Dong Ki Yoon

Chirality-Preserving Growth of Helical Filaments in the B4 Phase of Bent-Core Liquid Crystals

The growth of helical filaments in the B4 liquid-crystal phase was investigated in mixtures of the bent-core and calamitic mesogens NOBOW and 8CB. Freezing-point depression led to nucleation of the NOBOW B4 phase directly from the isotropic phase in the mixtures, forming large left- and right-handed chiral domains that were easily observed in the microscope. We show that these domains are composed of homochiral helical filaments formed in a nucleation and growth process that starts from a nucleus of arbitrary chirality and continues with chirality-preserving growth of the filaments. A model that accounts for the observed local homochirality and phase coherence of the branched filaments is proposed. This model will help in providing a better understanding of the nature of the B4 phase and controlling its growth and morphology for applications, such as the use of the helical nanophase as a nanoheterogeneous medium.

Organization of the polarization splay modulated smectic liquid crystal phase by topographic confinement

Recently, the topographic patterning of surfaces by lithography and nanoimprinting has emerged as a new and powerful tool for producing single structural domains of liquid crystals and other soft materials. Here the use of surface topography is extended to the organization of liquid crystals of bent-core molecules, soft materials that, on the one hand, exhibit a rich, exciting, and intensely studied array of novel phases, but that, on the other hand, have proved very difficult to align. Among the most notorious in this regard are the polarization splay modulated (B7) phases, in which the symmetry-required preference for ferroelectric polarization to be locally bouquet-like or “splayed” is expressed. Filling space with splay of a single sign requires defects and in the B7 splay is accommodated in the form of periodic splay stripes spaced by defects and coupled to smectic layer undulations. Upon cooling from the isotropic phase this structure grows via a first order transition in the form of an exotic array of twisted filaments and focal conic defects that are influenced very little by classic alignment methods. By contrast, growth under conditions of confinement in rectangular topographic channels is found to produce completely new growth morphology, generating highly ordered periodic layering patterns. The resulting macroscopic order will be of great use in further exploration of the physical properties of bent-core phases and offers a route for application of difficult-to-align soft materials as are encountered in organic electronic and optical applications.

Solvent‐Free Directed Patterning of a Highly Ordered Liquid Crystalline Organic Semiconductor via Template‐Assisted Self‐Assembly for Organic Transistors

Highly ordered organic semiconductor micropatterns of the liquid-crystalline small molecule 2,7-didecylbenzothienobenzothiophene (C10 -BTBT) are fabricated using a simple method based on template-assisted self-assembly (TASA). The liquid crystallinity of C10 -BTBT allows solvent-free fabrication of high-performance printed organic field-effect transistors (OFETs).

Spontaneous Chirality Induction and Enantiomer Separation in Liquid Crystals Composed of Achiral Rod-Shaped 4-Arylbenzoate Esters

The discovery of spontaneously induced chirality and enantiomeric separation in liquid crystal and soft crystal systems composed of achiral rod-shaped 4-arylbenzoate esters is described. Negligibly small circular dichroism (CD) signals are produced in the smectic A (SmA) phases of these substances, and the signals were found to increase with increasing smectic order. Since the advent of chirality occurs in freely suspended films, it is not a consequence of surface effects. Both positive and negative CD signals are observed with equal probability at different positions in these films. Vibrational CD spectroscopy and theoretical calculations are used to analyze the conformational changes that are associated with the induced chirality of the rod-shaped molecules. The results show that the phenomenon is caused by the twisting of biphenyl bond associated with the ester moiety in 4-arylbenzoate esters.

Multistep hierarchical self-assembly of chiral nanopore arrays

Significance Bent-core mesogenic molecules form smectic liquid crystal phases in which the molecular layers are locally polar and chiral, and have a built-in tendency for saddle splay curvature, a combination that fills bulk neat bent-core smectics with self-assembled helical nanofilament bundles of twisted layers. This observation led us to explore the growth mode of such smectics under conditions of nanoconfinement and the remarkable observation reported here that single nanofilaments readily grow in linear nanoscale pores, generating a new motif of hierarchically self-assembled hybrid organic/inorganic structures for applications in chiral synthesis and separation. A series of simple hierarchical self-assembly steps achieve self-organization from the centimeter to the subnanometer-length scales in the form of square-centimeter arrays of linear nanopores, each one having a single chiral helical nanofilament of large internal surface area and interfacial interactions based on chiral crystalline molecular arrangements.

Optically selective microlens photomasks using self-assembled smectic liquid crystal defect arrays.

2010 WILEY-VCH Verlag Gmb Microlens photolithographic fabrication using self-assembled materials has attracted considerable attention in recent years because the techniques involved are very simple, inexpensive, and provide a route to the fabrication of large-area patterns. Several materials, which include colloids, hydrogels, and liquid crystals (LCs), have been used to fabricate self-assembled microlens arrays for photolithographic use. Colloidal microspheres, 3mm in diameter, were embedded in a transparent polymer membrane. These spheres acted as lenses to reduce centimeter-scale images to micrometer-scale images in the image plane, providing a simple way to produce spontaneous assembly over a large area, with the appropriate feature size, using microlens arrays optimized for visible light. Biomimetic hydrogels were also used to fabricate microlens arrays for photolithography. These systems adopted a conventional microlens system morphology, containing spherical or hemispherical geometric shapes with a homogeneous refractive index. A more advanced microlens system has used LCs for the fabrication of active devices, because the molecular orientations within a LC can be easily controlled by an external electric field. A radial distribution of the refractive index can be attained through application of an axially distributed electric field. However, previous LC-based microlens systems required complex thick LC cell structures to control the molecular orientations of the nematic LC. Such cell structures included circular hole-patterned electrodes and polymer stabilizers. In this Communication, we report a new type of microstructure for the fabrication of optically selectivemicrolens arrays. This system uses a periodic toric focal conic domain (TFCD) of smectic LCs as a photomask, combining two imaging elements, microlens arrays and clear windows. The shape and focusing mechanism of the TFCD microlens photomasks are very different from those of conventional microlens photomasks, which use spherical (or hemispherical) structures with homogeneous refractive indices. TFCD microlens photomasks have several remarkable features. First, the periodic toroid-shaped holes of the TFCD structure act as microlenses due to the intrinsic molecular orientations of each TFCD, which can focus illuminated light. The flat regions between the toroidal holes act as clear windows and do not scatter light. Second, this system uses the advantages of a graded refractive index in LCs as well as periodic microscale arrays. The ordered TFCD structures are generated through the control of themolecular orientations in the LCs on surface modified substrates. The light passing through a TFCD is refracted and focused to the center of the TFCD by the graded refractive index according to the intrinsic LC molecular orientations in a TFCD. Therefore, LC-based TFCD microlenses are optically selective for the direction of polarization of the transmitted light, when used as a photomask. Accordingly, one can obtain a variety of microscale patterns with controlled domain sizes, geometries, and symmetries, by simply adjusting the illumination dose (intensity), the size of the TFCD photomask, the tone of the photoresist, and the direction of polarization of the illuminating light source. To generate the TFCD structure, we used a simple rodlike smectic LC material containing a rigid biphenyl core and a semifluorinated tail group, which was prepared by alkylation of ethyl 4-hydroxyphenylbenzoate with 1H,1H,2H,2H,3H,3H,4H, 4H-perfluorododecyl bromide (Fig. 1a). As reported previously, this material consistently yielded a hexagonal highly ordered structure of TFCDs on the surface of a treated glass substrate. Upon cooling ( 1 8Cmin ) from the isotropic to the SmAphase, ordered TFCD domain arrays were generated over large areas. Because the small LC components had a high mobility and responded rapidly in the smectic phase, the fabrication of TFCD microlens arrays was very fast and simple relative to other soft self-assembly building blocks. We found that the generation of a uniform TFCD large-scale array on a glass substrate required only a few seconds. Figure 1b shows representative polarized optical microscopy (POM) images of the TFCD domains of smectic LCs on a flat PEI-coated glass substrate and reveals the formation of highly ordered periodic TFCDs over a large area. Each small circular domain corresponds to a single TFCD. Close inspection of the POM images of the film formed by the LC revealed that the TFCD were identical in size and were present in a hexagonal array, a characteristic typical of SmA phases under surface anisotropy conditions. [17] Each TFCD produced a characteristic Maltese cross pattern (‘‘microlens’’ region), indicating that the projection of the director field onto the plane of the substrate was radial within the area bounded by the circular base of the TFCD. Outside the circular base (the ‘‘window’’ region), the molecules were vertically aligned to the

Fabrication of two-dimensional dimple and conical microlens arrays from a highly periodic toroidal-shaped liquid crystal defect array

A self-assembly fabrication method was developed for the preparation of microlens arrays (MLAs). The procedure used the focal conic structures of semi-fluorinated smectic liquid crystals (LCs), the periodic toric focal conic domains (TFCDs), which were prepared on a surface-modified substrate. This LC-based MLA system focuses light via the intrinsic molecular orientations of the TFCDs, leading to a highly efficient MLA with good optical properties. The thickness of the smectic LC film could be used to control both the microlens feature size, over the range 5–15 μm, and the microlens focal length, over the range 1–3 μm. In addition, we prepared two-dimensional hexagonally packed polymer MLAs with conical or dimple shapes by successive replica molding, using a UV-curable photopolymer (NOA63) and polydimethylsiloxane (PDMS), from a TFCD array template containing a dimple structure. The LC-based TFCD MLAs and the secondary replicated polymer MLAs, using NOA63 and PDMS molds, showed good lens performances. We anticipate that this LC self-assembly method will be applicable to the large-scale fabrication of MLAs. The method allows fabrication of dynamic MLAs that are responsive to external fields, such as electric or magnetic fields, or to thermal variations.

Self-assembled periodic liquid crystal defects array for soft lithographic template

We have developed highly periodic patterns with sub-micrometer features over large-areas using toric focal conic domains (TFCDs) originated from smectic liquid crystal (LC) as a new self-assembling building block. TFCDs are accomplished by precisely controlling the surface and interfacial properties of smectic LC. In order to apply the smectic liquid crystal defect arrays in soft lithography, the hexagonal arrays of domain patterns are used as molds for ultraviolet (UV) curable polymers, thereby providing LC defect stamps with high spatial resolution over large areas. Our method was further utilized to transfer patterns with sub-micrometer features from the polymer stamp surface to a secondary surface by microcontact printing (μ-CP). Accordingly, we show that such LC materials can be very strong candidates for the periodic templates, compared to other soft-building blocks such as block copolymers, colloids and surfactants. We anticipate our finding be a starting point for more sophisticated lithographic applications based on liquid crystalline materials.

Three-dimensional textures and defects of soft material layering revealed by thermal sublimation

Significance Techniques that enable direct visualization in three dimensions of internal nanometer- to micrometer-scale structure are highly prized in materials research and engineering. Here, we demonstrate an effective method for the 3D imaging of soft materials, revealing the alignment, textures, and defects in the organization of their internal interfaces: in this case, of a soft crystal and liquid crystal phase. This method employs combinations of quenching and sublimation generating surface topography that reflects the internal structure, exposing it for visualization using standard techniques. Layering is found and exploited in a variety of soft material systems, ranging from complex macromolecular self-assemblies to block copolymer and small-molecule liquid crystals. Because the control of layer structure is required for applications and characterization, and because defects reveal key features of the symmetries of layered phases, a variety of techniques have been developed for the study of soft-layer structure and defects, including X-ray diffraction and visualization using optical transmission and fluorescence confocal polarizing microscopy, atomic force microscopy, and SEM and transmission electron microscopy, including freeze-fracture transmission electron microscopy. Here, it is shown that thermal sublimation can be usefully combined with such techniques to enable visualization of the 3D structure of soft materials. Sequential sublimation removes material in a stepwise fashion, leaving a remnant layer structure largely unchanged and viewable using SEM, as demonstrated here using a lamellar smectic liquid crystal.

Alignment of helical nanofilaments on the surfaces of various self-assembled monolayers

We successfully prepared controlled layers of NOBOW molecules on a variety of self-assembled monolayers (SAMs) such that the layers formed oriented domain structures in which the complex chiral/polar crystalline helical nanofilaments (HNFs) were arranged to form a B4 bent-core liquid crystal (LC) phase. The alignment of the B2 phase, which formed at a higher temperature than the B4 phase, affected the direction of the HNFs. The HNFs formed on the B2 smectic layers and were aligned parallel or perpendicular, respectively, to the substrates with high or low surface energies (molecule-philic or -phobic SAM-treated substrates). The HNFs confined within rectangular microchannels modified by the SAMs were directly visualized by electron microscopy and X-ray diffraction studies. The orientations of the HNFs were found to be governed by the B2 smectic layer morphology.