Alignment Of Nematic Liquid Crystals Research Articles

Enhanced switching behavior of iron oxide nanoparticle-doped liquid-crystal display.

It is well known that doping nanoparticles (NPs) in liquid crystals (LCs) can easily change the physical and electro-optical properties of LC mixture. In this paper, we demonstrate homogeneous, aligned nematic LC (N-LC) system dispersed in iron oxide (Fe2O3) NPs. The prepared Fe2O3 NPs have an average particle size of 50 nm. By changing the doping concentration of Fe2O3 NPs, we observed the characteristics of LC systems. Electrooptical (EO) characteristics included faster rising and falling times (2.14 ms and 10.24 ms, respectively) and lower driving voltage (1.45 V) compared with a pure N-LC cell. We demonstrated these results via the relationship between dielectric con- stant and LC device properties. The results were verified by software simulation based on general physical properties. Moreover, we observed that LC system with Fe2O3 NPs could be accomplished without capacitance hysteresis by capturing charged impurities. Superior performance of LC cell with Fe2O3 NPs indicates that the proposed LC system have strong potential for use in the production of advanced LC displays.

A small-angle X-ray scattering study of nanoparticle assembly in an aligned nematic liquid crystal

The assembly of colloidal particles in a nematic liquid crystal has been investigated using small-angle X-ray scattering. The structure and orientation of nanoparticle assemblies in bulk samples of aligned nematic liquid crystal have been determined. The method offers some advantages over optical microscopy, which is usually restricted to investigations of thin cells and micron-sized particles. The scattering from chains of particles has been calculated, and comparison with experimental results has shown that suspensions of 48 and 105 nm diameter silica nanoparticles formed highly ordered structures perpendicular to the liquid crystal director, consistent with quadrupolar defect-induced assembly.

P‐116: Novel Approach to Achieve Conventional Polyimide‐less IPS/FFS LCDs

AbstractWe propose a novel approach to achieve a homogeneous alignment of nematic liquid crystal (LC). Two bare substrates are rubbed in antiparallel direction and a mixture of LC and reactive mesogen (RM) is filled into the cell using capillary injection, and then UV is exposed after confirming the clear homogenous alignment. The cell shows an excellent dark state and stable electro‐optic on/off switching in the IPS cell, which proves polymerized RM on surfaces plays role of alignment layer. Proposed fabrication method has several advantages such as simple process and low cost for IPS cell because alignment layer as well as the coating process is not required.

P‐4: Effects of Surface Polarity on Nematic Liquid Crystal Alignment

AbstractAlignment of liquid crystal molecules on a polymer substrate depend on various molecular interactions at the interface between liquid crystal and substrate, such as dispersion forces, dipolar interactions, steric interactions, hydrogen bonding, and physical interactions due to distortion of orientation field of liquid crystals by grooved surface of the substrate. Among these interactions, dispersion force is regarded as a long‐range interaction and other interactions are regarded as short‐range interactions which do not extend beyond a few tens of angstrom. The homeotropic alignment mechanisms for nematic liquid crystals were studied on oxygen plasma treated glass substrates. The homogenous alignment was obtained for positive nematic liquid crystals whereas the homeotropic alignment for negative liquid crystals. Our studies provide experimental evidences showing that there is competing forces to determine the orientation of liquid crystals, and alignment can be controlled by adjusting the substrate polarity. Relations between anchoring transition of liquid crystals and the polarity of substrates were studied by observing liquid crystal alignment directions using polarized optical microscopy, conoscope, and FTIR spectroscopy. These studies provide experimental evidences showing that short‐range dipolar interactions and long‐range van der Waals interactions competitively control the orientation of the LC director.

Conoscopic analysis of electric field driven planar aligned nematic liquid crystal.

This paper illustrates the conoscopic observation of a molecular reconstruction occurring across a nematic liquid crystal (NLC) medium in the presence of an external electric field. Conoscopy is an optical interferometric method, employed to determine the orientation of an optic axis in uniaxial crystals. Here a planar aligned NLC medium is used, and the topological changes with respect to various applied voltages are monitored simultaneously. Homogenous planar alignment is obtained by providing suitable surface treatments to the ITO coated cell walls. The variation in the conoscopic interferometric patterns clearly demonstrates the transition from planar to homeotropic state through various intermediate states.

Advanced Patterned Vertical Aligned Nematic Mode to Elevate Transmittance

This paper presents an advanced patterned vertical aligned nematic liquid crystal (APVA) mode as a way of improving the transmittance of the conventional patterned, vertically-aligned nematic liquid crystal (PVA) mode. The APVA was characterized by multi-electrodes, which play an important role in generating a horizontal electric field that prevents LC disclination in the vicinity of the middle region of electrodes between top and bottom slits in PVA mode. The proposed APVA mode focused on improving dramatically the transmittance with excellent image quality. In the simulation, the APVA mode improved the transmittance by more than 22%, compared to the conventional VA modes. Therefore, it is useful to upgrade the LCD display quality.

Ink‐Jet Printing: Ink‐Jet Printed Nanoparticle Alignment Layers: Easy Design and Fabrication of Patterned Alignment Layers for Nematic Liquid Crystals (Part. Part. Syst. Charact. 2/2014)

Ink-jet printing of monolayer-capped nanoparticles is introduced as a versatile and highly efficient means to pattern the alignment of nematic liquid crystals. As discussed by Torsten Hegmann and co-workers on page 257, any homeotropic alignment patterns that can be switched by applying AC or DC electric field can quickly be created with high accuracy ranging in size from a few micron (>850 dpi) to several square inch.

UV 배향된 액정셀에서의 전기광학 특성

In this study, we investigated the electro-optical(EO) characteristic of fringe-field switching(FFS) mode cell by the two kinds of ultraviolet(UV) alignment method on the organic thin film(polyimide: PI). The suitable organic layersfor FFS cell and the aligning capabilities of nematic liquid crystal(NLC) using the in-situ photoalignment method were studied Disclination is observed after conventional photoalignment method for 1h, and in-situ photoalignment method for 1h. Monodomain alignment of the NLC can be observed via in-situ photoalignment method for 2h and 3h. It is considered that NLC alignment is due to photo-depolymerization of the polymer with oblique non-polarized UV irradiation on PI surface. An unstable V-T curve of UV-aligned FFS-LCD with conventional photoalignment method can be achieved. However, a stable V-T curve of UV-aligned FFS-LCD with in-situ photoalignment method(1h), and V-T curve of UV-aligned FFS-LCD with in-situphotoalignment method was much stable comparing with that of other UV-aligned FFS-LCD’s. As a result, more stable EO performanceof UV-aligned FFS-LCD with in-situphotoalignment method for 3h is obtained than that of the other UV-aligned FFS-LCD"s.

Self‐aligning liquid crystals in polymer composite systems

ABSTRACTSoft composite materials combined with a holographic photopolymerization process are used for realizing an innovative switchable periodic structure made of slices of almost pure polymer alternated with films of well aligned nematic liquid crystals named POLICRYPS. It exhibits negligible scattering losses, while the effect of the spatial modulation of the refractive index (from polymer to nematic liquid crystal) can be switched on and off by applying a low (few V/μm) electric field. The diffractive properties of the POLICRYPS structure are characterized in terms of cell thickness, impinging probe angle and wavelength revealing a strong correlation between the diffraction efficiency and all the above mentioned parameters. These results are very attractive for many applications such as switchable Bragg gratings for telecom devices, phase modulators, and displays. Other advantages of the technology include absence of an alignment layer, absence of haze, robust structure, and inexpensive manufacturing. In addition, no special alignment layers are required. This is a unique opportunity and a big advantage compared to conventional liquid crystal devices. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2014,52, 158–162

Large-area bulk self-assembly of plasmonic nanorods in nematic liquid crystal via surface-mediated alignment

In this paper, we introduce a novel method to organize gold nanorods (GNRs) on a large scale by means of nematic lyotropic liquid crystal (LC) and surface-induced bulk alignment. LC is unidirectionally aligned via surface-treatment, and then gets GNRs organized with a negative order parameter in bulk through elastic force. The self-assembled GNRs (with a considerable high concentration of about 6 wt%) in LC exhibit a strong polarization-dependent localized surface plasmon resonance (LSPR). Polarized optical microscopic imaging, polarization-dependent extinction and scattering spectra of LSPR are utilized to characterize the spatial orientation of GNRs. This method is compatible with the state-of-the-art LC display technique and allows for large-scale fabrication of optical metamaterials, plasmonic polarizers comprised of self-aligned anisometric nanoparticles with extremely low cost.

Ink‐Jet Printed Nanoparticle Alignment Layers: Easy Design and Fabrication of Patterned Alignment Layers for Nematic Liquid Crystals

Ink‐jet printing of monolayer‐capped gold nanoparticles is introduced as a versatile and highly efficient means to pattern the alignment of nematic liquid crystals. Any homeotropic alignment patterns can be created quickly ranging in size from 30 μm (850 dpi) to several square inches, with high accuracy that does not deteriorate with time. Depending on the alignment underlayer, intermediate configurations between homeotropic and homogeneous are also feasible. Nematic liquid crystals with both positive and negative dielectric anisotropy can be switched by applying a DC or AC electric field in the printed vertical domains with the substrate configuration determining the electro‐optic response.

Microchannel-flowed-plasma modification of octadecyltrichlorosilane self-assembled-monolayers for liquid crystal alignment

We report that a chemical patterning technique based on local plasma modification of self-assembled monolayers has been utilized to fabricate surfaces for domain liquid crystal alignment. Highly hydrophobic octadecyltrichlorosilane monolayers deposited on glass substrates coated with Indium-Tin-Oxide were brought into contact with elastomeric stamps comprising trenches on a micro scale, and then exposed to an oxygen plasma. In the regions exposed to the plasma the monolayer was etched away leaving a patterned surface that exhibited surface energy differences between surface domains. The surfaces that bear the micropatterns have been shown to be capable of producing patterned alignment of nematic liquid crystal.

Domain Structures in Nematic Liquid Crystals on a Polycarbonate Surface

Alignment of nematic liquid crystals on polycarbonate films obtained with the use of solvents with different solvations is studied. Domain structures occurring during the growth on the polymer surface against the background of the initial thread-like or schlieren texture are demonstrated. It is established by optical methods that the domains are stable formations visualizing the polymer surface structures. In nematic droplets, the temperature-induced transition from the domain structure with two extinction bands to the structure with four bands is observed. This transition is shown to be caused by reorientation of the nematic director in the liquid crystal volume from the planar alignment to the homeotropic state with the pronounced radial configuration of nematic molecules on the surface. The observed textures are compared with different combinations of the volume LC orientations and the radial distribution of the director field and the disclination lines at the polycarbonate surface.

Electric field effects in nematic liquid crystals doped with carbon nanotubes

The aim of this paper was to investigate electric field induced effects in mixtures of nematic liquid crystals (NLCs) with positive electric anisotropies (MCL 6601 Merck) with carbon nanotubes (MWCNT from Aldrich). In planar alignment, the current–electric field dependence and the current–temperature dependence were explained by assuming a Poole–Frenkel effect (i.e. a tunnelling mechanism) and good agreement with the experimental data was obtained. Within this high field range it resulted that in planar aligned NLC–CNTs mixture the conductivity decreases when the temperature was increased. In homeotropic aligned mixture, the conduction mechanism is similar to the one occurring in a semiconductor: the conductivity increases when increasing temperature. This happens because in thin liquid crystal cells there is a possibility to realize an inner contact between nanotubes and electrodes so the mixture behaves like a semiconductor.

In Situ Homeotropic Alignment of Nematic Liquid Crystals Based on Photoisomerization of Azo‐Dye, Physical Adsorption of Aggregates, and Consequent Topographical Modification

In situ homeotropic alignment is achieved by photochromic trans- to cis-isomerization of an azo-dye doped in a nematic host. The augmented dipole moment of the cis-isomer formed under UV-irradiation expedites molecular assembly into crystalline aggregates. Subsequent deposition of the aggregates creates a roughened surface and induces an anchoring transition from the initial planar to a homeotropic alignment of the LCs.

On alignment of nematic liquid crystals infiltrating chiral sculptured thin films

A structurally right-handed chiral sculptured thin film (STF) with a central 90 deg-twist defect was made by thermal evaporation of chalcogenide glass and the use of a serial bi-deposition process to exhibit a narrowband hole in the spectrum of the right-circularly polarized light reflected when right-circularly polarized light is normally incident on the chiral STF. In an attempt to build a tunable filter, the chiral STF was then infiltrated with a highly birefringent nematic liquid crystal (LC), which caused a linear reflectance peak to redshift by ∼350 nm . But the circular Bragg phenomenon exhibited by the uninfiltrated chiral STF was greatly diminished owing to the similarity in the constitutive properties of the LC and the chalcogenide glass. No temperature dependence of the shifted peak was observed, which provided clear evidence that the LC molecules are not ordered inside the chiral STF but are randomly aligned instead.

Superior electro-optic properties of liquid crystal system using cobalt oxide nanoparticle dispersion

In this article, we study the electro-optical (EO) properties of the homogeneous aligned nematic liquid crystal (N-LC) doped with cobalt oxide (Co3O4) nanoparticles (NPs). The EO characteristics of Co3O4 doped N-LC are higher performance, indicating lower threshold voltage (1.33 V), faster rising time (1.479 ms), and faster falling time (9.343 ms) than pure liquid crystal (LC) cells. We have demonstrated these results by investigating the relationship between dielectric constants and LC device properties. Furthermore, we proved NPs doped N-LC cells drive low power operation without capacitance hysteresis. Our experimental results were verified by software simulation based on general physical properties.

Star colloids in nematic liquid crystals

We present a study of the elastic alignment, accompanying director field distortions, and elastic pair interactions of star-shaped colloids suspended in aligned nematic liquid crystals. We design and fabricate lithographic colloids, “N-stars”, containing N rod-like protrusions (i.e. “rays” or “arms”) each having a constant angle between adjacent rays. N-star geometries contain concave regions while retaining the rotational and mirror symmetries of regular polygonal platelets having N sides. Planar anchoring of the nematic director at N-star surfaces induces elastic deformations of the uniform background director, resulting in distinct orientational states and pair interactions that depend upon N. Director fields around isolated N-stars are characterized using polarized optical microscopy. For each N-star, we observe long-lived metastable orientational states with accompanying metastable director configurations, which are topologically distinct from the ground state director field. We develop a model, based on a superposition of the elastic energy of rod-like inclusions at appropriate angles to the far-field director, to estimate the energies in both cases. Numerical calculations of the director field around an individual ray elucidate the effect of azimuthal degeneracy in the anchoring and cross-sectional shape of the ray. The analytical results agree with the simulations, however, we find that the total elastic energy must be rescaled to account for weaker anchoring. The long-range elastic pair interactions between N-stars are probed using optical tweezers and video microscopy. We observe a distinct multipole depending on whether N is even or odd, which dominates the distance-dependence for attractive elastic forces between pairs of N-stars. Finally, we discuss assemblies made up of mixtures of different types of N-stars that display a variety of aggregated states.

All-optical control of localized plasmonic resonance realized by photoalignment of liquid crystals

Large shifts in the plasmonic resonances of a thin film of gold nanorods (GNRs) are induced through the modulation of the local refractive index of the neighboring dielectric medium. This change is enabled through light-induced surface reorientation of a nematic liquid crystal in contact with a nanometer-thin photoalignment layer coating the GNR film. The presence of isolated and well distributed GNRs, both before and after the photoalignment layer deposition, is shown through atomic force and scanning electron microscopy. Exposure of the photoalignment layer to polarized light is shown to reorient aligned nematic liquid crystal molecules to an orthogonal direction, thereby changing the local refractive index of the medium in close proximity to the GNR film. This large change in the local dielectric strength is shown to cause a broad red shift of the localized plasmonic longitudinal resonance and almost no shift in the local transverse resonance. The ability to remotely and quickly change the plasmonic properties of this GNR system marks a breakthrough towards the realization of all-optical plasmonic and photonic devices such as plasmonic colour filters.

Competition of elasticity and flexoelectricity for bistable alignment of nematic liquid crystals on patterned substrates

We show that patterned surfaces can promote bistable configurations of nematics for reasons other than the symmetry of the surface. Numerical and analytical calculations reveal that a nematic liquid crystal in contact with a striped surface is subject to the competing aligning influences of elastic anisotropy, differing energy cost of various types of deformation, and flexoelectricity, curvature-induced spontaneous polarization. These effects favor opposing ground states where the azimuthal alignment is, respectively, parallel or perpendicular to the stripes. Material parameters for which the effect might be observed lie within the range measured for bent-core nematogens.