Optical properties of poly(para-phenylene vinylene) and single-walled carbon nanotube composite films : effects of conversion temperature, precursor dilution, and nanotube concentrations

Optical absorption, photoluminescence, and Raman scattering spectra of poly(para-phenylene vinylene) (PPV) and single-walled carbon nanotube (SWNT) composite films are investigated at room temperature. Samples have been prepared at different precursor conversion temperatures, T-c, (300, 180,and 120 degrees C) and with SWNT mass concentrations from x = 0% up to 64%. In each sample, we observe drastic changes in all optical absorption spectra of PPV and composite films. In particular, after conversion at T, = 120 degrees C, PPV samples exhibit photoluminescence (PL) with a new feature at about 2.55 eV together with less-intense ones at about 2.37 and 2.20 eV, respectively. The most-intense at 2.55 eV is due to a radiative recombination on the shorter conjugated segments and interpreted from a theoretical model based on a distribution of conjugated lengths. This distribution, which allows an assignment of all PL peaks, is also able to explain all experimental data including Raman scattering and optical absorption spectra in a given sample. Also, further changes in PL and optical absorption spectra are observed by increasing the SWNT concentration in composite films converted at the same temperature. We have also investigated the effect of the dilution of the precursor polymer solution. From the theoretical analysis of the optical absorption, PL, and resonance Raman spectra, we show that PPV samples are characterized by a decrease of the effective conjugation lengths when the precursor dilution increases. All experimental data are explained well with a bimodal distribution model reflecting an effective inhomogeneity in the polymer as suggested already from morphological pictures issued in particular from X-ray data.