Nanoparticle-based hollow microstructures formed by two-stage nematic nucleation and phase separation

Dynamic magnetic field alignment and polarized emission of semiconductor nanoplatelets in a liquid crystal polymer

Reconfigurable arrays of 2D nanomaterials are essential for the realization of switchable and intelligent material systems. Using liquid crystals (LCs) as a medium represents a promising approach, in principle, to enable such control. In practice, however, this approach is hampered by the difficulty of achieving stable dispersions of nanomaterials. Here, we report on good dispersions of pristine CdSe nanoplatelets (NPLs) in LCs, and reversible, rapid control of their alignment and associated anisotropic photoluminescence, using a magnetic field. We reveal that dispersion stability is greatly enhanced using polymeric, rather than small molecule, LCs and is considerably greater in the smectic phases of the resulting systems relative to the nematic phases. Aligned composites exhibit highly polarized emission that is readily manipulated by field-realignment. Such dynamic alignment of optically-active 2D nanomaterials may enable the development of programmable materials for photonic applications and the methodology can guide designs for anisotropic nanomaterial composites for a broad set of related nanomaterials.

Liquid crystals (LC) are promising materials for the fabrication of reconfigurable arrays of 2D nanomaterials but it remains difficult to achieve stable dispersions of nanomaterials. Here, the authors report on good dispersions of pristine CdSe nanoplatelets (NPLs) in LCs, and reversible, rapid control of their alignment and associated anisotropic photoluminescence using a magnetic field.

Ordering hard-sphere particle suspensions by medium crystallization: Effect of size and interaction strength

While microstructure in soft materials is usually given by the self-assembly of their constituting building blocks, colloidal assembly can also be obtained via templating a morphology in a disordered suspension of particles by solidification of the melt. This order-templating process is applicable to different soft-matter systems with a variety of characteristic length scales, including particle suspensions in water, liquid-crystal materials, and polymer melts. In this work, we numerically investigate the effect of particle size and solvent size in the process of solidification templating by implementing a coarse-grain model of hard-sphere particles at a moving melt-crystal interface. This approach captures the dependence of crystallization templating on speed of crystal growth, showing the existence of a threshold speed for crystallization templating to occur. Results in this work show that the threshold speed changes following a power form as solvent size and particle size change. Furthermore, this work analyzes and reports the effect of particle-crystal interaction strength in combination with size effects. This scaling study from a numerical perspective sets a starting point for the development of hybrid soft materials via structural templating, allowing solidification-driven particle ordering in different systems with a variety of length scales.

Experimental Advances in Nanoparticle-Driven Stabilization of Liquid-Crystalline Blue Phases and Twist-Grain Boundary Phases

Recent advances in experimental studies of nanoparticle-driven stabilization of chiral liquid-crystalline phases are highlighted. The stabilization is achieved via the nanoparticles' assembly in the defect lattices of the soft liquid-crystalline hosts. This is of significant importance for understanding the interactions of nanoparticles with topological defects and for envisioned technological applications. We demonstrate that blue phases are stabilized and twist-grain boundary phases are induced by dispersing surface-functionalized CdSSe quantum dots, spherical Au nanoparticles, as well as MoS2 nanoplatelets and reduced-graphene oxide nanosheets in chiral liquid crystals. Phase diagrams are shown based on calorimetric and optical measurements. Our findings related to the role of the nanoparticle core composition, size, shape, and surface coating on the stabilization effect are presented, followed by an overview of and comparison with other related studies in the literature. Moreover, the key points of the underlying mechanisms are summarized and prospects in the field are briefly discussed.

Colloidal aggregation in anisotropic liquid crystal solvent

The mutual attraction between colloidal particles in an anisotropic fluid, such as the nematic liquid crystal phase, leads to the formation of hierarchical aggregate morphologies distinct from those that tend to form in isotropic fluids. Previously it was difficult to study this aggregation process for a large number of colloids due to the difficulty of achieving a well dispersed initial colloid distribution under good imaging conditions. In this paper, we report the use of a recently developed self-assembling colloidal system to investigate this process. Hollow, micron-scale colloids are formed in situ in the nematic phase and subsequently aggregate to produce fractal structures and colloidal gels, the structures of which are determined by colloid concentration and temperature quench depth through the isotropic to nematic phase transition point. This self-assembling colloidal system provides a unique method to study particle aggregation in liquid crystal over large length scales. We use fluorescence microscopy over a range of length scales to measure aggregate structure as a function of temperature quench depth, observe ageing mechanisms and explore the driving mechanisms in this unique system. Our analyses suggest that aggregate dynamics depend on a combination of Frank elasticity relaxation, spontaneous defect line annihilation and internal aggregate fracturing.

STEM Tomography of Au Helical Assemblies

Composite, helical nanostructures formed using cooperative interactions of liquid crystals and Au nanoparticles were studied using a scanning transmission electron microscopy (STEM) mode. The investigated helical assemblies exhibit long-range hierarchical order across length scales, as a result of the crystallization (freezing) directed growth mechanism of nanoparticle-coated twisted nanoribbons and their ability to form organized bundles. Here, STEM methods were used to reproduce the 3D structure of the Au nanoparticle double helix.

Understanding and Controlling the Crystallization Process in Reconfigurable Plasmonic Superlattices

The crystallization of nanomaterials is a primary source of solid-state, photonic structures. Thus, a detailed understanding of this process is of paramount importance for the successful application of photonic nanomaterials in emerging optoelectronic technologies. While colloidal crystallization has been thoroughly studied, for example, with advanced in situ electron microscopy methods, the noncolloidal crystallization (freezing) of nanoparticles (NPs) remains so far unexplored. To fill this gap, in this work, we present proof-of-principle experiments decoding a crystallization of reconfigurable assemblies of NPs at a solid state. The chosen material corresponds to an excellent testing bed, as it enables both in situ and ex situ investigation using X-ray diffraction (XRD), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atomic force microscopy (AFM), and optical spectroscopy in visible and ultraviolet range (UV-vis) techniques. In particular, ensemble measurements with small-angle XRD highlighted the dependence of the correlation length in the NPs assemblies on the number of heating/cooling cycles and the rate of cooling. Ex situ TEM imaging further supported these results by revealing a dependence of domain size and structure on the sample preparation route and by showing we can control the domain size over 2 orders of magnitude. The application of HAADF-STEM tomography, combined with in situ thermal control, provided three-dimensional single-particle level information on the positional order evolution within assemblies. This combination of real and reciprocal space provides insightful information on the anisotropic, reversibly reconfigurable assemblies of NPs. TEM measurements also highlighted the importance of interfaces in the polydomain structure of nanoparticle solids, allowing us to understand experimentally observed differences in UV-vis extinction spectra of the differently prepared crystallites. Overall, the obtained results show that the combination of in situ heating HAADF-STEM tomography with XRD and ex situ TEM techniques is a powerful approach to study nanoparticle freezing processes and to reveal the crucial impact of disorder in the solid-state aggregates of NPs on their plasmonic properties.

Swelling Cholesteric Liquid Crystal Shells to Direct the Assembly of Particles at the Interface

Cholesteric liquid crystals can exhibit spatial patterns in molecular alignment at interfaces that can be exploited for particle assembly. These patterns emerge from the competition between bulk and surface energies, tunable with the system geometry. In this work, we use the osmotic swelling of cholesteric double emulsions to assemble colloidal particles through a pathway-dependent process. Particles can be repositioned from a surface-mediated to an elasticity-mediated state through dynamically thinning the cholesteric shell at a rate comparable to that of colloidal adsorption. By tuning the balance between surface and bulk energies with the system geometry, colloidal assemblies on the cholesteric interface can be molded by the underlying elastic field to form linear aggregates. The transition of adsorbed particles from surface regions with homeotropic anchoring to defect regions is accompanied by a reduction in particle mobility. The arrested assemblies subsequently map out and stabilize topological defects. These results demonstrate the kinetic arrest of interfacial particles within definable patterns by regulating the energetic frustration within cholesterics. This work highlights the importance of kinetic pathways for particle assembly in liquid crystals, of relevance to optical and energy applications.

Shaping Liquid Crystals with Gold Nanoparticles: Helical Assemblies with Tunable and Hierarchical Structures Via Thin-Film Cooperative Interactions

The availability of helical assemblies of plasmonic nanoparticles with precisely controlled and tunable structures can play a key role in the future development of chiral plasmonics and metamaterials. Here, a strategy to efficiently yield helical structures based on the cooperative interactions of liquid crystals and gold nanoparticles in thin films is developed. These nanocomposites exhibit exceptional long-range hierarchical order across length scales, which results from the growth mechanism of nanoparticle-coated twisted nanoribbons and their ability to form organized bundles. The helical assembly formation is governed by the presence of rationally functionalized nanoparticles. Importantly, the thickness of the achieved nanocomposites can be reversibly reconfigured owing to the polymorphic nature of the liquid crystal. The versatility of the proposed approach is demonstrated by preparing helices assembled from nanoparticles of different geometries and dimensions (spherical and rod-like). The described strategy may become an enabling technology for structuring nanoparticle assemblies with high precision and fabricating optically active materials.

Self-Organized, One-Dimensional Periodic Structures in a Gold Nanoparticle-Doped Nematic Liquid Crystal Composite

Composite structures exhibiting a periodic arrangement of building blocks can be found in natural systems at different length scales. Recreating such systems in artificial composites using the principles of self-assembly has been a great challenge, especially for 1D microscale systems. Here, we present a purposely designed composite material consisting of gold nanoparticles and a nematic liquid crystal matrix that has the ability to self-create a periodic structure in the form of a one-dimensional photonic lattice through a phase separation process occurring in a confined space. Our strategy is based on the use of a thermoswitchable medium that reversibly and quickly responds to both heating and cooling. We find that the period of the structure is strongly related to the size of the confining space. We believe that our findings will allow us to not only better understand the phase separation process in multicomponent soft/colloid mixtures with useful optical properties but also improve our understanding of the precise assembly of advanced materials into one-dimensional periodic systems, with prospective applications in future photonic technologies.