Optimization of the Intensity of Luminescence Emission from Silica/Poly(ethylene oxide) and Silica/Poly(propylene oxide) Nanocomposite Gels

Abstract

Transparent nanocomposite gels have been synthesized by the sol−gel method using hybrid precursors composed of two triethoxysilane groups and a mid polyether chain [poly(ethylene oxide) or poly(propylene oxide)] of various chain lengths. The end silicate groups are linked with the polyether chain through urea bridges (Ureasils). These nanocomposite materials can be visualized as silica nanoparticles dispersed in the organic phase provided by polyether chains. The gels are important room-temperature luminescent materials. Luminescence is the result of delocalized electron−hole recombination processes. The emitting centers are, most probably, located on the surface of silica clusters, where there is a concentration of NH and CO groups. Larger clusters emit at longer wavelengths than smaller clusters. Precursor molecules tend to aggregate and they also emit luminescence. Gels obtained by hydrolysis in the presence of NH4F favor larger cluster formation than gels obtained by hydrolysis in the presence of HCl and tend to emit at longer wavelengths. It has been found that luminescence intensity can be increased by modifying two major parameters. Shorter polyether chains give samples with higher luminescence intensity while larger chains cause a dilution effect that acts against luminescence efficiency. Doping with divalent or trivalent elemental cations of large atomic number results in an important increase of luminescence intensity. Heavy cations are then attracted close to the silica cluster surface and enhance luminescence by eliminating surface defects.