Yun Ho Kim

Direct visualization of large-area graphene domains and boundaries by optical birefringency.

The boundaries between domains in single-layer graphene strongly influence its electronic properties. However, existing approaches for domain visualization, which are based on microscopy and spectroscopy, are only effective for domains that are less than a few micrometres in size. Here, we report a simple method for the visualization of arbitrarily large graphene domains by imaging the birefringence of a graphene surface covered with nematic liquid crystals. The method relies on a correspondence between the orientation of the liquid crystals and that of the underlying graphene, which we use to determine the boundaries of macroscopic domains.

Robust Microfluidic Encapsulation of Cholesteric Liquid Crystals Toward Photonic Ink Capsules

Robust photonic microcapsules are created by microfluidic encapsulation of cholesteric liquid crystals with a hydrogel membrane. The membrane encloses the cholesteric core without leakage in water and the core exhibits pronounced structural colors. The photonic ink capsules, which have a precisely controlled bandgap position and size, provide new opportunities in colorimetric micro-thermometers and optoelectric applications.

Growth mode and structural characterization of GaSb on Si (001) substrate: A transmission electron microscopy study

Growth mode and structural properties of GaSb layers grown on silicon substrate by molecular beam epitaxy method are investigated by transmission electron microscopy. It is found that the GaSb grows to three-dimensional islands and grains are tilted to reduce a lattice mismatch through twin boundaries when they are directly grown on Si substrate. A low-temperature (LT) AlSb buffer plays a key role in transferring the growth mode from a three-dimensional island to a layer-by-layer structure. When the LT AlSb layer is used as a buffer, 90° misfit dislocations, with the Burgers vector b of 1∕2a⟨110⟩, are observed on the interface.

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).

Template‐Guided Solution‐Shearing Method for Enhanced Charge Carrier Mobility in Diketopyrrolopyrrole‐Based Polymer Field‐Effect Transistors

Template-guided solution-shearing (TGSS) is used to fabricate field-effect transistors (FETs) composed of micropatterned prisms as active channels. The prisms comprise highly crystalline PTDPP-DTTE, in which diketopyrrolopyrrole (DPP) is flanked by thiophene. The FET has a maximum mobility of approximately 7.43 cm(2) V(-1) s(-1) , which is much higher than the mobility values of the thin-film transistors with solution-sheared or spin-coated films of PTDPP-DTTE annealed at 200 °C.

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.

Reconfigurable Photonic Capsules Containing Cholesteric Liquid Crystals with Planar Alignment

Cholesteric liquid crystals (CLCs) reflect selected wavelengths of light owing to their periodic helical structures. The encapsulation of CLCs leads to photonic devices that can be easily processed and might be used as stand-alone microsensors. However, when CLCs are enclosed by polymeric membranes, they usually lose their planar alignment, leading to a deterioration of the optical performance. A microfluidics approach was employed to integrate an ultrathin alignment layer into microcapsules to separate the CLC core and the elastomeric solid membrane using triple-emulsion drops as the templates. The thinness of the alignment layer provides high lubrication resistance, preserving the layer integrity during elastic deformation of the membrane. The CLCs in the microcapsules can thus maintain their planar alignment, rendering the shape and optical properties highly reconfigurable.

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

Recent advances in the fabrication of nanotemplates from supramolecular self-organization

The use of self-assembling materials, including block copolymers, colloids, surfactants and supramolecular materials, has had a profound impact on the fabrication of nanopatterned surfaces. Despite receiving intense attention, and unlike when other self-assembling methods are employed, the construction of nanoscale templates from supramolecular materials, such as dendrimers, discotics, rod–coils and liquid-crystals, is critically limited by the lack of long-range order and orientation of the materials as well as the methodology for creating lithographic templates. This review focuses on recent progress that has been made in developing strategies that can be used to create supramolecular templates from a large number of supramolecular units with unique size and shape based on supramolecular self-organization. The large number of approaches that have been devised to control order and orientation of supramolecular structures as part of nano-fabrication processes, such as surface anchoring and external fields, will be discussed. Finally, the review ends with a discussion of the supramolecular templates that have been made in this manner and used for the construction inorganic arrays, high performance opto-electronic materials, mesoporous templates and nanoparticle trapping.

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.