Optical properties of silica sol-gel materials singly-and doubly-doped with Eu~(3+)and Gd~(3+) ions

In present work, the optical and structural properties of silica sol-gel glasses and glass-ceramic materials singly- and doubly-doped with Eu~(3+) and Gd~(3+) ions were investigated. The optical properties of studied systems were determined based on absorption, excitation and emission spectra as well as luminescence decay analysis. Conducted studies clearly indicated a significant enhancement of visible emission originated from Eu~(3+) ions as a result of changing the excitation mechanism, via Gd~(3+)→Eu~(3+) energy transfer. The luminescence intensity R-ratio was analyzed before and after heat-treatment process upon excitation at λ_(ex)=393 nm and λ_(ex)=273 nm. Moreover, the influence of excitation wavelength on luminescence decay time of the ~5D0 excited state was also analyzed. The Gd~(3+)→Eu~(3+) energy transfer efficiencies for precursor and annealed samples were calculated based on luminescence lifetime of the ~6P_(7/2) level of Gd~(3+) ions. The X-ray diffraction measurements were conducted to verify the nature of obtained sol-gel materials. In result, the formation of orthorhombic GdF_3 nanocrystal phase dispersed in amorphous silica glass host was identified after annealing. Obtained results clearly indicated an incorporation of Eu~(3+) activators into formed GdF_3 nanocrystals. Thus, conducted heat-treatment process led to considerable changes in surrounding environment around Eu~(3+) ions. Actually, it was found that energy transfer phenomenon and heat-treatment process were responsible for significant improvement of Eu~(3+) luminescence in studied sol-gel samples.     

Structural characterization and temperature-dependent luminescence of CaF_2:Tb~(3+)/Eu~(3+) glass ceramics

Tb~(3+)/Eu~(3+) co-doped transparent glass ceramics containing CaF_2 nanocrystals were successfully synthesized by high temperature melt-quenching method and subsequent heating. The structure and morphology of the samples were investigated by X-ray diffraction(XRD), transmittance electron microscopy(TEM), high resolution TEM(HRTEM) and selected area electron diffraction(SAED). The photoluminescence properties and energy transfer process from Tb~(3+) to Eu~(3+) of CaF_2:Tb~(3+),Eu~(3+) phosphors were also investigated through excitation spectra and decay curves. In addition, the emission spectra of the glass ceramics in a wide temperature range from 21 to 320 K were recorded under the excitation of 485 nm. It was found that the fluorescence intensity ratios of Tb~(3+) at 545 nm(~5D_4→~7F_5) to Eu~(3+) at 615 nm(~5D_0→~7F_2) was highly temperature-dependent with an approximate linear relationship, and the temperature sensitivity was about 0.4%/K. It is expected that the investigated Tb~(3+)/Eu~(3+) co-doped CaF_2 glass ceramics may have prospective application in optical thermometry.     

Synthesis,photoluminescence properties and thermal investigation by TG-MS of RE(DAS)_3·xH_2O(RE=Eu~(3+),Tb~(3+))

Rare earth(Ⅲ) diphenyl-4-amine sulfonates(RE(DAS)_3·xH_2O,RE=Eu~(3+),Tb~(3+))phosphors were synthesized by precursor method from barium diphenyl-4-amine sulfonate and rare earth sulfates.FTIR,TG/DSC coupled to mass spectrometry(TG/DSC/MS),X-ray powder diffraction(XPD),scanning electron microscopy(SEM) and photo luminesce nce(PL) spectroscopy were utilized to structurally and morphologically characterize the samples.Thermal decomposition of Eu(DAS)_3·7H_2O and Tb(DAS)_3·2H_2O at 973 K under dynamic air atmosphere results in crystalline Eu_2O_2SO_4 and Tb_2O_2SO_4 materials,respectively.Accordingly,MS spectra reveal the liberation of thermal decomposition products of precursors,largely as CO_2,NO_2 and SO_2 gases.The diphenyl-4-amine sulfonate(DAS) ligand demonstrats a good stabilizing property for Eu~(3+) and Tb~(3+) ions.The Eu(DAS)_3·7H_2O and Tb(DAS)_3·2H_2O compounds display efficient red and green emissions,under UV excitation,arising from the ~5D_0→~7F_J(J=0-4) and ~5D_4→~7F_J(J=0-6) transitions of the Eu~(3+) and Tb~(3+) ions,respectively.     

Synthesis and luminescence of β-SrGe(PO_4)_2:RE(RE=Eu~(2+),Eu~(3+),Tb~(3+)) phosphors for UV light-emitting diodes

A new series of β-Sr Ge(PO_4)_2:RE(RE=Eu~(2+),Eu~(3+),Tb~(3+)) phosphors were synthesized and characterized by using X-ray powder diffraction as well as excitation, and emission spectroscopy. The results exhibited that the singly doping Eu2+, Tb~(3+) and Eu~(3+) of β-Sr Ge(PO_4)_2 emit strong blue, green and red light under UV irradiation, respectively. Based on the charge transfer transitions of O~(2–)→RE~(3+), an overlapping excitation band of the as-obtained phosphors could be found in UV region, which made β-Sr Ge(PO_4)_2:RE(RE=Eu~(2+),Eu~(3+),Tb~(3+)) serve as a new series of RGB phosphors. Meanwhile, these phosphors could also be excited by 380 nm excitation simultaneously, and hence the three phosphors mixed physically could achieve the tunable hues from blue to white region by adjusting the mixed ratios.     

Multi-color luminescence in Eu3+/Tb3+ co-doped ZnAl amorphous materials and their annealed samples

Multi-color luminescence basing on amorphous Eu~(3+)/Tb~(3+) co-doped Zn-Al hydroxides and their annealed samples were studied in detail. Results suggest that excellent red emissions due to Eu~(3+) and green emissions attributed to Tb~(3+) appear under the excitation of favorable wavelength for all the asprepared amorphous samples. Moreover, the emission intensity depends on the Eu~(3+)/Tb~(3+) molar ratio. The samples annealed at 300, 500, and 700 ℃ still exhibit amorphous state,and multi-color luminescence kept in the samples annealed at 300 ℃, while luminescence quenched for the samples annealed at 500 and 700 ℃. However, a broad emission ranging from 450 to 650 nm occurs in some samples annealed at 900 ℃. Further, the fluorescence decay and lifetimes for the as-prepared samples and the samples annealed at 300 ℃ were investigated. It is found that all the decay curves of emissions due to Tb~(3+) and Eu~(3+) present characteristic double exponential function despite their different lifetimes.The present work may be a good example for developing new multi-color even white light emitting materials.     

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.     

Visible-light emission properties of α-TeO_2:Ho~(3+)/Yb~(3+)/Eu~(3+) nanoparticles via both up-and down-conversion modes

In this study,α-TeO_2:Ho~(3+)/Yb~(3+),α-TeO_2:Eu~(3+) and α-TeO_2:Ho~(3+)/Yb~(3+)/Eu~(3+) nanoparticles were prepared via a simple hydrothermal process.The up- and down-conversion properties of the as-prepared nanoparticles were tested at room temperature under a near-infrared photo source(980 nm) and UV-vis photo source,respectively.The results indicated that α-TeO_2 NPs were a kind of outstanding host material for both up- and down-conversion luminescence.The α-TeO_2:Ho~(3+)/Yb~(3+) nanoparticles showed sharp up-conversion emission at 545 and 660 nm under 980 nm excitation,ascribed to the ~5S_2→~5I_8 and ~5F_5→~5I_8(Ho~(3+)) transitions,and weaker down-conversion emission at 545 nm under 455 nm excitation,ascribed to the ~5S_2→~5I_8(Ro~(3+)) transitions.The α-TeO_2:Eu~(3+)nanoparticles showed strong down-conversion emission at 592 and 615 nm under 395 nm excitation,attributed to the ~5D_0→~7F_1 and~5D_0→~7F_2(Eu~(3+)) transitions.Possessing the advantages of these two luminescent materials,the as-prepared tri-doped samples ofα-TeO_2:0.5Ho~(3+)/10Yb~(3+)/3Eu~(3+)(mol.%) nanoparticles could successfully emit visible light via both up- and down-conversion modes.     

Efficient near-infrared quantum cutting in Tb~(3+),Yb~(3+) codoped LuPO_4 phosphors

Tb~(3+) and Yb~(3+) codoped LuPO_4 phosphors were prepared by the reverse-strike co-precipitation method.The obtained LuPO_4:Tb~(3+),Yb~(3+) phosphors were characterized by X-ray diffraction(XRD),photoluminescence(PL) spectra and decay kinetics to understand the near-infrared quantum cutting(QC) phenomena.The XRD results showed that all the phosphors exhibited good crystallinity and had a pure tetragonal phase of LuPO_4.The experimental results showed that the strong green emission around 545 nm from Tb~(3+)(~5D_4→~7F_5) and near-infrared(NIR) emission at 1003 nm from Yb~(3+)(~2F_(5/2)→~2F_(7/2)) of LuPO_4:Tb~(3+),Yb~(3+)phosphors were observed under 489 nm excitation,respectively.The Yb~(3+) concentration dependence on luminescent properties and lifetimes of both the visible and NIR emissions were also investigated.The quenching concentration of Yb~(3+) ions approached as high as 10 mol.%.The excellent luminescence properties of the LuPO_4:Tb~(3+),Yb~(3+) indicated its potential application in improving the energy conversion efficiency of the silicon based solar cells by converting one blue photon to two NIR ones.     

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.     

Luminescent properties of Ce~(3+)/Tb~(3+) co-doped glass ceramics containing YPO_4 nanocrystals for W-LEDs

Ce~(3+)/Tb~(3+) co-doped transparent glass ceramics containing YPO_4 nanocrystals were prepared using high temperature melting method,and their structural and luminous properties were investigated.XRD analysis and TEM images confirmed the existence of YPO_4 nanocrystals in glass ceramics.The transmission spectra proved that the glass ceramics specimens still maintained a high transparency.Then the excitation and emission spectra of the Ce~(3+) and Tb~(3+) single-doped and co-doped glass and glass ceramics were discussed,which proved that the glass ceramics had better luminescent properties.Under the near ultraviolet(331 nm)excitation,the broadband emission located at 385 nm was observed which was ascribed to 5d→~2F_(5/2) and ~2F_(7/2) transition of Ce~(3+) ions.Several characteristic sharp peaks centered at 489,543,578 and 620 nm originated from the ~5D_4 to ~7F_J(J=6,5,4,3)of Tb~(3+) ions.The decay time of Tb~(3+) ions at 543 nm and the relevant energy levels of Ce~(3+) ions and Tb~(3+) ions illustrated the transfer process from Ce~(3+) ions to Tb~(3+) ions.The best CIE chromaticity coordinate of the glass ceramics specimen was calculated as(x=0.3201,y=0.3749),which was close to the NTSC standard values for white(x=0.333,y=0.333).All the results suggested that the YPO~4-based Ce~(3+)/Tb~(3+) co-doped glass ceramics could act as potential luminescent materials for white light-emitting diodes.     

Optical and luminescence characteristics of Eu3+-doped B2O3:SiO2:Y2O3:CaO glasses for visible red laser and scintillation material applications

Europium(Eu~(3+)) doped glasses of chemical compositions(55-x)B_2O_3:10 SiO_2:25 Y_2O_3:10CaO:xEu_2O_3,where x denotes mol% and ranges 0≤ x ≤ 2.5, were synthesized by adopting conventional melt quenching technique, Physical properties like density, molar volume, polaron radius, inter-ionic distance and field strength of the glass samples were investigated to assess the impact of Eu_2O_3. Optical and luminescence properties of the glasses were characterized with optical absorption, photoluminescence,X-ray induced emission spectra, temperature dependence emission spectra and decay times. Judd-Ofelt(JO) intensity parameters(Ω_λ) of the glasses were evaluated based on the absorption spectrum of 0.5 mol%. JO parameters, calculated from absorption spectra with thermal corrections on oscillator strength, were used to evaluate radiative properties such as radiative transition probability(A_R),branching ratio(β_R), stimulated cross section emission(σ) and radiative lifetime(τ_R) for ~5D_0→~7 F_J(J = 0,1,2,3 and 4) transitions. The decay rate of ~5D_0 fluorescent level for all the glass samples was single exponential. Lifetimes of the ~5D_0 level were decreased with increasing concentrations of Eu~(3+)ions from 0.05 mol% to 2.5 mol% which might be due to energy transfer through cross-relaxation in the glasses. The chromaticity coordinates(x, y) were similar for all BSYCaEu glasses and were located at the red region of CIE 1931 color chromaticity diagram. Hence, these results confirm that the Eu~(3+) doped BSYCaEu glasses could be useful for visible red lasers and glass scintillation applications.     

Luminescence properties of Eu~(3+) doped YBO_3 for temperature sensing

YBO_3:2 at.% Eu~(3+) was prepared by the solid state reaction and its temperature dependent luminescence was investigated for possible applications in temperature sensing. Phase composition of this material was confirmed by X-ray powder diffraction analysis and excitation and emission spectra were also provided. Under excitation of 355 nm, the fluorescence originating from ~5D_0 and ~5D_1 states varied as the temperature rose in a region from 333 to 773 K. The fluorescence intensity ratio(FIR) of ~5D_0 and ~5D_1 was investigated which increased significantly with the rise of temperature. The maximal relative sensitivity in the whole temperature range was 1.8% K~(–1)(at 333 K). The results recommended YBO_3:Eu~(3+) as a new material of the FIR method for non-contact optical thermometry.     

Synthesis and optical characterization of Eu2+,Tb3+-codoped Sr3Y(PO4)3 green phosphors

A series of Eu~(2+),Tb~(3+)-codoped Sr_3 Y(PO_4)_3(SYP) green phosphors were synthesized by hightemperature solid-state reaction. Several techniques, such as X-ray diffraction, UV-vis spectrum,and photoluminescence spectrum, were used to investigate the obtained phosphors. The present study investigates in detail photoluminescence excitation and emission properties, energy transfer between the two dopants, and effects of doping ions on optical band gap. SYP:0.05 Eu2+ phosphor shows an intense and broad excitation band ranging from 220 to 400 nm and exhibits a bright green emission band with CIE chromaticity coordinates(0.189, 0.359) under 350 nm excitation. Green emission of SYP:0.03 Tb3+ is intensified by codoping with Eu~(2+), and energy transfer mechanism between them is demonstrated to be a dipole-dipole interaction. Upon 350 nm excitation, SYP:Eu~(2+),Tb~(3+) phosphors exhibits two dominating bands peaking at 466 and 545 nm, which are assigned to 4 f~65 d~1→4 f~7 transition of Eu~(2+) ions and ~5 D_4→~7 F_5 transition of Tb~(3+) ions, respectively. Optimal doping concentrations of Eu~(2+) and Tb~(3+) in the SYP host are 5 mol% and 15 mol%, respectively. Results indicate that SYP:Eu~(2+),Tb~(3+) phosphors are potentially used as green-emitting phosphors for white light-emitting diodes.     

Synthesis and thermometric properties of Yb~(3+)-Er~(3+) co-doped K_2GdF_5 up-conversion phosphors

Yb~(3+)-Er~(3+) co-doped K_2GdF_5 up-conversion phosphor was successfully synthesized by a solid-state reaction method. The phase purity and structure of the sample were characterized by powder X-ray diffraction. The sample emitted orange light at room temperature and its up-conversion spectra at different temperatures were recorded under the excitation of a 980 nm diode laser. The energy transfer from Yb~(3+) to Er~(3+) notably enhanced the up-conversion luminescence intensity. The possible up-conversion mechanisms and processes were proposed based on the power dependence of the luminescence intensities. The temperature-dependent up-conversion luminescence and temperature sensing performances of the sample were discussed according to the fluorescence intensity ratio of green emissions originating from ~2H_(11/2)/~4S_(3/2)→~4I_(15/2) transitions of Er~(3+) in the range from 307 K to 570 K under the excitation of 980 nm laser with power of 260 mW. The dependence of the fluorescence intensity ratio on temperature was fitted with an exponential function and the effective energy difference obtained was 690 cm~(–1), which further gave a relative temperature sensitivity of 1.1%/K at 307 K. The results suggested that the Yb~(3+)-Er~(3+) co-doped K_2GdF_5 sample is a promising candidate for optical temperature sensor.     

High thermal stability and quantum yields of green-emitting Sr_3Gd_2(Si_3O_9)_2:Tb~(3+) phosphor by co-doping Ce~(3+)

A series of Tb~(3+) mono-doped and Ce~(3+)-Tb~(3+) co-doped Sr_3Gd_2(Si_3O_9)_2 phosphors with high thermal stability and quantum yields were successfully prepared via the solid state reaction. The as-prepared Sr_3Gd_2(Si_3O_9)_2:Tb~(3+) samples showed broad excitation spectrum from 250 to 400 nm and presented characteristic emission transitions ~5D_4→~7F_J(J=6, 5, 4, 3) of Tb~(3+) under 313 nm excitation, which were located at about 488, 541, 584 and 620 nm. The emission intensities of Tb~(3+) rose steadily in Sr_3Gd_2(Si_3O_9)_2 host with the increase of Tb~(3+) concentration even though Gd~(3+) ions were completely replaced by Tb~(3+) ions. The Ce~(3+) ion as a sensitizer could efficiently improve the performance of Tb~(3+) ion. First, with Ce~(3+) co-doping, the excitation spectrum of Tb~(3+) monitored at 541 nm showed a similar band that responds to the violet emission of Ce~(3+) monitored at 416 nm. Second, the quantum yields of Sr_3Gd_2(Si_3O_9)_2:Tb~(3+) phosphors could be enhanced from 26.6% to 80.2% by co-doping Ce~(3+). Finally, the co-doping of Ce~(3+) was also effective to improve the thermal stability of Sr_3Gd_2(Si_3O_9)_2:Tb~(3+). As the temperature rose to 150 oC, the emission intensity of Tb~(3+) remained at about 83.6% of that measured at room temperature, which was better than the commercial YAG:Ce phosphor in terms of their thermal quenching properties. These results indicated that the as-prepared Sr_3Gd_2(Si_3O_9)_2:Tb~(3+),Ce~(3+) samples could be used as green emission phosphors for possible applications in near ultraviolet based WLEDs.     

Color-tunable Ca10Na(PO4)7:Ce3+/Tb3+/Mn2+ phosphor via energy transfer

A series of Ca_(10)Na(PO_4)_7:Ce~(3+)/Tb~(3+)/Mn~(2+)(CNPO:Ce~(3+)/Tb~(3+)/Mn~(2+)) phosphors with high brightness were synthesized by high-temperature solid-state method. X-ray diffraction(XRD), scanning electron microscopy(SEM), diffuse reflectance spectra(DRS), photo luminescence(PL) spectra, luminescence decay curves and thermally stability were performed to characterize the as-prepared samples. For Ce~(3+)-doped samples, an intense and broad band emission is present under 265 nm excitation. When Ce~(3+) and Tb~(3+)are codoped, energy transfer(ET) process from Ce3+ to Tb3+ is demonstrated with electric dipole-dipole interaction. The internal and external quantum efficiencies(QEs) of CNPO:0.15 Ce~(3+), 0.04 Tb~(3+), 0.005 Mn~(2+)are measured to 76.79% and 54.11% under 265 nm excitation and temperature-dependent PL intensity can remain 51.78% at 150 ℃ of its initial intensity at 25 ℃. It is indicated that single-phased white lightemitting CNPO:Ce~(3+)/Tb~(3+)/Mn~(2+) phosphor can serve as a promising phosphor for illumination devices.     

Effect of Eu~(3+) contents on structure and luminescence properties of Na_3Bi_(2–x)(PO_4)_3:xEu~(3+) and Na_3Bi_(1–x)(PO_4)_2:xEu~(3+) phosphors

A series of Na_3Bi_(2–x)(PO_4)_3:xEu~(3+) and Na_3Bi_(1–x)(PO_4)_2:xEu~(3+) phosphors were successfully synthesized by solid-state method.The structure and luminescence properties were carefully investigated.The excitation spectra presented an obvious excitation band,and the peak was located at 396 nm,which matched well with the popular emissions from near-UV light-emitting diode chips.With the phase of Na_3Bi_(2–x)(PO_4)_3:xEu~(3+) changing to that of Na_3Bi_(1–x)(PO_4)_2:xEu~(3+),the intensity of magnetic dipole transition(~5D_0→F_1) at 598 nm became stronger than that of electric dipole transition(~5D_0→~7F_2) at 621 nm.Under 396 nm excitation,the chromaticity coordinates and the decay curves of the entitled phosphors were also investigated.Based on all experiments without concentration quenching,we could control the luminescence intensity of the material by adjusting the doping amount of the active ions.All results indicated that Na_3Bi_(2–x)(PO_4)_3:xEu~(3+) and Na_3Bi_(1–x)(PO_4)_2:xEu~(3+) phosphors have potential application as red phosphors in near UV chip-based white light emitting diodes.     

Study on white-light emission and light-emitting mechanism of hydrothermally synthesized YVO_4:1 mol.%Dy~(3+),x mol.%Eu~(3+) phosphor powders

White light-emitting YVO_4:1 mol.%Dy~(3+),x mol.%Eu~(3+) phosphor powders with order morphology and well crystallization were hydrothermally synthesized at 180°C. The microstructure, white-light emission, and light-emitting mechanism of the powders were carefully studied using X-ray diffractometry, scanning electron microscopy and photoluminescence spectra. The excitation and emission spectra of the phosphor powders indicated the coexistence of efficient energy transfer from Eu~(3+) to Dy~(3+) and inefficient energy transfer from Dy~(3+) to Eu~(3+) besides the energy transfer from VO_4~(3–) to Eu~(3+). Increasing the Eu~(~(3+)) concentration initially enhanced and then weakened the luminescent intensity of Dy~(3+). The white-light emissions of YVO_4:1 mol.%Dy~(3+),x mol.%Eu~(3+) phosphor powders were both related to the energy transfer between VO_4~(3–) and Dy~(3+)/Eu~(3+), as well as between Eu~(3+) and Dy~(3+). The inefficient energy transfer from Dy~(3+) to Eu~(3+) was first found.     

Influence of reaction parameters on the luminescence properties of monodisperse NaGd(1−x−y)(WO4)2:xEu3+,yTb3+ phosphor by molten salt synthesis

Eu~(3+) activated and Eu~(3+), Tb~(3+) co-activated monodisperse sodium double tungstates NaGd(WO4)2 phosphors were prepared by molten salt method at 750 ℃ for 10 h using NaCl as a flux. The crystal structure and morphology of the as-synthesized phosphors were measured by XRD and SEM, respectively. The photoluminescence properties were characterized by PL spectra, decay lifetime and CIE. The presence of NaCl plays an important role in the morphology and luminescence properties. In this work,NaCl and one of the raw material Na_2 CO_3 in a certain proportion will form a low eutectic salt to decrease the reaction temperature and benefit the formation of monodisperse NaGd(WO_4)_2 crystals. The color of Eu~(3+) and Tb~(3+) co-doped NaGd(WO_4)_2 phosphors can be tuned from creamy white to orange, red and green by adjusting the doping concentration of rare earth ions, since the emission contain the broad blue-green emission origin from NaGd(WO_4)_2 host and characteristic red and green emission origin from Eu~(3+) and Tb~(3+) ions. The electroluminescent spectra and CIE measurement shows that the LED device with NaGd_((1-x))(WO_4)_2:xEu~(3+)(x = 0.24) phosphor can be excited by 365 nm and 380 nm LED chip, and their CIE coordinate is(x = 0.45, y = 0.45) and(x = 0.36, y = 0.37), Ra is 80.3 and 86.3, T_c is 3196 and4556 K, respectively. As a single-component phosphor, NaGd(WO_4)_2:Eu~(3+),Tb~(3+) have potential application in UV-pumped WLEDs.     

Luminescent properties and energy transfer mechanism from Tb~(3+) to Eu~(3+) in CaMoO_4:Tb~(3+),Eu~(3+) phosphors

A series of CaMoO_4:Tb~(3+),Eu~(3+) phosphors were prepared by the method of precipitation. The structure and morphology of the phosphors were characterized by the X-ray diffraction(XRD) and field emission scanning electron microscopy(FE-SEM). The photoluminescence properties of the prepared products were researched, and the energy transfer from Tb~(3+) to Eu~(3+) in CaMoO_4 phosphors was studied. By adjusting the doping concentration of Eu~(3+) ions in CaMoO_4:Tb~(3+),Eu~(3+) phosphors, the emitting color of the phosphors could be easily tuned from green to red. With Tb~(3+) doped in the phosphors, the red luminescence of Eu~(3+) by near UV excitation was significantly enhanced. The energy transfer efficiency, rate and average distance between Tb~(3+) and Eu~(3+) in CaMoO_4:5%Tb~(3+),x%Eu~(3+)(mole percent) phosphors(x=0.3–10) were calculated. It was found that the interaction type between Tb~(3+) and Eu~(3+) was electric dipole-dipole interaction in the phosphors.