Liquid Crystal Alignment Research Articles

Label-free liquid crystal-based optical detection of norfloxacin using an aptamer recognition probe in soil and lake water.

Norfloxacin (NOX), a broad spectrum fluoroquinolone (FQ) antibiotic, is commonly detected in environmental residues, potentially contributing to biological drug resistance. In this paper, an aptamer recognition probe has been used to develop a label-free liquid crystal-based biosensor for simple and robust optical detection of NOX in aqueous solutions. Stimuli-receptive liquid crystals (LCs) have been employed to report aptamer-target binding events at the LC-aqueous interface. The homeotropic alignment of LCs at the aqueous-LC interface is due to the self-assembly of the cationic surfactant cetyltrimethylammonium bromide (CTAB). In the presence of the negatively charged NOX aptamer, the ordering changes to planar/tilted. On addition of NOX, the aptamer-NOX binding causes redistribution of CTAB at the LC-aqueous interface and the homeotropic orientation is restored. This results in a bright-to-dark optical transition under a polarized optical microscope (POM). This optical transition serves as a visual indicator to mark the presence of NOX. The devised aptasensor demonstrates high specificity with a minimum detection limit of 5 nM (1.596 ppb). Moreover, the application of the developed aptasensor for the detection of NOX in freshwater and soil samples underscores its practical utility in environmental monitoring. This proposed LC-based method offers several advantages over conventional detection techniques for a rapid, feasible and convenient way to detect norfloxacin.

Dielectric Meta-Atoms with Liquid Crystal Alignment Effect for Electrically Tunable Metasurface

Dielectric metasurfaces with tunable functions are increasingly appealing to both the academia and industrial communities. Among the many tuning mechanisms reported so far, metasurface immersed in liquid crystal (LC) stands out as one of the most viable approaches for industrial applications such as Lidar and 3D sensing. However, as with traditional LC devices, alignment processes are necessary for LC-based tunable metasurfaces, leading to additional costs in terms of time and materials. In this paper, we present a novel LC-based tunable metasurface, wherein the alignment of LC is provided by the metasurface structures. LC serves as the dielectric environment for the metasurface with an electrically tunable refractive index. Simultaneously, purposely engineered metasurface induces alignment effects within the LC. The alignment effect of the metasurface on the LC was quantitatively characterized using crossed polarizers. A marked transmission contrast was observed, indicating the high-quality alignment provided solely by the metasurface. Electrical tunability was also measured, recording a maximum modulation depth in transmission amplitude of 94% at near-infrared telecommunication wavelengths. This work highlights the remarkable advantages of LC-based tunable metasurface devices, making them highly valuable for next-generation metasurface-integrated LC on silicon devices.

Cost-effective photolithography-based dual liquid crystal alignment for versatile electro-optic applications

Nematic liquid crystals (NLCs) containing rod-shaped molecules exhibit remarkable birefringence and tunability. Therefore, they are of high interest for numerous electro-optic applications. Uniform molecular orientation is of utmost importance for their effective use. Current techniques for LC alignment, such as mechanical rubbing or photo-alignment, have limitations in creating specifically complex LC orientation patterns. To address this issue, we have developed a cost-effective photolithography-based technique for dual LC alignment which combines planar and vertical alignments. Our process involves inexpensive exposure systems and a lift-off process and employs commercially-available alignment materials, thereby reducing production costs. We utilized this process to generate a tunable phase grating with diffraction efficiencies which can be varied through voltage application, thus demonstrating the potential of dual alignment. Furthermore, we investigated smart windows that are insensitive to polarization, utilizing a two-dimensional grating structure, achieving high haze values and dynamic shading without relying on incident polarization.

Spontaneously homogeneous alignment of liquid crystals on self-assembly organic rubrene

A novel spontaneously homogeneous liquid crystal (LC) alignment with self-assembled organic rubrene layer on the substrate surface has been reported. The spontaneous alignment technique without surface pre-treatment results in low cost, short processing time, and the room-temperature processing extends application to plastic device. The rubrene-nematic LC mixture is used to fabricate the LC cell. During the capillary filling, the LC molecules align in the flow direction. Meanwhile, the flow-induced LC alignment enforces the tetracene of the rubrene dopants along the flow direction because of the van der Waals and π–π interactions between the rubrene and LC molecules. Afterwards, the rubrene dopants spontaneously migrate onto substrate surface and self-assemble as the triclinic crystalline rubrene (TCR) with ribbon morphology. The LC alignment on TCR has a low pretilt angle, thereby providing a homogeneous LC alignment spontaneously. LC alignment on TCR shows a higher thermal stability compared to conventional polyimide due to the strong interactions of the rubrene and LC molecules. Moreover, the rubrene molecules remain in the bulk suppress the free-ions, decrease the rotational viscosity of LCs and hence shortening the response time of cell. The suppressed free-ions also decrease the screening effect on the substrate surfaces and hence the operation voltage of cell. The TCR structure on the substrate surface and the residual rubrene molecules dispersed in the bulk simultaneously contribute to the enhancement in electro-optical performance of the LC cell.

Harnessing Smectic Ordering for Electric-Field-Driven Guided-Growth of Surface Topography in a Liquid Crystal Polymer.

The guided-growth strategy has been widely explored and proved its efficacy in fabricating surface micro/nanostructures in a variety of systems. However, soft materials like polymers are much less investigated partly due to the lack of strong internal driving mechanisms. Herein, the possibility of utilizing liquid crystal (LC) ordering of smectic liquid crystal polymers (LCPs) to induce guided growth of surface topography during the formation of electrohydrodynamic (EHD) patterns is demonstrated. In a two-stage growth, regular stripes are first found to selectively emerge from the homogeneously aligned region of an initially flat LCP film, and then extend neatly along the normal direction of the boundary line between homogeneous and homeotropic alignments. The stripes can maintain their directions for quite a distance before deviating. Coupled with the advanced tools for controlling LC alignment, intricate surface topographies can be produced in LCP films starting from relatively simple designs. The regularity of grown pattern is determined by the LC ordering of the polymer material, and influenced by conditions of EHD growth. The proposed approach provides new opportunities to employ LCPs in optical and electrical applications.

Elimination of Halo mura in homogeneous alignment liquid crystal displays based on dichroic absorption

Elimination of Halo mura in homogeneous alignment liquid crystal displays based on dichroic absorption

High electrical characteristics through graphene oxide doping process on physicochemically reformed inorganic thin films.

This study investigated the improvement of the electro-optical properties of a liquid crystal (LC) cell fabricated through brush coating using graphene oxide (GO) doping. The physical deformation of the surface was analyzed using atomic force microscopy. The size of the groove increased as the GO dopant concentration increased, but the direction of the groove along the brush direction was maintained. X-ray photoelectron spectroscopy analysis confirmed that the number of C-C and O-Sn bonds increased as the GO concentration increased. Since the van der Waals force on the surface increases as the number of O-metal bonds increases, we were able to determine why the anchoring energy of the LC alignment layer increased. This was confirmed by residual DC voltage and anchoring energy measurements that were later performed. As the GO concentration increased, the width of the hysteresis curve decreased, indicating that the residual DC voltage decreased. Additionally, the 15% GO-doped sample exhibited a significant increase in its anchoring energy up to 1.34 × 10-3J/m2, which is similar to that of rubbed polyimide. It also secured a high level of electro-optical properties and demonstrated potential as a next-generation thin-film display despite being produced via a simple brush-coating process.

Influence of orange azo dichroic dye on morphological, electro-optical, phase transition and absorption behaviour of ZnO nanoparticles-induced homeotropically aligned liquid crystal display cell

Influence of orange azo dichroic dye on morphological, electro-optical, phase transition and absorption behaviour of ZnO nanoparticles-induced homeotropically aligned liquid crystal display cell

Bismuth-magnesium-oxide-based graphene oxide hybrid film for liquid crystal device application

Hybrid films containing bismuth magnesium oxide (BiMgO) and graphene oxide (GO) are investigated in this study. Solution-processed brush coating is used to fabricate the hybrid films, with the graphene oxide weight ratios adjusted to 0, 5, and 15%. The hybrid films show more stable and higher optical transmittances than the pure BiMgO film. A directional micro/nanogroove structure is observed on the hybrid film surface, which is derived from the shear stress during the brush-coating process; this anisotropic structure is used as a uniform liquid crystal (LC) alignment layer. Uniform and homogeneous LC alignment is then demonstrated via polarized optical microscopy and pretilt angle analyses, along with perfect light-control performance. Enhanced LC polar anchoring energy and hysteresis-free characteristics are observed for the brush-coated hybrid film as the GO-doping concentration is increased. Based on this perspective, the BiMgO-based GO hybrid film achieved via brush coating has potential for applications in next-generation LC systems.

Multifunctional Al-doped ZnO thin films for vertically aligned liquid crystal devices

The integration of ITO-free transparent conductive layers remains a big challenge for development of next generation technologies. Here, we demonstrate the feasibility of transparent and conductive Aluminum-doped Zinc Oxide (AZO) thin films to operate simultaneously as electrode and alignment layer in Liquid Crystal (LC) device configuration. Controlling the growth process of AZO thin films by Atomic Layer Deposition (ALD) technique results of predominantly (100) crystallographic oriented AZO with very high transparency and excellent conductivity. The exceptionally low surface free energy of (100) oriented AZO, along with the topological modification following the mechanical rubbing treatment were used to elucidate the mechanism of LC molecules alignment on the surface of AZO film. The uniform vertical orientation of the nematic LC director was confirmed by polarized optical microscopy and pretilt angle measurements. The competitive electro-optical performance in terms of phase modulation, threshold and saturation voltages and contrast ratio of the assembled LC device reveals the great potential of AZO thin films as transparent electrode and alignment layer for future LC display applications with versatile functionality.

Dynamic propagation of an Airy beam in metasurface-enabled gradiently-aligned liquid crystals

Abstract Due to the unique self-acceleration, self-healing, and non-diffraction properties, Airy beams have been explored extensively and found applications in various fields. It has been proven as an essential aspect to tune the trajectory of Airy beams for extensive applications. In this paper, we propose a method based on liquid crystal (LC) alignment with metasurfaces, which enables dynamic tuning of the trajectory of Airy beams. Benefiting from both the tunable property of LCs and the compact alignment of metasurfaces, we achieve a sizeable linear potential in a short distance, which leads to the effective tuning of the trajectory of Airy beams dynamically. The introduction of metasurfaces into the alignment of LCs provides a promising method to manipulate the planar optical field.

Graphene-oxide-doped oriented nickel oxide film achieved through brush coating for liquid crystal system

Hybrid films containing nickel oxide and graphene oxide are introduced in this work. The hybrid films are fabricated through a one-step brush-coating process, and the weight ratios of graphene oxide are controlled to 0, 5, and 15 %. The fabricated hybrid films show oriented nano/microgroove structures on their surfaces by directional shear stress, which originates from the brush hair movements during the solution-processed brush-coating process. The hybrid film formation is confirmed using X-ray photoelectron spectroscopy. The films also demonstrate high optical transmittance with amorphous structures. The hybrid films are then used to form liquid crystal (LC) alignment layers, and uniform as well as homogeneous LC alignment is confirmed via polarized optical microscopy and pretilt angle analyses. Compared to the pure nickel-oxide film, the hybrid film with graphene oxide enhances LC alignment stability by increasing the polar anchoring energy of the surface LC molecules. From these observations, the hybrid film containing metal oxide and graphene oxide is expected to be a promising alternative for functional LC systems.

Ultrathin, Large‐Area, and Multifunctional Polarizer Based on Highly Ordered Carbon Nanotubes Produced by Simple Shear Flow

AbstractAligning carbon nanotubes (CNTs) with high orientational ordering in a 2D array is essential for realizing optical polarizers with high polarization efficiency over a wide spectral range. Two methods are reported: dispersing CNTs in a polymer matrix followed by stretching, and mechanical stretching of a vertically grown CNT forest and then transferring it onto a substrate. However, neither can realize nanometer‐thin or large polarizers. Herein, a novel approach is demonstrated to construct a large and thin (150 nm) conductive CNT polarizer by achieving liquid crystal (LC) phase of size‐controlled CNTs in chlorosulfonic acid, unidirectional shearing, and then drying. The polarizer exhibits an excellent polarization efficiency of 98.4% for ultraviolet (365 nm) and 96.0% for visible light (550 nm). This performance is maintained even at 400 °C, while the conventional iodine‐type polarizer starts to lose efficiency over 100 °C. The CNT polarizer with high polarization efficiency for UV can replace the costly nano‐patterned wire‐grid polarizers currently used for photoaligning LCs and also its multifunction playing role of polarizer, electrode, and LC alignment is confirmed with switching of LC device. This study provides a new paradigm for realizing ultrathin and large CNT polarizers for broadband applications with outstanding heat resistance.

Surface confinement of vertically aligned liquid crystals between flexible silver nanoparticles hybridized polyimide thin layers

Alignment of liquid crystals (LCs) has emerged as an essentially fundamental approach for exploring the electro-optical performance and fabricating novel LC devices. Growing evidence suggests that the surface wettability of the alignment layer plays a key role in tuning the tilt angles of LCs. In this work, silver (Ag) nanoparticles (d ≤ 100 nm) are blended in polyimide (PI) by a wide range of concentration from 0.1% to 1.0% to tune surface wettability and align LCs. Ag nanoparticles agglomerate and result in a bumpy surface topology and somewhat reduce the transparency of hybrid thin layer to 85.1% by 1.0% doping because of their shielding effect. Nevertheless, Ag nanoparticle agglomerates change the thickness of hybrid thin layers less, and the hybrid thin layers are ∼96 nm thick like a pure PI thin layer. LCs are vertically aligned on hybrid thin layers after rubbing. Benefit from the declined anchoring energy and the outstanding surface plasmon polariton (SPP) of Ag nanoparticles, the external electric field to switch LCs is substantial boosted, and hence remarkably diminish the operating voltage and likewise notably shorten the response time. In particular, LCs that vertically aligned between PI hybrid alignment layers by 0.5% doping could be electrically switched to become planner at 3.38 V and go back to their original vertical alignment state within 73.21 ms. Ag nanoparticles agglomerates in hybrid thin layers also fairly hinder the drift of LCs when cells are mechanically bent with various radii. The hybrid alignment layers cell that blended with 0.5% Ag nanoparticles could be bent under 1500 mm without any new evident light leakages and fluctuating electro-optical characteristics. These results indicate the promising applications of PI hybrid thin layers blended with Ag nanoparticles for flexible LC devices.

Ferroelectric liquid crystal array driven by a two‐layer electrode with a 1 × 1 μm pixel pitch for light modulation in electro‐holography

AbstractWe clarified that a ferroelectric liquid crystal (FLC) has high resolution display capability as small as 1 × 1 μm pixel pitch using an FLC pixel array with a two‐layer electrode, which has a 1 × 1‐μm‐checkered apertured electrode and a plane electrode separated by an insulation layer. By applying +2 V to the apertured electrode and −10 V to the plane electrode in the two‐layer electrode and 0 V to the transparent common electrode, a checkered pattern was clearly observed, which indicates the successful individual pixel driving with a pixel pitch of 1 × 1 μm. When fabricating 1 × 2‐μm‐pitch rectangular FLC pixels, we elucidated that the liquid crystal alignment direction should be along the shorter side of the pixels to avoid asymmetric transmittance distribution in each pixel. Moreover, we successfully reconstructed a 3D holographic image using 10 × 10 k FLC pixel array with a pitch of 1 × 1 μm driven by the two‐layer electrode with hologram‐patterned apertures. We showed that FLC is a strong candidate material for realizing spatial light modulator with extremely small pixel pitches, which is essential for holographic displays with wide‐viewing‐zone angles.

Effect of Azo Dye on Electro-Optic and Optical Band Gap Characteristics in ZnO Nanoparticles Induced Vertically Aligned Liquid Crystal Cells

Alignment of liquid crystal (LC) molecules in vertical direction is the most promising and interesting aspect for display devices in view of scientific and technological growth. Moreover, in accordance with the photovoltaic device applications, liquid crystals (LCs) show numerous applications. In the present work, the impact of dye as dopant with appropriate amount, over the electro-optic (E-O) and band gap properties as well as phase behaviour of vertically aligned liquid crystals (VALCs) has been studied and discussed. Initially, zinc oxide (ZnO) nanoparticles (NPs) were mixed to induce the vertical alignment (VA) in confined cell. Then, to prepare the dye doped LC sample, 0.125% of azo dye as dopant was uniformly mixed in the host sample. The results showed enhanced E-O characteristics with reduction in optical band gap as calculated using UV Visible study in 0.125% dye doped cell as compared to host sample cell.

The dichotomous role of anisotropic sensing in pattern generation and disruption

In this work, we explore the effects of an anisotropic sensing range on the behavior of migrating particles. We use a lattice-gas cellular automaton (LGCA) with liquid-crystal and ferromagnetic velocity alignment, and a sensing range modeled by a von Mises distribution. By performing simulations and mathematical analysis, we study the evolution of the system and the different patterns formed in each case. We observe that, increasing the anisotropy of the sensing range can have different effects on the density patterns. When particles interact ferromagnetically, sensing anisotropy has a deleterious effect which destabilizes patterns observed with an isotropic sensing range; while patterns change from thick bands to small clusters when anisotropy is increased with liquid-crystal interactions. The results suggest that organisms which coordinate through visual information could either benefit from, or be hidered by a restricted vision field depending on the nature of their pairwise interactions.

Parameters Affecting Interfacial Assembly and Alignment of Nanotubes.

Tangential flow interfacial self-assembly (TaFISA) is a promising scalable technique enabling uniformly aligned carbon nanotubes for high-performance semiconductor electronics. In this process, flow is utilized to induce global alignment in two-dimensional nematic carbon nanotube assemblies trapped at a liquid/liquid interface, and these assemblies are subsequently deposited on target substrates. Here, we present an observational study of experimental parameters that affect the interfacial assembly and subsequent aligned nanotube deposition. We specifically study the water contact angle (WCA) of the substrate, nanotube ink composition, and water subphase and examine their effects on liquid crystal defects, overall and local alignment, and nanotube bunching or crowding. By varying the substrate chemical functionalization, we determine that highly aligned, densely packed, individualized nanotubes deposit only at relatively small WCA between 35 and 65°. At WCA (< 10°), high nanotube bunching or crowding occurs, and the film is nonuniform, while aligned deposition ceases to occur at higher WCA (>65°). We find that the best alignment, with minimal liquid crystal defects, occurs when the polymer-wrapped nanotubes are dispersed in chloroform at a low (0.6:1) wrapper polymer to nanotube ratio. We also demonstrate that modifying the water subphase through the addition of glycerol not only improves overall alignment and reduces liquid crystal defects but also increases local nanotube bunching. These observations provide important guidance for the implementation of TaFISA and its use toward creating technologies based on aligned semiconducting carbon nanotubes.

Electrically driven kinklike distorting waves in microsized liquid crystals.

Electrically driven kinklike distortion regimes in a microsized liquid crystal channel have been investigated both experimentally and analytically. Kinklike distortion waves were excited by the interaction between the electric field E and the gradient ∇n[over ̂] of the director field in a homogeneously aligned liquid crystal (HALC) channel. Having obtained the evolution of the normalized light intensity, which was recorded by the high-speed camera, the process of excitation and evolution of the traveling wave in the HALC channel was visualized for the first time. It was shown, based on a nonlinear extension of the classical Ericksen-Leslie theory, that in the case when the electric field E≫E_{th}, the flow of liquid crystal material completely stops and a new mechanism for converting the electric field arises in the form of the electrically driven distorting traveling kinklike wave, which can be excited in the LC channel, composed of 4-n-pentyl-4^{'}-cyanobiphenyl molecules.

Nanoantenna induced liquid crystal alignment for high performance tunable metasurface

Abstract Liquid crystal (LC) based spatial light modulators (SLMs) are a type of versatile device capable of arbitrarily reconfiguring the wavefront of light. For current commercial LC-SLM devices, the large pixel size limits their application to diffractive optics and 3D holographic displays. Pixel miniaturization of these devices is challenging due to emerging inter-pixel crosstalk, ultimately linked to the thick LC layer necessary for full phase (or amplitude) control. Integration of metasurfaces, i.e., 2D arrangements of resonant nanoantennas, with thin LC has emerged as a promising platform to boost light modulation, enabling realization of sub-wavelength pixel size SLMs with full phase (or amplitude) control. In most devices realized so far, however, the presence of an alignment layer, necessary to induce a preferential initial LC orientation, increases the voltage requirement for resonance tuning and reduces the efficiency of light modulation, something that accentuates for an ultra-thin (e.g., submicron) metasurface-LC cell. Here, we present an alternative strategy by which the LC molecular alignment is purely controlled by the periodicity and geometry of the nanoantenna without any additional alignment layer. The nanoantennas are specifically designed for the double purpose of sustaining optical resonances that are used for light modulation and to, simultaneously, induce the required LC pre-alignment. The proposed device structure allows lower voltage and reduced switching times (sub-millisecond) compared to devices including the alignment layer. This novel strategy thus helps to improve the performance of these miniaturized-pixel devices, which have emerged as one of the potential candidates for the next generation of products in a wide range of applications, from virtual/augmented reality (VR/AR) and solid-state light detection and ranging (LiDAR), to 3D holographic displays and beyond.