Optical processes in SiO2 sol-gel glass colored with organic dyes

Colored SiO2 coatings were prepared using the sol-gel process. The color wa s obtained by adding organic dyes to the starting solutions. The dyes incor porated were the Brilliant Blue, Brilliant Black, Fast Green, Yellow 5, Tar trazine, and Erythrozine. It is observed that mechanical treatment of the s tarting solution using ball milling reduces the size of aggregated dye part icles in the coatings. The material obtained reveals an efficient photolumi nescence in the visible and infrared regions of the spectrum. Investigation s of the optical absorption, luminescence excitation, and emission spectra show that each of these systems is characterized by a well-defined set of d iscrete electronic energy levels. A relation is found between the level sep aration and the structure of the colorant's molecule and also with the dopi ng 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 tra nsitions. The results from this simple approach are compared with those obt ained using the modified FEMO and LCAO approaches. A reasonable agreement o f 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 discret e energy levels of the wells correspond to the molecular electronic transit ions active in the visible region.