Hole traps in Lu2O3:Eu ceramic scintillators. II. Radioluminescence and thermoluminescence

A sample of Eu³⁺-activated lutetium sesquioxide transparent ceramic has been investigated by combined scintillation and thermoluminescence excited by prolonged gamma-ray irradiation. The thermoluminescence glow curve partially confirms and extends a previous model for afterglow following pulsed X-ray excitation. The initial concentration of hole traps, tentatively attributed to anion Frenkel defects in thermodynamic equilibrium, is found to be substantially augmented by reversible radiation damage.     

Synthesis, characterization, and luminescent properties of Lu2O3:Eu phosphors

Eu-doped lutetia (Lu₂O₃:Eu) nano-phosphors were synthesized by the sol-gel combustion process from a mixed aqueous solution of europium and lutetium nitrates, using organic glycine as the fuel. Powder X-ray diffraction shows that cubic Lu₂O₃:Eu crystallites are directly obtained by the sol-gel combustion process without further calcination. Electron microscopy reveals that the as-prepared phosphors are agglomerated and have a fluffy, fine, and porous morphology, consisting of primary particle size of 8-10 nm. The excitation spectrum is characterized by three dominant bands centered at 395, 466, and 534 nm, respectively. Both the photoluminescent and radioluminescent spectra are very similar and exhibit intense emission peaks centered at 612 nm due to ⁵D₀→⁷F₂ transition of Eu³⁺ ions. The energy transfer from Lu₂O₃ host to Eu³⁺ activator is more efficient in the case of calcined phosphors than for the as-prepared phosphors due to their improved lattice perfection.     

Multiplication of electronic excitations in nanophosphors Lu2O3:Eu and Lu2O3:Tb

Luminescence properties of Lu₂O₃:Eu³⁺ and Lu₂O₃:Tb³⁺ nanocrystalline powders with the particle size varying from 46 to 6 nm were studied under excitation by synchrotron radiation in the photon energy range (up to ∼22.5 eV) covering the region where the processes of multiplication of electronic excitation occur. It was found that the excitation spectra of Tb³⁺ emission from all Lu₂O₃:Tb³⁺ nanopowders have similar behavior, whereas the shape of the excitation spectra of Eu³⁺ emission from Lu₂O₃:Eu³⁺ nanopowders strongly depends on the particle size. The difference in the behavior of Lu₂O₃:Eu³⁺ and Lu₂O₃:Tb³⁺ nanophosphor systems was explained by different mechanisms of the energy transfer from the host to Eu³⁺ or Tb³⁺ ions (either the hole or electron recombination mechanism, respectively), which are differently influenced by losses of electronic excitations near the particle surface.     

Radioluminescence of Lu2O3:Eu nanocrystalline powder and vacuum-sintered ceramic

Nanocrystalline powders of Lu₂O₃:Eu with the activator content varying in the range of 0.2–10% were prepared through a combustion technique. The powders were only slightly agglomerated and the size of crystallites were about 30 nm. Some of the powders were co-doped with Mg, Ca, Sr, Ba, La of various concentrations. Such powders were cold-pressed and sintered at 1750°C for 5 h in vacuum. X-ray-excited luminescence spectra of both the powders and the sintered ceramics were recorded and the efficiency was compared to the commercial standard Gd₂O₂S:Pr,Ce,F X-ray phosphor. It was found that the nanocrystalline powders of Lu₂O₃:Eu,Ca emit photons four-times less than the commercial micron-sized Gd₂O₂S:Pr,Ce,F powder. In the case of sintered materials the emission efficiency from our Lu₂O₃:Eu5%, Ca was roughly equal to the efficiency of the commercial-sintered Gd₂O₂S:Pr,Ce,F. The co-doping ions were shown to have various effects on the transparency of the sintered Lu₂O₃:Eu. Mg hindered the sintering process producing completely opaque pellets. Other ions facilitated the sintering course and the best results were obtained by co-doping the samples with 0.5% of Ba. Sr and La also significantly stimulated the sintering and the final pellets were only slightly cloudy.     

Silica-coated Y2O3:Eu nanoparticles and their luminescence properties

Crystalline Y₂O₃:Eu is of paramount significance in rare earth materials and research on luminescence spectra. In this work, the nanocrystalline Y₂O₃:Eu was coated with silica by a facile solid state reaction method at room temperature. The transmission electron microscope (TEM) photographs showed that the prepared Y₂O₃:Eu particle is polycrystalline with the size of 20 nm, the size of silica-coated particle is about 25 nm. The XPS spectra indicated that the silica layer is likely to interact with Y₂O₃:Eu by a Si-O-Y chemical bond. The luminescence spectra showed that the intensity of ground samples is lower than that of unground ones, the intensity of silica-coated phosphors is higher than that of the ground samples, while almost the same as that of the unground ones. Therefore, the silica coating decreases the surface defects of nanoparticles of the nanocrystalline Y₂O₃:Eu, thus increasing their luminescent intensity.     

The influence of Pr co-doping on the luminescent properties of Lu2O3:5 mol% Eu films

Uniform and crack free polycrystalline lutetium oxide (Lu₂O₃:(Eu,Pr)) films were fabricated by Pechini sol-gel method combined with the spin-coating technique. X-ray diffraction (XRD) and atomic force microscope (AFM) characterizations indicated that the obtained film was composed of polycrystalline cubic Lu₂O₃ phase with an average grain size around 30 nm. The photoluminescence(PL) spectra and decay performances of the Lu₂O₃:5 mol% Eu films co-doped by 0-0.5 mol% Pr³⁺ with different concentrations were characterized. It was found that the afterglow was reduced obviously due to the introduction of 0-0.5 mol% Pr³⁺ in the Lu₂O₃:5 mol% Eu films coupled by decrease in the emission intensity at 612 nm. The mechanism of afterglow diminishing was discussed based on the thermoluminescence measurements.     

Optical properties of (ZnO)0.5(P2O5)0.5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3

Binary (ZnO)_0.5(P₂O₅)_0.5 glasses doped with Eu₂O₃ and nanoparticles of Gd₂O₃:Eu were prepared by conventional melt-quench method and their luminescence properties were compared. Undoped (ZnO)_0.5(P₂O₅)_0.5 glass is characterized by a luminescent defect centre (similar to L-centre present in Na₂O-SiO₂ glasses) with emission around 324 nm and having an excited state lifetime of 18 ns. Such defect centres can transfer the energy to Eu³⁺ ions leading to improved Eu³⁺ luminescence from such glasses. Based on the decay curves corresponding to the ⁵D₀ level of Eu³⁺ ions in both Gd₂O₃:Eu nanoparticles incorporated as well as Eu₂O₃ incorporated glasses, a significant clustering of Eu³⁺ ions taking place with the latter sample is confirmed. From the lifetime studies of the excited state of L-centre emission from (ZnO)_0.5(P₂O₅)_0.5 glass doped with Gd₂O₃:Eu nanoparticles, it is established that there exists weak energy transfer from L-centres to Eu³⁺ ions. Poor energy transfer from the defect centres to Eu³⁺ ions in Gd₂O₃:Eu nanoparticles doped (ZnO)_0.5(P₂O₅)_0.5 glass has been attributed to effective shielding of Eu³⁺ ions from the luminescence centre by Gd-O-P type of linkages, leading to an increased distance between luminescent centre and Eu³⁺ ions.     

Synthesis of Y2Si2O7:Eu nanocrystal and its optical properties

Nanocrystalline Y₂Si₂O₇:Eu phosphor with an average size about 60 nm is easily prepared using silica aerogel as raw material under ultrasonic irradiation and annealing temperature at 300-600 °C and this nanocrystalline decomposes into Y₂O₃:Eu and silica by heat treatment at 700-900 °C. The excitation broad band centered at 283 and 254 nm results from Eu³⁺ substituting for Y³⁺ in Y₂Si₂O₇ and Y₂O₃/SiO₂, respectively. Compared with Y₂O₃:Eu/SiO₂ crystalline, the PL excitation and emission peaks of Y₂Si₂O₇:Eu nanocrystalline red-shift and lead to the enhance of its luminescence intensity due to the different chemical surroundings of Eu³⁺ in above nanocrystallines. The decrease of PL intensity may be ascribed to quenching effect resulting from more defects in Y₂O₃:Eu/SiO₂ crystalline.     

Preparation and luminescent properties of cubic Eu:Y2O3 nanocrystals and comparison to bulk Eu:Y2O3

Y₂O₃:Eu³⁺ nanocrystals were prepared by combustion synthesis. The particle size estimated by X-ray powder diffraction (XRD) was about 10 nm. A blue-shift of the charge-transfer (CT) band in excitation spectra was observed in Y₂O₃:Eu³⁺ nanocrystals compared with bulk Y₂O₃:Eu³⁺. The electronic structure of Y₂O₃ is calculated by density functional method and exchange and correlation have been treated by the generalized gradient approximation (GGA) within the scheme due to Perdew-Burke-Ernzerhof (PBE). The calculated results show that the energy centroid of 5d orbital in nanocrystal has increasing trend compared with that in the bulk material. The bond length and bond covalency are calculated by chemical bond theory. The bond lengths of Y₂O₃:Eu³⁺ nanocrystal are shorter than those of the bulk counterpart and the bond covalency of Y₂O₃:Eu³⁺ nanocrystal also has an increasing trend. By combining centroid shift and crystal-field splitting, the blue-shift of the CT band is interpreted.     

Influence of precipitant solution pH on the structural, morphological and upconversion luminescent properties of Lu2O3:2%Yb, 0.2%Tm nanopowders

Lutetium oxide nanopowders codoped with Tm³⁺ and Yb³⁺ were synthesized by the reverse-strike co-precipitation method. Effects of precipitant solution pH on the structural, morphological and upconversion luminescent properties of Lu₂O₃:2%Yb, 0.2%Tm nanopowders had been investigated. The results show that pH value of the precipitant (NH₄HCO₃) solution has a significant effect on the particle size, morphology and upconversion emission intensity of the Lu₂O₃:2%Yb, 0.2%Tm nanopowders. All the samples obtained from different pH value of precipitant solution can be readily indexed to pure cubic phase of Lu₂O₃, indicating good crystallinity. The upconversion emission intensity of Lu₂O₃:2%Yb, 0.2%Tm nanopowders obtained from the precipitant solution with pH=11 is the strongest. The enhancement of the upconversion luminescence is suggested to be the consequence of reducing the number of OH⁻ groups and the enlarged nanopowder size. The strong blue, weak red and near infrared emissions from the prepared nanopowders were observed under 980 nm laser excitation, and attributed to the ¹G₄→³H₆, ¹G₄→³F₄ and ³H₄→³H₆ transitions of Tm³⁺ ion, respectively.     

Metal-like to insulator transition in Lu3Fe5O12

Polycrystalline Lu₃Fe₅O₁₂, prepared using the solid state reaction method, has Fe in the mixed valence state as inferred from the X-ray photoelectron spectroscopy. A spectral change in the impedance plot at 343 K is attributed to metal-like to insulator transition (MIT), which is analyzed in terms of localized and delocalized _eg$e_g$ electrons. The change in the slope at 343 K in the DC conductivity plot also proves the MIT. The dependence of Z^′${\mathrm{Z}}^{\prime }$ on temperature and Z^″${\mathrm{Z}}^{″}$ on frequency clearly substantiates the presence of localized electrons up to 343 K and delocalized electrons above 343 K.     

Chemical quenching of positronium in Fe2O3/Al2O3 catalysts

Fe₂O₃/Al₂O₃ catalysts were prepared by solid state reaction method using α-Fe₂O₃ and γ-Al₂O₃ nano powders. The microstructure and surface properties of the catalyst were studied using positron lifetime and coincidence Doppler broadening annihilation radiation measurements. The positron lifetime spectrum shows four components. The two long lifetimes τ₃ and τ₄ are attributed to positronium annihilation in two types of pores distributed inside Al₂O₃ grain and between the grains, respectively. With increasing Fe₂O₃ content from 3 wt% to 40 wt%, the lifetime τ₃ keeps nearly unchanged, while the longest lifetime τ₄ shows decrease from 96 ns to 64 ns. Its intensity decreases drastically from 24% to less than 8%. The Doppler broadening S parameter shows also a continuous decrease. Further analysis of the Doppler broadening spectra reveals a decrease in the p-Ps intensity with increasing Fe₂O₃ content, which rules out the possibility of spin-conversion of positronium. Therefore the decrease of τ₄ is most probably due to the chemical quenching reaction of positronium with Fe ions on the surface of the large pores.     

Synthesis of Y2O3:(Li,Eu) films using phosphor powders coated with SiO2 nano particles

Y_1.9−xLi_0.1Eu_xO₃ (x=0.02, 0.05, 0.08, and 0.12) films were fabricated by spin-coating method. A colloidal silica suspension with Y_1.9−xLi_0.1Eu_xO₃ phosphor powder was exploited to obtain the highly stable and effective luminescent films onto the glass substrate. After heating as-prepared Y_1.9−xLi_0.1Eu_xO₃ films at 700 °C for 1 h, the phosphor films exhibit a high luminescent brightness as well as a strong adhesiveness on the glass substrate. The emission spectra of spin-coated and pulse-laser deposited Y_1.82Li_0.1Eu_0.08O₃ films were compared. The cathodoluminescence of the phosphor films was carried out at the anode voltage 1 kV.     

Luminescence studies on ZnO-P2O5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3

Zinc phosphate glasses doped with Gd₂O₃:Eu nanoparticles and Eu₂O₃ were prepared by conventional melt-quench method and characterized for their luminescence properties. Binary ZnO-P₂O₅ glass is characterized by an intrinsic defect centre emission around 324 nm. Strong energy transfer from these defect centres to Eu³⁺ ions has been observed when Eu₂O₃ is incorporated in ZnO-P₂O₅ glasses. Lack of energy transfer from these defect centres to Eu³⁺ in Gd₂O₃:Eu nanoparticles doped ZnO-P₂O₅ glass has been attributed to effective shielding of Eu³⁺ ions from the luminescence centre by Gd-O-P type of linkages, leading to an increased distance between the luminescent centre and Eu³⁺ ions. Both doped and undoped glasses have the same glass transition temperature, suggesting that the phosphate network is not significantly affected by the Gd₂O₃:Eu nanoparticles or Eu₂O₃ incorporation.     

Hydrothermal synthesis, characterization and Raman studies of Eu activated Gd2O3 nanorods

Eu³⁺ (8 mol%) activated gadolinium oxide nanorods have been prepared by hydrothermal method without and with surfactant, cityl trimethyl ammonium bromide (CTAB). Powder X-ray diffraction (PXRD) studies reveal that the as-formed product is in hexagonal Gd(OH)₃:Eu phase and subsequent heat treatment at 350 and 600 °C transforms the sample to monoclinic GdOOH:Eu and cubic Gd₂O₃:Eu phases, respectively. The structural data and refinement parameters for cubic Gd₂O₃:Eu nanorods were calculated by the Rietveld refinement. SEM and TEM micrographs show that as-obtained Gd(OH)₃:Eu consists of uniform nanorods in high yield with uniform diameters of about 15 nm and lengths of about 50-150 nm. The temperature dependent morphological evolution of Gd₂O₃:Eu without and with CTAB surfactant was studied. FTIR studies reveal that CTAB surfactant plays an important role in converting cubic Gd₂O₃:Eu to hexagonal Gd(OH)₃:Eu. The strong and intense Raman peak at 489 cm⁻¹ has been assigned to A_g mode, which is attributed to the hexagonal phase of Gd₂O₃. The peak at ∼360 cm⁻¹ has been assigned to the combination of F_g and E_g modes, which is mainly attributed to the cubic Gd₂O₃ phase. The shift in frequency and broadening of the Raman modes have been attributed to the decrease in crystallite dimension to the nanometer scale as a result of phonon confinement.     

Photoluminescent properties of Y2O3:Eu phosphors prepared via urea precipitation in non-aqueous solution

Europium-doped yttrium oxide phosphors were obtained by firing precursors prepared by urea precipitation in ethanol and ethylenediamine. The precipitation in non-aqueous solution was carried out in an autoclave at 150°C to allow the decomposition of urea. The photoluminescent intensities of the phosphors prepared in ethanol and ethylenediamine increased by about 30% compared to that of the phosphor prepared by the conventional urea homogeneous precipitation in aqueous solution. Amorphous carbonates and amorphous hydroxides/carbonates mixtures were identified as precursors from ethanol and ethylenediamine, respectively. The morphology and particle size were studied by SEM and dynamic laser scattering method.     

Excitation energy dependent chromaticity of BaZr(BO3)2:Eu

The luminescence properties of BaZr(BO₃)₂:5% Eu were investigated under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation and different luminescence behaviors were observed by different excitation energies. After the analyses of the luminescence spectra, the result indicates that Eu³⁺ occupying non-centrosymmetric sites Ba²⁺ can be excited preferentially under 254 nm excitation, while Eu³⁺ occupying centrosymmetric sites Zr⁴⁺ can be excited preferentially under 147 nm excitation.     

Y2O3:Eu纳米晶中能量传递相互作用的研究

通过浓度猝灭曲线确定了引起Y₂O₃纳米晶中Eu³⁺发光浓度猝灭的是交换相互作用.测量了两种颗粒尺寸下Eu³⁺的⁵D₀—⁷F₂跃迁发光衰减曲线随掺杂浓度的变化,利用交换相互作用的理论衰减曲线对实验衰减曲线进行拟合.计算Eu³⁺离子的交换相互作用能量传递的效率,分析了Y₂O_{3关键词： 能量传递 2}O₃Eu纳米晶')" href="#">Y₂O₃Eu纳米晶 发光衰减     

Non-magnetic impurity effect in two-gap superconductor Lu2Fe3Si5

We report detailed study of non-magnetic impurity effects in a two-gap superconductor Lu₂Fe₃Si₅ by replacing Lu with Sc. We find that the superconducting transition temperature T_c is drastically suppressed by slight substitution of Sc, while lattice constants change linearly with the substitution. These results strongly indicate that a slight substitution of Sc increases the inter-band scattering and causes averaging the amplitude of two gaps, which leads to the drastic suppression of T_c.     

The central role of oxygen on H-irradiated Lu2SiO5 luminescence

The behavior of self-trapped defects (STDs) in ion-beam irradiated Lu₂SiO₅ (LSO) crystal has been investigated via temperature-dependent radioluminescence (RL) measurements. Production of oxygen vacancies is the major effect of H⁺ irradiation on luminescencent properties of this phosphor. Luminescence centers for self-trapped exciton (STE) and self-trapped hole emission are assigned to oxygen vacancies and oxygen ions, respectively. Ion-induced structural damage modifies the thermal stability of the STDs and creates perturbed STEs. A striking effect of ion irradiation is the approximate factor-of-two enhancement of STE RL intensity that results from implantation of only a thin (∼250 nm) surface layer of LSO. This enhancement is attributed to ion-beam modification of a surface dead layer.