Effect of Aqueous Media over Stability and Optical Performance of Mesoporous 1D Photonic Crystals

Abstract

Photonic crystals based on mesoporous thin-film multilayers have generated great interest in the field of sensing due to their high sensibility to small changes in the medium refractive index. However, they usually include silica-based porous oxides in their composition, a material whose stability in water is low. This feature could limit the sensors performance in long-term operation. In this work, various photonic crystals based on Si, Ti, and Zr mesoporous pure and hybrid oxide thin films were exposed to water and their optical, structural, and mechanical properties were evaluated as a function of contact time, either in batch or in flow conditions. The thickness, porosity, and pore ordering of the layers were followed by means of electron microscopy, small-angle X-ray scattering, and X-ray reflectivity, and the mechanical properties were evaluated by nanoindentation. In addition, the aqueous media in contact with the multilayered systems was characterized by X-ray fluorescence to determine the presence of dissolved species. Finally, the vapor detection capabilities of the different systems was evaluated. The obtained results indicate that the photonic bands move toward shorter wavelengths as the contact time with water increases, indicating material loss, in accordance with the behavior of the photonic crystal building blocks. Moreover, the mechanical properties of the systems are affected by changes in the composition of the layers as a result of the selective dissolution of specific components. However, no structural collapse of the porous materials is observed that also remain responsive to vapors, indicating that the multilayers are affected in a limited way by water contact.