The importance of inversion disorder in the visible light induced persistent luminescence in Cr3+^{3+} doped AB2_2O4_4 (A = Zn or Mg and B = Ga or Al)

Cr$^{3+}$ doped spinel compounds AB$_2$O$_4$ with A=Zn, Mg and B=Ga, Al exhibit a long near infrared persistent luminescence when excited with UV or X-rays. In addition, persistent luminescence of ZnGa$_2$O$_4$ and to a lesser extent MgGa$_2$O$_4$, can also be induced by visible light excitation via $^4$A$_2$ $\rightarrow$ $^4$T$_2$ transition of Cr$^{3+}$, which makes these compounds suitable as biomarkers for in vivo optical imaging of small animals. We correlate this peculiar optical property with the presence of antisite defects, which are present in ZnGa$_2$O$_4$ and MgGa$_2$O$_4$. By using X-ray absorption fine structure (XAFS) spectroscopy, associated with electron paramagnetic resonance (EPR) and optical emission spectroscopy, it is shown that an increase in antisite defects concentration results in a decrease in the Cr-O bond length and the octahedral crystal field energy. A part of the defects are in the close environment of Cr$^{3+}$ ions, as shown by the increasing strain broadening of EPR and XAFS peaks observed upon increasing antisite disorder. It appears that ZnAl$_2$O$_4$, which exhibits the largest crystal field splitting of Cr$^{3+}$ and the smallest antisite disorder, does not show considerable persistent luminescence upon visible light excitation as compared to ZnGa$_2$O$_4$ and MgGa$_2$O$_4$. These results highlight the importance of Cr$^{3+}$ ions with neighboring antisite defects in the mechanism of persistent luminescence exhibited by Cr$^{3+}$ doped AB$_2$O$_4$ spinel compounds.Comment: 10 pages + supplementary (available on request

Dynamics of Charges in Superlong Blacklight-Emitting CaB2O4:Ce3+ Persistent Phosphor

The optical and persistent luminescence properties of CaB2O4:Ce3+ phosphor are presented. The optical emission for excitation in the 250-340 nm wavelength region is dominated by two bands at 365 and 460 nm. Lifetime measurements suggested that the 365 nm emission band is due to interconfigurational Ce3+ 5d -> 4f transitions. Upon excitation with a 254 nm UV lamp, a superlong persistent luminescence in the UVA1 region (340-400 nm, blacklight) was observed, lasting for at least 15 h, and with excellent reproducibility, which is perfectly suitable for phototherapy application. The initial-rise method was applied on the thermoluminescence glow curves to determine the trap distribution and trap depth. The results suggest that one distinct trap, with an activation energy of similar to 0.52 eV, was solely responsible for the persistent luminescence in the CaB2O4:Ce3+ phosphor. The other traps had a quasi-continuous distribution, with activation energies between 0.56 and 1.15 eV. The proposed persistent luminescence and the thermoluminescence mechanisms are elucidated using experimental parameters obtained from the optical and thermoluminescence results and the theoretically calculated electronic structure of the Ce3+ ion in CaB2O4. The lowest Ce3+ 5d(1) level was found to be similar to 0.97 eV below the conduction band, and the persistent luminescence/thermoluminescence emission was dominated by the radiative transitions between Ce3+ energy levels, 5d -> F-2(5/2),(7/2)

Detailed of X-ray diffraction and photoluminescence studies of Ce doped ZnO nanocrystals

Pristine and Ce doped ZnO nanopowders were synthesized by simple refluxing technique. A detailed X-ray diffraction analysis was carried out to evaluate the contribution of dopant ion concentration on strain and lattice parameters. A particle size of 20 nm was obtained via TEM studies. Spherical morphology was obtained for Ce doped ZnO nanocrystals. Multiple emission bands were observed for pristine and doped ZnO samples where, 401 nm emission band corresponds to a contribution due to free excitons recombination through an exciton–exciton collisions. With the Ce addition, the surface defects increases and the emission intensity decreases

Persistent luminescence induced by near infra-red photostimulation in chromium-doped zinc gallate for in vivo optical imaging

International audienceThe analysis of the optical spectroscopy of the Cr3+ doped spinel was initiated by Prof. Georges Boulon more than twenty years ago. More recently persistent luminescence nanoparticles of Cr doped zinc gallate have found interest for in vivo imaging of small animals. Here we evaluated near infra-red (NIR) excitation (or NIR photostimulation) via photo-transfer mechanism as an additional tool for in vivo optical imaging. Investigation of the persistent luminescence induced by NIR photostimulation is studied after either a primary UV (band-to-band excitation) or visible irradiation (direct Cr 3d-3d excitation). UV or visible pre-excited ZnGa2O4:Cr (ZGO:Cr) nanoparticles are kept active during several days thanks to deep traps (with depths 1 eV -1.2 eV) observed in these samples which can be probed through thermally stimulated luminescence (TSL) technique showing glow curve maximums at 470 K and 530 K upon visible light excitation. These deep traps are stable at room temperature but can be emptied by NIR light photostimulation. Experiments were carried out to study the photostimulation induced trapping-detrapping in the ZGO:Cr phosphor. Photostimulation was also tested in vivo for small animal optical imaging to offer new perspectives and modalities. (C) 2016 Elsevier B.V. All rights reserved

Library of UV-Visible Absorption Spectra of Rare Earth Orthophosphates, LnPO4 (Ln = La-Lu, except Pm)

In recent times, rare earth orthophosphates ( L n PO 4 ) have shown great potential as efficient optical materials. They possess either m o n a z i t e or x e n o t i m e –type structures. These light or heavy rare earth bearing orthophosphates also exhibit an extraordinary stability over geological time scale in nature, ∼10 9 years. In the present contribution, we measure, collect, and present a library of absorption spectra of all the L n PO 4 hosts ( L n = La–Lu, except Pm) using their single crystal samples, to conclude that the observed spectral features for wavelengths longer than 200 nm were attributable to either Ln- or defect related centers, which corroborate the fact that they have a bandgap higher than 8.0 eV. The absorption band around wavelength, 275 nm, corresponds to defect absorption related to PO 3 centers and/or oxygen vacancies. The hosts can potentially be used to study and interpret unperturbed rare earth emissions due to absence of host related absorption above 300 nm. The information presented herein is expected to serve as a library of absorption spectra for geologists, physicists, material scientists, and chemists working in the field of rare earths

Understanding the Reasons for Variations in Luminescence Sensitivity of Natural Quartz Using Spectroscopic and Chemical Studies

Natural quartz from diverse provenances exhibit variations in their luminescence sensitivity (photon flux/mg.Gy) that span over ten orders of magnitude. A range of factors (such as crystallization/recrystallization process, irradiation, thermal and optical history) that modify luminescence sensitivity have been considered to explain such differences in luminescence sensitivity. However, a clearly definable/identifiable reason is still awaited; such as, these large variations in luminescence sensitivity are related to commonly occurring luminescence quenchers/enhancers in natural quartz lattice. Towards this, quartz mineral grains extracts from different provenances and from varied depositional environments were examined spectroscopically using Fourier Transform infra-red spectroscopy (FTIR), optically stimulated luminescence (OSL), photoluminescence (PL), time resolved PL (TRPL) decay, thermoluminescence (TL) and the trace element analysis. The present study exhibited an anti-correlation between OSL sensitivity and water content in quartz (estimated through integrated intensity of normalized FTIR signal in the wave number region 3000-3600 cm(-1)), such that a sensitivity change of over 5 orders of magnitude corresponded to a change in integrated water absorption signal by 5 times, but in opposite direction. PL and TRPL results enabled further insights

Synthesis and thermoluminescence studies of Sr₄Al₁₄O₂₅:Eu²⁺ phosphor

165-168Trapping parameter, the activation energy (E), the order of kinetics (b) and the frequency factor (s), are determined for the solution combustion synthesized Sr4Al14O25: Eu2+ phosphor using 60Co irradiation source. The glow curve is a double peak structure with prominent peak at 487 K with shoulder at 423 K. Non-shifting Tm property is utilized for the trap parameter determination. Glow curve deconvolution (GCD) is carried out on recorded glow curves after gamma radiations. The trap depths calculated ranges between 0.5-1.36 eV. The linearity characteristic of the present phosphor and the fading experiments suggests the aptness of present phosphor towards dosimetric applications

Controlling disorder in the ZnGa2O4:Cr3+ persistent phosphor by Mg2+ substitution

We have studied in this work the effect of increasing structural disorder on the persistent luminescence of a Cr3+ doped zinc gallate spinel. This disorder was introduced by progressive substitution of Zn2+ by Mg2+ ions, and was studied by photoluminescence, X-ray diffraction, extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) and electron paramagnetic resonance (EPR) spectroscopy. It was found that increasing the Mg/Zn substitution decreases the number of Cr3+ in undistorted sites and increases the number of Cr3+ with neighbouring antisite defects and with neighbouring Cr3+ ions (referred to as Cr clusters), which in turn decreases the intensity of persistent luminescence. Both XANES and EPR spectra could be simulated by a linear combination of Cr3+ spectra with three types of Cr3+ environments. The increasing disorder was found to be correlated with a decrease of the average Cr-O bond length and a decrease of crystal field strength experienced by Cr3+ ions