Preparation And Luminescent Properties Of Eu～(3+) Doped Inorganic And Organic Nanoparticles

For trivalent rare earth doped inorganic nano-materials, surface effect is the key factor affecting their luminescent properties. However, restricted by the differences of material system and preparation method, knowledge on surface effect is still somewhat discrepant and hazy. To further study the luminescent properties of nanocrystals under surface effect, systems with high symmetry such as La₂O₂S: Eu³⁺ and YVO₄: Eu³⁺ are studied. The main research is as follows:1. La₂O₂S: Eu³⁺ nanocrystals with a mean size of 18 nm are prepared by gel thermolysis. The surface Eu³⁺ ions are first detected by time-resolved spectra in the ⁵D₀→⁷F₁ region. Because the symmetry of the sites occupied by surface Eu³⁺ ions is lower, the ⁵D₀→⁷F₁ line, which is doubly degenerate in the bulk crystal, is split, and the fluorescence lifetime becomes shorter. The results of the laser-selective excitation indicate that the degradation of the site symmetry of Eu³⁺ seems to be abrupt, which means the as-synthesized La₂O₂S: Eu³⁺ nanocrystals might be of the La₂O₂S /La₂O_2-xS_1+x core-shell structure and the shell is not in a disordered state but a rather pure one.2. By simulating surface defects in YVO₄: Eu³⁺ nanocrystals with point charges, ⁷F₂ crystal field levels of Eu³⁺ under different local microstructures are calculated employing a modified point charge model. From the calculation results, the gradual splitting and shift of ⁷F₂ energy levels can be seen clearly with the strengthening of disturbation of simulated charge. Based on the theoretical ⁷F₂ levels, surface Eu³⁺ indicated by the ⁵D₀→ ⁷F₂ selective excitation spectra are attributed, and Eu-O² broken bond is assumed to be responsible for the surface defects in YVO₄: Eu³⁺ nanocrystals. In addition, Furthermore, it is proposed from the attribution that relaxation of selection rules by surface effect is feeble for near surface Eu³⁺, whereas it is more intense for surface Eu³⁺.Lanthanide chelates are widely used as fluorescent labels in bioassay. Though several advantages are favorable, their shortcomings in stability and lower quantum yield hinder their further application. If lanthanide chelates are doped into nanoparticles formed with inert material, particle layer of particle would separate chelate molecules from surroundings. Hence, stability and luminescent yield would be greatly improved. The second part of this paper is about the preparation of fluorescent nanoparticles from Eu³⁺ chelate -Eu(DBM)₃TPPO (DT-Eu³⁺), and studies of fluorescent properties.Firstly, small sized DT-Eu³⁺ nanoparticle (10 nm) suspension is prepared by a novel reprecipitation-encapsulation method. It is found that the pure nanoparticles are easy to aggregate in aqueous, and their fluorescent are severely quenched by surrounding water molecules. Then an alkyl alkoxysilane encapsulation agent is included during the nanoparticle formation process, and the resultant DT-Eu³⁺@OTS nanoparticles are well dispersed, along with even size and intense luminescence. The developed encapsulation layer around nanoparticle inhibits aggregation and quenching from water. The emission spectrum of hybrid nanoparticle is the same as that of DT-Eu³⁺ molecule. However, their luminescent yield is nearly 20 % higher than that in THF solution.The formation mechanism of nanoparticles is studied with the time-correlated UV-vis spectroscopy and timebased luminescence. The evenly distributed microjets of stock solution formed in the mixture by sonication are believed to be the pre-nanoparticles, which nucleate and grow into nanoparticles under hydrophobic interaction with the diffusion of THF.