Fabrication and Characterization of Silica Opals

We present the details of the sol-gel processing used to realize inverse silica opal, where the silica was activated with 0.3 mol% of Er3+ ions. The template (direct opal) was obtained assembling polystyrene spheres of the dimensions of 260 nm by means of a vertical deposition technique. The Er3+-activated silica inverse opal was obtained infiltrating, into the void of the template, the silica sol doped with Er3+ ions and subsequently removing the polystyrene spheres by means of calcinations. Scanning electron microscope showed that the inverse opals possess an fcc structure with a air hollow of about 210 nm. A photonic band gap in the visible range was observed from reflectance measurements. Spectroscopic properties of Er 3+activated silica inverse opal were investigated by photoluminescence spectroscopy. A bandwidth of 21 nm was measured for the 4I13/2 -> 4I15/2 transition of Er3+ ions upon excitation at 514.5 nm. The luminescence decay curve of the 4I13/2 metastable state of the Er3+ ions presents a lifetime ? = 16.8 ± 0.1 ms giving a very high quantum efficiency of the fabricated system. Core-shell Er3+-activated silica spheres, where the core is the silica sphere and the shell is an Er 2O3-SiO2 coating is proposed as a possible route for opal fabrication. For core-shell system a quantum efficiency of about 70% was estimated.