Synthesis and optical studies of gamma irradiated Eu doped nanocrystalline CaF2

Europium (Eu) doped Calcium fluoride (CaF₂) nanoparticles are synthesized by co-precipitation method and characterized by powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Also, optical absorption and photoluminescence (PL) results on gamma irradiated Eu doped CaF₂ nanoparticles are presented. The PXRD patterns confirmed the cubic crystallinity of the samples and the particle size is found to be ∼25 nm. The purity of the synthesized nanoparticles is confirmed from FTIR spectrum. The morphological features studied using SEM revealed that nanoparticles are agglomerated and porous. Optical absorption spectrum shows that γ-rayed Eu doped CaF₂ nanoparticles exhibit absorption bands at ∼279, 360 and 434 nm. The PL studies showed emission at ∼420, 525 and 552 nm.     

Scintillation and luminescence properties of Ce-activated K3Lu(PO4)2

K₃Lu(PO₄)₂:Ce single crystals have been studied under gamma, X-ray, VUV, and UV excitation. For all of the excitation forms, the luminescence of these materials is dominated by the d–f emission bands of Ce³⁺. The shape and position of these bands, however, depends on the form of the excitation and on temperature due to crystallographic structural phase changes and multiple types of Lu³⁺ ions sites in the which the Ce³⁺ ion can substitute for Lu. The highly efficient and fast scintillation of these materials is based on radiative recombination of electron-hole pairs via Ce ions, and the scintillation characteristics identify K₃Lu(PO₄)₂:Ce as a promising fast and efficient scintillator.     

What makes the luminescence of Prdifferent in CaTiO3 and CaZrO3 ?

Although orthorhombic CaZrO₃ and CaTiO₃ perovskites are characterised by very similar structural features, they show different luminescence properties when doped with Pr³⁺ ions: the former exhibits the typical greenish-blue ³P₀→³H₄ emission, while the latter shows a single red ¹D₂→³H₄ luminescence. The explanation of this difference requires to understand the mechanism responsible for total quenching of ³P₀ emission in CaTiO₃ host. Among the different possible relaxation pathways from ³P₀ to ¹D₂, intersystem crossing through low lying 4f¹5d¹ levels has been proposed in a previous study. Owing to new spectroscopic results on CaZrO₃:Pr³⁺ and CaTiO₃:Pr³⁺, the role of the 4f5d band is reconsidered and an alternative relaxation channel is proposed via low lying intervalence charge transfer state.     

Upconversion luminescence in Er/Yb codoped Y2CaGe4O12

Calcium yttrium tetrametagermanates Y₂CaGe₄O₁₂ doped with Er³⁺ and Er³⁺/Yb³⁺ reveal upconversion emission in visible spectral range under near-infrared excitation, λ_ex = 980 nm. For the solid solution Er_xY_2−xCaGe₄O₁₂ concentration dependencies for the green and red lines of the visible emission around 526 nm (²H_11/2 → ⁴I_15/2), 545 nm (⁴S_3/2 → ⁴I_15/2) and 670 nm (⁴F_9/2 → ⁴I_15/2) show the optimal value for the sample x = 0.2. The power dependence of the visible luminescence measured at room temperature in the low-power limit indicates two-photon upconversion process. Direct intensification of the upconversion emission signals has been achieved by ytterbium sensitizing. The other upconversion excitation mechanism in Y₂CaGe₄O₁₂:Er³⁺ is discussed for an 808 nm incident laser irradiation. A scheme of excitation and emission routes involving ground/excited state absorption, energy transfer upconversion, nonradiative multiphonon relaxation processes in trivalent lanthanide ions in Y₂CaGe₄O₁₂:Er³⁺ and Y₂CaGe₄O₁₂:Er³⁺, Yb³⁺ has been proposed. Conditions for visible emission occurrence under quasi-resonance λ_ex = 1064 nm excitation depending on pump power values are considered. In the low-power regime only near-infrared emission caused by the transition ⁴I_13/2 → ⁴I_15/2 in erbium ions has been detected.     

A general hydrothermal route to synthesis of nanocrystalline lanthanide stannates: Ln2Sn2O7 (Ln = Y, La–Yb)

A general hydrothermal process with use of lanthanide (III) nitrates and Na₂SnO₃ as precursors has been proposed for synthesizing nanocrystalline lanthanide stannates with general formula: Ln₂Sn₂O₇ (Ln = Y, La–Yb). Stannates of all lanthanides except for the radioactive promethium were successfully synthesized. Characterization by XRD and TEM revealed that all the products were phase-pure nanocrystalline lanthanide stannates with pyrochlore-type structure. Photoluminescent properties of three samples (i.e. Tb₂Sn₂O₇, Dy₂Sn₂O₇ and Yb₂Sn₂O₇) are also presented. The mole ratio of Ln(NO₃)₃:Na₂SnO₃ and hydrothermal temperature were two key factors for this general hydrothermal route.     

Amplitude dependent damping in HgI2 wide band gap semiconductor

Amplitude dependent damping in crystals is considered as one of the methods of mechanical spectroscopy for investigating different mechanisms of dislocation microplasticity. A short review of the amplitude dependent damping studies in semiconductors is presented. A new acousto–optic experimental technique for studying a dislocation point defect interaction developed over the last several years is described. Experimental data of photoacoustic investigations of HgI₂ single crystals obtained in a wide amplitude range at frequencies of longitudinal vibrations of about 100 kHz are reviewed. More recent results are presented as well. Besides, acousto–electric and acousto–optic (for a band of photoluminescence spectrum) effects which result from a high amplitude ultrasonic treatment (HAUT) with the amplitudes from the amplitude dependent damping range illustrate the possibilities of the technique. A model of an electron or hole trapping center which transforms in a dislocation pinning point during the sample illumination is assumed to be the most convenient to explain the experimental data. Under the influence of HAUT, one can destroy (due to dislocation depinning) the center which consists of the dislocation, point defect (it is, surely, a lattice, iodine or mercury, vacancy or interstitial) and the trapped electron or hole. A binding energy between the dislocation and the photosensitive pinning point defect has been estimated by displacement under the HAUT of specific points in the luminescence excitation spectrum. A possible practical use of HAUT for a restoration of performance of γ-ray HgI₂ detectors is briefly discussed.     

Enhancement of the upconversion luminescence intensity in Er doped BaTiO3 nanocrystals by codoping with Li ions

Infrared-to-visible upconversion luminescence spectra were investigated in Er³⁺ doped and Er³⁺-Li⁺ codoped BaTiO₃ nanocrystals following excitation with 976 nm. By introducing Li⁺ ions, the upconverted emission intensity is found to be greatly enhanced compared to that of the nanocrystals without Li⁺ ions. The enhanced luminescence might be attributed to the oxygen vacancy generated by Li⁺ ion incorporation in the lattices and the distortion of the local asymmetry around Er³⁺. We observe that excitation power dependence and decay time are increased by the incorporation of Li⁺ ions. Li⁺ ions also can reduce the OH groups in specimen, which decrease nonradiative decay from the ⁴S_3/2 to ⁴F_9/2, enhancing the upconversion emission intensities.     

Energy levels and crystal field calculations of Er in LaAlO3

Absorption and emission of Er³⁺ doped LaAlO₃ crystal was measured at room and at low temperature (10 K). Based on these experimental results preliminary crystal field calculations were performed. The energy levels of Er³⁺ in LaAlO₃ were well reproduced.     

High quality TbMnO3 films deposited on YAlO3

High quality thin films of TbMnO₃ were grown by pulsed laser deposition on orthorhombicYAlO₃ (1 0 0). The interface and surface roughness of a 55 nm thick film were probed by X-ray reflectometry and atomic force microscopy, yielding a roughness of 1 nm. X-ray diffraction revealed untwinned films and a small mosaic spread of 0.04° and 0.2° for out-of-plane and in-plane reflections, respectively. This high degree of epitaxy was also confirmed by Rutherford backscattering spectrometry. Using polarized neutron diffraction we could identify a magnetic structure with the propagation vector (0 0.27 0), identical to the bulk magnetic structure of TbMnO₃.     

Correlation between the surface state and optical properties of S6 site and C2 site in nanocrystalline Eu:Y2O3

Eu³⁺ doped Y₂O₃ nanocrystals are synthesized via precipitation and hydrothermal methods. With the increase of hydrothermal temperature, the number of surface states decreases when nanorods are formed. The decrease of surface states induces the increase in the ratio of S₆ site to C₂ site, and meanwhile results in the spectral red-shift of charge transfer band as well as the enhancement of red emission. The results indicate that the surface states and optical properties have close correlations.     

Classical molecular dynamics simulation of UO2 to predict thermophysical properties

Interatomic potential parameters of UO₂, for a partially ionic potential model, were found out by a fitting method using isothermal compressibility data up to 1600 K. The inter-atomic potential used here is essentially a combined potential model comprising of Bushing–Ida and Morse-type potential to simulate UO₂ system using classical molecular dynamics. It has been shown that the deviation in the estimated lattice parameter from the experimental data as reported in literature is due to the inadequacy of the potential parameter. The calculated deviations in lattice parameter, due to the generation of point defects, with variation of temperature have been discussed. The calculated lattice parameters, thermal expansivity, isothermal compressibility of UO₂, using the current potential parameters are in close agreement with the experimental values. The model also successfully predicts Bredig transition.     

Influence of gadolinium concentration on the EMR spectrum of Gd in La2O3

Electron magnetic resonance (EMR) spectra of gadolinium-doped lanthanum oxide powders have been studied at room-temperature for Gd concentrations between 0.02 and 2.00 mol.%. The results suggest that Gd³⁺ ions occupy substitutional sites, that the EMR linewidth and intensity increase with increasing Gd concentration and that the range of the exchange interaction between Gd³⁺ ions is about 0.39 nm, much smaller than that of the same ion in other rare earth oxide.     

Upconversion-induced ultraviolet emission in Ho doped SrLaGa3O7 and SrLaGaO4 crystals

An analysis of the red to ultraviolet wavelength upconversion in Ho³⁺ in SrLaGa₃O₇ and SrLaGaO₄ crystals is given. Upconverted, ultraviolet emission from the ³D₃ level under cw 647 nm excitation at room temperature was observed. Excitation of the ⁵F₅ level, corresponding to the ⁵I₈→⁵F₅ transition, leads to intense emission from the ⁵I₇, ⁵I₆, ⁵F₅, ⁵S₂, ⁵F₃, ³G₅ and ³D₃ levels. Based on the energy level diagram of Ho³⁺, the pump intensity dependencies and experimental time dependencies of the observed emissions, an excitation scheme is proposed.     

Effect of Bi doping on the quenching concentration of H11/2/S3/2 level of Er

Bi³⁺ and Er³⁺ codoped Y₂O₃ was prepared by sol-gel method. The upconversion emission was investigated under 980 nm excitation. For samples without Bi³⁺, the quenching concentration of ²H_11/2/⁴S_3/2 level of Er³⁺ is 3.0 mol%. However, by 1.5 mol% Bi³⁺ doping the quenching concentration increases to 5.0 mol%; meanwhile, the green emission is enhanced 1.9 times. The results indicate that both the quenching concentration and the emission intensity of ²H_11/2/⁴S_3/2 level can be increased by Bi³⁺ doping.     

Population of excited erbium levels in Er:Na0.4Y0.6F2.2 (Er:NYF) laser crystals

We have studied theoretically and experimentally the population dynamics of six radiative erbium levels in concentration series of Er³⁺:NYF crystals. The radiative transition probabilities were calculated by the Judd–Ofelt method; the nonradiative transition probabilities were estimated; energy transfer (ET) microparameters (for migration, selfquenching (SQ), and upconversion (UC) processes) in studied crystals were estimated theoretically using the method of quantum–mechanical model calculation. The rate equations for the six lowest energy levels of erbium in Er³⁺:NYF crystals, including the 3 μm laser transition levels, were numerically solved for the case of excitation by nanosecond pulse at λ_pump=1.53 μm. The theoretical and experimental results for Er³⁺:NYF crystals are in good agreement. Single mode CW laser action was achieved in Er:NYF crystal at 2.8 μm with InGaAs LD pumping of the ⁴I_11/2 level. Output power about 72 mW with slope efficiency 4% was achieved in longitudinal pump scheme.     

Effects of Bi doping on the optical properties of Er:Y2O3

The influences of Bi³⁺ doping on the optical properties of Er³⁺:Y₂O₃ are investigated under UV and IR excitations. The emission intensity of Er³⁺ is remarkably enhanced by the introduction of Bi³⁺ under both two excitations. The emission enhancement under UV excitation originates from the energy transfer from Bi³⁺ to Er³⁺, while under IR excitation it can be attributed to the modification of the local crystal field around the Er³⁺.     

Size dependent luminescence of nanocrystalline Y2O3:Eu and connection to temperature stimulus

In the emission spectra of nanocrystalline Y₂O₃:Eu with decreasing the particle size, the ⁵D₀-⁷F₀ transition of Eu³⁺ shifts towards blue. This size effect is explained by lattice expansion of the nanocrystallines, more precisely, by larger Eu-O distance. Meanwhile, increasing temperature also expands the lattice constant and shifts the emission peak to blue. Then nephelauxetic effect is employed to organize the size effect and temperature dependence. It is also introduced to indicate the change of Eu³⁺ coordination number in the nanosized phosphor. Based on the experimental results, possible applications of temperature sensing and stress mapping with Eu³⁺ doped nanocrystalline phosphors are proposed.     

Synthesis, characterization and microwave dielectric properties of nanocrystalline CaZrO3 ceramics

The energy-transfer mechanisms and frequency upconversion emissions in 0.5Er³⁺/xHo³⁺ co-doped tellurite glasses by exciting at 980 nm have been investigated. Three intense upconversion luminescence emissions are observed at around 525, 548, and 660 nm, which correspond to Er³⁺:²H_11/2 → ⁴I_15/2, Er³⁺:⁴S_3/2 → ⁴I_15/2 + Ho³⁺:⁵S₂(⁵F₄) → ⁵I₈, and Er³⁺:⁴F_9/2 → ⁴I_15/2 + Ho³⁺:⁵F₅ → ⁵I₈ transitions, respectively. The upconversion emissions reach the maximum values when Ho₂O₃ is 0.5 mol%, and the intensities of the green and red light emissions were about 4.5 and 6 times stronger than those un-doped Ho₂O₃, respectively. The possible upconversion mechanisms and energy transfer between Er³⁺ and Ho³⁺ were also estimated and evaluated. All the three emissions are based on two photon absorption processes.