Analysis of cell gap-dependent driving voltage in a fringe field-driven homogeneously aligned nematic liquid crystal display

We have studied cell gap-dependent driving voltage characteristics in a homogeneously aligned nematic liquid crystal (LC) cell driven by a fringe electric field, termed the fringe field switching (FFS) mode. The results show that for the FFS mode using a LC with positive dielectric anisotropy, the operating voltage decreases as the cell gap decreases, whereas it increases with a decreasing cell gap when using a LC with negative dielectric anisotropy. The difference between LCs is explained by simulation and experiment.     

Analysis of cell gap-dependent driving voltage in a fringe field-driven homogeneously aligned nematic liquid crystal display

We have studied cell gap-dependent driving voltage characteristics in a homogeneously aligned nematic liquid crystal (LC) cell driven by a fringe electric field, termed the fringe field switching (FFS) mode. The results show that for the FFS mode using a LC with positive dielectric anisotropy, the operating voltage decreases as the cell gap decreases, whereas it increases with a decreasing cell gap when using a LC with negative dielectric anisotropy. The difference between LCs is explained by simulation and experiment.     

Polarised UV-induced homogeneous alignment for fringe-field switching liquid crystal display with a new azobenzene monomer

A novel preparation method of homogeneous alignment polymer film (HAPF) was proposed by polymerisation of the monomer, 4,4?-di-methacryloyl-oxy azobenzene (4,4?-DMOAz), being dissolved in the liquid crystal (LC) material of positive dielectric anisotropy. For obtaining the homogeneous alignment, exposure of the polarised UV light was carried out to the LC cell above the nematic to isotropic transition temperature of the LC material. The fringe-field switching (FFS) mode LC cell with the HAPF formed from the monomer 4,4?-DMOAz (FFS-HAPF-LC cell) exhibited enough level of alignment state, electro-optical and response properties compared with the FFS-LC cell carrying the conventional polyimide-type alignment layer. FFS-HAPF-LC cell can be expected to be useful for next-generation displays such as flexible LC displays.     

Stability of liquid crystal micro-droplets based optical microresonators

Liquid crystal (LC) based tuneable optical microresonators are potential for being used as crucial components in photonic devices. In this article, we report experimental studies on LC micro-droplets dispersed in several dispersing media. We find that the size of the micro-droplets formed in a low refractive index and optically transparent perfluoropolymer are most stable with time than commonly used dispersing media. Using a negative dielectric anisotropy nematic liquid crystal, we show that the whispering gallery mode optical resonance properties such as the quality factor and the free spectral range of stable micro-droplets are independent of the strength of the applied electric field. The optical resonance properties under applied field are significantly different than that of the liquid crystals with positive dielectric anisotropy and are explained based on the elastic deformation of the micro-droplets.     

Cell gap‐dependent transmission characteristics of a fringe‐electric field‐driven homogeneously aligned liquid crystal cell,for a liquid crystal with negative dielectric anisotropy

Transmittance characteristics were studied as a function of cell gap for a homogeneously aligned liquid crystal (LC) cell driven by a fringe‐electric field—named fringe‐field switching (FFS) mode. The light efficiency of a conventional LC cell using in‐plane switching and twisted nematic modes, where the LC director is determined by competition between elastic energy and electrical energy, does not depend on cell gap as long as the cell retardation value remains the same; i.e. only dielectric torque contributes to the deformation of the LC director. However, the transmittance of the FFS mode is dependent on the cell gap such that it decreases as the cell gap decreases, although the cell retardation value remains the same. This unusual behaviour (unlike that of conventional LC cells) arises because in the device the elastic and dielectric torques have the role of determining the LC director, such that the driving voltage giving rise to maximum transmittance becomes strongly dependent on the electrode position when the cell gap is as small as 2?µm. In addition, the LCs at the centre of the pixel and common electrodes are not sufficiently twisted because of a competition between the two elastic forces, which tries to twist the LCs in plane and hold them in their initial state by surface anchoring.     

Graphene and liquid crystal mediated interactions

ABSTRACT

The two-dimensional graphene-honeycomb structure can interact with the liquid crystal’s (LC) benzene rings through π–π electron stacking. This LC–graphene interaction gives rise to a number of interesting physical and optical phenomena in the LC. In this paper, we present a combination of a review and original research of the exploration of novel themes of LC ordering at the nanoscale graphene surface and its macroscopic effects on the LC’s nematic and smectic phases. We show that monolayer graphene films impose planar alignment on the LC, creating pseudo-nematic domains (PNDs) at the surface of graphene. In a graphene-nematic suspension, these PNDs enhance the orientational order parameter, exhibiting a giant enhancement in the dielectric anisotropy of the LC. These anisotropic domains interact with the external electric field, resulting in a non-zero dielectric anisotropy in the isotropic phase as well. We also show that graphene flakes in an LC reduce the free ion concentration in the nematic media by an ion-trapping process. The reduction of mobile ions in the LC is found to have subsequent impacts on the LC’s rotational viscosity, allowing the nematic director to respond quicker on switching the electric field on and off. In a ferroelectric LC (smectic-C* phase), suspended graphene flakes enhance the spontaneous polarisation by improving the tilted smectic-C* ordering resulting from the π–π electron stacking. This effect accelerates the ferroelectric-switching phenomenon. Graphene can possess strain chirality due to a soft shear mode. This surface chirality of graphene can be transmitted into LC molecules exhibiting two types of chiral signatures in the LCs: an electroclinic effect (a polar tilt of the LC director perpendicular to, and linear in, an applied electric field) in the smectic-A phase, and a macroscopic helical twist of the LC director in the nematic phase. Finally, we show that a graphene-based LC cell can be fabricated without using any aligning layers and ITO electrodes. Graphene itself can be used as the electrodes as well as the aligning layers, obtaining an electro-optic effect of the LC inside the cell.     

Surface polymer-stabilised in-plane field driven vertical alignment liquid crystal device

In-plane field driven vertical alignment device using a liquid crystal with positive dielectric anisotropy has been studied. In the device, the distance between inter-digital electrodes needs to be increased to achieve higher transmittance; however, such a design results in an increase in operating voltage and slower response time. In this work, we use polymer stabilisation technique, which generates surface tilt angle other than 90^o, to improve upon these drawbacks. As a result, the proposed device shows lower operating voltage and faster response time while keeping transmittance at the same level, compared to those prior to polymer stabilisation.     

Effects of carbon nanotubes on electro-optic characteristics in vertically aligned liquid crystal display

Size- and aggregation-controlled dispersion of thin multiwalled carbon nanotube (t-MWCNT) in negative dielectric anisotropic liquid crystal (LC) material exhibits remarkable improvement in electro-optic response time in vertically aligned LC cells. The physical properties such as birefringence, dielectric anisotropy and clearing temperature of nanotube dispersed LC material appear to be almost invariant to that of pristine LC. Nevertheless, the response time shows noticeable improvement, especially in decaying time associated with transition from maximum to minimum transmission, hence important for faster switching LC devices. The effect is attributed to that vertically aligned t-MWCNTs along the field direction play role of vertical alignment layer between LCs, consequently resulting in increased bend elastic constant of LCs.     

Mixed-field-switching liquid crystal mode using in-plane and fringe fields self-adjusted by bottom floating electrode for transmittance enhancement

We demonstrate a liquid crystal (LC) mode switched by mixed electric fields of in-plane and fringe fields, which are self-adjusted by adopting a bottom floating electrode for enhanced electro-optical properties. In our LC mode structure, conventional in-plane switching (IPS) electrodes are formed as pixel electrodes and common electrodes on an insulating layer and floating electrodes that are patterned per the sub-pixels. When the areas of the pixel and common electrodes are identical, the voltage of the bottom floating electrode is spontaneously determined to be half the value of the pixel voltage, which ideally generates symmetric fringe fields with both pixel and common electrodes. Due to the in-plane fields additionally generated between the pixel and common electrodes, the proposed LC structure operates by mixed-field switching (MFS), which shows higher transmittance than fringe-field switching (FFS) and IPS LC modes. Transmittance of the conventional FFS and IPS LC modes is highly sensitive to the in-plane electrode’s width (w) and spacing (l) condition, but the proposed MFS LC mode shows good transmittance without degradation with large variations of the in-plane electrode’s spacing-to-width ratio (l/w).     

Reverse mode operation polymer dispersed liquid crystal with a positive dielectric anisotropy liquid crystal

The development of electrically activated chromogenic materials is important for their potential applications in smart windows. Several previous works have reported on reverse mode operation polymer dispersed liquid crystals (PDLCs) based on negative dielectric anisotropy liquid crystals. They have a transparent OFF state, which turns opaque after the application of a suitable external electric field. Nevertheless, these devices have some limitations such as the use of large amount of expensive liquid crystals with peculiar physical‐chemical properties. In addition, a good matching between the refractive index of liquid crystal and the polymer matrix one is required. The main result of this work is the achievement of reverse mode operation devices prepared with a positive dielectric anisotropy liquid crystal and characterized by a high OFF state transmittance obtained by the onset of high intensity built‐in DC electric fields in a direct mode operation PDLC, which allows the OFF state homeotropic alignment of liquid crystal directors. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

The effect of surface polarity of glass on liquid crystal alignment

The effects of the surface polarity of a glass substrate on the orientation of nematic liquid crystals (LCs) were studied using the polarised optical microscope and Fourier-transform infrared spectroscopy. On the surface of oxygen plasma treated glass, a homeotropic alignment of LCs was induced for LCs with negative dielectric anisotropy. This suggests that vertical orientation of LCs could be induced on a polar glass substrate without using an LC alignment layer. Upon cooling towards the isotropic–nematic transition, E7 with positive dielectric anisotropy changes its LC arrangement to isotropic, homeotropic, planar orientations in order. The nematic LC anchoring transition of E7 was interpreted by considering the competition between van der Waals forces and dipole interactions that control the alignment of LC molecules on a polar glass surface.     

Optimal pixel design for low driving, single gamma curve and single cell-gap transflective fringe-field switching liquid crystal display

When a dielectric layer, in-cell retarder (ICR) is formed between the electrode and LC layer to obtain a single-gap transflective fringe-field switching (FFS) display, the driving voltage is highly increased due to the thickness of the dielectric material. In particular, the driving voltage of the transmissive part becomes very high, and goes beyond the driver integrated circuit (IC) range for mobile application because the homogenously aligned liquid crystal director should rotate twice as far as that in the reflective part. The correlation between the driving voltage and electrode structures was investigated. It was found that the problem could be solved by optimisation of the common electrode structure such that the electrode structure changed from a plane to slit shape (in-plane field is mainly used instead of fringe field), realising a high performance FFS transflective display.     

Photo-reversible liquid crystal alignment using azobenzene-based self-assembled monolayers: comparison of the bare monolayer and liquid crystal reorientation dynamics

Photosensitive surfaces treated to have in-plane structural anisotropy by illumination with polarized light can be used to orient liquid crystals (LCs). Here we report a detailed study of the dynamic behavior of this process at both short and long times, comparing the ordering induced in the bare active surface with that of the LC in contact with the surface using a high-sensitivity polarimeter that enables detailed characterization of the anisotropy of the active surface. The experiments were carried out using self-assembled monolayers (SAMs) made from dimethylaminoazobenzene covalently bonded to a glass surface through a triethoxysilane terminus. This surface gives planar alignment of the liquid crystal director with an azimuthal orientation that can be controlled by the polarization of actinic light. We find a remarkable long-term collective interaction between the orientationally ordered SAM and the director field of the LC: while an azobenzene based SAM in contact with an isotropic gas or liquid relaxes to an azimuthally isotropic state in the absence of light due to thermal fluctuations, an orientationally written SAM in contact with LC in the absence of light can maintain the LC director twist permanently, that is, the SAM is capable of providing azimuthal anchoring to the LC even in the presence of a torque about the surface normal. We find that the short-time, transient LC reorientation is limited by the weak azimuthal anchoring strength of the SAM and by the LC viscosity.     

Dielectric and elastic properties of fluorine-containing tricyclic isothiocyanate liquid crystal

The dielectric and elastic properties of liquid crystals (LCs) generally depend on the molecule structure, such as polar group and carbon chain length. For further investigation of the influence of molecular structure on the dielectric and elastic properties of fluorine-containing tricyclic isothiocyanate LC, the experimental temperature was controlled at 25°C by precision hot stage, and a precision LCR meter was used to measure the capacitance of six LC cells under the voltage from 0.1 to 20 V at 1 kHz. An LC cell capacitance model and a dual-cell model were adopted to obtain the dielectric anisotropy, and the capacitance–voltage curves of six LC materials were plotted. The threshold voltage of Fréedericksz transition was analysed, and a finite difference iterative method was used to attain specific values of three elastic constants. The influence of molecular structure on the dielectric and elastic constants was finally analysed. Experimental results showed that the introduction of meta-difluoro group would increase the dielectric anisotropy and reduce the threshold voltage of LC. As the length of the alkyl carbon chain increased, the dielectric anisotropy would have an odd–even alternation effect, which would lead to changes in the elastic constants of LC.     

Improvement of asymmetric viewing angle properties in single-domain fringe-field-switching liquid crystal mode by using parallel-rubbed alignment surfaces

In this paper, we discuss the viewing angle properties of single-domain fringe-field-switching (FFS) liquid crystal (LC) mode aligned by using parallel-rubbed polyimide surfaces. Due to the reduced initial tilting angle distribution in the bulk LC layer under parallel-rubbed surface alignment conditions, the problems of greyscale inversion and off-axis colour shift in the dark state and luminance asymmetry distribution in the low grey level, observed in the conventional single-domain FFS LC mode, can be improved effectively. The viewing angle properties of the proposed structure were analysed by using the Póincare sphere and fringe-field-induced LC distribution.     

Analysis of electro-optical and dielectric parameters of TiO2 nanoparticles dispersed nematic liquid crystal

The present investigation is focused on to find out the role of TiO₂ nanoparticles (NPs) on altering the dielectric and electro-optical parameters of nematic liquid crystal (NLC). In addition to this, we also optimized the concentration of dopant (0.25 wt%) for a saturation value of permittivity and dielectric anisotropy in the doped system. Dielectric spectroscopy has been performed with the variation of frequency and temperature to investigate the various dielectric parameters, which demonstrate that the investigated NLC is of positive dielectric anisotropy; the observed result shows a decrement in the value of relative permittivity and dielectric anisotropy; however, the permittivity value increases for higher concentration of dopant but remains less than that of pure NLC. Electro-optical measurements have also been performed to compute the optical response of pure and dispersed NLC. It is found that optical response decreases for the NP-doped systems. This optimized concentration of NPs in NLC matrix can have various credential applications in the field of active matrix display and holography.     

Effects of silver nanoparticle doping on the electro-optical properties of polymer stabilized liquid crystal devices

ABSTRACT

Polymer stabilised liquid crystal (PSLC) devices are widely used in various smart light modulation occasions. Their electro-optical properties can still be improved to address future challenges. It is well known that doping liquid crystal (LC) materials with nanoparticles can change the material’s electro-optical performance. In this study, silver nanoparticles (AgNPs) with size about 15–20 nm and surfactant were doped into PSLC devices. The effects of AgNPs doping on the PSLC electro-optical performances were studied. The result shows that AgNPs and surfactant doping can increase the dielectric anisotropy of LC mixture. PSLC devices with AgNPs doping have lower driving voltage and response time than un-doped PSLC devices. Therefore, doping of a specific concentration of AgNPs resulted in PSLC devices with improved electro-optical performance.     

Dielectric relaxation spectroscopy of a nematic liquid crystal doped with ferroelectric Sn 2 P 2 S 6 nanoparticles

It was found that doping a nematic liquid crystal (LC) with a small amount of ferroelectric nanoparticles strongly affects the dielectric properties of the system. In particular, adding the ferroelectric particles results in a shift of the absorption bands corresponding to the rotation of liquid crystal molecules around their short axes to lower frequencies and in an increase of the amplitude and with of the absorption bands. This suggests that strong interactions occur between the LC molecules and the particles, caused by the large dipole moment and high polarizability of the ferro-particles. The ferroelectric particles affect not only dielectric losses, but also dielectric permittivity of the system. Specifically, the static dielectric permittivity and the dielectric anisotropy of the suspension are more than twice that of the pure LC.     

Dielectric spectroscopy analysis in employing liquid crystal phthalonitrile derivative in nematic liquid crystals

Dielectric anizotropy and relaxation properties of 2,3-dicyano-1,4-di[3,4,5-tri(dodecyloxy)phenylcarbonyloxy] benzene (DCDPB)-doped E7 and E7 liquid crystal have been investigated by the dielectric spectroscopy method. Dielectric anisotropy property of the LCs changes from the positive type to negative type. Dielectric relaxation properties suggest that LCs exhibit a monodispersive dielectric property. The relaxation frequency of E7 and E7/DCDPB liquid crystals was calculated by means of Cole-Cole plots. Consequently, DCDPB dopant changes the dielectric anizotropy and relaxation parameters of E7 LC.     

Dielectric relaxation spectroscopy of a nematic liquid crystal doped with ferroelectric Sn 2 P 2 S 6 nanoparticles

It was found that doping a nematic liquid crystal (LC) with a small amount of ferroelectric nanoparticles strongly affects the dielectric properties of the system. In particular, adding the ferroelectric particles results in a shift of the absorption bands corresponding to the rotation of liquid crystal molecules around their short axes to lower frequencies and in an increase of the amplitude and with of the absorption bands. This suggests that strong interactions occur between the LC molecules and the particles, caused by the large dipole moment and high polarizability of the ferro-particles. The ferroelectric particles affect not only dielectric losses, but also dielectric permittivity of the system. Specifically, the static dielectric permittivity and the dielectric anisotropy of the suspension are more than twice that of the pure LC.