Ferroelectric liquid-crystal waveguide modulation based on a switchable uniaxial-uniaxial interface

Liquid crystals have effective electro-optic coefficients that are orders of magnitude larger than other integrated optical materials such as lithium niobate. However, previous studies of liquid-crystal waveguides have mainly focused on nematic liquid crystals, which exhibit impractically large scattering losses as waveguides. Studies of smectic liquid crystals and liquid crystals under strong confinement suggest the losses in these materials may be more manageable. In this study, the possibility of using ferroelectric liquid crystals in active waveguide modulators is explored through the analysis of several modulator configurations: a cutoff modulator, a deflection modulator, and an input coupler. As a way to study these structures, a mode-matching technique was developed to analyze the effects of a discontinuity in a uniaxial slab waveguide whose optic axis is in the plane of the waveguide. The results from the mode-matching technique were compared with those from simple bulk models. The analysis shows that ferroelectric liquid-crystal modulators have many desirable performance characteristics and could form the basis for practical waveguide modulators.     

Smectic Layer Origami via Preprogrammed Photoalignment

Hierarchical architecture is of vital importance in soft materials. Focal conic domains (FCDs) of smectic liquid crystals, characterized by an ordered lamellar structure, attract intensive attention. Simultaneously tailoring the geometry and clustering characteristics of FCDs remains a challenge. Here, the 3D smectic layer origami via a 2D preprogrammed photoalignment film is accomplished. Full control of hierarchical superstructures is demonstrated, including the domain size, shape, and orientation, and the lattice symmetry of fragmented toric FCDs. The unique symmetry breaking of resultant superstructures combined with the optical anisotropy of the liquid crystals induces an intriguing polarization‐dependent diffraction. This work broadens the scientific understanding of self‐assembled soft materials and may inspire new opportunities for advanced functional materials and devices.     

Pillar-assisted epitaxial assembly of toric focal conic domains of smectic-a liquid crystals

SU-8 pillar-assisted epitaxial assembly of toric focal conic domains (TFCDs) arrays of smectic-A liquid crystals is studied. The 3D nature of the pillar array is crucial to confine and direct the formation of TFCDs on the top of each pillar and between neighboring pillars, leading to highly ordered square and hexagonal array TFCDs. Excellent agreement between the experimentally obtained critical pillar diameter and elasticity calculation is found.     

Directed Self‐Assembly of Liquid‐Crystalline Molecular Building Blocks for Sub‐5 nm Nanopatterning

The thin‐film directed self‐assembly of molecular building blocks into oriented nanostructure arrays enables next‐generation lithography at the sub‐5 nm scale. Currently, the fabrication of inorganic arrays from molecular building blocks is restricted by the limited long‐range order and orientation of the materials, as well as suitable methodologies for creating lithographic templates at sub‐5 nm dimensions. In recent years, higher‐order liquid crystals have emerged as functional thin films for organic electronics, nanoporous membranes, and templated synthesis, which provide opportunities for their use as lithographic templates. By choosing examples from these fields, recent progress toward the design of molecular building blocks is highlighted, with an emphasis on liquid crystals, to access sub‐5 nm features, their directed self‐assembly into oriented thin films, and, importantly, the fabrication of inorganic arrays. Finally, future challenges regarding sub‐5 nm patterning with liquid crystals are discussed.     

Morphology-controllable fabrication of ordered platinum nanoshells with reduced symmetry

This paper presents a novel method for fabricating an ordered array of metallic nanoshells with a controllable shape by a combination of a porous polymer template and a nanocrystal-seeded electroless plating technique. The morphology of hollow particles has a strong dependence on the seed-deposition time onto the surfaces of the original colloidal template. Using this method, ordered Pt nanobowls (bowl-shaped shells) and, alternatively, nanocups (cup-shaped shells) are prepared. These materials show some intriguing properties: (i) reduced symmetry of the building blocks; (ii) a well-ordered structure; and (iii) a high ratio of surface area to volume, all of which are useful in many areas such as catalysts, sensors, and photonic crystals.     

Liquid Crystals for Charge Transport,Luminescence, and Photonics

Ordered molecular materials are increasingly used in active electronic and photonic organic devices. In this progress report we discuss whether the self‐assembling properties and supramolecular structures of liquid crystals can be tailored to improve such devices. Recent developments in charge‐transporting and luminescent liquid crystals are discussed in the context of material requirements for organic light‐emitting devices, photovoltaics, and thin film transistors. We identify high carrier mobility, polarized emission, and enhanced output‐coupling as the key advantages of nematic and smectic liquid crystals for electroluminescence. The formation of anisotropic polymer networks gives the added benefits of multilayer capability and photopatternability. The anisotropic transport and high carrier mobilities of columnar liquid crystals make them promising candidates for photovoltaics and transistors. We also outline some of the issues in material design and processing that these applications demand. The photonic properties of chiral liquid crystals and their use as mirror‐less lasers are also discussed.     

Long-range structuring of nanoparticles by mimicry of a cholesteric liquid crystal

Patterning nano-objects is an exciting interdisciplinary research area in current materials science, arising from new optical and optoelectronic properties and the need to miniaturize electronic components. Many techniques have been developed for assembling nanoparticles into two- and three-dimensional arrays. Most studies involving liquid crystals as templates have dealt with colloidal particles and nematic and smectic phases. Here, we demonstrate the long-range ordering of nanoparticle assemblies that adopt the helical configuration of the cholesteric liquid crystalline phase. Because we used glass-forming cholesterics, the nanostructures could be examined by transmission electron microscopy. The platinum nanoparticles form periodic ribbons that mimic the well-known 'fingerprint' cholesteric texture. Surprisingly, the nanoparticles do not decorate the original cholesteric texture but create a novel helical structure with a larger helical pitch. By varying the molar fraction of cholesterol-containing mesogen in the liquid crystal host, we show that the distance between the ribbons is directly correlated to the pitch. Therefore this inherent lengthscale becomes a simple control parameter to tune the structuring of nanoparticles. These results demonstrate how such an assembly process could be modulated, providing a versatile route to new materials systems.     

Electric field induced wavelike instability in smectic C liquid crystals

We studied 4-p-decyloxybenzoic acid 4′-p-hexyloxyphenyl ester liquid crystals of the smectic C (smC) type in which a wavelike instability is induced by an external electric field in the vicinity of the smC-smA phase transition. The smectic layers in a liquid-crystal sample placed in a sandwich type cell were parallel to the cell plates and the director was oriented at an angle θ to the layer normal. The second-order smC-smA phase transition exhibits a critical character. A wavelike instability in the form of a band structure was observed in both alternating and constant electric fields. The band structure period depends on the electric field strength.     

Memory and topological frustration in nematic liquid crystals confined in porous materials

Orientational ordering is key to functional materials with switching capability, such as nematic liquid crystals and ferromagnetic and ferroelectric materials. We explored the confinement of nematic liquid crystals in bicontinuous porous structures with smooth surfaces that locally impose normal orientational order on the liquid crystal. We find that frustration leads to a high density of topological defect lines permeating the porous structures, and that most defect lines are made stable by looping around solid portions of the confining material. Because many defect trajectories are possible, these systems are highly metastable and efficient in memorizing the alignment forced by external fields. Such memory effects have their origin in the topology of the confining surface and are maximized in a simple periodic bicontinuous cubic structure. We also show that nematic liquid crystals in random porous networks exhibit a disorder-induced slowing-down typical of glasses that originates from activated collisions and rearrangements of defect lines. Our findings offer the possibility to functionalize orientationally ordered materials through topological confinement.     

Highly ordered nanostructures with tunable size, shape and properties: A new way to surface nano-patterning using ultra-thin alumina masks

Large-scale arrays of nanostructures on substrates, such as semiconductor or metal nano-particle arrays, have attracted considerable interest due to their unique physical properties and many potential applications in areas such as electronics, optoelectronics, sensing, high-density storage, and ultra-thin display devices. In the last two decades, the search for a highly efficient and low-cost nano-patterning method in fabricating ordered surface nanostructures with tunable dimensions and properties, has involved interdisciplinary and cross-disciplinary research and development with emerging technologies such as lithographic methods, self-assembly processes, and scanning probe techniques. Here, we review a new surface nano-patterning approach in fabricating ordered nanostructures, in which ultra-thin anodic alumina membranes are used as fabrication masks. Using the method, large-scale arrays of highly ordered nanostructures in the range of square centimeters can be fabricated on any substrate in a massive parallel way. The resulting nanostructures are characterized by highly defined and controllable size, shape, composition, and spacing of the nanostructures. Tuning of the properties of the arrayed nanostructures can be obtained by controlled adjustment of the structural parameters of the arrayed nanostructures. Compared to conventional lithographic methods, the present nano-patterning approach offers attractive advantages, such as large pattern area, high throughput, low equipment costs, and high flexibility and control options for ordered nanostructures with tunable properties. This new non-lithographic nano-patterning approach will be shown to be a general method in fabricating a wide range of ordered surface nanostructures with tunable and unique physical and chemical properties that could be used in the fabrication of nano-devices with high performance and controllability.     

Monodispersed Colloidal Spheres: Old Materials with New Applications

This article presents an overview of current research activities that center on monodispersed colloidal spheres whose diameter falls anywhere in the range of 10 nm to 1 μm. It is organized into three parts: The first part briefly discusses several useful methods that have been developed for producing monodispersed colloidal spheres with tightly controlled sizes and well‐defined properties (both surface and bulk). The second part surveys some techniques that have been demonstrated for organizing these colloidal spheres into two‐ and three‐dimensionally ordered lattices. The third part highlights a number of unique applications of these crystalline assemblies, such as their uses as photonic bandgap (PBG) crystals; as removable templates to fabricate macroporous materials with highly ordered and three‐dimensionally interconnected porous structures; as physical masks in lithographic patterning; and as diffractive elements to fabricate new types of optical sensors. Finally, we conclude with some personal perspectives on the directions towards which future research in this area might be directed.     

Atomistic simulations of spinodal phase separation preceding polymer crystallization

Many polymeric materials crystallize when cooled below their melting temperature. Although progress has been made in our understanding of the crystallization process through both experimental and theoretical efforts, these studies have focused mainly on the crystal nucleation and growth mechanism, where critical nuclei are formed from a metastable state during the first stages of crystallization, leading ultimately to the growth of crystal domains. Attention has also been given to the structure during the precrystallization (induction period). A pretransition state occurring before crystallization has been characterized as an unstable phase separation initiated by density and orientational fluctuations. These fluctuations are caused by an increase in the average length of rigid trans segments along the polymer backbone during the induction period. These observations are consistent with the theory proposed in ref. 14 on the isotropic-to-nematic transition of polymer liquid crystals, that is, the parallel ordering of polymers is caused by an increase in chain rigidity. Here we use large-scale computer simulations to investigate melts of polymers in the early ordering stages (induction period) before crystallization. In the ordered domains we identify growing dense regions similar to smectic liquid crystals. Our simulations reveal a 'coexistence period' in the ordering before crystallization, where nucleation and growth mechanisms coexist with a phase-separation mechanism.     

Fabrication of nanoplasmonic arrays with square symmetry using spin-coating method

A spin-coating method was applied for the first time to prepare a colloid monolayer on the optical crossed gratings used as a template. Four polystyrene colloids of various nominal sizes and different surface charges were spin-coated on templates with periods matched to the particles size. Three types of coverage were described depending on the spin-coating parameters and particles type. The optimal coverage was obtained for all four particles sizes. A way of finding the right spin-coating parameters was proposed. The analysis of a coverage capability of polystyrene particles showed that neutral particles have the highest ability to order on the templates used. Large monolayered areas of ordered particles were used as a lithographic mask for generating a pattern of gold nanoparticles with a square symmetry. A few hundred square micrometers large, continuous and fully defect-free areas of gold nanoparticles were produced on the nearly entire surface of the templated substrates.     

Thin films of liquid crystal fumarates and a related acrylate

Five fumarate esters and one acrylate ester have been synthesized and their properties examined. Four of them can be deposited as multilayers by evaporation in vacuo, and three of this group behave as thermotropic smectic liquid crystals. We have attempted to stabilize the homeotropic smectic phase by polymerizing the film using UV light. In two cases we have been successful and have produced films which are resistant to prolonged heating in THF. We infer that these materials are both polymerized and cross linked. They retain their regular layer structure after polymerization, a behaviour which we have demonstrated by X-ray diffraction. We have also attempted to form Langmuir-Blodgett multilayers with these compounds and in four cases have been successful.     

Stressed liquid-crystal optical phased array for fast tip-tilt wavefront correction

A liquid-crystal optical phased-array technology that uses stressed liquid crystals provides a new type of tip-tilt wavefront corrector. It demonstrates a very fast time response (10 kHz) and high beam-steering efficiency (approximately 91%). The new technology presented here will allow for a nonmechanical, high-speed correction with simple device construction.     

The role of nanopore shape in surface-induced crystallization

Crystallization of a molecular liquid from solution often initiates at solid-liquid interfaces, and nucleation rates are generally believed to be enhanced by surface roughness. Here we show that, on a rough surface, the shape of surface nanopores can also alter nucleation kinetics. Using lithographic methods, we patterned polymer films with nanopores of various shapes and found that spherical nanopores 15-120 nm in diameter hindered nucleation of aspirin crystals, whereas angular nanopores of the same size promoted it. We also show that favourable surface-solute interactions are required for angular nanopores to promote nucleation, and propose that pore shape affects nucleation kinetics through the alteration of the orientational order of the crystallizing molecule near the angles of the pores. Our findings have clear technological implications, for instance in the control of pharmaceutical polymorphism and in the design of 'seed' particles for the regulation of crystallization of fine chemicals.     

Structured Porous Materials via Colloidal Crystal Templating: From Inorganic Oxides to Metals

The formation of nanostructured materials by using colloidal crystals as templates is a relatively new but rapidly growing area of materials science. Colloid crystalline templates are three‐dimensional close‐packed crystals of submicrometer spheres, whose long‐ranged ordered structure is replicated in a solid matrix, to yield materials with ordered pores. These materials hold promise for use as photonic crystals, advanced catalysts, and in a variety of other applications. Here we review the wide range of materials that have been made following the original synthesis of structured porous silica. This method has been recently modified to produce porous metals.     

Cover Picture: Functionalization of Crystalline Colloidal Arrays through Click Chemistry (Adv. Mater. 21/2007)

Photonic crystals based on electrostatically‐stabilized colloidal arrays dispersed in a liquid medium are of interest to materials scientists partly because of the optical tuning afforded to theses systems with a variation in interparticle distance. On p. 3507, Stephen Foulger and co‐workers from Clemson University, USA report on a general strategy for the preparation of well‐defined and regioselectively functionalized ordered colloidal particles through the exploitation of “click” chemistry. Click transformations have found utility in the synthesis and/or functionalization of a range of systems. In addition, the solvochromic tuning of the ordered arrays is employed to modify the emission spectra of the surface‐attached photoluminescent dyes.     

Block copolymer templated etching on silicon

The use of self-assembled polymer structures to direct the formation of mesoscopic (1-100 nm) features on silicon could provide a fabrication-compatible means to produce nanoscale patterns, supplementing conventional lithographic techniques. Here we demonstrate nanoscale etching of silicon, applying standard aqueous-based fluoride etchants, to produce three-dimensional nanoscale features with controllable shapes, sizes, average spacing, and chemical functionalization. The block copolymers serve to direct the silicon surface chemistry by controlling the spatial location of the reaction as well as concentration of reagents. The interiors of the resulting etched nanoscale features may be selectively functionalized with organic monolayers, metal nanoparticles, and other materials, leading to a range of ordered arrays on silicon.     

Template‐Free,Surfactant‐Mediated Orientation of Self‐Assembled Supercrystals of Metal–Organic Framework Particles

Mesoscale self‐assembly of particles into supercrystals is important for the design of functional materials such as photonic and plasmonic crystals. However, while much progress has been made in self‐assembling supercrystals adopting diverse lattices and using different types of particles, controlling their growth orientation on surfaces has received limited success. Most of the latter orientation control has been achieved via templating methods in which lithographic processes are used to form a patterned surface that acts as a template for particle assembly. Herein, a template‐free method to self‐assemble (111)‐, (100)‐, and (110)‐oriented face‐centered cubic supercrystals of the metal–organic framework ZIF‐8 particles by adjusting the amount of surfactant (cetyltrimethylammonium bromide) used is described. It is shown that these supercrystals behave as photonic crystals whose properties depend on their growth orientation. This control on the orientation of the supercrystals dictates the orientation of the composing porous particles that might ultimately facilitate pore orientation on surfaces for designing membranes and sensors.