Luminescence Studies and Judd-Ofelt Analysis on SiO@LaPO:Eu@SiO Submicro-spheres with Different Size of Intermediate Shells.

The novel submicro-spheres SiO@LaPO:Eu@SiO with core-shell-shell structures were prepared by connecting the SiO submicro-spheres and the rare earth ions through an organosilane HOOCCHN(CONH(CH)Si(OCHCH) (MABA-Si). The as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and infrared spectroscopy (IR). It is found that the intermediate shell of the submicro-spheres was composed by LaPO:Eu nanoparticles with the size of about 4, 5-7, or 15-34 nm. A possible formation mechanism for the SiO@LaPO:Eu@SiO submicro-spheres has been proposed. The dependence of the photoluminescence intensity on the size of the LaPO:Eu nanoparticles has been investigated. The intensity ratios of electrical dipole transition D → F to magnetic dipole transition D → F of Eu ions were increased with decreasing the size of LaPO:Eu nanoparticles. According to the Judd-Ofelt (J-O) theory, when the size of LaPO:Eu nanoparticles was about 4, 5-7 and 15-34 nm, the calculated J-O parameter Ω (optical transition intensity parameter) was 2.30 × 10, 1.80 × 10 and 1.20 × 10, respectively. The increase of Ω indicates that the symmetry of Eu in the LaPO lattice was gradually reduced. The photoluminescence intensity of the SiO@LaPO:Eu@SiO submicro-spheres was unquenched in aqueous solution even after 15 days.