Importance of evaluating the intensity dependency of the quantum efficiency : impact on LEDs and persistent phosphors

The quantum efficiency is a key metric in lighting technology and for the quantification of luminescent processes, indicating how many photons are emitted with respect to the number of absorbed photons. Ideally, this value should approach unity to reduce losses, for instance in the common phosphor converted white LEDs. In this work we demonstrate that in luminescent materials where energy can be stored at defect centers, like in the extreme case of persistent phosphors, the quantum efficiency depends on the excitation intensity. For the green emitting SrAl2O4:Eu2+,Dy3+, which has been proposed for use in AC-LEDs, the internal quantum efficiency drops for increasing excitation intensity from 71% to 54%. At elevated excitation intensities, as encountered in LEDs, the trapped charge carriers can be optically detrapped by the excitation light, leading to this strong reduction of the overall quantum efficiency. Considering that the absorption cross section for this process is 6-29X larger than the absorption cross section for the luminescent ion, the efficiency of LED phosphors can be increased by avoiding the presence of defects acting as trapping centers. Finally, designing persistent phosphors with defects that show a limited optical response for the excitation light could strongly increase their energy storage capacity

Het archeologisch vooronderzoek te Willebroek-Hoeikensstraat. Eindrapport

Dit rapport werd ingediend bij het agentschap samen met een aantal afzonderlijke digitale bijlagen. Een aantal van deze bijlagen zijn niet inbegrepen in dit pdf document en zijn niet online beschikbaar. Sommige bijlagen (grondplannen, fotos, spoorbeschrijvingen, enz.) kunnen van belang zijn voor een betere lezing en interpretatie van dit rapport. Indien u deze bijlagen wenst te raadplegen kan u daarvoor contact opnemen met: [email protected]

Het archeologisch onderzoek op de Grote Markt te Aarschot. Rapportage van de bekomen resultaten

Dit rapport werd ingediend bij het agentschap samen met een aantal afzonderlijke digitale bijlagen. Een aantal van deze bijlagen zijn niet inbegrepen in dit pdf document en zijn niet online beschikbaar. Sommige bijlagen (grondplannen, fotos, spoorbeschrijvingen, enz.) kunnen van belang zijn voor een betere lezing en interpretatie van dit rapport. Indien u deze bijlagen wenst te raadplegen kan u daarvoor contact opnemen met: [email protected]

Archeologische vooronderzoek te Lier-Transvaalstraat

Dit rapport werd ingediend bij het agentschap samen met een aantal afzonderlijke digitale bijlagen. Een aantal van deze bijlagen zijn niet inbegrepen in dit pdf document en zijn niet online beschikbaar. Sommige bijlagen (grondplannen, fotos, spoorbeschrijvingen, enz.) kunnen van belang zijn voor een betere lezing en interpretatie van dit rapport. Indien u deze bijlagen wenst te raadplegen kan u daarvoor contact opnemen met: [email protected]

Optically stimulated nanodosimeters with high storage capacity

In this work we report on the thermoluminescence (TL) and optically stimulated luminescence (OSL) properties of beta-Na(Gd,Lu)F-4:Tb3+ nanophosphors prepared via a standard high-temperature coprecipitation route. Irradiating this phosphor with X-rays not only produces radioluminescence but also leads to a bright green afterglow that is detectable up to hours after excitation has stopped. The storage capacity of the phosphor was found to be (2.83 +/- 0.05) x 10(16) photons/gram, which is extraordinarily high for nano-sized particles and comparable to the benchmark bulk phosphor SrAl2O4:Eu2+,Dy3+. By combining TL with OSL, we show that the relatively shallow traps, which dominate the TL glow curves and are responsible for the bright afterglow, can also be emptied optically using 808 or 980 nm infrared light while the deeper traps can only be emptied thermally. This OSL at therapeutically relevant radiation doses is of high interest to the medical dosimetry community, and is demonstrated here in uniform, solution-processable nanocrystals

Lead toxicity in Saccharomyces cerevisiae

The effect of Pb on Saccharomyces cerevisiae cell structure and function was examined. Membrane integrity was assessed by the release of UV-absorbing compounds and by the intracellular K+ efflux. No leakage of UV260-absorbing compounds or loss of K+ were observed in Pb (until 1,000 μmol/l) treated cells up to 30 min; these results suggest that plasma membrane seems not to be the immediate and primary target of Pb toxicity. The effect of Pb on yeast metabolism was examined using the fluorescent probe FUN-1 and compared with the ability to reproduce, evaluated by colony-forming units counting. The exposition of yeast cells, during 60 min to 1,000 μmol/l Pb, induces a decrease in the ability to process FUN-1 although the cells retain its proliferation capacity. A more prolonged contact time (120 min) of yeast cells with Pb induces a marked (> 50%) loss of yeast cells metabolic activity and replication competence through a mechanism which most likely requires protein synthesis

Microwave-assisted synthesis followed by a reduction step : making persistent phosphors with a large storage capacity

The performance of impurity doped luminescent materials, or phosphors, depends on the composition and crystallinity of the host compound, as well as on the distribution and valence state of the dopant ions. This is particularly true for persistent phosphors, where both luminescence centers and charge trapping defects are required. Here we show that splitting the synthesis procedure in two separate steps offers a simple way to obtain efficient persistent phosphors which are superior to phosphors prepared via a conventional solid state synthesis using a single step. The storage capacity of the persistent phosphor benefits from using a microwave assisted solid state synthesis (MASS) to achieve superior compositional homogeneity, followed by a short heat treatment in a reducing atmosphere to reduce the activators. In this work, the approach is demonstrated for the efficient blue-emitting Eu2+,Dy3+ co-doped Sr2MgSi2O7 persistent phosphor. The enhanced ionic diffusion during the MASS not only improves the homogeneity and dopant distribution, but also allows the phosphor to be obtained in considerably shorter times (ca. 25 minutes). The storage capacity of the as-obtained phosphors prepared by MASS is slightly higher than those obtained by the conventional solid-state method. Cathodoluminescence (CL) measurements evidenced however the existence of a large fraction of unreduced europium activators. Using a short reducing step at 900 degrees C, the Eu3+ emission was almost fully suppressed in CL and as a consequence, the storage capacity of the MASS-obtained material showed a ten fold increase, confirming the benefit of decoupling compositional homogeneity and the dopant reduction step for phosphor synthesis

Counting the photons : determining the absolute storage capacity of persistent phosphors

The performance of a persistent phosphor is often determined by comparing luminance decay curves, expressed in cd/m(2). However, these photometric units do not enable a straightforward, objective comparison between different phosphors in terms of the total number of emitted photons, as these units are dependent on the emission spectrum of the phosphor. This may lead to incorrect conclusions regarding the storage capacity of the phosphor. An alternative and convenient technique of characterizing the performance of a phosphor was developed on the basis of the absolute storage capacity of phosphors. In this technique, the phosphor is incorporated in a transparent polymer and the measured afterglow is converted into an absolute number of emitted photons, effectively quantifying the amount of energy that can be stored in the material. This method was applied to the benchmark phosphor SrAl2O4: Eu, Dy and to the nano-sized phosphor CaS: Eu. The results indicated that only a fraction of the Eu ions (around 1.6% in the case of SrAl2O4: Eu, Dy) participated in the energy storage process, which is in line with earlier reports based on X-ray absorption spectroscopy. These findings imply that there is still a significant margin for improving the storage capacity of persistent phosphors