Alignment Control Of Liquid Crystals Research Articles

Pretilt angle modulation of liquid crystals based on single crystal rubrene with incorporation of homeotropic polyimides

The alignment control of liquid crystals (LCs) is critical for various practical applications. The pretilt angle modulation of LCs typically requires a mechanical rubbing on substrates to orient the LCs. This study presents a contact-free approach to achieve pretilt angle modulation of LCs. Initially, a single crystal rubrene (SCR) film is deposited on the substrate. Subsequently, a mixture of nematic LCs and homeotropic polyimides (HPIs) is introduced between two SCR substrates via capillary action. During capillary filling, the synergy of capillary flow and the interaction between LCs and SCR ensures specific LC orientation. Subsequently, HPI dopants migrate toward and organize on the SCR through vertical phase separation, enhancing surface hydrophobicity and thereby increasing the pretilt angle of LCs. The pretilt angle of LCs can be continuously adjusted over a wide range from 2° to 90° by varying the HPI concentration. The contact-free process preserves against electrostatic charges, dust contamination, and surface damage typical of rubbed LC cells. This believed to be novel technique shows promise for developing no-bias-bend and bistable bend-splay LC displays.

Effects of CuInS2 nanoparticles on the alignment control of liquid crystals

Effects of CuInS2 nanoparticles on the alignment control of liquid crystals

Effects of ZnO nanostructures on the alignment control of liquid crystals

Effects of ZnO nanostructures on the alignment control of liquid crystals

6‐3: Distinguished Paper: Alignment Control of Liquid Crystals in a 1.0‐µm‐pitch Spatial Light Modulator by Lattice‐shaped Dielectric Wall Structure

We achieved uniform liquid crystal (LC) alignment in lattice‐shaped dielectric walls 1‐µm in pitch; this is a prerequisite when driving the individual pixels of spatial light modulators, facilitating the development of practical electronic holographic displays with a wide field of view.

Polyimide-Free Planar Alignment of Nematic Liquid Crystals: Sequential Interfacial Modifications through Dual-Wavelength in Situ Photoalignment.

High-quality alignment control of liquid crystals (LCs) for ultrahigh-definition large-sized display is a challenging task. A conventional rubbing method has obvious limitations for fabricating large-sized displays with a small pixel size and an uneven inner surface. To comply with the current trend, we propose a simple and reliable polyimide-less in situ photoalignment. It was achieved using a visible-light-sensitive azo-dye and a mesogenic acrylate, both doped to host LCs. Without using a pretreated alignment layer, mono- and multidomain uniaxial alignments of LC molecules were induced by linearly polarized visible light (LPVL) and subsequently stabilized by unpolarized UV-light irradiation. The stepwise process was monitored by adopting a fluorescent indicator. By loading the mixture into a confined cell, azo-dyes were spontaneously adsorbed at inner surfaces of the cell, whereas reactive mesogens (RMs) were homogeneously dissolved in an LC host. The molecular orientational anisotropy of dyes at the surface, induced by LPVL, aligned the LC director perpendicular to the polarization direction. Upon the second step, UV-irradiation, the RMs in an LC host were photopolymerized into thin interfacial layers, stabilizing the aligned LC director. The overlaid cross-linked RM layers secured a thermal and a radiative stability of LC alignment. The RM layers completely screened the effect of azo-dyes, which can be easily randomized by heat and irradiation. The interfacial RM layer functioned as a permanently stable alignment layer. It provided sufficient azimuthal anchoring strength together with heat and light stabilities, which are essential for practical applications. Such sequential interfacial modifications through dual-wavelength processes can completely avoid interference between forming alignment and stabilization layers, inevitable if the same wavelength light is used. The proposed method provides a simple fabrication process and reliable alignment characteristics by employing effective in situ photoalignment and without using a traditional alignment layer. Therefore, it meets a current trend in the display market toward ultrahigh-resolution and large-area displays.

Alignment control of liquid crystals in a 1.0‐μm‐pitch spatial light modulator by lattice‐shaped dielectric wall structure

AbstractWe achieved uniform liquid crystal (LC) alignment in lattice‐shaped dielectric walls 1 μm in pitch; this is a prerequisite when driving the individual pixels of spatial light modulators, facilitating the development of practical electronic holographic displays with a wide field of view. In lattice‐shaped dielectric walls, LC alignment becomes unstable, particularly on the bottom and the walls; the LC directors tend to align parallel to the walls. To overcome this problem, we created lattice‐shaped walls featuring partition plates that allow uniform LC alignment. When the plates confine LCs to small regions exhibiting spatial anisotropy, the LC elastic effect and wall anchoring forces align the LC directors parallel to the long anisotropic axis. We found that pixels 0.5 μm × 1.0 μm in pitch formed if the partition plates were sufficiently thick to allow shielding of electric field leakage.

Control of liquid crystal alignment using surface-localized low-density polymer networks and its applications to electro-optical devices

We developed an alignment method for control of the polar pretilt angle at the boundaries of a liquid crystal (LC) layer in a continuous range 2–90° by introducing a low-density polymer network. The network was formed as a result of the electric field-induced segregation of a photo-reactive monomer added in concentrations of 0.5–1% to the LC host and subsequent in-situ UV-light-induced polymerization. LC director was “frozen” within a less than a micron-thick layer in the vicinity of the cell substrates at high average tilt angles anchoring the LC in the bulk similarly to a high-pretilt alignment layer. The resultant pretilt angle was determined by the voltage applied before and during polymerization and the duration of voltage application before UV-light exposure. The desired pretilt angle could be set over a small area of a sample which allowed for the fabrication of LC devices with spatially variable optical retardation. Using this method, we fabricated high-pretilt splay and bend LC cells, optically recorded converging lenses and bi-prisms with electrically controlled optical characteristics, and phase diffraction gratings with resolution better than 50lines/mm.

Surface Dipole Control of Liquid Crystal Alignment

Detailed understanding and control of the intermolecular forces that govern molecular assembly are necessary to engineer structure and function at the nanoscale. Liquid crystal (LC) assembly is exceptionally sensitive to surface properties, capable of transducing nanoscale intermolecular interactions into a macroscopic optical readout. Self-assembled monolayers (SAMs) modify surface interactions and are known to influence LC alignment. Here, we exploit the different dipole magnitudes and orientations of carboranethiol and -dithiol positional isomers to deconvolve the influence of SAM-LC dipolar coupling from variations in molecular geometry, tilt, and order. Director orientations and anchoring energies are measured for LC cells employing various carboranethiol and -dithiol isomer alignment layers. The normal component of the molecular dipole in the SAM, toward or away from the underlying substrate, was found to determine the in-plane LC director orientation relative to the anisotropy axis of the surface. By using LC alignment as a probe of interaction strength, we elucidate the role of dipolar coupling of molecular monolayers to their environment in determining molecular orientations. We apply this understanding to advance the engineering of molecular interactions at the nanoscale.

Anchoring energy enhancement and pretilt angle control of liquid crystal alignment on polymerized surfaces

We demonstrate enhanced surface anchoring energy and control of pretilt angle in a nematic liquid crystal cell with vertical alignment and polymerized surfaces (PS-VA). The polymerized surfaces are formed by ultraviolet (UV) irradiation-induced phase separation of a minute amount of a reactive monomer in the vertical-aligned nematic liquid crystal. By introducing a bias voltage during UV curing, surface-localized polymer protrusions with a dimension of 100nm and a field-induced pretilt angle are observed. Experimental evidences and theoretical analyses validate that PS-VA has increased surface anchoring strength by two folds and pretilt angle has been changed from 89° to 86° compared to those of a VA cell. The enabling PS-VA cell technique with excel electro-optical properties such as very good dark state, high optical contrast, and fast rise and decay times may lead to development of a wide range of applications.

P‐95: Anchoring Energy Enhancement and Pre‐tilt Angle Control of Liquid Crystal Alignment on Polymerized Surfaces

We investigated the effect of the polymerized surfaces on the anchoring energy and pre‐tilt angle of liquid crystal alignment. Ultraviolet light‐illuminated cell initiates polymerization and produces sub‐micron sized polymer protrusions. Experimental evidences reveal the relation between the curing conditions and the resulted pre‐tilt angle and anchoring energy of cells.

Light‐Driven Reversible Alignment Switching of Liquid Crystals Enabled by Azo Thiol Grafted Gold Nanoparticles

Stimuli-directed alignment control of liquid crystals (LCs) with desired molecular orientation is currently in the limelight for the development of smart functional materials and devices. Here, photoresponsive azo thiol (AzoSH) was grafted onto gold nanoparticles (GNPs). The resulting hybrid GNPs were able to homogeneously mix with a commercially available nematic LC host, as evidenced by Cryo-TEM. Interestingly, the LC nanocomposites were found to undergo reversible alignment transition upon light irradiation as a consequence of the trans-cis photoisomerization of the azo groups on the GNP surface. LC molecules in either planar or bare glass cells were able to change their alignment to vertical upon UV irradiation, while the vertically aligned LC molecules returned to the planar or random orientation under visible irradiation. Neither the azo thiol molecules nor the unfunctionalized GNPs alone promoted the alignment of the LC molecules in the system upon light irradiation. The photoinduced vertical alignment without applied electric or magnetic field was very stable over time and with respect to temperature. Furthermore, an optically switchable device based on the photostimulated reversible alignment control of LCs was demonstrated.

Spatial and orientational control of liquid crystal alignment using a surface localized polymer layer

We present an alignment method for the surface contacting liquid crystal (LC) director. This method allows complete control of the polar pretilt angle as a function of position in a liquid crystal device, and has the potential of controlling the azimuthal orientation of LC. Important considerations of this method are to form a thin layer of reactive monomers at the LC cell interior surface, and to control the deleterious effects of flow due to polymerization induced concentration gradients. To achieve these, the voltage and frequency of the applied electric field and the UV intensity during the polymerization process are significant.

Optical near-field excitation using liquid crystals on nanostructured photoreactive molecular thin films

Optical near-field excitations were investigated on the basis of molecular alignment control of liquid crystals (LCs) on an optically rewritable nanostructure of photoreactive molecular thin films. Twisted nematic (TN) cells of LC molecules were constructed utilizing ITO substrates with 260 nm gratings of an azobenzene molecular thin film, fabricated using standing evanescent waves. The polarization changes of light transmitted through the TN cells, which were due to the alignment changes of LC molecules locally rubbed by the azobenzene nanogratings, were observed. Furthermore, we demonstrated local plasmon excitation of Au nanowires deposited on the azobenzene nanogratings using oblique vacuum evaporation, a phenomenon that produced strong anti-optical absorption spectra. The modulation of the local plasmon resonance in metallic nanowires decorated with LC molecules was confirmed.

フッ素化DLC（F-DLC）膜を用いた液晶配向制御技術

フッ素化DLC（F-DLC）膜を用いた液晶配向制御技術

Control of Liquid Crystal Alignment on SiOx Film Surfaces by Low-Energy Ion Beam

In this paper, liquid crystal alignment on SiOx film surfaces irradiated by a low-energy ion beam is systematically examined experimentally. As a consequence of low-energy ion beam exposure, LCs can be aligned vertically when a high rms of electric potential is present on SiOx film surfaces on the basis of electric force microscopy data, while they can be aligned homogeneously when a low rms of electric potential is present on those surfaces. It is also found by X-ray photoemission spectroscopy and the contact angle method that, for the vertical alignment, the absorption curve shifts to a lower binding energy and a higher contact angle, respectively. All experimental results consistently show that the Coulomb interaction between LC molecules and inorganic film surfaces has a dominant effect on LC alignment on films irradiated by a low-energy ion beam.

Alignment Control of Liquid Crystals Using Conductive Atomic Force Microscopy Nanolithography

Conductive atomic force microscopy (CAFM) nanolithography was used to modify a silicon surface. This approach generating the silicon oxide grating by CAFM gives a control of liquid crystal (LC) alignment in the micron or submicron region. It establishes a pixel with a smaller size to achieve high-resolution images. Compared with the conventional cloth rubbing and AFM scratching techniques, the CAFM nanolithography prevents scratching damage, dust contamination and residual static electricity problems. Furthermore, this inorganic alignment method can also avoid the damage caused by UV light exposure and high-temperature environment.

Control of liquid crystal alignment by deposition of silicon oxide thin film

We investigated liquid crystal alignment on a-SiO x thin films by means of X-ray diffraction and X-ray photoemission spectroscopy as we varied the deposition temperature and the target-to-substrate distance. We found that liquid crystal molecules can be aligned vertically on a-SiO x film when the stoichiometry parameter x of a-SiO x is smaller than 1.56, but they can be aligned homogeneously when x is larger than 1.56. We also found that whether liquid crystals can be aligned vertically or homogeneously on a-SiO x film can be predicted simply by measuring the change in optical transmittance by deposition of a-SiO x thin film layers.

Liquid-Crystal Alignment on Anisotropic Homeotropic-Planar Patterned Substrates

ABSTRACT The development of patterned surfaces for liquid crystal (LC) alignment has attracted much attention because it is applicable to bistable displays allowing the complete control of LC alignment. In this work we propose a method of preparing patterned substrates for LC alignment combining two surface treatments that lead to alternating homeotropic–planar alignment potentials. By superimposing two similar patterned substrates with crossed directions of treatment, a twisted nematic texture was observed in the regions of planar alignment in both surfaces, indicating the possibility of technological applications.

Control of Liquid Crystal Alignment on Crosslinkable Polymer Surfaces with Photo and Plasma Modification

Control of Liquid Crystal Alignment on Crosslinkable Polymer Surfaces with Photo and Plasma Modification

Alignment mechanism of liquid crystal in a stretched porous polymer film

This article discusses the mechanism of nematic liquid crystal alignment in stretched porous polymer films. The polymer films were formed by extreme stretching of an isotropic porous polyolefin, such that the draw ratio was 12:1. A 6-μm-thick porous film with a high porosity coefficient of 92% revealed fine string-shaped areas that exhibited optical anisotropy due to their possessing a high degree of molecular alignment. The porous film was filled with nematic liquid crystal and then the composite film was sandwiched between transparent electrodes coated onto glass substrates, without the use of conventional alignment layers. From polarizing microscopy observations it was found that the string-like polymer areas induce liquid crystal molecular alignment. The liquid crystal cells can exhibit an electrically controlled birefringence effect. This alignment technique enables us to realize three-dimensional control of liquid crystal alignment.