Planar Aligned Cells Research Articles

Studies on phase transitions and dielectric properties of biowaste synthesized porous carbon nanoparticles – ferroelectric liquid crystal mixture

Ferroelectric liquid crystals(FLCs), an exciting class of liquid crystals(LCs), found potential applications in the display as well as non-display regimes due to their fast response, low driving voltage and nonvolatile memory. The amalgamation of nanoparticles into FLCs has opened up new avenues in the LCs research field by alterations/modification of the existing properties of LCs. In this work, porous carbon nanoparticles (PCNPs) were dispersed into FLC mixture (W206E) and investigated their doping effect on FLC’s textural, phase transition temperatures and dielectric studies in planar-aligned cells. Dielectric spectroscopy was carried out in the frequency range of 20 Hz to 10 MHz to explore the frequency as well as the temperature dependent of FLC in the entire SmC* region. The transition temperature of FLC mixture is increased by 4 °C in PCNPs doped FLC sample then undoped FLC sample. Nearly 8.42% increase in permittivity is observed. A Gold stone relaxation mode at ∼627 Hz was observed at lower frequency.

Optical activity and linear electrooptic response in the paraelectric subphase of an achiral ferroelectric nematic compound

ABSTRACTPolarizing optical microscopy and electro optics are used to study a compound with constituent non-chiral molecules, DIO, known to exhibit three nematic phases, namely N, Nx and ferronematic NF. N is found to be a conventional nematic but shows two additional unusual features: the presence of optically active domains and a linear electro optic response to the applied signal. The optical activity is explained by a segregation of the stable helical conformers of the opposite sense; these preferring to form domains with an optical rotation of ±4°/μm. This is the first example of helical segregation observed in non-chiral molecules in the high temperature nematic phase. Unlike the ordinary nematic liquid crystalline material, this compound shows a linear electro optical response to the applied electric field, thereby confirming the polar nature of this phase. NF is the ferroelectric nematic as reported previously. The strong polar azimuthal surface interaction energy in the NF phase stabilises a homogeneous structure in planar-aligned cells that are rubbed parallel and gives a twisted structure in cells rubbed antiparallel.

Tailoring the physical parameters of ferroelectric liquid crystal mixture with cadmium selenide quantum dots

ABSTRACT Present studies demonstrate the dispersion effect of cadmium selenide quantum dots on ferroelectric liquid crystal mixture properties. Varying concentrations of cadmium selenide quantum dots (0.05 wt.%, 0.10 wt.% and 0.50 wt.%) have been taken to investigate its influence on the physical parameters of ferroelectric liquid crystal mixture KCFLC 10R. Phase transition, textural, electro-optical and photoluminescence studies have been performed in planar aligned cells. The presence of QDs disturbs the geometrical symmetry and molecular tilt of ferroelectric liquid crystal molecules which results in the modification of physical parameters of ferroelectric liquid crystal. It has been observed that doped samples have shown decrement in spontaneous polarisation and rotational viscosity. However, there is an improvement in the response time of the doped samples. The reduction in the spontaneous polarisation with voltage signifies that doped samples can operate at low voltage (~6 V) as compared to ferroelectric liquid crystal mixture (~12 V). Photoluminescence studies reveal that all the samples absorb UV light ~340 nm and produce visible emission at ~ 400 nm which can behave as UV light storage and high contrast display. This would be beneficial for luminescent displays as well as UV to visible light conversion applications.

Zinc Oleate Nanorod-Induced Vertical Alignment of Nematic Liquid Crystal.

Nanocomposite zinc oxide nanorods capped with oleic acid (ZOR) with positive dielectric anisotropy liquid crystal (LC) 4'-octyl-4-biphenylcarbonitrile (8CB) filled in unaligned cells exhibit homeotropic alignment of host LC molecules. Further, systematic investigation of the textural, dielectric, and conductivity properties of nanocomposites filled in planar cells is performed with increasing concentration of nanorods. At a nanorod concentration ≤0.2 wt % in 8CB, the order parameter of nanocomposite samples is found to be increasing and ionic conductivity is found to be decreasing as compared to pure LC. Beyond 0.3 wt % concentration of nanorods in 8CB, vertical alignment (VA) of host LC is observed even in a planar aligned cell. The VA of LC molecules in ZOR nanocomposites is confirmed through attenuated total reflection-Fourier transform infrared absorption spectra studies.

Mesomorphic, electro-optic and dielectric behavior of self-assembled nanocomposite materials: Nematic mixture doped with carbon coated cobalt nanoparticles

In the present study, different concentrations of carbon coated cobalt nanoparticles (0.25 wt% & 0.5 wt%) into eutectic nematic liquid crystal mixture (E7) have been dispersed and studied their mesomorphic, electro-optic and dielectric properties in planar as well as homeotropic aligned cells. Optical transmission method has been employed in planar aligned cell to study the electro-optic responses while dielectric spectroscopy in the frequency span 20 Hz-10 MHz has been performed in planar and homeotropic cells. Nearly 10°C increase in nematic-isotropic transition temperature is noticed at higher doping concentration. Dispersion of cobalt nanoparticles induces changes in dielectric permittivity, dielectric loss and relaxation frequency. Suppression of free ion concentration (in case of 0.5 wt% Co) suggests large dielectric anisotropy values. Entropy of the system is more or less affected with the nano inclusions but the free energy of the doped samples has been decreased. An improvement in threshold voltage, birefringence and order parameter by ∼ 10%, 15% & 5% respectively have been observed at low doping concentration. However, contrast ratio is reduced with the doping of cobalt nanoparticles. The present study shows that the doped systems have improved response than pure E7 and find potential applications in phase shifters as well as display devices.

Electrooptic, pyroelectric and dielectric spectroscopic studies of nematic and twist bend nematic phases of achiral hockey-shaped bent-core liquid crystal

Experimental and theoretical investigations of the twist-bend nematic (NTB) phase in liquid crystals have recently received significant attention for reasons of unusual and interesting characteristics of the NTB. Though NTB was predicted to exist for rigid bent-core LCs by Ivan Dozov in 2001, nevertheless it has unambiguously been proven to exist only in a few bent-core systems so far. Here the characteristics of both the nematic (N) and NTB phases are investigated for a hockey shaped achiral rigid bent-core LC, called BCI, in planar and homeotropic aligned cells, using polarizing optical microscopy, electro-optics, pyroelectricity and wide band dielectric spectroscopy. Spontaneous polarization measured for a bias field of 2.2 V/μm of a chiral domain using the pyroelectric effect is found to be only ∼5 nC/cm2. A measurement of the spontaneous polarization using pyroelectricity in general is unaffected by the sample’s dc conductivity except when its conductance dominates the admittance. As helix of NTB is partially unwound by the field, results lead to the identification of NTB as polar and chiral. The first and the second harmonics of the applied field are observed of higher amplitudes in NTB and N phases, respectively. The hockey-shaped bent-core system exhibits a large negative dielectric anisotropy over a wide range of temperatures than observed for any other compound so far. The large negative dielectric anisotropy is the highly desirable characteristic parameter of the NTB for exploring the field induced phases at relatively lower field strengths. The orientational order parameter characteristically jumps at the N-NTB transition temperature. The complex dielectric permittivity is measured as a function of frequency in the range 1 Hz to 10 MHz, the analysis of results reveales two collective modes in the dielectric spectra. Amplitude of the lower frequency mode is much higher than of higher frequency, former is assigned to the hydrodynamic mode q=qzz^ with z-dependent rotation of the heloconical director n^(r), this in turn involves compression and dilation of the pseudo-layers. The higher frequency mode corresponds to fluctuations of the tilt director and closer to the transition temperature, it exhibits a typical soft mode characteristic feature.

The Beauty of Twist-Bend Nematic Phase: Fast Switching Domains, First Order Fréedericksz Transition and a Hierarchy of Structures

The twist-bend nematic phase (NTB) exhibits a complicated hierarchy of structures responsible for several intriguing properties presented here. These are: the observation of a fast electrooptic response, the exhibition of a large electroclinic effect, and the observation of an unusual pattern of the temperature dependence of birefringence of bent-shaped bimesogens in parallel-rubbed planar-aligned cells. These unusual effects inspired the use of highly sophisticated techniques that led to the discovery of the twist-bend nematic phase. Results of the optical retardation of a parallel-rubbed planar-aligned cell show that the ‘heliconical angle’ (the angle the local director makes with the optical axis) starts increasing in the high temperature N phase, it exhibits a jump at the N–NTB transition temperature and continues to increase in magnitude with a further reduction in temperature. The liquid crystalline parallel-rubbed planar-aligned and twist-aligned cells in this phase exhibit fascinating phenomena such as a demonstration of the beautiful stripes and dependence of their periodicity on temperature. The Fréedericksz transition in the NTB phase is found to be of the first order both in rubbed planar and homeotropic-aligned cells, in contrast to the second order transition exhibited by a conventional nematic phase. This transition shows a significant hysteresis as well as an abrupt change in the orientation of the director as a function of the applied electric field. Hierarchical structures are revealed using the technique of polymer templating the structure of the liquid crystalline phase of interest, and imaging of the resulting structure by scanning electron microscopy.

P‐148: Investigation of photoluminescence and birefringence of nanoparticles dispersed nematic liquid crystal and its application towards liquid crystal display and optoelectronic devices

In the present investigation nanoparticles (NPs) have been dispersed into the pure nematic liquid crystal (NLC). NPs have been dispersed into the NLC with two different concentrations. Polarizing optical microscope (POD imnages have been recorded for pure NLC and NP dispersed system in the planar aligned liquid crystal cell. The dispersion of NPs in NLC increases the photoluminescence intensity which is an important finding of this study. Birefringence has also been measured here with the help of transmittance technique and it has been observed that the birefringence got increased after the dispersion of NPs into pure NLC. This is the key finding of the present reported work. Relative electric permittivity has also been measured here and found to be increased for the dispersed system as compared to the pure NLC. It is suggested that the output of the present work may be very useful in liquid crystal display and photonic applications.

Comprehensive studies on dielectric properties of p-methoxy benzylidene p-decyl aniline with function of temperature and frequency in planar geometry: A potential nematic liquid crystal for display devices

The dielectric properties of the nematic mesophase, p-methoxy benzylidene p-decyl aniline (MBDA), measured in planar geometry with a function of frequency and temperature are investigated in detail. The complex dielectric permittivity (ε′ and ε″) is also studied at a bias voltage of 10 V for planar aligned sample cell of nematic mesophase. The dielectric permittivity with bias voltage attains a higher (> 2 times) value than that without bias voltage at a temperature of 56 °C, which is due to the fact that the linking group of nematic molecules is internally interacted with an applied bias voltage. This is supported by observing an enhanced dielectric permittivity of nematic liquid crystal (LC) in the presence of bias voltage, which can be fully explained as the increasing of the corresponding dipole moment. The dielectric relaxation behaviors of nematic LC are also demonstrated for planar aligned sample cell. The remarkable results are observed that the relaxation frequency shifts into low frequency region with the increase of the bias voltage applied to the planar aligned sample cells. The dielectric relaxation spectra are fitted by Cole–Cole nonlinear curve fitting for nematic mesophase in order to determine the dielectric strength.

Impact of twisted alignment on the smectic layer structure of ferroelectric liquid crystal

The twisted aligned cell of ferroelectric liquid crystal is assessed through electro-optical and dielectric spectroscopy and a comparison is made with antiparallel planar aligned cells of the same thickness around 8 μm. The study has been carried in two types of twisted aligned samples, one is natural cooled sample in the heating chamber and another is slow cooled at the rate of 0.05 °C/min. The influential distinct results have been observed in natural cooled twisted cells. In natural cooled cells, the helicoidal structure at the surface and in the bulk of the cell is distinct and contributes separately to the response of the cell. On the other hand, at the same cooling condition in twisted cell, the smectic layers are in a twisted state resulting into a non-uniform helical structure, therefore, the partial helicoidal unwound structure at the surface and in the bulk of the cell are not observed contributing to the dielectric permittivity separately unlike in planar aligned cell. The twisted cell is found to induce the restoring force within the sample. However, the slow cooling cycles on the twisted cell changes to the strain free equilibrium state, whereas the behavior of the planar sample cell remains unchanged. The studies are significant for the display devices based on twisted alignment of FLC cells.

In-plane Switching Deformed Helix Ferroelectric Liquid Crystal Display Cells

In-plane electro-optical switching (IPS) is a natural feature of a conventional planar-aligned display cell based on the deformed helix ferroelectric liquid crystal effect (DHFLC-effect) with a sub-wavelength helix pitch, if the tilt angle is close to 40 degrees.

Dielectric response of electric-field distortions of the twist-bend nematic phase for LC dimers

Wide band dielectric spectroscopy of bent-shaped achiral liquid-crystal dimers 1″-n″-bis(4-cyanobiphenyl-4'-yl) n-alkanes (CBnCB n = 7, 9, 11) has been investigated in a frequency range 0.1 Hz-100 MHz using planar-aligned cells of sample thicknesses ranging from 2 to 10 (μm) over a temperature range that covers both nematic and twist bend nematic phases. Two peaks in the dielectric spectrum in the higher frequency range are assigned to the molecular relaxation processes. The peak at the highest frequency, ∼40 to 80 MHz, is assigned to an internal precessional rotation of a single unit of the dimer around the director. The mode in the next lower frequency range of 2-10 MHz is assigned to the spinning rotation of the dimer around its long axis. This involves fluctuations of the dipole moment of the bent-shaped conformation that is directed along its arrow direction of the bow shape formed by the dimer. The peak in the frequency range 100 kHz-1 MHz can be assigned to the collective fluctuations of the local director with reference to the helical axis of the NTB structure. The dependence of its frequency on temperature is reminiscent of the soft mode observed at the SmA* to SmC* phase transition. This result clearly corresponds to the electro-clinic effect-the response of the director to the applied electric field in an electro-optic experiment. The lowest frequency mode, observed in the frequency range of 0.1 Hz-100 Hz, is identified with the Goldstone mode. This mode is concerned with the long range azimuthal angle fluctuations of the local director. This leads to an alternating compression and expansion of the periodic structure of the NTB phase.

A modified Langevin-Debye model for investigating the electro-optic behaviour of de Vries smectic liquid crystals

ABSTRACTAn external electric field applied across a planar-aligned cell in Smectic A* phase of de Vries smectic liquid crystal induces director redistribution over a cone, resulting in a substantial increase in the birefringence and the apparent optical tilt angle. Such an electro-optic response is modelled by Shen et al. [Y. Shen et al., Phys. Rev. E 88, 062504 (2013)], who modified their previous hollow cone with a diffuse cone model by introducing the molecular distribution function limited over a range of tilt angles, that lie in between θmin and θmax. The limits in these two tilt angles are assumed to be temperature independent though the tilt angle in between the two values can be temperature dependent. However, the high resolution measurements of birefringence and the layer thickness indicate the presence of temperature dependent diffuse cone angle in SmA* phase.. In the proposed model, we replace θmin by θT, a temperature dependent fitting parameter and the change shows that a better fit of the experimental data to the model is obtained. We determine the temperature dependence of θmin and show that this angle increases as SmA* to SmC* phase transition temperature is approached.

Electro-thermally tunable reflective colors in a self-organized cholesteric helical superstructure

We propose to dynamically control the reflective color of a cholesteric liquid crystal (CLC) by electrically tuning the center wavelength (λc) of the bandgap. The CLC, sandwiched in a planar-aligned cell with indium–tin-oxide electrodes, possesses negative dielectric anisotropy and thermo-responsive spectral properties. The helix in the Grandjean planar state, which is subject to vertically applied voltage, should be undisturbed in that the long molecular axis is initially perpendicular to the direction of the electric field. Surprisingly, when the frequency of the applied voltage is higher than a critical value, λc of the CLC cell varies as a function of the voltage. The underlying mechanism is the voltage-induced temperature change through dielectric heating in the frequency regime of pseudo-dielectric relaxation, attributable to the significant equivalent resistance–capacitance circuit of the cell due to the use of electrode layers with finite conductance. The driving voltage enabling the tuning of λc in the entire visible spectrum is as low as 12 Vrms in a 5 μm thick cell at a frequency of 2 MHz. The proposed CLC cell, exhibiting a broad electrically tunable spectral range from the near-infrared to ultraviolet, holds great promise for developing tunable photonic devices such as multicolor reflectors, filters, and sensors.

Resonant x-ray scattering observation of transitional subphases during the electric-field-induced phase transition in a mixture of Se-containing chiral smectic liquid crystals.

Using resonant x-ray scattering techniques, transitional subphases during the electric-field-induced phase transition of a mixture of Se-containing chiral liquid crystals, 80% AS657 and 20% AS620, in a planar-aligned cell geometry were investigated, where the prototypical phase sequence SmC_{A}^{*}-SmC_{γ}^{*}-AF-SmC^{*} was observed; the transitional subphases were formed during the transition from the three-layer periodicity phase to the ferroelectric phase. In the lower-temperature range where the three-layer SmCγ^{*} phase appeared under the low electric field, nine- and six-layer subphases and a "streak" pattern appeared in sequence after the transition from the SmCγ^{*} phase with increasing applied electric field; the ferroelectric phase was realized. In the higher-temperature range where the four-layer AF phase appeared under a low electric field, the AF phase changed to a three-layer phase at the medium electric field. The twelve-, nine-, and six-layer subphases subsequently appeared in sequence, and finally the ferroelectric phase was generated with increasing electric field. The molecular arrangements of the field-induced subphases, especially the newly found nine-layer periodicity phase, was analyzed. The successive field-induced phase transition of the present results was compared with that of our previous results for pure Se-containing and Br-containing liquid crystals, and the relation to the three-layer ferrielectric phase was discussed.

Shear-enhanced diffraction from thin permanent gratings in dye-doped nematic liquid crystal cells

Permanent gratings are recorded in planar-aligned dye-doped nematic liquid crystal cells under visible light illumination. By increasing the irradiation intensity and exposure time, several diffraction orders of the recorded gratings are obtained in the Raman-Nath diffraction regime. By applying a dynamic transverse shear on one of the confining plates of the cell, an enhancement of the diffraction efficiency is achieved, which follows the period of the grating. By microscope inspection under static displacement of the upper plate, surface gratings formed by the dye adsorption are revealed in both the front and rear windows of the cell, indicating that the diffraction amplification originates from a coherent superposition of the diffracted orders when the gratings are displaced half a period. The effect provides self-matched amplification of diffraction with a simple cell, a single photo-inscription stage, and elementary displacement steps.

Tunable angular shearing interferometer based on wedged liquid crystal cells.

The concept of a liquid crystal wedge as a tunable angular shearing interferometer is introduced and demonstrated to combine both high stability and high tunability. Different wedges are fabricated from planar aligned nematic liquid crystal cells with thickness gradients. These wedges are shown to produce stable interferograms from the polarization interference between the ordinary and extraordinary waves propagating in different directions at the output of the cell. The fringe periods, ranging from 70μm to 1.25mm, can be precisely controlled by a low voltage. Despite the wedge-shaped structure, no inhomogeneity has been detected when the wedge is driven adiabatically and the interferograms are uniform over regions as large as 5×5 mm. Moreover, dynamical measurements show that the wedges behave as a succession of multiple cells with different thickness, giving rise to a moving front of stabilizing fringes when driven dynamically. All the observations show that the device is suitable for large beam size and tunable shearing interferometry, with attractive features for applications such as phase sensing, photoalignment or photolithography.

Development of ferroelectricity in the smectic phases of 4-cyanoresorcinol derived achiral bent-core liquid crystals with long terminal alkyl chains

Bent-core molecules can form numerous polar and symmetry broken liquid crystalline phases with fascinating properties. Here we report the characteristics of a previously unknown polar synclinic smectic phase, $\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{X}}$, found in the phase sequence of an achiral bent-core material with a 4-cyanoresorcinol bisbenzoate core, terminated by long linear alkyl chains ($n=18$) on both ends. This phase exists over a narrow range of temperatures and is sandwiched in between the random polar synclinic smectic phase ($\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{R}}$) and polar synclinic ferroelectric ($\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{F}}$) phase with ${\mathbit{P}}_{S}\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{nC}/\mathrm{c}{\mathrm{m}}^{2}$. In a planar-aligned cell it exhibits only chirality flipping on application of a conventional ac field but it also exhibits optical switching by rotation of the molecular directors on the tilt cone subjected to a modified sequence of bipolar pulses. This changeover is discussed in terms of the model given by Nakata et al. [Phys. Rev. Lett. 96, 067802 (2006)], involving a competition between the two forms of switching: the rotation around the long molecular axis and the switching through rotation of the molecular directors on the tilt cone. The model is modified to take account of the azimuthal pretilt and the molecular tilt angles. In addition to it, characteristics of $\mathrm{Sm}{C}_{\mathrm{S}}{P}_{\mathrm{R}}$ are also discussed.

Textural, thermal, optical and electrical properties of Iron nanoparticles dispersed 4′-(Hexyloxy)-4-biphenylcarbonitrile liquid crystal mixture

ABSTRACTIn this work, the effect of Iron nanoparticles (Fe NPs) dispersion in 4′-(Hexyloxy)-4-biphenylcarbonitrile (6OCB) nematic liquid crystal properties has been studied. Inclusion of Fe NPs (0.25 wt. %) in 6OCB liquid crystal (LC) on textures, isotropic–nematic transition temperature (TI–N), electro-optical and dielectric properties have been investigated in planar aligned cell. The threshold voltage (Vth) and TI–N decrease after dispersion of Fe NPs. Dielectric spectroscopy in nematic phase show that relaxation frequency (fr) also decreases after dispersion of Fe NPs in 6OCB. The observed relaxation mode is due to the flip-flop motion of LC molecules about their short axis. The band gap and AC conductivity in case of 6OCB-Fe sample increase over pure 6OCB sample. A decrease in activation energy is also noticed.

Textural and optical studies of magneto-mesogen material for display applications

ABSTRACTIn this work, a magneto-mesogen material has been prepared by nematic liquid crystal (6OCB) and magnetic nanoparticles (Fe). The effect of small amount of Fe doping on textural behaviour and optical responses has been investigated in a planar aligned cell in the nematic phase. The doped sample shows relatively higher transmission intensity than pure 6OCB and decreases a threshold voltage from 1.4 V to 1.0 V. The UV-Vis spectra show that after dispersion the absorbance peak shift from 319 nm to 315 nm. The measured band gap in 6OCB and Fe-6OCB composite was found 3.71 eV and 3.72 eV respectively.