Zirconia nanoparticles/ferroelectric liquid crystal composites for ionic impurity-free memory applications

We observed an ionic impurity-free memory effect using a zirconia nanoparticles (ZNPs)/ferroelectric liquid crystal (FLC) composite. The pure and ZNPs doped FLC cells have been analyzed by means of dielectric spectroscopy, polarizing optical microscopy and electrical resistance/conductivity measurements. The memory behavior in ZNPs/FLC composite was confirmed by dielectric dispersion, electrical, and optical studies, whereas dielectric loss spectra confirmed the disappearance of the low-frequency relaxation peak, which appears due to the presence of ionic impurities in FLC materials. The observed memory effect has been attributed to minimization of the depolarization field and ionic charges, whereas the reduction of ionic effects has been attributed to the strong adsorption of ionic impurities on the surface of ZNPs. The ZNPs dispersed in FLCs may play a role in trapping the impurity ions (minimize the depolarization fields) under applied voltage and cause a better memory effect in ZNPs doped FLC material. Moreover, the ion adsorption capability of ZNPs is found to be almost independent of temperature as the value of resistance did not change remarkably on increasing the temperature. The reduction of ionic impurities of FLCs by doping ZNPs did not show degradation over time, as we repeated the experiments on the same sample cells after many days and did not find ionic effects in the ZNPs doped FLC materials. These studies would be helpful to provide an idea for designing ionic impurity-free memory devices

Scientific developments of liquid crystal-based optical memory: a review

The memory behavior in liquid crystals (LCs), although rarely observed, has made very significant headway over the past three decades since their discovery in nematic type LCs. It has gone from a mere scientific curiosity to application in variety of commodities. The memory element formed by numerous LCs have been protected by patents, and some commercialized, and used as compensation to non-volatile memory devices, and as memory in personal computers and digital cameras. They also have the low cost, large area, high speed, and high density memory needed for advanced computers and digital electronics. Short and long duration memory behavior for industrial applications have been obtained from several LC materials, and an LC memory with interesting features and applications has been demonstrated using numerous LCs. However, considerable challenges still exist in searching for highly efficient, stable, and long-lifespan materials and methods so that the development of useful memory devices is possible. This review focuses on the scientific and technological approach of fascinating applications of LC-based memory. We address the introduction, development status, novel design and engineering principles, and parameters of LC memory. We also address how the amalgamation of LCs could bring significant change/improvement in memory effects in the emerging field of nanotechnology, and the application of LC memory as the active component for futuristic and interesting memory devices

Self assembled monolayer based liquid crystal biosensor for free cholesterol detection

A unique cholesterol oxidase (ChOx) liquid crystal (LC) biosensor, based on the disruption of orientation in LCs, is developed for cholesterol detection. A self-assembled monolayer (SAM) of Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP) and (3-Aminopropyl)trimethoxy-silane (APTMS) is prepared on a glass plate by adsorption. The enzyme (ChOx) is immobilized on SAM surface for 12 h before utilizing the film for biosensing purpose. LC based biosensing study is conducted on SAM/ChOx/LC (5CB) cells for cholesterol concentrations ranging from 10 mg/dl to 250 mg/dl. The sensing mechanism has been verified through polarizing optical microscopy, scanning electron microscopy, and spectrometric techniques

Probing the effect of temperature and electric field on the low frequency dielectric relaxation in a ferroelectric liquid crystal mesogen

We present the characterization and dielectric spectroscopic investigations of a ferroelectric liquid crystal (PLC) material, namely KCFLC 10S. We experimentally demonstrate the observation of a low frequency dielectric relaxation mode along with Goldstone mode in the smectic C* phase of the FLC material under investigation. The behavior of low frequency dielectric relaxation mode has been systematically studied with temperature and applied bias field. The relaxation frequency of the low frequency dielectric relaxation has been found to be shifted toward the higher frequency side with an increase in temperature and field strength. This shift has been attributed to the single particle diffusion of fast ions. Further investigations on electrical conductivity (a) with frequency at different electric field strengths revealed a Schottky-type of charge injection at electrode even under a small DC bias. The studies carried out in the present paper would be helpful to provide ideas for designing and developing advanced liquid crystal materials

Role of cell thickness in tailoring the dielectric and electro-optical parameters of ferroelectric liquid crystals

Here, we report thickness dependence of dielectric and electro-optical parameters in ferroelectric liquid crystals (FLCs) without surface stabilisation. The dependence of dielectric and electro-optical parameters on cell thickness is observed by dielectric spectroscopy and electro-optical measurements. The dielectric permittivity (epsilon') measured by varying the cell thickness showed increase of epsilon' with increase of cell thickness which is attributed to the presence of more ions and larger contributions of Goldstone mode in thick cells. The spontaneous polarisation also shows increment with increase of cell gap up to certain thickness range. The rotational viscosity decreases with increase in the cell thickness whereas the response time is more for thicker cells. The decrease in the rotational viscosity is attributed to lowering of elastic deformation with increase in cell thickness and the response time is directly proportional to cell gap. These studies would be utilised to understand the effect of cell thickness on dielectric and electro-optical properties of FLC materials and optimising the material parameters with cell thickness for better and efficient liquid-crystal-based devices

Interfacial behavior of confined mesogens at smectic-C*-water boundary

In this paper, we have investigated the behavior of mesogens at smectic-C*-water interface confined in a liquid crystal (LC) cell with interfacial geometry. Polarized optical microscopy was used to probe the appearance of various smectic-C* domain patterns at water interface owing to the reorientation of mesogens. The undulated stripe domains observed at the air interface of smectic-C* meniscus vanished as the water entered into the smectic layers and focal conical domain patterns appeared at smectic-C*-water boundary. A spatially variable electro-optical switching of LC molecules was also observed outside the electrode area of the interfacial cell. The electrode region at the interface, as well as on the water side, was damaged upon application of an electric field of magnitude more than 150 kV/m. The change in dielectric parameters of mesogens was extensively studied at interface after evaporating the water. These studies give fundamental insights into smectic-C*-water interface and also will be helpful in fabricating better LC devices for electro-optical and sensing applications

Probing the dynamics of geometrically confined ferroelectric mesogens at the air interface

This article focuses on the alignment and dynamics of mesogens at the ferroelectric liquid crystal (FLC)/air interface in a confined geometry. The interface has been systematically prepared and characterised with provision for applying an electric field separately to the bulk and air interface of the FLC. Polarizing optical microscopy (POM) investigations done at the FLC/air interface have exposed the concave geometry, cell thickness dependent boundary width and phase dependent optical textures of the FLC meniscus at the interface. Dielectric spectroscopy investigations revealed the presence of an additional molecular relaxation mode at the FLC/air interface, which is attributed to the short axis rotation of homeotropically aligned mesogens at the interface. Based on the observations from the POM, dielectric spectroscopy and X-ray diffraction profiles, we schematically envisaged the molecular arrangement and dynamics of the FLC/air boundary. These studies would be helpful for innovations in liquid crystal based devices and also for many other applications, where soft surfaces, interfaces and confinement play a momentous role

Preparation and characterization of MgO nanoparticles/ferroelectric liquid crystal composites for faster display devices with improved contrast

In this article, we present the formulation and characterization of a ferroelectric liquid crystal (FLC) mixture W301 composed of pyrimidine compounds. We observed that upon doping magnesium oxide nanoparticles (MgO NPs) into the host FLC, the MgO NPs/FLC composite showed significantly faster response and improved optical tilt angle. The decreased response time in the MgO NPs/FLC composite has been attributed to the decrease in rotational viscosity and increase in surface anchoring energy. The decrease in rotational viscosity of the composite is due to the torque experienced by both MgO NPs and FLC in the presence of an electric field and perturbations of order parameters of FLC. Due to the enhanced surface interaction of MgO NPs having surface defects with mesogens, strong surface anchoring is experienced on the FLC molecules that not only increased the speed of the response but also improved the optical tilt angle of the MgO NPs/FLC composites, which ultimately resulted in improved contrast. A systematic approach has been followed to elucidate the idea of designing faster display devices with improved contrast based on MgO NPs/FLC composites

Effect of Nickel Oxide Nanoparticles on Dielectric and Optical Properties of Nematic Liquid Crystal

In the present paper, we have studied the improvement in dielectric and optical properties of nematic liquid crystal (NLC) by doping of nickel oxide (NiO) nanoparticles. We have observed the dielectric and optical properties of pure and doped cells in order to understand the influence of NiO nanoparticles in the pure NLC. The experimental results have been analyzed through dielectric spectroscopic and optical texural methods. Detailed studies of dielectric parameters such as dielectric permittivity, dielectric loss and dielectric loss factor as a function of frequency with temperature were carried out. It has been observed that on doping the nanoparticles in NLC, the value of dielectric parameters (dielectric permittivity, dielectric loss and dielectric loss factor) decreases. The impedance and resistance of both pure and nanoparticles doped NLC cells were studied and found that for doped NLC, these parameter have low value. In addition to this, optical textures of the pure and doped samples have also been observed with a polarizing optical microscope at room temperature. All the results i.e. related to the investigation of dielectric and electro-optic properties have been explained by using existing theory of NLC

Low-temperature nematic phase in asymmetrical 1,3,4-oxadiazole bent-core liquid crystals possessing lateral methoxy group

Asymmetrical bent-core molecules based on 1,3,4-oxadiazole bent-core unit have been synthesised as a new design with a lateral methoxy group at outer phenyl ring of the molecule. These new asymmetrical bent-core molecules resemble hockey-stick shaped due to the presence of two different arms of different lengths. One arm of these molecules is elongated having two phenyl rings and possesses a 4-n-alkyloxy chain of a different number of carbon atoms (n=4, 8, 12 and 18) and other arm is short and has one phenyl ring with fixed 4-n-octyloxy chain. The bent-core molecules possess a lateral polar methoxy group at the elongated arm of the molecule. These bent-core compounds exhibited fluorescence emission in the UV wavelength region (similar to 377-386nm) whereas in acetonitrile and dimethylformamide, solvent displays blue emission peak with a large stoke shift.The bent-core molecules with the number of carbon atoms (n=4, 8 and 12) at the elongated arm exhibited monotropic nematic phase at low temperature, while the 4-n-octadecyloxy chain at the elongated arm displayed smectic A phase. Dielectric studies were performed in the nematic phase of the bent-core mesogens confirm the formation of the cybotactic cluster in the nematic mesophases