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.     

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.     

Review Paper: Emerging vertical‐alignment liquid‐crystal technology associated with surface modification using UV‐curable monomer

Abstract— The recent development of polymer‐induced pretilt angle in multi‐domain vertical‐alignment liquid‐crystal (LC) structures is reviewed. To create a small but well‐defined pretilt angle, ～0.1 wt.% of a photo‐curable monomer was mixed in an LC host and a bias voltage was applied to reorient the LC directors within each domain. The monomers are polymerized near the substrate surfaces by UV exposure. The formed polymer layers change the surface pretilt angle of the LC from 90° to about 89° with a defined azimuthal orientation. Consequently, within each domain the LC reorientation direction responding to the external field is well‐defined which leads to faster rise time and higher transmittance. This new technology overcomes the long standing problems of conventional MVA devices and is therefore expected to play a dominant role in the future.     

Medical displays by using plasma‐beam‐alignment technology

Abstract— Novel anode layer plasma within minimum chamber space was developed for non‐contact alignment process. The plasma‐treated polyimide (PI) surface showed no particle contamination and no micro‐scratches. Surface morphology was investigated by using scanning electron microscope (SEM), an atomic force microscope (AFM), and X‐ray photoemission spectroscopy. The different oxygen‐to‐carbon ratio ([O]/[C] ratio) for XPS spectra indicated a composition change after plasma treatment. Surface pretilt angles were varied from 0 to 2.1° under different plasma exposure times. Finally, a prototype 20.8‐in. QXGA IPS‐mode gray‐scale medical liquid‐crystal display was successfully demonstrated with high contrast ratio, excellent uniformity, and wide viewing angle using this new plasma‐beam‐alignment technique.     

Surface modified solution‐derived nickel oxide film via ion‐beam irradiation as a liquid crystal alignment layer

In this article, we demonstrate the liquid crystal (LC) alignment characteristics of solution‐derived nickel oxide (NiO) film modified with ion‐beam (IB) irradiation. Cross‐polarized optical microscopy and pretilt angle measurements verified that uniform LC alignment was achieved using the NiO film as an alignment layer regardless of IB incidence angle. Contact angle measurements revealed that all of the NiO films had a deionized water contact angle below 90°, which indicates that they had hydrophilic surfaces that had an effect on the homogeneous LC alignment. Atomic force microscopy was conducted to determine the physical surface modification due to the IB irradiation, which showed that it reduced the size of the surface grains with agglomerations depending on the surface tilt from the IB incidence angle. Furthermore, microgroove structures strongly related to uniform LC alignment were observed after IB irradiation. Chemical surface modification was investigated via an X‐ray photoelectron spectroscopy analysis which revealed that IB irradiation modified the chemical bonds in the NiO film, and this affected the LC alignment state. Thus, these results indicate that using NiO film exposed to IB irradiation as an alignment layer is a suitable method for LC applications.     

Alignment materials with controllable anchoring energy

New polymers with photo‐controllable anchoring energy and tunable pretilt angle within 90°–0° range for nematic liquid crystals alignment were developed. The functional properties of polymers are provided by the effect of photoinduced planar alignment and the presence of side hydrocarbon chains in macromolecules that create a homeotropic alignment effect. Applying photosensitive alignment layers based on polymers with side benzaldehyde and hydrocarbon groups, fabrication of optical devices with refractive index gradient, uniform cell gap, and low operation voltage is possible. The developed materials are suitable for simple fabrication of tunable liquid crystal lenses.     

O films with adjustable tilt of optical axis for LCD compensation

Abstract— A method of preparation of positive O films with the tilt angle of the optic axis continuously controlled in the range 0–90° is proposed. It is based on the use of reactive mesogens and alignment materials that provide a wide range of pretilt angles. The method developed allows for further improvement in the viewing‐angle characteristics of LCDs with O compensation films.     

Azo‐dye aligning layers for liquid‐crystal cells

Abstract— The photo‐induced alignment of liquid crystal onto a photochemical stable azo‐dye film was studied for liquid‐crystal display (LCD) applications. The photo‐aligning of azo dye takes place due to the pure reorientation of the molecular absorption oscillators perpendicular to the UV‐light polarization. The order parameters S = ?0.4 (80% of the maximum absolute value S_m = ?0.5) was measured at a wavelength of 372 nm from the polarized absorption spectra. The temperature‐stable pretilt angle of 5.3° was obtained by a two‐step exposure of azo‐dye film using normally incident polarized light followed by oblique non‐polarized light. The azimuthal anchoring energy of the photo‐aligned substrate was A_? > 10^?4 J/m², which is of the same magnitude as the anchoring of the rubbed PI layer. The VHR value of the photo‐aligned LC cell was also found to be very high (98–99%) at room temperature and more than 95% at T=80°C. The thermal stability of the photo‐aligned azo‐dye layers is sufficiently high, while UV stability has to be improved, _e.g., by polymerization. The new LCD aligning technology based on photochemical stable azo‐dye layers is envisaged.     

Photoalignment of LCoS LCDs

Because the pixel area of liquid‐crystal‐on‐silicon (LCoS) microdisplays is about 100 times smaller than that of direct‐view liquid‐crystal displays (LCDs), the limitations of the conventional alignment by brushing are obvious: Scratches and particle contamination caused by brushing become visible due to the strong optical magnification required for LCoS LCDs both in front or rear projection. As an alternative, photoalignment with the linear photo‐polymerization (LPP) technology avoids the generation of defects, thus increasing production yields considerably. For application in LCoS LCDs, alignment layer materials must match the high‐voltage holding ratio (VHR) specifications of TFT‐LCDs. The VHR performance of the newly developed second‐generation LPP materials is shown to be similar to standard TFT polyimides used for conventional brushed alignment. We report investigations of pretilt‐angle generation by photoalignment on reflective CMOS substrates. UV light reflected from the CMOS surface during LPP exposure affects the resulting pretilt angle. Compared to pretilt angles on transmissive substrates, the reflected UV light can induce lower, higher, or identical pretilt angles, depending on LPP material properties. In any case, the pretilt angles are well defined, which results in perfect LCD alignment.     

Photoalignment and photo‐patterning of planar and homeotropic liquid‐crystal‐display configurations

Abstract— Optical alignment and micro‐patterning of the alignment of liquid‐crystal displays (LCDs) by linear photopolymerization (LPP) technology renders high‐quality multi‐domain twisted‐nematic (TN) and supertwisted‐nematic (STN) displays with broad fields of view over wide temperature ranges feasible. The prerequisites are the generation of photo‐induced high‐resolution azimuthal alignment patterns with defined bias‐tilt angles 0° ≤ θ ≤ 90°. For the first time, LPP‐aligned single‐ and dual‐domain vertically aligned nematic LCDs (VAN‐LCDs) are presented. Dual‐domain VAN‐LCDs are shown to exhibit broad fields of view which are further broadened by combining the displays with LPP‐aligned optical compensators made of liquid‐crystal polymers.     

Diamond‐like‐carbon LC‐alignment layers for application in LCOS microdisplays

Abstract— To improve the lifetime and yield of LCOS microdisplays, non‐contact LC alignment techniques using inorganic materials are under investigation. This report focuses on oblique ion‐beam treatment of diamond‐like carbon (DLC) layers, and in particular on the influence of the ion dose on the LC alignment on DLC, keeping the ion‐beam angle (40°) and ion‐beam energy (170 eV) the same. LC alignment on ion‐milled DLC layers is uniform if the ion dose is between 3.8 × 10^?4 C/cm² and 5.5 × 10^?3 C/cm². Above and below this ion dose range, non‐uniform alignment is observed. NEXAFS experiments show that this is caused by lack of molecular anisotropy on the surface of the ion‐milled DLC layers. By varying the ion dose between 3.8 × 10^?4 C/cm² and 5.5 × 10^?3 C/cm², LC molecules have an average pre‐tilt between 3° and 5°, which is within the desired range for application in LCOS microdisplays. The lifetime of the LCOS microdisplays with ion‐milled DLC for projection‐TV application is, however, shorter than the lifetime of microdisplays with PI layers. Ion milling probably creates a reactive surface that is unstable under the high light fluxes used in projection TVs. A solution for this problem could be chemical passivation of the ion‐milled alignment layers. Initial experiments with passivation of ion‐milled PI resulted in an increase in lifetime, but the lifetime after passivation was still lower than the lifetime of rubbed PI layers (factor 0.7). Nevertheless, ion‐milling of DLC or PI can be a good alternative LC alignment technique in other LCD applications. LC‐alignment layers based on inorganic layers such as obliquely deposited SiO₂ films would be a better option for application in LCOS microdisplays due to their higher light stability.     

Ferroelectric liquid crystal aligned on SiO2 thin films using ion‐beam deposition

Abstract— Results for a ferroelectric‐liquid‐crystal (FLC) display cell, aligned on inorganic SiO₂ thin‐film surfaces by using oblique ion‐beam sputtering deposition on the substrates, is presented. A large deposition angle from 60° to 80° can be employed for the thin alignment layer, with thicknesses varying from 5 to 40 nm. Two types of uniform alignment, chevron (before electrical treatment) and quazi‐bookshelf (after electrical treatment), were studied. High‐quality alignment on large‐sized substrates was also easily be achieved because of the linear design of the ion‐beam sputtering source, which was previously a significant challenge for FLC on SiO_x layers.     

A single‐cell‐gap transflective liquid‐crystal display using different pretilt angles in transmissive and reflective regions

Abstract— A single‐cel l‐gap transflective liquid‐crystal display with two types of liquid‐crystal alignment based on an in‐plane‐switching structure is proposed. The transmissive region is almost homeotropically aligned with the rubbed surfaces at parallel directions while the reflective region has a homeotropic liquid‐crystal alignment. For every driving voltage for a positive‐dielectric‐anisotropy nematic liquid crystal, the effective cell‐retardation value in the transmissive region becomes larger than that in the reflective region because of optical compensation film which is generated by low‐pretilt‐angle liquid crystal in the transmissive region. Under the optimization of the liquid‐crystal cell and alignment used in the transmissive and reflective areas, the transmissive and reflective parts have similar gamma curves. An identical response time in both the transmissive and reflective regions and a desirable viewing angle for personal portable displays can also be obtained.     

Bistable nano‐structured photoalignment surface by nanoimprint lithography

A novel nano‐structured photoalignment surface is proposed and demonstrated. Such alignment surface has bistable azimuthal alignment directions for the liquid crystal molecules. The new alignment surface has a structure of stacking a photo‐polymerizable photoalignment polymer on top of a nano‐sized groove surface. The photoalignment polymer and groove surface have different azimuthal alignment directions but the same azimuthal anchoring energies. The fabrication of the nano‐sized groove is based on nano‐imprint lithography. Hence, the size and depth are controllable, where no random process is involved. The alignment surface is robust, stable, reliable, reproducible and suitable for mass manufacturing. Such alignment surface can be applied to fabricate a π/2 bistable twisted nematic (π/2‐BTN) display which has better optical performances than the traditional π‐BTN display.     

Liquid‐crystal photoaligning by azo dyes (Invited Paper)

Abstract— Liquid‐crystal (LC) photoalignment using azo dyes is described. It will be shown that this photoaligning method can provide a highly uniform alignment with a controllable pretilt angle and strong anchoring energy of the LC cell, as well as a high thermal and UV stability. The application of LC photoalignment to the fabrication of various types of liquid‐crystal displays, such as VAN‐LCDs, FLCDs, TN‐LCDs, and microdisplays, on glass and plastic substrates is also discussed. Azo‐dye photoaligned super‐thin polarizers and phase retarders are considered as new optical elements in LCD production, in particular for transflective displays.     

Arbitrary pretilt and azimuth angles generation by stacked photoalignment layers

We report a method of fabricating a nano‐sized stack alignment layer. The stacked alignment layer consists of nano‐domains of vertical and planar alignment materials. Experiments reveal that photoalignment thin film can undergo dewetting and form discrete nano‐sized domains. Such self‐organized structure creates a discontinuous layer stacked on top of a continuous layer and hence produces an inhomogeneous alignment surface. With two or more different principle alignment directions, this new alignment layer is capable of producing multiple pretilt and azimuth angle domains on a single substrate.     

Control of the pretilt angle for liquid crystal alignment using novel electrospray deposition methods with two capillaries

The electrospray deposition (ESD) method is an established film‐forming technique. To control the pretilt angle on films, we developed two novel ESD methods for spraying two kinds of solutions containing alignment materials. One method is a simultaneous spraying method, while the other is a time‐divided spraying method. When we used the simultaneous‐spraying ESD method, we observed numerous fine liquid crystal (LC) domains with a diameter of approximately 10 µm in the LC cell. These LC domains were substantially smaller than those of LCs fabricated using the conventional ESD method. However, the pretilt angle could not be controlled over a wide range because multiple parameters could not be simultaneously controlled to achieve a stable spray. Using the time‐division ESD method, we controlled the pretilt angle over a wide range from 5° to 40°, with tiny domains. As a demonstration, we fabricated a 270° super‐twisted nematic mode cell using this method.     

Liquid‐crystal alignment on rubbed polyimide films with various side chains

Abstract— A series of polyimides containing various side chains was synthesized in order to investigate the effect of side chains on the alignment of liquid crystals on the rubbed surface. Here, the side chains include short flexible alkyl spacers and isomeric biphenyl mesogens. The pretilting of liquid‐crystal (LC) molecules was found to be very sensitive to the isomeric structure of biphenyl mesogen end groups as well as the conformation and length of flexible spacers, in addition to the rubbing process. The pretilt angle of LC molecules in the LC cell was achieved in a wide angle range of 8–27°, depending upon the rubbing density as well as the side chains. The high performance in the pretilt and alignment of LCs might be attributed mainly to a strong interaction between the biphenyl mesogen end group in the side chain and the LC molecule in addition to the microgrooves generated in the rubbing direction.     

Nanostructured alignment layer for liquid crystals

A new type of nanostructured alignment surface is proposed and demonstrated. As in previous nanostructured alignment surfaces, a large pretilt angle of near 45° can be produced reliably. Moreover, this new structure is fabricated with a mask, and there is no randomness involved. The uniformity and anchoring properties of this new surface have been measured to be of excellent quality.