Unique luminescence properties of rare earth ion-doped polyimide nanoparticles and investigation of their mechanism

We fabricated the rare earth ion (Eu³⁺, Tb³⁺)-doped polyimide (PI) nanoparticles by reprecipitation method. The nanoparticles showed unique luminescence properties. It was observed that luminescence intensity assigned to rare earth ion increased with increase in UV-irradiation time. In contrast, the intensity decreased with elevation of heat-treatment temperature and the luminescence vanished after treatment at 200°C. Broad luminescence bands ascribed to a matrix were observed and these intensities were also varied by UV-irradiation and heat-treatment. These luminescence intensities were kept for several months. The excitation spectra indicated that energy transfer from a matrix to rare earth ion occurred. The above phenomena are explained that a reversible change in the intensity of luminescence ascribed to a matrix results in that assigned to rare earth ion through energy transfer. IR spectra showed generation of OH groups after UV irradiation. It is considered that COOH groups were introduced into matrix by UV-irradiation and rare earth ion coordinated to COOH group and energy transfer from a matrix to rare earth ion was allowed.