Fabrication and characterization of opals and core-shell silica spheres

We present the details of the sol-gel process used to realize inverse silica opal, where the silica was activated with 0.3 mol% of Er3+ ions. The template 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 calcination. Scanning electron microscope analysis 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 Er3+-activated silica inverse opal were investigated by photoluminescence spectroscopy. The lifetime of the 4I13/2 metastable state of the Er3+ ions resulted to be 16.8 ± 0.1 ms giving a quantum efficiency of about 90%. Core-shell Er3+-activated silica spheres, where the core is the silica sphere and the shell is an Er2O3-SiO2 coating, are proposed as a possible route for opal fabrication. For the core-shell system a quantum efficiency of about 70% is estimated.