A novel route to luminescent opals for controlling spontaneous emission

Abstract

Fabrication of luminescent opals that enable redistribution of spontaneous emission of rare-earth ions is presented. Highly monodispersed silica-based composite spheres containing rare-earth oxide nanoparticles (Y₂O₃:Tb) were successfully synthesized using monodispersed mesoporous silica spheres (MMSSs) as hosts. Rare-earth oxide nanoparticles were embedded inside silica spheres through the ion exchange of rare earth ions with surfactant ions inside as-made MMSSs and subsequent calcination at high temperatures. The resultant Y₂O₃:Tb/SiO₂ composite spheres exhibited green emission with narrow bands corresponding to the intra-4f transitions, and the emission intensity increased with increasing calcination temperature. Even after calcination at 1000 °C, the composite spheres could be sufficiently dispersed in water, and self-assembled into a face-centered cubic opal structure with its (111) plane oriented parallel to the substrate. The luminescent opal exhibited a well-defined stop band corresponding to the Bragg equation. The particle size of composite spheres was adjusted so that the Bragg stop band of the opal overlapped the emission of the rare-earth oxide, and the directional effect of the stop band on the spontaneous emission was investigated. It was found that the emission from the luminescent opal was strongly modulated by the presence of the Bragg stop band and that the overlapping of the emission and the Bragg stop band resulted in the wavelength redistribution of the photoluminescence intensity.