M. B. Pandey

Influence of low concentration silver nanoparticles on the electrical and electro-optical parameters of nematic liquid crystals

Thermodynamical, optical, dielectric and electro-optical characterisation of nematic liquid crystals (LCs) and silver nanoparticle (NP) composites have been carried out. Transition temperatures of pure and composites systems have been measured. Thermodynamical studies suggest increase of clearing temperature of the composite material as compared to the pure material. Threshold voltage for switching from bright to dark state and splay elastic constant of the pure and composite materials have been determined. From frequency dependence of dielectric measurements, permittivity, loss, relaxation frequency and dielectric strength of flip-flop mechanism of LC molecules in the nematic phase have been calculated. Dielectric properties of composites have been explained in reference of Maier and Meier theory. The effects of doping of NPs on dielectric and electro-optic properties of LC-NP composites have been discussed.

Dielectric spectroscopy of unsymmetrical liquid crystal dimers showing wide temperature range TGBA and TGBC* phases

The electrical properties of frustrated twist grain boundary (TGB) phase are a matter of curiosity. Some studies have indicated the existence of soft and Goldstone modes in TGBA and TGBC* phases respectively. However, the experimental results are still not very conclusive. In the present work, we report dielectric studies of wide temperature range TGBA and TGBC* phases of an optically active dimeric compound 4‐n‐decyloxy‐4′‐(cholesteryloxycarbonyl‐1‐butyloxy) chalcone in the frequency range of 1 Hz to 35 MHz for the planar and homeotropic anchoring of the molecules. Two different relaxation processes have been detected for the planar anchoring of molecules in the TGBA and TGBC* phases. The soft mode like behaviour is obtained due to tilt fluctuation of molecules in the megahertz region for both TGBA and TGBC* phases. Goldstone mode like behaviour due to phase fluctuation of molecules has been detected for the TGBC* phase in the low frequency region (∼200–300 Hz). Activation energies for DC conductivity have also been determined for various phases of the material.

Enhanced electro-optical properties of a nematic liquid crystals in presence of BaTiO3 nanoparticles

Composites of nematic liquid crystals (NLCs) and ferroelectric barium titanate (BaTiO3) nanoparticles (NPs) have been prepared. The alignments of NPs in the host medium have been demonstrated. Effect of NPs doping on various display parameters of NLCs, namely, threshold voltage, dielectric anisotropy and splay elastic constant has been studied using electro-optical and dielectric studies. The nematic ordering of host supports alignment of NPs parallel to the director which consequently improves electro-optical parameters in the composite system. The dielectric and electro-optic properties of LC–NPs composites have been discussed in frame of conventional theories of NLCs.

Temperature and bias electric field dependence of dielectric parameters of ( S )-(+)-4-(1-methylheptyloxycarbonyl) phenyl-4’-(3-butanoiloxyprop-1-oxy)biphenyl-4’-carboxylate

Temperature and bias electric field-dependent dielectric behaviour of an antiferroelectric liquid crystal compound was investigated in the frequency range 1 Hz to 10 MHz. The investigated compound possessed paraelectric (SmA*) and antiferroelectric ( ) phases, in addition to a monotropic hexatic antiferroelectric smectic ( ) phase. The dielectric study showed the presence of a soft mode in the SmA* phase. In the phase, two weak relaxation modes were observed for a planar orientation of molecules. These two modes were related to antiferroelectric ordering and the helical structure of the phase. Two modes, related to the hexatic order of the phase, were also observed and characterized in terms of bond-orientational order and coupling to the molecular tilt direction. With respect to non-collective modes, rotation around the short axis of molecules was detected in the SmA* and phases for a homeotropic alignment.

Characteristics of the collective dielectric relaxation mode of the incommensurate SmCα* phase

Employing the dielectric relaxation spectroscopy technique, we have observed two different characteristics of the collective relaxation mode of an incommensurate SmC(α)(*) phase. Recent theoretical advances predict two different natures for the dielectric relaxation mode in the SmC(α)(*) phase. Our experimental results confirm two different behaviours of the relaxation mode in different temperature ranges for this phase for one unique compound having an exceptionally wide temperature range (∼9 °C) for the SmC(α)(*) phase.

Chiral Smectics: As Observed through Dielectric Spectroscopy

Chiral compounds show rich variety of smectic phases viz. SmCα*, SmCβ*, SmCγ*, SmC*, SmCA* due to different correlation between adjacent layers and various TGB phases due to different orientation of blocks of layers. Dielectric spectroscopy has proven as an important tool for the characterization of various smectic phases posses by chiral compounds. Dielectric spectra of these phases are sometimes complex but rich in information and are very helpful in the determination of the structure of these phases. Dielectric spectroscopy of the above phases observed in the recently synthesized materials is discussed with regards to their structure property relationship.

Amplitude and phase fluctuation modes in the smectic Cα* phase of an antiferroelectric compound

Dielectric measurements have been carried out on the chiral smectic Cα (SmCα*) phase of a MHPOBC analogous compound. Two relaxation modes have been observed in this phase for planar orientation of the molecules. One process has been observed at frequency lower than that of the soft mode of the chiral smectic A (SmA*) phase. This relaxation process is connected with the helicity of the SmCα* phase. In the high‐frequency region, another relaxation process has been observed in the SmCα* phase for which bias field dependence is similar to that of the soft mode at the SmA*–SmC* phase transition. The experimental observations are in agreement with a recently proposed dielectric theory for the SmCα* phase and theoretical dielectric results obtained by numerical simulations. Thus, we report here experimental verification of two theoretically predicted dielectric modes in the SmCα* phase.

Effect of dispersion of CdSe quantum dots on phase transition, electrical and electro-optical properties of 4PP4OB

ABSTRACT We prepared composites of a liquid crystalline material, 4-pentylphenyl 4-octyloxybenzoate (4PP4OB) and cadmium selenide quantum dots (CdSe-QDs) and investigated their thermodynamic, electro-optical and dielectric properties. The effect of QDs on transition temperature from isotropic to nematic and nematic to smectic A phases was evaluated in this study. The effect of CdSe-QDs inclusion on various display parameters on host liquid crystals was studied in the nematic phase. The electrical parameters of the composites – relative permittivity, dielectric anisotropy, dielectric loss and dielectric relaxation – were investigated in the nematic and smectic phases. The changes in dielectric parameters of the composites are explained in terms of Maier–Meier theory. GRAPHICAL ABSTRACT

Switching and Dielectric Behavior of Phase

Switching and dielectric properties of a compound (S)-(+)-4-(1-methylheptyl) 4-[4-(3-hexanoyloxy prop-1-oxy) benzoyloxy]biphenylate are studied by means of polarization reversal methods and dielectric spectroscopy respectively. Two characteristics polarization peaks have been observed on application of triangular pulse in the phase. The separation between two peaks has been decreases with increase in frequency and electric field. These two peaks merge into each other after a critical field of ∼200 kV/cm. A relaxation mechanism has been detected under planar orientation of molecules for phase in the high kHz region. This mode shows two different behaviors in two temperature ranges.

Characteristics of electrical properties of wide temperature range TGB phases in liquid crystal dimers

Abstract4-n-decyloxy-4′-(cholesteryloxycarbonyl-1-butyloxy) chalcone and its two successive homologous are optically active dimeric compound derived from cholesterol. They possess wide temperature ranges of two twist grain boundary (TGB) phases namely TGBA and TGBC*. Comprehensive dielectric studies have been carried out for these compounds in the frequency range of 1 Hz to 10 MHz for different conditions of molecular anchoring. Various electrical parameter viz. dielectric permittivity, dielectric anisotropy, dc conductivity, and activation energy have been determined for these two TGB phases. Weak relaxation processes have been detected under planar anchoring of molecules in the TGBA and TGBC* phases presumably due to amplitude (soft mode) and phase (Goldstone mode) fluctuations.