Rare-earth-doped SiO2 colloids can be grown in a wet chemical procedure by adding the appropriate rare earth chloride to a solution of tetraethoxysilane, glacial acetic acid, and water. The size polydispersity of the particles can be controlled in a seeded growth process, and monodisperse Er3+-doped particles were grown by covering 183-nm-diameter seed particles with a 5-6-nm-thick (Er-doped) shell. After annealing, typical Er luminescence is observed with long luminescence lifetimes.

Chem. Mater.
Photonic Materials

de Dood, M.J.A, Berkhout, B, van Kats, C.M, Polman, A, & van Blaaderen, A. (2002). Acid based synthesis of monodisperse rare earth doped colloidal SiO2 spheres. Chem. Mater., 14, 2849–2853.