Synthesis and luminescence properties of YVO4:Eu,Bi phosphor with enhanced photoluminescence by Bi doping

YVO₄:Eu³⁺,Bi³⁺ phosphors have been prepared by the high-temperature solid-state (HT) method and the Pechini-type sol-gel (SG) method. Spherical SiO₂ particles have been further coated with YVO₄:Eu³⁺,Bi³⁺ phosphor layers by the Pechini-type SG process, and it leads to the formation of core-shell structured SiO₂/YVO₄:Eu³⁺,Bi³⁺ phosphors. Therefore, the phase formations, structures, morphologies, and photoluminescence properties of the three types of as-prepared YVO₄:Eu³⁺,Bi³⁺ phosphors were studied in detail. The average diameters for the phosphor particles are 2-4 μm for HT method, 0.1-0.4 μm for SG method, and 0.5 μm for core-shell structured SiO₂/YVO₄:Eu³⁺,Bi³⁺ particles, respectively. Photoluminescence spectra show that effective energy transfer takes place between Bi³⁺ and Eu³⁺ ions in each type of as-prepared YVO₄:Eu³⁺,Bi³⁺ phosphors. Introduction of Bi³⁺ into YVO₄:Eu³⁺ leads to the shift of excitation band to the long-wavelength region, thus the emission intensities of ⁵D₀-⁷F₂ electric dipole transition of Eu³⁺ at 615 nm upon 365 nm excitation increases sharply, which makes this phosphor a suitable red-emitting materials that can be pumped with near-UV light emitting diodes (LEDs).     

Correlation of photoluminescence of (Y, Ln)VO4:Eu (Ln=Gd and La) phosphors with their crystal structures

We have studied the photoluminescence (PL) of (Y, Ln)VO₄:Eu³⁺ (Ln=La and Gd) phosphors and the correlation of the PL of those phosphor with their crystal structure. It is found that (Y, Gd)VO₄:Eu³⁺ phosphors have the same crystal structure as YVO₄:Eu³⁺, which is tetragonal with a little different lattice parameters. In the case of (Y, La)VO₄:Eu³⁺ phosphors, however, the gradual change from tetragonal to monoclinic structure of host lattice was observed as the amount of La ion increased. To investigate the PL property of (Y, Ln)VO₄:Eu³⁺ (Ln=La and Gd) phosphors, vacuum ultraviolet (VUV) and ultraviolet (UV) excitation were used. The favorable crystal structure for the PL intensity of orthovanadate phosphor under 147 and 254 nm excitation was tetragonal containing Gd ion and under 365 nm excitation was monoclinic containing La ion which might have the lowest site symmetry for Eu³⁺ ion.     

Enhancement of luminescence properties of Eu:YVO4 in polymeric nanocomposites upon UV excitation

Highly emissive Eu³⁺:YVO₄ nanocrystals were successfully prepared by the hydrothermal method. The average diameter of grains was determined to be 15 nm. Transparent polymer nanocomposites composed of PMMA and well dispersed Eu³⁺:YVO₄ nanocrystals were fabricated by in situ polymerization. The nanocrystalline powders and nanocomposites were characterized by scanning and transmission electron microscopy (TEM) and X-ray diffraction (XRD). The luminescence properties of the obtained nanocomposites were investigated and compared with the starting powders and Eu³⁺:YVO₄ single crystal. The effect of the polymeric host on the luminescence properties of Eu³⁺:YVO₄ is presented and discussed.     

纳米棒状GdPO4:Eu3+荧光粉的合成及其发光性能的研究

采用溶解热法合成出了纳米棒状GdPO₄:Eu³⁺荧光粉,其中不同磷源和pH值对最终产物的形成起着关键的作用. 将纳米棒状和块体GdPO₄:Eu³⁺荧光粉的发光性能进行了对比,其中与块体GdPO₄:Eu³⁺荧光粉相比,纳米棒状GdPO₄:Eu³⁺荧光粉的色纯度得到了改善,而在激发光谱中,纳米棒状GdPO₄:Eu< 关键词： 纳米棒 磷酸钆 发光性能     

Improved photoluminescence of pulsed-laser-ablated Y<Subscript>1-x</Subscript>Gd<Subscript>x</Subscript>VO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> thin film phosphors by Gd substitution

Gd-substituted Y_1-xGd_xVO₄:Eu³⁺ luminescent thin films have been grown on Al₂O₃(0001) substrates using pulsed-laser deposition. The films grown under different deposition conditions have been characterized using microstructural and luminescent measurements. The crystallinity, surface morphology, and photoluminescence (PL) of the films are highly dependent on the amount of Gd. The photoluminescence (PL) brightness data obtained from Y_1-xGd_xVO₄:Eu³⁺ films grown under optimized conditions have indicated that the PL brightness is more dependent on the surface roughness than the crystallinity of the films. In particular, the incorporation of Gd into the YVO₄ lattice could induce a remarkable increase of PL. The highest emission intensity was observed with Y_0.57Gd_0.40Eu_0.03VO₄ thin film whose brightness was increased by a factor of 2.5 and 1.9 in comparison with that of YVO₄:Eu³⁺ and GdVO₄:Eu³⁺ films, respectively. This phosphor have application to flat panel displays. PACS 78.20.-e; 78.55.-m; 78.66.-w     

Preparation and photoluminescence properties of a new orange–red Ba3P4O13:Eu phosphor

A new orange–red Ba₃P₄O₁₃:Eu³⁺ phosphor has been synthesized by solid-state technique, and its photoluminescence properties were investigated. X-ray powder diffraction (XRD) analysis indicates that doping Eu³⁺ does not change the lattice of Ba₃P₄O₁₃. Field-emission scanning electron microscope (FE-SEM) images illustrate that microstructure of the phosphor consists of oval grains with average diameter of 1 μm and heavy agglomerate phenomenon. The excitation spectra indicate the phosphor can be effectively excited by near ultraviolet (NUV) light, making it attractive as conversion phosphor for LED applications. The phosphor exhibits a bright orange–red emission excited by 394 nm light. The CIE chromaticity can be varied slightly by adjusting the content of Eu³⁺, which is attributed to the different lattice sites occupied by Eu³⁺ in Ba₃P₄O₁₃ host. The photoluminescence studies indicate that Ba₃P₄O₁₃:Eu³⁺ is a potential orange–red phosphor for near-ultraviolet InGaN-based white light-emitting diodes (WLEDs).     

Controlled oxidation of graphite to graphene oxide with novel oxidants in a bulk scale

A series of different concentrations of Eu³⁺ and Dy³⁺ ions co-doping yttrium vanadate phosphors coated with Fe₃O₄ (YVO₄:Eu³⁺, Dy³⁺@Fe₃O₄) was successful prepared by using two steps route including sol?Cgel method and hydrothermal method. The resulting phase formation, particle morphology, structure, luminescent, and magnetic properties were examined by X-ray diffraction, transmission electron microscopy, photoluminescence spectra, and vibrating sample magnetometer. The results indicate that the diameter of the YVO₄:Eu³⁺, Dy³⁺@Fe₃O₄ nanocomposites is 100?C300?nm. The special saturation magnetization Ms of the nanocomposites is 53?emu/g. Additionally, the emission intensities of YVO₄:Eu³⁺ or Dy³⁺ ions are regularly changed with the emission doping concentrations. After coating with Fe₃O₄, the variation of the luminescent intensity of YVO₄:Eu³⁺, Dy³⁺@Fe₃O₄ magnetic phosphors is different.     

Synthesis and luminescent properties of hexagonal BaZnSiO4:Eu2+ phosphor

A green-emitting phosphor of hexagonal BaZnSiO₄:Eu²⁺ was prepared by a combustion-assisted synthesis method and an efficient green emission from ultraviolet to visible light was observed. The luminescence and crystallinity were investigated by using luminescence spectrometry and X-ray diffractometry. In the hexagonal structure of BaZnSiO₄:Eu²⁺ phosphor, Eu²⁺ ions occupy three different lattice sites by substitution for Ba²⁺ ions. Eu²⁺ ions on Ba (1) and Ba (2) sites gave emissions at about 505 nm while Eu²⁺ ions on Ba (3) sites showed an emission band at 403 nm. The excitation spectrum is a broad band extending from 260 to 465 nm, which matches the emission of ultraviolet light-emitting diodes. The critical quenching concentration of Eu²⁺ in BaZnSiO₄:Eu²⁺ phosphor is about 0.05 mol. The value of the critical transfer distance is calculated as 10.97 Å. The corresponding concentration quenching mechanism is verified to be the electric multipole–multipole interaction. The CIE coordinates of the optimized sample $\mathrm{Ba}_{0.95}\mathrm{ZnSiO}_{4}{:}\mathrm{Eu}_{0.05}^{2+}$ were calculated as (x,y)=(0.172,0.463).     

A potential red phosphor LiGd(MoO4)2:Eu for light-emitting diode application

Eu³⁺-doped LiGd(MoO₄)₂ red phosphor was synthesized by solid-state reaction, and its photoluminescent properties were measured. The effect of Eu³⁺ doping concentration on PL intensity was investigated, and the optimum concentration of Eu³⁺ doped in LiGd(MoO₄)₂ was found to be 30 mol%. Compared with Y₂O₂S:0.05Eu³⁺, Na_0.5Gd_0.5MoO₄:Eu³⁺ and KGd(MoO₄)₂:Eu³⁺, the LiGd(MoO₄)₂:Eu³⁺ phosphor showed a stronger excitation band around 395 nm and a higher intensity red emission of Eu³⁺ under 395 nm light excitation. For the first time, intensive red light-emitting diodes (LEDs) were fabricated by combining phosphor and a 395 nm InGaN chip, confirming that the LiGd(MoO₄)₂:Eu³⁺ phosphor is a good candidate for LED applications.     

Effects of x and R3+ on the luminescent properties of Eu3+ in nanocrystalline YVxP1-xO4:Eu3+ and RVO4:Eu3+ thin-film phosphors

Using inorganic oxides and salts instead of alkoxides as the main starting materials, we prepared nanocrystalline YV_xP_1-xO₄:Eu³⁺ and RVO₄:Eu³⁺ (0x1; R=Y,La,Gd) thin-film phosphors by the Pechini sol–gel dip-coating process. The resulting films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and photoluminescence excitation and emission spectra as well as luminescence decay. The results of XRD showed that a solid solution formed in the YV_xP_1-xO₄:Eu³⁺ film series from x=0 to x=1 with zircon structure. The same structure also held for the GdVO₄:Eu³⁺ film, but the LaVO₄:Eu³⁺ film crystallized with a different structure, monazite. AFM and SEM studies revealed that the phosphor films consisted of spherical particles ranging from 90 to 400 nm depending on the film compositions. With the increase of x values in YV_xP_1-xO₄:Eu³⁺ films, the integrated emission intensity and the red (⁵D₀–⁷F₂)-to-orange (⁵D₀–⁷F₁) intensity ratio of Eu³⁺ increase due to the increased energy-transfer probability from VO₄^3- to Eu³⁺ and the increased polarizability of the surrounding oxygen ions, respectively. The x values also have an influence on the decay behavior of Eu³⁺. The YVO₄:Eu³⁺ and GdVO₄:Eu³⁺ films showed very similar luminescence properties due to their same crystal structures. However, the LaVO₄:Eu³⁺ film exhibited a much different emission property from those of the YVO₄:Eu³⁺ and GdVO₄:Eu³⁺ films due to the structural effects. PACS 73.63.Bd; 78.55.Hx; 78.66.Nk; 81.15.Lm; 81.20.Fw     

Photoluminescence of colloidal YVO4:Eu/SiO2 core/shell nanocrystals

YVO₄:Eu, and YVO₄:Eu/SiO₂ nanocrystals (NCs) were prepared by hydrothermal method with citrate as capping ligands. Their morphologies, structures, components, and photoluminescence properties were investigated and presented in this paper. A remarkable fluorescence enhancement up to 2.17 times was observed in colloidal YVO₄:Eu/SiO₂ NCs, compared to that of colloidal YVO₄:Eu NCs. This is mainly attributed to the formation of the outer protecting layers of biocompatible SiO₂ shells; which shield the Eu³⁺ ions effectively from water and thus reduces the deleterious effects of water on the luminescence. Meanwhile, on the basis of laser selective excitation, two kinds of luminescent centers were confirmed in the NCs, namely, inner Eu³⁺ ions and surface Eu³⁺ ions. The surface modifications for YVO₄:Eu NCs effectively reduced the surface defects and accordingly enhanced the luminescence. The core/shell NCs exhibited long fluorescence lifetime and high photostability under ultraviolet radiation.     

Luminescent properties of red-emitting LiSr4B3O(9−3x/2)Nx:Eu2+ phosphor for white-LEDs

An Eu²⁺-activated oxynitride LiSr_(4?y)B₃O_(9?3x/2)N_x:yEu²⁺ red-emitting phosphor was synthesized by solid-state reactions. The synthesized phosphor crystallized in a cubic system with space group Ia–3d. The LiSr₄B₃O_(9?3x/2)N_x:Eu²⁺ phosphors exhibited a broad red emission band with a peak at 610 nm and a full width at half maximum of 106 nm under 410 nm excitation, which is ascribed to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. The optimal doped nitrogen concentration was observed to be x=0.75. The average decay times of two different emission centers were estimated to be 568 and 489 ns in the LiSr_3.99B₃O_8.25N_0.5:0.01Eu²⁺ phosphors, respectively. Concentration quenching of Eu²⁺ ions occurred at y=0.07, and the critical distance was determined as 17.86 Å. The non-radiative transitions via dipole–dipole interactions resulted in the concentration quenching of Eu²⁺-site emission centers in the LiSr₄B₃O₉ host. These results indicate LiSr₄B₃O_(9?3x/2)N_x:Eu²⁺ phosphor is promising for application in white near-UV LEDs.     

Synthesis, luminescence and crystallographic structure of Eu-doped garnet-type yafsoanite Ca3Te2(ZnO4)3

A red-emitting phosphor of Eu³⁺-doped calcium–tellurium–zinc oxide, Ca₃Te₂(ZnO₄)₃, with a garnet-type structure was synthesized by high temperature solid-state reactions. This phosphor exhibited a strong red emission. The photoluminescence excitation spectrum showed that Ca₃Te₂(ZnO₄)₃:Eu³⁺ can be effectively excited by UV–visible light. The property of long-wavelength excitation for this material has a benefit as a red phosphor in application of white light-emitting diodes. The colour coordinates were calculated. The excitation and emission spectra and luminescence decay curves were obtained using a pulsed, tunable, narrowband dye laser. Crystallographic sites and charge compensation mechanism of Eu³⁺ ions were discussed. The emission line from Eu³⁺ in intrinsic crystallographic site in the lattice was located at 579.56 nm. The emission line from Eu³⁺ in another disturbed site, which is created by the defects created by the charge-compensation, was located at 580.88 nm. The disordered crystallographic sites of Eu³⁺ are benefit for their strong red luminescence corresponding to the ⁵D₀→⁷F₂ transition.     

Low-temperature wet chemical synthesis and photoluminescence properties of YVO4: Bi, Eu nanophosphors

YVO₄: Bi³⁺, Eu³⁺nanophosphors are prepared by the citrate-assisted low-temperature wet chemical synthesis. When the colloidal solution is aged at 60 °C, the crystalline YVO₄: Bi³⁺, Eu³⁺ nanorods are formed from the amorphous gel precursors, as confirmed by transmission electron microscopy and X-ray diffractometry (XRD). YVO₄: Bi³⁺, Eu³⁺ nanophosphors emit red through energy transfer from Bi³⁺ to Eu³⁺ under near-UV-light excitation. The emission intensity increases with increasing the fraction of the crystalline phase during aging. The excitation peak corresponding to Bi³⁺-V⁵⁺ charge transfer relative to those of O²⁻-V⁵⁺ and O²⁻-Eu³⁺ charge transfers gradually becomes strong until the completion of the crystallization, although the contents of individual Bi³⁺ and Eu³⁺ ions incorporated into YVO₄ keep constant. When the aging is continued after the completion of the crystallization, the content of incorporated Bi³⁺ gradually increases, and hence the emission intensity decreases as a result of the energy migration among Bi³⁺ ions. These results suggest that in addition to the fraction of the crystalline phase and the contents of incorporated Bi³⁺ and Eu³⁺ ions, the local chemical states around Bi³⁺ play significant roles in photoluminescence properties.     

Analysis of surface effect on luminescent properties of Eu in YVO4 nanocrystals

In this Letter, ⁷F₂ crystal field (CF) levels of surface Eu³⁺ in YVO₄ nanocrystals are calculated employing a refined electrostatic point charge model, where surface states are simulated by point charges. Based on the theoretical ⁷F₂ CF levels, emission spectra of YVO₄: Eu³⁺ nanocrystals are assigned to Eu³⁺ under different local environments. and relaxation of selection rules by surface effect is discussed.     

Spectral properties and emission efficiencies of GdVO4 phosphors

GdVO₄ with activators Eu, Dy, Sm and Bi has been synthesised by a solid-state reaction. GdVO₄:Eu³⁺ (3%) yielded the highest quantum efficiency of 95%. Interesting energy-transfer properties have been revealed in the mixed-activator phosphor (GdVO₄:Eu³⁺, Sm³⁺) when excited in the 4f shell of Sm³⁺ at 408 nm. Bismuth-activated GdVO₄ gives rise to a broad-band emission peaking at 525 nm in comparison to YVO₄:Bi³⁺, which gives an emission peak at 570 nm under UV excitation. The quantum efficiency of GdVO₄:Bi³⁺ increases gradually with bismuth concentration and reaches a maximum of 80% for a bismuth concentration of ≈0.5%. There is a shift in the excitation band of GdVO₄:Bi³⁺ towards longer wavelengths with increasing concentration of bismuth, which can lead to energy transfer from bismuth to europium in a phosphor with both these activators. Heat treatment of GdVO₄:Bi³⁺ at 1500 °C for 3–3.5 h resulted in a large percentage of bismuth being lost from the lattice as evaluated by X-ray fluorescence. However, if a large percentage of bismuth (of the order of 3% or more) is initially added, a sufficient quantity of bismuth can still be retained after heat treatment, which can lead to the development of ceramic scintillators for X-ray tomographic applications. Addition of 3–5% boron gives a white GdVO₄ phosphor without any chemical treatment. Received: 27 Feruary 2001 / Accepted: 1 August 2001 / Published online: 30 October 2001     

Cathodoluminescence characteristics of particles and film of (Y, Zn)2O3:Eu phosphor prepared by spray pyrolysis

Eu-doped Y₂O₃ particles with spherical shape and fine size were prepared by spray pyrolysis. The cathodoluminescence of Y₂O₃:Eu³⁺ powder was optimized by substituting small amount of zinc atoms in place of yttrium sites. As a result, the optimized (Y, Zn)₂O₃:Eu³⁺ phosphor showed 60% improved cathodoluminescence compared with Y₂O₃:Eu³⁺ particles. The prepared (Y, Zn)₂O₃:Eu³⁺ phosphor had spherical shape and 0.726 μm in mean size. Using these particles, the thickness of the phosphor film was controlled by varying the phosphor loading. The brightness and luminous efficiency of phosphor films prepared were monitored with varying the accelerating voltage ranges from 4 to 14 kV. The dependency of the luminous efficiency on the accelerating voltage was very sensitive to the phosphor loading. As increasing the accelerating voltage from 4 to 14 kV, the brightness of phosphor films prepared was monotonically increased from 200 to 1085 cd/cm²_, but the saturation in the luminous efficiency appeared at 10 kV. The highest efficiency was achieved when the number of phosphor-particles layer was about 3. More details about the luminous efficiency and brightness were discussed with changing the phosphor loading.     

Luminescent properties and energy transfer of double-emitting NaSrPO4:Eu2+,Tb3+ phosphor for near-UV white LEDs

Novel blue/green NaSrPO₄ phosphors co-doped with Eu²⁺ and Tb³⁺ were synthesized by a conventional solid-state reaction. Their luminescent properties were characterized by using powder X-ray diffraction, photoluminescence excitation and emission spectra, lifetime, and temperature dependent emission spectra, respectively. The NaSrPO₄:Eu²⁺,Tb³⁺,Na⁺ phosphor showed an intense broad excitation band between 250 and 430 nm, which was in agreement with the near-UV chip (350–420 nm), and it exhibited two dominating emission bands at 445 and 545 nm, corresponding to the allowed 4f⁶5d¹→4f⁷(⁸S_7/2) transition of Eu²⁺ ion and the ⁵D₄→⁷F₅ transition of Tb³⁺ ion, respectively. The emission intensity and lifetime of Eu²⁺ ion decreased with the increasing concentration of Tb³⁺ ion, which strongly indicated that an effective energy transfer occurred from Eu²⁺ to Tb³⁺ in NaSrPO₄ host. The principle of the energy transfer should be the combined effect of the non-radiative resonant energy transfer and the phonon-assisted non-radiative process.     

Luminescent properties of Mg3Ca3(PO4)4: Eu2+ blue-emitting phosphor for white light emitting diodes

A blue-emitting phosphor, Eu²⁺-activated Mg₃Ca₃(PO₄)₄ phosphor was synthesized by conventional solid-state reaction. X-ray powder diffraction (XRD) analysis confirmed the phase formation. Photoluminescence (PL) results showed that Mg₃Ca₃(PO₄)₄: Eu²⁺ could be efficiently excited by UV–visible light from 250 to 430 nm, which matched well with the emission wavelengths of near-UV and UV LED chips. The effects of the doped-Eu²⁺ concentration in Mg₃Ca₃(PO₄)₄: Eu²⁺ on the PL were also investigated. The result reveals that Mg₃Ca₃(PO₄)₄: Eu²⁺ is a potential blue-emitting phosphor for white LEDs.     

Effect of nano-sized SiO2 on the optical properties of YVO4:Eu3+ phosphors

In this paper, YVO₄: 1%Eu³⁺ phosphor was synthesized via solid state method at 1100 °C. Then TEOS was used as the source of silica, to coat the phosphors, using sol–gel approach. HRTEM analysis confirmed the formation of adhered and smooth coating layer with the thickness of 40–50 nm. From the experiments and characterizations, we found that although the amounts of added SiO₂ to the phosphors were not remarkable, but it resulted in enhancement of photoluminescence properties. Interestingly, under the excitation wavelength of 310 nm, the efficiency of the phosphors increased by about 20%. Also, a considerable effect of coating layer on decrease in surface oxygen vacancies was studied using ESR technique. Finally it was found that SiO₂ coating of YVO₄:Eu³⁺ phosphors, improves both chemical stability and thermal quenching, effectively.