Improving the electro‐optical properties of bistable reflective cholesteric liquid crystal displays

We have improved the electro‐optical properties of a bistable cholesteric liquid crystal display (Ch‐LCD) that is driven by a 3+2 dynamic driving scheme (3+2 DDS). The best contrast ratio is achieved at the selection period of 1.2–1.5 ms/line and the temperature range 26–30°C. The suitable preparation period and evolution period for contrast ratio are 40 times and 20 times the selection time, respectively. In the 3+2 DDS, rubbing the homeotropic alignment layer increases reflectivity of the ON state and keeps the reflectivity of the OFF state at the same level, so reflectivity and contrast ratio alike increase as a result. However, in a delayed homeotropic reset driving method, when the homeotropic alignment layer is rubbed, the reflectivity of both the ON and OFF states increases, thus the contrast ratio decreases. The combination of driving method and aligned surface morphology influences the relaxation mechanism in the cholesteric texture. By optimizing panel condition, we have demonstrated an 8.4″ foldable VGA Ch‐LCD that exhibits high reflectivity and contrast ratio with an addressing speed of around 1.2 ms/line.     

Nematic liquid crystal alignment on chemical patterns

Patterned Self-Assembled Monolayers (SAMs) promoting both homeotropic and planar degenerate alignment of 6CB and 9CB in their nematic phase were created using microcontact printing of functionalized organothiols on gold films. The effects of a range of different pattern geometries and sizes were investigated, including stripes, circles and checkerboards. Evanescent wave ellipsometry was used to study the orientation of the liquid crystal (LC) on these patterned surfaces during the isotropic-nematic phase transition. Pretransitional growth of a homeotropic layer was observed on 1 µm homeotropic aligning stripes, followed by a homeotropic monodomain state prior to the bulk phase transition. Accompanying Monte Carlo simulations of LCs aligned on nanoscale-patterned surfaces were also performed. These simulations also showed the presence of the homeotropic monodomain state prior to the transition.     

Arch-texture in cholesteric liquid crystals

We have observed an arch-texture in cholesteric liquid crystals sandwiched between two glass plates making a small wedge angle. The anchoring is homeotropic on one plate and planar on the opposite one. This texture is locally periodic and composed of parallel stripes whose average direction rotates by 180° each time the sample thickness increases by p/2, where p is the equilibrium pitch of the cholesteric phase. This texture is due to a periodic modulation of the elastic boundary layer which forms near the plate treated for homeotropic anchoring.     

Electro-optical properties of holographically patterned polymer-stabilized cholesteric liquid crystals

Electro-optical properties of cholesteric liquid crystals (LCs) with holographically patterned polymer stabilization were examined. It is hypothesized that increasing the LC domain size in a single dimension, relative to a random three-dimensional network of LC pockets separated by polymer strands, will allow enhanced electro-optical properties of the final device. Prior to holographic patterning, polymer stabilization with large elastic memory was generated by way of high irradiation intensities and optimized material choices. High irradiation intensities are required for the holographic patterning process to maintain polymer layer formation. At optimized conditions, polymer patterning of the stabilization allowed for an approximate 20% increase in the clear state transmission of the device, and allowed for an approximate 3 V µm^-1 reduction in the overall switching voltage as compared to an analogous floodlit irradiated sample. Switching times were increased at most threefold with holographic patterning, but all relaxation times were below 20 ms. The enhanced electro-optical properties appear to stem from a single dimension domain size increase, which allows for a reduction in the LC/polymer interaction.     

Forming process and stability of bubble domains in dielectrically positive cholesteric liquid crystals

The nucleation of bubble domains in homeotropic samples of a dielectrically positive cholesteric liquid crystal is described. These domains are found to be more stable in an electric field than the rectilinear double-twisted fingers. The electric field-induced transformation of a looped finger into a bubble domain is described in detail: it is discontinuous and irreversible, and operates only at a large enough confinement ratio C = d/p, where d is the sample thickness and p the quiescent cholesteric pitch. Finally, in contrast with Stieb's model [4], we propose that there are two point defects along the bubble axis and not a disclination line.     

Ternary Conductance Switching Realized by a Pillar[5]arene-Functionalized Two-Dimensional Imine Polymer Film

Nowadays, most manufacturing memory devices are based on materials with electrical bistability (i. e., “0” and “1”) in response to an applied electric field. Memory devices with multilevel states are highly desired so as to produce high-density and efficient memory devices. Herein, we report the first multichannel strategy to realize a ternary-state memristor. We make use of the intrinsic sub-nanometer channel of pillar[5]arene and nanometer channel of a two-dimensional imine polymer to construct an active layer with multilevel channels for ternary memory devices. Low threshold voltage, long retention time, clearly distinguishable resistance states, high ON/OFF ratio (OFF/ON1/ON2=1 : 10 : 10³), and high ternary yield (75 %) were obtained. In addition, the flexible memory device based on 2DP_TPAZ+TAPB can maintain its stable ternary memory performance after being bent 500 times. The device also exhibits excellent thermal stability and can tolerate a temperature as high as 300 °C. It is envisioned that the results of this work will open up possibilities for multistate, flexible resistive memories with good thermal stability and low energy consumption, and broaden the application of pillar[n]arene.     

High contrast ratio of a vertically aligned liquid crystal cell using photocrosslinking alignment

A photo-induced alignment layer for LCDs has been successfully fabricated using the polarized UV photoreaction of a photo-crosslinkable polymer with the incident UV light at some angle with respect to the cell normal. The surface alignment and electro-optic properties were investigated for various UV exposure times. The homeotropic alignment layer showed a discrete anisotropic dichroic ratio, its surface morphology became smoother as the UV exposure time increased, and the LCs were arranged in a perpendicular direction to the PUVL direction with a proper pretilt angle. The cell showed no defects under cross-polarized microscopy and the contrast ratio was as high as 550:1 in transmittance. The contrast ratio pattern was found to be very similar to that of dichroic ratio as a function of UV exposure time and depended upon the frequency change to some degree. The response time was also investigated.     

High contrast ratio of a vertically aligned liquid crystal cell using photocrosslinking alignment

A photo-induced alignment layer for LCDs has been successfully fabricated using the polarized UV photoreaction of a photo-crosslinkable polymer with the incident UV light at some angle with respect to the cell normal. The surface alignment and electro-optic properties were investigated for various UV exposure times. The homeotropic alignment layer showed a discrete anisotropic dichroic ratio, its surface morphology became smoother as the UV exposure time increased, and the LCs were arranged in a perpendicular direction to the PUVL direction with a proper pretilt angle. The cell showed no defects under cross-polarized microscopy and the contrast ratio was as high as 550:1 in transmittance. The contrast ratio pattern was found to be very similar to that of dichroic ratio as a function of UV exposure time and depended upon the frequency change to some degree. The response time was also investigated.     

Electrically induced anchoring transition in cholesteric liquid crystal cells with different confinement ratios

Reorientation of cholesteric liquid crystal induced by the electrically controlled ionic modification of surface anchoring within the cell with confinement ratio exceeding 1 has been studied. The change of homeotropic surface anchoring to the planar one on the electrode-anode substrate under the action of DC voltage causes the formation of the modulated hybrid-aligned cholesteric layer in the cell. Optical texture of the liquid crystal layer with such an orientation structure is the linear periodic stripes. Homogeneity of emerging optical texture depending on the confinement ratio as well as on the prehistory of voltage application has been considered. It has been found that the ionic modification of surface anchoring results in total transformation of the diffraction pattern observed after the laser beam passing through the sample.     

Instabilities across the isotropic conductivity point in a nematic phenyl benzoate under AC driving

We characterize the sequence of bifurcations generated by ac fields in a nematic layer held between unidirectionally rubbed ITO electrodes. The material, which possesses a negative dielectric anisotropy epsilona and an inversion temperature for electrical conductivity anisotropy sigmaa, exhibits a monostable tilted alignment near TIN, the isotropic-nematic point. On cooling, an anchoring transition to the homeotropic configuration occurs close to the underlying smectic phase. The field experiments are performed for (i) negative sigmaa and homeotropic alignment, and (ii) weakly positive sigmaa and nearly homeotropic alignment. Under ac driving, the Freedericksz transition is followed by bifurcation into various patterned states. Among them are the striped states that seem to belong to the dielectric regime and localized hybrid instabilities. Very significantly, the patterned instabilities are not excited by dc fields, indicating their possible gradient flexoelectric origin. The Carr-Helfrich mechanism-based theories that take account of flexoelectric terms can explain the observed electroconvective effects only in part.     

Orientational transition in the cholesteric layer induced by electrically controlled ionic modification of the surface anchoring

A reorientation of cholesteric liquid crystal with a large helix pitch induced by the electrically controlled ionic modification of the surface anchoring has been studied. In initial state, the cholesteric helix is untwisted completely owing to the normal surface anchoring specified by the cations adsorbed at the substrates. As a result, the homeotropic director configuration is observed within the cell. Under the action of DC electric field, one of the substrates becomes free from the layer of surface active cations, therefore, setting the planar surface anchoring. The latter, in turn, leads to the formation of the hybrid chiral structure. The threshold value and dynamic parameters have been estimated for this process as well as the range of control voltages, which do not allow the electrohydrodynamic instabilities. The twisted hybrid director configuration observed in the experiment has been analysed by means of the simulation of polarisation change of light propagating through the cholesteric layer with asymmetric (planar and homeotropic) surface anchoring on the cell substrates.     

Covalent Functionalization of Black Phosphorus with Conjugated Polymer for Information Storage

Major disadvantages of black phosphorus (BP) are its poor air‐stability and poor solubility in common organic solvents. The best way to solve this problem is to incorporate BP into a polymer backbone or a polymer matrix to form novel functional materials that can provide both challenges and opportunities for new innovation in optoelectronic and photonic applications. As a proof‐of concept application, we synthesized in situ the first highly soluble conjugated polymer‐covalently functionalized BP derivative (PDDF‐g‐BP) which was used to fabricate a resistive random access memory (RRAM) device with a configuration of Au/PDDF‐g‐BP/ITO. In contrast to PDDF without memory effect, PDDF‐g‐BP‐based device exhibits a nonvolatile rewritable memory performance, with a turn‐on and turn‐off voltages of +1.95 V and ?2.34 V, and an ON/OFF current ratio of 10⁴. The current through the device in both the ON and OFF states is still kept unchanged even at 200th switching cycle. The PDDF/BP blends show a very unstable memory performance with a very small ON/OFF current ratio.     

A Metalloregulated Four‐State Nanoswitch Controls Two‐Step Sequential Catalysis in an Eleven‐Component System

The nanomechanical switch 1 with its three orthogonal binding motifs—the zinc(II) porphyrin, azaterpyridine, and shielded phenanthroline binding station—is quantitatively and reversibly toggled back and forth between four different switching states by means of addition and removal of appropriate metal‐ion inputs. Two of the four switching stages are able to initiate catalytic transformations (ON1, ON2), while the two others shut down any reaction (OFF1, OFF2). Thus, in a cyclic four‐state switching process the sequential transformation A + B + C → AB + C → ABC can be controlled, which proceeds stepwise along the switching states OFF1→ON1 (click reaction: A + B → AB )→OFF2→ON2 (Michael addition: AB + C → ABC )→OFF1. Two consecutive cycles of the sequential catalysis were realized without loss in activity in a reaction system with eleven different components.     

Control of liquid crystal pretilt angles by chemical derivatization reaction of polyvinyl alcohol films

A chemical derivatization technique was used to control the pretilt angle of a liquid crystal. A polyvinyl alcohol (PVA) alignment layer, which gives a very low pretilt angle when in contact with the liquid crystal (LC), was reacted with trifluoroacetic anhydride (TFAA) in the gas phase to change polar -OH groups to -OCOCF₃ groups. By introduction of the -OCOCF₃ groups in to the PVA, we obtained homeotropic alignment of the E7 LC molecules. The homeotropic alignment of E7 LC molecules in contact with the derivatized PVA alignment layer was confirmed by FTIR and microscopy with crossed polarizers. The change of liquid crystal molecules from homogeneous to homeotropic alignment may be caused by the decrease in surface tension of the PVA alignment layer, due to substitution of the polar -OH groups by -OCOCF₃ groups in the gas phase derivatization reaction.     

Two-dimensional director configurations in a nematic-filled cylindrical capillary with the hybrid director alignment on its surface

ABSTRACT

We analytically calculate two-dimensional equilibrium configurations of the nematic liquid crystal (NLC) director in a right cylindrical capillary with the hybrid director alignment on its inner surface. A part of this surface is treated to impose the strong homeotropic alignment, while on the remaining part the strong circular alignment, perpendicular to the capillary axis, is imposed. Owing to the system homogeneity along the capillary axis, a disclination line emerges in the bulk of the NLC, which runs parallel to the axis. It is shown that there exist either one or two equilibrium locations of the disclination line depending on the ratio of capillary surface parts with homeotropic and circular alignment. The technique that allows to obtain an analytical expression for the system free energy as a function of a disclination line location is presented. It considerably simplifies calculations and can be used while solving a variety of problems in which a defect location is sought.     

Electrical bistability of organic devices with a polymeric thin film for nonvolatile data storage

In this study, organic memory devices with a single active layer between the two external electrodes were fabricated using an electron‐donor type conjugated polymer and an electron‐acceptor type small organic molecule. The active layer of the memory device was prepared by blending polystyrene, poly[10‐(2′‐ethylhexyl)phenothiazine‐3,7‐diyl], and tetracyanoquinodimethane in 1,2‐dichlorobenzene. The device initially showed a low‐conductance state (OFF state) in the low‐voltage range, and an abrupt current increase, corresponding to the transition to a high‐conductance state (ON state), occurred at a certain voltage (V_th). The ON state could be reverted to the OFF state by applying a voltage higher than V_th. The current ratio between the two states was about 10³ (up to 10⁵). After this transition, the device remained in the ON state even after the applied voltage was removed, and this indicated the nonvolatile characteristics of the device. There was no sharp current degradation in the OFF or ON states for 4500 s of continuous bias. The device‐to‐device performance fluctuation was measured, and the conduction mechanisms in the ON and OFF states were examined by fitting the data to well‐known theoretical models. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

In situ self-assembled homeotropic alignment layer for fast-switching liquid crystal devices

We propose a novel method for homeotropic alignment of liquid crystals (LCs) utilising in situ self-assembly of a low concentration of 4-(4-heptylphenyl)benzoic acids that form hydrogen bond with the indium tin oxide (ITO) substrates. Stable homeotropic alignment in the LC device is achieved with a simple mixing process of benzoic acid derivative in LC media, and it yields electro-optical performance similar to that achieved with the conventional alignment method using polyimides. It is experimentally confirmed that an ultrathin self-assembled molecular layer of 4-(4-heptylphenyl)benzoic acid formed by hydrogen bonding on ITO substrate makes it possible to attain a reliable homeotropic alignment of LCs. Furthermore, this simple approach provides a cost-effective and stable LC alignment layer with fast response time and thermal stability.     

Optical and electrooptical properties of homeoplanar layers of cholesteric liquid crystals

Abstract

A wedge shaped layer of a cholesteric liquid crystal, with the director surface orientations first planar and the other homeotropic, shows two distinctive textures depending on the relation between the local thickness, d, and the equilibrium pitch, P ₀. If d/P ₀ < 1, the texture does not show any domains; the director distribution is reminiscent of a corkscrew. If d/P ₀ > 1, there are linear periodic domains. The domain direction rotates as the thickness of the layer increases. The voltage dependence of light transmission of the homeoplanar cholesteric layer placed between crossed polarizers is less pronounced and more linear than the corresponding dependence for the twisted nematic effect.     

Application of photosensitive polyimides as alignment layer to optical switching devices of a nematic liquid crystal

We have explored the change in alignment of a nematic liquid crystal, 4'-pentyl-4-cyanobiphenyl (5CB) with three types of photosensitive polyimide as the alignment layer by photoirradiation at 366 nm. The photosensitive polyimide alignment layer induced a reversible change in alignment of 5CB. It was observed that the 5CB molecules became aligned from homogeneous alignment to homeotropic on photoirradiation with a d.c. electric field as a bias, and reversed to the homogeneous state when photoirradiation was ceased. This result indicates that optical switching could be repeated by on and off switching of the excitation light at 366 nm. The optical switching of the nematic liquid crystal might be mainly due to a photophysical change in the polyimide surface which is affected by the chemical structures of the polyimides at the temperature at which 5CB exhibits a nematic phase. The optical switching of nematic liquid crystals with photosensitive polyimides as the alignment layer is a novel driving method for nematic liquid crystals.     

Application of photosensitive polyimides as alignment layer to optical switching devices of a nematic liquid crystal

We have explored the change in alignment of a nematic liquid crystal, 4'-pentyl-4-cyanobiphenyl (5CB) with three types of photosensitive polyimide as the alignment layer by photoirradiation at 366 nm. The photosensitive polyimide alignment layer induced a reversible change in alignment of 5CB. It was observed that the 5CB molecules became aligned from homogeneous alignment to homeotropic on photoirradiation with a d.c. electric field as a bias, and reversed to the homogeneous state when photoirradiation was ceased. This result indicates that optical switching could be repeated by on and off switching of the excitation light at 366 nm. The optical switching of the nematic liquid crystal might be mainly due to a photophysical change in the polyimide surface which is affected by the chemical structures of the polyimides at the temperature at which 5CB exhibits a nematic phase. The optical switching of nematic liquid crystals with photosensitive polyimides as the alignment layer is a novel driving method for nematic liquid crystals.