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Systèmes luminescents BaTiO3 : Ln(Ln= Er3+, Yb3+, Eu3+) préparés par chimie douce : poudres, nanopoudres et films nanostructurés

Abstract : This work was focused to the preparation of BaTiO3 powders, nanoparticles and films synthesized by solid state route, hydrothermal and sol-gel methods as well as to the study to structural and optical properties. BaTiO3 perovskite has the capability to incorporate rare earth ions of different size exhibiting luminescent properties, specifically when is doped by Eu3+, Er3+ and Yb3+ ions, also, these ions can be employed as optical probe in this matrix. Europium doped barium titanate prepared by hydrothermal route and solid state reaction presented cubic and/or tetragonal phases depending of the used method. The size of the powder obtained by hydrothermal route was 25 nm meanwhile by using the solid state reaction the particle size was increased up to 200 nm after 1150 °C heat treatment. The presence of the erbium and ytterbium ions in the BaTiO3 matrix provokes red, green and blue emissions which are originated by 4S3/2 and 4F9/2 transitions. In this work emission spectra and lifetime decays of single and co-doped Er3+ and Yb3+ ions in BaTiO3 polycrystalline sol-gel matrix were measured to describe the mechanisms that drive the fluorescence and the observed quenching. For both systems single and co-doped samples were a cross-relaxation mechanism, 4S3/2 + 4 I15/2  4 I9/2 + 4I13/2, which quenches the green fluorescence and it was demonstrated that this mechanism depends on Er3+ concentration. The enhanced concentration quenching in co-doped samples is due to energy transfer between neighbouring Er3+ -Yb3+ ions. For this case, emission of the 4I13/2 and 4I11/2 levels diminished as Er3+ concentration increases; but this behavior is the contrary for single doped Er3+ samples. The lengthening of the lifetime decay of 4S3/2 level, in co-doped samples, is due to an energy back transfer, which also affects the quenching of the 4I11/2 level. Further, when single doped Yb3+ samples were pumped at 940 nm, an emission from Yb3+ pairs was measured at about 500 nm. This process should be present in co-doped Er3+-Yb3+ samples and contributes to the quenching of emission at about 1.0 m. The heat treatment at intermediate temperature and the use of chelating agents allows to control the shape of the particles in the sol-gel systems. The particle shape and size could be tailored in the un-doped and codoped BaTiO3 system by tailoring the chelating agents and the heat treatment temperature. The morphology of BaTiO3 (undoped and codoped) compounds with and without chelating agents differs greatly. The microstructure of the derived materials is more spherical than non-chelating agents and the size of the particles is lower (96-337 nm) than that obtained without (AcAc)H and acetic acid (137-312 nm). Further, the morphology of the undoped and codoped BaTiO3 particles in presence of chelating agents at 700 and 950 °C results in a combination of nanorods and nearly spherical structures associated with the cubic and tetragonal phase, respectively. At 1150 °C, the BaTiO3 and BaTiO3:Er, Yb powders exhibit a tetragonal phase (predominant), and only nearly spherical particles were observed. Isolated cubic structure un-doped and codoped BaTiO3 nanorods, with diameters ranging from 100 to 120 nm and reaching 800 nm in length, presenting for both undoped and codoped system. From the addition of chelating agents as the acetylacetone and acetic acid were observed an important change in the morphology of the powders was observed, i.e. the presence of the chelating agents allow to form quasi spherical particles (tetragonal structure). Tetragonal structure, can be fabricated by this route when Ba(C5H7O2)2, Ti(OPri)4 are selected as starting materials and (AcAc)H, H-(OAc) are chosen as chelating agents. The influence of calcination temperature and chelating agents on the surface area, pore radius and pore volume of BaTiO3:Er, Yb were also investigated A notable result associated with the elaboration of BaTiO3:Er, Yb films was the influence of the chelating agents in the synthesis on the films. BaTiO3:Er, Yb optical thin films are only obtained in the presence of chelating agents, in contrast with the films prepared without them, which were characterized by granular networks showing poor adhesive properties toward the substrates and cracks. The best film, in terms of homogeneity in morphology, was obtained using chelating agents at 950 °C, resulting in the formation of highly cross-linked and uniform films, composition, low porosity, and a mean surface roughness of 12.4 nm. The good optical films formation of BaTiO3:Er, Yb system concerning the high quality, do not allow the high thickness, so it was establish the addition of polyvinylpirrolidone (PVP) during the process arising for the first time monolayer of high thickness (828 nm), value determined by m-lines spectroscopy technique. Using PVP the morphology resulted in a flower-like morphology characterized by high homogeneity, low porosity, and even distribution of doping ions. XPS analysis indicates that the compensating mechanism in BaTiO3:Er3+ can be expressed by the dynamic equilibrium . The optical studies showed the up-conversion properties of Er3+ doped BaTiO3 films upon 974 nm infrared excitation and demonstrated that the green emission at 548 nm was predominant under this excitation. This effect was explained in terms of cooperative energy transfer between two Er3+ ions. Finally, BaTiO3:Eu3+ systems were obtained in powders and films using sol-gel method incorporating a viscosity modifier (PVP) in the sol. Both systems exhibited crystalline BaTiO3 phase at 700 °C within 2 h as revealed the HT-XRD studies. The powders were mainly spherical with some rod shapes; however, the films presented good surface morphology as detected by SEM. The obtained films exhibit the room temperature photoluminescence of the europium ions, with the predominant band at 615 nm (5D0 7F2) transition). The obtained thickness (~500 nm) of BaTiO3:Eu3+ films must be optimized to be promising for luminescent applications.
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Submitted on : Thursday, July 19, 2012 - 11:50:46 AM
Last modification on : Tuesday, April 20, 2021 - 11:24:03 AM
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  • HAL Id : tel-00719196, version 1


Margarita Garcia-Hernandez. Systèmes luminescents BaTiO3 : Ln(Ln= Er3+, Yb3+, Eu3+) préparés par chimie douce : poudres, nanopoudres et films nanostructurés. Matériaux. Université Blaise Pascal - Clermont-Ferrand II; National Polytechnic Institut, Mexico, 2010. Français. ⟨NNT : 2010CLF22040⟩. ⟨tel-00719196⟩



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