Synthetic Metals 139 (2003) 283–286 Large blue shift in the absorption spectra of BEH-PPV films containing gold nanoparticles L.A. Cury ∗ , L.O. Ladeira, A. Righi Departamento de F´ ısica, Instituto de Ciˆ encias Exatas, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais C.P. 702 30123-970, Brazil Received 9 August 2002; received in revised form 21 January 2003; accepted 4 March 2003 Abstract 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. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Conjugated polymers; BEH-PPV; Absorption blue shift; Photoluminescence; Gold colloidal nanoparticles 1. Introduction Conjugated polymers, due to their mechanical and optical properties, have been extensively explored as active media in light-emitting [1] and laser [2] devices. A new branch of research in hybrid polymer systems containing titanium dioxide [3] or gold [4] colloidal nanoparticles is relatively more recent. As described in [3], scattering off the ran- domly distributed, high index of refraction titanium dioxide nanoparticles, increases the path length traversed by the emitted light, allowing a larger stimulated emission gain if the medium is pumped above threshold and the scattering length exceeds the gain length. The inclusion of relatively small gold nanoparticles in polythiophene derivatives [4] leads to the formation of circular fluorescence centers of pure polymer dispersed throughout the film, which are sur- rounded by regions with a higher density of nanoparticles. From the nucleus of these centers to the periphery, a large blue shift is observed in the spatially resolved fluorescence spectra. The correlation between the spatially resolved fluo- ∗ Corresponding author. E-mail address: cury@fisica.ufmg.br (L.A. Cury). rescence and TEM observations indicates that the blue shift is controlled by the Au nanoparticles distribution. In this letter, we focus on the optical properties of poly- (2,5-bis(2 ′ -ethyl-hexyl)-1,4-phenylenevinylene) (BEH-PPV) films containing relatively large gold colloidal nanoparti- cles. This study partially resolves the question of the role of gold nanoparticles in a conjugated polymer matrix, in particular in a PPV derivative matrix. Based on our experi- mental results, we interpret the relatively large blue shift in the absorption characteristics of the BEH-PPV/Au film as a consequence of disorder effects induced by these metal nanoparticles in the polymer environment. Micro-Raman measurements clearly show a broadening of the more in- tense phonon mode lines, which is a signature of the dis- order effect in our BEH-PPV/Au sample. In addition, our photoluminescence (PL) results show a visible inhomoge- neous broadening of the spectra and an effective loss in the emission efficiency for the BEH-PPV/Au film, which are also related to the disorder effects induced by the gold nanoparticles. Based on our experimental results and on [4,5], we interpret the observed blue shift as a result of the disordered phase in the BEH-PPV/Au film, leading to a decrease in the effective conjugation length, with a reduced delocalization of the -electrons, compared to the pure 0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00169-3