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Role of silver nanoparticles in the laser-induced reversible colour-marking and controlled crystallization of mesoporous titania films

Due to their surface plasmon resonance silver nanoparticles are known to absorb visible light and give glasses various colors. Grown in mesoporous titania films, they give the material a photochromic behaviour that can be used to produce rewritable data carriers. On the one hand, UV light forms silver nanoparticles thanks to the photo-induced generation of electrons by titania matrix. On the other hand, visible light oxidizes the silver nanoparticles via the photoexcitation of electrons on Ag and their stabilization by oxygen molecules. The well controlled porosity of the mesoporous films allows to tune the nanoparticles size and to obtain, under UV illumination, homogenous distributions of small nanoparticles embedded within the titania matrix, which color the films. As all nanoparticles absorb light similarly, the film can then be completely bleached under exposure to a visible laser beam whose wavelength falls in the SPR band of the particles. Therefore, CW UV and visible focused-laser radiations, respectively, can repeatedly print and completely erase colored micropatterns within TiO2/Ag films. The paper shows patterns printed under different conditions, deals with the reproducibility of the process and the inscription stability, and explains the nanoscale mechanisms, including silver migration during exposures, leading to the reversible color changes on the basis of TEM, SEM, absorption spectroscopy and Raman micro-spectroscopy characterizations. This paper also evidences that CW laser illuminations at higher intensity locally crystallize the titania matrix and investigates the influence of the absorption-induced heating around nanoparticles.