Persistent luminescence dosimetric properties of UV-irradiated SrAl2O4:Eu, Dy phosphor

Current radiation dosimetry methods involve the release of trapped charge carriers in the form of electrons-holes pairs generated by irradiation exposure of the dosimetric materials. Thermal and optical stimulations of the irradiated material freed the trapped charges that eventually recombine with interband centers producing the emission of light. The integrated intensity of the emitted light is proportional to the radiation dose exposure. In this work, we present an UV radiation dosimetry technique based on the characteristic persistence luminescence (PLUM) 4f⁶5d¹→4f⁷ electronic transition of Eu²⁺ ions in SrAl₂O₄:Eu²⁺, Dy³⁺. The dose assessment is carried out by measuring the PLUM signal integrated during a certain time. The PLUM performance of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor exhibited a linear behavior for the first 50 s of UV irradiation. For higher UV time exposure the behavior is sublinear with no apparent saturation during a 10 min period. The PLUM dosimetry response was performed at 400 nm that corresponds to the main band component of the PLUM excitation spectrum in the 250-500 nm range. The main advantage of a dosimeter device based on the PLUM of SrAl₂O₄:Eu²⁺, Dy³⁺ is that neither thermal nor optical stimulation is required, avoiding the need of cumbersome electronic photo/thermal stimulation equipment. Due to the highly efficient 250-500 nm PLUM response of SrAl₂O₄:Eu²⁺, Dy³⁺, it could have potential application as UV radiation dosimeter in the UV range of grate human health concerns caused by UV solar radiation.     

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.     

Synthesis and optical properties of red emitting Eu doped CaZrO3 phosphor

Eu³⁺ activated Ca_1−xEu_xZrO₃ (x = 0.01–0.05) phosphor with perovskite structure has been synthesized by sol–gel combustion method. The structure, morphology and optical properties of materials were characterized by X-ray diffraction, scanning electron microscopy and fluorescence spectrometry. The XRD results indicate that crystals of CaZrO₃:Eu³⁺ belongs to orthorhombic perovskite structure. The phosphors can be effectively excited by UV light and the emission spectra results indicate that red luminescence of CaZrO₃:Eu³⁺ due to electric dipole transition ⁵D₀ → ⁷F₂ at 616 nm is dominant. Thus, these prepared phosphors show remarkable luminescent properties which find applications in display devices.     

Synthesis and characterization of Eu doped SrY2O4 phosphor

The present paper reports that TL glow curve and kinetic parameter of Eu³⁺ doped SrY₂O₄ phosphor irradiated by beta source. Sample was prepared by solid state preparation method. Sample was characterized by XRD analysis and particle size was calculated by Debye–Scherrer formula. The sample was irradiated with Sr-90 beta source giving a dose of 10 Gy and the heating rate used for TL measurements are 6.7 °C/s. The samples display good TL peaks at 106 °C, 225 °C and 382 °C. The corresponding kinetic parameters are calculated. The photoluminescence excitation spectrum at 247 and 364 nm monitored with 400 nm excitation and the corresponding emission peaks at 590, 612 and 624 nm are reported.     

Effect of temperature on the luminescence processes of SrAl2O4:Eu

The UV excited and persistent luminescence properties as well as thermoluminescence (TL) of Eu²⁺ doped strontium aluminates, SrAl₂O₄:Eu²⁺ were studied at different temperatures. Two luminescence bands peaking at 445 and 520 nm were observed at 20 K but only the latter at 295 K. Both Sr-sites in the lattice are thus occupied by Eu²⁺ but at room temperature efficient energy transfer occurs between the two sites. The UV excited and persistent luminescence spectra were similar at 295 K but the excitation spectra were different. Thus the luminescent centre is the same in both phenomena but excitation processes are different. Two TL peaks were observed between 50 and 250 °C in the glow curve. Multiple traps were, however, observed by preheating and initial rise methods. With longer delay times only the high temperature TL peak was observed. The persistent luminescence is mainly due to slow fading of the low temperature TL peak but the step-wise feeding process from high temperature traps is also probable.     

Broad yellow orange emission from SrAl2O4:Pr phosphor with blue excitation for application to white LEDs

Photoluminescence excitation to intermediate atomic levels of rare earth activator ion (praseodymium) situated intragap in alkaline earth aluminate (AEA) SrAl₂O₄ has been tailored. This lead to blue excitation (2.7 eV) of large band gap AEA possible. Photoluminescence (PL) emission in the visible region extends from 525 to 650 nm corresponding to transition from ³P₀ and ¹D₂ excited states to different ³H_J and ³F_J states of Pr³⁺, broadened by crystal field effect of SrAl₂O₄. Thus SrAl₂O₄:Pr³⁺ promise to be a good candidate for solid state lighting in conjunction with blue LED.     

Synthesis and luminescence properties of SrAl2O4:Eu,Dy nanosheets

We report an effective method to synthesize SrAl₂O₄:Eu²⁺,Dy³⁺ nanosheets. Sheet-like precursors were firstly synthesized by the solvothermal method, and acetate was used as the raw material. Then the final products were obtained by calcinating the precursor in a weak reductive atmosphere of H₂. The crystal structure and particle morphology were investigated by X-ray diffraction (XRD) patterns, field-emission scanning electron microscopy and transmission electron microscopy (FE-SEM and TEM) , respectively. A possible growth mechanism was proposed to reveal the formation process. Luminescence properties of the SrAl₂O₄:Eu²⁺,Dy³⁺ long-lasting phosphor were analyzed by measuring the excitation spectra, the emission spectra, the afterglow decay curve and the thermoluminescence curve. Both the photoluminescence (PL) spectra and luminance decay revealed that the phosphors had efficient luminescent and long lasting properties compared with the phosphors prepared by using nitrates as the raw material. Furthermore, the photoluminescence intensity was even a little higher than commercial phosphors.     

Photoluminescence and afterglow behavior of Eu, Dy and Eu, Dy in Sr3Al2O6 matrix

This paper reports the photoluminescence and afterglow behavior of Eu²⁺ and Eu³⁺ in Sr₃Al₂O₆ matrix co-doped with Dy³⁺. The samples containing Eu²⁺ and Eu³⁺ were prepared via solid-state reaction. X-ray diffraction (XRD), photo luminescent spectroscope (PLS) and thermal luminescent spectroscope (TLS) were employed to characterize the phosphors. The comparison between the emission spectra revealed that Sr₃Al₂O₆ phosphors doped with Eu²⁺, Dy³⁺ and Eu³⁺, Dy³⁺ showed different photoluminescence. The phosphor doped with Eu³⁺, Dy³⁺ showed an intrinsic f-f transition generated from Eu³⁺, with two significant emissions at 591 and 610 nm. However, the phosphor doped with Eu²⁺, Dy³⁺ revealed a broad d-f emission centering around 512 nm. After the UV source was turned off, Eu²⁺, Dy³⁺ activated Sr₃Al₂O₆ phosphor showed excellent afterglow while Eu³⁺, Dy³⁺ activated phosphor almost showed no afterglow. Thermal simulated luminescence study indicated that the persistent afterglow of Sr₃Al₂O₆: Eu²⁺, Dy³⁺ phosphor was generated by suitable electron traps formed by the co-doped rare-earth ions (Dy³⁺) within the host.     

Spectral and defect analysis of Cu-doped combustion synthesized new SrAl4O7 phosphor

A simple solution combustion synthesis route for preparation of Cu-doped SrAl₄O₇ has been described. Average particle size of 33 nm and platelet-like morphology has been observed. ESR studies confirm the presence of Cu in polycrystalline SrAl₄O₇. Bright-green luminescence under near-UV irradiation arising due to transition between Cu⁺ levels with microsecond level decay time makes it suitable for application in phosphor converted light emitting diode (LED). TL measurements show broad glow peaks which when deconvoluted indicate activation energies in the range of 0.6-1.1 eV and elucidate the trapping dynamics.     

Photoluminescence properties of nanocrystalline La2TeO6:Eu phosphor prepared by Pechini sol-gel method

La₂TeO₆:Eu³⁺ nanophosphors were prepared by Pechini sol-gel process, using lanthanide nitrates and telluric acid as precursor. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), thermogravimetric analysis (TG), photoluminescence spectra (PL) and fluorescence lifetime were used to characterize the resulting phosphors. The results of XRD indicate that all samples crystallized completely at 1023 K and are isostructural with the orthorhombic La₂TeO₆. SEM study reveals that the samples have a strong tendency to form agglomerates with an average size ranging from 50 to 80 nm. The photoluminescence intensity and chromaticity were improved for excitation at 254 and 395 nm. The optimized phosphor La_1.80Eu_0.10TeO₆ could be considered as an efficient red-emitting phosphor for solid-state lighting devices based on GaN LEDs.     

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.     

First principles calculations of optical and magnetic properties of SrFe2O4 compound under pressure

In this research, the magnetic and optical properties of SrFe₂O₄ ceramic were studied. The calculations were performed by Full Potential Linearized Augmented Plane Wave method in Density Functional Theory framework with generalized gradient (GGA), GGA+U and modified Becke–Johnson approximations for the exchange and correlation functionals. The results show that SrFe₂O₄ is a ferrimagnetic ceramics with six different spin configurations. The Hubbard parameter was calculated (_Ueff=4.5 eV$U_{\mathit{eff}}=4.5\text{eV}$) by an ab initio method. The optical properties such as dielectric function, refraction index, electron energy-loss function, reflectivity, absorption coefficient and optical conductivity were investigated at zero up to 20 GPa pressure in x, y and z directions. The pressure coefficients of optical band, static dielectric constant, plasmon peak, static refraction index and the maximum of absorption were determined. Moreover, the pressure dependence of the static dielectric constant, plasmon peak, static refraction index, the maximum of absorption and the optical gap were investigated.     

Concentration quenching of Eu in SrO·Al2O3:Eu phosphor

The luminescence of Eu²⁺ in alkaline earth aluminates of the type SrO·Al₂O₃ has been studied. In SrO·Al₂O₃:Eu²⁺ phosphor, green Eu²⁺ luminescence is observed from Eu²⁺ on the two different strontium sites present in the lattice. Their concentration quenching processes of the two inequivalent Eu²⁺ ions are investigated, respectively, and the corresponding concentration quenching mechanism is verified as dipole-dipole interaction. The value of the critical transfer distance is calculated.     

Growth and optical properties of Eu/Li-co-doped SrB4O7 single crystals

This paper reports the growth and optical properties of Eu²⁺/Li⁺-co-doped SrB₄O₇ single crystals. High-quality Eu,Li:SrB₄O₇ crystals without macro-defects or cracks were grown using the top-seeded solution growth (TSSG) method. The absorption and luminescent properties were measured and different spectra were observed in the as-grown crystals. As the doping amount of lithium increases, the absorption peak at 300 nm becomes stronger and the emission peak shifts to a longer wavelength. This phenomenon could be attributed to the doping lithium ions, which might affect the electric field distribution in the lattice structure.     

Enhanced luminescence from spontaneously ordered Gd2O3:Eu based nanostructures

Nanostructured Gd₂O₃:Eu³⁺ and Li⁺ doped Gd₂O₃:Eu³⁺ thin films were prepared by pulsed laser ablation technique. The effects of annealing and Li⁺ doping on the structural, morphological, optical and luminescent properties are discussed. X-ray diffraction and Micro-Raman investigations indicate a phase transformation from amorphous to nanocrystalline phase and an early crystallization was observed in Li⁺ doped Gd₂O₃:Eu³⁺ thin films on annealing. AFM images of Li⁺ doped Gd₂O₃:Eu³⁺ films annealed at different temperatures especially at 973 K show a spontaneous ordering of the nanocrystals distributed uniformly all over the surface, with a hillocks (or tips) like self-assembly of nanoparticles driven by thermodynamic and kinetic considerations. Enhanced photoemission from locations corresponding to the tips suggest their use in high resolution display devices. An investigation on the photoluminescence of Gd_2−xEu_xO₃ (x=0.10) and Gd_2−x−yEu_xLi_yO₃ (x=0.10, y=0.08) thin films annealed at 973 K reveals that the enhancement in luminescence intensity of about 3.04 times on Li⁺ doping is solely due to the increase in oxygen vacancies and the flux effect of Li⁺ ions. The observed decrease in the values of asymmetric ratio from the luminescence spectra of Li⁺ doped Gd₂O₃:Eu³⁺ films at high temperature region is discussed in terms of increased EuO bond length as a result of Li⁺ doping.     

Effect of hydroxyl groups on Er doped Bi2O3-B2O3-SiO2 glasses

A series of Er³⁺-doped Bi₂O₃-B₂O₃-SiO₂-Na₂O glasses with different hydroxyl groups were prepared and the interaction between the Er³⁺ ions and OH groups was investigated. Infrared spectra were measured in order to calculate the exact content of OH groups in samples. The observed increase of the fluorescence lifetime with the oxygen bubbling time has been related to the reduction in the OH content concentration evidenced by infrared (IR) absorption spectra, which confirmed that the OH groups were dominant quenching centers of excited Er³⁺ and a cause of concentration quenching of 1.5 μm band emission. Various nonradiative decay rates from ⁴I_13/2 of Er³⁺ with the change of OH content were determined from the fluorescence lifetimes and radiative decay rates, which were calculated on the basis of Judd-Ofelt theory.     

Synthesis and luminescence properties of needle-like SrAl2O4:Eu, Dy phosphor via a hydrothermal co-precipitation method

Needle-like SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors had been prepared by calcining the precursors obtained from hydrothermal process at the temperature of 1100 °C in a weak reductive atmosphere of active carbon. The crystal structure, morphology and optical properties of the composites were characterized. X-ray diffraction (XRD) patterns illustrated that the single-phase SrAl₂O₄ was formed at 1100 °C, which is much lower than that prepared by the traditional method. The transmission electron microscope (TEM) observation revealed the precursors and the resulted SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors had well-dispersed distribution and needle-like morphology with an average diameter about 150 nm at the center and the length up to 1 μm. After irradiation by ultraviolet radiation with 350 nm for 5 min, the phosphors emit green color long-lasting phosphorescence corresponding to the typical emission of Eu²⁺ ion, both the PL spectra and luminance decay revealed that the phosphors had efficient luminescent and long lasting properties.     

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.     

Modification on luminescent properties of SrAl2O4:Eu, Dy phosphor by Yb ions doping

The modification effect of the doping of Yb³⁺ ions, as an auxiliary activator, onto the luminescent properties of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor was studied for the first time. The phosphorescent nanoparticles were prepared by the combustion method. The experimental results indicate that the appropriate doping of Yb³⁺ ions largely improves phosphorescence of the phosphors with more intense luminescence, higher brightness, and no change in emission spectrum peaked at 513 nm. Meanwhile the decay speed of the phosphor nanoparticles rises increasingly with the doping ratio of Yb³⁺ ions, whereas an excessive Yb³⁺ ions doping leads to the disappearance of the pure monoclinic phase of SrAl₂O₄ and the appearance of the weak diffraction lines of the YbAlO₃ phase. The phosphorescent mechanism of the phosphors could be well understood based on the hole, thermally released from the trap levels of Dy³⁺ and Yb³⁺.     

Self-organized ZnAl2O4 nanostructures grown on -sapphire

Self-organized ZnAl₂O₄ nanostructures with the appearance (in SEM) of high aspect ratio horizontal nanowires are grown on uncatalysed c-sapphire by vapour phase transport. The nanostructures grow as three equivalent crystallographic variants on c-sapphire. Raman and cathodoluminescence spectroscopy confirm that the nanostructures are not ZnO and TEM shows that they are the cubic spinel, zinc aluminate, ZnAl₂O₄, formed by the reaction of Zn and O with the sapphire substrate.