SYNTHESIS AND APPLICATION OF PHOTOLUMINESCENT DOPED LANTHANIDE OXIDE MICROSPHERES USING THE REACTION-DIFFUSION FRAMEWORK (RDF)

Abstract

Self-organization is the spontaneous process whereby some form of overall spatiotem poral order arises from synergies between parts of an initially disordered system. It occurs in open systems driven away from thermal equilibrium. Liesegang banding is a type of far-from-equilibrium self-organizing phenomenon that emerges due to the coupling of diffusion to precipitation chemical reactions[1]. We can exploit Liesegang banding to make crystalline material via the so-called reaction-diffusion framework (RDF); a method which allows control over the size, morphology, and composition of crystalline solids[2]. In this work, we are particularly interested in studying the optical properties of lanthanide-doped lanthanum oxide mono-dispersed spheres synthesized in an agar hydrogel matrix via the reaction-diffusion framework (RDF). Having long excited state lifetimes in the millisecond range, resistance to photobleaching, and low toxicity, Lanthanide-doped luminescent inorganic nanoparticles give rise to promising possibilities for light-emitting applications, bioassays, and bioimaging[3], [4]. First, the lanthanum hydroxide crystals (pure and doped) are precipitated by pouring an outer electrolyte of concentrated ammonia onto agar gel containing the lanthanide salts in given initial proportions. Macroscopically, the resulting lanthanum hydroxide material forms a set of Liesegang bands. SEM images of individual bands reveal a microscopic hierarchical structure of nano-platelets selfassembling into exquisitely shaped nano- and micro- spheres[2].The average size of the spheres depends on the selection of a specific Liesegang band and the resulting set. As expected, RDF provides a green, fast and easy synthetic route to obtain lanthanide-doped nanoparticles. The dopants used include erbium, samarium, europium and terbium ions. The lanthanide doped La2O3 :Ln3+ (Ln = Eu, Sm, Eu, Tb) phosphors[5] are obtained from the corresponding hydroxide by calcination. The emission spectra of the lanthanide-doped La2O3 phosphors are found to exhibit 2 classic sharp easily recognizable inner-shell 4f-4f transitions which span both the visible and near-infrared (NIR) ranges[4].

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