Liquid crystal alignment in nanoporous anodic aluminum oxide layer for LCD panel applications

Homeotropic orientation of an ion-channel forming mesophase induced by nanotemplate wetting

Anodic aluminum oxide (AAO) membranes were used as templates to control orientation of an ion-channel forming columnar mesophase obtained by self assembly of a wedge-shaped sulfonate molecule. Inside the AAO structure, the director vector of the mesophase is oriented parallel to the pore axis due to the confinement effect. The molecular arrangement induced by the spatial confinement within the pores is extended over several microns into the remnant film on the AAO surface. The homeotropic alignment of the channels promotes unidimensional ion conduction through the film plane, which is manifested by a considerable increase in conductivity relative to isotropic samples.

Electro-optic hybrid aligned nematic device utilizing carbon nanotube arrays and two-dimensional hexagonal boron nitride nanosheet as alignment substrates

Hybrid-aligned nematic (HAN) liquid crystal (LC) devices have both fundamental and technological importance for their applications in LC adaptive lenses, low voltage LC displays, smart windows, and many more. We report the fabrication and characterization of a nanostructure-based HAN device employing vertically aligned carbon nanotube (VA-CNT) arrays as the homeotropic alignment agent on one side and two-dimensional (2D) hexagonal boron nitride (h-BN) as the planar alignment agent on the other side of the LC cell. The LC achieves the HAN configuration in the cell, i.e., homeotropic alignment at the VA-CNT side due to the π-π stacking interaction between the LC and CNTs, and planar alignment at the h-BN side due to the π-π stacking interaction between the LC and h-BN. When an applied electric field is ramped up across this VA-CNT/h-BN HAN cell, the LC (positive anisotropic) obtains a homeotropic state, requiring no threshold voltage to start the reorientation process; this effect is similar to that of a traditional polyimide (PI)-based HAN device. This VA-CNT/h-BN HAN cell successfully demonstrates the optical, electro-optical operations and the electric field-induced dynamic response. This study reveals that two inorganic nanostructured surfaces, VA-CNT arrays and 2D h-BN, can efficiently replace the organic PI alignment agents when needed and retain the HAN device's necessary electro-optical performances. These results substantially expand the fundamental understanding and the scope of utilizing various nanostructured surfaces for LC alignment mechanisms.

Controlling the Multiscale Topography of Anodized Aluminum Oxide Nanowire Structures for Surface-Enhanced Raman Scattering and Perfect Absorbers

In this study, a plasmonically active substrate is developed with the aim of controlling the perfect absorption and manipulating its optical properties for application in SERS (in NIR regime) and colorimetry. Based on modified fabrication method of anodized aluminum oxide (AAO), the cost-effective self-aggregation technique is presented to fabricate unique topography of bone-fire-like funnel-shaped collapsed and vertically aligned nanowire structures. The length of the nanowire and the modification of surface topography induced by capillary force inside the nanowire are set to structural parameters, and the effect of their changes is closely studied. After deposition of 40 nm gold (Au) film on numerous AAO nanowire structures with different wire lengths and unique topography, the localized surface plasmon resonance excitation is generated, and also its application on reflection and SERS spectra have been shown quantitatively. The length of the wire and surface topography modification are identified as suitable parameters to tune the reflection/absorption (from <40 to >90%) as well as colorimetric effect. Finally, an optimized wire length of Au-coated AAO substrate in SERS sensing application with 3.92 × 10⁵ order of enhancement of rhodamine 6G (R6G) Raman signal is demonstrated.

The toolbox of porous anodic aluminum oxide–based nanocomposites: from preparation to application

Anodic aluminum oxide (AAO) templates have been intensively investigated during the past decades and have meanwhile been widely applied through both sacrificial and non-sacrificial pathways. In numerous non-sacrificial applications, the AAO membrane is maintained as part of the obtained composite materials; hence, the template structure and topography determine to a great extent the potential applications. Through-hole isotropic AAO features nanochannels that promote transfer of matter, while anisotropic AAO with barrier layer exhibits nanocavities suitable as independent and homogenous containers. By combining the two kinds of AAO membranes with diverse organic and inorganic materials through physical interactions or chemical bonds, AAO composites are designed and applied in versatile fields such as catalysis, drug release platform, separation membrane, optical appliances, sensors, cell culture, energy, and electronic devices. Therefore, within this review, a perspective on exhilarating prospect for complementary advancement on AAO composites both in preparation and application is provided.

Homeotropic liquid crystal device employing vertically aligned carbon nanotube arrays as the alignment agent

Vertically aligned carbon nanotube (VA-CNT) arrays were grown on several chromium (Cr)-coated glass substrates using a plasma-enhanced chemical vapor deposition system. The CNTs were 2μm long and had a site density of 2×10^{9}cm^{-2} on the substrates. Two VA-CNT slides on Cr glass substrates were put together to design a homeotropic electro-optic liquid crystal (LC) device. A negative dielectric anisotropic LC was used in the device. The π-π stacking interaction between the LC and the VA-CNTs allows the LC material to align homeotropically in the cell. When an external electric field was applied using the transparent conducting Cr layers, the LC achieves a planar orientation above a threshold field. These results successfully demonstrate the optical, electro-optical operations, and the field-induced dynamic response of a homeotropic LC device employing the VA-CNT arrays as the homeotropic-alignment agent. This study significantly advances the range and understanding of nanostructured surfaces that provide vertical alignment of LCs.

Adsorption of the Cationic Dye Methylene Blue on Anodic Porous Alumina in Sodium Dodecyl Sulfate Solutions

Dyeing of anodic porous alumina (APA) prepared by aluminum anodization is generally achieved by dipping the positively charged APA surface into a negatively charged dye solution. We have proposed a new method to adsorb dyes and molecules onto APA using negatively charged sodium dodecyl sulfate (SDS). In this study, we found that cationic methylene blue (MB) can be adsorbed onto the positively charged APA surface using SDS aqueous solutions. We investigated two adsorption methods: dipping APA into aqueous solutions containing both MB and SDS (method 1) and successive dipping of APA into SDS and then MB aqueous solutions (method 2). The two methods produced different adsorption characteristics. Method 1 adsorption profile reflected formation of dye-rich induced micelles below the critical micellar concentration (CMC) and electrostatic interaction of micelles with MB above CMC. Method 2 adsorption was explained by electrostatic interaction of preadsorbed SDS with APA and MB.

Periodic nanostructure-induced change of director profiles and variable stop bands of photonic crystals infiltrated by nematic liquid crystals

Photonic crystals with periodic nanostructures infiltrated by nematic liquid crystals were investigated based on Landau-de Gennes theory. We studied the fine structures of the system within different amplitudes on the sinusoidal boundaries. When modulating the amplitude, the location of the defects will change. Two new director profiles occurred, and the state observed in Appl. Phys. Lett.87, 241108 (2005)APPLAB0003-695110.1063/1.2139846 also appeared. The transmittance curves show a redshift of ${\sim} {0.1}\,\,\unicode{x00B5}{\rm m}$∼0.1µm in the mid-infrared spectra when increasing the amplitude. The location change of defect rings will induce a shift of ${\sim} 22.4\,\,{\rm nm}$∼22.4nm. Variations in sinusoidal boundaries will have an effect on the transmittance spectrum. Elastic anisotropic will also induce a small shift when the structure is fixed. Results could be useful in designing new types of photonic crystal devices or sensors.

Fast Response and Spontaneous Alignment in Liquid Crystals Doped with 12-Hydroxystearic Acid Gelators

The spontaneous vertical alignment of liquid crystals (LCs) in gelator (12-hydroxystearic acid)-doped LC cells was studied. Gelator-induced alignment can be used in both positive and negative LC cells. The electro-optical characteristics of the gelator-doped negative LC cell were similar to those of an LC cell that contained a vertically aligned (VA) host. The rise time of the gelator-doped LC cell was two orders of magnitude shorter than that of the VA host LC cell. The experimental results indicate that the gelator-induced vertical alignment of LC molecules occurred not only on the surface of the indium tin oxide (ITO) but also on the homogeneous alignment layer. Various LC alignments (planar, hybrid, multistable hybrid, and vertical alignments) were achieved by modulating the doped gelator concentrations. The multistable characteristic of LCs doped with the gelator is also presented. The alignment by doping with a gelator reduces the manufacturing costs and provides a means of fabricating fast-responding, flexible LC displays using a low-temperature process.

Molecular Orientation of Liquid Crystals on Topographic Nanopatterns

Controlling the orientation of building blocks in soft matter on the substrate has been a big challenge in material sciences. We have controlled the molecular orientation of liquid crystal (LC) materials on the porous anodic aluminum oxide (AAO) film having hexagonal pore arrays on the top surface. In our method, anchoring conditions can be varied by changing the pore size (Dp) and the porosity (P). As a proof-of-concept, the orientation of smectic A (SmA) structure at different anchoring conditions was successfully controlled in a sandwich cell consisting of AAO and a glass substrate, which has not been successfully controlled by conventional methods.

Growth and characterization of zeolitic imidazolate framework-8 nanocrystalline layers on microstructured surfaces for liquid crystal alignment

The coverage of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals deposited on patterned sol–gel films is significantly affected by the surface morphology. The ZIF-8 layer can induce vertical alignment of a typical nematic liquid crystal (LC) E7.

Physico-chemical confinement of helical nanofilaments

Helical nanofilaments (HNFs) have attracted much interest because of their unique optical properties, but there have been many hurdles to overcome in using them for the practical applications due to their structural complexity. Here we demonstrate that the molecular configuration and layer conformation of a modulated HNF (HNFs(mod)) can be studied using a physicochemical confinement system. The layer directions affected by the chemical affinity between the mesogen and surface were drastically controlled in surface-modified nanochannels. Furthermore, an in situ experiment using grazing-incidence X-ray diffraction (GIXD) was carried out to investigate in detail the structural evolution through thermal transitions. The results demonstrate that the HNF(mod) structure can be perfectly controlled for functional HNF device applications, and a combined system with chemical and physical confinement effects will be helpful to better understand the fundamentals of soft matter.

Tunable Surface Wettability of ZnO Nanoparticle Arrays for Controlling the Alignment of Liquid Crystals

The control of the liquid crystal (LC) alignment is very important for both academic research and practical applications. LC molecules aligned on the ZnO nanoparticle arrays (ZnO NPAs) are demonstrated and the pretilt angles of LCs can be controlled by using ZnO NPAs with different surface wettability. The wettability of ZnO NPAs fabricated by the solution-based hydrothermal method can be controlled by changing the annealing temperature of the as-prepared ZnO NPAs. The measurements of the energy-dispersive spectra and photoluminescence have shown that the chemical properties of ZnO NPAs have been changed with the annealing temperature. Our results show that the pretilt angle of LCs can be tuned continuously from ∼0 to ∼90° as the contact angle of water on ZnO NPAs changes from 33 to 108°.

Liquid crystal alignment on zinc oxide nanowire arrays for LCDs applications

The zinc oxide (ZnO) nanowire arrays on the indium tin oxide (ITO) glass substrates were fabricated by using the two-step hydrothermal method. A high transmittance ~92% of ZnO nanowire arrays on ITO substrate in the visible region was obtained. It was observed that the liquid crystal (LC) directors were aligned vertically to the (ZnO) nanowire arrays. The properties of ZnO nanowire arrays as vertical liquid crystal (LC) alignment layers and their applications for hybrid-aligned nematic LC modes were investigated in this work.

Observation of two isotropic-nematic phase transitions near a surface

Using specified conditions, we succeeded in observing the isotropic-nematic (Iso-N) liquid crystal phase transition at surfaces followed by that in bulk for the first time. An additional heat anomaly peak was found at a higher temperature side of a main phase transition peak using highly sensitive differential scanning calorimetry (HS-DSC). The peak is pronounced particularly in the cooling process, since the transition starts at surfaces on cooling. The temperature dependence of retardation allows us to safely conclude that the higher temperature peak that appeared in HS-DSC is attributed to the Iso-N transition at surfaces. These measurements also indicate that the surface transition is of first order. These behaviors were theoretically explained by generalized Maier-Saupe theory.