Synthesis and Characterization of Side-Chain Liquid-Crystalline Block Copolymers Containing Cyano-Terminated Phenyl Benzoate Moieties.

Block copolymers, known for their capacity to undergo microphase separation, spontaneously yield various periodic nanostructures. These precisely controlled nanostructures have attracted considerable interest due to their potential applications in microfabrication templates, conducting films, filter membranes, and other areas. However, it is crucial to acknowledge that microphase-separated structures typically exhibit random alignment, making alignment control a pivotal factor in functional material development. To address this challenge, researchers have explored the use of block copolymers containing liquid-crystalline (LC) polymers, which offer a promising technique for alignment control. The molecular structure and LC behavior of these polymers significantly impact the morphology and alignment of microphase-separated structures. In this study, we synthesized LC diblock copolymers with cyano-terminated phenyl benzoate moieties and evaluated the microphase-separated structures and molecular alignment behaviors. The LC diblock copolymers with a narrow molecular weight distribution were synthesized by atom transfer radical polymerization. Small angle X-ray scattering measurements revealed that the block copolymers exhibit smectic LC phases and form cylinder structures with a lattice period of about 18 nm by microphase separation. The examination of block copolymer films using polarized optical microscopy and polarized UV-visible absorption spectroscopy corroborated that the LC moieties were uniaxially aligned along the alignment treatment direction.

Direct Surface Patterning of Microscale Well and Canal Structures by Photopolymerization of Liquid Crystals with Structured Light.

Precise control of the surface topographies of polymer materials is key to developing high-performance materials and devices for a wide variety of applications, such as optical displays, micro/nanofabrication, photonic devices, and microscale actuators. In particular, photocontrolled polymer surfaces, such as photoinduced surface relief, have been extensively studied mainly through photochemical mass transport. In this study, we propose a novel method triggering the mass transport by photopolymerization of liquid crystals with structured light and demonstrate the direct formation of microscale well and canal structures on the surface of polymer films. The wells and canals with depths of several micrometers and high aspect ratios, which are 10 times larger than those of previously reported structures, were found to be aligned in the center of non-irradiated areas. Furthermore, such well and canal structures can be arranged in two dimensions by designing light patterns. Real-time observations of canal structure formation reveal that anisotropic molecular diffusion during photopolymerization leads to a directed molecular alignment and subsequent surface structure formation. We believe that our proposed approach to designing microscale surface topographies has promising applications in advanced optical and mechanical devices.

Effect of Host Structure on Optical Freedericksz Transition in Dye-Doped Liquid Crystals.

The optical Freedericksz transition (OFT) can reversibly control the molecular orientation of liquid crystals (LCs) only by light irradiation, leading to the development of all-optical devices, such as smart windows. In particular, oligothiophene-doped LCs show the highly sensitive OFT due to the interaction between dyes and an optical-electric field. However, the sensitivity is still low for the application to optical devices. It is necessary to understand the factors in LCs affecting the OFT behavior to reduce the sensitivity. In this study, we investigated the effect of the host LC structure on the OFT in oligothiophene-doped LCs. The threshold light intensity for the OFT in trifluorinated LCs was 42% lower than that in LCs without fluorine substituents. This result contributes to the material design for the low-threshold optical devices utilizing the OFT of dye-doped LCs.

Liquid crystalline 2D borophene oxide for inorganic optical devices.

Borophene has been recently proposed as a next-generation two-dimensional material with promising electronic and optical properties. However, its instability has thus far limited its large-scale applications. Here, we investigate a liquid-state borophene analogue with an ordered layer structure derived from two-dimensional borophene oxide. The material structure, phase transition features and basic properties are revealed by using X-ray analysis, optical and electron microscopy, and thermal characterization. The obtained liquid crystal exhibits high thermal stability at temperatures up to 350 °C and an optical switching behaviour driven by a low voltage of 1 V.

Unidirectional Alignment of Surface-Grafted ZnO Nanorods in Micrometer-Thick Cells Using Low-Molecular-Weight Liquid Crystals.

Inorganic nanomaterials such as nanotubes and nanorods have attracted great attention due to their anisotropic properties. Although the alignment control of inorganic nanomaterials is key to the development of functional devices utilizing their fascinating properties, there is still difficulty in achieving uniform alignment over a large area with a micrometer thickness. To overcome this problem, we focused on liquid crystals (LCs) to promote the alignment of anisotropic nanomaterials, taking advantage of the cooperative motion of LCs. We present the uniform, one-dimensional alignment of ZnO nanorods along the direction of LCs in micrometer-thick cells by grafting nematic LC polymers from the nanorod surfaces to provide miscibility with the host LCs. Polarized optical microscopy and polarized UV-visible absorption spectroscopy revealed the unidirectional alignment of nematic LC polymer-grafted ZnO nanorods parallel to the nematic host LCs.

Dependence of the Growth Mode in Epitaxial FePt Films on Surface Free Energy.

Epitaxial thin films of L10-ordered FePt alloys are one of the most important materials in magnetic recording and spintronics applications due to their large perpendicular magnetic anisotropy (PMA). The key to the production of these required superior properties lies in the control of the growth mode of the films. Further, it is necessary to distinguish between the effect of lattice mismatch and surface free energy on the growth mode because of their strong correlation. In this study, the effect of surface free energy on the growth mode of FePt epitaxial films was investigated using MgO, NiO, and MgON surfaces with almost the same lattice constant to exclude the effect of lattice mismatch. It was found that the growth mode can be tuned from a three-dimensional (3D) island mode on MgO to a more two-dimensional (2D)-like mode on MgON and NiO. Contact angle measurements revealed that MgON and NiO show larger surface free energy than MgO, indicating that the difference in the growth mode is due to their larger surface free energy. In addition, MgON was found to induce not only a flat surface as FePt grown on SrTiO3 (STO), which has a small lattice mismatch, but also a larger PMA than that of STO/FePt. As larger lattice mismatch is favored to induce a higher PMA into the FePt films, MgO substrates are exclusively used, but 3D island growth is indispensable. This work demonstrates that tuning the surface free energy enables us to achieve a large PMA and flat film surface in FePt epitaxial films on MgO. The results also indicate that modifying the surface free energy is pertinent for the flexible functional design of thin films.

Photoinduced Reorientation in Thin Films of a Nematic Liquid Crystalline Polymer Anchored to Interfaces and Enhancement Using Small Liquid Crystalline Molecules.

The photoinduced reorientation of the side-chain mesogens in nematic liquid crystalline (LC) polymer thin films triggered by the axis-selective photo-Fries rearrangements of side-chain phenyl benzoate moieties is studied to understand the regulation of the anisotropic nanostructures supported by LC polymers. The influence of the substrate surface in anchoring the side-chain mesogens near the interfaces is examined by comparing the reorientation of 30- and 120-nm-thick films. Irradiation with linearly polarized ultraviolet (UV) light and subsequent annealing causes the side-chain mesogen reorientation to align perpendicular to the electric field of the incident UV light. The inplane order in the 30-nm-thick films is lower than that in the 120-nm ones. On the other hand, the annealing period required for mesogen alignment is independent of the film thickness. It is suggested that the substrate surfaces anchor the LC mesogens to fix their orientation, rather than slowing down the reorientational motion. In addition, it is demonstrated that small LC molecules miscible with the nematic LC polymer enhance photoinduced reorientation through cooperative molecular interaction with the side-chain mesogens, remarkably accelerating the orientation and improving the inplane order of the unidirectionally aligned mesogens.

Selection of di(meth)acrylate monomers for low pollution of fluorinated mold surfaces in ultraviolet nanoimprint lithography.

We used fluorescence microscopy to show that low adsorption of resin components by a mold surface was necessary for continuous ultraviolet (UV) nanoimprinting, as well as generation of a low release energy on detachment of a cured resin from a template mold. This is because with low mold pollution, fracture on demolding occurred at the interface between the mold and cured resin surfaces rather than at the outermost part of the cured resin. To achieve low mold pollution, we investigated the radical photopolymerization behaviors of fluorescent UV-curable resins and the mechanical properties (fracture toughness, surface hardness, and release energy) of the cured resin films for six types of di(meth)acrylate-based monomers with similar chemical structures, in which polar hydroxy and aromatic bulky bisphenol moieties and methacryloyl or acryloyl reactive groups were present or absent. As a result, we selected bisphenol A glycerolate dimethacrylate (BPAGDM), which contains hydroxy, bisphenol, and methacryloyl moieties, which give good mechanical properties, monomer bulkiness, and mild reactivity, respectively, as a suitable base monomer for UV nanoimprinting under an easily condensable alternative chlorofluorocarbon (HFC-245fa) atmosphere. The fluorescent UV-curable BPAGDM resin was used for UV nanoimprinting and lithographic reactive ion etching of a silicon surface with 32 nm line-and-space patterns without a hard metal layer.

Surface-assisted unidirectional orientation of ZnO nanorods hybridized with nematic liquid crystals.

Inorganic semiconductor nanorods are regarded as the primary components of optical and electrical nanoscale devices. In this paper, we demonstrate the unidirectional alignment of monolayered and dispersed ZnO nanorods on a rubbed polyimide alignment layer, which was achieved by a conventional liquid crystal alignment technique. The outermost surfaces of the ZnO nanorods (average diameter 7 nm; length 50 nm) were modified by polymerization initiator moieties, and nematic liquid crystalline (LC) methacrylate polymers were grown by atom transfer radical polymerization. By regulating the densities of the polymerization initiator moieties, we successfully hybridized LC-polymer-grafted ZnO nanorods and small nematic LC molecules. The LC-polymer-modified ZnO nanorods were hierarchically aligned on the substrate via cooperative molecular interactions among the liquid crystal mesogens, which induced molecular orientation on the rubbed polyimide alignment layer.

Photochemically grafted polystyrene layer assisting selective Au electrodeposition.

We describe the selective electrodeposition of submicrometer gold (Au) patterns achieved by a thin film resist layer of polystyrene (PS) that was exposed to ultraviolet (UV) light on a photoreactive monolayer of a benzophenone-containing alkylthiol formed on a Au-plated substrate and patterned by thermal nanoimprint lithography. The presence of a PS graft layer caused by the benzophenone monolayer photochemistry at an interface between the PS resist layer and photoreactive monolayer played the important role of suppressing the unfavorable growth of tiny Au grains in regions masked with the PS resist layer, resulting in the selective Au electrodeposition in aperture regions of PS resist patterns. The suppressive effect on selective Au electrodeposition depended on the molecular weight of PS used as a resist material. Among unimodal PSs having weight-average molecular weights (M(w)'s) of 2100, 10,900, and 106,000 g mol(-1), the PS of M(w) = 10,900 g mol(-1) functioned most effectively as the resist layer. Au electrodeposition at a low current density allowed the preparation of Au lines having widths of submicrometers and a uniform height independent of line widths in resist aperture regions. Submicrometer bump structures of Au lines could be fabricated on transparent silica substrates by the subsequent wet etching of a Au electrode layer and then a chromium adhesive layer.

Location of the Azobenzene Moieties within the Cross-Linked Liquid-Crystalline Polymers Can Dictate the Direction of Photoinduced Bending.

We present a simple way to control the photoinduced bending direction of azobenzene-containing cross-linked liquid-crystalline polymers. By changing the location of the photoactive azobenzene moieties from cross-links to side-chains, the bending direction of the sample is reversed under identical irradiation conditions. In addition to providing a versatile route toward directionality control of the photoinduced macroscopic motions, this observation highlights the complicated nature of the photomechanical response of azobenzene-containing cross-linked liquid-crystalline polymers, showing that the photomobile behavior can be determined by seemingly small details on the materials design.

Structural color films with lotus effects, superhydrophilicity, and tunable stop-bands.

The structural blue color of a Morpho butterfly originates from the diffraction of light and interference effects due to the presence of the microstructures on the wing of the butterfly. Structural color on the surface of a damselfish reversibly changes between green and blue. Inspired by these creatures, we have been trying to prepare high-quality and functional structural color films. We describe our efforts in this Account. A useful technique to prepare such structural color films in colloidal solution is a "lifting" method, which allows us to quickly fabricate brilliant colloidal crystal films. The thicknesses of the films can be controlled by precisely adjusting the particle concentration and the lifting speed. Moreover, in order to prepare a complicated structure, we have used template methods. Indeed, we have successfully prepared the inverse structure of the wing of a Morpho butterfly with this technique. Initially, however, our structural color films had a whitish appearance due to the scattering of light by defects in the colloidal crystal film. Later, we were able to prepare a non-whitish structural color film by doping an appropriate dye in the colloidal particles to absorb the scattering light. In addition to the structural blue color, the wing of the Morpho butterfly has superhydrophobic properties. According to Wenzel's equation, the hydrophobic and hydrophilic properties are enhanced when the roughness of the hydrophobic and hydrophilic surface is increased, respectively. Based on this mechanism, we have successfully prepared structural color films with superhydrophobic properties, as well as with superhydrophilic properties. Another important property that can be seen in nature is tunable structural color, such as the color change that can be seen on the surface of a damselfish. In order to mimic such color change, we have developed several tunable structural color films. In particular, we have successfully prepared phototunable photonic crystals using photoresponsive azobenzene derivatives. In order to apply these structural color films, we developed a technique for patterning them by taking advantage of the wettability of the substrate surface. These materials can be used in the future for self-cleaning pigments and tunable photonic crystals.

Tunability of the refractive index of gold nanoparticle dispersions.

Alkanethiol-capped gold nanoparticles dispersed in n-dodecane were studied by spectroscopic ellipsometry and were modeled using Mie scattering theory. The refractive index in the visible and near-infrared depended on the volume fraction of gold nanoparticles, in good agreement with the theoretical expectation that such dispersed plasmonic nanoparticles can act as low or tunable refractive index materials at specific optical wavelengths.

Electric-field-induced conductance switching in FeCo Prussian blue analogues.

FeCo Prussian blue analogues, which are known as typical molecule-based magnets, exhibited abrupt conductance switching by applying a high electric field as well as by varying the temperature. The current density versus electric field (J-E) curves of FeCo Prussian blue with Rb cations in interstitial sites shows so-called negative resistance effects at electric fields higher than the threshold voltage. This means that the FeCo Prussian blue analogues are multiproperty materials in the sense that their conducting, magnetic, and optical properties can be reversibly controlled by certain external stimuli.

Tunable photonic band gap crystals based on a liquid crystal-infiltrated inverse opal structure.

Composite materials comprised of nematic liquid crystals (LCs) and SiO(2) inverse opal films were fabricated. Their optical properties were quite different from those of inverse opal films without the LCs. The optical properties could be controlled by changing the refractive indices of the LCs, which vary with orientation, phase, and temperature. In particular, the optical properties were drastically changed by thermal or photoinduced isothermal phase transitions of the LCs. This means that the photonic band structure could be controlled, and tunable photonic crystals have been achieved, based on the inverse opal structure. The mechanism of this change was investigated by the evaluation of the effective refractive indices. As a result, it was found that the change in optical properties was derived from the orientation of the LC molecules in the voids in the inverse opal film. Furthermore, once the mechanism was understood, it was also possible to control the position of the reflection peak by changing the alignment of the LCs. Such materials have the possibility for practical use in optical devices and fundamental research systems.

Control of the optical band structure of liquid crystal infiltrated inverse opal by a photoinduced nematic-isotropic phase transition.

Recently, photonic band gap (PBG) crystals with lattice parameters comparable to the wavelength of light have attracted much attention, because they offer unique ways in which to control the propagation of light. PBG crystals have applications in laser, quantum optical devices, and so on. For many of these applications, it is important to have the capability of tuning the photonic band structures. The fabrication of such tunable PBG crystals is still a challenge. In this paper, we proposed that switchable PBG crystals could be realized by taking advantage of the phase transition in liquid crystals induced by the photoisomerization of azo dyes. A dynamic change in the optical stop band was demonstrated. Such photoswitchable PBG crystals provide a method by which light can be routed using light.