Crystal Alignment Research Articles

Planar and Vertical Alignment of Rod-like and Bent-core Liquid Crystals Using Functionalized Indium Tin Oxide Substrates

ABSTRACT Indium tin oxide (ITO) substrates are chemically modified with bent-shaped molecules and influence of these surface immobilised molecules on the alignment of bulk liquid crystal (LC) samples has been investigated. Bent-core (BC) compounds containing a dimethylchlorosilane moiety at the end of a terminal alkyl chain and a lateral alkyl chain are successfully attached to transparent conducting ITO substrates in an end-on (ITO-BC-t) and side-on (ITO-BC-l) oriented fashion to create stable permanent self-assembled BC layers on the substrates. Surface functionalization is thoroughly characterised and aligning ability of these BC-modified ITO substrates for orientation of bulk LC sample is evaluated for nematic phase of both rod-like and BCLC samples. The BC-modified substrates are effective in aligning the rod-like compounds in vertical (ITO-BC-t), planar orientations (ITO-BC-l) and the BC compounds in homeotropic (ITO-BC-t) alignment.

Physical Properties of Thermally Crosslinked Fluorinated Polyimide and Its Application to a Liquid Crystal Alignment Layer.

This study demonstrated the use of a thermally crosslinked polyimide (PI) for the liquid crystal (LC) alignment layer of an LC display (LCD) cell. Polyamic acid was prepared using 4,4′-oxydianiline (ODA) and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA). The 6FDA−ODA-based polyimide (PI) prepared by the thermal cyclic dehydration of the polyamic acid (PAA) was soluble in various polar solvents. After forming a thin film by mixing trifunctional epoxide [4-(oxiran-2-ylmethoxy)-N,N-bis(oxiran-2-ylmethyl)aniline] with the 6FDA−ODA-based PAA, it was confirmed that thermal curing at −110 °C caused an epoxy ring opening reaction, which could result in the formation of a networked polyimide not soluble in tetrahydrofuran. The crosslinked PI film showed a higher rigidity than the neat PI films, as measured by the elastic modulus. Furthermore, based on a dynamic mechanical analysis of the neat PI and crosslinked PI films, the glass transition temperatures (Tgs) were 217 and 339 °C, respectively, which provided further evidence of the formation of crosslinking by the addition of the epoxy reagent. After mechanical rubbing using these two PI films, an LC cell was fabricated using an anisotropic PI film as an LC alignment film. LC cells with crosslinked PI layers showed a high voltage holding ratio and low residual direct current voltage. This suggests that the crosslinked PI has good potential for use as an LC alignment layer material in advanced LCD technologies that require high performance and reliability.

Effect of atmospheric plasma and rubbing coprocessing on liquid crystal alignment on a polyimide layer

We investigated the effect of combining a rubbing process with an atmospheric plasma process on liquid crystal (LC) alignment on a polyimide layer. When the two processes were used together, the pretilt angles of 2.150° and 2.601° were excellent than when the rubbing-only process was performed. Concerning electro-optical characteristics, the low-power driving voltage was less than 2.5 V and the effective switching voltage was more than 20% lower than for the conventional rubbing-only process. In addition, the thermal stability and alignment characteristics after treatment at 150 °C were superior to those obtained when the atmospheric plasma-only process was used. This study suggests a route to the manufacture of LC displays having better performance and efficiency than achievable with current LC alignment methods.

Self-aligned liquid crystal smart windows with dual modes

A smart window composed of guest–host liquid crystals (GHLCs) is a promising technology but has several limitations, such as a relatively narrow tuning range of optical transmittance and the difficulty in adopting a cost-effective roll-to-roll process due to the polyimide (PI) alignment layer process. In this study, we introduce a self-alignment (SA) technology that uses a thiol-group additive into GHLC and successfully demonstrate a PI-less GHLC device by skipping the PI layer coating step. The SA additive molecules attach firmly to the inner surfaces of the devices, forming a self-assembled monolayer, and induce an excellent vertical alignment of the liquid crystals. Additionally, during the SA additive attachment process, the ionic concentration increases within the cell, and the ions play a role in inducing hydrodynamic instability at high applied voltages. Consequently, the SA-GHLC device can be operated in dual modes: a transmittance control mode at low applied voltages and a haze control mode at high applied voltages. Thus, the PI-less SA-GHLC exhibits excellent smart-window properties as can be fabricated using a simple process without a high-temperature PI coating process.

Formation of a Fast Charge Transfer Channel in Quasi-2D Perovskite Solar Cells through External Electric Field Modulation

Quasi-2D perovskites solar cells exhibit excellent environmental stability, but relatively low photovoltaic properties, compared with 3D perovskites solar cells. However, charge transport and extraction in quasi-2D perovskite solar cells are still limited by the inevitable quantum well effect, resulting in low power conversion efficiency (PCE). To date, most efforts concentrate on crystal orientation and favorable alignment during materials and films processing. In this paper, we demonstrated that the quasi-2D perovskite [(BA)2(MA)3Pb4I13 (n = 4)] solar cells show an optimized device performance through forming a fast charge transfer channel among 2D quantum wells through external electric field modulation, with appropriate modulation bias and time after the device has been fabricated. Essentially, ions will move directionally due to local polarization in quasi-2D perovskite solar cells under the action of electric field modulation. More importantly, the mobile ions function as a dopant to de-passivate the defects when releasing at grain boundaries, while decreasing built-in potential by applying forward modulation bias with proper modulation time. The capacitance-voltage characteristics indicate that electric field modulation can decrease the charge accumulation and improve the charge collection in quasi-2D perovskite solar cells. Photoluminescence (PL) studies confirm that the non-radiative recombination is reduced by electric field modulation, leading to enhanced charge transfer. Our work indicates that external electric field modulation is an effective method to form a fast charge transfer channel among 2D quantum wells, leading to enhanced charge transfer and charge collection through local polarization toward developing high–performance quasi-2D perovskite devices.

Nanopattern transfer on bismuth gallium oxide surface via sol-gel stamp process applied for uniform liquid crystal alignment

A convenient aligning method using nanoimprint lithography is introduced. Sol-gel-processed bismuth gallium oxide (BGO) thin films that are one-dimensionally nanopatterned by polydimethylsiloxane molds are used for the liquid crystal (LC) alignment layer. The one-dimensional nanopatterns are transferred to the films at a curing temperature of 250 °C. Atomic force microscopy data present clear line grooves on the surfaces, which allow the LCs to align parallel to this pattern in one direction. Polarized optical microscopy and pre-tilt angle data support uniformity of the LC alignment, indicating that the surface nanopatterns act as alignment guides for LC molecules. X-ray photoelectron spectroscopy data shows active thermal oxidation at a curing temperature of 250 °C, which contributes to nanopattern transfer on BGO film. Thermal endurance tests of the BGO films show good thermal stability until 180 °C. The twisted nematic cells in the BGO layer also show superior electro-optical performances compared with the conventional rubbed polyimide layer. From these characteristics of the nanopatterned BGO alignment layer and owing to its simple fabrication, we expect that this new alignment technique can be used for producing next-generation LC devices to replace the conventional rubbing process.

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.

Electric Field Dependent Textural Variation inside the Liquid Crystal Droplets with Homeotropic Alignment

The alignment of liquid crystal inside the droplets highly influences the electro-optical behaviour of polymer dispersed liquid crystals (PDLCs). In PDLCs with initial transparent state, LC droplets exhibit homeotropic boundary conditions with darker zone at the centre with ring shaped boundary. In the present work, the textures were observed under parallel and crossed polarizers. The captured information revealed that there are no changes in the central zone of the droplets due to the perfect homeotropic alignment of liquid crystals inside the droplet. The count of the droplets with different ranges was measured using ImageJ software. Further, the effect of electric field on textural variation inside the droplets, measuring the ratio of the size of darker zone to the size of droplet (a/d) was analysed by applying image processing. The response curve was obtained for different range of sizes of droplets from the plot of a/d ratio vs applied voltage and found supportive to the measure of the textural variation inside the LC droplets. Therefore, the a/d ratio can be the valuable parameter for optimizing the parameters such as droplet size, area of darker zone and required voltage for energy efficient PDLC devices.

Improvement of charge transport in organic TIPS semiconductor device using crystal alignment eco-friendly plastic cellulose nano-whisker

Improvement of charge transport in organic TIPS semiconductor device using crystal alignment eco-friendly plastic cellulose nano-whisker

Versatile homeotropic liquid crystal alignment with tunable functionality prepared by one-step method

Alignment layers are vital to the function of numerous devices based on liquid crystal (LC) materials. The pursue of versatile, effective and even flexible alignment layers, preferably prepared by simple methods, is still actively ongoing. Herein, we propose a facile one-step method by mixing silanes into the starting LC mixtures, which in contact with a glass substrate secede and self-assemble in-situ to form a stable and highly effective homeotropic alignment layer at the interface. Tetradecyldimethyl(3-trimethoxysilylpropyl)ammonium chloride (TDTA) is selected as the example to demonstrate the method, although a number of other silanes can produce similar results. With only 0.05 vol% of TDTA added to a mixture of liquid crystals and reactive mesogens, a uniform monolayer is chemically attached to the substrate, which automatically aligns the LCs homeotropically. Furthermore, by blending the TDTA with acrylate functionalized silanes like 3-(trimethoxysilyl)propyl methacrylate (A174), additional reactive functional groups can be easily introduced into the alignment layer, therefore offering opportunities to adjust the interface properties. An electro-responsive smart window based on the polymer stabilized liquid crystals (PSLCs) is successfully prepared using a one-step method, demonstrating excellent electro-optic performances and notably enhanced adhesion between the substrate and the in-situ formed polymer network. These findings are valuable especially for the development of flexible LC devices.

Large rewritable liquid crystal pretilt angle by in situ photoalignment of brilliant yellow films

The ability to control the alignment of liquid crystals (LCs) is a challenge that is near-universal in LC research and applications. While several practical solutions exist for controlling the azimuthal (in-plane) alignment of LCs at an interface, control over the polar, or pretilt angle of the LC often requires a combination of materials or expensive and intricate processing. Here, we introduce a method for exerting control over large pretilt angles utilizing a two-step exposure method applied to Brilliant Yellow (BY) photoalignment films. We demonstrate that BY photoalignment films have the unique ability to enforce a large range of pretilt angles spanning homeotropic (90°) and planar (0°) anchoring conditions at the LC-substrate interface. The alignment is stable over time, rewritable, and patternable. Ultimately the control established here provides a powerful and low-cost means to align and take full advantage of the anisotropic and electro-optic properties of LCs for a wide range of applications.

Hierarchical and automatic construction of ultrathin polymer nanoarchitecture with islands of alkyl chains for spontaneous interfacial molecular alignment

Hierarchical ultrathin nanoarchitectures are structurally stable and functionally versatile, which are supramolecular building blocks with a broad range of applications from optoelectronic devices to tunable biosensors. Here, we present a facile and scalable strategy to fabricate an ultrathin polymer nanoarchitecture with islands of alkyl chains for spontaneous interfacial molecular orientation by using the hierarchical and automatic self-assembly of amphiphilic comb-like copolymers. The intriguing self-constructed nanoarchitecture is achieved by facile doping and interfacial self-assembly of amphiphilic poly(dodecyl acrylate)–poly(acrylic acid) comb-like copolymers on hydrophilic indium tin oxide substrate. It is experimentally revealed that an ultrathin and spiky polymer nanoarchitecture with a hydrophobic segment surface formed by hydrophilic and cooperative interactions of amphiphilic copolymers ensures an automatic vertical molecular alignment of liquid crystals during hierarchical self-construction process. Furthermore, this effective approach affords a fast responsive and low-voltage driving polymer nanoarchitecture with an enhanced electro-optical performance compared to common polyimide layer.

Mechano-chemical regulation of bat wing bones for flight

Distal phalanges in bat wings have been hypothesized to be cartilaginous to allow for flight. We provide new evidence on how bat wing development might facilitate flight though protein-based regulation of bone mineralization and lead to more deflection at phalanx than humerus. Between Pteropus poliocephalus and Pteropus hypomelanus, two large bat species, we detected 112 proteins including 11 associated with mineralization and analyzed their distribution between the wing bones. Here, in contrast to previous reports, we found no cartilage-specific proteins and demonstrate that distal phalanges in bat wings are in fact low density bone that contain collagen I (the main constituent of bone's organic matrix) and proteins associated with mineralization in bone such as osteomodulin, bone-specific protein osteocalcin. The functional relevance of these changes was explored by measuring changes in mineral (crystal sizes, packing and density), material (Young's modulus and hardness) and structural characteristics. Consistent with changes in proteins associated with mineralization, mineral crystal thickness and alignment decreased from humerus to phalanges, and the mineral platelets were less densely packed along the wing length. Crystal thickness was negatively correlated with proteins associated with inhibition of mineralization as well as with two types of small leucine-rich proteoglycans, indicating the mineral growth and maturity is down regulated by these proteins independent of mineral quantity. The Young's modulus decreased across the wing and was significantly correlated with bone mineral density. Thus, the results from two bat species, studied here, demonstrate progressive alterations in bone mineralization occur in concert with the changes in secretion of bone regulatory proteins along the wing length. This altered mineralization together with structural changes serve to lighten the limb bone and optimize biomechanical properties conducive to flight.

Hydrogen bond assisted anchoring transitions in nematic liquid crystals at the aqueous interface

Homeotropic alignment of liquid crystals (LCs) at the aqueous interface and their fundamental understanding is still limited known, which is a prerequisite for the development of new kinds of LC-based sensors for the detection of biomolecules in the aqueous phase. Herein, we have investigated the orientational ordering of nematic LC at LC-aqueous interface in contact with the different hydrogen bonding strength of para-substituted phenol derivatives (X; functional group attached to phenol at para position = -NO2, -CN, -Cl, -F, -CHO, -H) intending to understand how the intermolecular hydrogen bonding interactions at LC-aqueous interface amplified into supramolecular ordering in LCs. The hydrogen bonding interactions between -CN group of 5CB and -OH group of phenol derivatives lead to a change in tilt angle from planar to homeotropic ordering of 5CB molecules at LC-aqueous interface. Raman measurement of different mixtures between 5CB and para-substituted phenol derivatives have been recorded to correlate the experimental observations at the LC-aqueous interface as well as direct evidence of hydrogen bonding at the LC-aqueous interface. Density functional theory (DFT) has been used to calculate hydrogen bonding strength and charge transfer mechanism between 5CB and phenol derivatives. The orientation of LC at the aqueous interface depends on the strength of the interfacial hydrogen bonding between phenol derivatives and LC, which varies on substitution of phenol and concentration of the phenol molecules.

Study of the molecular orientation of steroidal side chains at polyimide surfaces using sum frequency generation vibrational spectroscopy.

The molecular orientation of steroidal side chains at rubbed polyimide (PI) surfaces is studied by sum frequency generation (SFG) vibrational spectroscopy. The main objective is to find a correlation between the molecular structure of the PI film and the liquid crystal alignment on the polymer. Analysis of the SFG spectra shows that rubbing of the polymer film appears to cause conformational changes in the methyl group of the polymer side chain near the steroidal structure. However, rubbing does not significantly influence the orientation of the isopropyl group at the end of the polymer side chain. This shows that the liquid crystal alignment is not correlated with the orientation of the isopropyl group.

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.

Influence of the flexoelectric effect on director alignment of nematic liquid crystals confined to the planar anchoring cylinder

ABSTRACT The influence of the flexoelectric effect on the director distribution of nematic liquid crystals confined to a cylinder with planar anchoring under an applied electric field is investigated by the continuum theory and the difference iterative method. Numerical analyses of the influence of the dielectric effect, flexoelectric effect and the coupling of these two effects on director alignment of nematic liquid crystal in a cylinder are obtained on the basis of the equilibrium equations using continuum theory. The analyses revealed that the change of director of nematic liquid crystals was caused by flexoelectric effect and dielectric effect in different directions. It provides a good theoretical basis for controlling the orientation of liquid crystal director in a cylinder and measuring of flexoelectric coefficient of liquid crystal materials.

Phase Transition Modulation and Defect Suppression in Perovskite Solar Cells Enabled by a Self‐Sacrificed Template

Tunable crystal growth offering highly aligned perovskite crystallites with suppressed deep‐level defects is vital for efficient charge transport, which in turn significantly influences the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Herein, a “precursor to perovskite‐like template to perovskite” (PPP) growth strategy is developed, using either MAAc or GuaCl precursor to induce a sacrificial thermal–unstable perovskite‐like template for (FAPbI3)x(MAPbI3)y perovskite growth. The self‐sacrificed intermediate template induces the formation of highly aligned perovskite crystals with greatly enhanced film crystallinity and suppresses deep‐level defect formation. Furthermore, it is proved that MAAc or GuaCl completely evaporates during the high‐temperature annealing process. The reduction in defect densities and nonradiative recombination enhances both carrier lifetime and charge dynamics, yielding impressive PCEs of 22.3% and 22.8% with a high open‐circuit voltage (VOC) of 1.16 V and an incredible fill factor (FF) of 81.5% and 79.4% for MAAc‐ and GuaCl‐based devices, respectively. These results suggest that the formation of the thermal–unstable perovskite‐like sacrificial template is a promising strategy to restrain the deep‐level defects in perovskite films toward the attainment of highly efficient and stable large‐scale PSCs as well as other perovskite‐based electronics.

Experimental verification of the Landau–Lifshitz equation

The Landau–Lifshitz (LL) equation has been proposed as the classical equation to describe the dynamics of a charged particle in a strong electromagnetic field when influenced by radiation reaction. Until recently, there has been no clear experimental verification. However, aligned crystals have remedied the situation: here, as in Nielsen et al CERN NA63 Collaboration (2020 Phys. Rev. D 102 052004), we report on a quantitative experimental test of the LL equation by measuring the emission spectra of electrons and positrons penetrating aligned single crystals. The recorded spectra are in remarkable agreement with simulations based on the LL equation of motion with moderate quantum corrections for recoil and, in the case of electrons in axially aligned crystals, spin and reduced radiation intensity.