Metal-oxide nanoparticles dispersed in a polyimide

Abstract

We developed a simple method of fabricating metal-oxide nanoparticles by reacting a polyamic acid (PAA) with metal films. Polyamic acid (PAA) in N-methyl-2-pyrrdione was spin-coated on Cu, Zn, Sn, or Fe films (5-30 nm). The samples were baked at 135℃ for 30 min to remove the solvent and then fully cured to form polyimide(PI) in N2 atmosphere at 400℃ for 1 hour. Particle size, distribution, and morphology were characterized by transmission electron microscopy (TEM). The diameter of metal-oxide particles formed during curing was 9-30 nm. The electron diffraction patterns were indexed as cubic Cu2O, hexagonal ZnO, tetragonal SnO2, and cubic ?-Fe2O3. Cu2O or ZnO particles were relatively uniform in size and randomly dispersed within the polyimide. On the other hand, SnO2 and ?-Fe2O3 particles were observed only at the interface between PI and substrate. The SnO2 particles showed a bimodal particle size distribution. Below the Fe thickness of 7 nm, curing broke up the Fe film into fine oxide particles whose size was ~10 nm with narrow size distribution. The difference in the particle distributions was ascribed to the degree of reactivity between thin metal film and PAA. Cu and Zn reacted with PAA and were completely dissolved in the PAA. During curing, polyamic acid will dehydrates and imidizes, forming polyimide, accompanied by precipitation of Cu2O or ZnO particles. In case of SnO2 and ?-Fe2O3 particles, the coated polyamic acid attacks the metal surfaces through the grain boundaries. This reaction drives the continuous etching of the metal surface and the polyamic acid reacts through the grain boundaries of Sn or Fe and laterally separates the continuous metal layer into particles. The effect of metal thickness on the oxide nanoparticle formation was also discussed.