Radioluminescence properties of rare earths doped SrAl2O4 nanopowders

Nanopowders of SrAl₂O₄ pure and doped with rare earths were prepared via a proteic sol-gel methodology. The prepared materials presented a single crystalline phase, confirmed by XRD measurements. AFM results indicate that the average particle size is about 53 nm for SrAl₂O₄ powders. The radioluminescence spectrum of SrAl₂O₄: Eu²⁺, Dy³⁺ is composed by two intense peaks around 520 and 570 nm followed by a weaker emission peaking at 615 nm. It was observed that the intensity of RL emission during irradiation with X-rays decreased as a function of the irradiation time, indicating the build up of radiation damage in the nanopowders. The irradiated samples exhibited a persistent radiation damage that changes the colour of the sample, and also influenced the reduction in the scintillation efficiency. The saturation level of SrAl₂O₄: Eu²⁺ is 96%, exhibiting good resistance to radiation damage.     

Concentration effect of Tm on cathodoluminescence properties of SiO2:Tm and SiO2:Ho,Tm systems

Cathodoluminescence (CL) properties of SiO₂ powders activated with thulium (Tm³⁺) and holmium (Ho³⁺) ions prepared by a sol–gel process were investigated. Different molar concentrations of Tm³⁺ co-doped with Ho³⁺ were studied. The 460 nm peak was monitored and the influence of the beam energy and concentration of Tm³⁺ ions on the emission properties of this peak was also monitored. The peculiar behavior whereby the 460 nm emission peak decreases and the increase in the 705 and 865 nm peaks with the increase in the concentration of Tm³⁺ ions is reported. The relationship between the accelerating beam voltage and the CL intensity of the blue emission peak (460 nm peak) is established. Morphology, particle size and optical properties were characterized with Scanning electron microscopy (SEM), UV/VIS Lambda 750 S spectrometer and Auger electron spectroscopy (AES) equipped with Ocean Optics S2000, respectively.     

Luminescence spectra and optical properties of TeO2-WO3-Li2O glasses doped with Nd, Sm and Er rare earth ions

New tellurite glass series of the form (70-x)TeO₂-20WO₃-10Li₂O-xLn₂O₃, where x=0, 1, 3 and 5 mol% and Ln=Nd, Sm and Er, were prepared. Density of the prepared glasses was measured and molar volume was calculated. Luminescence spectra of the prepared glasses were measured at room temperature using a micro-Raman spectrometer. The obtained luminescence intensity ratio was correlated with the rare earth ion concentration, the short distance between the identical rare earth ions r(Ln-Ln) and the glass density. Optical properties like refractive index, molar refractivity and optical polarizability were theoretically calculated in order to interpret the dependence of these properties on the rare earth ion content.     

Synthesis and optical properties of Er and Eu doped SrAl2O4 phosphor ceramic

We report, for the first time on luminescence from a Er³⁺ doped SrAl₂O₄ phosphor. Effects of Eu³⁺ doping were also studied. The influence of rare-earth doping in crystal structure and its optical properties were analysed by means of X-ray diffraction (XRD), Raman scattering, optical absorption, excitation and emission (PL) spectroscopy, thermally stimulated luminescence (TSL) and scanning electron microscope (SEM). Luminescence spectra and luminescence decay curves for Er³⁺ transitions in the near infrared region were recorded. The PL maximum for Eu doped SrAl₂O₄ is obtained at 620 nm and corresponds to the orange region of the spectrum. Diffraction patterns reveal a dominant phase, characteristic of the monoclinic SrAl₂O₄ compound and the presence of dopants has no effect on the basic crystal structure of SrAl₂O₄. The shapes of the glow curves are different for each dopant irradiated with either a ⁹⁰Sr-⁹⁰Y beta source, or UV light at 311 nm, and in detail the TL signals differ somewhat between Er and Eu dopants.     

Influence of Yb doping concentration on upconversion luminescence of Er in Lu2O3 phosphor

Cubic phase Lu₂O₃:Er³⁺/Yb³⁺ nanocrystal phosphors were prepared by sol–gel method. Fourier transform infrared (FT-IR) spectra were measured to evaluate the vibrational feature of the samples. Green and red radiations were observed upon 980 nm diode laser excitation. Laser power and Er³⁺ or Yb³⁺ doping concentration dependence of upconversion luminescence were studied to understand upconversion mechanisms. Excited state absorption, cross relaxation and energy transfer processes are the possible mechanisms for the visible emissions.     

Luminescence properties of SrSi2O2N2 doped with divalent rare earth ions

The optical properties of SrSi₂O₂N₂ doped with divalent Eu²⁺ and Yb²⁺ are investigated. The Eu²⁺ doped material shows efficient green emission peaking at around 540 nm that is consistent with 4f⁷→4f⁶5d transitions of Eu²⁺. Due to the high quantum yield (90%) and high quenching temperature (>500 K) of luminescence, SrSi₂O₂N₂:Eu²⁺ is a promising material for application in phosphor conversion LEDs. The Yb²⁺ luminescence is markedly different from Eu²⁺ and is characterized by a larger Stokes shift and a lower quenching temperature. The anomalous luminescence properties are ascribed to impurity trapped exciton emission. Based on temperature and time dependent luminescence measurements, a schematic energy level diagram is derived for both Eu²⁺ and Yb²⁺ relative to the valence and conduction bands of the oxonitridosilicate host material.     

High field electrical properties of CdF2 crystals doped with rare earth ions

The electrical properties of CdF₂ crystals doped with Eu³⁺ and Gd³⁺ are described. A reversible “forming” effect is observed under high electric fields. The effect seems to be connected with a reversible modification of the potential barrier at the metal-insulator contact.     

Effect of accelerating voltage on crystallization of self-standing W-nanodendrites fabricated on SiO2 substrate with electron-beam-induced deposition

Self-standing W-nanodendrite structures were grown on SiO₂ substrate using an electron-beam-induced deposition (EBID) process with various accelerating voltages from 400 to 1000 kV. Effect of accelerating voltage on crystallization of the nanodendrites was investigated. The nanodendrites consisted of nano-sized grains and amorphous structures. The nano-sized grains were determined to be W crystal in BCC structure. The higher was electron beam accelerating voltage, the higher was crystallinity of the as-fabricated nanodendrites. It is suggested that high-energy electron irradiation enhances diffusion of W atoms in the nanodendrites, promotes crystallization of W grains.     

Luminescence center transformation in wet and dry SiO2

The main luminescent centers in SiO₂ films are the red luminescence R (1.85 eV) of the nonbridging oxygen hole center (NBOHC) and the oxygen deficient center (ODC) with a blue B (2.7 eV) and a UV band (4.4 eV). By means of a new “track-stop” technique we have investigated especially the initial luminescence behavior at the beginning of irradiation. Thus the blue-emitting center is produced under irradiation, but from existing precursors. Contrary to that, the dose behavior of the red (R) luminescence in wet and dry oxide is quite different, decreasing in wet oxide from a high initial level and increasing in dry oxide from almost zero. We propose a model of luminescence center transformation based on radiolytic dissociation and the reactions of mobile oxygen and hydrogen.     

Improving monochromaticity of upconversion luminescence by codoping Eu ions in Y2O3:Ho, Yb nanocrystals

Upconversion (UC) luminescence of Y₂O₃:Ho³⁺, Yb³⁺ nanocrystals codoped with different concentrations of Eu³⁺ ions were investigated to improve the monochromaticity of the UC emission. The results show that the monochromaticity, quantified by a parameter SR, increases as the concentration of Eu³⁺ ions becomes higher, which is due to the energy transfer between ⁵I₇ (Ho³⁺) and ⁷F₆ (Eu³⁺). The energy transfer accelerates the relaxation of Ho³⁺ ions from the ⁵I₇ to ⁵I₈ state and then quenches the red emission. The influence of the Eu³⁺ concentration on the pump power dependence of the red UC fluorescence in Y₂O₃:Ho³⁺, Yb³⁺, Eu³⁺ nanocrystals is verified using the steady-state rate equation theory.     

Synthesis process and the luminescence properties of rare earth doped NaLa(WO4)2 nanoparticles

Rare earth doped NaLa(WO₄)₂ nanoparticles have been prepared by a simply hydrothermal synthesis procedure. The X-ray diffraction (XRD) pattern shows that the Eu³⁺-doped NaLa(WO₄)₂ nanoparticles with an average size of 10-30 nm can be obtained via hydrothermal treatment for different time at 180 °C. The luminescence intensity of Eu³⁺-doped NaLa(WO₄)₂ nanoparticles depended on the size of the nanoparticles. The bright upconversion luminescence of the 2 mol% Er³⁺ and 20 mol% Yb³⁺ codoped NaLa(WO₄)₂ nanoparticles under 980 nm excitation could also be observed. The Yb³⁺-Er³⁺ codoped NaLa(WO₄)₂ nanoparticles prepared by the hydrothermal treatment at 180 °C and then heated at 600 °C shows a 20 times stronger upconversion luminescence than those prepared by hydrothermal treatment at 180 °C or by hydrothermal treatment at 180 °C and then heated at 400 °C.     

Energy transfer between Sm and Er in orthophosphate YPO4

The energy transfer between Sm³⁺ and Er³⁺ ions in yttrium orthophosphate is studied. This choice of ions is based on the possibility of quantum cutting processes and the host material is selected according to the position of the 5d bands of the Sm³⁺ ion. The Sm³⁺ and Er³⁺ doped and Sm³⁺, Er³⁺ co-doped YPO₄ have been synthesized. Spectroscopic studies were done in the ultraviolet and vacuum ultraviolet ranges. The energy transfer between Sm³⁺ and Er³⁺ is very efficient but it does not lead to Er³⁺ visible emission. Whatever the excitation wavelength, the emission of co-doped samples mainly occurs in the infrared range.     

Kinetics of luminescence of interface defects and resonant Er ions in nanostructured SnO2:SiO2

Silica glass with SnO₂ nanocrystals and Er³⁺ ions are prepared by the sol-gel route and treatment above 1000 °C. Transmission electron microscopy evidences a homogeneous dispersion of nanoclusters 4-6 nm in size in the amorphous silica matrix. Photoluminescence spectra excited at 3.5 eV, outside erbium transitions, show an inhomogeneous spectral distribution of light emission from interface defects, in the range 1.9-2.4 eV, resonant with transitions of erbium ions. The analysis of kinetics and temperature dependence of luminescence allows to quantify the efficiency of the energy transfer channel between nanoclusters and erbium ions.     

Magnetic resonance study of Ce and Gd doped NiFe2O4 nanoparticles

Present work is a study of temperature dependent electron paramagnetic resonance spectra of Ce and Gd doped nickel ferrite nanoparticles. The samples, synthesised by chemical route were characterised by X-ray diffractometer, electron paramagnetic resonance spectroscopy (EPR) and vibrating sample magnetometer (VSM). The average crystallite size of pure nickel ferrite is ∼64 nm and for Gd and Ce doped samples it is ∼20 nm and ∼14 nm, respectively. The EPR spectra were recorded from 120 to 300 K. Doping with Gd and Ce reduces the line width and g-value in comparison to that of pure nickel ferrite. Ce doped samples have the lowest values of both these parameters at room temperature. This indicates that Ce doped samples show lowest loss and is suitable for high frequency devices. EPR spin numbers are reduced while the spin relaxation time is increased after doping with rare earth ions. Gd doped samples have higher values of relaxation time and lower spin numbers in comparison to that of Ce doped samples. VSM results show that the magnetisation and coercivity are reduced after doping with both Ce and Gd rare earth ions.     

Magnetic structures of R3Cu4Si4 (R=Dy, Ho and Er)

Polycrystalline samples of R₃Cu₄Si₄ (R=Dy, Ho, Er) intermetallics were studied with neutron diffraction methods. All of them crystallize in the orthorhombic structure of Gd₃Cu₄Ge₄-type and order antiferromagnetically at low temperatures. Magnetic moments localized at the rare earth atoms, that occupy two non-equivalent 2d and 4e sublattices, order simultaneously in Dy₃Cu₄Si₄. The order is described by the propagation vector accompanied by , δ=0.025(2). In Ho₃Cu₄Si₄ two propagation vectors are needed to model the magnetic order. These are: for the 4e sublattice, which disorders as the first when the temperature increases, and for the 2d sublattice. A similar situation is observed for Er₃Cu₄Si₄ where the propagation vectors are: k=(0,1−δ,0), δ=0.097(2) for the 4e sublattice, which disorders as the first with increasing temperature, and , δ=0.0015(6) for the 2d sublattice.     

Effect of CdS nanoparticles on fluorescence from Sm doped SiO2 glass

Sm³⁺ doped CdS nanoparticles have been prepared by sol-gel method. The effect of annealing temperatures and doping concentrations of CdS on the photoluminescence spectra of Sm³⁺ were studied. From the measurement of its optical absorption, three phenomenological Judd-Ofelt intensity parameters (Ω₂, Ω₄, and Ω₆) have been computed and used to parameterize the radiative properties. The influences of CdS on Sm³⁺ ions were studied by fluorescence spectroscopy. The fluorescence spectra revealed that the emission intensity of samarium increased considerably in the presence of CdS nanoparticles. The evaluation of radiative properties of Sm³⁺ containing CdS showed that the ⁴G_5/2→⁶H_7/2 transition in silica matrix had the potential to be a laser transition.     

Photoluminescence analysis of α-Al2O3 powders doped with Eu and Eu ions

Alumina (Al₂O₃) powders doped with europium trivalent (Eu³⁺) were prepared by a low-temperature (∼280 °C) combustion synthesis technique. When the powder was heat treated at 1200 °C for 2 h in the presence of flowing ammonia (NH₃), α-Al₂O₃ crystalline ceramic powders was obtained. The analysis of the luminescence showed that Eu³⁺ was reduced to europium divalent (Eu²⁺) after the heat-treatment process. Under ultraviolet (UV) lamp excitation (λ=254 nm) these powders containing sub-microcrystalline structures present bright red (Al₂O₃:Eu³⁺) and green (Al₂O₃:Eu²⁺) luminescence indicating that this material is a potential candidate for applications in phosphor technology.     

Photoluminescence spectra of rare earth doped CaGa2S4 single crystals

Six kind CaGa₂S₄ single crystals doped with different rare earth (RE) elements are grown by the horizontal Bridgman method, and their photoluminescence (PL) spectra are measured in the temperature range from 10 to 300 K. The PL spectra of Ce or Eu doped crystals have broad line shapes due to the phonon assisted 4f-5d transitions. On the other hand, those of Pr³⁺, Tb³⁺, Er³⁺ or Tm³⁺ doped samples show narrow ones owing to the 4f-4f transitions. The assignments of the electronic levels are made in reference to the reported data of RE 4f multiplets observed in same materials.     

Synthesis and characterization of Ce doped silica (SiO2) nanoparticles

A series of silica doped with different mol percentages of Ce³⁺ concentration was synthesized using a sol-gel method to determine the dependence of photoluminescence wavelengths and intensity on the concentrations of the dopants. Sol-gel glasses are porous networks that have been densified through chemical processing and heat treatment. Rare-earths (REs) are insoluble in silica; due to this insolubility RE ions in silicate glasses enter as network modifiers and compete for non-bridging oxygen in order to complete their coordination. The morphological, structural, thermal and optical properties of the phosphors were characterized by X-ray diffraction, scanning electron microscopy, UV-vis absorption, photoluminescence, thermogravimetric analyses and differential scanning calorimeter. Silica (SiO₂) gel containing Ce³⁺ ions was sputter coated with Au (gold) in order to monitor surface morphology of the samples. The highest emission intensity was found for the sample with a composition of 0.5 mol% Ce³⁺. Cerium doped silica glasses had broad blue emission corresponding to the ²D_3/2-²F_J transition at 448 nm but exhibited apparent concentration quenching above concentrations of 0.5 mol% Ce³⁺.     

Sensitizing effects of ZnO quantum dots on red-emitting Pr-doped SiO2 phosphor

In this study, red cathodoluminescence (CL) (λ_emission=614 nm) was observed from Pr³⁺ ions in a glassy (amorphous) SiO₂ host. This emission was enhanced considerably when ZnO quantum dots (QDs) were incorporated in the SiO₂:Pr³⁺ suggesting that the ZnO QDs transferred excitation energy to Pr³⁺ ions. That is, ZnO QDs acted to sensitize the Pr³⁺ emission. The sol–gel method was used to prepare ZnO–SiO₂:Pr³⁺ phosphors with different molar ratios of Zn to Si. The effects of the ZnO QDs concentration and the possible mechanisms of energy transfer from ZnO to Pr³⁺ are discussed. In addition, the electronic states and the chemical composition of the ZnO–SiO₂:Pr³⁺ phosphors were analyzed using X-ray photoelectron spectroscopy (XPS).