Carbon nanotubes and liquid crystals

nanotubes or graphene particles. On the other hand, the organization of nanocarbons as liquid crystals providesnew methodologies for the still challenging structural characterization of such materials.In this talk, we will present a review of our recent results in the preparation and characterization of lyotropicLC made from concentrated aqueous suspensions, stabilized by surfactants, of single-walled carbon nanotubes(SWNT) or reduced graphene oxide (RGO) [1-5]. Homogeneously oriented SWNT LC can be prepared by shearinga drop between two glass plates [1]. The order parameter of SWNT LC was found to be very sensitive to wavinessand entanglement of the nanotubes and the best orientational order was achieved after disentangling, shorteningand sorting the nanotubes using ultrasonication and ultracentrifugation [2]. Anisotropic conductive films ofnanotubes can be prepared by drying the LC. The anisotropy of conductivity of the anisotropic layers can be relatedto the order parameter through a simple analytical model assuming that the electrical resistance in a percolationpath is proportional to the number of contacts [3].Homogeneous orientation of RGO LC was achieved by centrifugating capillaries. Structural and thermodynamiccharacterizations provide statistical information on the dimensions of the graphene flakes, which are found to becomparable with those of neat graphene oxide flakes [4,5].[1] C. Zamora-Ledezma et al., Nano Lett. 8 (2008) 4103-4107 [2] N. Puech et al., J. Phys. Chem. C. 115 (2011) 3272-3278 [3] C. Zamora-Ledezma et al., Phys. Rev. E 85 (2012) 062701(5) [4] C. Zamora-Ledezma et al., J. Phys. Chem.Lett., 3 (2012) 2425-2430 [5] C. Zakri et al., Phil. Trans. R. Soc. A. 371 (2013) 201204995(15)

Most of the current SWNTs production today is used to process bulk composite materials and thin films for exploiting their intrinsic properties at the macroscopic scale. However, the poor control of their distribution and orientation lead to unorganized architectures and limited properties [1]. Besides mechanical properties, electrical and optical properties are the most widely studied. However, various kinds of photoluminescent (PL) nanotube-based composites can be prepared only when the tubes are well dispersed as individuals in the composites [2-4]. Indeed, contacts between nanotubes quench the PL intensity. On the other hand, contacts are required to obtain some electrically conductive materials.In this work we propose and compare different strategies to sort and control the number and nature of contacts in order to control both the PL and electrical (conductivity) properties of SWNTs-based composites [2-4]. These approaches include the control of the quantity of SWNTs, the quantity and nature of the dispersing agents used to disperse them, and their orientational order. We especially work on thin films and thin layers of polymer composites, as well as inorganic gels. Different optical spectroscopy techniques, i.e. absorption, Raman and photoluminescence spectroscopies, are used to study the dispersion and orientation of the nanotubes [2-4]. Electrical measurements are achieved using two and four point probes.[1] Michael F. L. De Volder et al. Science 339, (2013) 535[2] C. Zamora-Ledezma et al, J. Phys. Chem. C, 116 (25), (2012) 13760[3] C. Zamora-Ledezma et al, Phys. Rev. E 84, (2011) 062701[4] C. Zamora-Ledezma et al, Carbon 46, (2008) 1253-1269

In this work, we experimentally investigate the ability of topological defects to guide interfacial assembly of spherical particles with homeotropic anchoring confined to nematic interfaces. We propose two different systems: In the first one, particles are trapped at an air/nematic interface where they spontaneously form various 2D patterns. We demonstrate that the phase transition between these patterns can be controlled by defects formed in the nematic bulk. In the second system, we explore the behavior of particles at the surface of bipolar nematic drops. We found that particles assemble into linear chains and interact with surface defects at the North and South poles of the drop, giving rise to the formation of star structures in a self-assembly process. We detail the mechanism that guides the behavior of particles and discuss the role of defects in the formation of the observed patterns.