Photoluminescence of Bismuth Germanate Phosphors with a Silica-shell Structure

An intense bluish-white emitting phosphor Bi4Ge3O12 (BGO) was synthesized by tartaric acid-assisted solgel method and covered with a SiO2 nanolayer by the sol-gel method. The cubic crystal structure of BGO was confirmed by X-ray diffraction. Transmission electron microscopy analysis revealed the particle size to be in the range of 6-20 nm. Under excitation with 4.3 eV photons the BGO powders showed a PL contribution centered at 2.43 eV that corresponds to the typical BGO luminescence associated with 3P1à1S0 of Bi3+ electron transitions. Radioluminescence measurements showed the predominant emission centered at 2.43 eV due to the intra-ionic Bi3+ transitions. The PL emission of the silica coated BGO nanoparticles presented a luminescence enhancement of more than 200% compared with the uncoated nanoparticles. This enhancement in photoluminescence is proposed to be related to the reflectivity (refractive index) between air-silica-BGO layers.