Raman studies of solutions of single-wall carbon nanotube salts.

Polyelectrolyte solutions of Na-doped single-wall carbon nanotube (SWNT) salts are studied by Raman spectroscopy. Their Raman signature is first compared to undoped SWNT suspensions and dry alkali-doped SWNT powders, and the results indicate that the nanotube solutions consist of heavily doped (charged) SWNT. Raman signature of doping is then used to monitor in situ the oxidation reaction of the nanotube salt solutions upon exposure to air and to an acceptor molecule (benzoquinone). The results indicate a direct charge-transfer reaction from the acceptor molecule to the SWNT, leading to their gradual charge neutralization and eventual precipitation in solution. The results are consistent with a simple redox titration process occurring at the thermodynamical equilibrium.

Influence of structure on the optical limiting properties of nanotubes.

We investigate the role of carbon nanotubes structure on their optical limiting properties. Samples of different and well-characterized structural features are studied by optical limiting and pump-probe experiments. The influence of the diameter's size on the nano-object is demonstrated. Indeed, both nucleation and growth of gas bubbles are expected to be sensitive to diameter.

Structure and texture of anisotropic nematic gels.

Anisotropic nematic gels are prepared via in situ polymerization of diacrylate monomers in an oriented nematic liquid crystal (LC) matrix. The structure of the gels is studied from micrometer to nanometer scales by optical microscopy, small angle neutron scattering, and theta/2theta light scattering. A strong anisotropy is evidenced at all scales without electric field for both mesogenic and nonmesogenic monomers. The gel network can be pictured as an ordered but strongly distorted and polydisperse structure with two characteristic sizes: the mean size of the polymer objects and a correlation length between these objects, corresponding to the mean-size of the LC domains, which are estimated from neutron and light scattering results to be of the order of some tens of nanometers and some micrometers, respectively. Moreover, a sheet-like structure of the polymer network is evidenced. When an electric field is applied, one part of the LC switches while the other part remains anchored to the polymer network. The electric field dependence of the volume fraction of anchored LC is estimated from the analysis of the light scattering data. We emphasize systematic correlations between structure and electro-optical properties of the gels.

Raman investigation of stable and metastable states of 4-octyl-4'-cyanobiphenyl confined in porous silica matrices.

We study the polymorphism of solid phases of 4-octyl-4(')-cyanobiphenyl (8CB) by Raman spectroscopy. For bulk 8CB, the Raman spectrum of the CN stretch is featured by a single peak, which shifts abruptly at the smectic-A-crystal transition. In confinement, the CN peak splits both at high and low temperatures. In the isotropic and liquid crystal phases, the signature of the liquid crystal bulk (LC) coexists with another peak that is assigned to LC molecules interacting with the matrice interface. We find correlations between the volumic fractions of interfacial liquid and the texture of the matrices. At low temperatures, we assign the splitting of the CN peak to the coexistence of different metastable solid phases. For strong confinements, the temperature dependence of the CN stretching frequency extends to that of the liquid, which suggests the existence of frozen-in smecticlike solid phases. We discuss the structure of these metastable solid phases in the light of neutron diffraction measurements. We also report on the peculiar analogy between the effect of quenched disorder due to the porous matrices and the effect of thermal quenching.

Solid state polymorphism of liquid crystals in confined geometries.

Solid polymorphism of 4-alkyl-4'-cyanobiphenyl (nCB) was studied so far as a function of thermal history. In this paper we show that metastable solid phases of 4-octyl-4'-cyanobiphenyl (8CB) are also formed when the mesogens are confined in porous silica matrices and we study their structure by neutron diffraction and by Raman spectroscopy. Three metastable solid states are identified: one crystalline phase K', two frozen-in smectic-like phases K(s) and K'(s). We discuss the relation between the structure of the metastable solid phases and that of the mesomorph phases.

Pulse duration and wavelength effects on the optical limiting behavior of carbon nanotube suspensions.

We investigate pulse duration and wavelength effects on the optical limiting behavior of single-wall carbon nanotubes suspended either in chloroform or in water. The principal optical limiting effect in carbon nanotube suspensions is nonlinear scattering that is due to heat transfer from particles to solvent, leading to solvent-bubble formation and to sublimation of carbon nanotubes. We report on nonlinear transmittance measurements for pulse durations ranging from 3 to 100 ns and for wavelengths from 430 to 1064 nm. The dependence of optical limiting behavior on pulse duration and wavelength is analyzed and discussed in terms of nonlinear mechanisms.

Switching behavior and electro-optical properties of liquid crystals in nematic gels

Anisotropic nematic gels are prepared via in situ polymerization of diacrylate monomers in an orientated nematic liquid crystal (LC) matrix. The switching behavior of the LC molecules under electric field is probed in polarized Raman spectroscopy and straight theta-2straight theta elastic light scattering experiments. The electro-optical characteristics of the gels are directly related to the electric field dependence of the fraction of switched molecules. The electro-optical contrast relates to the coexistence of switched LC domains and LC domains anchored to the polymer network.

Phonon density of states of single-wall carbon nanotubes

The vibrational density of states of single-wall carbon nanotubes (SWNT) was obtained from inelastic neutron scattering data from 0 to 225 meV. The spectrum is similar to that of graphite above 40 meV, while intratube features are clearly observed at 22 and 36 meV. An unusual energy dependence below 10 meV is assigned to contributions from intertube modes in the 2D triangular lattice of SWNT bundles, and from intertube coupling to intratube excitations. Good agreement between experiment and a calculated density of states for the SWNT lattice is found over the entire energy range.