Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micro...

Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials

Recent advances and challenges in designing stimuli-responsive polymers

The ability to generate regular spatial arrangements of particles is an important technological and fundamental aspect of colloidal science We showed that colloidal particles confined to a few-micrometer-thick layer of a nematic liquid crystal form two-dimensional crystal structures that are bound by topological defects Two basic crystalline structures were observed, depending on the ordering of the liquid crystal around the particle Colloids inducing quadrupolar order crystallize into weakly bound two-dimensional ordered structure, where the particle interaction is mediated by the sharing of localized topological defects Colloids inducing dipolar order are strongly bound into antiferroelectric-like two-dimensional crystallites of dipolar colloidal chains Self-assembly by topological defects could be applied to other systems with similar symmetry

https://science.sciencemag.org/content/sci/313/5789/954.full.pdf

Two-Dimensional Nematic Colloidal Crystals Self-Assembled by Topological Defects

Colloidal particles may be considered as building blocks for materials, just like atoms are the bricks of molecules, macromolecules, and crystals. Periodic arrays of colloids (colloidal crystals) have attracted much interest over the last two decades, largely because of their unique photonic properties. The archetype opal structures are based on close-packed arrays of spheres of submicrometer diameter. Interest in structuring materials at this length scale, but with more complex features and ideally by self-assembly processes, has led to much progress in controlling features of both building blocks and assemblies. The necessary ingredients include colloids, colloidal clusters, and colloidal "molecules" which have special shapes and the ability to bind directionally, the control over short-range and long-range interactions, and the capability to place and orientate these bricks. This Review highlights recent experimental and theoretical progress in the assembly of colloids larger than 50 nm.

Colloidal Assembly: The Road from Particles to Colloidal Molecules and Crystals

https://orbilu.uni.lu/bitstream/10993/20503/1/Lagerwall%20%26%20Scalia%202012.pdf

A new era for liquid crystal research: Applications of liquid crystals in soft matter nano-, bio- and microtechnology

We describe the experimental observation of a crystal structure formed by glycerol droplets suspended in a nematic liquid crystal. The structure exhibits a high density hexagonal ordering. We have experimentally observed a noticeable interaction between droplets with tangential boundary conditions. Within the scope of known models we discuss the nature of appropriate mechanisms of the interaction.

Crystal structure in nematic emulsion.

Colloidal crystals are 3D periodic structures formed from small colloidal particles as basic building blocks and exhibit unique optical and electronic properties. Nych et al. report a laser controlled assembly of 3D colloidal crystals, which can be compressed and rotated in a collective manner.

/pdf/assembly-and-control-of-3d-nematic-dipolar-colloidal-20r7ndak7h.pdf

Assembly and control of 3D nematic dipolar colloidal crystals

We demonstrate a variety of ordered patterns, including hexagonal structures and chains, formed by colloidal particles (droplets) at the free surface of a nematic liquid crystal (LC). The surface placement introduces a new type of particle interaction as compared to particles entirely in the LC bulk. Namely, director deformations caused by the particles lead to distortions of the interface and thus to capillary attraction. The elastic-capillary coupling is strong enough to remain relevant even at the micron-scale when its buoyancy-capillary counterpart becomes irrelevant.

/pdf/ordered-droplet-structures-at-the-liquid-crystal-surface-and-4glnkgfkxe.pdf

Ordered droplet structures at the liquid crystal surface and elastic-capillary colloidal interactions.

In this Letter, we demonstrate that the symmetry of the elastic interaction between the dipolar and quadrupolar colloidal particles in the nematic liquid crystal leads to a novel variety of 2D nematic ``binary'' colloidal crystals, which have not been observed in any colloidal system. The dipolar-quadrupolar interaction is highly anisotropic and shows a power-law dependence when the particles approach each other along the director field with a pair-binding energy of the order of several thousands of ${k}_{B}T$ for $4\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ diameter colloids.

2D interactions and binary crystals of dipolar and quadrupolar nematic colloids.

Skyrmions are coreless vortex-like excitations emerging in diverse condensed-matter systems, and real-time observation of their dynamics is still challenging. Here we report the first direct optical observation of the spontaneous formation of half-skyrmions. In a thin film of a chiral liquid crystal, depending on experimental conditions including film thickness, they form a hexagonal lattice whose lattice constant is a few hundred nanometres, or appear as isolated entities with topological defects compensating their charge. These half-skyrmions exhibit intriguing dynamical behaviour driven by thermal fluctuations. Numerical calculations of real-space images successfully corroborate the experimental observations despite the challenge because of the characteristic scale of the structures close to the optical resolution limit. A thin film of a chiral liquid crystal thus offers an intriguing platform that facilitates a direct investigation of the dynamics of topological excitations such as half-skyrmions and their manipulation with optical techniques. Spontaneous formation of a half-skyrmion lattice is observed in a thin-film chiral liquid crystal. The dynamics are shown to be thermally driven — presenting a platform to study the thermal fluctuations of topological defects.

A. Nych

Papers

Crystal structure in nematic emulsion.

Assembly and control of 3D nematic dipolar colloidal crystals

Ordered droplet structures at the liquid crystal surface and elastic-capillary colloidal interactions.

2D interactions and binary crystals of dipolar and quadrupolar nematic colloids.

Spontaneous formation and dynamics of half-skyrmions in a chiral liquid-crystal film