Sol-gel growth of Gd2MoO6:Eu nanocrystalline layers on SiO2 spheres (SiO2@Gd2MoO6:Eu) and their luminescent properties

SiO₂@Gd₂MoO₆:Eu³⁺ core-shell phosphors were prepared by the sol-gel process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectra (EDS), transmission electron microscopy (TEM), photoluminescence (PL) spectra as well as kinetic decays were used to characterize the resulting SiO₂@Gd₂MoO₆:Eu³⁺ core-shell phosphors. The XRD results demonstrate that the Gd₂MoO₆:Eu³⁺ layers on the SiO₂ spheres begin to crystallize after annealing at 600 °C and the crystallinity increases with raising the annealing temperature. The obtained core-shell phosphors have a near perfect spherical shape with narrow size distribution (average size ca. 600 nm), are not agglomerated, and have a smooth surface. The thickness of the Gd₂MoO₆:Eu³⁺ shells on the SiO₂ cores could be easily tailored by varying the number of deposition cycles (50 nm for four deposition cycles). The Eu³⁺ shows a strong PL luminescence (dominated by ⁵D₀-⁷F₂ red emission at 613 nm) under the excitation of 307 nm UV light. The PL intensity of Eu³⁺ increases with increasing the annealing temperature and the number of coating cycles.     

Preparation and luminescent properties of phosphor MGd2(MoO4)4: Eu (M=Ca, Sr and Ba)

Intense red emitting phosphors MGd₂(MoO₄)₄: Eu³⁺ (M=Ca, Sr and Ba) have been synthesized by the simple sol-gel technique. The formation processes and the phase impurity of phosphors are characterized by thermogravimetry-differential thermal analysis (TG-DTA) and power X-ray diffraction (XRD). The narrower size distribution and the regular shape of the phosphor particles are also measured by Field emission scanning electronic microscopy (FE-SEM). Photo-luminescent properties of the phosphors are performed at room temperature. Their excitation spectra present strong absorption at 395 nm near-UV light and 465 nm blue light, which match well with commercial LED chips. The phosphors exhibit satisfactory and excellent red light dominated by 616 nm and their photoluminescence intensity is about 3-4 times stronger than that of phosphor YAG under the 465 nm excitation. In addition, the optimal concentrations of Eu³⁺ for phosphors MGd₂(MoO₄)₄ (M=Ca, Sr and Ba) have also been determined.     

Synthesis and characterization of monodisperse spherical core-shell structured SiO2@Y3Al5O12:Ce3+/Tb3+ phosphors for field emission displays

Nanocrystalline Y₃Al₅O₁₂: Ce³⁺/Tb³⁺ (average crystalline size 30 nm) phosphor layers were coated on non-aggregated, monodisperse and spherical SiO₂ particles by the sol-gel method, resulting in the formation of core-shell structured SiO₂@Y₃Al₅O₁₂:Ce³⁺/Tb³⁺ particles. X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, photoluminescence, cathodoluminescence spectra, as well as lifetimes were utilized to characterize the core-shell structured SiO₂@Y₃Al₅O₁₂:Ce³⁺/Tb³⁺ phosphor particles. The obtained core-shell structured phosphors consist of well-dispersed submicron spherical particles with a narrow size distribution. The thickness of the Y₃Al₅O₁₂:Ce³⁺/Tb³⁺ shells on the SiO₂ cores (average size about 500 nm, crystalline size about 30 nm) could be easily tailored by varying the number of deposition cycles (100 nm for four deposition cycles). Under the excitation of ultraviolet and low-voltage electron beams (1–3 kV), the core-shell SiO₂@Y₃Al₅O₁₂:Ce³⁺/Tb³⁺ particles show strong yellow-green and green emission corresponding to the 5d–4f emission of Ce³⁺ and ⁵D₄–⁷F _J (J = 6, 5, 4, 3) emission of Tb³⁺, respectively. These phosphors may have potential application in field emission displays.     

Spherical Zn2SiO4:Eu@SiO2 phosphor particles in core-shell structure: Synthesis and characterization

Spherical SiO₂ particles have been coated with Zn₂SiO₄:Eu³⁺ phosphor layers by a Pechini sol-gel process. The microstructure and luminescent properties of the obtained Zn₂SiO₄:Eu³⁺@SiO₂ particles were well characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), photoluminescence (PL) spectra, and lifetime. The results demonstrate that the Zn₂SiO₄:Eu³⁺@SiO₂ particles, which have regular and uniform spherical morphology, emitted an intensive red light emission at 613 nm under excitation at 395 nm. Besides, the effects of the Eu³⁺ concentration, annealing temperature and charge compensators of Li⁺ ions on the PL emission intensities were investigated in detail.     

Synthesis and characterization of core-shell structured SiO2@YVO4:Yb,Er microspheres

In this paper, the core-shell structured SiO₂@YVO₄:Yb³⁺,Er³⁺ microspheres have been successfully prepared via a facile sol-gel process followed by a heat treatment. X-ray diffraction, field emission scanning electron microscopy, energy disperse X-ray spectroscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and photoluminescence spectra were used to characterize the samples. The results reveal that the SiO₂ spheres have been successfully coated by YVO₄:Yb³⁺,Er³⁺ phosphors to form core-shell structures and the size of obtained microspheres has a uniform distribution. Additionally, the samples exhibit bright green luminescence under the excitation of a 980 nm laser diode. The photoluminescence intensity increases with the number of coatings. These core-shell structured SiO₂@YVO₄:Yb³⁺,Er³⁺ microspheres may have great potential in the fields of infrared detection and display devices.     

花状NaY(MoO4)2的合成和(MoO4)2:Eu3+的光学性质

用微波辅助水热-煅烧法成功合成了花状NaY(MoO₄)₂颗粒,用XRD、XPS、FESEM进行了表征,提出了花状NaY(MoO₄)₂颗粒可能的形成机理. 采用相同的方法合成了NaY(MoO₄)₂:Eu³⁺荧光体,该荧光材料在612 nm处有一个强的发射峰,可用作白色发光二极管的红色磷光剂. 此外,微波辅助水热-煅烧法可能发展成为制备其他花状稀土钼酸盐的有效途径.     

Preparation of phosphors AEu(MoO4)2 (A?=?Li, Na, K and Ag) by?sol-gel method

A series of double molybdates phosphors AEu(MoO₄)₂ (A = Li, Na, K and Ag) have been prepared by sol-gel method. Their crystal structure and luminescent properties have also been investigated in a comparable way. The crystallization processes of the phosphor precursors were characterized by X-ray diffraction (XRD) and thermogravimetry-differential thermal analysis (TG-DTA). Field emission scanning electron microscopy (FE-SEM) was also used to characterize the shape and size distribution of the phosphors. Samples except KEu(MoO₄)₂ showed tetragonal scheelite structure in the range of our experiments, and no phase transition appeared. Phosphor KEu(MoO₄)₂ possessed two structures, and the phase transition took place at about 800°C. All samples with high purity could be obtained at about 500°C for 5 hours, and they all showed intense red light peaked at 616 nm originated from ⁵D₀→⁷F₂ emission of Eu³⁺ under the excitation of 465 nm or 394 nm light. The excitation spectra of phosphors AEu(MoO₄)₂ (A = Li, Na, and K) are composed of a strong broad charge transfer (CT) band and some sharp lines, and the relative intensity of CT band, the two strongest absorption lines at 395 nm and 465 nm are comparative, so these three phosphors are good red phosphor candidates for violet or blue LEDs. For the excitation spectrum of phosphor AgEu(MoO₄)₂, intensities of CT band and the absorption line at 395 nm are much weaker than that of line at 465 nm, thus phosphor AgEu(MoO₄)₂ is only suit for GaN-based blue LED.     

Luminescent properties of R2(MoO4)3:Eu (R=La, Y, Gd) phosphors prepared by sol-gel process

A series of red phosphors R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) have been synthesized by sol-gel method. The crystallization processes of the phosphor precursors were characterized by X-ray diffraction (XRD) and thermogravimetry-differential thermal analysis (TG-DTA), and the properties of these resulting phosphors have also been characterized by photoluminescence (PL) spectra and reflectance spectra. Field emission scanning electron microscopy (FE-SEM) was also used to characterize the shape and the size of the samples. The results of TG-DTA and XRD indicated that all of the R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) phosphors crystallized completely at 650 °C. Y_0.8Eu_1.2(MoO₄)₃ and Gd_0.8Eu_1.2(MoO₄)₃ have two structures, monoclinic and orthorhombic, while La_0.8Eu_1.2(MoO₄)₃ only adopts monoclinic structure. The luminescent properties of phosphors R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) are dependent on their structures to some extent. The orthorhombic Y_0.8Eu_1.2(MoO₄)₃ and Gd_0.8Eu_1.2(MoO₄)₃ phosphors show very similar luminescent properties, which differ from those of phosphors with monoclinic structure. For all of R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) phosphors, intense red emission is obtained by exciting at ∼394 and ∼465 nm which are owing to the sharp ⁷F₀→⁵L₆ and ⁷F₀→⁵D₂ lines of Eu³⁺. Two strongest lines at 394 and 465 nm in excitation spectra of these phosphors match well with the two popular emissions from near-UV and blue GaN-based LEDs, so they could be used as red components for white light-emitting diodes.     

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).     

The improvement of moisture resistance and thermal stability of Ca3SiO4Cl2:Eu phosphor coated with SiO2

Green-emitting phosphors Ca₃SiO₄Cl₂:Eu²⁺ were prepared by the high temperature solid-state method. Sol-gel process was adopted to encapsulate the as-prepared phosphors with tetraethylorthosilicate (TEOS) as silicon coating reagent. Fluorescence spectrometer, scanning electron microscopy (SEM) and powder X-ray diffraction (XRD) patterns were employed to characterize the emission spectra, the surface morphologies and the phase structures, respectively. The chemical stability testing was operated by the method of soaking the phosphors in deionized water and roasting them at different temperatures. The results indicated that the surfaces of the green phosphors were evenly coated by SiO₂ and the phase structure of the coated phosphors remained the same as the uncoated samples. The luminance centre of Eu²⁺ did not shift after surface treatment and the luminance intensity of coated phosphors was lower than that of the uncoated samples. The results demonstrated that the water-resistance stability of the coated phosphor was improved to some degree because the pH value and the luminance intensity variation were both smaller than the uncoated phosphor after steeping within the same time. Moreover, the thermal stability of coated phosphors was enhanced obviously compared to the original samples based on the temperature dependent emission spectra measurement.     

Combustion synthesis and luminescent properties of a new material Li2(Ba0.99,Eu0.01)SiO4:B for ultraviolet light emitting diodes

Using urea as fuel and boric as flux, a novel bluish green emitting phosphor Li₂(Ba_0.99,Eu_0.01)SiO₄:B³⁺ has been successfully synthesized using a combustion method. The material has potential application as the fluorescent material for ultraviolet light-emitting diodes (UV-LEDs). The dependence of the properties of Li₂(Ba_0.99,Eu_0.01)SiO₄:B³⁺ phosphors upon urea concentration, boric acid doping and initiating combustion temperature were investigated. The crystallization and particle sizes of Li₂(Ba_0.99,Eu_0.01)SiO₄:B³⁺ have been investigated by using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). Luminescence measurements showed that the phosphors can be efficiently excited by ultraviolet (UV) to visible region, emitting a bluish green light with peak wavelength of 490 nm. The results showed that the boric acid was effective in improving the luminescence intensity of Li₂(Ba_0.99,Eu_0.01)SiO₄ and the optimum molar ratio of boric acid to barium nitrate was about 0.06. The optimized phosphors Li₂(Ba_0.99,Eu_0.01)SiO₄:B_0.06³⁺ showed 160% improved emission intensity compared with that of the Li₂(Ba_0.99,Eu_0.01)SiO₄ phosphors under UV (λ_ex=350 nm) excitation.     

Luminescence characteristics of Eu2+ activated Ca2SiO4 Sr2SiO4 and Ba2SiO4 phosphorsfor white LEDs

<正>This paper investigates the luminescence characteristics of Eu²⁺ activated Ca₂SiO₄,Sr₂SiO₄ and Ba₂SiO₄ phosphors. Two emission bands are assigned to the f-d transitions of Eu²⁺ ions doped into two different cation sites in host lattices,and show different emission colour variation caused by substituting M²⁺ cations for smaller cations.This behaviour is discussed in terms of two competing factors of the crystal field strength and covalence.These phosphors with maximum excitation of around 370 nm can be applied as a colour-tunable phosphor for light-emitting diodes（LEDs） based on ultraviolet chip/phosphor technology.     

A potential green-emitting phosphor Ca8Mg(SiO4)4Cl2:Eu for white light emitting diodes prepared by sol-gel method

A potential green emitting phosphor Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺ was prepared by modified sol-gel method. The factors those affect the photoluminescence intensity including heating temperature, the usage of the chlorine source CaCl₂ and the concentration of dopant Eu²⁺ were also investigated in detail. As comparison, the phosphor prepared by solid-state reaction was also prepared. The phosphors show intense absorption in the range of 375-450 nm, which makes it a potential candidate of green emitting phosphor used for near-UV or blue light excited white LEDs.     

Synthesis and VUV spectral properties of nanoscaled Zn2SiO4:Mn green phosphor

Nanoscaled Zn₂SiO₄:Mn²⁺ green phosphor with regular and uniform morphology was synthesized by hydrothermal method at a low temperature of 140 °C. The structure and morphology of the phosphor was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effects of the hydrothermal temperature and the time on the crystallite structure and the vacuum ultraviolet (VUV) photoluminescence (PL) properties were evaluated. The as-synthesized nanoscaled Zn₂SiO₄:Mn²⁺ phosphor exhibited intensive broad emission around 523 nm, which was attributed to the ⁴T₁→⁶A₁ transition of Mn²⁺. The PL intensity increased along with the increasing hydrothermal temperature and time. The heat-treated phosphors exhibited higher PL intensity than the corresponding samples prepared using the conventional solid-state reaction.     

Ionic liquid-assisted hydrothermal synthesis of dendrite-like NaY(MoO4)2:Tb phosphor

Micro-sized NaY(MoO₄)₂:Tb³⁺ phosphors with dendritic morphology was synthesized by a ionic liquid-assisted hydrothermal process. X-ray diffraction (XRD) indicated that the as-prepared product is pure tetragonal phase of NaY(MoO₄)₂. Field emission scanning electron microscopy (FE-SEM) images showed that the as-prepared NaY(MoO₄)₂:Tb³⁺ phosphors have dendritic morphology. The photoluminescent (PL) spectra displayed that the as-prepared NaY(MoO₄)₂:Tb³⁺ phosphors show a stronger green emission with main emission wavelength 545 nm corresponding to the ⁵D₄→⁷F₅ transition of Tb³⁺ ion, and the optimal Tb³⁺ doping concentration for obtaining maximum emission intensity was confirmed to be 10 mol%. Based on Van Uitert's and Dexter's models the electric dipole–dipole (D–D) interaction was confirmed to be responsible for the concentration quenching of ⁵D₄ fluorescence of Tb³⁺ in the NaY(MoO₄)₂:Tb³⁺ phosphors. The intrinsic radiative transition lifetime of ⁵D₄ level is found to be 0.703 ms.     

Synthesis and characterization of Mn-doped Zn2SiO4/SiO2 phosphor particles in core-shell structure

Manganese-doped zinc silicate (Zn₂SiO₄:Mn) is a kind of phosphor material that has a photo-luminescent (PL) and cathode-luminescent (CL) properties with intensive green light emission at 520 nm. The particles consisting of SiO₂@Zn₂SiO₄:Mn (SiO₂ core-Zn₂SiO₄:Mn shell) were synthesized via colloidal process and forced precipitation. After drying, the Zn/Mn precipitates were coated on the surface of SiO₂ particles. The Zn/Mn precipitates reacted with SiO₂ and transformed to Zn₂SiO₄:Mn by suitable calcination. The microstructure, crystalline phase, and luminescent characteristics of the products were studied. Besides, a CL device consisting of the core-shell powder was characterized.     

NaEu0.96Sm0.04(MoO4)2 as a promising red-emitting phosphor for LED solid-state lighting prepared by the Pechini process

NaEu_0.96Sm_0.04(MoO₄)₂ was prepared by the Pechini method (P phosphor) and as a comparison, also by solid-state reaction technique (S phosphor). The photo-luminescent properties, the morphology and the grain size were investigated. The phosphors show broadened excitation band around 400 nm, high intensity of Eu³⁺⁵D₀→⁷F₂ emission upon excitation around 400 nm, and appropriate CIE chromaticity coordinates. Intensive red light-emitting diodes (LEDs) were fabricated by combining the phosphor and a 400 nm InGaN chip for the first time, which confirm that the phosphor is a good candidate for near UV LED. The luminescent intensity of P phosphor prepared at 700 °C is near that of S phosphor prepared at 800 °C. In addition, P phosphor shows advantages of lower calcining temperature, shorter heating time, and smaller grain size. Considering all these factors, the suitable method for preparing the promising near UV LED phosphor NaEu_0.96Sm_0.04(MoO₄)₂ is recommended to be the Pechini process at 700 °C.     

Intense green/yellow emission in Ca8Zn(SiO4)4Cl2:Eu, Mn through energy transfer for blue-LED lighting

Eu²⁺ and Mn²⁺ co-doped Ca₈Zn(SiO₄)₄Cl₂ phosphors have been synthesized by a high temperature solid state reaction. Energy transfer from Eu²⁺ to Mn²⁺ is observed. The emission spectra of the phosphors show a green band at 505 nm of Eu²⁺ and a yellow band at 550 nm of Mn²⁺. The excitation spectra corresponding to 4f⁷-4f⁶5d transition of Eu²⁺ cover the spectral range of 370-470 nm, well matching UV and/or blue LEDs. The shortening of fluorescent lifetimes of Eu²⁺ followed by simultaneous increase of fluorescent intensity of Mn²⁺ with increasing Mn²⁺ concentrations is studied based on energy transfer. Upon blue light excitation the present phosphor can emit intense green/yellow in comparison with other chlorosilicate phosphors such as Eu²⁺ and Mn²⁺ co-doped Ca₈Mg(SiO₄)₄Cl₂ and Ca₃SiO₄Cl₂, demonstrating a potential application in phosphor converted white LEDs.     

Yellow phosphors coated with TiO2 for the enhancement of photoluminescence and thermal stability

In order to improve the phosphor efficiency of yellow emission of the phosphor-converted white light emitting diode (pcW-LED), the Ba²⁺ Mg²⁺ co-doped Sr₂SiO₄:Eu phosphors were synthesized and were coated with thin and uniform TiO₂. The TiO₂ layer with 20 nm was uniformly coated over the phosphor surface. The photoluminescence (PL) properties of the TiO₂-coated phosphors showed improved yellow-emission intensity compared to the pristine phosphors. The temperature dependence of photoluminescence was measured from 25 to 150 °C. The TiO₂-coated phosphors showed superior thermal quenching property compared to pristine phosphors. We concluded that the TiO₂-coated surface of the phosphor is an effective way to improve the phosphor efficiency and enhance the thermal quenching stability.     

Enhanced blue emission and EPR study of LaMgAl11O19:Eu phosphors

Europium doped LaMgAl₁₁O₁₉ phosphor was prepared by the combustion method. The as-prepared and post-treated (1350 °C 10 h 5% H₂+95% N₂) phosphors were investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), photoluminescence (PL) and electron paramagnetic resonance (EPR) techniques. XRD patterns show that LaMgAl₁₁O₁₉:Eu phosphors have hexagonal structure. FT-IR spectrum exhibits absorption bands corresponding to the stretching vibration of AlO₄ and AlO₆. Morphological studies reveal that this phosphor has faceted plates of varying sizes and shapes. The as-prepared LaMgAl₁₁O₁₉:Eu phosphor consists of both Eu³⁺ and Eu²⁺ ions. The phosphor exhibits a bright blue emission at 450 nm (4f⁶5d→4f⁷ transition of Eu²⁺). On post-treating the phosphor we are able to enhance the blue emission efficiency by 330%. The process was detected from the evolution of excitation, emission and EPR spectra and the results are discussed.