Vertical Alignment Liquid Crystal Research Articles

Effect of varying concentration of dichroic dye on textural, electro-optical, phase transition and band gap studies of zinc oxide nanoparticles induced vertically aligned liquid crystals

Vertically aligned liquid crystal (VALC) perpetually boosts the advancement for display devices and optoelectronics. Herein, initially, the nanoparticles (NPs) induced VALC (nVALC) cell was prepared via doping of 0.3 (wt./wt.%) of zinc oxide NPs in nematic liquid crystal (LC). After that, a series of orange azo dichroic dye doped nVALC cells were prepared with varying concentrations of dye such as 0.0625, 0.125, 0.25 and 0.5 wt.%. The effect of dye concentrations on LC textures, phase transition, electro-optical and band gap properties were investigated, as well as resulted in a significantly reduced value of the threshold and operating voltages as a function of dye concentrations. The contrast ratio was also enhanced up to 0.25 wt.% dye doped nVALC cells. Additionally, the transition temperature from nematic to isotropic phase and band gap studies were also performed. The present work provides improved properties of nVALC systems for next-generation display technologies.

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.

Optical efficiency modulation of vertical alignment liquid crystal displays with transparent shielding and storage electrode

A high-transmittance vertical alignment liquid crystal display (VA-LCD) using a transparent shielding and storage electrode (TSS) design is proposed. The aperture ratio of the proposed four domain TSS pixel with size of 52 × 156 μm improves approximately by 40% over the conventional pixel due to the shielding and storage metal electrode can be replaced by TSS electrode. Experimental results shown that the transmittance of TSS designed panel with 3.2 μm cell gap is 35% higher than that with conventional pixel design, whereas it is only enhanced by 21% when the cell gap is 2.6 μm. The liquid crystal dynamics simulation results revealed that both the effective birefringence and the azimuthally controlled director profile of the liquid crystal layer decreases with TSS pixel design, and these adverse impacts are magnified as the cell gap decreases. This is due to that the introduction of TSS common electrode not only weakens the vertical electric field but also affects the horizontal electric field of liquid crystal layer, which leads to a greater loss of optical efficiency under lower cell gap, resulting in the worse transmittance promotion. The proposed method is expected to be advantageously employed in enhancing transmittance of VA-LCDs for power saving.

Analysis of morphological and electro-optical properties of silica nanoparticles induced vertically aligned liquid crystal - effect of doping and coating techniques

Conventionally, polyimide (PI) coating materials are used on the indium tin oxide (ITO) glass substrates to induce the vertical alignment (VA) of nematic liquid crystals (LCs). However, this coating technique requires a couple of steps such as spin coating, high-temperature baking and mechanical rubbing which limits their applicability to flexible LC display device applications. In the present work, VA of nematic LCs in a confined cell is induced using two different approaches: one via doping of silica nanoparticles (SNPs) as guest particles in LCs and the other with the deposition of SNPs layer on ITO glass substrates. Further, morphological and electro-optical (E-O) studies have been studied for the SNPs doped and SNPs coated vertically aligned liquid crystals (VALCs). The morphological study shows the identical degree of dark (black) and bright (white) textures of SNPs doped and coated VALC cells at OFF and ON states, respectively under the crossed polarizers. Also, the VA of nematic LC molecules in confined cells has been confirmed from the conoscopic behaviour. Consequently, morphological and E-O results revealed that the threshold (V th ) and operating (V o ) voltages required to switch the LCs from their initial VA (black state) towards minimum transition planar state and complete planar (bright) state, respectively, are reduced with better contrast ratio (CR) in SNPs coated VALC cell as compared with SNPs doped VALC cell.

Zinc Oleate Nanorod-Induced Vertical Alignment of Nematic Liquid Crystal.

Nanocomposite zinc oxide nanorods capped with oleic acid (ZOR) with positive dielectric anisotropy liquid crystal (LC) 4'-octyl-4-biphenylcarbonitrile (8CB) filled in unaligned cells exhibit homeotropic alignment of host LC molecules. Further, systematic investigation of the textural, dielectric, and conductivity properties of nanocomposites filled in planar cells is performed with increasing concentration of nanorods. At a nanorod concentration ≤0.2 wt % in 8CB, the order parameter of nanocomposite samples is found to be increasing and ionic conductivity is found to be decreasing as compared to pure LC. Beyond 0.3 wt % concentration of nanorods in 8CB, vertical alignment (VA) of host LC is observed even in a planar aligned cell. The VA of LC molecules in ZOR nanocomposites is confirmed through attenuated total reflection-Fourier transform infrared absorption spectra studies.

Vertical-aligned liquid crystal devices for ocular wavefront correction and simulation

The potential of vertical-aligned spatial light modulators for use in adaptive-optics visual simulators is demonstrated. We performed visual acuity and contrast sensitivity tests in different subjects, with their eye’s optics corrected by the custom adaptive-optics system and with induced aberrations similar to those in severe keratoconus. We tested the system in a see-through configuration, achieving a total corrected field of view of ∼13 deg. The applicability of these modulation devices for a wearable visual adaptive optics system is discussed.

Self-Folding Liquid Crystal Network Filaments Patterned with Vertically Aligned Mesogens.

Fibrous soft actuators with high molecular anisotropy are of interest for shape morphing from 1D to 2D and 3D in response to external stimuli with high actuation efficiency. Nevertheless, few have fabricated fibrous actuators with controlled molecular orientations and stiffness. Here, we fabricate filaments from liquid crystal networks (LCNs) with segmental crosslinking density and gradient porosity from a mixture of di-acrylate mesogenic monomers and small-molecule nematic or smectic liquid crystals (LCs) filled in a capillary. During photopolymerization, phase separation between the small-molecule LCs and LCN occurs, making one side of the filament considerably denser than the other side. To direct its folding mode (bending or twisting), we control the alignment of LC molecules within the capillary, either along or perpendicular to the filament long axis. We show that the direction of UV exposure can determine the direction of phase separation, which in turn direct the deformation of the filament after removal of the small-molecule LCs. We find that the vertical alignment of LCs within the filament is essential to efficiently direct bending deformation. By photopatterning the filament with segmental crosslinking density, we can induce a reversible folding/unfolding into 2D and 3D geometries triggered by deswelling/swelling in an organic solvent. Moreover, by taking advantage of the large elastic modulus of LCNs and large contrast of the modulus before and after swelling, we show that the self-folded LCP filament could act as a strong gripper.

Polymer sustained alignment for vertical aligned liquid crystal devices with a fast response and high contrast

ABSTRACT We demonstrate fast response and high contrast vertical alignment (VA) liquid crystal devices (LCDs) with polymer sustained alignment (PSA). In general, the rise time of a liquid crystal (LC) is a trade-off relationship with the decay time. In the present study, an appropriate tilt angle of LCs was anchored by polymerisation using UV irradiation with the application of a threshold voltage and optimisation of the reactive mesogen (RM) concentration. The PSA VA-LCDs showed a fast optical response to the applied electric field because the backflow (optical bounce) was suppressed by anchoring with the polymer. In addition, the fast response was maintained until a high voltage was applied. The transmittance of these PSA VA-LCDs at 0 V was identical to that of VA-LCDs without a polymer and the opaque phenomena at the applied voltage were eliminated because alignment of LCs and RMs was improved. The findings of this work are expected to enable the development of displays and phase modulators with improved performance.

Studies of Electro-Optic and Thermal Behaviour of Azo Dye Doped Vertically Aligned Liquid Crystal

The performance of liquid crystal (LC) display devices significantly depends on the usage of an optimized concentration of dopant. Thus, in the present study, vertically aligned liquid crystals (VALCs) display cells were prepared using dispersion of zinc oxide (ZnO) nano particles into host nematic liquid crystal (NLC) without any surface treatment. Further, the sample mixture was doped with fixed concentration of 0.5% of azo dye. Both the sample cells were analyzed and compared for morphological, electro-optical (E-O) and thermal behaviour. Results show the improvement in voltage-transmittance characteristics with lowering of threshold and operating voltage, however the contrast ratio (CR) for dye doped sample cell was found to be degraded. This degradation may be possibly due to much lower transition temperature at this concentration of dye, which further might because of higher OFF state and lesser ON state transmittance due to disturbance in intrinsic LC order. As, the thermal study showed the doping of 0.5% of azo dye reduced the transition temperature to 410 for dye doped VALC from 680 of VALC sample. Hence, these results indicate that a control amount of dichroic dye is a crucial measure for an adequate performance of VALC display.

Vertical Alignment of Liquid Crystal on Sustainable 2,4-Di-tert-butylphenoxymethyl-Substituted Polystyrene Films.

We synthesized sustainable 2,4-di-tert-butylphenoxymethyl-substituted polystyrenes (PDtBP#, # = 88, 68, 35, and 19, where # is molar percent contents of 2,4-di-tert-butylphenoxymethyl moiety), using post-polymerization modification reactions in order to study their liquid crystal (LC) alignment behaviors. In general, LC cells fabricated using polymer film with higher molar content of 2,4-di-tert-butylphenoxymethyl side groups showed vertical LC alignment behavior. LC alignment behavior in LC cell was related to the surface energy of the polymer alignment layer. For example, when the total surface energy value of the polymer layer was smaller than about 29.4 mJ/m2, vertical alignment behaviors were observed, generated by the nonpolar 2,4-di-tert-butylphenoxymethyl moiety with long and bulky carbon groups. Orientation stability was observed at 200 °C in the LC cells fabricated using PDtBP88 as the LC alignment layer. Therefore, as a natural compound modified polymer, PDtBP# can be used as a candidate LC alignment layer for environmentally friendly applications.

Evaporation induced self-assembly of silica nanoparticles on ITO substrates in a confined cell for vertical alignment of liquid crystals and performance analysis

The work reported herein advances the approach to achieve the vertical alignment (VA) of liquid crystals (LCs) by directly forming the multilayer self-assembly of nanoparticles (NPs) on the indium tin oxide (ITO) substrates in a confined cell. In this method, for confined evaporative self-assembly of NPs, the colloidal solution of uniformly dispersed 0.3 wt% silica NPs (SNPs) in deionized (DI) water was injected in between the two ITO coated glass substrates by capillary method. The ITO cell filled with colloidal solution was kept at a fixed temperature of 60°C for uniform thermal evaporation of water to deposit the multilayer of NPs on ITO substrates. The multilayered self-assembly of SNPs on ITO substrates was confirmed through field emission scanning electron microscopy (FESEM) and fourier transform infrared (FTIR) spectroscopy studies. The surface roughness profile of the deposited SNPs-multilayer was analysed by atomic force microscopy (AFM). Further, the contact angle (CA) and surface energy values of SNPs-multilayer deposited ITO substrates were also measured and analyzed comparatively with the values of polyimide (PI) coated ITO substrates. After complete evaporation of water, a negative dielectric anisotropy nematic LC was filled in to the cell using capillary method again and the versatility of prepared substrates was assessed for the VA of the liquid crystal (LC). Inquisitively, confirmation of VA of LC in the present approach was found similar with the PI coated VA (conventional approach) using polarized optical microscopy (POM) and conoscopy studies. In addition, effects of external applied electric field were observed well in order with the orientation of LCs to switch vertically aligned LC (VALC) cells from opaque OFF state to transparent ON state, under crossed polarizers. Also, the obtained results in terms of transmission, threshold voltage, operating voltage, and contrast ratio (CR) of VALC cell prepared with this easy and new approach were found comparable to the conventional approach.

Vertical Alignment of Liquid Crystals on Phenylphenoxymethyl-Substituted Polystyrene—PS Derivatives Structurally Similar to LC Molecules

A series of polystyrene derivatives containing precursors of liquid crystal (LC) molecules, phenylphenoxymethyl-substituted polystyrene (PPHE#; # = 5, 15, 25, 50, 75, and 100)—where # is the molar content of 4-phenylphenol using polymer modification reactions—were prepared in order to examine the effect of the polymer film, which possess similar LC molecular structure on the LC alignment properties. It was found that the Tg values of the PPHE# were higher than 100 °C due to their aromatic structure in the biphenyl-based PHE moiety. The LC cells fabricated with PPHE5 and PPHE15 films exhibited planar LC alignment. Conversely, LC molecules showed a vertical alignment in LC cells made using the polymer films with phenylphenoxymethyl side groups in the range of 25–100 mol %. The polar surface energies on the PPHE# films can be associated with the vertical LC alignment on the PPHE# films. For example, vertical LC alignment was exhibited when the polar surface energy of the polymer films was less than approximately 4.2 mJ/m2. Aligning stability was observed at 200 °C and UV irradiation of 20 J/cm2 for LC cells made using the PPHE100 film. Therefore, it was found that biphenyl, one of the LC precursors, modified polystyrene derivatives and can produce a next-generation vertical LC alignment system.

Vanadium Dioxide Nanoparticle Doped Polyimide Hybrid Alignment Layers for Flexible Liquid Crystal Displays

In this paper, a flexible liquid crystal display (LCD) is developed by applying hybrid alignment layers made of vanadium dioxide (VO2) nanoparticles and homeotropic-polyimide (h-PI) onto colorless plastic substrates, and the proposed hybrid thin layers with randomly embedded agglomerated VO2 nanoparticles exhibit a transparency over 85% at 550 nm including the substrate and a strong surface irregularity. After rubbing, liquid crystals (LCs) are vertically aligned between the hybrid thin layers with a tilt angle of ∼90°, and due to the significant surface plasmon polariton (SPP) of VO2 nanoparticles, the switch of LCs under the external electrical field has been remarkably accelerated. The operating voltage to switch LCs in a 5 μm cell assembled using a pair of 0.5% VO2 nanoparticle doped h-PI hybrid alignment layers has been reduced to 3.536 V, and simultaneously, the decaying time is also shortened more than 18%. Notably, the vertical alignment of LCs was also stabilized under bending thanks to the use of undulating hybrid alignment layers. The fabricated flexible LC cell using 0.5% VO2 nanoparticle doped h-PI alignment layers yielded good flexibility at a bending radius of 1500 mm without aggravated light leakages and deduced electro-optical performance, and the VO2 agglomerates hindering the slide of LCs by providing adequate obstructions are believed as the main contributors in firmly stabilizing the vertical alignment of LCs during bending. Compared to micropatterned substrates, ultrathin VO2 nanoparticle doped h-PI alignment layers employed in flexible LC cells demonstrating their priorities in easy investigation and displaying quality improvement are emerging, highly appreciated, and of great interest for the potential applications in flexible LCDs.

Control over Vertical Orientation of Liquid Crystal using Spin Coated Silica Nanoparticles Layer on Conductive Substrates

Recently, various techniques such as self-assembly, chemical vapour deposition and capillary method etc. are used for nanoparticles (NPs) deposition on the conductive substrates for the vertical alignment (VA) of liquid crystals (LCs). The initial VA of LCs in display devices have the potential impact on industry and various electro-optic devices applications. In the present work, vertical orientation of LCs was controlled on silica (SiO2) nanoparticles (NPs) layer deposited on indium tin oxide (ITO) glass substrates using spin coating method. The VA LC cell was also prepared for conventional polyimide coated ITO glass substrates. The morphological images showed the initial darker textures under crossed polarizers and conoscopic image with dark cross confirmed the VA of LC on SiO2 NPs coated substrates analogous to the conventional VA LC cell. Further, electro-optic results revealed the reduction in applied voltage and better contrast ratio for SiO2 NPs coated VA LC cell compared with polyimide coated VA LC cell.

Evaporative deposition of SiO2 nanoparticles multilayer on ITO substrates and application for vertical alignment of liquid crystals - Effect of dichroic dye

In this work the SiO2 nanoparticles layer was deposited by solvent evaporation on ITO substrates and used for vertical alignment of pure and dye doped liquid crystal (LC) in confined cells. The pure nematic liquid crystal (NLC) material was filled into SiO2 layer coated cell by the capillary injection method. The 0.5 wt.% of dichroic dye added into the LC mixture and studied its effect on electro-optic behaviour of display cell. The FESEM study reveals the multilayered deposition of the SiO2 on Indium tin oxide (ITO) substrates. Conoscopic and polarised optical microscopy (POM) study show good dark black and bright textures at OFF and ON state under the cross polarizers and confirm vertical alignment (VA) of LC molecules. The electro-optical (E-O) study shows the lower threshold and operating voltage with higher contrast ratio in dye doped vertically aligned LC (VALC) cell as compare to the pure SiO2 coated VALC cell.

Polyimide-free liquid crystal devices with carbon nanotube-based electrodes

A novel type of polyimide-free liquid crystal (LC) device has been fabricated using layer-by-layer (LBL) deposition of carbon nanotube-based electrodes and in situ formation of self-aligning amphiphilic graft copolymer layer. The LBL-processed organic electrodes showed high transparency comparable to indium tin oxide (ITO) and the self-aligned layer of amphiphilic graft copolymer on organic electrodes provided a stable vertical alignment of LCs. Moreover, the proposed LC device resulted in voltage-dependent driving characteristics suitable for display application. This functional organic electrode layer has potential for application in a flexible display to replace conventional ITO electrode and polyimide alignment layer.

Effect of Position and Structure of the Terminal Moieties in the Side Group on the Liquid Crystal Alignment Behavior of Polystyrene Derivatives.

We synthesized a series of polystyrene derivatives containing various side groups, such as the 4-(tert-butyl)-phenoxymethyl, 3-(tert-butyl)-phenoxymethyl, 2-(tert-butyl)-phenoxymethyl, 4-cumyl-phenoxymethyl, and 4-trityl-phenoxymethyl groups, through a polymer modification reaction to examine the liquid crystal (LC) alignment of these derivatives. In general, the vertical LC alignment on polymer films can be affected by the position and structure of the terminal moiety of the polymer side group. For example, the LC cells fabricated with 4-(tert-butyl)-phenoxymethyl-substituted polystyrene having a tert-butyl moiety as a para-type attachment to the phenoxy groups of the polystyrene derivatives exhibited vertical LC alignment, whereas the LC cells prepared from 3-(tert-butyl)- and 2-(tert-butyl)-phenoxymethyl-substituted polystyrene films exhibited planar LC alignment. In addition, the LC cells fabricated from 4-cumyl- and 4-trityl-phenoxymethyl-substituted polystyrene films with additional phenyl rings in the side groups exhibited planar LC alignment, in contrast to the LC alignment of the (tert-butyl)-phenoxymethyl-substituted polystyrene series. The vertical LC orientation was well correlated with the surface energy of these polymer films. For example, vertical LC orientation, which mainly originates due to the nonpolar tertiary carbon moiety having bulky groups, was observed when the surface energy of the polymer was lower than 36.6 mJ/m2.

Hierarchical self-constructed biomolecular nanolayers comprising cholesterol and cholesteryl hemisuccinate for automatic alignment of liquid crystals

Hierarchical supramolecular nanostructures are architecturally robust and functionally tunable, and therefore are widely utilized as functional building blocks for optoelectronic and biological applications. Herein, we report a simple and effective fabrication strategy to construct hierarchical biomolecular nanolayers comprising cholesterol and cholesteryl hemisuccinate for automatic alignment of liquid crystals (LCs) by using the programmed interfacial self-assembly of biomolecular amphiphiles on the polar electrode surface. The spontaneous formation of ultrathin biomolecular nanolayer was successfully achieved by simple doping and in situ self-organization of a small amount of steroid-based amphiphiles in the confined LC cell. The surface analysis demonstrated that the amphiphilic biomolecules of cholesterol and cholesteryl hemisuccinate formed hydrophobic self-constructed nanolayers with a dense surface coverage on the surface of indium tin oxide (ITO) electrode via hydrogen bonding. These nanolayers, bearing hydrophobic steroid groups and alkyl chains on their surface, induced uniform vertical alignment of LCs via van der Waals forces. Notably, the biomolecular nanolayer containing cholesteryl hemisuccinate exhibited more stably oriented LCs than cholesterol-based nanolayer at the same concentration owing to the strong interfacial hydrogen bonding with polar ITO electrode surface. The electro-optical properties of the LC cells containing the steroid-based amphiphiles revealed that hierarchical self-constructed nanolayers based on the amphiphilic biomolecules afforded low threshold voltage and fast response time comparable to those of conventional polyimide alignment layer.

High-transmittance vertical-alignment liquid crystal display with subpixel electrode shielding electric field design

A proposed pixel structure named ‘Sub-pixel Electrode Shielding (SES)’ is developed to improve the transmittance of eight-domain vertical alignment liquid crystal display (LCD). In the SES pixel, the sub pixel electrode is designed to partially overlap with data line and encircle the main pixel to shield the effects of data signal on the common signal. Therefore, the array common electrode and data signal shielding lines of conventional pixel can be removed to enhance the aperture ratio. Results show that the 65 inch ultra-high-definition (UHD) LCD panel with this proposed SES pixel design achieves a transmittance of 5.9%, which is 7.3% higher than the panel with conventional pixel design, and shows the same viewing angle characteristic as the conventional one.

Vertical Alignment of Liquid Crystals on Comb-Like Renewable Chavicol-Modified Polystyrene.

This study demonstrates liquid crystal (LC) alignment behaviors on the surface of phytochemical-based and renewable chavicol-modified polystyrene (PCHA#, # = 20, 40, 60, 80, and 100, where # represent the molar content of chavicol moiety in the side group) via polymer modification reactions. Generally, a LC cell fabricated with a polymer film containing a high molar content of the chavicol side group exhibited a vertical LC alignment property. There is a correlation between the vertical alignment of LC molecules and the polar surface energy value of the polymer films. Therefore, vertical LC alignment was observed when the polar surface energy values of these polymer films were smaller than about 1.3 mJ/m2, induced by the nonpolar chavicol moiety having long and bulky carbon groups. Aligning stability under harsh conditions such as ultraviolet (UV) irradiation of about 5 J/cm2 was observed in the LC cells fabricated from PCHA100 film. Therefore, it was found that the plant-based chavicol-substituted polymer system can produce an eco-friendly and sustainable LC alignment layer for next-generation applications.