THERMOCHEMICALLY INDUCED PHOTOLUMINESCENCE IN SOL GEL-DERIVED OXIDE NETWORKS

Chemical and thermal processes can lead to the creation of photoluminescence centers in sol-gel-derived oxide systems. Photosensitivity commences upon heating the gel to approximately 300-350-degrees-C, coinciding with the chemical bond cleavage temperatures, and diminishes with heat treatments about approximately 650-700-degrees-C in air. Photoluminescence absorption peaks occur at approximately 360 and approximately 225 nm (3.4 and 5.5 eV) in binary Al2O3-SiO2. The emission spectra cover the entire visible spectrum, peaking at approximately 425 and approximately 500 nm (2.9-2.5 eV). A strong phosphorescence is also associated with these centers. Luminescence lifetime measurements show relatively long decay time, and indicate at least two mechanisms for the phosphorescence. Electron paramagnetic resonance measurements indicate paramagnetic states which are attributed to paramagnetic states of oxygen, carbon and E' centers. The initial observations are consistent with the model that photosensitivity is related to chemical bond cleavage and resultant carbon formation and/or non-stoichiometry. Coordination states appear to play a significant role in the alumina system.