Nanoparticles (NPs) can now be synthesized with a wide array of controlled sizes, shapes, and properties. However, turning them into nanomaterials often requires packing them into ordered assemblies to manifest specific electronic or optical properties for applications in nanoelectronics, optics, and metamaterials. Colloidal self-assembly (1) of NPs is relatively simple but is often restricted to high-symmetry crystals by the lack of specific directional bonds, especially for dilute NP solutions. To obtain lower symmetries that confer useful optical or electronic properties, long-range directional interactions must be imparted. On page 69 of this issue, Mundoor et al. (2) make clever use of an anisotropic host fluid, a liquid crystal, to promote the formation of a low-symmetry crystal in a dilute dispersion of nanorods.

We review the optical spectroscopy signatures of carbon nanotubes and graphene dispersed in aqueous suspensions.

Processing single-walled carbon nanotubes (SWNT) into thin films or composite nanomaterials, are key issues to take advantage of their mechanical, electrical and optical properties. In this talk, we will review our recent results on the preparation, structure and optical properties of SWNT-based inks, nanocomposites and thin films.

Strategic approaches for dispersing and/or orienting SWNT in films and composites are essential for controling and optimizing their properties [1,2]. Dispersion in a liquid phase is the most efficient way to exfoliate the bundles into individual SWNT. Additives, most often surfactants, have to be used because of the poor solubility of SWNT in all solvents. However, the presence of surfactants in the final films/composites can drastically change their properties. In this talk, we show that a simple dialysis process opens a new route for stabilizing aqueous inks with high concentrations of individual SWNT without surfactant, and for preparing surfactant-free polymer-based composites [3]. Coupled Raman/ photoluminescence spectroscopy studies allow to probe in situ both the dispersion of the nanotubes and the nature of the molecules adsorbed on their surface. We demonstrate a spontaneous exchange between surfactant molecules (bile salts) and polyvinyl alcohol (PVA) chains at the surface of the nanotubes, and a forced exchange by dialysis for both PVA and polyvinyl pyrrolidone (PVP) [3].
On the other hand, we study the processing of SWNT aqueous inks into patterned deposits by inkjet printing. We show that a good control of both the morphology of inkjet printing lines and of the alignment of SWNT in these lines can be achieved by tuning the SWNT concentration, inter-drop distance and substrate temperature. Using polarized micro-Raman spectroscopy, we present an original method to determine the spatial distribution of nanotubes in the deposits as well as their orientation. We show that two types of line morphologies can be obtained: homogeneous deposits or pairs of ‘‘railway tracks’’. Highly oriented SWNT patterns can be observed in the “railways tracks” and at the edge of the homogeneous lines.
References:
[1] Ph. Avouris. Acc. Chem. Res. (2002) 35, 1026
[2] P. M Ajayan et al, Nature. (2007) 447, 1066
[3] F. Torres-Canas et al, J. Phys. Chem. C 119, 703 (2015)
[4] F. Torres-Canas et al, in preparation

In this work, we develop the concept of evaporation-induced self-structuring as a novel approach forproducing organised films by exploiting cooperative physical and chemical interactions under far-fromequilibriumconditions (spin-coating), using sol–gel precursors with multiple functional groups. Thin filmsof self-structured silsesquioxane nanohybrids have been deposited by spin coating through the sol–gelhydrolysis and condensation of a bridged organosilane bearing self-assembling urea groups. The resultingnanostructure, investigated by FTIR, AFM and SEM, is shown to be highly dependent on the catalyst used(nucleophilic or acidic), and can be further modulated by varying the spinning rate. FTIR studies revealed thepresence of highly organised structures under acidic catalysis due to strong hydrogen bonding between ureagroups and hydrophobic interactions between long alkylene chains. The preferential orientation of the ureacross-links parallel to the substrate is shown using polarized FTIR experiments.

Active colloids are an emerging and promising class of colloidal particles, which are designed to perform autonomous motion as biological microswimmers by transforming chemical or other form of energies into work and propulsion. When the particle size is about a micrometer or smaller, the directional propulsion competes with the translational and rotational Brownian motion. Here we explore the possibility of enhancing the directional motion of self-propelled Janus colloids by slowing down their rotational diffusion.We have investigated the active motion of self-propelled colloids confined at the air-water interface. The two dimensional motion of micron-sized Silica-Platinum Janus colloids has been experimentally measured by particle tracking video-microscopy under increasing concentration of the catalytic fuel, i.e. H2O2. Comparing to the motion in bulk, a dramatic enhancement of both the persistence length of trajectories and the speed has been observed. The interplay of colloid self-propulsion, due to an asymmetric catalytic reaction occurring on the colloid, and interfacial frictions controls the enhancement of the directional movement. The slowing down of the rotational diffusion at the interface, also measured experimentally, plays a pivotal role in the control and enhancement of active motion.

Directed and true self-assembly mechanisms in nematic liquid crystal colloids rely on specific interactions between microparticles and the topological defects of the matrix. Most ordered structures formed in thin nematic cells are thus based on elastic multipoles consisting of a particle and nearby defects. Here, we report, for the first time to our knowledge, the existence of giant elastic dipoles arising from particles dispersed in free nematic liquid crystal films. We discuss the role of capillarity and film thickness on the dimensions of the dipoles and explain their main features with a simple 2D model. Coupling of capillarity with nematic elasticity could offer ways to tune finely the spatial organization of complex colloidal systems.