Comparison between the UV and X-ray Photosensitivities of Hybrid TiO₂-SiO₂ Thin Layers

The photo-induced effects on sol-gel-based organo TiO₂-SiO₂ thin layers deposited by the dip-coating technique have been investigated using two very different light sources: A light-emitting diode (LED) emitting in the UV (at 365 nm, 3.4 eV) and an X-ray tube producing 40 keV mean-energy photons. The impact of adding a photo-initiator (2,2-dimethoxy-2-phenylacetophenone-DMPA) on the sol-gel photosensitivity is characterized namely in terms of the photo-induced refractive index measured through M-line spectroscopy. Results show that both silica-titania sol-gel films with or without the photo-initiator are photosensitive to both photon sources. The induced refractive index values reveal several features where slightly higher refractive indexes are obtained for the sol-gel containing the photo-initiator. UV and X-ray-induced polymerization degrees are discussed using Fourier-transform infrared (FTIR) spectroscopy where the densification of hybrid TiO₂-SiO₂ layers is related to the consumption of the CH=C groups and to the decomposition of Si-OH and Si-O-CH₃ bonds. X-rays are more efficient at densifying the TiO₂-SiO₂ inorganic and organic network with respect to the UV photons. Hard X-ray photolithography, where no cracks or damages are observed after intense exposition, can be a promising technique to design submicronic-structure patterns on TiO₂-SiO₂ thin layers for the building of optical sensors.