Optical processes in SiO2 Sol-Gel Glass Colored with organic dyes

Colored SiO2 coatings were prepared using the sol-gel process. The color was obtained by adding organic dyes to the starting solutions. The dyes incorporated were the Brilliant Blue, Brilliant Black, Fast Green, Yellow 5, Tartrazine, and Erythrozine. It is observed that mechanical treatment of the starting solution using ball milling reduces the size of aggregated dye particles in the coatings. The material obtained reveals an efficient photoluminescence in the visible and infrared regions of the spectrum. Investigations of the optical absorption, luminescence excitation, and emission spectra show that each of these systems is characterized by a well-defined set of discrete electronic energy levels. A relation is found between the level separation and the structure of the colorant’s molecule and also with the doping level. Besides, the size of the molecular aggregates greatly influences the efficiency of light absorption and emission. It is shown that a simple quantum-mechanical description of the system, treating the organic molecule as a two-dimensional potential well, accounts for the observed optical transitions. The results from this simple approach are compared with those obtained using the modified FEMO and LCAO approaches. A reasonable agreement of theory with experiment was obtained. From this work it is concluded that, by using the sol-gel technique, it is possible to produce systems in which nanometer-scale potential wells are embedded in a SiO2 matrix. The discrete energy levels of the wells correspond to the molecular electronic transitions active in the visible region.