substituted cobalt-zinc ferrite on photodegradation of methyl orange and cytotoxicity tests

Gd~(3+)-,Pr~(3+)-or Sm~(3+)-doped Co-Zn(Co_(0.5)Zn_(0.5)Fe_2 O_4) magnetic ferrites(i.e.,Co_(0.5)Zn_(0.5)Gd_(0.1)Fe_(1.9)O_4,Co_(0.5)Zn_(0.5)Pr_(0.1)Fe_(1.9)O_4 and Co_(0.5)Zn_(0.5)Sm_(0.1)Fe_(1.9)O_4) were prepared using a facile sol-gel approach,and the structure,surface morphology and chemical composition of the products were studied by means of scanning electron microscopy(SEM),energy dispersive X-ray analysis(EDX),X-ray diffraction(XRD),UVvisible diffuse reflectance spectroscopy(DRS),photoluminescence(PL) spectroscopy,Fourier transform infrared spectroscopy(FT-IR) and vibrating sample magnetometer(VSM) spectroscopy.XRD patterns show the Co-Zn product is composed of cubic spinel phases with few impurities or secondary phases,and the average crystallite sizes of the samples are determined to be approximately～51—80,～99—181,～68—103 and～83—133 nm.Also the coercivity and remnant and saturation magnetizations,evaluated by vibrating sample magnetometer(VSM),are found to increase linearly with the incorporation of Gd3+,Pr3+and Sm3+in the product formulation.The CO_(1-x)Zn_xFe_(2-y)R_yO_4 photocatalyst sample is found to display a red shift in its absorption,and exhibits outstanding photocatalytic effects in the degradation of MO under ultraviolet(UV) light.This is attributed to the reduction of the band gap of cobalt-zinc ferrite due to the presence of rare earth ions.Further in vitro evaluations of the cytotoxic effects of the synthesized nanoparticles were performed on a HeLa cell line.     

Red-emitting CaWO4:Eu3+,Tm3+ phosphor for solid-state lighting: Luminescent properties and morphology evolution

CaWO₄:xEu³⁺,yTm³⁺crystals were obtained by facile synthesis at low temperature by the microwaveassisted hydrothermal method(MAH).The phase formation,morphology,luminescent properties and ene rgy transfer were investigated.The X-ray diffraction(XRD)re sults show the formation of a scheelitelike tetragonal structure without the presence of secondary phases.The growth mechanism of hierarchical micro structures based on self-assembly and Ostwald-ripening processes was evaluated,obtaining different types of morphologies.The luminescence spectra of CaWO₄:Eu³⁺,Tm³⁺at 325 nm excitation show the predominance of red emission at the 5 D0→7 F2(Eu³⁺)transition at 624 nm.This feature signals dominant behavior of the electric dipole type.The presence of Tm³⁺is notably evident in the absorption spectra by the related excitation transitions:3 H6→1 G4,3 H6→3 F3 and 3 H6→3 H4.Color parameters are discussed to characterize CaWO₄:Eu³⁺,Tm³⁺emission.The study of the emission spectrum as a function of the concentration of Eu³⁺(x mol%)and Tm³⁺(y mol%)indicates that the CaWO₄:Eu³⁺,Tm³⁺phosphors show stronger red emission intensity and exhibit the CIE value of x=0.63 and y=0.35.The photoluminescence results show 97%high color purity for CaWO₄:4 mol%Eu³⁺,a high CRI(92%)and a low CCT of 1085 K.These results demonstrate that the CaWO₄:Eu³⁺,Tm³⁺red phosphors are promising as color converters for application in white light-emitting diodes and display devices.     

Enhanced luminescence performances of Mn~(4+)-activated Sr_4Al_(14)O_(25) red phosphors by doping with Sc~(3+) ions

A series of non-rare earth Mn⁴⁺-activated strontium aluminate phosphors Sr₄Al₁₄O₂₅:Mn⁴⁺co-doped with Sc³⁺ions were successfully synthesized by a high-temperature solid-state reaction method.XRD result reveals that there is no introduction of additional phase but expansion of lattice with incorporation of Sc34 ions.Excitation and emission spectrum measurement shows that the synthesized phosphors can be efficiently excited by near-ultraviolet and blue light,and a deep red emission centered at 652 nm with a narrow full width at half maximum(FWHM)can be obtained,which is attributed to the transition²E→⁴A₂of Mn⁴⁺ions.In addition,the crystal field strength parameter(Dq)and Racah parameters(B,C)and energies of states were calculated based on experimental data.Moreover,the luminous intensity of Sr₄Al_14-xSCxO₂₅:Mn⁴⁺is enhanced and increased by 60%compared with Mn⁴⁺single incorporated sample at x=0.06.A phenomenon of redshift is observed in the excitation spectrum and discussed systematically.Finally,the mechanism of the positive effects with Sc³⁺ions incorporated into lattice is discussed in detail.All the results suggest that the Sr₄Al_13.94Sc_0.06O₂₅:Mn⁴⁺phosphor will become one of the great candidates for backlight of LCD.     

Photoluminescence of BaGdB9O（16）：Eu^3＋ co-doped Al^3＋ or Sc^3＋ under UV-VUV excitation

Single phase of BaGd0.9-xMxEu0.1B9O16 (M=Al or Sc, 0≤x≤0.3) powder was prepared by the solid-state reaction and its photoluminescence (PL) properties were investigated under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation. Monitored with 613 nm emission, the excitation spectra of BaGd0.9-xMxEu0.1B9O16 consisted of three broad bands peaking at about 242, 208, and 142 nm, respectively. The one at about 242 nm originated from the charge transfer band (CTB) of O2-→Eu3+. The other two were assigned to the absorption of the host, which was overlapped with absorptions among borate groups, f→d transition of RE3+ (RE=Gd, Eu), and the charge transfer transition of O2-→Gd3+. The maximum emission peak was observed at about 613 nm in the emission spectra of BaGd0.9-xMxEu0.1B9O16 under both 254 and 147 nm excitation, which originated from the electric dipole 5D0→7F2 transition of Eu3+. When excited with 254 nm, the integral emission intensity of Eu3+ increased after Al3+ or Sc3+ substituting Gd3+ partly in BaGd0.9Eu0.1B9O16. Under 147 nm excitation, the integral emission intensity of Eu3+ decreased after some Gd3+ was replaced by Sc3+, but increased after adding appropriate Al3+ into BaGd0.9Eu0.1B9O16.     

Green and red phosphor for LED backlight in wide color gamut LCD

Wide color gamut(WCG) backlight for liquid crystal display(LCD) utilizing white light-emitting diodes(LED) has attracted considerable attention for their high efficiency and color reduction.In this review,recent developments in crystal structure,luminescence and applications of phosphors for wide color gamut LED backlight are introduced.As novel red phosphors,Mn~(4+)activate fluoride and aluminate phosphors are advanced in quantum efficiency,thermal quenching and color saturation for their characteristic spectrum with broad excitation band and linear emission.The crystal structure and fluorescence properties of Mn~(4+)doped fluosilicate,fluorogermanate,fluotitanate,as well as Sr_4 Al_(14)O_(25),CaAl_(12)O_(19) and BaMgAl_(10)O_(17) phosphors are discussed in detail.A serial of narrow-band red-emitting Eu~(2+),Eu~(3+)and Pr~(3+)-doped nitride silicates and molybdate phosphors are also introduced.Rare-earth-doped oxynitride and silicate green-emitting phosphors have attracted more and more attention because of the wide excitation,narrow emission,high quenching temperature,high quantum efficiency,such as β-sialon:Eu~(2+),Ba_3Si_6O_(12)N_2:Eu~(2+),MSi_2O_2N_2:Eu~(2+)(M=Ca,Sr,Ba),y-AlON:Mn~(2+)and Ca_3Sc_2Si_3O_(12):Ce~(3+).All above phosphors demonstrate their adaptability in wide color gamut LCD display.Especially for Mn~(4+)doped fluosilicate red phosphor and β-sialon:Eu~(2+)green phosphor.To achieve an ultra-high color gamut in white LED backlight and against the OLED,innovative narrow-band-emission red and green phosphor materials with independent intellectual property rights are continuously pursed.     

Emission tunable of Mg_(0.695)Si_(0.695)Al_(1.39)O_(3.65)N_(0.35):Eu~(2+),Mn~(2+) oxynitride phosphors via energy transfer for WLEDs

A series of Eu~(2+)doped and Eu~(2+)/Mn~(2+) co-doped Mg_(0.695)Si_(0.695)Al_(1.39)O_(3.65)N_(0.35)(MSAON) phosphors were synthesized by solid-state reaction at a lower temperature of 1500℃.The crystal morphology and structure of MSAON host were characterized by SEM,TEM and XRD.The quantum yield(QY) for Eu~(2+)doped MSAON phosphors was measured as high as 62%,indicating the excellent luminous efficiency.For the Eu~(2+)/Mn~(2+)co-doped MSAON phosphor,the photoluminescence spectrum and delay curves reveal the efficient energy transfer(ET) process from Eu2+to Mn~(2+)ions.Meanwhile,the corresponding energy transfer efficiency,critical distance and mechanism are discussed in detail.Temperature-dependent emission spectrum shows the thermal and color stabilities.The emission color of MSAON:Eu~(2+),Mn~(2+)phosphors could be tuned from blue through white to red via varying the concentration of Mn~(2+) ions.White-light-emitting diodes(WLEDs) were successfully fabricated by encapsulating the phosphors in nUV LED(365 nm) devices obtaining white light with color rendering index(CRI) as high as 87.7.The results reveal that the MSAON:Eu~(2+),Mn~(2+)phosphors could have potential application in the field of n-UV WLEDs.     

A potential red-emitting phosphor Ca_2YTaO_6:Eu~(3+):Luminescence properties,thermal stability and applications for white LEDs

A novel red-emitting phosphor tantalate Ca_2 YTaO_6:Eu~(3+)was synthesized by a solid-state reaction.The purity and surface morphology of the phosphors were characterized.The Ca_2 YTaO_6:Eu~(3+)phosphors show a sharp emission peak at 612 nm under near-ultraviolet(n-UV) at 395 nm because of the ~5 D_0→~7 F_2 transition of Eu3+.The optimal Eu3+doping concentration in Ca2 YTaO_6 is 40 mol% and the critical energy-transfer distance of Eu3+ions was calculated to be 0.9 nm.The emission spectra of Ca_2 YTaO_6:Eu3+from 300 to 480 K were investigated.The thermal-quenching temperature(T_(0.5)) of Ca_2 YTaO_6:Eu~(3+)is above 480 K.The color purity of Ca_2 YTaO_6:40 mol%Eu3+is as high as 99.8%.The luminescence lifetime of Ca_2 YTaO_6:40 mol%Eu~(3+)was also discussed.The high color purity and high thermal stability of Eu~(3+)-doped Ca2 YTaO6 phosphors contribute to its application value in white lightemitting diodes(w-LEDs).     

Upconversion multicolor tuning of NaY(WO4)2:Tb3+ with Eu3+ doping

A series of Tb~(3+) and Eu~(3+) co-doped NaY(WO_4)_2 phosphors were synthesized by hydrothermal reactions.The crystal structure,morphology,upconversion luminescent properties,the energy transfer from Tb~(3+) to Eu~(3+)ions and the ~5 D_4→ ~7 F_5 transition of the Tb~(3+) ion in NaY(WO_4)_2:Tb~(3+),Eu~(3+) phosphors were investigated in details.The results indicate that all the synthesized samples are of pure tetragonal phase NaY(WO_4)2.Furthermore,the micrometer-sized needle spheres and excellent dispersion of the particles are obtained by adding polyethylene glycol(PEG-2000) as the surfactant.Phosphors of NaY(WO_4)_2:Tb~(3+),Eu~(3+) exhibit the492 nm blue emission peak,546 nm green emission peak,595 nm orange emission peak and 616 nm red emission peak under 790 nm excitation.The energy transfer from Tb~(3+) to Eu~(3+) is a resonant transfer,in which electric dipole-dipole interaction plays a leading role.By adjusting the doping concentration of Eu~(3+) in NaY(WO_4)_2: 1.0 mol%Tb~(3+),xmol%Eu~(3+) phosphors,the emitting color of UC phosphors can be tuned from green to red.     

Luminescence thermal quenching of M_2SiO_4:Eu~(2+)(M=Sr,Ba) phosphors

In this paper,luminescence thermal quenching of M_2SiO_4:Eu~(2+)(M=Sr,Ba) orthosilicate phosphors and mechanisms for thermal quenching proposed by different authors are briefly reviewed.Depending on preparation conditions and/or Eu~(2+)-doping concentrations,the quenching temperature(T_(1/2)) and activation energy for thermal quenching of the same orthosilicate phosphor reported by different authors are inconsistent.The common conclusion is that T_(1/2) of the intermediate compound(Ba_(1-x)Sr_x)_2 SiO_4:Eu~(2+)(x≈0.5) is higher than that of either Sr_2So_4;Eu~(2+)or Ba_2SiO_4;Eu~(2+)end-member.Moreover,T_(1/2) of the best-performing SrBaSiO_4:Eu~(2+)is evidently lower than that of YAG:Ce~(3+)and some Eu~(2+)-doped nitride phosphors.Rega rding the quenching mechanism,most of the investigators attributed thermal quenching to a thermally assisted 4f-5d cross-over in the configuration coordinate diagram.Only a few authors ascribed thermal quenching to a thermally assisted photoionization of 5 d electron to conduction band of the host.Nonetheless,a close inspection of T_(1/2) and Stokes shift derived from the vibrational spectra of the intermediate compound and end-member phosphors indicates that the 5d electron photo ionization model instead of the 4f-5d crossing decay model should be the genuine mechanism for the thermal quenching of M_2 SiO_4:Eu~(2+)(M=Sr,Ba) phosphors.Since the relationship between T_(1/2) and Stokes shift of the phosphors does not support the 4 f-5 d crossing decay model.The ionization probability of the 5 d electron depends on the energy gap(E_(dC)) between 5 d_1 level of the Eu~(2+)and conduction band minimum(CBM) of the host at higher temperatures.Lattice thermal expansion would result in an elevating 5 d_1 level of the Eu~(2+)along with a diminishing CBM of the host and as a consequence a reduction in E_(dC) and an enhanced photo ionization probability at elevated temperatures.A less rigid lattice and hence a larger coefficient of thermal expansion of M_2SiO_4 hosts should be the physical origin of poorer thermal quenching properties of the orthosilicate phosphors.