Supertwisted Nematic Research Articles

The 60th Anniversary of TFTs and the Evolution of High‐Resolution Displays

The 60th Anniversary of TFTs and the Evolution of High‐Resolution Displays

Stabilization of Long-Pitch Supertwisted Nematic Structures

Stabilized reverse twisted nematic liquid crystal devices (RTN-LCDs) were fabricated using formation of a polymer matrix under UV irradiation with an applied voltage (sustain voltage) in the vicinity of the alignment layers. In the absence of an applied voltage, the non-stabilized RTN structure gradually returns to a splay twist structure. The sustain voltage was decreased with an increase in temperature. A stabilized long-pitch supertwisted nematic (LPSTN) structure could also be formed during the RTN structure stabilization process with a much lower sustain voltage at a temperature near the clearing point. The chiral pitch for the LPSTN structure is longer than that for a typical STN structure. LPSTN-LCDs similar to RTN-LCDs show a large reduction in both the threshold and saturation voltage compared with those for TN-LCDs consisted of the same LC materials. Furthermore, a notable feature of LPSTN-LCDs is a change to a TN structure when a high voltage is applied. A black state can be realized due to the change from the LPSTN structure to the RTN structure unlike the typical STN mode under the crossed nicols condition. In contrast STN-LCDs retain their color due to the retardation because the RTN and LPSTN states are considered topologically equivalent.

Microwave modulation characteristics of twisted liquid crystals with chiral dopant

Adding a chiral dopant in twisted nematic (TN) liquid crystal cell can stabilize the orientation of liquid crystal molecules, particularly in high TN (HTN) or super TN (STN) liquid crystal cells. The difference in pitches in liquid crystal is induced by the chiral dopant, and these different pitches affect the orientation of liquid crystal director under an external applied voltage and influence the characteristics of microwave modulation. To illustrate this point, the microwave phase shift per unit length (MPSL) versus voltage is calculated on the basis of the elastic theory of liquid crystal and the finite-difference iterative method. Enhancing the pitch induced by the chiral dopant in liquid crystal increases the MPSLs, but the stability of the twisted structures is decreased. Thus, appropriate pitches of 100d, 4d, and 2d can be applied in TN, HTN, and STN cells with cell gap d to enhance the characteristics of microwave modulation and stabilize the structures in twisted cell. This method can improve the characteristics of liquid crystal microwave modulators such that the operating voltage and the size of such phase shifters can be decreased.

A simple method to measure the twist elastic constant of a nematic liquid crystal

We demonstrate a simple method for measuring the twist elastic constant (K22) of a nematic liquid crystal (LC). By adding some chiral dopant to an LC host, the LC directors rotate 180° in a homogeneous cell, which is known as 180° super-twisted nematic (STN) cell. By preparing two such STN cells with different chiral concentrations and measuring their Fréedericksz threshold voltages, we can obtain the K22 and helical twisting power simultaneously. In the whole process, there is no need to measure the pitch length. Our obtained K22 values agree well with those reported by using other methods.

Failure mode and effects analysis using intuitionistic fuzzy hybrid TOPSIS approach

Failure mode and effects analysis (FMEA) is an effective reliability analysis technique used to identify and evaluate potential failures in systems, products, processes, and/or designs. In traditional FMEA, prioritization of failure modes is carried out by utilizing risk priority numbers (RPNs), which can be acquired by the multiplication of three risk factors: occurrence (O), severity (S) and detection (D). However, there are some inherent deficiencies in the conventional RPN method, which affect its effectiveness and thus limit its applications. In response, this paper introduces a new modified TOPSIS method, named intuitionistic fuzzy hybrid TOPSIS approach, to determine the risk priorities of failure modes identified in FMEA. Moreover, both the subjective and objective weights of risk factors are taken into consideration in the process of risk and failure analysis. A product example of the color super twisted nematic is presented at last to demonstrate the potential applications of the proposed approach, and the merits are highlighted by comparing with some existing methods.

Fast-response twisted nematic liquid crystal displays with ultrashort pitch liquid crystalline materials

A new method to improve the response time of twisted nematic (TN) liquid crystal displays (LCDs) with liquid crystalline (LC) materials of pitch length shorter than twice the cell gap is proposed. It was known that by using LC materials possessing shorter pitch length, the response time could be improved. However, in previous studies, the pitch length p was limited to values larger than the twice of the cell gap d. This is because 90° TN arrangements become unstable and transform into a 270° super-twisted nematic (STN) arrangement for p < 2d. It was found that by applying voltage to 270° STN arrangement, the 90° TN arrangement could be formed and the obtained TN-LCDs could be stabilised by photopolymerisation. This is because the 270° STN arrangement contains splay deformation. The obtained TN-LCDs exhibit a fast response. By using this method, fast-response TN-LCDs could be realised maintaining moderate driving voltage.

P‐27: Light Printer for Optical Rewritable Electronic Paper

AbstractComposed of a transmissive super‐twisted‐nematic(STN) liquid crystal display (LCD) and an external driving circuit system, the light printer for optical rewritable (ORW) electronic paper (e‐paper) can easily hold or change the display image, which is used as the mask pattern of ORW electronic paper.

Electrically Tunable and Dual-Wavelength Semiconductor Laser with a Liquid Crystal Display

An electrically tunable semiconductor laser with a liquid crystal display (LCD) and a commercially available laser diode is presented. We tested two types of LCD under strong laser light and, on the basis of the results, selected the supertwist-nematic (STN) LCD with a passive matrix. Tuning is achieved up to 5 nm by moving a single stripe pattern displayed on the LCD. The wavelength shifts in 0.553 nm/cell, which agrees well with the calculated resolution derived from the diffraction grating formula. Dual-wavelength oscillation is also achieved up to a wavelength separation of 4.62 nm using the double stripe pattern.

Characterization of the single and double films consisting of Al, Sc-co-doped ZnO/Al-doped ZnO and Al-doped ZnO/Al, Sc-co-doped ZnO

Single and double films consisting of Al, Sc-co-doped ZnO (denoted as S) and Al-doped ZnO (denoted as A) layers were deposited on the Super Twisted Nematic (STN) glass by dc-sputtering on the pure aluminum (99.999%) or Al-0.8 wt.% Sc eutectic alloy target (99.999%) combined with rf-sputtering on the pure zinc oxide (99.99%). The thickness of each layer in the double film was controlled at roughly 230 nm but the depositing order changed. A double film fabricated by depositing first with a layer of Al-doped ZnO on STN then with a layer of Al, Sc-co-doped ZnO on the top was expressed as SA. The film deposited with the same layers but inverse the order was represented as AS. The characterization of the films revealed that the electrical resistivity is lower (i.e., 1.43×10−3–1.58×10−3 Ω-cm) for the double layers (both AS and SA) than the single layer (i.e., 3.8×10−3 Ω-cm for A and 1.5×10−3 Ω-cm for S). The transmission of visible light exceeds 80% for all the films deposited on the glass but the transmission decreases substantially at short wavelength near the ultraviolet range. X-ray diffraction (XRD) revealed that both AS and SA specimens all belong to hexagonal wurtzite textured at (002) and (103). Transmission electron microscopy (TEM) of the cross-sections depicted the films are composed of columnar grains. The electrical resistivity for all the films decreased to a minimum with increasing the annealing temperature from 200 to 300 °C, and increased beyond 300 °C. No sacrifice to the optical transparency (more than 80 % transmittance for the visible light) but a blue-shift of optical transmittance was found for the double-layered films, especially for AS specimen. Annealing of the double films at 300 °C intensified their blue-shift. Based on X-ray photoelectron spectroscopy (XPS) on the surface film we inferred that a decrease of electrical resistivity of the double films is due to an increase in the relative strength of O (III).

Fast and high-contrast liquid-crystal shutters with low power consumption based on an optical-mode-interference cell

Three electro-optical modes of nematic liquid crystal (LC) were investigated as applications for LC shutters: the waveguide mode (TN), optical mode interference (OMI), and supertwist nematic (STN). The characteristics of the three modes under dynamic driving were simulated by solving the Ericksen-Leslie hydrodynamic equations numerically. Prototypes of the three types of LC were fabricated for verification. The experimental data show good coincidence with the simulation results. Among the three LC modes, the OMI was found to have the greatest potential for LC shutter applications. The prototype that used the OMI mode showed high contrast, fast response times, low power consumption owing to the pseudomemory effect under an optimal driving scheme, and a low optical retardation of the LC cell.

The Influence of the Driving Voltage and Ion Concentration on the Lateral Ion Transport in Nematic Liquid Crystal Displays

Nematic liquid crystal displays (LCDs) contain ions that influence the electrooptical characteristics of the display. A typical super-twisted nematic (STN) display for mobile phone applications becomes darker at a standard driving frequency if it contains many impurity ions. We have discovered that ions can travel in the plane of the glass plates in the absence of a lateral electric field, leading to lateral nonhomogeneity in transmission (dark and bright stripes). In this paper, we present our research on the lateral ion transport dependence on the driving square wave (SQW) amplitude and dc component at a wide range of ion concentrations. The existence of a dc component, a high ion concentration and high SQW amplitudes increase the lateral ion speed.

New measuring method for determining alignment parameters in twisted-nematic and super-twisted-nematic cells

— A new method for determining the thickness, twist angle, and azimuth of the director for a substrate with twisted-nematic (TN) and super-twisted-nematic (STN) liquid-crystal (LC) cells has been proposed. The measurement was carried out by using an optical system which consisted of a monochromatic light source, polarizer, rotating analyzer, and software programs. The polarization transfer matrix, or Jones matrix, of a TN or an STN LC cell is formulated by the Fourier coefficient of the signal that is modulated by the rotating analyzer. These three alignment parameters are derived directly from components of the Jones matrix. We made a special effort to design the system to ensure accurate, stable, and quick measurement. Thus, the three parameters were determined simultaneously within a few seconds with good reproducibility; e.g., the standard deviation of the twist angle in the TN cell is 0.02° or less.

Dynamical process of switch-off in a supertwisted nematic cell

The switch-off dynamics of a 180° supertwisted nematic (STN) are explored in detail by using a convergent beam system and the fully leaky guided-mode technique. From the dynamical guided-mode data and using the Ericksen–Leslie theory, the director structure in the cell at different times during switch-off is obtained. We have studied the dynamic switch-off from different applied voltages and have found that the relaxation time is independent of the voltage. From fitting the optical data, the effective viscosities are obtained. It appears that two viscosity coefficients, γ and η1, largely control the dynamics of switch-off. From the analysis of the director tilt profile in the cell, it is found that there is no backflow-induced increased tilt at the cell center in the STN cell during the switch-off. From liquid crystal hydrodynamics, this no-backflow dynamic process in the STN cell is well explained.

Generalized spectroscopic ellipsometry and Mueller-matrix study of twisted nematic and super twisted nematic liquid crystals

Generalized spectroscopic ellipsometry (g-SE) and Mueller-matrix (MM) measurements are applied to twisted nematic (TN) and super twisted nematic (STN) displays. Transmitted measurements near normal incidence are used to extract twist, φ( d), and anisotropy for each display type. Angle-dependent measurements are used to determine tilt distribution, θ( d). Oblique angle measurements allow characterization of both ordinary and extraordinary indices. Finally, the liquid crystal layer thickness is accurately determined from coherent interference oscillations.

Improved Liquid Crystal Mixtures for STN Displays

Supertwisted nematic (STN) liquid crystal displays remain a leading technology for mobile communications applications, personal digital assistants (PDA) and personal convergence devices. Higher demands on display information content place ever increasing demands on the physical parameters of the liquid crystal mixture. To satisfy current and future display requirements, it is necessary to develop mixtures showing: ? Fast response times achieved by rotational viscosity reduction and birefringence increase. ? Small temperature dependence of the operating voltage (dV/dT), especially at low temperatures. ? Small frequency dependence of the operating voltage (dV/df), necessary for multi-level greyscale operation. ? Improved steepness (V90/V10) of the electro-optical curve to enable higher duty multiplexing of the display. In order to fulfil these requirements, liquid crystal mixtures with improved physical properties were developed. Examples of these mixtures are presented.

Electro-Optical Bistability in Ferroelectric Liquid Crystal Switching Devices for Use in Displays and Real-Time Holography

Liquid crystalline (LC) materials which exhibit the ferroelectric Sc* phase showed early promise for device applications because of their fast electro-optic response and potential long-term electro-optical bistability. In combination these features should allow fast line at a time addressing and therefore potentially large passive matrix displays. Bistability should also facilitate DC balancing without the need for inverse frame or blanking procedures used in active matrix addressed liquid crystal over silicon devices. Such a feature is very valuable in the application of these materials to telecom devices where signal interruption for DC balancing purposes is very problematic. Long term bistability is also important in realising active tiling based displays. However the realisation of acceptable bistability has not been straightforward. Partly as a result, commercial passive matrix displays have been dominated by super twisted nematic (STN) devices, which have a limited voltage selection ratio that decreases with the number of pixel rows addressed. High quality flat panel LC displays for laptop computers, computer monitors and television has been delivered using active matrix thin film transistor twisted nematic (TFT TN) technology. The first generation of commercially important ferroelectric liquid crystal displays (for near-eye and projection applications) have in fact utilised active matrix liquid crystal over silicon (LCoS) device structures, where the speed of the FLC electro-optic response permits the use of frame sequential methods for generating colour and greyscales in images. Here we discuss the problems encountered in achieving useful bistability within FLC devices, the new applications that would greatly benefit from true FLC bistability and some interesting new results from some siloxane oligomers which suggest new possibilities in achieving true bistability. We hope to highlight the need for a deeper understanding of this problem.

Influence of cell and LC parameters on the dynamics of STN liquid crystal displays

The dynamics of super-twisted nematic (STN) liquid crystal displays have rarely been studied. In this article, the dynamic response of STN is analysed in detail. The evolution of director configuration with time was obtained by solving Ericksen–Leslie hydrodynamic equations. The time varying midlayer tilt angle is presented as a measure of dynamic response. The influence on STN dynamics of cell parameters including pretilt angle, twist angle, cell thickness, and of material parameters including d/p, K 22, K 33, were studied.

Monte Carlo simulation of twisted nematic and supertwisted nematic liquid crystal cellsPresented at the 19th International Liquid Crystal Conference, 30 June–5 July 2002, Edinburgh, UK.

The orientational properties of liquid crystals in twisted nematic (TN) and supertwisted nematic (STN) cells were studied by Monte Carlo (MC) simulation depending on the strength of an external field. The liquid crystals sandwiched between planar surfaces were described by three-dimensional lattice systems of dimensions 10 × 10 × 13 up to 10 × 10 × 33 with pairwise interactions given by the chiral Lebwohl–Lasher potential between objects taken as directors representing a cluster of neighbouring molecules in a cubic unit cell. The electro-distortional curves showing the midlayer tilt angle as function of the applied voltage have been investigated depending on a set of cell and material parameters including e.g. the cell twist angle, the surface pretilt angle, and the thickness to pitch ratio. At very low temperature, in spite of the small system sizes studied a very good agreement has been obtained for MC simulation results and continuum theoretical predictions, both with respect to the dependency of the obtained threshold voltages necessary for the field-induced unwinding transition on cell twist angle and thickness to pitch ratio even for non-zero surface pretilt angles. This enabled the calculation of the elastic constants of the chiral Lebwohl–Lasher model. The applied method, the analysis of director arrangements in liquid crystal cells by means of Monte Carlo simulations starting from pairwise interactions, will be helpful as an alternative route to a continuum theoretical approach in designing advanced liquid crystal displays.

Reflective liquid-crystal display using an in-plane-switching super-twisted nematic cell

Two in-plane-switching (IPS) super-twisted nematic (STN) cells are proposed for reflective liquid-crystal displays. The IPS 180°-STN cell exhibits a normally black display with a relatively low-operating voltage, fast response time, and wide viewing angle. Potential applications for large screen reflective displays are foreseeable.

Dynamic response of super-twisted nematic liquid crystal displays

The dynamics of super-twisted nematic (STN) liquid crystal displays was studied by detailed computer simulation. The time evolution of director configuration and velocity of flow as obtained by solving Ericksen-Leslie hydrodynamic equations. The influence of d/p value and pretilt angle on the dynamic response was also studied. A comparison was also made between twisted nematic and STN liquid crystal displays.