Luminescence studies on nitride quaternary alloys double quantum wells

We present theoretical photoluminescence (PL) spectra of undoped and p-doped Al(x)In(1-xy)Ga(y)N/Al(X)In(1) (X) (Y)Ga(Y)N double quantum wells (DQWs). The calculations were performed within the k.p method by means of solving a full eight-band Kane Hamiltonian together with the Poisson equation in a plane wave representation, including exchange-correlation effects within the local density approximation. Strain effects due to the lattice mismatch are also taken into account. We show the calculated PL spectra, analyzing the blue and red-shifts in energy as one varies the spike and the well widths, as well as the acceptor doping concentration. We found a transition between a regime of isolated quantum wells and that of interacting DQWs. Since there are few studies of optical properties of quantum wells based on nitride quaternary alloys, the results reported here will provide guidelines for the interpretation of forthcoming experiments. (C) 2008 Elsevier B.V. All rights reserved.

Synthesis, Characterization, Luminescence and Defect Centres in CaYAl(3)O(7):Eu(3+) Red Phosphor

CaYAl(3)O(7):Eu(3+) phosphor was prepared at furnace temperatures as low as 550A degrees C by a solution combustion method. The formation of crystalline CaYAl(3)O(7):Eu(3+) was confirmed by powder X-Ray diffraction pattern. The prepared phosphor was characterized by SEM, FT-IR and photoluminescence techniques. Photoluminescence measurements indicated that emission spectrum is dominated by the red peak located at 618 nm due to the (5)D(0)-(7)F(2) electric dipole transition of Eu(3+) ions. Electron Spin Resonance (ESR) studies were carried out to identify the centres responsible for the thermoluminescence (TL) peaks. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of two distinct centres. One of the centres (centre I) with principal g-value 2.0126 is identified as an O(-) ion while centre II with an isotropic g-factor 2.0060 is assigned to an F(+) centre (singly ionized oxygen vacancy). An additional defect centre is observed during thermal annealing experiments and this centre (assigned to F(+) centre) seems to originate from an F centre (oxygen vacancy with two electrons). The F(+) centre appears to correlate with the observed high temperature TL peak in CaYAl(3)O(7):Eu(3+) phosphor.

Characterization, photoluminescence, thermally stimulated luminescence and electron spin resonance studies of Eu(3+) doped LaAlO(3) phosphor

Europium-doped lanthanum aluminate (LaAlO(3)) powder was prepared by using a combustion method. The crystallization, surface morphology, specific surface area and luminescence properties of the samples have been investigated. Photoluminescence studies of Eu doped LaAlO(3) showed orange-reddish emission due to Eu(3+) ions. LaAlO(3):Eu(3+) exhibits one thermally stimulated luminescence (TSL) peak around 400 degrees C. Room temperature electron spin resonance spectrum of irradiated phosphor appears to be a superposition of two centres. One of them (centre I) with principal g-value 2.017 is identified as an O(-) centre while centre II with an isotropic g-value 2.011 is assigned to an F(+) centre (singly ionized oxygen vacancy). An additional defect centre observed during thermal annealing around 300 degrees C grows with the annealing temperature. This centre (assigned to F(+) centre) originates from an F-centre (oxygen vacancy with two electrons) and the F-centre along with the associated F(+) centre appear to correlate with the observed TSL peak in LaAlO(3):Eu(3+) phosphor. The activation energy for this peak has been determined to be 1.54 eV from TSL data. (C) 2010 Elsevier Masson SAS. All rights reserved.

Luminescence and defect centres in Tb(3+) doped LaMgAl(11)O(19) phosphors

Terbium (Tb) doped LaMgAl(11)O(19) phosphors have been prepared by the combustion of corresponding metal nitrates (oxidizer) and urea (fuel) at furnace temperature as low as 500 C Combustion synthesized powder phosphor was characterized by X-ray diffraction and field emission scanning electron microscopy techniques LaMgAl(11)O(19) doped with trivalent terbium ions emit weakly in blue and orange light region and strongly in green light region when excited by the ultraviolet light of 261 nm Electron Spin Resonance (ESR) studies were carried out to study the defect centres Induced in the phosphor by gamma irradiation and also to identify the defect centres responsible for the thermally stimulated luminescence (TSL) process Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of at least two defect centres One of the centres (centre I) with principal g-values g(parallel to) = 2 0417 and g(perpendicular to) = 2 0041 is identified as O(2)(-) ion while centre II with an axially symmetric g-tensor with principal values g(parallel to) = 19698 and g(perpendicular to) = 1 9653 is assigned to an F(+) centre (singly ionized oxygen vacancy) An additional defect centre is observed during thermal annealing experiments and this centre (assigned to F(+) centre) seems to originate from an F centre (oxygen vacancy with two electrons) The F centre and also the F+ centre appear to correlate with the observed high temperature TSL peak in LaMgAl(11)O(19) Tb phosphor (C) 2010 Elsevier Masson SAS All rights reserved

Luminescence and defect centres in MgSrAl(10)O(17):Sm(3+) phosphor

An efficient reddish orange emission MgSrAl(10)O(17):Sm(3+) phosphor was prepared by the combustion method. The phosphor has been characterized by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis measurements. Photoluminescence spectrum revealed that samarium ions are present in trivalent oxidation states. The phosphor exhibits two thermally stimulated luminescence (TSL) peaks at 210 degrees C and 450 degrees C. Electron spin resonance studies were carried out to identify the defect centres responsible for the TSL process in MgSrAl(10)O(17):Sm(3+) phosphor. Three defect centres have been identified in irradiated phosphor and these centres are tentatively assigned to an O(-) ion and F(+) centres. O(-) ion (hole centre) correlates with the 210 degrees C TSL peak while one of the F+ centres (electron centre) appears to relate to the 450 degrees C TSL peak. (C) 2010 Elsevier B.V. All rights reserved.

Comparison between blue and green stimulated luminescence of Al(2)O(3):C

This paper presents a systematic comparison of OSL signals from Al(2)O(3):C when stimulated with blue and green light. Al(2)O(3):C detectors were irradiated with various doses and submitted to various bleaching regimes using yellow, green and blue light. Most of the investigations were carried out using Luxel (TM)-type detectors used in the commercial Luxet (TM) and InLight (TM) dosimetry systems (Landauer Inc.). Al(2)O(3):C single crystals and Al(2)O(3):C powder were also used to complement the investigations. The results show that, although blue stimulation provides faster readout times (OSL curves that decayed faster) and higher initial OSL intensity than green stimulation, blue stimulation introduced complicating factors. These include incomplete bleaching of the dosimetric trap when the Al(2)O(3):C detectors are bleached with yellow or green light and the OSL is recorded with blue light stimulation, and an increased residual level due to stimulation of charge carriers from deep traps. The results warrant caution when using blue stimulation to measure the OSL signal from Al(2)O(3):C detectors, particularly if the doses involved are low and the detectors have been previously exposed to high doses. (C) 2010 Elsevier Ltd. All rights reserved.

Infrared luminescence, thermoluminescence and defect centres in Er and Yb co-doped ZnAl(2)O(4) phosphor

Er and Yb co-doped ZnAl(2)O(4) phosphors were prepared by solution combustion synthesis and the identification of Er and Yb were done by energy-dispersive X-ray analysis (EDX) studies. A luminescence at 1.5 mu m, due to the (4)I(13/2) ->(4)I(15/2) transition, has been studied in the NIR region in Er and Yb co-doped ZnAl(2)O(4) phosphors upon 980 nm CW pumping. Er-doped ZnAl(2)O(4) exhibits two thermally stimulated luminescence (TSL) peaks around 174A degrees C and 483A degrees C, while Yb co-doped ZnAl(2)O(4) exhibits TSL peaks around 170A degrees C and 423A degrees C. Electron spin resonance (ESR) studies were carried out to identify defect centres responsible for TSL peaks observed in the phosphors. Room temperature ESR spectrum appears to be a superposition of two distinct centres. These centres are assigned to an O(-) ion and F(+) centre. O(-) ion appears to correlate with the 174A degrees C TSL peak and F(+) centre appears to relate with the high temperature TSL peak at 483A degrees C in ZnAl(2)O(4):Er phosphor.

NIR to visible up-conversion, infrared luminescence, thermoluminescence and defect centres in Y(2)O(3) : Er phosphor

Er(3+) doped Y(2)O(3) phosphor was prepared by the solution combustion method and characterized using powder x-ray diffraction and energy-dispersive analysis of x-ray mapping studies. Room temperature near infrared (NIR) to green up-conversion (UC) emissions in the region 520-580 nm {((2)H(11/2), (4)S(3/2)) -> (4)I(15/2)} and red UC emissions in the region 650-700 nm ((4)F(9/2) -> (4)I(15/2)) of Er(3+) ions have been observed upon direct excitation to the (4)I(11/2) level using similar to 972 nm laser radiation of nanosecond pulses. The possible mechanisms for the UC processes have been discussed on the basis of the energy level scheme, the pump power dependence as well as based on the temporal evolution. The excited state absorption is observed to be the dominant mechanism for the UC process. Y(2)O(3) : Er exhibits one thermally stimulated luminescence (TSL) peak around 367 degrees C. Electron spin resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the centres responsible for the TSL peak. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of at least three distinct centres. One of them (centre I) with principal g-values g(parallel to) = 2.0415 and g(perpendicular to) = 2.0056 is identified as O(2)(-) centre while centre II with an isotropic g-factor 2.0096 is assigned to an F(+)-centre (singly ionized oxygen vacancy). Centre III is also assigned to an F(+)-centre with a small g-factor anisotropy (g(parallel to) = 1.974 and g(perpendicular to) = 1.967). Additional defect centres are observed during thermal annealing experiments and one of them appearing around 330 degrees C grows with the annealing temperature. This centre (assigned to an F(+)-centre) seems to originate from an F-centre (oxygen vacancy with two electrons) and the F-centre appears to correlate with the observed TSL peak in Y2O3 : Er phosphor. The trap depth for this peak has been determined to be 0.97 eV from TSL data.

Thermo luminescence of natural and synthetic diopside

Diopside, a natural silicate mineral of formula CaMgSi2O6, has been investigated concerning its thermoluminescence (TL) and electron paramagnetic resonance (EPR) properties. Glow curves and TL vs. gamma-dose were obtained irradiating natural samples to additional dose varying from 50 to 10,000Gy. Except for a 410 degrees C peak found in the Al-doped artificial diopside, all the other peaks grow linearly with radiation dose, but saturate beyond -1 kGy. To investigate high-temperature effect before irradiation, measurements of TL intensity in samples annealed at 500-900 degrees C and then irradiated to I kGy gamma-dose were carried out. Also the TL emission spectrum has been obtained. To compare with natural diopside, a synthetic pure polycrystal was produced and further those doped with iron, aluminum and manganese were also produced. (c) 2007 Elsevier B.V. All rights reserved.

Cr3+ and Cr4+ luminescence in glass ceramic silica

This paper reports on the effect of glass ceramic silica matrix on [CrO4](4-) and Cr2O3 NIR and visible luminescence. Chromium-containing silica was obtained by precipitation from water-glass and chromium nitrate acid solution with thermal treatment at 1000 degrees C. From XRD results silica and silica-chromium samples are crystalline. The chromium emission spectrum presents two main broad bands: one in the NIR region (1.1-1.7 mu m) and other in the visible region (0.6-0.7 mu m) assigned to Cr4+ and to Cr3+, respectively. This thermal treated glass ceramic silica-chromium sample stabilizes the [CrO4](4-) where Cr4+ substitutes for Si4+ and also hexacoordinated Cr3+ group probably as segregated phase in the system. It can be pointed out that luminescence spectroscopy is a powerful toot for detecting the two chromium optical centers in the glass ceramic silica. (C) 2008 Elsevier B.V. All rights reserved.

Luminescence characteristics of YAP:Ce scintillator powders and composites

Cerium doped yttrium aluminate perovskite YAlO(3) (YAP) powders are pursued as interesting alternatives to bulk crystals for application in scintillating devices. The emissions of these materials in the near-UV and visible spectral regions originate from electric dipole transitions between 4f and 5d energy levels of Ce(3+) and largely depend on the environment occupied by the ion. In search for improved synthesis conditions that can lead to phase pure powders with optimized structural and spectroscopic characteristics, in this work we have employed the polymeric precursor (Pechini) method to prepare crystalline and amorphous YAP:Ce powders doped with 1-10 mol% Ce(3+). Interesting composite materials were also obtained by dispersing some of the YAP:Ce powders in silica xerogels. A comparative structural and spectroscopic study of all the samples was done by XRD, FT-IR, emission, excitation and excited state lifetime measurements. In agreement with previous reports, excitation at 296 nm results in intense emission in the range 315-425 nm with an average decay time of 30 ns. (c) 2010 Elsevier B.V. All rights reserved.

Thermoluminescence and synchrotron radiation studies on the persistent luminescence of BaAl(2)O(4)/Eu(2+),Dy(3+)

The persistent luminescence materials, barium aluminates doped with Eu(2+) and Dy(3+) (BaAl(2)O(4): Eu(2+),Dy(3+)), were prepared with the combustion synthesis at temperatures between 400 and 600 degrees C as well as with the solid state reaction at 1500 degrees C. The concentrations of Eu(2+)/Dy(3+) (in mol% of the Ba amount) ranged from 0.1/0.1 to 1.0/3.0. The electronic and defect energy level structures were studied with thermoluminescence (TL) and synchrotron radiation (SR) spectroscopies: UV-VUV excitation and emission, as well as with X-ray absorption near-edge structure (XANES) methods. Theoretical calculations using the density functional theory (DFT) were carried out in order to compare with the experimental data. (C) 2010 Elsevier Inc. All rights reserved.

Optical energy storage properties of Sr(2)MgSi(2)O(7):Eu(2+),R(3+) persistent luminescence materials

The details of the mechanism of persistent luminescence were probed by investigating the trap level structure of Sr(2)MgSi(2)O(7):Eu(2+),R(3+) materials (R: Y, La-Lu, excluding Pm and Eu) with thermoluminescence (TL) measurements and Density Functional Theory (DFT) calculations. The TL results indicated that the shallowest traps for each Sr(2)MgSi(2)O(7):Eu(2+),R(3+) material above room temperature were always ca. 0.7 eV corresponding to a strong TL maximum at ca. 90 A degrees C. This main trap energy was only slightly modified by the different co-dopants, which, in contrast, had a significant effect on the depths of the deeper traps. The combined results of the trap level energies obtained from the experimental data and DFT calculations suggest that the main trap responsible for the persistent luminescence of the Sr(2)MgSi(2)O(7):Eu(2+),R(3+) materials is created by charge compensation lattice defects, identified tentatively as oxygen vacancies, induced by the R(3+) co-dopants.

Persistent luminescence of Eu(2+) and Dy(3+) doped barium aluminate (BaAl(2)O(4):Eu(2+),Dy(3+)) materials

Polycrystalline Eu(2+) and Dy(3+) doped barium aluminate materials, BaAl(2)O(4):Eu(2+),Dy(3+), were prepared with solid state reactions at temperatures between 700 and 1500 degrees C. The influence of the thermal treatments on the stability, homogeneity and structure as well as to the UV-excited and persistent luminescence of the materials was investigated by X-ray powder diffraction, SEM imaging and infrared spectroscopies as well as by steady state luminescence spectroscopy and persistent luminescence decay curves, respectively. The IR spectra of the materials prepared at 250, 700, and 1500 degrees C follow the formation of BaAl(2)O(4) composition whereas the X-ray powder diffraction of compounds revealed how the hexagonal structure was obtained. The morphology of the materials at high temperatures indicated important aggregation due to sintering. The luminescence decay of the quite narrow Eu(2+) band at ca. 500 nm shows the presence of persistent luminescence after UV irradiation. The dopant (Eu(2+)) and co-clopant (Dy(3+)) concentrations affect the crystallinity and luminescence properties of the materials. (C) 2009 Elsevier B.V. All rights reserved.

Biolabeling with nanoparticles based on Y(2)O(3):Nd(3+) and luminescence detection in the near-infrared

Neodymium based fluorescence presents several advantages in comparison to conventional rare earth or enzyme-substrate based fluorescence emitting sources (e.g.Tb, HRP). Based on this fact we have herein explored a Nd-based fluoroimmunoassay. We efficiently detected the presence of an oxidized low-density lipoprotein (oxLDL) in human plasma a well-known marker for cardiovascular diseases, which causes around 30% of deaths worldwide. Conventional fluoroimmunoassay uses time-resolved luminescence techniques, with detection in the visible range, to eliminate the fluorescence background from the biological specimens. By using an immunoassay based on functionalized Y(2)O(3):Nd(3+) nanoparticles, where the excitation and emission processes in the Nd(3+) ion occur in the near-infrared (NIR) region, we have succeeded in eliminating the interferences from the biological fluorescence background, avoiding the use of time-resolved techniques. This yields higher emission intensity from the Nd(3+)-nanolabels and efficient detection of anti-oxidized low-density lipoproteins (anti-oxLDL) by Y(2)O(3):Nd(3+)-antibody-antigen conjugation, leading to a novel biolabeling method. (C) 2010 Elsevier B.V. All rights reserved.