This tutorial review highlights the formation of chiral molecular patterns at the liquid–solid interface, revealed at the submolecular level with scanning tunnelling microscopy. It is shown that chiral patterns can be formed by both chiral and achiral molecules. The assembly of mixtures of mirror-image-like molecules gets special attention. Finally, non-standard methods to induce surface chirality in achiral systems are discussed.

Two-dimensional chirality at liquid–solid interfaces

Besides nanostructured materials, individual particles are key elements for nanosciences. The structuring properties of liquid crystals (LCs) are appealing to assemble them, to organize them on substrates or to design functional composites. We present here an overview on particles/LC systems, the size of the dispersed particles being larger than the typical LC length. We first summarize the large number of advances made these last 10 years concerning microparticles assemblies. We then discuss the evolution of the relevant interactions between nanoparticles (NPs) dispersed in LCs when their size decreases from micrometers to nanometers. Various NPs assemblies obtained, either in LC bulk, at interfaces or within LC distorted areas or topological defects are then reported and discussed. Finally, we consider the recent possibilities to use NPs as building elements of complex fluids. We discuss accordingly the LC phases, which can be obtained with pure inorganic NPs in concentrated solution, as well as the sel...

Ordering nano- and microparticles assemblies with liquid crystals

The ability to control light direction with tailored precision via facile means is long-desired in science and industry. With the advances in optics, a periodic structure called diffraction grating gains prominence and renders a more flexible control over light propagation when compared to prisms. Today, diffraction gratings are common components in wavelength division multiplexing devices, monochromators, lasers, spectrometers, media storage, beam steering, and many other applications. Next-generation optical devices, however, demand nonmechanical, full and remote control, besides generating higher than 1D diffraction patterns with as few optical elements as possible. Liquid crystals (LCs) are great candidates for light control since they can form various patterns under different stimuli, including periodic structures capable of behaving as diffraction gratings. The characteristics of such gratings depend on several physical properties of the LCs such as film thickness, periodicity, and molecular orientation, all resulting from the internal constraints of the sample, and all of these are easily controllable. In this review, the authors summarize the research and development on stimuli-controllable diffraction gratings and beam steering using LCs as the active optical materials. Dynamic gratings fabricated by applying external field forces or surface treatments and made of chiral and nonchiral LCs with and without polymer networks are described. LC gratings capable of switching under external stimuli such as light, electric and magnetic fields, heat, and chemical composition are discussed. The focus is on the materials, designs, applications, and future prospects of diffraction gratings using LC materials as active layers.

Dynamic Control of Light Direction Enabled by Stimuli-Responsive Liquid Crystal Gratings.

We used oily streaks defects of smectic films, oriented by the underlying substrates to create chains of nano-particles of diameter 5nm, gold nano-spheres and quantum dots. Combination of Scanning Electronic Microscopy, fluorescence microscopy and micro-specto-photometry evidenced formation of straight chains of hundreds of micrometers, parallel to the oily streaks, They are constituted of single nano-particles of separation ranging from some micrometers (c'est bien cela?) to 1.5nm when the concentration is increased by ten thousands, in agreement with the most stable sites of trapping located within defects cores, not larger than 5nm. A hierarchy of trapping efficiency is demonstrated in relation with the defect nature. When concentration is further increased, single chains are thickened, but still anisotropic, thanks to the presence of trapping sites around the defccts cores. The hybrid systems with gold nano-particles are consequently optically active and the LSPR can be tuned by light polarization, in a matter controllable by the concentration of GNPs.

https://hal.archives-ouvertes.fr/hal-00777951/document

Linear self-assembly of nanoparticles within liquid crystal defect arrays.

This chapter deals with the applications of the density functional (DF) formalism to the study of inhomogeneous systems with hard core interactions It includes a brief tutorial on the fundamentals of the method, and the exact free energy DF for one-dimensional hard rods obtained by Percus The development of DF approximations for the free energy of hard spheres (HS) is presented through its milestones in the weighted density approximation (WDA) and the fundamental measure theory (FMT) The extensions of these approaches to HS mixtures include the FMT treatment of polydisperse systems and the approximations for mixtures with non-additive core radii The DF treatment of non-spherical hard core systems is presented within the generic context of the study of liquid crystals phases The chapter is directed to the potential users of these theoretical techniques, with clear explanations of the practical implementation details of the most successful approximations

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Density Functional Theories of Hard Particle Systems

Through the combination of three different, complementary techniques (optical microscopy, x-ray diffraction and atomic force microscopy), we reveal the deformations inside thin smectic films frustrated between two interfaces imposing antagonistic anchorings. We study the model system, 4-n-octyl-4'-cyanobiphenyl (8CB) between MoS2 and air, which is characterized by the competition between homeotropic anchoring at air and planar unidirectional anchoring on the substrate, with thicknesses varying around 0.3 microm. Optical microscopy and x-ray diffraction demonstrate the continuous topology of smectic layers between the interfaces, which are stacked into periodic flattened hemicylinders. These latter are one-dimensional (1D) focal conic domains which form an optical grating in the smectic film, of a period ranging from 1 to 2.5 microm. The interpretation of our results through an energetic model, associated with the atomic force microscopy (AFM) measurements, shows the presence below a critical thickness of a new type of curvature wall between neighboring hemicylinders.

Optical gratings formed in thin smectic films frustrated on a single crystalline substrate.

We study the structure of very thin liquid crystal films frustrated by antagonistic anchorings in the smectic phase. In a cylindrical geometry, the structure is dominated by the defects for film thicknesses smaller than 150 nm and the detailed topology of the defects' cores can be revealed by x-ray diffraction. They appear to be split in half tube-shaped rotating grain boundaries (RGB). We determine the RGB spatial extension and evaluate its energy per unit length. Both are significantly larger than the ones usually proposed in the literature.

https://hal.archives-ouvertes.fr/hal-00012699/document

Structure of smectic defect cores: x-ray study of 8CB liquid crystal ultrathin films.

We have studied the anchoring directions imposed on 4-n-octyl-${4}^{\ensuremath{'}}$-cyanobiphenyl (8CB) smectic-A and nematic phases by a single crystal of molybdenum disulfide $({\mathrm{MoS}}_{2}).$ Combining optical microscopy and x-ray diffraction under grazing incidence we have demonstrated the occurrence of a bistable planar anchoring. A previous study of the two-dimensional (2D) network of adsorbed 8CB molecules under the liquid crystal film allows a direct connection to be made between the interface structure and the anchoring directions, demonstrating that bistability is induced by the presence of two dipolar groups in the skeleton of the 2D network. It is demonstrated that the Landau--de Gennes theory cannot account for the observed anchoring in the nematic phase. The Landau--de Gennes free energy has to be associated with a coupling with both the surface order and the ${\mathrm{MoS}}_{2}$ substrate to explain the experimental observations. The hypothesis of a nematic layer under the liquid crystal bulk is postulated in the smectic phase.

Bistable nematic and smectic anchoring in the liquid crystal octylcyanobiphenyl (8CB) adsorbed on a MoS2 single crystal.

We have studied by x-ray diffraction the deformations of thin smectic films induced by antagonistic anchorings, as a function of thickness and temperature. The structure of the cores is revealed for thicknesses of the order of 0.15 μm when the deformations are dominated by the presence of the focal conics cores. In a cylindrical geometry imposed by the unidirectional anchoring on the substrate, the cores are tube-shaped rotating grain boundaries (RGB). The spatial extension is of the order of 0.14 × 0.22 μm2, larger than the ones usually proposed in the literature. A drastic evolution of the x-ray scans with temperature is also revealed. It corresponds to a variation of the location of the RGB and is analyzed as the result of a weak variation of the 8CB/air surface tension with temperature.

Marc de Boissieu

Papers

Optical gratings formed in thin smectic films frustrated on a single crystalline substrate.

Structure of smectic defect cores: x-ray study of 8CB liquid crystal ultrathin films.

Bistable nematic and smectic anchoring in the liquid crystal octylcyanobiphenyl (8CB) adsorbed on a MoS2 single crystal.

Revealing the Structure of Focal Conics Cores and their Influence on the Evolution with Temperature: An X-ray Study of Ultra-thin 8CB Films