Abstract
TiO2 thin films deposited by atomic layer deposition (ALD) at low temperatures (<100 °C) are, in general, amorphous and exhibit a smaller refractive index in comparison to their crystalline counterparts. Nonetheless, low-temperature ALD is needed when the substrates or templates are based on polymeric materials, as the deposition has to be performed below their glass transition or melting temperatures. This is the case for photonic crystals generated via ALD infiltration of self-assembled polystyrene templates. When heated up, crystal phase transformations take place in the thin films or photonic structures, and the accompanying volume reduction as well as the burn-out of residual impurities can lead to mechanical instability. The introduction of cation doping (e.g., Al or Nb) in bulk TiO2 parts is known to alter phase transitions and to stabilize crystalline phases. In this work, we have developed low-temperature ALD super-cycles to introduce Al2O3 into TiO2 thin films and photonic crystals. The aluminum oxide content was adjusted by varying the TiO2:Al2O3 internal loop ratio within the ALD super-cycle. Both thin films and inverse opal photonic crystal structures were subjected to thermal treatments ranging from 200 to 1200 °C and were characterized by in- and ex-situ X-ray diffraction, spectroscopic ellipsometry, and spectroscopic reflectance measurements. The results show that the introduction of alumina affects the crystallization and phase transition temperatures of titania as well as the optical properties of the inverse opal photonic crystals (iPhC). The thermal stability of the titania iPhCs was increased by the alumina introduction, maintaining their photonic bandgap even after heat treatment at 900 °C and outperforming the pure titania, with the best results being achieved with the super-cycles corresponding to an estimated alumina content of 26 wt.%.
Highlights
Photonic crystals and glasses are periodic or disordered arrangements of refractive index perturbations
We demonstrate the infiltration of inverse opal photonic crystals (iPhC) templates with alumina–titanium oxide films at a very low deposition temperature (95 ◦ C)
All ternary compositions as well as the pure TiO2 thin film showed an increase in the refractive index, which can be related to two factors: on the one hand, heat treatments can promote the removal of common film impurities, such as nitrogen, carbon, as well as hydrogen and could lead to further densification of the thin film
Summary
Photonic crystals and glasses are periodic or disordered arrangements of refractive index perturbations. They are present in natural materials such as bird feathers, butterfly wings, and beetle wing-cases, presenting a so-called structural color. Photonic crystals and glasses based on inversion of colloidal templates ( referred to as colloidal-based porous materials) can be used as sensors, membranes, self-cleaning surfaces, and in batteries and water purification systems [11].
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
