Orientational order of single-wall carbon nanotubes in stretch-aligned photoluminescent composite films
Résumé
Aqueous suspensions of single-wall carbon nanotubes (SWNT) are mixed with solutions of polyvinylalcohol (PVA) and dried to prepare free standing SWNT/PVA composite films (~100 μm thick). For nanotube concentrations up to 1wt %, the composites are homogeneous and the nanotubes are well dispersed both at mesoscopic scales and down to the individual scale as revealed by microscopic pictures and strong photoluminescence (PL) signals, repectively. We study the alignment of SWNT in the composites using polarized Raman and PL spectroscopies. Alignment can be continuously adjusted by hot-stretching the PVA composites. We propose a simple relation to measure the order parameter, based on the assumption that only one single component of the Raman polarizability tensor is non-zero. This assumption is validated experimentally by the observation of similar profiles for spectra measured in different polarization configurations. We modelize the orientation of the nanotubes under uniaxial stretching by a simple affine model assuming deformation at constant volume. For high nanotube concentrations, we discuss the effect of absorption on the Raman/PL results and show how it can be taken into account to measure the order parameter accurately.