Large blue shift in the absorption spectra of BEH-PPV films containing gold nanoparticles

We report on a relatively large spectral blue shift observed in the absorption of conjugated polymer films of poly (2,5-bis (2'-ethyl-hexyl)1,4-phenylenevinylene) containing gold colloidal nanoparticles (BEH-PPV/Au). The range of the photoluminescence (PL) emission energy, contrarily, does not change when compared to that of the pure BEH-PPV films. A large broadening effect in the PL peaks was observed for the BEH-PPV/Au film, which was attributed to a larger structural disorder induced in the polymer matrix by the colloidal nanoparticles. Micro-Raman spectroscopy revealed a broadening in the most intense phonon modes, which corroborates the disorder effects in the BEH-PPV/Au film structure. At low temperatures and also at higher excitation intensities, however, the PL vibronic peaks recover their relatively sharp characteristics, indicating a relative suppression of the disorder effects on the optical properties of the BEH-PPV/Au film. Our experimental results support the statement that gold colloidal nanoparticles, interspersed in the polymer matrix, can effectively lower the barrier for a crossover to a structurally disordered phase, resulting in a decrease in the effective conjugation length and consequently leading to a blue shift in the absorption. (C) 2003 Elsevier Science B.V. All rights reserved.