Unique orientation of 1D and 2D nanoparticle assemblies confined in smectic topological defects.

New collective optical properties have emerged recently from organized and oriented arrays of closely packed semiconducting and metallic nanoparticles (NPs). However, it is still challenging to obtain NP assemblies which are similar everywhere on a given sample and, most importantly, share a unique common orientation that would guarantee a unique behavior everywhere on the sample. In this context, by combining optical microscopy, fluorescence microscopy and synchrotron-based grazing incidence X-ray scattering (GISAXS) of assemblies of gold nanospheres and of fluorescent nanorods, we study the interactions between NPs and liquid crystal smectic topological defects that can ultimately lead to unique NP orientations. We demonstrate that arrays of one-dimensional - 1D (dislocations) and two-dimensional - 2D (grain boundaries) topological defects oriented along one single direction confine and organize NPs in closely packed networks but also orient both single nanorods and NP networks along the same direction. Through the comparison between smectic films associated with different kinds of topological defects, we highlight that the coupling between the NP ligands and the smectic layers below the grain boundaries may be necessary to allow for fixed NP orientation. This is in contrast with 1D defects, where the induced orientation of the NPs is intrinsically induced by the confinement independently of the ligand nature. We thus succeeded in achieving the fixed polarization of assemblies of single photon emitters in defects. For gold nanospheres confined in grain boundaries, a strict orientation of hexagonal networks has been obtained with the 〈10〉 direction strictly parallel to the defects. With such closely packed and oriented NPs, new collective properties are now foreseen.

From Chains to Monolayers: Nanoparticle Assembly Driven by Smectic Topological Defects.

In this Letter, we show how advanced hierarchical structures of topological defects in the so-called smectic oily streaks can be used to sequentially transfer their geometrical features to gold nanospheres. We use two kinds of topological defects, 1D dislocations and 2D ribbon-like topological defects. The large trapping efficiency of the smectic dislocation cores not only surpasses that of the elastically distorted zones around the cores but also surpasses the one of the 2D ribbon-like topological defect. This enables the formation of a large number of aligned NP chains within the dislocation cores that can be quasi-fully filled without any significant aggregation outside of the cores. When the NP concentration is large enough to entirely fill the dislocation cores, the LC confinement varies from 1D to 2D. We demonstrate that the 2D topological defect cores induce a confinement that leads to planar hexagonal networks of NPs. We then draw the phase diagram driven by NP concentration, associated with the sequential confinements induced by these two kinds of topological defects. Owing to the excellent large-scale order of these defect cores, not only the NP chains but also the NP hexagonal networks can be oriented along the desired direction, suggesting a possible new route for the creation of either 1D or 2D highly anisotropic NP networks. In addition, these results open rich perspectives based on the possible creation of coexisting NP assemblies of different kinds, localized in different confining areas of a same smectic film that would thus interact thanks to their proximity but also would interact via the surrounding soft matter matrix.

Colloidal Particles in Thin Nematic Wetting Films.

We experimentally and theoretically study the variety of elastic deformations that appear when colloidal inclusions are embedded in thin wetting films of a nematic liquid crystal with hybrid anchoring conditions. In the thickest films, the elastic dipoles formed by particles and their accompanying defects share features with the patterns commonly observed in liquid crystal cells. When the film gets thinner than the particles size, however, the capillary effects strongly modify the appearance of the elastic dipoles and the birefringence patterns. The influence of the film thickness and particles sizes on the patterns has been explored. The main experimental features and the transitions observed at large scale-with respect to the inclusions' size-are explained with a simple two-dimensional Ansatz, combining capillarity and nematic elasticity. In a second step, we discuss the origin of the variety of observed textures. Developing a three-dimensional Landau-de Gennes model at the scale of the particles, we show that the presence of free interfaces and the beads confinement yield metastable configurations that are quenched during the film spreading or the beads trapping at interfaces.

Capillary-induced giant elastic dipoles in thin nematic films.

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.