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.

Carbon nanotubes exhibit exceptional physical properties at the individual scale. In order to optimize these properties at the macroscopic scale, a good control of both the dispersion and alignment of the nanotubes is required. In this work we study the dispersion and alignment of single-wall carbon nanotubes in lyotropic nematic suspensions and we use these suspensions for the elaboration of anisotropic thin films. Individual nanotubes are stabilized by denaturated DNA in aqueous suspensions. Lyotropic nematic phases are prepared by concentrating nanotubes/DNA aqueous suspensions and the nematic is shear-aligned in thin cells. At each step of the preparation, the dispersion and alignment are studied by polarized optical microscopy, photoluminescence and Raman measurements. We show that thin homogeneous anisotropic films can be prepared from wet aligned suspensions. In a first series of experiments, the nematic and the corresponding films exhibited a quite weak order parameter (S about 0.2) [1], which was assigned to some entanglements of the nanotube network. In this talk, we will show that the order parameter of the nematic and the corresponding films can be significantly improved by controling the aspect ratio and/or the bundling of the nanotubes, for example using fractionation through controled ultra-centrifugation during the nematic phase formation. [1] C. Zamora-Ledezma, C. Blanc, M. Maugey, C. Zakri, P. Poulin, E. Anglaret. Nanoletters 8 (12), 4103-4107, (2008).

In situ Raman measurements were used to investigate the kinetics and the self-deactivation of the growth of single-walled carbon nanotubes during catalytic chemical vapor deposition. The kinetics appear controlled by the mass-transport of the gaseous precursor at low precursor pressure and high temperature and by the catalytic decomposition of the precursor at high precursor pressure and low temperature. The initial growth rate and the lifetime display inversely correlated evolutions with the growth parameters. In addition, we measured the activation energy for the healing of defects during the growth and discuss it in comparison to the apparent activation energies measured for the initial growth rate and the lifetime. Our results support that the healing of the edge defects controls both the crystalline order and the growth lifetime.

We study the alignment of single-wall carbon nanotubes in polyvinylalcohol composites using polarized Raman and photoluminescence spectroscopies. Nanotubes are well dispersed down to the individual scale in the composites as revealed by the strong photoluminescence signal. Alignment can be continuously adjusted by hot stretching the 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 nonzero. The similar profiles of spectra measured in different polarization configurations validate this assumption. The orientation of the nanotubes under uniaxial stretching is well modelized by a simple affine model assuming deformation at constant volume.