Photoluminescence properties of Y2O3:Tb and YBO3:Tb green phosphors synthesized by hydrothermal method

In this work, two Tb³⁺ activated green phosphors: Y₂O₃:Tb³⁺ and YBO₃:Tb³⁺ were prepared by hydrothermal method. Photoluminescence properties of both phosphors were studied in details. Both phosphors exhibit similar luminescent characteristics symbolized by the dominant green emission at 545 nm. Concentration quenching occurs at the Tb³⁺ concentration of 1.60 atomic% and 2.57 atomic% for Y₂O₃:Tb³⁺ and YBO₃:Tb³⁺, respectively. Luminescence decay properties were characterized to better understand the mechanism of concentration quenching. Based on the calculation, the concentration quenching in both phosphors was caused by the dipole–dipole interaction between Tb³⁺ ions.     

Growth and optical properties of Er,Yb:YAl3(BO3)4 crystal

Single crystal of erbium, ytterbium-codoped yttrium aluminum tetraborate Er,Yb:YAl₃(BO₃)₄(Er,Yb:YAB) has been grown by the flux method. The absorption spectrum in the visible and NIR regions of Er,Yb:YAl₃(BO₃)₄ crystal are measured at room temperature and fluorescence spectrum of Er,Yb:YAl₃(BO₃)₄ crystal are also measured at room temperature, excited by 976 nm laser. Not only the strong NIR emission peaks located at 1548 nm was observed, but also the visible up-conversion luminescence has been found. The specific heat of the Er/Yb:YAB crystal at room temperature is 0.81 J/g °C.     

Factors for determining photoluminescence properties of YBO3:Ce phosphor prepared by hydrothermal method

YBO₃:Ce³⁺ blue-emitting phosphors were prepared from boric acid and nitrates of yttrium and cerium(III) by hydrothermal method. An excess amount of boric acid, prolonged aging, high temperature, and a high pH value promote the formation of crystalline YBO₃. The higher crystallinity results in the higher photoluminescence (PL) intensity corresponding to the 5d-4f transition of Ce³⁺ under the irradiation of near-UV light. The PL intensity also depends on the pH value of precursor suspension and the nominal Ce³⁺ concentration, where the sample prepared at pH = 8 and Ce/(Y + Ce) = 0.25-0.5 at% shows the maximum PL intensity. In addition, the hydrothermally prepared sample shows the characteristic photobleaching behavior under the continuous irradiation of near-UV light. These results suggest that the crystallinity of the host YBO₃ crystal and the homogeneity of substituted Ce³⁺ ions play significant roles in the PL properties.     

Luminescence properties of Y0.95? x M x Eu0.05B1? y R y O3 (M?=?Ca, Sr, Ba, Zn, Al, 0?≤ x ≤?0.1; R?=?Si, P, 0?≤ y ≤?0.1) in UV and VUV regions

The phosphors Y_0.95−x M _x Eu_0.05B_1−y R _y O₃:Eu³⁺ (M = Ca, Sr, Ba, Zn, Al, 0 ≤ x ≤ 0.1; R = Si, P, 0 ≤ y ≤ 0.1) are successfully synthesized by solid-state reaction. All the solid samples are identified as isomorphs. Their luminescent properties are studied under UV and VUV excitation. With the incorporation of metallic or nonmetallic cations, the chromaticity of YBO₃:Eu³⁺ is remarkably improved as well as the luminescent intensity increased. When 10% of Al³⁺ is doped into YBO₃ host lattice, the best ratio of the red emission at 610 nm to the orange one at 591 nm is obtained and the luminescent intensity increased simultaneously. The reason is analyzed, and can be explained by the decrease of the symmetry around Eu³⁺ site in YBO₃ host lattice.     

Color tunability of upconversion emission in YBO3: Yb, Er inverse opal

Upconversion (C) light-emitting photonic band gap materials (YBO₃: Yb, Er) with inverse opal structure were prepared by a self-assembly technique in combination with a sol-gel method. The effect of the photonic stop-band on the upconversion luminescence of Er³⁺ ions has been investigated in the YBO₃: Yb, Er inverse opals. Significant suppression of the green or red UC emission was detected if the photonic band-gap overlaps with the Er³⁺ ions emission band. We successfully achieved the color tuning of the UC optical properties of the inverse opal by controlling the structure of the photonic crystal.     

Structural investigations on TiO2 and Fe-doped TiO2 nanoparticles synthesized by laser pyrolysis

Undoped and Fe-doped TiO₂ nanopowders with Fe/Ti (atomic ratio) precursor concentration ranging from 7% up to 25% have been prepared by the IR laser pyrolysis technique. A sensitized mixture of TiCl₄ and Fe (CO)₅ was used as titanium and iron precursor, respectively. Reference undoped titania samples with a major concentration of anatase phase (about 90%) were obtained by the same technique by using very high flows of the oxidizing agent (air). The effects of the iron-dopant concentration on the essential structural properties of the resultant powders such as the phase formation, the crystallinity, the average particle size and distributions were systematically investigated by X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The decrease of the TiO₂-anatase crystalline phase, the simultaneous increase of the amorphous phase and the decrease in size of particle mean diameter appear as main effects induced by the Fe-dopant concentration.     

Synthesis of Sb2Te3 nanopowders by vacuum arc plasma evaporation

Taking elemental antimony and tellurium as source materials, Sb₂Te₃ nanopowders were prepared by vacuum arc plasma evaporation technique. Microstructure and morphology of the samples were characterized via X-ray diffraction and field emission scanning electron microscope. Compositional analysis was carried out by energy dispersive analysis of X-rays. Lattice constants (a = 4.267 Å, c = 30.469 Å) of Sb₂Te₃ nanopowders are calculated by (015) and (110) diffraction peaks of X-ray diffraction patterns. Field emission scanning electron microscope surface morphology of the nanopowders shows the irregular polyhedron and rhombohedral structure. Atoms percentage of the nanopowders is calculated by quantitative analysis using energy dispersive analysis of X-rays spectrum. The experimental results show that the percentages of Sb and Te atoms are 41.3% and 58.7% respectively.     

Enhanced luminescence properties of YBO3:Eu phosphors by Li-doping

Different concentrations of Li-doped YBO₃:Eu³⁺ phosphors have been prepared by the conventional solid state reaction method and were characterized by X-ray diffraction, field emission scanning electron microscopy, photoluminescence excitation and emission measurements. An intense reddish orange emission is observed under UV excitation and the emitted radiation was dominated by an orange peak at 594 nm resulted from the ⁵D₀ → ⁷F₁ transitions of Eu³⁺ ions. The brightness of the YBO₃:Eu³⁺ phosphor was found greatly improved with Li-doping accompanied by slight improvement in the purity of the color which might be attributed to improvement in crystallinity, grain sizes and creation of oxygen vacancies with Li-doping. The observed results have been discussed in comparison with similar reported works.     

Structure and microstructure of In4Te3 nanopowders prepared by solid state reaction

In this paper, we investigated the structure and microstructure of In₄Te₃ nanopowders obtained by mechanically alloying an In₇₅Te₂₅ powder mixture. Structural, chemical, thermal and vibrational studies of the In₇₅Te₂₅ powder mixture were carried out using X-ray diffraction, energy dispersive spectroscopy, transmission electron microscopy, differential scanning calorimetry and Raman spectroscopy. The orthorhombic In₄Te₃ phase (In₃Se₄-type) was nucleated in 2 h of synthesis, although non-reacted tetragonal indium (In) was still present at that time. Small amounts of cubic In₂O₃ phase were observed after 31 h of synthesis. Rietveld analyses allowed the measurement of mean crystallites sizes and phase fraction variations when milling times were increased. These analyses showed that, after 31 h of synthesis, about 65 wt% of In₄Te₃ phase contained mean crystallite sizes smaller than 27 nm and microstrains greater than 1.5%. The crystallite and interfacial components sizes were determined by high resolution transmission electron microscopy. Differential scanning calorimetry measurements showed the influence of nanometric crystallite sizes on the melting of the In₄Te₃ and non-reacted In phases. Raman measurements showed that the trigonal Te and α-TeO₂ modes, observed for the precursor Te powder, are absent for the sample milled for 31 h. The structural stability of the nanocrystalline phases of the In₇₅Te₂₅ sample milled for 31 h was attested by X-ray diffraction measurements performed twelve months after its production.     

Preparation of γ-Fe2O3 nanopowders by direct thermal decomposition of Fe-urea complex: reaction mechanism and magnetic properties

In this work, a novel method of producing maghemite (γ-Fe₂O₃) nanopowders has been developed, which can be performed by the direct thermal decomposition of an Fe–urea complex ([Fe(CON₂H₄)₆](NO₃)₃) in a single step. The reaction mechanism, particle morphology, and the magnetic properties of the γ-Fe₂O₃ nanopowders have been studied by using thermogravimetric (TG), differential scanning calorimetry (DSC), fourier transformed infrared (FTIR) spectroscopy, elemental analysis, X-ray powder diffraction (XRD), transmission electron micrograph (TEM) observations, and magnetic measurements. Thermal analyses together with the results of XRD show that the formation of γ-Fe₂O₃ occurs at ~200 °C through a two-stage thermal decomposition of the [Fe(CON₂H₄)₆](NO₃)₃ complex. The resulting iron oxide phases (i.e., γ-Fe₂O₃ and α-Fe₂O₃) are strongly dependent on the synthesis conditions of the [Fe(CON₂H₄)₆](NO₃)₃. When the molar ratio of Fe(NO₃)₃ · 9H₂O to CON₂H₄ that is used for the synthesis of [Fe(CON₂H₄)₆](NO₃)₃ is 1:6 (i.e., molar ratio in stoichiometry), a mixed phase of γ-Fe₂O₃ and α-Fe₂O₃ is formed. When the molar ratio is 1:6.2 (i.e., using an excess CON₂H₄), on the other hand, a pure γ-Fe₂O₃ is obtained. Magnetic measurements show that resulting nanopowders exhibit a ferromagnetic characteristic and their maximum saturation magnetization increases from 47.2 to 67.4 emu/g with an increase in the molar ratio of Fe(NO₃)₃ · 9H₂O to CON₂H₄ from 1:6 to 1:6.2.     

Luminescent and structural properties of Yb-doped Al2O3 nanopowders

In this work, we discuss structural and luminescent properties of Al₂O₃ nanopowders doped with Yb³⁺ ions prepared by a novel method, in which organic compounds were used as a solvent and lanthanide organic derivatives, served as a rare-earth ion source. The set of samples differing in activator concentrations and particle sizes was carefully studied by means of structural and optical characterization methods. In particular, the high resolution electron and transmission microscopy has been deployed together with X-ray diffraction technique to determine fundamental structural properties of nanopowders. The optical characterization was focused mainly on basic excitation and emission features and their sensitiveness on dopant concentration and the average nanoparticle size.     

Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4

An extensive theoretical study is performed for wide bandgap crystalline oxides and nitrides, namely, SiO₂, GeO₂, Al₂O₃, Si₃N₄, and Ge₃N₄. Their important polymorphs are considered which are for SiO₂: α-quartz, α- and β-cristobalite and stishovite, for GeO₂: α-quartz, and rutile, for Al₂O₃: α-phase, for Si₃N₄ and Ge₃N₄: α- and β-phases. This work constitutes a comprehensive account of both electronic structure and the elastic properties of these important insulating oxides and nitrides obtained with high accuracy based on density functional theory within the local density approximation. Two different norm-conserving ab initio pseudopotentials have been tested which agree in all respects with the only exception arising for the elastic properties of rutile GeO₂. The agreement with experimental values, when available, are seen to be highly satisfactory. The uniformity and the well convergence of this approach enables an unbiased assessment of important physical parameters within each material and among different insulating oxide and nitrides. The computed static electric susceptibilities are observed to display a strong correlation with their mass densities. There is a marked discrepancy between the considered oxides and nitrides with the latter having sudden increase of density of states away from the respective band edges. This is expected to give rise to excessive carrier scattering which can practically preclude bulk impact ionization process in Si₃N₄ and Ge₃N₄.     

Synthesis,structure and properties of heavily Mn-doped perovskite-type SrTiO3 nanoparticles

Cubic perovskite SrTi_1−xMn_xO₃ nanopowders with x = 0–0.5, which is much higher than the conventionally believed Mn incorporation limit, and particles sizes 10–80 nm have been successfully synthesized using the citrate sol–gel method. The crystalline structure, the morphology, the chemical composition and the lattice constant behavior versus the annealing temperature and the Mn concentration have been characterized using X-ray diffraction, scanning electron microscopy and proton-induced X-ray emission techniques. Studies of Raman light scattering and the magnetic properties revealed the activation of the TO2 polar mode and the magnetic ordering effects at low temperatures. Such features are treated as possible evidence of non-d⁰ Mn-driven transition to a polar phase with multiferroicity in heavily concentrated perovskite SrTi_1−xMn_xO₃ nanopowders.     

Preparation and characterization of rare earth orthoborates, LnBO3 (Ln = Tb, La, Pr, Nd, Sm, Eu, Gd, Dy, Y) and LaBO3:Gd, Tb, Eu by metathesis reaction: ESR of LaBO3:Gd and luminescence of LaBO3:Tb, Eu

Lanthanide orthoborates of composition LnBO₃ (Ln = Tb, La, Pr, Nd, Sm, Eu, Gd, Dy, Y) and LaBO₃:Gd, Tb, Eu have been prepared by metathesis reaction. This method provides a convenient route for the synthesis of orthoborates and its solid solutions at low temperatures. Powder X-ray diffraction and FT-IR spectroscopy were used to characterize these borates. Rare earth borates, (LnBO₃) are isomorphous with different forms of CaCO₃ depending on the radius of rare earth ion. LaBO₃, LaBO₃:Gd, Tb, Eu, PrBO₃, NdBO₃ crystallized in aragonite structure, SmBO₃ crystallized in H-form and TbBO₃, EuBO₃, GdBO₃, DyBO₃, YBO₃ crystallized in vaterite structure. The structural analysis of TbBO₃ was carried out. The morphology of these borates was obtained from Scanning electron microscopy. Spin-Hamiltonian parameters for Gd³⁺ are deduced from room temperature electron spin resonance spectrum of LaBO₃:Gd. The luminescence of LaBO₃:Tb, Eu gave characteristics peaks corresponding to Tb³⁺, Eu³⁺ respectively.     

A novel approach to prepare tetragonal BaTiO3 nanopowders

BaTiO₃ nanopowders were attempted to synthesize by using a novel straight-forward, solvent free reactions under autogenic pressure at elevated temperature (RAPET) approach. The as-prepared BaTiO₃ nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, high-resolution TEM, and convergent-beam electron diffraction. It was found that Ba(OH)₂·8H₂O and Ti(OBu)₄ could be appropriate starting materials to synthesize BaTiO₃ at 973 K for 2 h by using RAPET approach. Pure tetragonal BaTiO₃ nanopowders could be obtained by exceeding moderate amount of Ba(OH)₂·8H₂O in the starting materials. The obtained BaTiO₃ nanoparticles had well dispersion and crystallinity, possessed a tetragonal perovskite structure at room temperature and relatively narrow particle size distribution.     

Thermoelectric properties of Zn4Sb3 prepared by hot pressing

Polycrystalline specimens of the thermoelectric material Zn₄Sb₃ were prepared by the hot-pressing method at various temperatures and pressures and their thermoelectric properties were evaluated in a temperature range from 298 K to 673 K. A single phase of Zn₄Sb₃ was obtained in the samples prepared at 673 K with a pressure above 150 MPa, whereas ZnSb was placed in the Zn₄Sb₃ matrix for the samples prepared at 100 MPa. The electrical transport properties of the single phase compound showed p-type conduction and metallic transport behavior based on the temperature dependence. The sample produced at 673 K under a pressure of 200 MPa exhibited the highest ZT value of 1.36 at 673 K. This study suggests that the dense and single-phase Zn₄Sb₃ compound is a route to achieve a high thermoelectric performance.     

Upconversion and infrared luminescences in Er/Yb codoped Y2O3 and (Y0.9La0.1)2O3 transparent ceramics

Er³⁺ and Yb³⁺ codoped Y₂O₃ and (Y_0.9La_0.1)₂O₃ transparent ceramics were fabricated by the conventional ceramics processing with nanopowders. Compared to Er/Yb:Y₂O₃, Er/Yb:(Y_0.9La_0.1)₂O₃ ceramics had higher transmittance. Intense upconversion (UC) and infrared emission (1543 nm) were observed under excitation of 980 nm. According to three intensity parameters Ω₂, Ω₄, and Ω₆ fitted by the Judd-Ofelt theory, the spectroscopic quality parameters (X), radiative lifetimes (τ_rad), and emission cross-sections (α_em) were determined. Er/Yb:(Y_0.9La_0.1)₂O₃ ceramics owned broader peaks and longer lifetime (12.3 ms) at 1548 nm due to the glass-like structure of (Y_0.9La_0.1)₂O₃ ceramics. The results showed Y₂O₃ and Y_1.8La_0.2O₃ transparent ceramics are promising gain media for developing the solid-state 1.5 μm optical amplifiers and tunable UC lasers.     

Mechanochemical synthesis and characterization of TiB2 and VB2 nanopowders

A novel method for the synthesis of transition-metal boride nanopowder has been developed using a mechanochemical reaction between LiBH₄, LiH and transition-metal chloride (TiCl₃ and VCl₃) by high energy ball milling. This method successfully produces TiB₂ and VB₂ particles dispersed within a soluble LiCl matrix. Subsequent washing with distilled water, ethanol and acetone to remove the LiCl matrix phase yields TiB₂ and VB₂ nanopowders of 15-60 nm particle size. From the X-ray diffraction patterns and high resolution transmission electron microscopy image, it is found that each particle is polycrystalline consisting of 3-5 nm crystallites. Neither particle nor crystallite size are increased significantly after heating at 680 °C.     

Low-temperature synthesis,phonon and luminescence properties of Eu doped Y3Al5O12 (YAG) nanopowders

This contribution presents two simple and cost-effective routes for the low-temperature and large-scale production of pure and Eu-doped Y₃Al₅O₁₂ (yttrium aluminum garnet YAG) nanopowders. The proposed methodologies combine a mechanically assisted metathesis reaction or coprecipitation from solution followed by crystallization of the obtained precursors from molten sodium nitrate/nitrite. Both procedures allow obtaining pure and/or doped YAG nanopowders at remarkably low temperatures, i.e. already at 350 °C although firing at 500 °C is needed in order to get single phase and fully crystalline materials. As-obtained samples were characterized by XRD, TEM, Raman, IR and luminescence methods. These methods showed that the mean crystallite size is near 23–31 and 51 nm, when crystallization is performed from the amorphous precursor obtained by a mechanically assisted metathesis reaction and coprecipitation, respectively. Raman and IR spectra indicated better crystallinity of the powders prepared at 500 °C. The emission study showed that the intensity ratio between hypersensitive ⁵D₀ → ⁷F₂ and magnetic-dipole ⁵D₀ → ⁷F₁ transitions of Eu³⁺ is significantly larger than expected for well-crystallized YAG. Origin of this behavior is discussed.     

Thermal characterization of KClO3 KBrO3 and KIO3

The specific heats, thermal decompositions and thermal properties of the phase transitions in KClO₃ KBrO₃ and KIO₃ crystals were measured. The thermal stability of the compounds decreased in the order KIO₃ > KCIO₃ > KBrO₃. Over the temperature range 250–600 K, no phase transitions were detected for KBrO₃: one at 545 K for KClO₃ and two for KIO₃ at 345 and 487 K.