Luminescent Sol-Gel Coatings Based on Nanophosphors
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Luminescent Sol-Gel Coatings Based on Nanophosphors Anne-Laure Pénard, Thierry Gacoin, and Jean-Pierre Boilot LPMC, Ecole Polytechnique, Route de Saclay, Palaiseau, 91128, France
ABSTRACT Nanoparticles of lanthanide vanadate (YVO4:Ln) and phosphate (LaPO4:Ln) are obtained by simple aqueous syntheses. The nanoparticles obtained are stabilized in colloidal suspensions and deposited in sol-gel films by spin-coating using a silicate as inorganic binder. The influence of the thermal treatment on the brightness and the ability to achieve multilayer coatings are discussed. INTRODUCTION Thin phosphor films have been prepared by a variety of deposition techniques (chemical vapor deposition, laser ablation,…) These techniques are often difficult to control for complex oxide matrices or doping compositions so, the liquid route seems to be more appropriate to produce transparent films. Different luminescent coatings have already been prepared by sol-gel processing from alkoxide precursors or from inorganic salts, using the Pechini method.[1-2] However, the production of dense and well-crystallized layers generally requires a thermal treatment (> 800°C) which is not compatible with the thermal stability of industrial substrates (glasses and plastics). An alternative way consists in the dispersion of nanophosphors, i.e. luminescent nanocrystals, in transparent polymer or sol-gel matrices.[3] This requires first to synthesize perfectly individualized and crystallized nanophosphors. The development of colloidal syntheses to prepare size-controlled, monodisperse and bright nanoparticles has been largely developed during the last years, especially for semi-conducting quantum dots.[4] Lanthanide-doped insulator oxides constitute an important other class of nanophosphors for which different types of liquid phase-syntheses led to the production of colloidal suspensions.[5] The preparation of stable colloids makes possible the second key step which is the elaboration of transparent luminescent films from a dispersion of nanophosphors in cement, which can either be constituted of organic or inorganic polymerizable species.[6] The silica-type sol-gel chemistry enables a fine control of chemical and physical properties of the matrix in which the nanophosphors are dispersed, and is thus an easy way to obtain transparent and luminescent thin films by spin or dip-coating. In spite of the high quality of these films, the complex functionalization and the use of alcoholic solvents are quite important drawbacks for an industrial application. A simpler route consists in mixing aqueous luminescent colloidal suspensions with an inorganic binder that will ensure the good mechanical properties of the film and preserve the good dispersion state of the particles in water. In this study, we use a simple aqueous route using competition between precipitation and complexation reactions to synthesize lanthanide vanadate (YVO4:Ln) and phosphate (LaPO4:Ln) nanoparticles.[7] Thin films of YVO4:Eu and LaPO4:(Ce,Tb), emitting respectively in red an
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