Synthesis, optical and electrical characterization of organically soluble silver nanoparticles and its poly(3-hexylthiophene) nanocomposites : enhanced luminescence property in the nanocomposite thin films

Abstract

The hexadecyl amine capped Ag nanoparticles in chloroform are prepared by reducing [Ag(hexadecyl amine)₂]NO₃ complex with sodium borohydride in a chloroform/water medium. Poly(3-hexyl thiophene) (P3HT)-silver nanoparticle composites at five different compositions [Ag10, Ag25, Ag50, Ag75, and Ag90; the number indicates percentage (w/w) of silver in the composite] are prepared by mixing the chloroform solutions of P3HT and hexadecyl amine capped Ag nanoparticles at appropriate proportions. Thin films (average thickness ~50 nm) of the composites were prepared by spin-coating the above homogeneous solutions at 2000 rpm. The UV-vis spectra of the composite solutions exhibit a homogeneous mixing of the components as an intermediate absorption peak of those of the plasmon band of Ag and π-π∗ transition band of P3HT are observed in all cases. But the thin films retain the individual plasmon peak of Ag nanoparticle and the π-π∗ transition band of P3HT, showing a small red shift in both peaks at certain compositions. The TEM pictures indicate hexagonal array of Ag nanoparticles in the pure hexadecyl amine capped Ag nanoparticle film, but in the composites the Ag nanoparticles are distributed homogeneously. The histograms indicate an increase in nanoparticle size with increasing nanoparticle concentration and AFM study indicates cluster formation by organization of P3HT lamella surrounding the Ag nanoparticles. The photoluminescence spectra in the solution state shows a continuous decrease of luminescence intensity with increase in Ag nanoparticle concentration but the thin films of the nanocomposite indicate a significant (~4 times for Ag50 and on considering P3HT concentration only it is an 8 times increase compared to that of pure P3HT) increase in fluorescence intensity. A small blue shift of the luminescence band of P3HT with increasing Ag nanoparticle concentration is also observed possibly for the coupling of plasmon vibration with the electronic levels of the excitons of P3HT. The pure hexadecyl amine capped Ag nanoparticles thin film exhibits electrical bistability, signifying switching effect in the system but the nanocomposite thin films do not show these characteristics. They show a sharp increase in current at a threshold voltage, which increase with an increase in Ag nanoparticle concentration except for the Ag75 composite. The characteristics of I-V curves of the nanocomposites promise the probable use of nanocomposite thin films as efficient light-emitting diode compared to that of pure P3HT.