Aqueous routes to lanthanide-doped oxide nanophosphors

Besides well-known quantum dots, lanthanide-doped oxide nanocrystals form a new promising class of nanophosphors. Lanthanide vanadate and phosphate nanoparticles are prepared as well-dispersed and highly concentrated colloids by a very simple aqueous route, using competition between precipitation and complexation reactions. Under UV excitation, the three basic colors are furnished by YVO₄: Eu (red), LaPO₄: Ce·0.7H ₂O (blue-violet) and LaPO₄: CeTb·0.7H₂O (green). In the case of cerium-doped phosphate nanoparticles, the epitaxial growth of a LaPO₄·0.7H₂O shell leads to "core-shell" nanocrystals presenting improved stability against cerium oxidation which is detrimental for the luminescence. Nanocrystals exhibiting passivation and chemical or biological functionalization are easily prepared using the silane strategy. The optimized nanophosphors are either dispersed in transparent sol-gel films or used as isolated particles for biological labelling. The luminescence properties of nanoparticles differ from corresponding bulk materials in several ways: (i) spectroscopic changes, as a systematic broadening of the luminescence bands in sub-20 nm nanocrystals, resulting from a structural disorder intrinsic to the small size; (ii) a lower luminescence quantum yield related to the presence of chemical species adsorbed at the surface, especially the OH groups; (iii) a shift towards higher values of the optimum doping concentration, due to the alteration of energy transfers either in the vanadate or cerium sub-networks.