High-performance Liquid Crystal Research Articles

The TV in Your Pocket: Development of Liquid‐Crystal Materials for the New Millennium

New liquid crystals with very low viscosity, good mesophase behavior, and high reliability are necessary to achieve the breakthrough from flat computer monitors to large displays for television. Fluorine plays a decisive role not only because of the polarity it induces in organic molecules but also because of its low polarizability and weak propensity for ion solvation. In addition, subtle stereoelectronic effects in fluorine-containing liquid crystals influence material properties and allow these to be tuned to some extent to achieve the desired outcome. Some fairly sophisticated chemistry is required that is normally ruled out in the specialty chemicals industry because of cost. The television display market is now entering a phase of saturation. The broad availability of the internet has led to an ever increasing tendency for mobile products. Tablet PCs and smartphones require touch-panel functionality and low power consumption. New LCD modes with high-performance liquid crystals and additional components, such as polymerizable materials, can be used in such products.

3,4-Difluoropyrrole-, 3,3,4,4-tetrafluoropyrrolidine- and pyrrolidine-based liquid crystals

A new class of liquid crystals was formed with N-heterocycles as the terminal group, such groups as a 3,4-difluoropyrrole, 3,3,4,4-tetrafluoropyrrolidine, or pyrrolidine. Their properties were modified by varying the terminal heterocycles and/or the length of the alkyl chains on the cyclohexane liquid crystal building block. A comparison of the non-fluorinated and fluorinated pyrrolidine and fluorinated pyrrole as terminal group in the cyclohexane liquid crystals was studied. The mesophase behaviour was affected by altering the polarity of the terminal N-heterocycle. Compounds n/m-4H having the terminal pyrrolidine show a nematic phase. Comparatively, compounds 3/2-4F and 5/2-4F, having a fluorinated pyrrolidine, exhibit SmG phase. With 3,4-difluoropyrrole as terminal group, n/m-2F exhibit the broadest nematic phase range and good thermal stability, thus meeting the criterion for high performance liquid crystals materials.

CIS–ZnS quantum dots for self-aligned liquid crystal molecules with superior electro-optic properties

We demonstrate self-aligned and high-performance liquid crystal (LC) systems doped with 1-dimensional (1D) chain-like clusters of CuInS(2) (CIS)-ZnS core-shell quantum dots (QDs). By changing the cell fabrication method of the LC-QD composites, we can selectively control the orientation of the LC molecules between the homogeneous and homeotropic states without conventional LC alignment layers. The homeotropic alignment of LCs was achieved by random dropcasting and the homogeneous alignment was performed using a capillary injection of LC-QDs due to the random or linear diffusion of QD clusters into ITO defects. The electrically compensated bend (ECB)- and vertically aligned (VA) mode LC displays (LCDs) containing our LC-QD composite both showed superior electro-optic (EO) properties. A 37.1% reduction in the threshold voltage (V(th)) and a 36.6% decrease in the response time were observed for ECB mode LCDs, and a 47.0% reduction in the V(th) and a 38.3% decrease in the response time were observed for VA mode LCDs, meaning that the proposed LC-QD composites have a great potential for the production of advanced flexible LCDs.

Surface reformation on solution-derived zinc oxide films for liquid crystal systems via ion-beam irradiation

We investigated solution-derived ZnO films as liquid crystal (LC) alignment layers via ion-beam (IB) irradiation. Solution processing was adopted to replace the sputtering method for the deposition of ZnO films as LC alignment layers. Uniform and homogeneous LC alignment was achieved, and a high-performance LC system was achieved using these substrates. X-ray photoelectron spectroscopy (XPS) analysis showed that surface reformation of ZnO films by annealing and IB irradiation affects the uniformity of the LC alignment and allows control of the pretilt angle. Superior electro-optical (EO) characteristics of twisted nematic-LC cells constructed with IB-irradiated ZnO films were observed, which indicates that the proposed solution-derived ZnO films have strong potential for use in the production of advanced LC displays.

The Influence of Thermal Annealing on Microstructure and Mechanical Properties in High Performance Liquid Crystal Copolyesters

ABSTRACTIn this research we have focused on the influence of thermal treatment for periods of time on the thermal and mechanical properties of extruded films of a series of high-performance thermotropic liquid crystal polymers (LCPs). The dependence of microstructure, thermal and mechanical properties on the extent of thermal treatment is investigated. Especially synthesized wholly aromatic LCPs based on hydroxybenzoic acid (B), hydroxynaphthoic acid (N), terephthalic acid (TA) and biphenol (BP) are kindly supplied by Hoechst Celanese Research Corp in the form of 50 μm thick extruded films. Thus, the influence of monomer composition is also studied in order to contrast the influence of molecular conformation. Thermal treatments are carried out at temperatures close to the solid-to-nematic transition (Ts→n) for up to several hours under dry air conditions. The results show a profound influence of thermal annealing on morphology and mechanical modulus when annealing is carried out c.a. 40ºC below Ts→n, where solid-to-nematic transition and Young’s modulus are significantly increased.

Devices and materials for high-performance mobile liquid crystal displays

Mobile liquid crystal displays (LCDs), often playing a role in hand-held computers, have become indispensable electronic tools to human beings in modern society. Innovative technological developments in the devices and materials have paved a successful road toward the development of mobile LCDs. Herein, after reviewing the trends and performance requirements of mobile LCDs, organic–inorganic materials as the key components of high performance mobile LCDs are addressed. Additionally, the developing trends in mobile LCDs are discussed with respect to these materials.

Frequency tunable S-band resonator using nematic liquid crystal

A tunable resonator based on change of substrate permittivity is fabricated using a new high performance liquid crystal GT3-23001. The results show a measured frequency tuning range of 8.2% compared with a simulated 8.5% around a centre frequency of 2.8%GHz, with a lower insertion loss compared to standard liquid crystals.

Super‐Fast Switching of Twisted Nematic Liquid Crystals on 2D Single Wall Carbon Nanotube Networks

AbstractWe have developed a high performance liquid crystal (LC) alignment layer of ultra‐thin single wall carbon nanotubes (SWNTs) and a conjugated block copolymer nanocomposite that is solution‐processible for conventional twisted nematic (TN) LC cells. The alignment layer is based on the non‐destructive solution dispersion of nanotubes with a poly(styrene‐b‐ paraphenylene) (PS‐b‐PPP) copolymer and subsequent spin coating, followed by conventional rubbing without a post‐annealing process. Topographically grooved nanocomposite films with two dimensionally (2D) networked SWNTs embedded in a block copolymer matrix were created using a rubbing process in which bundles of SWNTs on the composite surface were effectively removed. The LCs were well aligned with a stable pre‐tilt angle of approximately 2° on our extremely transparent nanocomposite, which gave rise to superfast switching of the TN LC molecules that was approximately 3.8 ms, or four times faster than that on a commercial polyimide layer. Furthermore, the TN LCD cells containing our SWNT nanocomposite alignment layers exhibited low power operation at an effective switching voltage amplitude of approximately 1.3 V without capacitance hysteresis.

High-performance composites based on all-aromatic liquid crystal thermosets

In this study, a new high-performance liquid crystal ester-based thermoset for composite applications was investigated. All-aromatic liquid crystalline thermosets (LCTs) are a promising class of polymers that offer a unique combination of properties such as solvent resistivity, high modulus, high strength, low coefficient of thermal expansion and high after cure glass-transition temperatures ( T g ⩾ 150 °C). Fully cured LCTs offer superior thermo-mechanical properties over high-performance thermoplastic polymers such as PPS, PEEK and PEI. For this study we used a 9000 g mol −1 ester-based LCT based on cheap and readily available monomers, i.e. 4-hydroxybenzoic acid (H), isophthalic acid (I) and hydroquinone (Q), abbreviated by us as HIQ-9. Composite panels prepared from T300 carbon fiber (5-harness satin weave) showed in-plane shear strength of 154 MPa and an in-plane shear modulus of 3.7 GPa. The tensile strength and modulus were measured to be 696 MPa and 57 GPa, respectively. A post-mortem inspection showed that the interfacial strength was excellent and no delamination was observed in the test specimen. Preliminary results show that LCT-based composites exhibit a better combination of (thermo) mechanical properties over PPS and PEI-based composites.

Low-Voltage Driving of Liquid Crystals Vertically Aligned by Ion-Beam Bombardment on High-k HfO2 Surface

Ion-beam irradiation (IB) on HfO2 surface induced high-performance liquid crystal (LC) driving at a 1-V threshold with vertical alignment of liquid crystals (LC). The high-k materials Atomic layer deposition was used to obtain LC orientation on ultrathin and high-quality films of HfO2 layers. To analyze surface morphological transition of HfO2 which can act as physical alignment effect of LC, atomic force microscopy images are employed with various IB intensities. The contact angle was measured to elucidate the mechanism of vertical alignment of LC on HfO2 with IB irradiation. Contact angle measurements show the surface energy changes via IB intensity increasing.

High-resolution photoaligned liquid-crystal micropolarizer array for polarization imaging in visible spectrum

We report a superior high-resolution micropolarizer array fabrication technology that combines a linear polarizer with a micropatterned liquid-crystal (LC) cell. A 2 microm pitch is achieved by using UV light to define the orientation of the micropolarizer elements. Reported experimental results validate the concept of high-performance photoaligned LC micropolarizer arrays with major principal transmittance of approximately 80% and extinction ratio as high as approximately 3200(35 dB).

Vertical alignment of liquid crystals on a fully oxidized HfO2 surface by ion bombardment

High-performance liquid crystals (LCs) driven at a 0.9 V threshold were demonstrated on very thin HfO2 films with vertical (homeotropic) alignment by ion bombardment. Atomic layer deposition was used to obtain LC orientation on ultrathin high-quality films of double-layer HfO2/Al2O3. X-ray photoelectron spectroscopy indicated that full oxidization of HfO2 film surfaces was induced by ion bombardment, shifting the Hf 4f spectra to lower binding energies. The increased intensities of the Hf 4f peaks after ion bombardment confirmed that nonstoichiometric HfOx was converted to the fully oxidized HfO2 surfaces.

P‐129: Study on Electro‐Optic Characteristics According to Length between Pixel Electrodes in the Fringe and In‐Plane Field Switching Mode

AbstractWe have studied electro‐optic characteristics of high performance liquid crystal display using combined fringe and in‐plane horizontal electric fields, named as fringe and In‐plane switching (FIS). The strong electric fields cause more liquid crystals to reorient above and between the pixel electrodes. Moreover, the operating voltage of FIS mode was decreased by using two thin film transistors (TFTs). To suggest novel structure of FIS mode, electro optic characteristics are analyzed as a function of length between pixel electrodes compared to IPS and FFS mode. Consequently FIS mode shows not only higher transmittance but also lower operating voltage than those of IPS and FFS mode while keeping wide viewing angle.

Liquid crystal display using combined fringe and in-plane electric fields

A high performance liquid crystal display using combined fringe and in-plane horizontal electric fields is proposed. The strong electric fields cause more liquid crystals to reorient almost in plane above and between the pixel electrodes. As a result, the operation voltage is lower and transmittance is higher than those of fringe field switching and in-plane switching modes, while preserving a wide viewing angle. Such a high performance device is particularly attractive for large panel liquid crystal displays.

Weak surface anchoring energy of 4-cyano-4′-pentyl-1,1′-biphenyl on perfluoropolyether Langmuir-Blodgett films

Functional director alignment layers are needed for high performance liquid crystal displays (LCDs). Reported herein is a novel polymer material for LC alignment, namely, perfluoropolyether (PFPE), which exhibits a weak surface anchoring energy for bend deformation and is amenable to simple fabrication of grooved surfaces by soft lithography, a surface topography desired for multistable LCDs. Liquid crystal optical cells fabricated using Langmuir-Blodgett films of PFPE (of variable thickness) exhibited weak surface anchoring energies on the order of 10(-5) Jm2 for the nematic liquid crystal 4-cyano-4'-pentyl-1,1'-biphenyl with no dependence on film thickness.

Aligned Polycrystalline Silicon Array Thin Film by XeCl Excimer Laser Annealing for AMOLED Displays

Low temperature polycrystalline silicon (LTPS) thin film transistors (TFTs) are demanded to fabricate high performance liquid crystal displays (LCD) and organic light-emitting diode displays (OLED). The mobility of poly-Si TFT can be two orders of magnitude higher than that of amorphous Si (a-Si) TFT. Excimer laser annealing has been studied to be the most promising technology to meet the stringent requirement in high speed operation. The process parameters were identified as a-Si thickness, laser energy density, overlap ratio, annealing atmosphere and pre-clean condition. The a-Si layer of 40-50 nm was deposited by plasma enhanced chemical vapor deposition (PECVD). The XeCl excimer laser was irradiated on the a-Si film at room temperature under N2 or N2/O2 environment. The energy density ranged 250-400 mJ/cm2, and the overlap ratio was 95-99%. The highly aligned poly-Si array thin film could be obtained. The grain size has been about 0.31x0.33 μm2, and the regular arrangement in poly-Si grains was discussed. In addition, the PMOS TFT has been fabricated from the aligned poly-Si array. The mobility was as high as 100 cm2/Vs and the sub-threshold swing was around 0.24 V/dec. The threshold voltage was -1.25 V and the on/off current ratio was about 106.

Design and Fabrication of High-Performance Liquid Crystal Gratings

We present basic principles of designing and fabricating high-performance liquid crystal (LC) gratings with binary alignment structures which produce a variety of the diffraction properties. The periodic binary structure in the LC layer was fabricated through a selective irradiation of the UV light using the single-masking process. An alternating homeotropic and hybrid alignment configuration produces the input polarization-dependent diffraction suitable for the polarization-separating devices while an oppositely twisted binary alignment configuration gives the input polarization-insensitive diffraction. It is found that a dye-doped bidirectional alignment configuration can be used for obtaining optically controllable polarization properties.

Optical engines for high-performance liquid crystal on silicon projection systems

Liquid crystal on silicon (LCoS) is a relatively new technology that can be used as the spatial light modulator in projection systems. It can offer high performance at low cost. Various optical architectures have been proposed that can be used with LCoS panels. All of these systems have their own specific advantages and disadvantages. Efficiency, color gamut, and contrast ratio are three quality parameters that are largely dependent on the optical engine that is used. An optical engine with a high performance that is relatively compact is proposed. Realistic optical simulations are used to demonstrate the performance of this system. Two different optical configurations are investigated.

25.2: Characterization of Physical Image Quality of 2‐ and 5‐Million‐Pixel Monochrome AMLCDs for Medical Imaging

AbstractThe physical image quality of two new high‐performance monochrome active matrix liquid crystal displays (AMLCDs), a 2‐ and a 5‐million‐pixel display, is reviewed. The performance is illustrated by examples (because of space limitations) of on‐axis and off‐axis characteristic curves, luminance range and contrast, luminance uniformity across the display screen, temporal modulation transfer function (MTF), spatial MTF, spatial noise power spectra and spatial signal‐to‐noise ratios. The LCDs are equipped with an internal photosensor that maintains a desired maximum luminance and calibration to a given display function. The systems offer aperture and temporal modulation to place luminance levels with more than 12‐bit precision on a desired display function and achieve uniform contrast distribution over the luminance range for a narrow viewing angle perpendicular to the LCD face. The image quality of the LCDs is compared with that of high‐performance high‐resolution cathode‐ray‐tube (CRT) displays.

Intensity filtering of a two-dimensional optical image in high-performance photorefractive mesogenic composites

Edge enhancement, a type of intensity filtering of a two-dimensional optical image, was demonstrated using a high-performance photorefractive polymer-dissolved liquid crystal. We calculated the expected images using Fourier transform holographic geometry and obtained good agreement between the observed images and the theoretical expectation.