Nanomorphology tuning of the thermal response of TiO2/SiO2 Bragg stacks

Herein, we present a comparative study of thermo- and environmentally responsive TiO2/SiO2 one-dimensional photonic crystals (Bragg stacks) fabricated by different deposition methods and fabrication schemes, featuring various multilayer nanomorphologies. These include dense multilayer systems processed by physical vapor deposition and wet-chemistry protocols, as well as porous systems, namely, nanoparticle-based optical filters exhibiting textural porosity, and evaporation-induced self-assembled mesoporous Bragg stacks exhibiting predominantly structural porosity, as well as hybrid structures comprising both dense and porous layers. We investigate the spectral shift of the photonic stop band for the different Bragg stack nanomorphologies induced by the humidity-enhanced thermo-optic effect in a temperature range from 15 to 60 degrees C. We also demonstrate the response and recovery kinetics of the multilayer systems during external changes in ambient humidity. Notably, the choice of fabrication method plays a significant role in the thermal and humidity response of the system. Taking advantage of different material nanomorphologies we can tune the thermal shift of the photonic stop band in the range 0.2-32.9 nm for the Bragg stacks at ambient relative humidity. In addition, we can design dense multilayer systems nonresponsive to humidity and achieve time responses of the porous systems to external changes in humidity ranging from about 1 to 3 s.