Alignment Of Ferroelectric Liquid Crystal Research Articles

Faster and low-power-operated highly luminescent CdSe nanoplatelets incorporated ferroelectric liquid crystals

ABSTRACT The ferroelectric liquid crystals (FLC) composites with improved electro-optic properties, obtained by incorporating CdSe (CS) nanoplatelets (NPLs), are reported. Noticeable improvementin optical contrast, saturation voltage, response time, and brightness of FLC have been found. The improvements of electro-optical properties are the results of enhanced molecular alignment of FLC due to the presence of CS NPLs. Time resolved photoluminescence (TRPL) measurements reveal NPLs surface energy transfer in FLC. Two types of Fӧrster resonant energy transfer (FRET) is observed in CS incorporated FLC. First is homo FRET which is the transfer of exciton within stacked CS NPLs and second is FRET between FLC and CS NPLs. FRET are supporting the CS incorporated FLC to get superior electro-optic properties. These findings may create opportunity of future CS NPLs incorporated FLC based display devices having better electro-optical properties.

Improvement in molecular alignment of ferroelectric liquid crystal by Co-ZnO/ZnO core/shell quantum dots

ABSTRACTThe ferroelectric liquid crystal (FLC) shows improvement in electro-optical properties on incorporation of Co-ZnO/ZnO core/shell (CZZ) quantum dots (QDs), which is a manifestation of better alignment for FLC molecules. A suitable amount of core/shell QDs was added into pure FLC and studied using polarising optical microscope, ferroelectric, photoluminescence (PL) and dielectric measurements. Significant increase in the order parameters (θ, PS) and PL intensity with reduced light leakage centres and dielectric loss factor are the consequences of enhanced molecular ordering of FLC molecules dispersed with CZZ QDs. FLC composites with luminescent core/shell QDs are a step forward for the improvement of low power operated highly photoluminescent FLC-based display devices. Such emissive liquid crystal displays would exhibit improved brightness, better contrast, wide viewing angle and lower operating power.

Electro-Optical Modulation in Planar Aligned Ferroelectric Liquid Crystals with Subwavelength Helix Pitch

Electro-Optical Modulation in Planar Aligned Ferroelectric Liquid Crystals with Subwavelength Helix Pitch

Fork gratings based on ferroelectric liquid crystals.

In this article, we disclose a fork grating (FG) based on the photo-aligned ferroelectric liquid crystal (FLC). The Digital Micro-mirror Device based system is used as a dynamic photomask to generated different holograms. Because of controlled anchoring energy, the photo alignment process offers optimal conditions for the multi-domain FLC alignment. Two different electro-optical modes namely DIFF/TRANS and DIFF/OFF switchable modes have been proposed where the diffraction can be switched either to no diffraction or to a completely black state, respectively. The FLC FG shows high diffraction efficiency and fast response time of 50µs that is relatively faster than existing technologies. Thus, the FLC FG may pave a good foundation toward optical vertices generation and manipulation that could find applications in a variety of devices.

Quenching of photoluminescence and enhanced contrast of ferroelectric liquid crystal dispersed with Cd1−XZnXS/ZnS core/shell nanocrystals

We report the dispersion of Cd1−xZnxS/ZnS core/shell nanocrystals (NCs) in ferroelectric liquid crystal (FLC) into different concentrations. Absorbance of white light, photoluminescence (PL), contrast, polarizing optical microscopy and Fourier transformed infrared spectroscopy (FTIR) have been performed to disclose the nature of FLC/NCs composite. The presence of NCs, in different concentration, quenches the PL of FLC material. The mechanism of quenching is explained by the modified Stern–Volmer quenching equation. The maximum contrast for FLC/NCs was found at 0.25wt% of dopant concentration which is the optimized concentration of NCs in FLC from application point of view. Contrast was further supported by polarizing optical micrographs (POMs). The change in POM also indicates the change in molecular alignment of FLC owing to concentration of NCs which also affect the optical properties of composites. FTIR spectra reveal that the NCs strongly interact with FLC molecules and changes their molecular dynamics associated with the central core and aliphatic chain. The present FLC/NCs composite has potential application in ultraviolet to visible light conversion, ultraviolet light storage applications and high contrast PL displays.

Reverse bistable effect in ferroelectric liquid crystal devices with ultra-fast switching at low driving voltage.

In this Letter, reverse bistable effect with deep-sub-millisecond switching time is first reported in ferroelectric liquid crystal (FLC) devices using a homogeneous photo-alignment technique. It is indicated by our experimental results that both the anchoring energy and the dielectric property of the FLC's alignment layer is critical for the existence of the reverse bistable effect. In addition, with the derived criteria of the reverse bistable effect, we quantitatively analyze the switching dynamics of the reverse bistable FLC and the transition condition between the traditional bistability and our presented reverse bistability. Moreover, the fabricated FLC device exhibits an ultra-fast switching of ∼160 μs and a high contrast ratio of 1000:1, both of which were measured at a low driving voltage of 11 V. The featured deep-sub-millisecond switching time is really advantageous for our presented reverse bistable FLC devices, which enables a significant quality improvement of the existing optical devices, as well as a wide range of new applications in photonics and display areas.

Photo-aligned ferroelectric liquid crystals in microchannels.

In this Letter we disclose a method to realize a good alignment of ferroelectric liquid crystals (FLCs) in microchannels, based on photo-alignment. The sulfonic azo dye used in our research offers variable anchoring energy depending on the irradiation energy and thus provides good control on the FLC alignment in microchannels. The good FLC alignment has been observed only when anchoring energy normalized to the capillary diameter is less than the elastic energy of the FLC helix. The same approach can also be used for the different microstructures viz. photonic crystal fibers, microwaveguides, etc. which gives an opportunity for designing a photonic devices based on FLC.

Fabrication of Uniformly Vertical Alignment FLC Cell

We focus on vertical alignment ferroelectric liquid crystal (FLC) cells fabricated without the rubbing technique to realize FLC displays having high contrast ratio. In this study, we research the conditions for the fabrication of uniformly vertical alignment FLC cells in terms of the tilt angle of FLC and the anchoring strength of alignment film materials.As a result, the uniformly vertical alignment of FLC can be obtained in the case of the small tilt angle and the strong anchoring strength. Furthermore, we investigate the effect of the application of electric field on the vertical alignment.

Mechanism of homeotropic alignment of ferroelectric liquid crystals doped with ferro-fluid and applications

We report homeotropic (HT) alignment of ferroelectric liquid crystal (FLC) doped with various concentrations of ferro-fluid (FF) without using any type of alignment layer. The FF induced HT alignment of FLC was found to be dependent on the doping concentration as revealed by optical micrographs, contact angle, and dielectric spectroscopy studies. Higher water contact angle of FF doped FLC films with respect to pure FLC film suggests higher surface energy of FF doped FLC than the surface energy of substrate. The physico-chemical mechanism together with steric model successfully explains the HT alignment of the studied FLC on the ITO substrate.

Molecular Alignment of Ferroelectric Liquid Crystal in Wide-Gap Cell for Liquid Crystal Lens

The high-speed response of ferroelectric liquid crystals (FLCs) is advantageous for optical devices such as an electrically controlled lens. We have investigated the molecular alignment structure of FLC in a wide-gap cell for application to the lens. As a result, it has been found that a zigzag defect-free FLC medium can be obtained by utilizing alignment films, which possess a smooth surface and give rise to the defect-free situation in a narrow-gap cell, and by cooling from the nematic to the smectic phase at a low rate of <1 °C/min. However, the transformation of the smectic layer structure has been observed under an electric field, particularly under a very low electric field in the wide-gap cell. We have attempted to suppress this transformation, which considerably degrades the optical performance of FLC, by the polymer stabilization technique.

Ferroelectric liquid crystal composites based on the porous stretched polyethylene films

Electro-optically active polymer–liquid crystal composites based on ferroelectric liquid crystals and stretched porous polyethylene films were developed. The alignment of ferroelectric liquid crystals incorporated into the porous polyethylene films with average porous diameter of around 200 nm was observed and studied. It was shown experimentally that these samples containing ferroelectric liquid crystals are flexible electro-optical films exhibiting a saturation electric field near 2·107 Vm−1 and a response time of about 30 μs under the action of the saturation field. A simple theoretical model of ferroelectric liquid crystal molecules' complete reorientation in electric fields inside pores of the films has been proposed and confirmed experimentally.

The influence of thermodynamic equilibrium state prior to the phase transition on the alignment of ferroelectric liquid crystal

In this experiment, devices of ferroelectric liquid crystal (FLC) were prepared by selecting ZLI-3654 FLC and SE-3140 alignment material. In order to study the influence of thermodynamic equilibrium state before the phase transition on the alignment of FLC, ten sets of experiments were carried out by changing the cooling rate during phase transition and thermodynamic equilibrium state before the phase transition. From experiments, a conclusion that the thermodynamic equilibrium state prior to chiral nematic-smectic A (N*-SmA) phase transition play an important role in realizing a uniform alignment of FLC devices, was obtained by comparing static contrasts of the devices. The static contrast ratio of our samples reached 620 ∶1 and the thermodynamic equilibrium state before N*-SmA phase transition was conducive to the formation of high contrast ratio FLC devices. The conclusion can provides a theoretical and practical guidance for the fabrication of FLC devices with I←→N*←→SmA←→S*mC←→Cr phase sequence.

Rubbed Polyimide Thin Films for Zigzag Free Surface Stabilized Ferroelectric Liquid Crystal

We investigate the effects of rubbing on the surface free energies of polyimide thin films, and how the surface energetic properties influence the molecular alignment of ferroelectric liquid crystals and the formation of zigzag defects. Rubbing breaks the uniformity in free energy at the surface of polyimide thin film. By properly rubbing the polymer, a diminishment of the difference between the polar part of the surface energies along and against the rubbing direction, respectively, can be achieved, and will favour the formation of zigzag free ferroelectric liquid crystal layer.

Molecular alignment of ferroelectric liquid crystals by polyimide thin film

Molecular alignment of ferroelectric liquid crystals by polyimide thin film

Fabrication of a Mono-Domain Alignment Ferroelectric Liquid Crystal Device Using a Polar Self-Assembled Monolayer

A mono-domain ferroelectric liquid crystal device (FLCD) is fabricated using a novel method. The cell used in this method is an asymmetric cell, typically the combination of a polar self-assembled monolayer (SAM) for one substrate and a rubbed polyimide for the other substrate. A defect-free alignment of ferroelectric liquid crystal is fabricated without applying a dc voltage to remove degeneracy in the layer structure. The contact angles of self-assembled monolayer and PI-2942 are measured and the polarity of SAM is higher than the PI alignment. It is found that the polarity of self-assembled monolayer is a key factor in the formation of mono-domain alignment of FLC.

Dynamic Processes of Ferroelectric Liquid Crystal Molecules under an Applied Electric Field

The dynamic changes in the molecular alignment of ferroelectric liquid crystal under an applied electric field are studied by observation of conoscopic figures with a temporal resolution of 0.1 ms. Comparison of the observed conoscopic figures with simulations indicate that deformation of the helical structures in the ferroelectric smectic C* phase initiates from the center of the liquid crystal cell within 0.1 ms under an electric field, propagating outward to the cell edges over a period of a few milliseconds. The fastest moving velocity (Δ φ/Δ t) is found to occur at molecular layers with azimuthal angles of π /2 and 3π /2 rad.

Molecular relaxation in homeotropically aligned ferroelectric liquid crystals

Uniform and defect-free homeotropic (HMT) alignment of ferroelectric liquid crystals (FLC) cells have been successfully achieved by a non-contact technique, namely, magnetic field-assisted alignment; otherwise such alignment is difficult by conventional methods. Frequency–temperature dependent dielectric studies, confirm the presence of stable molecular relaxation around the short axis of FLC molecule, throughout SmC*–SmA* phases with minimum low-frequency fluctuations, known as Goldstone modes. The proposed non-contact alignment method, combined with FLC-based HMT configuration, would be much promising for integrated-optoelectronic devices, such as micro-mirror displays.

P‐156: Alignment of Ferroelectric Liquid Crystals with the Substrates Processed by Plasma‐Beam

AbstractPlasma beam procedure earlier adapted for alignment of nematic liquid crystals and reactive mesogens is successfully applied for alignment of ferroelectric liquid crystals. Highly uniform alignment of “chevron” structure (before electrical treatment) and “quazi bookshelf” structure (after electrical treatment) are realized. In the latter case, the contrast of bistable switching is larger than 350:1 for 650 nm wavelength. This makes the non‐contact plasma alignment procedure especially attractive for high contrast bistable LCD on LCOS base particularly used in PDA and E‐books. Fast switching and realization of gray scale in the plasma beam aligned FLC cells makes this technique also promising for full color displays including color TV LCD.

A novel photoalignment film for ferroelectric liquid crystal based on self‐assembled monolayer method

A novel strategy based on self‐assembly technology was devised for design of photosensitive material as a ferroelectric liquid crystal (FLC) alignment layer. This development offers new tools for the study and control at the molecular level of the interaction of FLCs with solid surfaces. The photoreactive material was self‐assembled to the substrate by covalent bond linkage due to a special chemical adsorption reaction. Through ester bond linkage, a cyano group with strong polarity was introduced to be terminus of the film. Under irradiation of linearly polarised ultraviolet light, an optically anisotropic self‐assembled film was easily obtained. The irradiated film was demonstrated to result in homogenous alignment of FLC by optical transmittance measurements and polarising optical microscopy images of a FLC cell at different rotation angles. The alignment quality of the FLC on this self‐assembled monolayer film is comparable to that of commercial rubbed polyimide film. Furthermore, it was also found that the fine alignment of the FLC may be related to the smoothness of the self‐assembled film surface owing to its polar end.

P‐112: Low Driving Voltage with Clean Aligned Ferroelectric Liquid Crystal Devices

AbstractLow driving voltage FLC material, R3206 with Vsat. at 3.5V (AZ Electronic Materials), and its mixtures were under evaluation for potential TFT display application. Poor alignment issue was overcome by the diluted pitch mixtures and asymmetric hybrid alignment cells. In the series of diluted pitch mixtures, the contrast and alignment are better than pure compound in pre‐made cells (1.8±0.1μm, from EHC). In particular, the 70% R3206 mixture showed the best result with low driving voltage below 5V and fast response time (T on +T off) under 1.1 ms. The hybrid alignment cell was successfully suppressed horizontal chevron defects and achieved the best contrast ratio of 750. These results provide promising FLC materials with low driving voltage, well aligned, and fast switching for TFT‐LCD application.