Optimization of the intensity of luminescence emission from silica/poly(ethylene oxide) and silica/poly(propylene oxide) nanocomposite gels

Transparent nanocomposite gels have been synthesized by the sol-gel method using hybrid precursors composed of two triethoxysilane groups and a mid po lyether chain [poly(ethylene oxide) or poly(propylene oxide)] of various ch ain lengths. The end silicate groups are linked with the polyether chain th rough urea bridges (Ureasils). These nanocomposite materials can be visuali zed as silica nanoparticles dispersed in the organic phase provided by poly ether chains. The gels are important room-temperature luminescent materials . Luminescence is the result of delocalized electron-hole recombination pro cesses. The emitting centers are, most probably, located on the surface of silica clusters, where there is a concentration of NH and C=O groups. Large r clusters emit at longer wavelengths than smaller clusters. Precursor mole cules tend to aggregate and they also emit luminescence. Gels obtained by h ydrolysis 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 wa velengths. It has been found that luminescence intensity can be increased b y modifying two major parameters. Shorter polyether chains give samples wit h higher luminescence intensity while larger chains cause a dilution effect that acts against luminescence efficiency. Doping with divalent or trivale nt elemental cations of large atomic number results in an important increas e of luminescence intensity. Heavy cations are then attracted close to the silica cluster surface and enhance luminescence by eliminating surface defe cts.