Optical properties and microstructures of colloidal crystalline arrays composed of titania nanosheets coated core-shell structured spheres

Abstract

Colloidal crystalline arrays are three dimensionally periodic lattices of self-assembled monodisperse colloidal spheres. These periodic structures have been actively explored as functional components in fabricating new types of diffractive devices such as optical filters and chemical sensors, mechanical sensors, and photonic bandgap structures. We have demonstrated the synthesis of silica or polystyrene spheres uniformly coated with titania nanosheets and the fabrication of these spheres into close-packed colloidal crystalline arrays. We have also reported on the optical properties and microstructures of the colloidal crystalline array estimated by angle-resolved reflection spectra measurements. The titania nanosheets were synthesized by delamination of layered titanate crystallites. The titania nanosheets coated spheres were prepared by the LBL (Layer-By-Layer) assembly coating process, which consisted of alternately laminating cationic polyelectrolyte and anionic titania nanosheets on monodisperse silica or polystyrene spheres. The close-packed colloidal crystalline array was fabricated in the glass cell by drying process of the aqueous dispersion of the spheres. The Bragg diffraction peak of the colloidal crystalline array shifted to longer wavelengths with increasing thickness of titania nanosheets layers. Angle-resolved reflection spectra measurements showed that this red shift was caused by increasing the mean effective refractive index n eff of this crystalline lattice without changing interplanar spacing d 111 with increasing thickness of titania nanosheets layers. Since a wide range of coated colloids of different size, composition, and optical properties can be prepared via the LBL coating, the current work suggests new possibilities for the creation of advanced colloidal crystalline arrays with tunable optical properties.