Homogeneity of oxide air-sphere crystals from millimeter to 100-nm length scales: A probe for macroporous photonic crystal formation

We study the homogeneity of photonic air-sphere crystals made of oxides. This important class of macroporous materials is prepared via liquid precursor infiltration of opal templates and subsequent removal of the template by calcination. We have synthesized highly ordered structures of macropores in a solid backbone consisting of two consecutively infiltrated materials: the oxide pairs zirconia/titania (ZrO2/TiO2) and alumina/titania (Al2O3/TiO2). The ordered arrangement of the air spheres was confirmed by scanning electron microscopy. To determine the spatial distribution of the materials, energy-dispersive spectroscopy was used. It was observed that there are variations in the distribution of oxide materials throughout the sample on length scales ranging from 1 mm to 100 nm. We propose that the spatial variations originate from the reaction process where the precursors hydrolyze and possibly also from the densification that occurs in the calcination step. Since many light emitters are quenched on semiconductor surfaces, such inhomogeneities hamper a proper shielding of the emitters, thereby impeding spontaneous emission experiments in photonic crystals. On the other hand, inhomogeneities appear to cause minor variations in the optical Bragg conditions.