Luminescence Studies and Judd–Ofelt Analysis on SiO(2)@LaPO(4):Eu@SiO(2) Submicro-spheres with Different Size of Intermediate Shells

The novel submicro-spheres SiO(2)@LaPO(4):Eu@SiO(2) with core-shell-shell structures were prepared by connecting the SiO(2) submicro-spheres and the rare earth ions through an organosilane HOOCC(6)H(4)N(CONH(CH(2))(3)Si(OCH(2)CH(3))(3) (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(4):Eu nanoparticles with the size of about 4, 5–7, or 15–34 nm. A possible formation mechanism for the SiO(2)@LaPO(4):Eu@SiO(2) submicro-spheres has been proposed. The dependence of the photoluminescence intensity on the size of the LaPO(4):Eu nanoparticles has been investigated. The intensity ratios of electrical dipole transition (5)D(0) → (7)F(2) to magnetic dipole transition (5)D(0) → (7)F(1) of Eu(3+) ions were increased with decreasing the size of LaPO(4):Eu nanoparticles. According to the Judd-Ofelt (J-O) theory, when the size of LaPO(4):Eu nanoparticles was about 4, 5–7 and 15–34 nm, the calculated J-O parameter Ω(2) (optical transition intensity parameter) was 2.30 × 10(−20), 1.80 × 10(−20) and 1.20 × 10(−20), respectively. The increase of Ω(2) indicates that the symmetry of Eu(3+) in the LaPO(4) lattice was gradually reduced. The photoluminescence intensity of the SiO(2)@LaPO(4):Eu@SiO(2) submicro-spheres was unquenched in aqueous solution even after 15 days.