Local annealing of Ag-TiO2 nanocomposite films with plasmonic response by CW UV laser scanning

Semiconductor sol-gel films containing plasmonic nanoparticles are increasingly used in wet analytics (µ-TAS systems) as functional substrates for SERS, as optical elements, photovoltaic and photocatalytic devices. A local change in the structure of such materials with predictable properties of the modified region opens up new possibilities for the creation of integrated circuits and multifunctional systems. Here we considered the mechanism of local modification of TiO₂ thin films structure containing plasmon nanoparticles as a result of laser annealing. The material processing was carried out by scanning with a CW semiconductor laser at a wavelength of 405 nm and at radiation intensity from 35 kW/cm² to 85 kW/cm². The modification region differed in optical characteristics and structural features from the original film. As a result of the laser processing, a heat source was formed that ensured the crystal nucleation and growth of brookite up to an intensity of 55.4 kW/cm². A subsequent increase in intensity led to the transformation of brookite into anatase. The crystal phase formation in the obtained track was accompanied by a change in the relief in its cross section and a decrease of the plasmon resonance peak. The density of the film in the modified region increased, which was accompanied by a decrease in its thickness by 20% from the original film thickness. The disappearance of plasmon resonance in the modified region contributed to a decrease in the absorption capacity and, as a consequence, to a sharp decrease in temperature at the central part of the heat source.