Tailored circular Bragg phenomenon in TiO₂ sculptured thin films by Post-deposition processing

Chiral sculptured thin films (STFs) of TiO₂ deposited by the serial bi-deposition method can be modified post-deposition to tailor the circular Bragg phenomenon which occurs due to their periodic helical morphology. This post-deposition tailoring is accomplished through annealing and/or wet chemical etching. Annealing of the chiral STF in spectral-hole-filters (SHFs) transforms the material structure from non-crystalline to an anatase structure which is more stable and provides better optical performance. The transformation causes a blue-shift of the circular Bragg phenomenon (CBP) which is quantified and correlated with material properties. Further annealing causes a significant change in morphology, which resembles a "string of pearls", and may be the cause of a loss of transmission. Wet chemical etching is also capable of blue-shifting the Bragg regime of a non-crystalline SHF, while having very little effect on a crystalline SHF. The spectral shift of the Bragg regime is limited by the ability of the nanowires, of which the STFs are composed, to remain attached to the substrate. Wet chemical etching and annealing can be used in combination to allow more control of post-deposition processing.