A. Choudhary

Nonvolatile memory effect based on gold nanoparticles doped ferroelectric liquid crystal

Nonvolatile memory effect based on gold nanoparticles (GNPs) doped deformed helix ferroelectric liquid crystal (DHFLC) has been observed. This observed memory effect has been attributed to electric field induced charge transfer from liquid crystal molecules to the GNPs and the stabilization of helix deformation of DHFLC material, which occurs on the application of electric field beyond a critical field. The memory effect has been analyzed by polarizing optical microscopy, high-resolution transmission electron microscopy, and dielectric spectroscopy. The observed memory effect combines two active research areas: nanotechnology and liquid crystal.

Effect of graphene oxide nanomaterial in electroclinic liquid crystals

The collective dielectric relaxation studies have been carried out on the electroclinic liquid crystals (ELCs) doped with 0.1 to 0.2 wt % of graphene oxide (GO) in the frequency range of 20 Hz to 1 MHz. The GO favors for a good quality vertical alignment without any surface treatment of the substrates. The coupling of GO with indium tin oxide (ITO) substrate and ELC materials affects the molecular ordering and supports the ELC molecules to be aligned along the GO attached to the ITO surface in vertical direction. The vertical alignment can be changed to homogeneous by applying a high bias field to the sample and such converted homogeneous cell shows an additional dielectric relaxation peak in the low frequency side of Goldstone mode in SmC∗ phase due to presence of GO whereas in the pure material no such peak was observed. The frequency separation of both peaks (Goldstone mode and an additional peak) increases with temperature and low frequency peak vanishes near transition temperature.

Effect of ZnO nanoparticles on the SmC*-SmA* phase transition temperature in electroclinic liquid crystals

ZnO nanoparticles (NPs), synthesized in an alcoholic medium at room temperature, were added to electroclinic liquid crystal (ELC) materials. The addition of ZnO NPs in ELCs, caused a remarkable shift in SmC*-SmA* phase transition which was investigated from the dielectric and electro-optical measurements. The anchoring of ELC molecules around ZnO NPs creates orientational distortions near the surface, which may give additional ordering to the ELC molecular arrangement. After analyzing collective dielectric relaxation processes of ZnO NP doped ELCs, three distinct loss peaks were observed. The different behavior of ZnO NP doped ELC from pure ELC has been explained by determining the dielectric strength, the distribution parameter and the corresponding relaxation frequency, and so on, and then these results have been compared with the data calculated by using the theoretical model. The effect of ZnO NPs addition on physical parameters, such as spontaneous polarization (Ps) and rotational viscosity (η) has a...

Criticality of bistability phenomenon in deformed helix ferroelectric liquid crystal

The dynamics of bistability in deformed helix ferroelectric liquid crystal have been investigated by electro-optical method. The threshold value of the voltage required to switch the molecules of deformed helix ferroelectric liquid crystals (DHFLC) material has been studied by applying triangular wave pulse and texture observation under polarizing microscope. Two peaks have been observed in low and high frequency regimes on the application of triangular wave pulse to the sample. It has been proposed that the first peak is due to helix unwinding-winding process and the second is due to molecular reorientation process. The occurrence of double peak has been predicted to have close correlation with the observance of memory effect in DHFLC material, which is critically dependent on frequency and applied voltage, on the application of time delayed square wave pulse to the sample cell. The conditions for dynamic and static memory effects in DHFLC material have also been discussed in detail.

A dielectric mode in electroclinic liquid crystals

The dielectric properties of electroclinic and ferroelectric liquid crystal materials have been investigated in the frequency range of 100Hz–10MHz. A dielectric mode has been predicted in electroclinic liquid crystals near the transition temperature of Sm-C*–Sm-A phase. It has been observed that the investigated material has nonlayer shrinkage near the transition temperature of Sm-C*–Sm-A phase and shows anomalous behavior of dielectric spectra, tilt, and texture which is entirely different from the behavior of ferroelectric liquid crystals (FLCs). The dielectric relaxation frequency and tilt angle are almost constant with respect to temperature near the transition temperature under high bias field, which is characteristically different from FLCs. The high dielectric permittivity near Tc owing to the presence of mode contribution leads to the fact that there is an intralayer phason variation few degrees before transition. The mode has been named random mode due to its origin from randomization of tilt nea...

Dielectric and electro-optical studies of glycerol/ferroelectric liquid crystal mixture at room temperature

The optical memory effect based on glycerol mixed deformed helix ferroelectric liquid crystal (DHFLC) material has been investigated. The observed memory effect has also been compared with the glycerol mixed ferroelectric liquid crystals (FLCs). The observance of memory effect has been verified by experimental data using dielectric spectroscopy, electro-optical, and textural studies under polarizing optical microscope. The glycerol not only helps to improve the memory effect in DHFLC material but also alters the physical parameters such as rotational viscosity, spontaneous polarization, response time, etc. too in both glycerol/DHFLC and glycerol/FLC mixtures.

Enhancement of ferro-para transition in ethanol doped ferroelectric liquid crystals

The influence of ethanol (C2H5OH) doping in the electroclinic liquid crystal has been investigated using dielectric spectroscopy. A giant shift in ferroelectric (SmC∗) to paraelectric (SmA) phase transition has been observed. After analyzing collective relaxation processes, it is confirmed that Curie–Weiss law is obeyed in the vicinity of SmC∗ to SmA phase transition after doping with ethanol. It has been predicted that the dipole-dipole interaction due to ethanol molecules creates the uniformity in the randomized liquid crystal molecules in paraelectric phase, resulting in the increase in the ferroelectric phase in de Vries electroclinic liquid crystal materials.

Memory effect near transition temperature in Sm C∗ phase in nonsurface stabilized ferroelectric liquid crystals

Memory behavior in the ferroelectric liquid crystal (FLC) material, Felix 17/100, has been investigated by electro-optical, dielectric, and hysteresis methods at different temperatures ranging from room temperature to near ferro-paraelectric phase transition. Memory effect has been observed in the studied material near the transition temperature in Sm C∗ phase in the cells having thickness greater than the pitch value of the material. This is in contrast to the memory effect observed in conventional FLCs where thickness of the cell has to be less than the pitch value of the material. Electrical conductivity measurements elucidate that the steep increase in the conductivity near the transition temperature in Sm C∗ phase enhances the motion of free ions and probably weakens the depolarization field in the material, thereby showing memory effect.

Memory effect in Sm C* phase of electroclinic liquid crystals

Dielectric, textural, and electro-optical investigations have been carried out on a material exhibiting de Vries smectic A phase transition in sample cells of various thicknesses. Thickness independent memory effect has been observed in this special class of ferroelectric liquid crystals (FLCs) known as de Vries electroclinic liquid crystals. Unlike the FLC materials, surface anchoring energy does not play a role in the memory effect due to the fact that pitch is around 2 micron and basic assumption of SSFLC effect is not valid. The conversion of short-range azimuthal (Φ) ordering of molecules in Sm C* phase into a long-range azimuthal ordering under the influence of field could be one of the possible explanations. The transformation of molecular ordering in Sm C* phase under field may be mediated by dipolar interaction and viscoelastic properties.

Triboelectric activation of ferroelectric liquid crystal memory devices

The switching of ferroelectric liquid crystal (FLC) memory device is achieved by triboelectric effect of externally connected insulator to the sample through a conductor. The positive and negative charges are induced by triboelectrification of polymers, depending on the nature of materials, and utilized to get the bright and dark states of the FLC memory device. These switching states are analyzed by electro-optical studies. This technique of polarization switching proves to be powerful for FLC devices exhibiting long-term memory. The charges, developed on the electrodes by this nonconventional technique, generate enough potential to switch the FLC molecules in memory state.