Multi wall carbon nanotubes assisted synthesis of YVO4:Eu3+ nanocomposites for display device applications

YVO₄:Eu³⁺ nanocomposites have been synthesized by means of a modified co-precipitation method (CP-CNT). Multi walled carbon nanotubes (MWCNT’s) have been employed in the synthesis of the YVO₄:Eu³⁺ nanocomposites to enhance its photoluminescence efficiency. The prepared nanocomposites were thoroughly characterized using the characterization techniques namely XRD, SEM, FTIR and Raman scattering. To evaluate the potentiality of the prepared nanocomposites, the same phosphor has also been prepared by using co-precipitation (CP) method without employing multi walled carbon nanotubes and also by means of conventional solid state reaction method (SSR). The photoluminescence spectra of YVO₄:Eu³⁺ nanocomposites have shown stronger red emission at 619 nm (⁵D₀ → ⁷F₂) for both the excitation wavelengths (254 and 393 nm) than the other two prepared samples. The effect of MWCNT’s on photoluminescent properties of the YVO₄:Eu³⁺ nanocomposites is also explained.     

Luminescent properties of Eu doped α-Gd2(MoO4)3 phosphor for white light emitting diodes

A novel red emitting phosphor α-Gd₂(MoO₄)₃:Eu³⁺ was developed for white light emitting diodes (LEDs). The phosphor was prepared by solid-state reaction. The effects of the flux content and the activator concentration on the crystal structure, morphology and luminescent properties were investigated by using XRD, SEM, and fluorescent spectra. These results showed that this phosphor can be effectively excited by ultraviolet (UV) (395 nm) and blue (465 nm) light, matching the output wavelengths of ultraviolet or blue LED chips. The α-Gd₂ (MoO₄)₃ phosphor may be a better candidate for solid state lighting application.     

Luminescent properties of Eu3+ doped α-Gd2(MoO4)3 phosphor for white light emitting diodes

A novel red emitting phosphor α-Gd₂(MoO₄)₃:Eu³⁺ was developed for white light emitting diodes (LEDs). The phosphor was prepared by solid-state reaction. The effects of the flux content and the activator concentration on the crystal structure, morphology and luminescent properties were investigated by using XRD, SEM, and fluorescent spectra. These results showed that this phosphor can be effectively excited by ultraviolet (UV) (395 nm) and blue (465 nm) light, matching the output wavelengths of ultraviolet or blue LED chips. The α-Gd₂ (MoO₄)₃ phosphor may be a better candidate for solid state lighting application.     

Rietveld refinement and switching properties of Cr substituted NiFe2O4 ferrites

A series of ferrite samples of the chemical formula NiCr_xFe_2 − xO₄ (x = 0.0 to x = 1.0) were prepared by a wet chemical co-precipitation method and annealed at 600 °C for 4 h. The prepared samples were shown to have the cubic spinel structure by applying the full pattern fitting of the Rietveld method using the Full Prof program. The unit cell dimension, discrepancy factor and interatomic distance have been determined. It was observed that the unit cell dimensions decrease with an increase in Cr³⁺ content x. The grain size of the synthesized powder has been determined from XRD data and strain graph. The grain size of all samples is in the range of 26 to 40 nm. The IR spectra show three absorption bands in the wave number range of 200 to 800 cm^− 1. All samples were studied for electrical switching. Current controlled negative resistance type switching was observed for all samples.     

Synthesis and characterization Y2O3:Eu3+ nanocrystals prepared via solvothermal refluxing route

Y₂O₃:Eu³⁺ nanocrystals were prepared via co-precipitation–solvothermal refluxing–calcination method using three kinds of organic solvents, propylene glycol, 1,3-butanediol and polyethylene glycol and yttrium chloride hexahydrate and europium chloride as starting materials. The Y₂O₃:Eu³⁺ nanocrystals with diameter of 20–50 nm prepared by refluxing in polyethylene glycol followed by calcinations at 800–1000 °C exhibited the strongest luminescence at 611 nm under the excitation wavelength of 254 nm than the reference sample prepared via conventional co-precipitation method. The photoluminescence spectra of the samples were recorded at room temperature. The effect of concentration of Eu³⁺ (Eu³⁺/Y³⁺ atomic ratio: 0.01–0.1) on the photoluminescence intensity was also investigated. The samples with the Eu³⁺/Y³⁺ atomic ratio of 0.07 exhibited the strongest emission at 611 nm and quenching effect was observed above 0.10.     

Mössbauer studies of thiospinels VI. The system FexZn1−xCr2S4

The spinel system Fe_xZn_1?xCr₂S₄ with 0 ≦ x ≦ 1 has been prepared in polycristalline form. All samples are p type semiconductors. I.R. spectra have been measured between 200 cm^?1 and 600 cm^?1. Room temperature ⁵⁷Fe-Mössbauer spectra show the typical behaviour of tetrahedral site Fe(II) surrounded by different octahedral site neighbours. In comparison with Mössbauer spectra of the spinel system Fe_1+xCr_2?xS₄ the distribution, 0.01 < y < 0.02, is derived.     

Induced synthesis of ZnO:Tb3+ green hybrid phosphors by the assembly of polyethylene glycol matrices

The Tb³⁺, Li⁺ ion co-doped ZnO/PEG particles were prepared by a co-precipitation approach derived from Zn(CH₃COO)₂, NaOH, CH₃COOLi and PEG in ethanol solution at room temperature. The characteristic green luminescence spectra which corresponded to f–f transitions of Tb³⁺ were observed. The microstructure and luminescent properties were investigated by XRD, SEM and luminescent spectra. The strong emissions at 484 and 545 nm were noticed from the Tb³⁺ doped ZnO/PEG composites. With regard to the comparison between the emission spectra of pure and PEG directed ZnO:Tb hybrids, it is worthy of pointing out that the PEG matrix largely improves the emission intensity of Tb³⁺.     

EPR and photoluminescence properties of combustion-synthesized ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript>:Cr<Superscript>3+</Superscript> phosphors

An efficient red emitting ZnAl₂O₄:Cr³⁺ powder phosphor material was prepared at furnace temperatures as low as 500 °C by using the combustion method. The prepared powders were analyzed by X-ray diffraction and scanning electron microscopy techniques. The optical properties were studied using photoluminescence technique. The EPR spectra exhibit an intense resonance signal at g = 3.74 which is attributed to Cr³⁺–Cr³⁺ pairs, and the weak resonance signal of at g = 1.97 is attributed to Cr³⁺ single ion transition. The spin population (N) has been evaluated as a function of temperature. The excitation spectrum exhibits two broad bands in the visible region which are characteristic of Cr³⁺ ions in octahedral symmetry and the emission spectrum exhibits zero-phonon line frequencies along with vibronic frequencies. The crystal field parameter (Dq) and Racah parameters (B and C) have been evaluated and discussed.     

Saturation magnetization and structural analysis of Ni0.6Zn0.4NdyFe2?yO4 by XRD, IR and SEM techniques

Compositions having general formula Ni_0.6Zn_0.4Nd _y Fe_2−y O₄ (where y = 0, 0.01, 0.02 and 0.03) were prepared by oxalate co-precipitation method from high purity sulphates. The samples were characterized by XRD, IR and SEM techniques. X-ray diffraction measurements confirmed the formation of single phase cubic spinel structure. Lattice constant increases with rise in Nd³⁺ content and obeys Vegard’s law. Crystallite size of the samples lies in the range 29.98–31.15 nm. The IR spectra shows two strong absorption bands in the frequency range 400–600 cm⁻¹. Further, it shows that Nd³⁺ occupies B-site. SEM studies show that the grain size of the samples decreases with increase in Nd³⁺ content. Saturation magnetization of Nd³⁺ substituted Ni–Zn ferrites is higher than unsubstituted ferrite.     

Fluorescence spectroscopy of Er3+:LaOBr prepared by NH4Br solid state reaction

The steady luminescent materials La_1−xEr_xOBr (x = 0, 0.0003, 0.01) were synthesized by a new NH₄Br solid state reaction, and the structures were studied using XRD and Raman methods. Under 514.5 nm Ar⁺ laser excitation, the upconversion fluorescence spectra in LaOBr:Er³⁺ were recorded and investigated. It was found that four upconverted emission bands with peaks at 388 nm, 399 nm, 405 nm (violet) and 477 nm (blue) were observed. All these upconverted emissions were assigned, and the upconversion mechanism was deduced to be excited state absorption (ESA), by analyzing the energy level structures of Er³⁺ ions and measuring the power dependence of upconverted emission intensities.     

Synthesis of mesoporous Zn–Al spinel oxide nanorods with membrane like morphology

A well-aligned ZnAl₂O₄ spinel oxide nanorods with hierarchical pore structure was synthesized by a simple route using polycarbonate membrane as the sacrificial template. Spinel formation and the template removal occurred in one-step by calcination at 550 °C. SEM analysis of the calcined sample showed a membrane morphology made up of aligned rods with micron sized pores in between them. TEM analysis revealed that individual rods are 130 ± 30 nm in diameter and are nanoporous in nature. These rods are made up of nanoparticles of the sizes ranging from 5 to 12 nm. HRTEM analysis further showed that these nanoparticles are crystalline with the lattice spacing in commensurate with the ZnAl₂O₄ spinel oxide. Adsorption–desorption study shows Type IV isotherm with the pore size around 3.4 nm and high specific surface area of about 140 m²/g.     

UV-VUV excited luminescence of SrBPO5:Sm phosphor prepared in air

The high-resolution luminescent spectrum of divalent samarium excited by 355 nm UV light at 77 K, the VUV excitation spectra, the VUV excited emission spectra and EXAFS at Sm-L₃ edge were reported for samarium doped strontium borophosphate, SrBPO₅:Sm prepared by solid state reaction in air at high temperature. The high-resolution luminescent spectrum showed that the divalent samarium ions occupied the C_2υ lattice sites. The VUV excitation spectra indicated that the sample exhibited absorption bands with the maxima at 129 and 148 nm, respectively. The performance of EXAFS at Sm-L₃ absorption edge suggested that the samarium ions were nine-coordinated and the mean distances of bond SmO were 2.38 Å.     

Synthesis and luminescence of Sr2CeO4 fine particles

Fine particles of a blue emission phosphor Sr₂CeO₄ have been synthesized using a chemical co-precipitation technique, and the textual and luminescent properties were compared with the one synthesized by the conventional solid-state reaction method. Particle size and distribution of the Sr₂CeO₄ fine powder prepared by the co-precipitation process were smaller and narrower than those obtained by the samples prepared from the conventional one. The emission intensity of the fine particles was equal to that of the larger particles prepared from the solid-state reaction, on the contrary to the general tendency that emission intensity decrease with particle size reduction. Although no Ce³⁺ peaks were observed in EPR measurements, X-ray photoelectron spectra of the samples clearly elucidated the existence of Ce³⁺ only on the surface of Sr₂CeO₄.     

Photoluminescence properties of Sr3(PO4)2: Eu2+, Dy3+ double-emitting blue phosphor for white LEDs

Double-emitting blue phosphor Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ was synthesized by solid state reaction under H₂ atmosphere. XRD exhibited the pure hexagonal phase of the prepared phosphor. The photoluminescence results showed that all samples had intense broad absorption band between 250 and 450 nm, which matched well with the near-UV (350–420 nm) emission band of InGaN-based chips. The emission spectrum of Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ consisted of two broad bands, peaked at 485 nm and 410 nm, which originated from two luminescent centers, related to 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ in six-coordinated Sr(I) and ten-coordinated Sr(II) sites respectively. The intensity ratio of two emission bands could be easily tuned by adjusting Dy³⁺ co-doping content, which resulted in color-tunable luminescence in bluish green region to purplish blue region.     

SrHfO3-based phosphors and scintillators

The heavily Ce-doped SrHfO₃ (SHO) and the undoped non-stoichiometric SHO sample sets were prepared by solid state reaction in powder form and their luminescence spectra and decay kinetics were measured. Concentration quenching effects and thermally induced ionization of the Ce³⁺ excited states were followed and discussed in the former sample set. In the latter one new emission band at about 334 nm was found in Sr-deficient samples and temperature dependences of its emission intensity and decay times were studied in the detail. Room temperature decay time of about 180 ns and efficient and fast energy transfer from the host to the center of the 334 nm emission make this material promising candidate for X-ray phosphors.     

Al2O3:Cr3+ microfibers by hydrothermal route: Luminescence properties

Uniform Al₂O₃:Cr³⁺ microfibers were synthesized by using a hydrothermal route and thermal decomposition of a precursor of Cr³⁺ doped ammonium aluminum hydroxide carbonate (denoted as AAHC), and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curves. XRD indicated that Cr³⁺ doped samples calcined at 1473 K were the most of α-Al₂O₃ phase. SEM showed that the length and diameter of these Cr³⁺ doped alumina microfibers were about 3–9 μm and 300 nm, respectively. PL spectra showed that the Al₂O₃:Cr³⁺ microfibers presented a broad R band at 696 nm. It is shown that the 0.07 mol% of doping concentration of Cr³⁺ ions in α-Al₂O₃:Cr³⁺ was optimum. According to Dexter's theory, the critical distance between Cr³⁺ ions for energy transfer was determined to be 38 Å. It is found that the curve followed the single-exponential decay.     

Luminescence properties of monodispersed spherical BaWO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> microphosphors for white light-emitting diodes

Monodispersed spheres (1–4 μm in diameter) of BaWO₄:Eu³⁺ (hereafter BWO:Eu) red-phosphor exhibiting intense emission at 615 nm were synthesized via a mild hydrothermal method. X-ray diffraction, scanning electron microscope, photoluminescence excitation and emission spectra, and decay curve were used to characterize the properties of BWO:Eu phosphors. An intense red emission was obtained by exciting either into the ⁵L₆ state with 394 nm or the ⁵D₂ state with 465 nm, that correspond to two popular emission lines from near-UV and blue LED chips, respectively. The values of Ω _2,4 experimental intensity parameters (13.8 × 10⁻²⁰ and 8.2 × 10⁻²⁰ cm²) are determined. The high-emission quantum efficiency of the BWO:Eu phosphor suggests this material could be promising red phosphors for generating white light in phosphor-converted white light-emitting diodes.     

Synthesis of LiMn2O4 via high-temperature ball milling process

Spinel LiMn₂O₄ powder was prepared by a novel process of high-temperature ball milling. For comparison, the spinel LiMn₂O₄ powder was also synthesized by the traditional method of solid state reaction. It was found that high-temperature ball milling significantly decreased the synthesis temperature and time. LiMn₂O₄ with pure spinel phase could be successfully synthesized only by 2?h high-temperature ball milling at 500°C and 600°C. However, pure spinel LiMn₂O₄ could not be completely synthesized by 2?h solid state reaction at 800°C. The LiMn₂O₄ particles prepared by high-temperature ball milling are nano-sized (<100?nm) and much smaller than that prepared using solid state reaction. The electrochemical tests results indicated that the as-synthesized LiMn₂O₄ by 2?h high-temperature ball milling at 600°C showed a favorable initial discharge capacity of 124.2 mAh g^?1 at current rate of 0.1 C and still retained a capacity of 119.8 mAh g^?1 at 0.1 C after 80 continuous cycles from 0.1 to 2.0 C.     

Structural and magnetic properties of nanocrystalline Mg–Cd ferrites prepared by oxalate co-precipitation method

Polycrystalline spinel ferrites with general formula Mg_1−x Cd _x Fe₂O₄ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by oxalate co-precipitation method using high purity sulfates. The samples were sintered at 1,050 °C for 5 h. The structural properties of these samples were investigated by XRD, SEM and FTIR techniques. The X-ray diffraction analysis confirms the formation of single phase cubic spinel structure of all the samples. The lattice constant, X-ray density, physical density, porosity, crystallite size, site radii (r _A, r _B), bond length (A–O, B–O) on tetrahedral (A-site) and octahedral (B-site) were calculated for the samples. The lattice constant increases with increase in Cd²⁺ content. The X-ray density increases with increase in Cd²⁺ content. The crystallite size calculated by Scherrer formula is in the range of 27.79–30.40 nm. Physical densities are calculated by Archimedes principle. The SEM study shows that the grain size increases with increasing Cd²⁺ content. The FTIR spectra shows two strong absorption bands around 576 and 431 cm⁻¹ on the tetrahedral and octahedral sites, respectively. The dependence of saturation magnetization on Cd²⁺ content suggests that A–B and B–B super exchange interaction are comparable in strength. Neel’s two sub lattice model is applicable up to x ≤ 0.4, while Y–K three sub lattice models (canted spin) is predominant for x ≥ 0.4.     

Preparation and characterization of Co-doped ZnO-Al2O3-SiO2 glass-ceramics by the sol-gel method

Transparent glass-ceramics containing Co²⁺:ZnAl₂O₄ nanocrystals were obtained by the sol-gel process for the first time. The gels of composition 89SiO₂-5.9Al₂O₃-4.9ZnO-0.2CoO (ZAS) were prepared at room temperature, and heat-treated at different temperatures. The microstructure and optical properties of the heated samples were studied by using X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared spectroscopy, and optical absorption spectra. Co²⁺:ZnAl₂O₄ nanocrystals were precipitated from ZAS system and dispersed in the SiO₂-based glass during heat-treatment in the temperature range 900-1100 °C. Co²⁺ ions were located in tetrahedral sites in ZnAl₂O₄ nanocrystals. The Co²⁺:ZnAl₂O₄ crystallite size was in the range of 4-15 nm.