Improvement in the Luminescence Properties and Processability of LaF₃/Ln and LaPO₄/Ln Nanoparticles by Surface Modification

The surface of lanthanide(III)-doped LaPO₄ nanoparticles was modified by reaction with an alcohol, leading to a covalent bond between the ligand and the particle surface. The surface of lanthanide(III)-doped LaF₃ nanoparticles was modified to alter the solubility of the nanoparticles and study the influence of surface effects on the luminescence of lanthanide ions doped in the nanoparticles. The coordinated organic ligands can be modified by a quantitative exchange reaction in solution or by using functionalized ligands during the synthesis. Variation of the ratio of ligand to core reagents had a significant influence on the size of the nanoparticles. Smaller nanoparticles were formed with a higher ligand ratio. The optical properties of these nanoparticles show a strong dependence on nanoparticle size, indicating the influence of quenching probably by CH and OH groups at or near the surface of the nanoparticle cores. The luminescence lifetime of LaF₃/Eu nanoparticles varied from 6.5 to 7.4 ms for nanoparticles with an average size of 7.1 to 8.4 nm. A significant reduction of the quenching from the surface of the nanoparticles was obtained by the synthesis of core−shell nanoparticles, in which a shell of LaF₃ was grown epitaxially around the doped core nanoparticles. This leads to an increase in the luminescence lifetime of the Eu³⁺ ion and the observation of emissions from the ⁵D₂ energy level, in addition to emissions from the ⁵D₁ and ⁵D₀ levels. The quantum yield of LaF₃/Ce,Tb nanoparticles could be increased from 24 to 54% by the growth of a LaF₃ shell around the nanoparticles.