Rare earth doped LiYbF4 phosphors with controlled morphologies: Hydrothermal synthesis and luminescent properties

High quality monodisperse LiYbF₄ microparticles with shape of octahedron had been prepared via a facile hydrothermal route. The crystalline phase, size, morphology and luminescence properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectra and Commission Internationale de L’Eclairage (CIE 1931) chromaticity coordinates, respectively. The influences of reaction temperature, reaction time and the molar ratio of EDTA to Yb³⁺ on the crystal phases and shapes of as-prepared products had been investigated in detail. The upconversion (UC) luminescence properties of LiYb_1?xF₄:xEr³⁺ (x =0.1, 0.2, 0.5, 1, 2, 5 and 10 mol%) particles with octahedral microstructures were studied under 976 nm excitation. The results showed that the luminescence colors of the corresponding products could be tuned to bright green by changing the doping concentration of Er³⁺ ion. The luminescence mechanisms for the doped Er³⁺ ion were thoroughly analyzed, showing great potential in applications such as biolabels, displays and other optical technologies.     

用水热法制备的Ba1-xPbxWO4微晶及其发光性能研究

采用水热法制备了Ba1-xPbxWO4固溶体微晶,通过XRD、SEM、荧光分析等测试手段,研究了微晶的晶相结构、表面形貌及室温光致发光特性。结果表明,当x从0～1.0变化时,Ba1-xPbxWO4固溶体微晶的表面形貌由四方锥型向树枝状变化。在270nm紫外光激发下,其发射光谱均呈现两个发射峰,即位于320nm左右的弱发射峰和400～470nm左右的强发射峰,前者为WO42-四面体的本征发射,其强度和波长随x值变化不大;而后者为掺入的Pb2+引起的激活发射峰,掺入Pb2+量(即x)对其强度和发射波长有较大的影响。当x=0.03时,微晶发射强度达到最强。     

Hydrothermal synthesis and luminescent properties of GdVO4 : Eu3+ nanophosphors

Eu³⁺-doped GdVO₄ has been synthesised via hydrothermal method by altering the hydrothermal temperature, reaction time and surfactant. The microstructure and morphology information of the phosphors were investigated via the techniques of X-ray powder diffraction and scanning electronic microscopy, which show that the phosphors wear tetragonal phase and the products present various regular morphologies under different reaction conditions such as bulk and nanoparticle. Moreover, the morphologies of the products have been controlled by altering reaction temperature. In addition, the surfactant was also included to control the morphologies of the products and the phosphors present different morphologies. All the phosphors exhibit the characteristic fluorescence of Eu ion (⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₂). The electric dipole transition ⁵D₀ → ⁷F₂ of Eu³⁺ is dominant indicating that most sites of Eu³⁺ ions in GdVO₄ have no inversion centre. Furthermore, we found that the reaction time and the morphologies have great influence on optical properties.     

One-dimensional rare earth compounds and complexes: preparation and improved photoluminescence properties

One-dimensional nanosized phosphors demonstrate special structural and photoluminescence properties, which have application potential in some optical fields. In this article, we present our recent progress on preparation and luminescence properties of some one-dimensional rare earth compounds and complexes, the core-shell oxide nanowires prepared by a two-step hydrothermal route, the nanowires of some inorganic compounds doped with rare earths and rare earth complexes/PVP composites prepared by the electrospinning method, and the rare earth complexes in the SBA-15 mesoporous molecule sieves. In these systems, some novel or improved photoluminescence properties are observed such as improved luminescence quantum yield, thermal stability and/or photostability, and depressed thermal effect in upconversion luminescence.     

稀土激发的CaMgSi2O6的长余辉发光特性

使用固相反应法在还原气氛中制备了具有长余辉性能的CaMgSi2O6发光材料。研究了不同的共激发离子(Dy^3 和Nd^3 )对于材料发光性能的影响。光谱分析表明了这些磷光体在438nm处有一个宽的发射峰,这个发射峰是由Eu^2 的4f^6 5d^1能级到4f^7能级的跃迁所导致的,而Eu^2 在透辉石中形成六配位的发光中心。获得的3种磷光体都具有长余辉发光性能。其中共掺杂Eu^2 和Dy^3 材料比单掺Eu^2 和共掺Eu^2 扣Nd^3 的材料具有更好的余辉强度扣更长的余辉时间，其原因在于Dy^3 在CaMgSi2O6晶格中形成了更深的和更高密度的陷阱。     

Hydrothermal synthesis and luminescent properties of EuW2O6(OH)3 red micro-phosphors

Red phosphors EuW₂O₆(OH)₃ were synthesized via a facile hydrothermal method at different pH values (1, 2, 3, 4, 5, 6 and 7) at 180 °C for 24 h. The XRD patterns indicate that as-obtained phosphors exhibit monoclinic phase. The SEM results show that the as-obtained phosphors exhibit micro-spherical morphologies. The emission spectra under 394 nm excitation exhibit dominant peaks centered at 617 nm. pH value has a strong effect on the phosphor morphology and the emission intensity. The strongest 617 nm red emission was obtained at pH 6. The influence of morphology and crystal size on the intensity of red emission were discussed. The as-obtained micro-phosphors are potential candidate for red-emitting LED phosphors.     

水热法制备GdVO_4∶Tm蓝色纳米荧光粉体

采用水热法以十二烷基苯磺酸钠为活性剂合成了蓝色纳米荧光GdVO4:Tm粉体。用X射线衍射、扫描电镜、荧光光谱(PL)研究不同反应时间、反应物浓度和Tm3+掺杂量等条件下所得产物的结构和发光性能。结果说明,所得的产物为单一的四方锆型GdVO4:Tm晶体,颗粒大小约为20～50nm;GdVO4:Tm能够被200～320nm的紫外光激发,在278nm波长紫外光激发下发出波长为476nm的明亮蓝光,发光强度分别在反应物浓度为0.205mol/L时、反应时间为3d和Tm3+掺杂量为2%时最高。     

Hydrothermal synthesis and luminescent properties of microtubes constructed by fluffy Zns:Mn2+ with nanostructures

Tubular micrometer-sized ZnS:Mn2+ constructed by fluffy nanostructures were fabricated in the mixed solutions of water and ethanol in a fixed volume ratio with the aid of ethylenediamine. In the X-ray diffraction pattern, the products obtained in the presence and absence of ethylenediamine show the wurtzite and sphalerite phases, respectively. Field-emission scanning electron microscopic images reveal the evolution process from nanowires to fluffy ZnS:Mn2+ to microtubes with the reaction times of 2, 4, and 8 hours at 100 degrees C, and the basal nanowires are below 10 nm in diameter. Photoluminescence and photoluminescence excitation spectra were investigated. The results suggest that the wurtzite phase, instead of the sphalerite phase ZnS:Mn2+ is luminescence-active for the 4T1 -6A1 transition of the Mn2+ in the ZnS host. The excitation spectra monitored at orange emission bands exhibit sharp peaks at 320, 326 and 327 nm with increasing reaction times of 2, 4, and 8 hours, respectively, indicating the energy transfer from ZnS host to Mn2+ ions, and the blue-shifts compared with the band gap absorption of the bulk counterpart (344 nm) are also observed due to the quantum confinement effects. The formation mechanism of the wurtzite one-dimensional nanostructures at such a low temperature is proposed based on a molecular template mechanism involving the bidentate coordinating ligand, ethylenediamine, and the possible formation mechanism of novel tubular structure are also discussed.     

Thermal and dielectric properties of rare earth iodates

The thermal (TGA, DTA and DSC) and dielectric behaviour of all rare earth iodates (except prometheum) has been investigated. The iodates of lanthanum, cerium to europium and gadolinium to lutetium decompose differently and the activation energies for these decomposition reactions have been determined. The DSC measurements have shown that all of the anhydrous iodates undergo one or more exothermic/endothermic changes. From the nature of the DSC profiles, heat of transitions, and powder diffraction data the phase transformations of Ln (IO₃)₃ have been found to occur in four different ways (lanthanum, cerium to europium, gadolinium to terbium, dysprosium to lutetium). Dielectric measurements of Ln (IO₃)₃ at varying temperature have corroborated the observations made with DSC studies. Relatively low dielectric constants of the high-temperature phases probably indicate their centrosymmetric structures. The dielectric behaviour of noncentrosymmetric Ln (IO₃)₃·H₂O (Ln=cerium to samarium) has been investigated in the temperature range 30 to –160° C. They have high dielectric constants (_r, ~ 1800 to 2400) at the ambient temperature and do not show significant change in the entire temperature range of investigation.     

Magnetic properties of some rare earth ferrites

The tetragonal SrEu₂Fe₂O₇ and BaEu₂Fe₂O₇ have been investigated using Mössbauer spectroscopy and magnetic susceptibility techniques. The measurements of magnetic susceptibility have been carried out in the temperature range 100 to 700°K. The data have been fitted to Curie-Weiss law $\text{χ =}\text{C}\text{T}\text{+ θ +}\text{A}$ using a non-linear least square curve fitting procedure. These ferrite compounds are paramagnetic above 534°K and 537°K respectively, with antiferromagnetic ordering below these temperatures.     

Hydrothermal synthesis,characterization and luminescent properties of lanthanide-doped NaLaF<Subscript>4</Subscript> nanoparticles

Nanoparticles of sodium lanthanum (III) fluoride-doped and co-doped with Eu³⁺/Tb³⁺ were prepared by the hydrothermal method using citric acid as structure-directing agent. Structural aspects and optical properties of synthesized nanoparticles were studied by powder X-ray diffraction (XRPD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy-dispersive X-ray spectra (EDS), particle size by dynamic light scattering (DLS), Fourier transform infrared (FTIR) spectrum and photoluminescence (PL) techniques. Nanoparticles consist of well-crystallized hexagonal phase and the average crystallite size for undoped and doped-NaLaF₄ nanoparticles are in the range of 20–22 nm. TEM images show that nanoparticles have cylindrical shape and crystalline nature of nanoparticles was confirmed by SAED patterns. Down- conversion (DC) luminescent properties of doped NaLaF₄ were also investigated and impact of co-doping has been explored.     

Hydrothermal synthesis and luminescent properties of (La,Gd)PO4:Tb phosphors under VUV excitation

Tb doped orthophosphate phosphors with monoclinic system were successfully prepared by mild hydrothermal reaction at 240 °C. The emission characteristics of phosphors (La,Gd)PO₄:Tb under 147 nm excitation were studied. The quenching concentration of Tb in LaPO₄ is 15 mol% and when Gd is co-doped into the matrix the Tb emission at 543 nm is improved further. The emission intensity of the optimal composition of La_0.55Gd_0.3Tb_0.15PO₄ is comparable with that of commercial Zn₂SiO₄:Mn phosphor. The study of its optical properties suggests that it is a potential candidate for plasma display panels (PDPs) application.     

Hydrothermal derived nanostructure rare earth (Er,Yb)-doped ZnO: structural,optical and electrical properties

The structural, optical and electrical properties of undoped and rare-earth (Er, Yb) doped zinc oxide (ZnO) nanopowder samples synthesized by hydrothermal method were investigated. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy. The optical properties of undoped and rare-earth (Er, Yb) doped ZnO were carried out with UV–visible diffuse reflectance spectroscopy techniques. XRD results reveal that Yb and Er doped ZnO nanopowders have single phase hexagonal (Wurtzite) structure without any impurities. SEM analysis indicate that dopants with different radii affected the surface morphology of ZnO nanostructures. The optical band gap of all samples were calculated from UV–Vis diffuse reflectance spectroscopy data. We have obtained band gap values of undoped, Er and Yb doped ZnO as 3.24, 3.23, 3.22 eV, respectively. Electrical characterization of the samples were made in the 280–350 K temperature range using Van der Pauw method based on Hall effect measurement. The carrier concentrations decreased for both Er and Yb doping while the Hall mobility and electrical resistivity increased with Yb, Er doping compared to undoped ZnO nanopowder at room temperature. The temperature dependent resistivity measurements of Er doped ZnO showed a metal–semiconductor transition at about 295 K, while Yb doped ZnO showed characteristic semiconductor behavior.     

Catalytic properties of rare earth cobaltites and related compounds

The activity for conversion of methanol to CO₂ on rare-earth cobaltites is greatest for SmCoO₃, which has the highest room-temperature ratio of high-spin to low-spin cobalt ions. The sequence of the activities for Sm_0.5·M_0.5CoO₃ (M=Ca,Sr,Ba), Ba>Sr>Ca, is shown to vary as the amount of high-spin cobalt ion in these compounds at 200°C.     

Synthesis and properties of rare earth doped lamp phosphors

Red, blue and green emitting lamp phosphors such as Eu³⁺ doped Y₂O₃ (red phosphor), Eu²⁺ doped Ba_0·64Al₁₂O_18·64, BaMgAl₁₀O₁₇ and BaMg₂Al₁₆O₂₇ (blue phosphors) and Ce_0·67Tb_0·33MgAl₁₁O₁₉ and Eu²⁺, Mn²⁺ doped BaMgAl₁₀O₁₇ (green phosphors) have been prepared by the combustion of the corresponding metal nitrates (oxidizer) and oxalyl dihydrazide/urea/carbohydrazide (fuel) mixtures at 400°–500°C within 5 min. The formation of these phosphors has been confirmed by their characteristic powder X-ray diffraction patterns and fluorescence spectra. The phosphors showed characteristic emission bands at 611 nm (red emission), 430–450 nm (blue emission) and 515–540 nm (green emission). The fine-particle nature of the combustion derived phosphors has been investigated using powder density, particle size and BET surface area measurements. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Explaining texture weakening and improved formability in magnesium rare earth alloys

Wrought magnesium alloys are rarely used due to their poor formability which is caused by strong textures created during processing. Addition of rare earth (RE) elements including Y, Ce, La, Gd and Nd weakens these strong basal textures and significantly improves formability. Developing a mechanistic understanding of this effect is critical in leading alloy design towards a new class of highly formable magnesium alloys. This fall in texture intensity occurs during recrystallisation and only requires very low solute RE additions, 0·01 at.-% in the magnesium–Ce case. These additions retard dynamic recrystallisation and increase non-basal slip; however, a full understanding of the RE effect has yet to be obtained, with a variety of mechanisms proposed. Recent research in these areas is critically reviewed.     

Physical properties of some rare earth tellurite glasses

Mossbauer and IR spectra as well as the electrical conductivity have been measured to give an idea about the structure and the electrical properties of some rare earth tellurite glasses containing Fe₂O₃. The glasses denoted [1 – (2x + 0.05)] TeO₂ ·xFe₂O₃ · (x + 0.05) Ln₂O₃, wherex = 0.0 and 0.05 and Ln = lanthanum, neodymium, samarium, europium or gadolinium, were prepared by fusing a mixture of their respective reagent grade oxides in a platinum crucible at 800° C for one hour. The Mossbauer parameters such as isomer shift, quadruple splitting and line width were found to be a function of the polarizing power (charge/radius) of the rare earth cations. The Mossbauer parameters were not affected by the heat treatment of the glass samples. Both of the Te-O-Ln and Te-O-Fe stretching vibrations were obtained from the IR results which indicate that the rare earth oxides and iron oxide are partially covalent. The electrical resistivity was measured as a function of temperature from 293 to 520 K. Both the electrical resistivity and the activation energy were found to be a function of the atomic number (Z) of the rare earth cations. The results were interpreted on the basis of the electronic structure of the glass.     

Hydrothermal synthesis and optical properties of CuS nanoplates

CuS nanoplates have been successfully prepared in the presence of cation surfactant cetyltrimethylammonium bromide (CTAB) by hydrothermally treating the solution of CuCl₂·2H₂O and Na₂S·9H₂O at 180 °C for 24 h. The as-prepared CuS nanoplates are of hexagonal phase and are single crystal. The thickness and edge length of the nanoplates are about 15 nm and 60 nm, respectively. The products was characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, photoluminescence measurements and ultraviolet-visible light spectrophotometer. The influences of synthetic parameters, such as reaction time and CTAB, on the morphologies of the products have been investigated.     

Hydrothermal synthesis and luminescent properties of LaPO4:Eu 3D microstructures with controllable phase and morphology

Three types of Eu³⁺-doped LaPO₄ three-dimensional (3D) microstructures have been hydrothermally prepared by adjusting pH value and the molar ratio of La³⁺ and Eu³⁺ to phosphorus of Na₅P₃O₁₀ (Ln/P) at 180 °C. As the pH value was 0.15 and the molar ratio of Ln/P was in the range of 1/1 to 1/4, 5 μm urchinlike spheres composed of long nanorods of hexagonal LaPO₄:Eu were obtained. As the pH value was 1.00 and the molar ratio of Ln/P was 1/1, 3 μm hollow spheres consisting of short nanorods of monoclinic LaPO₄:Eu were prepared. Keeping the pH value at 1.0 and the molar ratio of Ln/P at 1/4, 6 μm core-shell spheres of monoclinic LaPO₄:Eu formed. Luminescent measurements were performed. Hollow spheres of monoclinic LaPO₄:Eu had the strongest emission intensity. However, the emission intensity of monoclinic core/shell structure was even lower than that of hexagonal LaPO₄:Eu urchinlike spheres.