Poonam Silotia

Enhancing the photoluminescence of ferroelectric liquid crystal by doping with ZnS quantum dots

We report the enhancement in photoluminescence (PL) intensity and shift in spectral energy band of ferroelectric liquid crystal (FLC) doped with ZnS quantum dots (QDs). The emission from ZnS QDs has combined constructively with that of FLC to give enhanced PL intensity. On the other hand, the smectic phases of FLC provided strong light scattering and hence improved the PL intensity of the composite. The change in effective refractive index of FLC due to different concentrations of ZnS QDs has caused the red shift. Our observations will certainly provide a promising tool in the realization of enhanced PL-LC display devices.

Sign reversal of dielectric anisotropy of ferroelectric liquid crystals doped with cadmium telluride quantum dots

A small amount of cadmium telluride quantum dots (CdTe QDs) has been doped into various ferroelectric liquid crystals (FLCs) to observe the modifications in the alignment and dielectric anisotropy (Δɛ) of the composites. The CdTe QDs have induced a uniform homeotropic (HMT) alignment in most of the FLC mixtures. We observed an unexpected switching (from HMT to homogeneous configuration) of CdTe QDs doped FLC CS1026 (having positive Δɛ) by the application of high dc bias. This reverse switching has been attributed to the interaction between FLC molecules and CdTe QDs which caused the sign reversal of Δɛ of FLC CS1026.

Polymeric-nanoparticles–induced vertical alignment in ferroelectric liquid crystals

Here, we report the polymeric (copolymer of benzene and pentacene) nanoparticles (PNPs) induced vertical alignment in ferroelectric liquid crystals (FLCs). The nanoparticles used in this study have been synthesized via chemical route method. The PNPs have been doped in FLC mixture. It has been observed that pentacene molecules (presented in PNPs used) prefer an upright orientation on the indium-tin-oxide–coated surfaces, which in turn provide assistance to align FLC molecules vertically (or homeotropically). It has also been observed that the addition of PNPs into the FLC materials improves the electro-optical response. However, the transition temperatures of the PNPs-doped FLC materials have been lowered. These findings will provide a fascinating tool to align FLC materials devoid of any surface treatment. Moreover, these studies would be helpful in the realization of low threshold and faster liquid crystal display devices.

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...

Effect of polymeric nanoparticles on dielectric and electro-optical properties of ferroelectric liquid crystals

It has been observed that the polymeric nanoparticles, copolymer of polybenzene and anthracene (PBA NPs), can induce a homeotropic (HMT) alignment in various ferroelectric liquid crystal (FLC) mixtures. The HMT alignment of various FLCs is attributable to the fact that the anthracene molecules favor an upright orientation (with a little tilt) on the surfaces of indium tin oxide and this upright orientation of anthracene molecules works as a template to align FLC molecules homeotropically. It has been concluded that the addition of ∼0.5 wt % of PBA NPs is enough to induce a HMT alignment in a FLC material Felix 17/100. The influence of the PBA NPs concentrations on the transition temperature, physical constants (such as spontaneous polarization, rotational viscosity, and response time) and dielectric relaxation processes of FLC material (Felix 17/100) has also been investigated.

Effect of cubic and planar collective and localized modes on the specific heat of C60 fullerite for 0.2≤ T≤ 300 K

A dynamical model for C60 polycrystalline fullerite is suggested to explain successfully the recent experimentally observed seven orders of variation in its specific heat for 0.2 ≤ T ≤ 300 K. The collective translational modes have both cubic and planar characters which yield correct T3 dependence of the observed specific heat for T < 1 K and over 60% contribution of the total specific heat for T ≥ 100 K. Other localized modes of the buckyballs: librational, orientational diffusive, tunneling and intramolecular vibrational, contribute significantly in different ranges of temperatures. It appears that surface modes persist in curled up graphite sheets that form fullerenes.

ANISOTROPIC TEMPERATURE DEPENDENT RAYLEIGH-MOSSBAUER RECOILLESS FRACTION IN FULLERITE

Abstract The Rayleigh-Mossbauer recoilless fraction, f , for 57 Fe Mossbauer radiation in the temperature range 0–300 K, has been computed for different dynamical models of fullerite and compared with the corresponding anisotropic f in graphite. The realistic dynamical model incorporating acoustic modes, librational, orientational diffusive, tunneling and intramolecular modes, which explains the measured temperature variation of the specific heat in the range 0.2–300 K, yields an anisotropic f which can be easily observed by performing a Rayleigh-Mossbauer scattering experiment in fullerite.

Density fluctuation spectra in Ar36 fluid having number density 21, 5 and 2 nm-3

Abstract Making use of the recently proposed modified microscopic theory by the authors, it has been possible to explain quantitatively the observed dynamics of a Van der Waals fluid, Ar36 when the number density of Ar atoms varies from 2, 5 to 21 particles/nm3 in the momentum transfer region extending to almost two orders of magnitude in the wave vector from the Rayleigh regime.

Adsorbed molecules in external fields: Effect of confining potential

We study the rotational excitation of a molecule adsorbed on a surface. As is well known the interaction potential between the surface and the molecule can be modeled in number of ways, depending on the molecular structure and the geometry under which the molecule is being adsorbed by the surface. We explore the effect of change of confining potential on the excitation, which is largely controlled by the static electric fields and continuous wave laser fields. We focus on dipolar molecules and hence we restrict ourselves to the first order interaction in field-molecule interaction potential either through permanent dipole moment or/and the molecular polarizability parameter. It is shown that confining potential shapes, strength of the confinement, strongly affect the excitation. We compare our results for different confining potentials.