Pure and doped LiNbO3 nanocrystals from alkoxides : reaction, nucleation and crystallization mechanisms by aqueous sol-gel route under solvothermal conditions ; Nanocristaux de LiNbO3 purs et dopés à partir d'alcoxydes : mécanismes réactionnels, de nucléation et de cristallisation par voie sol-gel aqueuse en conditions solvothermales

Optimisation of the synthesis conditions of multifunctional harmonic nanoparticles, based on lithium niobate (LiNbO3) for example, consists in deriving the relationships between the nucleation/growth kinetics and the sizes and morphologies of the final particles in order to optimise their properties of use. Bulk LiNbO3 crystals are widely used for their piezoelectric, acoustic, photorefractive, ., and optical properties. At the nanoscale, it is also an excellent candidate in terms of contrast agents for multiphoton biomedical imaging due to its biocompatibility and its excellent non-linear optical properties for the generation of specific second and third harmonic wavelengths. This work focuses on a detailed understanding of the nucleation, crystallisation and growth mechanisms of LiNbO3 nanocrystals prepared from metal alkoxides by an aqueous sol-gel route under solvothermal conditions. A systematic modification of the chemical composition of the reaction medium (addition of hydroxylated co-solvents such as alcohols and glycols, hydrolysis rate) not only allows to obtain nanocrystals of adjustable mean size (between 15 and 80 nm) but also to evidence the occurrence of non-classical nucleation and crystallization phenomena. The controlled hydrolysis of the reaction medium at room temperature indeed leads to the formation of oxoclusters and of a colloidal gel. After thermal treatment, time- and temperature-resolved measurements demonstrate an aggregation-induced crystallisation mechanism from primary crystallised reaction intermediates stabilised by alkoxy or glycoxy ligands. The interaction energy between the inorganic cores and the surface ligands varies according to their nature, thus modifying the crystallization and kinetic temperatures and the final sizes. Moreover, at high hydrolysis rates, an inter-particle Ostwald ripening phenomenon was also observed resulting to a morphology change, the primary platelet-shaped nanocrystals evolve towards pseudo-cubic structures with an exposure of the {012} facets. ...