Rachid Mahiou

Red luminescence in Pr3+-doped calcium titanates

A strong single red emission band (λ max = 613 nm, FWMH = 450 cm -1 ) was observed at room temperature in Pr 3+ -doped calcium titanates. The evolution of the intensity of this luminescence was analysed in compensated and uncompensated samples (using Na + , Ag + or TI + as charge compensators) and also as a function of Pr concentration. Emission, excitation and reflectivity spectral repartitions were investigated at various temperatures, as well as the luminescence decays. The origin of the red luminescence is discussed.

Luminescence Dynamics in Tb3+-Doped CaWO4 and CaMoO4 Crystals

Single crystals of CaWO(4) and CaMoO(4) doped with Tb(3+) have been grown by the flux growth method. Their luminescence properties have been investigated in the 10-600 K temperature range under different experimental conditions. In spite of very similar spectra at low temperature upon excitation at 365 nm, the crystals show a very different behavior as the temperature is raised or the excitation wavelength is changed. These differences have been accounted for on the basis of models that take into consideration the position of the energy levels of the rare earth relative to the bandgap of the host material.

Ce-Doped YAG Nanophosphor and Red Emitting CuInS2/ZnS Core/Shell Quantum Dots for Warm White Light-Emitting Diode with High Color Rendering Index

In this work, we report the solvothermal synthesis of Ce-doped YAG (YAG:Ce) nanoparticles (NPs) and their association with a free-Cd CuInS2/ZnS (CIS/ZnS) core/shell QDs for application into white light emitting diode (WLED). 1500 °C-annealed YAG:Ce NPs and CIS/ZnS core/shell QDs exhibited intense yellow and red emissions band with maxima at 545 and 667 nm, respectively. Both YAG:Ce nanophosphor and CIS/ZnS QDs showed high photoluminescence quantum yield (PL QY) of about 50% upon 460 nm excitation. YAG:Ce nanophosphor layer and bilayered YAG:Ce nanophosphor-CIS/ZnS QDs were applied on blue InGaN chip as converter wavelength to achieve WLED. While YAG:Ce nanophosphor converter layer showed low color rendering index (CRI) and cold white light, bilayered YAG:Ce nanophosphor-CIS/ZnS QDs displayed higher CRI of about 84 and warm white light with a correlated color temperature (CCT) of 2784 K. WLED characteristics were measured as a function of forward current from 20 to 1200 mA. The white light stability of bilayered nanophosphor-QDs-based WLED operated at 200 mA was also studied as a function of operating time up to 40 h. Interestingly, CRI and CCT of such device tend to remain constant after 7 h of operating time suggesting that layer-by-layer structure of YAG:Ce phosphor and red-emitting CIS/ZnS QDs could be a good solution to achieve stable warm WLED, especially when high current density is applied.

Synthesis dependent luminescence efficiency in Eu3+ doped polycrystalline YBO3

The influence of the synthesis method on the luminescence efficiency of the orthoborate YBO 3 :Eu 3+ was studied by comparing the optical performances of samples prepared by three routes: solid state reaction (SR), wet process (WP) and sol-gel processing (SG). On the basis of X-ray powder diffraction, SEM micrographs and photoluminescence measurements, SG appears to be the most efficient method to prepare high quality compounds for practical applications. Such a conclusion is reinforced by the existence of an important gain in the luminescence efficiency using this procedure.

Improvement of the optical performances of Pr3+ in CaTiO3

Abstract Trivalent praseodymium-doped calcium titanates are red emitting phosphors upon UV photon excitation in the range 150–300 nm, with a luminescence appearing as a single peak centered at 613 nm. The intensity of this emission is investigated in different samples prepared either by a solid state or a sol–gel method. The study includes analyses of microstructure, sintering temperature, charge compensation and Pr concentration effects.

Molecular design of luminescent organic-inorganic hybrid materials activated by europium (III) ions

Abstract Luminescent hybrid materials consisting in rare-earth (Eu 3+ , Gd 3+ ) organic complexes covalently attached to a silica-based network have been obtained by a sol–gel process. Four dicarboxylic acids with different aromatic subunits (dipicolinic acid, 4-phenyl-2,6-pyridinedicarboxylic acid, 4-(phenylethynyl)-2,6-pyridinedicarboxylic acid and 2,6-Bis(3-carboxy-1-pyrazolyl)pyridine) have been chosen as ligands for Ln 3+ ions. They were grafted to 3-aminopropyltriethoxysilane (APTES) to give organically modified alkoxysilanes that were used as molecular precursors for the preparation of hybrid materials. Ln 3+ first coordination sphere, composition of the siloxane matrix and connection between the organic and inorganic parts have been characterized by infrared spectroscopy, by 13 C 29 Si solid-state NMR as well as by elemental analyses. UV excitation in the organic component resulted in strong emission from Eu 3+ ions due to an efficient ligand-to-metal energy transfer. As compared to reference organic molecules, hybrid samples exhibited similar emission properties under UV excitation in addition to mainly unchanged excited states lifetimes. However, by direct excitation of the Eu 3+ - 5 D 0 energy level, the presence of two different site distributions were evidenced in the four hybrid compounds. Emission features related to each of these site distributions and their respective attribution were investigated. Variations in the relative emission intensities were observed according to the nature of the organic chromophore. These variations were discussed in relation to the ATE (Absorption-Transfer-Emission) mechanism and to the relative energy positions of the ligand and the rare-earth ions respectively.

About red afterglow in Pr3+ doped titanate perovskites

The red phosphor CaTiO3 : Pr3+ is known to show a persistent luminescence in the red spectral region upon irradiation in the UV. Although a number of studies have been dedicated to the enhancement of the afterglow efficiency in this perovskite by varying the preparation method, the chemical composition or the particle size, no clear explanation of the mechanisms involved in the afterglow process has been given yet. The purpose of this paper is to start investigating this aspect on the basis of a recently developed model and by using the luminescence and afterglow properties of the solid solution (Ca, Sr)TiO3 : Pr3+ as experimental support.

Molecular design of inorganic scintillators: from alkoxides to scintillating materials

In this paper we report on preliminary results obtained on sol–gel derived inorganic scintillators. A new general scheme based on the use of alkoxide chemistry is proposed for the preparation of various scintillating materials including LuBO3, GdBO3, LuPO4 and Lu2SiO5. The materials are obtained as monophasic powders and as homogeneous transparent thin films for borates and silicate. Thermal behaviour is analysed and a study of the oxidation state of cerium ions is proposed by combining X-ray photoelectron spectroscopy and X-ray absorption near edge structure spectroscopy. A mixture of Ce3+ and Ce4+ is observed in proportions depending on the material. This presence of Ce4+ ions does not seem to greatly affect the observed scintillation yields of the materials. Efficient sol–gel derived scintillators have been prepared both as powders and thin films. The preparation of optical quality scintillator thin films could be the starting point for a new generation of scintillating materials. Finally, the versatility of the proposed synthesis route will allow extension of this study towards various other materials.

Spectroscopy and upconversion processes in YAlO3:Ho3+ crystals

Abstract Emission from the high lying excited states, energy transfer, and upconversion processes are investigated in YAlO 3 :Ho 3+ . Selectively excited emission spectra in the range from 300 to 800 nm starting from the 3 D 3 , 3 G 5 , 5 F 3 , 5 S 2 and 5 F 5 multiplets were measured at 15 K. This, together with the detailed absorption and excitation measurements at 15 K allowed determination of the Stark energy levels of Ho 3+ ions in YAlO 3 up to UV energies. The 5 S 2 fluorescence decays were recorded as a function of temperature and Ho 3+ concentration in order to investigate the process of quenching of fluorescence due to cross relaxation among two ions. Conversion of red and infrared laser radiation to green 5 S 2 and blue 5 F 3 emission is reported. Under pulsed resonant excitation of the 5 F 5 or 5 I 5 levels the upconversion was found to be due to energy transfer process between two excited ions. The photon avalanche effect was observed under cw excitation around 585 nm.

Luminescence of the orthoborate YBO3:Eu3+. Relationship with crystal structure

Abstract Pseudo-vaterite structure of YBO3 has been determined from X-Ray data taken from a single crystal. This orthoborate crystallizes in the P6 3 m space group. The structure exhibits two types of [YO8] polyhedra due to the existence of two environments for the yttrium ions. The Eu3+ luminescence in this compound corroborates fairly well the structural determination by identification of two crystallographic sites both showing a C3 symmetry.