Alignment Of Nematic Liquid Crystals Research Articles

Liquid crystal device with 50 nm nanogroove structure fabricated by nanoimprint lithography

We have investigated the alignment of nematic liquid crystals (NLCs) using 50 nm ultrafine nanogrooves fabricated by electron-beam (EB) lithography and nanoimprint lithography. To improve the wettability of NLCs, silicon dioxide was sputtered onto the NLCs after nanogroove fabrication. The azimuthal and polar anchoring energies obtained were 9.4×10−5 J/m2 and 3.6×10−4 J/m2, respectively. From the electro-optical characteristic of a twisted nematic (TN) liquid crystal (LC) cell, the contrast ratio of the TN cell was found to be 44:1, which is a relatively low value because of the misalignment of the EB exposure. However, a steeper and common threshold voltage was obtained for the TN LC cell.

Liquid crystal alignment on a ZrO2 thin film as a function of ion beam incident angle

Nematic liquid crystal alignment capabilities and electro-optical performance characteristics have first been embodied in a ZrO2 layer using ion-beam irradiation. The study demonstrates that liquid crystal layers can be aligned homogeneously as a function of the incident angle of the ion beam device, which causes a uniform dense plasma. X-ray photoelectron spectroscopy indicated that full oxidation of the ZrO2 thin-film surfaces was produced by ion irradiation, shifting the Zr 3d spectra to lower binding energies. In addition, the electro-optical performance characteristics of twisted nematic cells on a ZrO2 thin-film layer after ion beam irradiation showed similar characteristics to those of a rubbed polyimide cell.

Gravitational field-induced orientational transition of aligned nematic liquid crystals

We demonstrate induced orientational transition of liquid crystal (LC) E7 between two optical fibre tips in a gravitational field as a result of minimising the free energy. The LC orientational transition is from a homeotropic state, with respect to fibre tip, to a tilted state as the spacing between two fibre tips is increased. The orientational distortion introduces birefringence in the optical fibre that changes the polarised direction of the output light. At short spacing, the undistorted homeotropic orientation of LC is preferred. However, at longer spacing, a distorted orientation is preferred. Once the LC director profile is known, a Finite Difference Time Domain method is used to calculate the optical properties, which agree well with the experimental results.

Nematic liquid crystal alignment on the ion beam-exposed ZnO film

This paper investigates the nematic liquid crystal (NLC) alignment on ion beam-exposed zinc oxide (ZnO) films. The ZnO films are deposited by a radio frequency magnetron sputtering. During the deposition of ZnO film, we supplied sufficient oxygen gas for high resistivity and transmittance. The deposited films show a high transmittance of over 90% and high resistivity of over 10 10 Ω cm. The ZnO films show a high deposition rate of 26.7 Å/min. Images obtained via scanning electron microscopy of the ZnO film surfaces, before and after the ion beam exposure, show that groove patterns are formed being to be parallel to the ion beam exposure direction. LC cells are fabricated with the ion beam-exposed ZnO films. The NLC molecules align parallel to the ion beam exposure direction. The electro-optic and response characteristics of fabricated cells show the possibility of application to liquid crystal displays.

Full Optical Control of Holographic Gratings Realized in Composite Materials Containing Photosensitive Liquid Crystals

We report on the characterization of an all optical switching effect that takes place when a UV light beam irradiates an azo-POLICRYPS diffraction grating. This high quality polymeric structure is utilized to confine and stabilize a well aligned nematic liquid crystal (NLC) film doped with a small percentage of azo-NLC molecules. The optical control of the grating diffraction efficiency is obtained by exploiting the photochemical phase transition between the nematic and the isotropic phases of the liquid crystal, based on the photoisomerization of the azobenzene guest molecules. The switch ON-OFF of the grating diffraction efficiency is triggered by a UV pump laser and is investigated by means of a low power red probe beam.

Microseconds–Nanoseconds All-Optical Switching of Visible-Near Infrared (0.5 µm–1.55 µm) Lasers with Dye-Doped Nematic Liquid Crystals

Laser induced disorder is an efficient and very fast mechanism to induce changes in the birefringence of nematic liquid crystals, which in turn enable many nonlinear optical processes. Using a 90-degrees twist alignment nematic liquid crystal doped with suitable dye to impart the required photonic absorption and order parameter modulation, we have demonstrated ultrafast all-optical shuttle operation for lasers spanning the visible to near-infrared spectral region (532 nm; 488 nm; 750 nm; 1060 nm; 1550 nm). With increasing intensity, the on-time of the switch decreases from microseconds to the nanoseconds regime, in such a manner that the transmitted light energy/intensity is clamped to below the eye- or sensor-safe levels.

Titanium dioxide surface modification via ion-beam bombardment for vertical alignment of nematic liquid crystal

We introduce the characteristics of the titanium dioxide (TiO(2)) inorganic film deposited by rf magnetron sputtering for liquid crystal display applications. The TiO(2) films demonstrated vertical alignment (VA) of the liquid crystals (LCs) obtained by using ion-beam (IB) bombardment. As observed by using x-ray photoelectron spectroscopy, the chemical structure of the TiO(2) was changed by IB bombardment, altering the Ti-O bonding of the Ti 2p spectra to lower intensity levels. Breaking Ti-O bonding by IB bombardment created pretilt angles between the TiO(2) film and LC molecules. The better voltage-transmittance characteristics of the VA LCDs based on TiO(2) film were measured and compared with the same characteristics of polyimide film.

A study of the transition of liquid-crystal alignment from homeotropic to planar on a polyimide layer

The alignment of nematic liquid crystals by rubbed polyimide surfaces has been well-studied and developed. A novel polyimide film which induced a homeotropic alignment of the nematic liquid crystal without rubbing or with weak rubbing strength was presented. However, there was a transition from homeotropic to planar alignment of the nematic liquid crystal after strong rubbing. In order to study the transition, the polyimide surface was investigated by atomic force microscopy, surface free energy measurement and angle-resolved analysis X-ray photo-electron spectroscopy before and after rubbing with a velvet fabric. It was found that both the change of surface polarity and surface morphology were not the reasons for the transition. The droop of the side chain on the polyimide surface after the rubbing treatment was detected by angle-resolved analysis X-ray photo-electron spectroscopy. Owing to the special structure of the novel polyimide, the X-ray photo-electron spectroscopy was successfully used for the first time to analyse the conformational change of the side chain of a polymer. In conclusion, the transition of nematic liquid crystal alignment from homeotropic to planar after rubbing was influenced by the side chain conformation of the polyimide.

Effective Rotational Viscosity of Vertical Alignment Nematic Liquid Crystal Cells

An effective rotational viscosity of vertical alignment (homeotropic) nematic liquid crystal (NLC) cells with negative dielectric anisotropy is derived from a theory of NLCs in which flow effects under the free-slip boundary condition are taken into account. The effective rotational viscosity is a function of the Leslie viscosity coefficients and is much smaller than the rotational viscosity at the initial stage of the director reorientation induced by external electric field to the NLC cells. This is the origin of fast response of vertical alignment of NLC cells.

Homeotropic alignment of nematic liquid crystals with negative dielectric anisotropy

We report here uniform and defect-free homeotropic alignment of nematic liquid crystal having negative dielectric anisotropy (Δe<0) on cleaned substrates, which can be changed to bright state (homogeneous alignment) by applying an electric field. However, non-uniformity in the bright state would not be desirable for display devices. Therefore, silane treated and gently rubbed substrates have been utilised to obtain uniform bright state. Several textural, dielectric, and optical transmission studies have been carried out to explore the applicability of studied material in the liquid crystal display applications. Furthermore, such materials could also be useful for making colourful displays without colour filters.

Calculation and Prediction: An Effective Security Device Consisted of Photo Alignment Nematic Liquid Crystal Films and Polarizers

The paper reports the methods for making a novel optical device consisted of photo alignment nematic liquid crystal films and polarizers. The color shift of the device depends on not only the viewing angle but also the viewing orientation. The retardation and intensity of the reflected light are theoretically calculated on the basis of Jones Matrix, and a spectral-RGB transition program is introduced to display colorful results directly. The theoretical results show excellent agreement with experimental data. It is an effective method for predicting interference color and optimizing key parameters of the device. The device can be used to protect valuable documents against fraud because of its special optical variable effects which is caused by interference of polarized light.

Programmable liquid crystal waveplate polarization gratings as elements for polarimetric and interference applications

We review the use of a parallel aligned nematic liquid crystal spatial light modulator as a very useful and flexible device for polarimetric and interferometric applications. The device acts as a programmable pixelated waveplate, and the encoding of a linear grating permits its use as a polarization beam splitter. When a grating with a reduced period is encoded, the diffracted beams are spatially separated and the device can be used for polarimetric analysis. On the contrary when a large period grating is displayed, the beams are not spatially separated, and they are useful to realize a common path interferometric system with polarization sensitivity. The flexibility offered by the programmability of the display allows non-conventional uses, including the analysis of light beams with structured spatial polarizations.

ITO layers modified in glow discharge plasma for Nematic Liquid Crystal alignment

ITO layers modified in glow discharge plasma for Nematic Liquid Crystal alignment

Synthesis, Characterization, and Liquid Crystal Alignment Properties of Photosensitive Polyimide

A photoreactive polyimide, bearing a cinnamate photoreactive moiety in its side group, was synthesized. The photochemical reactions of the PSPI in the films were investigated in detail by UV-Vis and IR spectroscopies; the cinnamate chromophores were confirmed to undergo photodimerization. Linearly polarized ultra-violet light irradiation induced anisotropic reorientations of both the polymer main chains and the side groups in the film. The PSPI thin films were found to have unidirectional orientation ability as a result of photo-exposure with linearly polarized ultraviolet light, with the photoaligned polymer chains in the PSPI films able to effectively induce the alignment of nematic liquid crystals.

Ion Beam Irradiation for Surface Modification of Alignment Layers in LCD

We investigated liquid crystal (LC) alignment as a function of ion beam (IB) incident angles and electro-optical (EO) characteristics of twisted nematic (TN)-liquid crystal displays (LCD) on polyimide (PI) surfaces. X-ray photoelectron spectroscopic analysis showed that the intensity of C=O chemical bonding decreased, while that of C–O chemical bonding increased for increasing IB incident angles. This indicates that the dipole-dipole interaction between C–O chemical bonding and the LC molecules has a chemical alignment effect. A good uniform alignment of nematic LCs on PI surfaces by the IB irradiation was observed, and good EO properties of IB-aligned TN-LCDs on PI surfaces were achieved.

Symmetries and alignment of biaxial nematic liquid crystals

The possible symmetries of the biaxial nematic phase are examined against the implications of the presently available experimental results. Contrary to the widespread notion that biaxial nematics have orthorhombic symmetry, our study shows that a monoclinic (C(2h)) symmetry is more likely to be the case for the recently observed phase biaxiality in thermotropic bent-core and calamitic-tetrapode nematic systems. The methodology for differentiating between the possible symmetries of the biaxial nematic phase by NMR and by IR spectroscopy measurements is presented in detail. The manifestations of the different symmetries on the alignment of the biaxial phase are identified and their implications on the measurement and quantification of biaxiality as well as on the potential use of biaxial nematic liquid crystals in electro-optic applications are discussed.

Multiple Alignment Modes for Nematic Liquid Crystals Doped with Alkylthiol-Capped Gold Nanoparticles

The ability of alkylthiol capped gold nanoparticles (Au NPs) to tune, alter, and reverse the alignment of nematic liquid crystals (LCs) has been investigated in detail. Adjusting the concentration of the suspended Au NPs in the nematic LC host, optimizing the sample preparation protocol, or providing different sample substrates (untreated glass slides, rubbed polyimide-coated LC test cell, or ITO-coated glass slides) results in several LC alignment scenarios (modes) including vertical alignment, planar alignment, and a thermally controlled alignment switch between these two alignment modes. The latter thermal switch between planar and homeotropic alignment was observed particularly for lower concentrations (i.e., around 1 to 2 wt %) of suspended NPs in the size regime of 1.5-2 nm and was found to be concentration-dependent and thermally reversible. Different scenarios are discussed that could explain these induced alignment modes. In one scenario, the NP-induced alignment is related to the temperature-dependent change of the order parameter, S, of the nematic phase (ordering in the bulk). In the second scenario, a change of the ordering of the nematic molecules around the NPs that reside at the interfaces is described. We also started to test spin coating as an alternative way of preparing nematic thin films with well-separated Au NPs on the substrate and found this to be a possible method for manufacturing of future NP-doped LC devices, as this method produced evenly distributed NPs on glass substrates. Together the presented findings continue to pave the way for LC display-related applications of Au NP-doped nematic LCs and provide insights for N-LC sensor applications.

Homeotropic Alignment of Nematic Liquid Crystals by a Photocross-Linkable Organic Monomer Containing Dual Photofunctional Groups

In this paper, we report a novel organic monomer containing dual photocross-linkable groups and success in realizing photoinduced homeotropic alignment of nematic liquid crystals (LCs) with it. It was first revealed that direct irradiation of the photoalignment thin film with nonpolarized ultraviolet (UV) light at 365.0 nm brought out homeotropic orientation of the photopolymer as a result of the photocross-linking of the dual photoreactive groups. When the thin film was obliquely irradiated with nonpolarized UV light, the pretilt angles of nematic LC were generated. Interestingly, we find that the hydrophobicity of the photopolymer increases with increasing irradiation time. In discussing the mechanism of the homeotropic alignment, it was found that the incorporation of the dual photofunctional group of the photoalignment molecules as well as the extreme hydrophobicity of the photopolymer play the essential roles. This monomer cross-linked film is expected as a promising homeotropic alignment film with rubbing-free processing for the fabrication of advanced vertical alignment LC displays.

Photoalignment of liquid crystals in a hydrogen‐bonding‐directed layer‐by‐layer ultrathin film

In this paper, a new type of hydrogen‐bonding‐directed layer‐by‐layer (LBL) ultrathin film with photosensitive cinnamoyl chromophores was prepared. It could induce uniform alignment of liquid crystals (LCs) after being irradiated by linearly polarised ultraviolet light (LPUVL). The hydrogen‐bonding‐directed formation of a photochromic multilayer film was achieved on the basis of an alternating assembly of poly(4‐vinylpyridine) (PVPy) and poly(4‐acryloyloxy)cinnamic acid (PCA). The assembly process was followed by UV–visible spectroscopy. The nature of the interaction between the two polymers was identified as a hydrogen bond through Fourier transform infrared spectroscopy. When the film was irradiated by LPUVL, the cinnamoyl units in the film underwent [2+2] cycloaddition along the electric vector direction of the LPUVL. The polarised UV–visible spectra proved that the film was anisotropic, and then the film could induce homogeneous alignment of nematic LCs. With the number increment of the bilayers, the alignment effect of the multilayer film was enhanced, and the contrast ratio became higher. The orientation direction of the LCs was always perpendicular to the electric vector of the irradiating LPUVL.

IPS mode investigation of liquid crystal alignment on organic hybrid overcoat layer via ion beam irradiation

The nematic liquid crystal alignment capability and electro‐optical (EO) performance characteristics of in‐plane switching (IPS) cells were first demonstrated on organic hybrid overcoat layers via ion beam irradiation. Usually the organic hybrid overcoat is used as the material for making a plate alignment layer in a colour filter process. To characterise the organic hybrid overcoat layer, we analysed the surface composition of the organic hybrid overcoat by X‐ray photoelectron spectroscopy. The results show that the intensity of C = O and C–O bonding undergoes a change corresponding to the changes in energy of the ion beam exposure. Moreover, EO performance characteristics of IPS cells on the organic hybrid overcoat layer via ion beam irradiation were observed to have similar characteristics to the polyimide.