Photoconductivity and optical properties of PPV and single-walled carbon nanotubes

We present photoconductivity and Raman scattering data obtained from composite films of poly(paraphenylene vinylene) (PPV) and single-walled carbon nanotubes at different weight concentrations from 0% to 64%. It is found that the introduction of nanotubes in the PPV precursor polymer solution, heated at 300 degrees C to perform conversion into PPV, yields drastic modifications in both the structural features of the composite components and in the electronic properties of the composites. The PPV polymer matrix becomes more disordered due to the introduction of nanotubes which induce a shortening of the polymer conjugated segments as shown by Raman scattering spectra. In addition, these spectra yield information about the evolution from small bundles to thick bundles of single-walled nanotubes as function of their concentration x. Photoconductivity data show that the percolation regime begins at x=2%, indicating that a migration network for the photogenerated charges is established above this threshold. By using a model based on distributions of PPV conjugated lengths and their changes as function of x, we calculate the Raman scattering band shapes and their relative intensities. The theoretical results lead to a comprehensive interpretation of experimental data.