Green-light emission of ZnO nanoparticles spontaneously precipitated in fluorinated polyimide films

The light emitting property of zinc oxide (ZnO) nanoparticles, which were spontaneously precipitated and dispersed in fluorinated polyimide films has been studied because this hybrid material exhibits interesting properties and could be widely applied in optoelectronics and photonics. Polyimides (PI) provide high thermal and chemical stability and outstanding electrical properties. In addition, ZnO presents excellent optical properties due to its wide band gap (3.37 eV) at room temperature and large exciton bonding energy (60 meV), which can be used in light-emitting diodes, transparent electrodes, and piezoelectric devices. By adding small amount of Zinc compounds (5 mol %), either Zinc hexafluoroacetylacetonate dihydrate or Zinc nitrate hexahydrate, to precursor solutions of polyimides followed by thermal curing at 350~390°C, the green light emission at ca. 520 nm of polyimides has been significantly enhanced by 10 to 15 times respectively. When zinc concentration excesses the saturation level, light emission decreases and emission peak was shifted to higher wavelengths due to the aggregation of ZnO. This can be explained by the quantum confinement mechanism and the interaction between the oxygen of ZnO and PI functional groups. Zinc compound precursors and curing temperature and atmosphere affect the light emitting behavior which will be discussed in detail.