Ferroelectric Liquid Crystal Material Research Articles

Multiferrocity in ferroelectric liquid crystal by incorporating core/shell CoFe2O4/ZnO nanocrystals

ABSTRACT Incorporation of magnetic core/shell CoFe2O4/ZnO (CFZ) nanocrystals (NCs) in ferroelectric liquid crystal (FLC) namely KCFLC 7S is studied. The core/shell nature of CFZ NCs causes effective confinement of charge carrier wavefunction into core that reduces possibility of charge leakage. The reduced charge leakage possibility in FLC retains ferroelectric nature of FLC material. The magnetic nature of CFZ NCs triggers magnetic interactions between NCs and FLC molecules and resulted into magnetic ordering. The presence of electric as well as magnetic ordering in FLC due to CFZ NCs forms novel multiferroic composite system. The response time of FLC is observed to reduce significantly by presence of CFZ NCs. Observed multiferroic property in NC-FLC composite would be beneficial for future display devices with unique capacity of transmission, storage and process of data.

Probing the impact of bent-core mesogen on the dielectric relaxations of ferroelectric liquid crystal

The modulation in dielectric relaxations of a conventional ferroelectric liquid crystal (FLC, SCE9) material by doping the bent-core mesogen (BCM) derived from the oxadiazole bisaniline moiety has been greatly investigated using frequency-temperature dependent dielectric spectroscopy. The application of probing ac voltage (i.e., oscillating voltage, 800 mV) has clearly shown the presence of partially unwound helical mode (p-UHM) along with Goldstone mode (GM). However, the temperature-dependent dielectric studies further demonstrate that p-UHM gets merged with GM at higher temperatures and hence becomes difficult to observe it separately. It appears that doping of BCM into FLC leads to the suppression of p-UHM at lower temperature, but with the increase in the concentration of dopant beyond a certain point, p-UHM mode reappears. Moreover, the significant decrement (∼9 °C) in SmC*-SmA* phase transition temperature of FLC with increase in the dopant BCM concentration up to 3 wt%. We believe that such studies would be certainly useful for the understanding of the plausible interaction between two different shaped mesogens (i.e., rod and bent-core) present in the mixture and could pave the way forward to the use of one type of mesogen in the modulation of the dielectric properties of other mesogen.

The Birefringence and Extinction Coefficient of Ferroelectric Liquid Crystals in the Terahertz Range

In this paper, the refractive index and extinction coefficient of ferroelectric liquid crystals have been examined by the terahertz time-domain spectroscopy system. Two modes of ferroelectric liquid crystal materials, deformed helix ferroelectric liquid crystal (DHFLC), and electric suppressed helix ferroelectric liquid crystal (ESHFLC) are tested as experimental samples. Nematic liquid crystal (NLC) was also investigated for comparison. The birefringence of DHFLC 587 slowly increases with the growth of frequency, and it averages at 0.115. Its extinction coefficients gradually incline to their stable states at 0.06 for o-wave and 0.04 for e-wave. The birefringence of ESHFLC FD4004N remains between around 0.165 and 0.175, and both of its e-wave and o-wave extinction coefficients are under 0.1, ranging from 0.05 to 0.09. These results of FLC will facilitate the examination and improve the response performance of THz devices using fast liquid crystal materials.

Smectic layers reorientation with stripes and heliconical structure in surface stabilised ferroelectric liquid crystal

ABSTRACT High-tilt angle ferroelectric liquid crystal material in surface stabilised geometry (SSFLC) is investigated for the smectic layer reorientation and its molecular dynamics by electro-optical and dielectric spectroscopy techniques. Smectic layer reorientation (5 o to 45 o ) is obtained by controlled cooling rate and applying electric field in the vicinity of transition temperature from chiral nematic ( N ∗ ) to chiral smectic C ( Sm C ∗ ) phase. After slow cooling down to room temperature, the stripe domain textures along the rubbing are observed with quasi-bistable states. Dielectric spectroscopy has revealed the unusual increment in the dielectric permittivity. A dielectric relaxation process at lower frequency than Goldstone mode with the increase in DC bias up to 2 V is observed in the sample cooled without any bias. This increment is found to be originated by the heliconical structure within the stripes which is seen in the optical textures recorded at the time of dielectric scan. However, this increment is not observed in the sample cooled under DC bias. Thus, there exist different phenomena in the high tilt angle SSFLC sample cooled in different conditions of DC bias; stripe domains, smectic layers reorientation, heliconical stricture and twisted structure within stripes which are significant to explore deeply in high-tilt angle ferroelectric liquid crystals.

Improved electro-optical properties of ferroelectric liquid crystal by incorporating alloyed quaternary ZnCuGaS2/ZnS core/shell quantum dots

ABSTRACT KCFLC 7S, a ferroelectric liquid crystal (FLC) shows improved dielectric and electro-optic properties by the incorporation of alloyed quaternary ZnCuGaS2/ZnS core/shell (ZCZ) quantum dots (QDs). Four different Zn alloyed ZCZ QDs (ZCZ-0, ZCZ-0.5, ZCZ-1, and ZCZ-1.5) incorporated in FLC and have been studied with the help of optical, dielectric and ferroelectric characterisations. Substantial changes of optical contrast, molecular ordering, photoluminescence, dielectric constant, dielectric loss factor, response time, and helical pitch of FLC are observed and attributed to varying molecular ordering. The effective confinement of exciton wavefunction in core of QDs results in improved molecular ordering of FLC material. The Zn alloyed ZCZ QDs incorporated FLC may be resulted for future brighter, high contrast, large viewing angle and memory based display device.

Design of a high-speed circular polarization converter with a large field of view and wavelength range.

A high-speed circular polarization converter (CPC) with a wide field of view (FOV) and wavelength range is designed and fabricated in this paper. The multi-waveplate combined structure is applied to constitute the basic configuration of the CPC for broadening the wavelength range. An electrically suppressed helix ferroelectric liquid crystal (ESHFLC) material with fast response is used as a medium for dynamic polarization operation. The compensation films are used to expand the FOV by attaching to the configuration. The simulation results demonstrate that the optimized CPC structure can achieve over 97% orthogonal circular polarization conversion efficiency in 300 nm bandwidth at a 90° viewing cone for both working states. Finally, we have experiments and the results show well consistency with the theoretical results.

Evidence of cholesterol crystallization along with smectic layers in ferroelectric liquid crystal

We report the crystallization of cholesterol (concentration in ethanol 100, 200, 300, and 400 mg/dl) in combination with ferroelectric liquid crystal (FLC) in the form of cholesterol needles. This has been investigated by dielectric spectroscopy, Raman spectroscopy, and texture analysis through optical micrographs. The foremost impression of adding cholesterol is observed with a reduction in the relative dielectric permittivity value and ferroelectric (chiral smectic C, SmC∗) to paraelectric (chiral smectic A, SmA∗) phase transition temperature with increasing concentration of cholesterol in FLC. In higher concentrations of 300 and 400 mg/dl, crystallized needles have been observed as a result of the supersaturation state of cholesterol in FLC matrix. Texture analysis has revealed that these needles are optically birefringent. Elongation of needles is found to be perpendicular to the FLC molecular director, and hence parallel to the smectic layers also. Temperature dependent investigation has shown that cholesterol-based needles disappear gradually with temperature and has not been observed in nematic and isotropic phases of the material. The results have confirmed that the lines are not only the supersaturation of cholesterol only but it is the composition of both cholesterol and FLC material. This is because when these needles disappear in SmA∗ phase the focal conic domains appear there. The understanding of the behavior of the cholesterol addition on the properties of FLC could be helpful in exploring the possibilities of using FLC as a sensing element in the biosensing of cholesterol and other molecules.

P-Terphenyl-containing symmetric tetraesters for nano-scale pitch ferroelectric liquid crystal materials

Design and studying of derivatives of fluorinated and non-fluorinated p-terphenyl-containing symmetric tetraesters as chiral components (CCs) of ferroelectric liquid crystal (FLC) materials working in deformed helix FLC (DHFLC) mode are described. The tetraesters combine a lactate and alcoholic residues in chiral tails. The new CCs induce short helix pitch (as short as 78 nm) and large spontaneous polarization (PS > 100 nC/cm2) at moderate concentrations (as low as 12 mol. %) in a non-chiral biphenylpyrimidine host. We have studied the effects of the (S)-lactate linker, terminal alkyl chains and fluorine atoms on helix pitch and other useful properties of the chiral tilted smectic (SmC*) phase induced by these CCs in the biphenylpyrimidine host. Depending on positions of the fluorine atoms, the fluorination of the p-terphenyl core shows little effect or even reduces the helical twisting power (HTP) of the CCs. At the same time, modifications in the chiral tails can essentially enhance the HTP and other CC performance characteristics. Thus, the combination of the (S)-lactate linker with a (S)-alkan-2-olic residue results in high HTP (up to 85 μm−1) even without fluorine atoms in the chiral tails. Substitution of the (S)-alkan-2-olic residue with a fluorinated analogue, viz. (R)-1,1,1-trifluoroalkan-2-ol derivative, provides further increase of the HTP in the induced SmC* phase up to the highest known to date value of 101 μm−1.

Luminance and chrominance parameters through image analysis in ferroelectric liquid crystals

ABSTRACT In this paper, luminance and chrominance characteristics of Chiral Ferro electric liquid crystals (FLCs): (S)-(-)-2-methylbutyl 4’-(4”-n-alkanoyloxybenzoyloxy) biphenyl-4-carboxylates (S-MB-nB-BC) where n = 16 and 18, are investigated. In optical applications of liquid crystal material, luminance and chrominance are the important characteristics and provide the information about the usage of material especially in displays. Here, ferroelectric liquid crystals' display properties based on their characteristics of luminance and chrominance are investigated using the image analysis technique. Optical anisotropy properties of the ferroelectric liquid crystal material are also computed since birefringence is an important parameter of liquid crystal material in all applications. Therefore, this investigation involves the measurement of luminance properties: luminance uniformity, luminance coefficient, luminance distribution and contrast sensitivity; and chrominance properties: chromacity coordinates, colour histogram and colour difference and optical anisotropic properties: birefringence and transient transmission. On the MATLAB platform, these properties are computed from the grey-level intensity values of the textures as a function of temperature and wavelength (red, green and blue) since chrominance and optical anisotropy are wavelength-dependent.

Electro-optical and dielectric characteristics of highly tilted ferroelectric liquid crystal material 4'-(3-pentanoyloxyprop-1-oxy)biphenyl-4-yl (S)-(+)-4-(1-methylheptyloxy) benzoate

The advancement on ferroelectric liquid crystals and their novel appliance is just changing from being largely focused on display devices and fast optical shutter elements to several other fields such as spatial light modulator, optical sensor, polarisation grating, photonic and optical devices. In the present work, we report the thermodynamical, textural, electrical and switching behaviour of a highly tilted FLC material. The studies have been performed on a 5 μm thick planar oriented sample. The above material possess cholesteric (N*), chiral smectic C* (SmC*) and hexatic (SmB*) phases in cooling cycle as confirmed by optical, thermodynamic and dielectric studies. The spontaneous polarisation is highly temperature dependent and increases while cooling in the SmC* phase. The maximum value of spontaneous polarisation is ∼117 nC cm−2 at 76 °C. The switching time is of the order of a few milli seconds (∼1.3 ms). The conductivity (ionic and dipolar both) of the material has also been determined in all the above exhibited phases.

Alignment layer and helix controlled unconventional operational switching in ferroelectric liquid crystal

Substrate interface driven helical unwinding led dynamical switching is observed and analysed for the low voltage operation switching of ferroelectric liquid crystal (FLC). The surface driven FLC molecules force the bulk molecules to switch the sample cell device at very low voltage as compared to conventional cells. It has been shown in the present investigation using three FLC materials that the surface anchoring of the FLC molecules and the thickness of the cell play a primary role for such devices because these conditions contrive the surface interface particle helical unwinding which is very essential for the low power switching. However, P s, helical pitch value, and the dielectric strength parameters of the FLC materials are also taken into account for the analysis of bulk switching assisted by interfacial switching. Dielectric and electro-optics investigation has proven that once the relaxation process associated with the interface is confirmed by dielectric spectra then only, the low power operation is observed in the sample which is interesting for fundamental and potential electro optical devices application.

Probing the effect of chiral dopant fluorination on dielectric and electro-optical properties of the ferroelectric liquid crystalline mixture

In this article, fluorinated chiral dopant-assisted tuning of dielectric and electro-optical properties of a ferroelectric liquid crystal (FLC) material is presented. Five new FLC mixtures with room temperature ferroelectric phase were formulated by fluorinating the phenyl core of chiral dopant at different positions and with a different number of fluorine atoms. The dielectric and electro-optical properties of all the FLC mixtures were systematically studied. It is found that the permittivity value of ferroelectric mesogens can be tuned between 5 and 36 (at 100 Hz) with fluorination. Also, the dielectric loss factor decreases owing to fluorination of chiral dopant with a shift in phason mode relaxation towards the low-frequency side. In addition, there is a significant change in material parameters of FLC, such as phase transition temperature, response time, spontaneous polarization, and rotational viscosity due to fluorination in chiral dopant. The presence of highly electronegative fluorine acts as a source of potentially repulsive force. It partially screens the molecular chirality of the dopant, which resulted in a change in the material parameters of FLCs. Interestingly, there is no considerable change in the mesogenic structure and orientation with the addition of fluorine, which is confirmed through wide-angle X-ray scattering. These studies open up a new avenue for tailoring the dielectric and electro-optical properties of mesogens for advanced device applications based on FLCs.

P‐12.4: Fast‐response LCD Based on a Liquid Crystal Material Doped with Modified Nanoclay

The object of study is nematic and ferroelectric liquid crystals containing chemically modified nanoclay based on montmorillonite. The aim of the work is to develop new composite materials with improved physico‐chemical and electro‐optical properties. During the study, chemical methods were developed for modification of the surface of nanoclay; the mesomorphic, dielectric, and electro‐optical properties of nematic and ferroelectric compositions doped with a nanoclay with a modified surface are studied. Based on experimental data, the regularities of the influence of functional groups grafted onto the surface of nanoclay on the mesomorphic, dielectric and electro‐optical properties of nematic and ferroelectric compositions were established. It has been shown experimentally that the addition of a small amount of nanoclay to a nematic and ferroelectric liquid crystal material can significantly improve the electro‐optical response time and reduce the threshold and saturation voltage values. For ferroelectric liquid crystals, the addition of nanoclay leads to an increase in the tilt angle in the smectic layer and an increase in spontaneous polarization.

Fast-switchable, high diffraction-efficiency ferroelectric liquid crystal Fibonacci grating.

Low-voltage fast switchable 1D and 2D Fibonacci grating (FbG), using an electrically suppressed helix ferroelectric liquid crystal (ESHFLC), with high diffraction efficiency for a super-resolution imaging system in far-field are disclosed in this paper. Specifically, the polarization-independent two-domain (0, π) structure is well designed based on photoalignment technology to maximize the total diffraction efficiency that can reach 97.4% (1st order:8.5%, 2nd order: 30%). Apart from that, the FLC gratings offer two tunable states: non-diffractive and diffractive states. Derived from the fast-response property of ferroelectric liquid crystal material, the switching speed of the 1D and 2D ESHFLC-FbG is 103µs at 4 V of the driving voltage. Furthermore, this system achieves the high-resolving power of (λ/2.25) for object detection based upon the intensity map received behind 1D ESHFLC-FbG at far-field. Contribution from the quasi-periodic FbG's special ability to translate the super-resolution information (including at evanescent wave) into the detectable far-field region. Concisely, the proposed ESHFLC-FbG can be a promising candidate for a super-resolution imaging system, superstructure fibre sensor, and other photonic applications.

Modification in different physical parameters of orthoconic antiferroelectric liquid crystal mixture via the dispersion of hexanethiol capped silver nanoparticles

Herein we investigate an orthoconic anti ferroelectric liquid crystal (OAFLC) material, W1000 and its mixtures with the dispersion of hexanethiol capped silver nanoparticles (Ag NPs) in three different concentrations of 0.1 wt%, 0.25 wt% and 0.50 wt%. The pristine AFLC and its mixtures are characterized by the dielectric, electro-optical and spectroscopic techniques. Various parameters viz. dielectric permittivity, electrical conductivity, spontaneous polarization, rotational viscosity, response time, anchoring energy coefficients, and photoluminescence of the pristine AFLC are altered due to the addition of Ag NPs. The polarizing optical micrographs show modification in the molecular distribution of the host AFLC after the addition of Ag NPs. The spontaneous polarization of the Ag NPs dispersed system was reduced due to the anti-parallel correlation amid dopant and AFLC dipoles as a consequence the dielectric permittivity of the Ag NPs-AFLC mixtures has also been decreased. The electrical conductivity of the pristine AFLC has increased after the dispersion of Ag NPs. Faster switching response of Ag NPs dispersed system comparing to that of the pristine AFLC is manifested by the decrement in the rotational viscosity. The anchoring energy coefficients were found to be reduced for the Ag NPs dispersed system. The photoluminescence (PL) intensity of the pristine AFLC material has been enhanced after the dispersion of Ag NPs which strongly depends upon the dopant concentration. Faster switching response, enhanced electrical conductivity and PL Intensity of the Ag NPs-AFLC mixtures are the remarkable findings of this study.

Characterization and applications of a newly formulated ferroelectric liquid crystal mixture W315

In this article, we present the formulation, characterization, and application of a newly prepared ferroelectric liquid crystal (FLC) mixture namely W315 composed of pyrimidine compounds. The characterization of FLC material under investigation has been performed through polarizing optical microscopic, dielectric spectroscopic, differential scanning calorimetric and other electro-optical methods. The temperature dependence of the dielectric loss factor of the homogeneously aligned FLC sample shows a phase change from Smectic C* to Smectic A that further confirms the transition temperature of FLC material. From electro-optical observations, the values of spontaneous polarization, rotational viscosity, and response time of W315 were found to be 8.03 nC/cm2, 86.56 mPa s, and 890 µs respectively. The material constants of FLC material under investigation can be utilized in various FLC based faster displays and non-display devices.

Nanoparticle surface energy transfer (NSET) in ferroelectric liquid crystal-metallic-silver nanoparticle composites: Effect of dopant concentration on NSET parameters.

In the recent past, the resonance energy transfer studies using metallic nanoparticles has become a matter of quintessence in modern technology, which considerably extends its applications in probing specific biological and chemical processes. In the present study, metallic-silver nanoparticles of 2-4 nm (diameter) capped with hexanethiol ligand are developed and dispersed in ferroelectric liquid crystal (FLC). The morphology of nanoparticles was characterized using HR-TEM and SEM techniques. Furthermore, a systematic study of energy transfer between the host FLC material (as donors) and metallic-silver nanoparticles (as acceptors) has been explored employing steady state and time resolved fluorescence spectroscopic techniques. The nanoparticle based surface energy transfer (NSET) parameters viz., transfer efficiency, transfer rate, and proximity distance between donor and acceptor, have been determined for NSET couples (FLC material-metallic-silver nanoparticle) composites. It is observed that various NSET parameters and quenching efficiency follow a linear dependence on the concentration of metallic-silver nanoparticles in host FLC material. The nonradiative energy transfer and superquenching effect were analyzed with the help of Stern-Volmer plots. The impact of present study about superquenching effect of the silver nanoparticles can be used for sensing applications that require high degree sensitivity.

Physico-chemical and electro-optical properties of liquid crystals doped with chemically modified nanocline minerals

The object of study is nematic and smectic (ferroelectric) liquid crystals containing chemically modified nanoclay based on montmorillonite. The aim of the work is to develop new composite materials with improved physico-chemical and electro-optical properties. During the study, chemical methods were developed for modification of the surface of nanoclay; the mesomorphic, dielectric, and electro-optical properties of nematic and ferroelectric compositions doped with a nanoclay with a modified surface are studied. On the basis of experimental data, the regularities of the influence of functional groups grafted onto the surface of nanoclay on the mesomorphic, dielectric and electro-optical properties of nematic and ferroelectric compositions are established. It has been shown experimentally that the addition of a small amount of nanoclay to a nematic and ferroelectric liquid crystal material can significantly improve the electro-optical response time and reduce the threshold and saturation voltage values. For ferroelectric liquid crystals, the addition of nanoclay leads to an increase in the tilt angle in the layer and an increase in spontaneous polarization.

Correlation between alignment geometries and memory effect in a surface-stabilized ferroelectric liquid crystal.

Memory effect in weakly aligned surface stabilized ferroelectric liquid crystal (SSFLC) material has been investigated by electro-optical and dielectric spectroscopy in three configurations of alignment: antiparallel, 90^{∘} twisted, and unaligned planar samples. It has been observed that two types of molecular dynamics exist in antiparallel rubbed cell in which memory effect is observed for longer duration than in other samples. One dielectric relaxation process is near the surface of the electrode and a second is in the bulk of the SSFLC. Both the molecular dynamics contribute in the switching process and affect the memory phenomenon in surface stabilized geometries. However, a single dielectric process is observed in twisted geometry in which the sample is showing shorter memory effect than in antiparallel and this is compared with unaligned samples also having cell thickness less than the pitch value of FLC. In an unaligned sample, a single dielectric process is observed and the smaple does not show memory effect at all. The investigation is significant to understand the anomalies occurring in memory observations in various geometries.

Dielectric and electro-optical studies of Au/SnO2 core/shell nanocrystals incorporated ferroelectric liquid crystal

ABSTRACT We study the effect of Au/SnO2 core/shell nanocrystals (NCs) on dielectric and electro-optical parameters of a ferroelectric liquid crystal (FLC) material, namely KCFLC 7S. The FLC composites are characterised through polarising optical microscope (POM), photoluminescence (PL) spectroscopy and dielectric spectroscopy. Incorporation of Au/SnO2 NCs enhances the optical contrast, optical tilt angle, PL intensity and reduces saturation voltage of a FLC mixture. This modification in electrical and optical properties is attributed to change in dynamics of FLC molecules in presence of the NCs. Dielectric measurement shows that chiral Smectic C (SmC*) phase of FLC is dominated by an applied oscillating field dependent partially unwound helical mode (p-UHM) process in FLC composite with Au/SnO2 NCs. These results would be useful to fabricate power efficient and highly luminescent NCs-FLC-based electro-optical devices.