Tuning the alignment of liquid crystals by means of nano‐structured surfaces (Invited Paper)

Abstract— The solid‐surface/liquid‐crystal interactions, defining the field‐free alignment of the liquid crystal in conventional liquid‐crystal displays, are playing a vital role in their optical appearance and performance. Nano‐scale changes in the solid‐surface structure induced by light have been recently shown to affect the anchoring strength and the easy‐axis direction. Fine tuning of the anchoring strength is also demonstrated by nano‐structuring of the Langmuir‐Blodgett monolayer employed as liquid‐crystal alignment layers promoting homeotropic orientation. On the basis of nano‐engineering of the surface alignment properties, two novel alignment concepts have been introduced: electrically commanded surfaces (ECS) and high‐performance alignment layers (HiPAL). Nano‐structured polymers related to these concepts have been designed, synthesized, and used as materials for alignment layers in LCDs. ECS materials belong to the category of active alignment materials designed to mediate switching of the liquid crystal, whereas the HiPAL materials make possible the control of the molecular tilt angle in a broad range, from 0° to 90°, and they seem to enable the control of the anchoring strength as well. The nano‐structured alignment materials are strong candidates for implementation in a new generation of advanced liquid‐crystal displays and devices.     

Roll‐to‐roll manufacturing considerations for flexible,cholesteric liquid‐crystal display (Ch‐LCD) media

Abstract— Roll‐to‐roll methods and equipment to manufacture a bistable, passively driven display media on a flexible substrate have been developed. Using continuous coating techniques and equipment, cholesteric liquid‐crystal droplets in a gelatin binder and a dark layer are simultaneously coated onto laser‐etched‐patterned transparent ITO conductors on a polymeric web. Second conductors are printed with a UV‐curable polymer thick‐film ink over the active display layers, followed by slitting and chopping to complete the manufacture of display media in a full roll‐to‐roll mode. Segmented‐ and matrix‐display media can be generated using these techniques. This paper will focus on the manufacturing considerations for producing matrix‐display media.     

Nano‐domains formation on photoalignment thin film by self‐organized dewetting

A novel discontinuous photoalignment surface with nano‐domains for liquid crystal is developed. The formation of the discontinuous structure is created by self‐organized dewetting, which is regarded as one of the most promising bottom‐up approaches to fabricate nano‐structure. Different dewetting conditions, such as surface roughness, thickness and viscosity, have been investigated. Such discontinuous photoalignment layer can be fabricated on top of another continuous alignment layer to form a new kind of heterogeneous nano‐structured alignment surface – stacked alignment layers. This heterogeneous alignment surface can be used to produce arbitrary pretilt angles for the liquid crystal display. Simulation model has been built to understand the dewetting mechanism. Experiments using photo‐aligned and photo‐polymerisable polymer have been done to verify the dewetting theory. The produced stacked alignment layers are proved to be robust. Moreover, the dewetting processing is a fully controllable process and is compatible with existing manufacturing techniques.     

Nano‐structured liquid‐crystal alignment layers (Invited Paper)

Abstract— The alignment of liquid crystal by nano‐structured surfaces is investigated. It is shown that reliable pretilt angles of any value between 0° and 90° can be produced with these surfaces. The physics and properties of such alignment layers are studied using a variety of techniques. The anchoring energy and temperature stability of the alignment are also measured. Dependence on various processing conditions is also characterized. It is shown that these nano‐structured alignment layers are useful for the production of high pretilt angles needed for a variety of applications.     

Suspension‐deposited carbon‐nanotube networks for flexible active‐matrix displays

Abstract— The unique properties of carbon nanotubes (CNTs) promise innovative solutions for a variety of display applications. The CNTs can be deposited from suspension. These simple and low‐cost techniques will replace time‐consuming and costly vacuum processes and can be applied to large‐area glass and flexible substrates. Single‐walled carbon nanotubes (SWNTs) have been used as conducting and transparent layers, replacing the brittle ITO, and as the semiconducting layer in thin‐film transistors (TFTs). There is no need for alignment because a CNT network is used instead of single CNTs. Both processes can be applied to glass and to flexible plastic substrates. The transparent and conductive nanotube layers can be produced with a sheet resistance of 400 Ω/□ at 80% transmittance. Such layers have been used to produce directly addressed liquid‐crystal displays and organic light‐emitting diodes (OLED). The CNT‐TFTs reach on/off ratios of more than 10⁵ and effective charge‐carrier mobilities of 1 cm²/V‐sec and above.     

Energy‐efficient liquid‐crystal displays (e2‐LCDs) using a photonic‐crystal backlight system

Abstract— Cholesteric liquid crystals automatically form one‐dimensional photonic crystals. For a photonic crystal in which light‐emitting moieties are embedded, unique properties such as microcavity effects and simultaneous light emission and light reflection can be expected. Three primary‐color photonic‐crystal films were prepared based on cholesteric liquid crystal in which fluorescent dye is incorporated. Microcavity effects, i.e., emission enhancement and spectrum narrowing, were observed. Two types of demonstration liquid‐crystal displays (LCDs) were fabricated using the prepared photonic‐crystal films in a backlight system. One is an area‐color LCD in which a single photonic‐crystal layer is used for each color pixel and the other is a full‐color TFT‐LCD in which three stacked photonic‐crystal layers are used as light‐conversion layers. The area‐color LCD was excited by using 365‐nm UV light, and the full‐color TFT‐LCD was excited by using 470‐nm blue LED light. Because of the photonic crystal's unique features that allow it to work as light‐emitting and light‐reflecting layers simultaneously, both LCDs demonstrate clear readable images even under strong ambient light, such as direct‐sunlight conditions, under which conventional displays including LCDs and OLED displays cannot demonstrate clear images. In particular, an area‐color LCD, which eliminated color filters, gives clear images under bright ambient light conditions even without backlight illumination. This fact suggests that a backlight system using novel photonic‐crystal layers will be suitable for energy‐efficient LCDs (e²‐LCDs), especially for displays designed for outdoor usage.     

Novel liquid‐crystal materials for AMLCDs

Abstract— We have developed novel liquid‐crystal (LC) materials with a difluoromethyleneoxy CF₂O) moiety as the linkage group in order to satisfy the various requirements of active‐matrix liquid‐crystal displays (AMLCDs). We measured the physical properties of the CF₂O LC materials. The novel CF₂O LC materials have excellent physical properties, that is, high dielectric anisotropy, low viscosity, a broad nematic range, high reliability, etc. It was revealed that several advantages can be obtained by the introduction of CF₂O moiety into the LC molecules. The mixture containing CF₂O LC materials for various applications indicated a decrease in rotational viscosity of about 15–25% compared with that for conventional LC mixtures. Also, these novel mixtures with CF₂O LCM demonstrated a higher voltage‐holding ratio (VHR). It was confirmed that mixtures containing CF₂O LC materials have suitable characteristics for various applications, for example, low‐driving‐voltage note‐book PCs, quick response monitors, and transflective applications.     

Photoconductor‐based (direct) large‐area x‐ray imagers

Abstract— A brief overview of the present status of active‐matrix flat‐panel direct x‐ray imagers (D‐AMFPI) is given. The spatial resolutions of direct and indirect imagers are compared, and it is pointed out that the lack of light scattering greatly improves resolution. Furthermore, the resolution does not degrade as layers of the x‐ray detector materials become thicker for better x‐ray absorption at higher x‐ray energies, opposite to that of indirect imagers. Different direct x‐ray conversion materials are compared, how the physical properties influence the x‐ray detection efficiency, and imager stability are discussed. Ghosting and image‐lag properties are also weighted. A few x‐ray‐sensitive photoconductor materials produce very‐high x‐ray conversion efficiency, which could be advantageous for low‐dose fluoroscopy to overcome the noise of the readout electronics. Last, but not least, the manufacturing advantage of the direct imagers is emphasized. The direct imagers do not need p‐i‐n photodiodes, so the a‐Si TFT matrices for these arrays can be manufactured at any LCD manufacturing sites and not only at a few, very specialized companies where the p‐layers for the photodiodes can be deposited.     

Printing of polymer thin‐film transistors for active‐matrix‐display applications

Abstract— We present a process for active‐matrix flat‐panel‐display manufacture based on solution processing and printing of polymer thin‐film transistors. In this process, transistors are fabricated using soluble semiconducting, conducting, and dielectric polymer materials. Accurate definition of the transistor channel and other circuit components are achieved by direct ink‐jet printing combined with surface‐energy patterning. We have used this process to create 4800‐pixel 50‐dpi active‐matrix backplanes. These backplanes were combined with polymer‐dispersed liquid crystal to create the first ink‐jet‐printed active‐matrix displays. Our process is, in principle, environmentally friendly, low temperature, compatible with flexible substrates, cost effective, and advantageous for short‐run length and large display sizes. As well as polymer‐dispersed liquid crystal, this technology is applicable to conventional liquid‐crystal and electrophoretic display effects.     

Local dimming light‐guiding plate type backlight system using alignment‐controlled polymer‐dispersed liquid crystals

A new technology which enables a local brightness control according to the displayed images has been expected in the thin and lightweight backlight systems to improve a contrast ratio and power consumption of the liquid crystal displays (LCDs). In this paper, we have proposed a novel local‐dimming backlight system using alignment‐controlled polymer‐dispersed liquid crystals as a light‐guiding plate and investigated the forming conditions of polymer‐dispersed liquid crystals to achieve both a high‐luminance ratio and a fast response speed. As a result, we found that a luminance ratio and response speed of the backlight system can be improved by using bifunctional LC monomer materials and forming fine and rigid polymer network in the LCs, and achieved high luminance ratio of 16:1 and fast response time less than 0.5 ms. In addition, we fabricated the twisted nematic‐mode LCD using the local dimming light‐guiding plate‐type backlight based on this design, and successfully realized eight times higher contrast ratio than that of the traditional twisted nematic‐mode LCD.     

Designing multilayered wire‐grid polarizers using a monochromatic recursive convolution finite‐difference time‐domain algorithm

Multilayered wire‐grid polarizers (WGP) find application as low‐reflection polarizers in projection‐type liquid crystal display devices. A multilayered WGP is formed by adding thin layers on top of the metal ridges of an ordinary WGP. The ordinary WGP consists of a periodic array of parallel metal ridges, where the period of the array and the width of any individual metal ridge are typically less than the wavelength of the incident light. Such WGPs are often used as efficient polarizers. However, in certain applications, it is important to reduce the reflection from the WGP while preserving the polarization efficiency. One of the ways to achieve this goal is to add thin layers on top of the metal ridges of the ordinary WGP. The reduction in reflection from the multilayered WGP depends on the number and material of these additional layers. In this paper, we describe a design method for multilayered WGPs based on an effective medium theory, thin‐film computation method and a monochromatic recursive convolution finite‐difference time‐domain algorithm. The goal of design process is to identify suitable materials and thicknesses for the additional thin layers needed to lower the reflection appreciably. The design method is explained with the help of bilayered WGPs.     

Real‐time Animation of Sand‐Water Interaction

Recent advances in physically‐based simulations have made it possible to generate realistic animations. However, in the case of solid‐fluid coupling, wetting effects have rarely been noticed despite their visual importance especially in interactions between fluids and granular materials. This paper presents a simple particle‐based method to model the physical mechanism of wetness propagating through granular materials; Fluid particles are absorbed in the spaces between the granular particles and these wetted granular particles then stick together due to liquid bridges that are caused by surface tension and which will subsequently disappear when over‐wetting occurs. Our method can handle these phenomena by introducing a wetness value for each granular particle and by integrating those aspects of behavior that are dependent on wetness into the simulation framework. Using this method, a GPU‐based simulator can achieve highly dynamic animations that include wetting effects in real time.     

Polymer‐stabilized pretilt angle on the surface of nanoparticle‐induced vertical‐alignment surface for multi‐domain vertical‐alignment liquid‐crystal display

Abstract— By introducing polyhedral oligomeric silsesquioxane (POSS) nanoparticles along with a controlled amount of UV‐curable reactive mesogen (RM) into a liquid‐crystalline (LC) medium, a multi‐domain vertical‐alignment LC device was successfully demonstrated. The device, possessing a vertically aligned LC director in four different azimuthal directions, exhibited a fast response time and wide‐viewing‐angle characteristics, in the absence of conventional polymer‐type vertical‐alignment layers. Electro‐optic characteristics of the fabricated device, before and after UV curing of the cell, were studied. The surface morphology of the substrate surfaces were analyzed by using field‐emission scanning electron microscopy (FESEM). The experimental results show that the technology will possibly be applicable to cost‐effective vertical‐alignment liquid‐crystal devices and is suitable for green‐technology liquid‐crystal displays.     

Single‐substrate encapsulated cholesteric LCDs: Coatable,drapable, and foldable

Abstract— The first ever, reflective cholesteric liquid‐crystal displays (ChLCDs) on single textile substrates made with simple coating processes have been developed. A novel approach for fabrication of ultra‐thin encapsulated ChLCDs with transparent conducting polymers as bottom and top electrodes will be reported. These displays are fabricated from the bottom‐up by sequential coating of various functional layers on fabric materials. Encapsulation of the cholesteric liquid‐crystal droplets in a polymer matrix and the mechanical flexibility of the conducting polymers allow for the creation of durable and highly conformable textile displays. The development and status of this next‐generation display technology for both monochrome and multicolor cholesteric displays will be discussed.     

Tailored liquid‐crystal switching on ferroelectric polymer films

Abstract— The development of voltage‐controlled visible‐wavelength progression in displays and optical data storage devices using ferroelectric polymers and liquid crystals is described. Ferroelectric polymers are materials that have a ready distribution of dipoles which can be oriented by manipulating material composition and external fields. Utilizing the charge polarization distribution, their performance as an alignment layer for inducing liquid‐crystal alignment is presented. The switching response of the devices was tailored by changing the material composition through copolymers and nanoclay doping.     

LCD applications of thin‐crystal‐film polarizers

Abstract— We have developed a new technique for the production of thin crystal film (TCF) by deposition, molecular alignment, and the drying of water‐based lyotropic‐liquid‐crystal (LLC) materials. TCF exhibits high optical anisotropy and birefringence. This paper presents liquid‐crystal‐display (LCD) applications and opportunities for TCF plastic sheet polarizers, retarders, and color‐correction films as well as LCD designs with TCF internal polarizers.     

High‐transmittance in‐plane switching liquid crystal display device driven by three‐level voltages

We propose a single‐layered electrode structure using three voltage levels instead of two to achieve high transmittance in an in‐plane switching liquid crystal display device. The proposed structure consists of two pixel electrodes and one common electrode. By using three voltage levels, we can generate an in‐plane electric field higher than that in a conventional in‐plane switching device. We confirmed that by using the proposed structure, the transmittance of a liquid crystal device can be increased from 29% to 35% at a slightly lower operating voltage without using the double‐layered electrode structure required for the fringe‐field switching mode. The transmittance of the proposed device is higher than that of the fringe‐field switching device.     

Wearable near‐field communication bracelet based on highly conductive graphene‐assembled films

Benefiting from the high conductivity and superb flexibility, graphene‐based materials are promising to replace metal for near‐field communication (NFC) applications. Herein, we report a flexible NFC tag antenna based on high‐conductivity graphene‐assembled films (HCGAFs) and investigate how the performance of the antenna is affected by antenna design and human body effect. The fabricated prototype via a one‐step laser‐direct mold engraving method shows a 10 dB bandwidth of 2.5 MHz centering at 13.70 MHz with a quality factor (Q) of 9.19. The maximum read range of the HCGAF NFC tag is measured to be around 7.5 cm, comparable to the commercially available metal NFC tags. Moreover, the flexible nature of HCGAFs guarantees excellent mechanical stability and deformation insensitivity, especially when compared to commercial metal‐based counterparts. We further demonstrate the practical applications of the HCGAF tag as key card and electronic business card in the vicinity of human body.     

Active matrix QD‐LED with top emission structure by UV lithography for RGB patterning

We have developed full colour top emitting quantum dot light‐emitting diode (QD‐LED) display driven by a 176‐ppi active matrix of metal oxide thin‐film transistors. Red, green and blue (RGB) QD‐LED subpixel emission layers are patterned by our original UV photolithography process and materials. We also demonstrate the potential to achieve high resolution such as 528 ppi using this process.