Crystal structure and luminescent properties of nanocrystalline YAG and YAG:Nd synthesized by sol–gel method

The work describes results of synthesis of undoped and Nd-doped YAG nanopowders by sol–gel method using different complexing agents (ethylene glycol and citric acid) and characterization of the material by X-ray powder diffraction, scanning electron microscopy, photoluminescence and thermoluminescence techniques. Utilization of citrate sol–gel procedure using yttrium and aluminum nitrate nonahydrates as starting substances allowed to obtain highly stoichiometric and non-defected YAG and YAG:Nd nanocrystalline samples with good luminescence performance and low radiation storage efficiency.     

Synthesis and characterization of nanocrystalline Nd3+-doped gadolinium scandium aluminum garnet powders by a gel-combustion method

Nd³⁺-doped gadolinium scandium aluminum garnet (Nd:GSAG) precursor was synthesized by a gel combustion method using metal nitrates and citric acid as raw materials. The structure and morphology of the precursor and the sintered powders were studied by means of X-ray diffraction (XRD), infrared spectroscopy (IR) and transmission electron microscopy (TEM). The results showed that the precursor transformed into pure GSAG polycrystalline phase at about 800 °C, and the powders sintered at 800–1000 °C were well-dispersed with average particle sizes in the range of 30–80 nm. Optical properties of Nd:GSAG nano-powders were characterized by using photoluminescence spectroscopy. The highest photoluminescence intensity was achieved for the powder sintered at 900 °C.     

Na2EDTA-assisted hydrothermal synthesis and luminescent properties of YVO4:Eu3+ with different morphologies in a wide pH range

Nano- and micro-scaled Eu-doped yttrium orthovanadate (YVO₄:Eu³⁺) powders had been fabricated via disodium ethylenediamine tetraacetate (Na₂EDTA)-assisted morphology controllable hydrothermal method in a wide pH range at 180 °C for 24 h. The as-synthesized samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence spectroscopy (PL). The results showed that the pH value of the synthesis solution played a key role in the formation of the final products with different morphologies, including ball-like micro-spheres, micro-spheres composed of submicron cubes and flower-like structures containing nano-plates. The photoluminescence measurement revealed that the luminescent properties of the samples were changed by varying their morphologies. The significant ball-like micro-spheres of YVO₄:Eu³⁺ particles had been synthesized, and the luminescence intensity of them is the strongest one among all products.     

The optimum conditions for solid-state-prepared (Y3−xCex)Al5O12 phosphor using the Taguchi method

Using the Taguchi method, the authors analyzed the optimum conditions for (Y_3?xCe_x)Al₅O₁₂ (YAG:Ce) phosphor, which is prepared using the solid-state reaction method. The controllable factors used in this study consisted of the following: (1) the duration of milling, (2) the quantity of substitution, (3) the duration of sintering, and (4) the temperature of sintering. Under optimum conditions, a confirmation experiment was carried out, and the average photoluminescence (PL) intensity of YAG:Ce phosphor was found to be up to 270.84 (a.u.). The percentage contribution of each controllable factor was also determined. Most interestingly, the temperature of sintering is the most influential factor within current investigation range to the solid-state-prepared YAG:Ce phosphor, and its value of percentage contribution is up to 70.90%. Aside from this, through the optimum conditions, the average PL intensity of YAG:Ce phosphor can be substantially promoted from 193.88 (a.u.), the average PL intensity of YAG:Ce phosphor sintered at 1500 °C for 6 h that was usually used to sinter YAG:Ce phosphor.     

Preparation of polycrystalline YAG/alumina composite fibers and YAG fiber by sol–gel method

Polycrystalline yttrium–aluminum garnet, Y₃Al₅O₁₂ (YAG) fiber and α-alumina and YAG matrix composite fiber were prepared by the sol–gel method. α-Alumina and YAG matrix composite fiber with fine and homogeneous microstructure could be successfully fabricated by interpenetrating YAG in alumina matrix and adding α-alumina of seed particles to fibers. Effect of α-alumina seed particles and YAG on crystallization and microstructure of composite fiber were discussed. The size of alumina matrix of the composite fibers heated at 1600°C for 4 h was below 2 μm. The tensile of strength alumina fiber heat-treated at 1500°C was 0.2 GPa, while that of the composite fiber was 1.1 GPa.     

Effect of the europium doping on the structural and luminescent properties of yttrium aluminum garnet

Europium activated yttrium aluminate (YAG:Eu) phosphors were synthesized using urea by wet chemical synthesis route. The luminescent behavior and the microstructural changes are monitored, depending on the amount of europium. The luminescent spectra of YAG:Eu phosphors possess four major emission bands in the range of 570–700 nm with maxima situated at 592 nm 598 nm, 611 nm and 631 nm. The intensity ratio between the 611 nm band as red component (⁵D₀ → ⁷F₂) and 592 nm band as orange component (⁵D₀ → ⁷F₁) is discussed, in order to obtain information about the chemical surroundings of the luminescent centers and their symmetry. X-ray diffraction showed that the main crystalline phase of the phosphors is yttrium aluminum garnet Y₃Al₅O₁₂ with cubic structure. Monoclinic Y₄Al₂O₉ phase was also found as impurity. The effect of the europium content on the microstructural parameters is revealed. The luminescent characteristics depend strongly on the structural purity, activator concentration and incorporation of europium ions in the host lattice. Additional investigations as FT-IR, BET, ICP-OES were performed for a better understanding of the luminescent and structural characteristics of YAG:Eu phosphor.     

Scintillation properties of Pr3+-doped lutetium and yttrium aluminum garnets: Comparison with Ce3+-doped ones

Scintillation properties of Pr³⁺-doped LuAG and YAG crystals were investigated and compared with those of Ce³⁺-doped ones. The highest L.Y.’s were observed with the longest shaping time 10 μs. They can reach up to ～16,000 ph/MeV or ～23,500 ph/MeV for LuAG:Pr and LuAG:Ce, respectively. Energy resolutions (FWHM) are a bit better with LuAG:Pr than those of LuAG:Ce, e.g. at 662 keV FWHM are around 6% and between 8–12%, respectively. There were observed no large changes in proportionality of Pr³⁺- or Ce³⁺-doped LuAG or YAG crystals but the best proportionality has YAP:Ce crystal. Pr³⁺- or Ce³⁺-doped LuAG crystals exhibit slow decay components in the time range 1.5–3.5 μs while those of YAG ones have shorter decay components between 0.3–1.7 μs.     

Structural transformation and photoluminescence behavior during calcination of the layered europium-doped yttrium hydroxide intercalate with organic-sensitizer

We report, for the first time, structural transformation and photoluminescence behavior by calcination of layered europium-doped yttrium hydroxide (LYH:Eu) intercalate with organic sensitizer terephthalate (TA). The calcined samples displayed tunable luminescent performance dependent on calcined temperatures. Calcination under low temperatures (200 and 300 °C) retained layered structure, while high temperatures (>400 °C) yielded oxide phase. The optimal fluorescence occurred in 200 °C-calcination, possibly resulting from an optimal arrangement of TA. Above 200 °C, the luminescence intensity was first weakened and then enhanced, due to gradual departure of TA and following occurrence of oxide phase. The energy transfer presented an intrinsic transition from TA-to-Eu³⁺ in the organic intercalate to O²⁻-to-Eu³⁺ charge transfer in as-transformed oxide. The predominant luminescence property of the hybrid material can provide valuable guide for developing tunable luminescent materials, especially flexible materials resulting from the containing organic component.     

Basic zinc carbonate as a precursor in the solvothermal synthesis of nano-zinc oxide

ZnO nanoparticles were synthesized solvothermally in various diols (ethylene glycol, di(ethylene glycol), tetra(ethylene glycol), 1,2-propanediol, 1,4-butanediol), using basic zinc carbonate (2ZnCO₃·3Zn(OH)₂) as a precursor for the first time. Since ZnCO₃ was sparingly soluble in diols the transformation reaction proceeded at a low reaction rate. Ethylene glycol was found as the most suitable medium among five diols studied yielding the smallest ZnO particles (~ 55 nm) and short reaction time, t_r (2 h). Diols with shorter chain length produced smaller ZnO particles. p-Toluene sulfonic acid (p-TSA) acted as a catalyst and reduced t_r from 8 h to 2 h in concentration of 0.02 M. Optimum reaction conditions for the synthesis in ethylene glycol were 185 °C and 2 h. At higher p-TSA concentrations (0.04–0.08 M) the size of ZnO particles was reduced from 500–800 nm to 50–100 nm and crystallite size to 25–30 nm. Benzene sulfonic acid (BSA) and inorganic bases (LiOH, NaOH, and KOH) also showed catalytic activities. Raman and photoluminescence spectroscopies revealed high concentration of defects on ZnO surface causing the emission of visible light and giving this type of ZnO higher potential in various (opto)-electronic application in comparison to Zn(II) acetate based ZnO.     

Study of the line intensity in the optical and magnetooptical spectra in holmium-containing paramagnetic garnets

Studies of line intensity in the optical and magneto-optical spectra in the holmium-containing paramagnetic garnet Ho³⁺:YAG were carried out within the visible spectrum at T = 85 K. Detailed investigation of the magnetic circularly polarized luminescence spectra at 85 and 300 K on ⁵S₂ → ⁵I₈ emission transition in Ho³⁺:YAG was carried out. A quasi-doublet state in the energy spectrum of the Ho³⁺ ions was observed, characterized by a significant magneto-optical activity, which is caused by a large Zeeman splitting of the quasi-doublet. The measurement of the magnetic circular polarized luminescence spectrum carried out within one of the emission lines of the luminescence band ⁵S₂ → ⁵I₈ in Ho³⁺:YAG at 85 K shows significant magneto-optical effects of the intensity change of the emitted light, compared to that measured for the other emission lines in the same luminescent band.     

Synthesis and characterization of nanocrystalline barium strontium titanate powders prepared by citrate precursor method

Nanocrystalline and well dispersed barium strontium titanate (BST) powders were prepared by a novel and simple citrate precursor method. This method involved direct crystallization of a white precursor from a stable solution in the citric acid (CA)–ethylene glycol (EG)–tetrabutyl titanate–M²⁺ (M = Ba, Sr) system under a specific pH value range. Subsequent heat treatment of the precursor at 850 °C led to a pure phase BST powder. TG/DTA was used to examine the decomposition behaviour of the precursor. The crystalline phase and morphology of the BST powders were investigated by XRD and TEM. It was found that the BST powders synthesized by citrate precursor process were more homogeneous and uniform than that obtained by the citrate gel method.     

Preparation and characterization of nanocrystalline Nd-YAG powder

Nanocrystalline materials have assumed notable importance in a wide variety of fields owing to numerous possible applications offered by them. These include transparent ceramics wherein they facilitate synthesis as well as sintering at significantly lower temperatures. We report preparation of nanocrystalline neodymium doped yttrium aluminum garnet (YAG) with an ultimate intent to make transparent Nd-YAG ceramic. The Liquid Mix method employed involves mixing of metal nitrates with excess amounts of citric acid followed by dissolution and polymerization in ethylene glycol to form complex chelates. Amorphous powder obtained by firing of polymeric chelate compound at 400 °C permits formation of nanoparticles of Nd:YAG at as low a crystallization temperature as 920 °C as shown by the thermal analysis. Progressive evolution of well crystallized phase-pure YAG was studied by XRD of amorphous powders subjected to different calcination temperatures. Scanning electron microscopic (SEM) study of the crystalline material shows that particle size ranges between 50 and 100 nm.     

Characterization of laser-induced damage in silicon solar cells during selective ablation processes

Selective laser ablation of silicon nitride layers on crystalline silicon wafers was investigated for solar cell fabrication. Laser processing was performed with a nanosecond UV laser at various energy densities ranging from 0.2 to 1.5 J cm^?2. Optical microscopy was used as a simple mean to assess the ablation threshold that was correlated to the temperature at the interface between the silicon nitride coating and the silicon substrate. Minority carrier lifetime measurements were performed using a microwave photo-conductance decay technique. Band to band photoluminescence spectroscopy proved to be a sensitive technique to qualify the laser-induced damage to the silicon substrate. The crystalline structure of silicon seemed to be maintained after silicon nitride ablation as shown by UV reflectivity measurements. Laser parameters corresponding to fluences of around 0.4 J cm^?2 were found to achieve selective ablation of SiN_x without causing detrimental damage to the surrounding material.     

Photoluminescence properties of Y3Al5O12:Eu nanocrystallites prepared by co-precipitation method using a mixed precipitator of NH4HCO3 and NH3·H2O

Europium-doped yttrium aluminum garnet (Y₃Al₅O₁₂:Eu, YAG:Eu) nanocrystallites were prepared by calcining the precursors obtained via a co-precipitation method using a mixed solution of NH₄HCO₃ and NH₃·H₂O as the precipitator. The results of XRD, FTIR and thermal analysis showed that phase-pure YAG:Eu without any other phases was obtained at 900 °C. TEM results indicated that the particle sizes are 50–100 nm. YAG:Eu nanocrystallites showed four emission bands ascribed to ⁵D₀ → ⁷F₁ transition (592 and 597 nm) and ⁵D₀ → ⁷F₂ transition (611 and 633 nm) of Eu³⁺, respectively. The intensity of the magnetic dipole transition (⁵D₀ → ⁷F₁) is stronger than that of the electric dipole transition (⁵D₀ → ⁷F₂). The influence of the precipitators with different molar ratios of NH₄HCO₃ to NH₃·H₂O on the thermal properties of the as-prepared precursors and luminescent properties of the resulting YAG:Eu nanocrystallites was also investigated.     

Photoluminescence properties of TiO2: Eu3 + thin films deposited on different substrates

Strong photoluminescence of Eu^3 + due to intra 4f transitions are obtained from amorphous xerogel TiO₂: Eu^3 + films prepared by sol–gel method and treated at a low temperature of 100 °C. The films are deposited on four different substrates: Si, Al, AAO (anodic alumina oxide) and porous silicon. We find that the luminescence intensity on AAO substrate increased 4 times comparing with that of Si or Al, and luminescence intensity decreases obviously on porous silicon substrate. Energy transfer mechanism from TiO₂ host to Eu^3 + is deduced through analysis of photoluminescence and photoluminescence excitation spectrum. Concentration quenching of Eu^3 + does not appear even at high atomic concentration of 7.69%.     

Luminescence studies of Ce:YAG using vacuum ultraviolet synchrotron radiation

Photoluminescence spectrum of Ce:YAG single crystal was studied employing vacuum ultraviolet (VUV) synchrotron radiation. Intrinsic absorption edge at about 52,000 cm⁻¹ was observed in the absorption spectrum. From the VUV excitation spectrum, the energy of the highest d-component of 53,191 cm⁻¹ (188 nm) for the Ce³⁺ ions in YAG was obtained at 300 K. The disappearance of the third 5d level at 37,735 cm⁻¹ (265 nm) in absorption and excitation spectra in our samples may be due to the impurity Fe³⁺ ions absorption.     

Modified sol–gel preparation of LiNbO3 target for PLD

When preparing LiNbO₃ thin layer by PLD, the maximal density of polycrystalline target is important. In this work, we prepared the precursor powders by sol–gel method and used them at PLD target synthesis. The gels were synthesized by Pechini polyesterification, using citric acid and ethylene glycol as an organic matrix constituents, and then decomposed using various temperatures (800, 600 and 400 °C) and atmospheres (air, nitrogen and oxygen in sequence). Out of several combinations tested, the decomposition at 800 °C in nitrogen followed by the oxidation at 400 °C in oxygen was found to be the best way to achieve the minimal particle size of powder precursor resulting in denser targets. The prepared targets were characterized by XRD and SEM.The optimized process was subsequently used for preparation of LiNbO₃ target. Thin layers prepared by PLD from this target resulted smoother than the layers prepared from commercial monocrystalline target.     

Annealing effect on the photoluminescence properties of ZnO nanorod array prepared by a PLD-assistant wet chemical method

Well-aligned ZnO nanorod arrays were synthesized by a wet chemical method on the glass substrate with ZnO thin film as seed layer prepared by pulsed laser deposition. The effect of annealing temperature on the luminescence characteristics was investigated. As the annealing temperature increased, the photoluminescence properties show a general enhancing tendency. The nanorod array with high ultraviolet emission and negligible visible light emission (designated by the photoluminescence intensity ratio of ultraviolet to visible emission of 66.4) is obtained by annealing the sample at 700 °C for 1 h. Based on the results of X-ray photoelectron spectroscopy and photoluminescence spectra, the mechanisms of visible emission were discussed.     

Luminescent properties of Al2O3:Ce single crystalline films under synchrotron radiation excitation

The paper is dedicated to study the luminescent and scintillation properties of the Al₂O₃:Ce single crystalline films (SCF) grown by LPE method onto saphire substrates from PbO based flux. The structural quality of SCF samples was investigated by XRD method. For characterization of luminescent properties of Al₂O₃:Ce SCFs the cathodoluminescence spectra, scintillation light yield (LY) and decay kinetics under excitation by α-particles of Pu²³⁹ source were used. We have found that the scintillation LY of Al₂O₃:Ce SCF samples is relatively large and can reach up to 50% of the value realized in the reference YAG:Ce SCF. Using the synchrotron radiation excitation in the 3.7–25 eV range at 10 K we have also determined the basic parameters of the Ce³⁺ luminescence in Al₂O₃ host.     

Photoluminescent properties of LiInO2 nanocrystals

Synthesis and luminescence properties of LiInO₂ nanocrystals by the sol–gel process were investigated. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence spectroscopy and absorption spectra. The well-crystallized tetragonal LiInO₂ can be obtained by heat treatment above 600 °C from XRD. The excitation wavelengths at about 246 nm were associated with charge transfer between In and O with In³⁺ ions in octahedral coordination. The PL spectra excited at 246 nm have a broad and strong emission band maximum at 391 nm, corresponding to the self-activated luminescence. The optical absorption spectra of the 600 °C sample exhibited the band gap energies of 3.7 eV.