Ferroelectric Liquid Crystal Material Research Articles

Manifestation of strong magneto-electric dipolar coupling in ferromagnetic nanoparticles−FLC composite: evaluation of time-dependent memory effect

ABSTRACTRod-shaped 5 wt.% copper-doped ZnO (ZnO:Cu2+) ferromagnetic nanoparticles (NPs), prepared by hydrothermal method, were dispersed in ferroelectric liquid crystal (FLC) named Felix 17/100. The effect of ferromagnetic NPs on the physical properties of FLC material (Felix 17/100) has been investigated by dielectric, electro-optical and polarising optical microscopic methods. A noteworthy time-dependent memory has been observed in the NPs-dispersed FLC composite attributed to the coupling of magnetic field associated to NPs with the director orientation of FLC. Improvement in spontaneous polarisation and dielectric susceptibility of FLC material has been ensued with the addition of ferromagnetic NPs. Faster electro-optic response, at lower applied voltage, has also been observed in NPs-dispersed FLC composite. These changes are accredited to the magneto-electric dipolar coupling existing due to the interactions between magnetic-dipole and electric-dipole moments of magnetic NPs and FLC material, respectively. The formation of periodic domains capable to show memory effect has been observed in composite. The observed time-dependent memory was confirmed by dielectric and electro-optical methods. FLC material enriched with the properties of ferromagnetic NPs can be utilised in advanced multifunctional optical devices, time-dependent memory-based security devices and computational purposes.

CdTe quantum dot dispersed ferroelectric liquid crystal: Transient memory with faster optical response and quenching of photoluminescence

In the present study, the effect of dispersing Octadecylamine capped Cadmium Telluride quantum dots (CdTe QDs) on the memory behaviour, physical parameters and dielectric relaxations of ferroelectric liquid crystal (FLC) W-327 with the variation of dopant concentration and applied voltage has been examined. Significant changes have been observed in visible light absorbance, photoluminescence (PL), spontaneous polarization, optical response and relative permittivity of the FLC material with the addition of QDs. An average of 19% (Mix.1) and 28% (Mix.2) fastening of optical response in QDs dispersed FLC mixtures was one of the remarkable findings. Enhancement in visible absorbance along with the quenching in PL intensity has also been noticed in FLC/QD mixtures in comparison to pure FLC. The study of tan δ curves confirms the reduction of ionic impurities in the presence of QDs. Pure FLC as well as FLC/QD mixtures show time dependent memory effects. The memory behaviour of mixtures significantly depends upon the concentration of dopant CdTe. The memory effect was confirmed with the help of dielectric spectroscopy, polarizing optical micrographs and electro-optical study. The observed memory effect has been attributed to minimization of the depolarization field and ionic charges. The QDs dispersed FLC plays a major role to enhance memory effects by trapping the impurity ions under the application of bias voltage. These studies would be helpful to provide an idea for ionic impurity free memory devices having micro-second optical response and in security coded transmission.

A comparison between size dependent paraelectric and ferroelectric BaTiO3 nanoparticle doped nematic and ferroelectric liquid crystals

The electro-optic and dielectric properties of barium titanate, BaTiO3, and nanoparticle (NP) doped nematic and ferroelectric liquid crystal materials were examined with respect to different size and concentrations of the NPs. Smaller size paraelectric NPs (≈80 nm) are compared to larger, ferroelectric NPs (≈240 nm). It is found that for concentrations larger than 0.5 vol. %, the ferroelectric NPs exhibit an increasing effect on the electro-optic response of the nematic liquid crystal, which is demonstrated by the enhancement of the dielectric anisotropy. This could be induced by the coupling of the electrical dipole moments in the spherical NPs with the LC director field. The electro-optical properties of the SmC* phase, such as the tilt angle Θ, switching time τs, and spontaneous polarisation Ps, are found to be independent of the concentration and size of the NPs. The rotational viscosity η calculated from response times, polarization reversal current, and dielectric properties is comparable for all three methods employed and practically independent of particle concentration and size. The relaxation frequency fR of the Goldstone mode is higher in the ferroelectric NPs suspensions of 2.0 vol. % as compared to the smaller paraelectric NPs.

Observation of macro-heterogeneities in surface-stabilized smectic C* with antagonistically patterned substrates

In our recent study, we theoretically considered equilibrium states in a monolayered sample of ferroelectric liquid crystal in smectic C* phase, confined between two differently patterned substrates with strong anchoring under the applied electric field. By using the continuum theory for a “bookshelf” aligned sample for smectic C*, we derived elliptic sine-Gordon equation. The solution of the Dirichlet problem has shown that due to the antagonistic boundary conditions at the substrates, competing boundary effects in the thin film generate a stable alignment of smectic C* director. This alignment can be controlled by the electric field. Our theoretical finding was described as the system with harmonically coupled “atoms” with external potential within the framework of the Frenkel-Kontorova model. To challenge our finding, we prepared a cell with differently patterned substrates, and ferroelectric liquid crystal material CS-1024 (Chisso Co.) was injected into the empty cell by the capillarity flow. Patterned monolayers exhibiting planar alignment of CS-1024 in its smectic C* phase were created using microcontact printing of functionalized organothiols on gold films. By patterning the surface with planar alignment of monolayers, the location and formation of smectic C* director macro-heterogeneities can be controlled by the electric field. The observed macro-heterogeneities continue to exist when the electric field is turned off. Polarizing microscopy and fluorescent microscopy were used to observe the formation of macro-heterogeneities in the alignment of SmC* director field.

ZnS quantum dot induced phase transitional changes and enhanced ferroelectric mesophase in QDs/FLC composites

In the present study, we report the dielectric and electro – optical (E – O) study of ZnS quantum dots (QDs) dispersed ferroelectric liquid crystal (FLC) material. Change in the SmC*- SmA phase transition temperature has been investigated by the thermal study. Width of SmC* phase is found to be slightly increased due to the dispersion of ZnS QDs, which has also been observed in the dielectric and E – O study of composites. Fitting of spontaneous polarization curves on the temperature scale has been done theoretically to obtain the change in SmC*- SmA phase transition temperature. A significant modification in the FLC material parameters (like spontaneous polarization, optical response time, tilt angle and rotational viscosity) has been observed after the dispersion of QDs. These modifications are the consequences of the strong dipolar interaction between the FLC molecule and QDs. Significant fastening of the optical response time for low conc. of QDs dispersed FLC composite shows its utilization in advanced display devices.

Nanoparticles induced multiferroicity in liquid crystal

Soft multiferroic character has been observed in a ferroelectric liquid crystal (FLC) dispersed with nickel nanoparticles (Ni NPs). A suitable amount of ferromagnetic Ni NPs has been added into FLC material, and the co-existence of ferroelectric and ferromagnetic ordering is examined using P-E and M-H hysteresis measurements. The magnitude of ferromagnetic order is found to depend strongly on the concentration of Ni NPs. Our theoretical approach indicated a strong dependence of helical pitch of FLC on the doping concentration of Ni NPs. We proposed that the intrinsic magnetic field of Ni NPs has been coupled with that of director field of the FLC molecules to result in the observed multiferroic behavior.

Core/shell quantum dots in ferroelectric liquid crystals matrix: effect of spontaneous polarisation coupling with dopant

ABSTRACTIn order to fabricate efficient and superior performance liquid crystal (LC) devices, the physical parameters of the LC mesogens can be duly altered by incorporating non-mesogenic materials like quantum dots (QDs), graphene and polymers. In the present work, the effect of adding core/shell QDs in two ferroelectric liquid crystals (FLCs), along with the change in their physical properties, has been investigated. A small concentration of QDs is dispersed into the two FLCs and temperature variations of vital parameters like spontaneous polarisation (Ps), rotational viscosity, response time, relative permittivity and relaxation strength have been measured for both the FLC materials. The contrast ratio, UV–near visible absorbance as well as photoluminescence (PL) of both the mesogens have also been determined and compared. A faster electro-optical response and the induced phenomenon of PL with a temperature-dependent low-frequency relaxation mode have been observed in Felix 17/100 after the addition of QDs. The present study also provides valuable information about the interaction between QDs and the two FLC systems depending upon polarisation–field (P–E) coupling. The same dopant can interact with FLCs in dissimilar fashion if the intrinsic properties of both the FLCs are different thereby producing different modifications in their respective physical parameters.

Mn2+ doped ZnS quantum dots in ferroelectric liquid crystal matrix: Analysis of new relaxation phenomenon, faster optical response, and concentration dependent quenching in photoluminescence

Phase transitional, dielectric, electro-optical, polarizing optical microscopic, photoluminescence (PL), and Fourier transformed infrared (FTIR) spectroscopic measurements have been carried out on ZnS:Mn quantum dots (QDs) dispersed ferroelectric liquid crystal (FLC). A new dielectric relaxation mode has been envisaged in FLC material due to the presence of 0.25 wt. % ZnS:Mn (40 mol. %) QDs. The characteristics of the new mode have been compared with those of the soft mode. A significant fastening of the electro-optical response (∼75%) has been observed in the case of 0.25 wt. % ZnS:Mn (20 mol. %) QDs doped FLC material. The induction of the new relaxation mode is attributed to the flexoelectric tilt fluctuations. The induced flexoelectric polarization in the FLC medium at the vicinity of QDs might be responsible for the enhanced spontaneous polarization in the FLC/QDs mixtures. Quenching in PL for the FLC/QDs mixtures has been observed, which strongly depends on Mn content in QDs. The change in FTIR spectra for the FLC/QDs composite clearly indicates the change in molecular dynamics of the pure FLC after the dispersion of QDs. The presented results will certainly pave a way to utilize QDs for obtaining faster response of the FLC material and potential material to tune the relaxation processes.

Quenching of photoluminescence and enhanced contrast of ferroelectric liquid crystal dispersed with Cd1−XZnXS/ZnS core/shell nanocrystals

We report the dispersion of Cd1−xZnxS/ZnS core/shell nanocrystals (NCs) in ferroelectric liquid crystal (FLC) into different concentrations. Absorbance of white light, photoluminescence (PL), contrast, polarizing optical microscopy and Fourier transformed infrared spectroscopy (FTIR) have been performed to disclose the nature of FLC/NCs composite. The presence of NCs, in different concentration, quenches the PL of FLC material. The mechanism of quenching is explained by the modified Stern–Volmer quenching equation. The maximum contrast for FLC/NCs was found at 0.25wt% of dopant concentration which is the optimized concentration of NCs in FLC from application point of view. Contrast was further supported by polarizing optical micrographs (POMs). The change in POM also indicates the change in molecular alignment of FLC owing to concentration of NCs which also affect the optical properties of composites. FTIR spectra reveal that the NCs strongly interact with FLC molecules and changes their molecular dynamics associated with the central core and aliphatic chain. The present FLC/NCs composite has potential application in ultraviolet to visible light conversion, ultraviolet light storage applications and high contrast PL displays.

Role of cell thickness in tailoring the dielectric and electro-optical parameters of ferroelectric liquid crystals

Here, we report thickness dependence of dielectric and electro-optical parameters in ferroelectric liquid crystals (FLCs) without surface stabilisation. The dependence of dielectric and electro-optical parameters on cell thickness is observed by dielectric spectroscopy and electro-optical measurements. The dielectric permittivity () measured by varying the cell thickness showed increase of with increase of cell thickness which is attributed to the presence of more ions and larger contributions of Goldstone mode in thick cells. The spontaneous polarisation also shows increment with increase of cell gap up to certain thickness range. The rotational viscosity decreases with increase in the cell thickness whereas the response time is more for thicker cells. The decrease in the rotational viscosity is attributed to lowering of elastic deformation with increase in cell thickness and the response time is directly proportional to cell gap. These studies would be utilised to understand the effect of cell thickness on dielectric and electro-optical properties of FLC materials and optimising the material parameters with cell thickness for better and efficient liquid-crystal-based devices.

Analysis of physical parameters and collective dielectric relaxations in core/shell quantum dot ferroelectric liquid crystal composite

The effect of doping of Cd1−xZnxS/ZnS core/shell quantum dots (QDs) on the physical parameters and dielectric relaxations of ferroelectric liquid crystal (FLC) with the variation of dopant concentration, temperature and frequency has been examined in the present study. Fourier transformed infrared spectroscopy reveals that the QDs interact with FLC molecules and affect their molecular dynamics. The UV absorbance of pure FLC material has diminished in the presence of QDs according to the dopant concentration in FLC. A substantial change has been observed in tilt angle, spontaneous polarization and relative permittivity with the addition of QDs. A faster optical response for FLC, when doped with 0.25wt.% quantum dots, is one of the interesting finding of the present study. In the SmC⁎phase, a low frequency relaxation mode at 20Hz along with the characteristic Goldstone mode has been observed. The occurrence of new low frequency relaxation mode is accredited to the ionization–recombination assisted diffusion of slow ions present in the FLC material. These results would certainly be helpful in the fabrication of low charge consumable and faster liquid crystal systems.

Switching Operation in Tunable Add-Drop Multiplexer With Si-Grating Waveguides Featuring Ferroelectric Liquid Crystal Cladding

We propose a tunable add-drop multiplexer in a silicon waveguide Mach-Zehnder interferometer that has a sampled grating structure and ferroelectric liquid crystal (FLC) cladding, which we fabricated. The effective coupling coefficient that was decreased by the sample grating structure led to a narrow stop bandwidth in the grating waveguide. The FLC material caused a change in the Bragg conditions for lightwave propagation in the waveguide according to the direction of the electric field applied to FLC. The switching operation of the fabricated device, which separated the input and reflected signals without a circulator, was successfully demonstrated. The time response, dependence on voltage, and memory operation of the fabricated device based on FLC characteristics are also reported.

Enhancement of electro-optical response of ferroelectric liquid crystal: the role of graphene quantum dots

We present a pioneer study depicting a significant improvement in the photoluminescence (PL) intensity and display of an expeditious electro-optic response of ferroelectric liquid crystal (FLC) when doped with graphene quantum dots (GQDs). Significant threefold enhancement in PL intensity of GQDs/FLC composite material can be ascribed to the additive combination of emissions from GQDs and FLCs. Furthermore, promptness in electro-optical response by a factor of 34% can be attributed to the lowering of rotational viscosity of the FLC material due to the incorporation of GQDs. These results would certainly be helpful in realisation of highly luminescent and faster generation of LC systems.

Dielectric, electro-optical, and photoluminescence characteristics of ferroelectric liquid crystals on a graphene-coated indium tin oxide substrate.

Multilayer graphene was deposited on indium tin oxide (ITO) -coated glass plates and characterized by suitable techniques. A liquid crystal sample cell was designed using graphene deposited ITO glass plates without any additional treatment for alignment. Ferroelectric liquid crystal (FLC) material was filled in the sample cell. The effect of multilayer graphene on the characteristics of FLC material was investigated. The extremely high relative permittivity of pristine graphene and charge transfer between graphene and FLC material were consequences of the enormous increase in relative permittivity for the graphene-FLC (GFLC) system as compared to pure FLC. The presence of multilayer graphene suppresses the ionic impurities, comprised in the FLC material at lower frequencies. The ionic charge annihilation mechanism might be responsible for the reduction of ionic impurities. The presence of graphene reduces the net ferroelectricity and results in a change in the spontaneous polarization of pure FLC. Rotational viscosity of the GFLC system also decreases due to the strong π-π interaction between the FLC molecule and multilayer graphene. The photoluminescence of the GFLC system is blueshifted as compared to pure FLC, which is due to the coupling of energy released in the process of charge annihilation and photon emission.

Probing the effect of temperature and electric field on the low frequency dielectric relaxation in a ferroelectric liquid crystal mesogen

We present the characterization and dielectric spectroscopic investigations of a ferroelectric liquid crystal (FLC) material, namely KCFLC 10S. We experimentally demonstrate the observation of a low frequency dielectric relaxation mode along with Goldstone mode in the smectic C* phase of the FLC material under investigation. The behavior of low frequency dielectric relaxation mode has been systematically studied with temperature and applied bias field. The relaxation frequency of the low frequency dielectric relaxation has been found to be shifted toward the higher frequency side with an increase in temperature and field strength. This shift has been attributed to the single particle diffusion of fast ions. Further investigations on electrical conductivity (σ) with frequency at different electric field strengths revealed a Schottky-type of charge injection at electrode even under a small DC bias. The studies carried out in the present paper would be helpful to provide ideas for designing and developing advanced liquid crystal materials.

Formation of periodic domains and change in physical properties of paramagnetic copper doped ZnO nanoparticle dispersed ferroelectric liquid crystal system

Copper doped paramagnetic ZnO nanoparticles (NPs) were dispersed in ferroelectric liquid crystal (FLC) matrix. The dielectric spectroscopy was performed by using the Cole–Cole equation. The experimental data was corrected for the low and high frequency limitations attributed to the ionic conductance and ITO resistance respectively. The presence of paramagnetic NPs reduces the net ferroelectricity of the pure FLC material which causes the lowering of relative permittivity for the NP dispersed FLC system as compared to pure FLC. NP assisted domains, having specific period and inclination, were also observed in the NP dispersed FLC system. These domains also contribute in the lowering of spontaneous polarization of the dispersed system. The domain formation also affects the response time and rotational viscosity of the pristine FLC material. The mobile ionic carriers, present in the FLC material, have been trapped on the surface of the NPs. The trapping of ionic carriers was verified by the a.c. conductivity measurement. The change in physical parameters of the pure FLC material due to the dispersion of NPs has been analyzed and explained in the present investigation.

Effect of cadmium selenide quantum dots on the dielectric and physical parameters of ferroelectric liquid crystal

The effect of cadmium selenide quantum dots (CdSe QDs) on the dielectric relaxation and material constants of a ferroelectric liquid crystal (FLC) has been investigated. Along with the characteristic Goldstone mode, a new relaxation mode has been induced in the FLC material due to the presence of CdSe QDs. This new relaxation mode is strongly dependent on the concentration of CdSe QDs but is found to be independent of the external bias voltage and temperature. The material constants have also been modified remarkably due to the presence of CdSe QDs. The appearance of this new relaxation phenomenon has been attributed to the concentration dependent interaction between CdSe QDs and FLC molecules.

Electro-optical, UV absorbance, and UV photoluminescence analysis of Se95In5 chalcogenide glass microparticle doped ferroelectric liquid crystal

The dandelion like Se95In5 chalcogenide glass microparticle (CGMPs) doped ferroelectric liquid crystal (FLC) has been investigated. The electro-optical parameters of the pure and doped FLC were carried out as a function of applied voltage. The experimental response time and polarization curves for the Se95In5 CGMPs doped FLC have also been theoretically fitted. The presence of Se95In5 CGMPs affects the molecular dynamics of the FLC molecules, which was proved by the Fourier transformed infrared spectroscopy. The UV absorbance of the pure FLC material has been enhanced in the presence of CGMPs, which is analogous to the coupling between phonons of CGMPs and radiation field. The photoluminescence (PL) of the pure FLC has also been enhanced and blue shifted with the addition of Se95In5 CGMPs. The enhanced PL is attributed to the constructive interaction between low energy phonons of the CGMPs and incident photons of the monochromatic light in the LC medium. The blue shifting of PL emission is due to the enhanced optical band gap of the Se95In5 CGMPs doped FLC. The enhancement in the optical band gap of the CGMPs doped FLC was explained by the model of density states in the composite. Ultra-violet lasing, UV filtering, and optical band gap engineering are the possible applications of the investigated Se95In5 CGMPs doped FLC material.

Reduced ionic contaminations in CdSe quantum dot dispersed ferroelectric liquid crystal and its applications

Octadecylamine capped cadmium selenide quantum dots (CdSe QDs) were dispersed in the ferroelectric liquid crystal (FLC) FELIX 16/100. The QD dispersed FLC system was investigated on the planar anchored cell. Addition of specific concentration of the QDs in the pure FLC induces a new relaxation mode along with the Goldstone relaxation mode. QDs assisted quantum fluctuations are probably responsible for the existence of this new relaxation mode in the QDs dispersed FLC system. The ionic contaminations associated with the FLC materials were trapped on the surface of QDs due to the ion-trapping character of QDs. The trapping of ionic contaminations was confirmed by the a.c. conductivity measurement. The physical properties of the pure and dispersed FLC were carried out as a function of doping concentration of QDs, temperature and frequency.

Electro-optic switching in iron oxide nanoparticle embedded paramagnetic chiral liquid crystal via magneto-electric coupling

The variation in optical texture, electro-optic, and dielectric properties of iron oxide nanoparticles (NPs) embedded ferroelectric liquid crystal (FLC) with respect to change in temperature and electrical bias conditions are demonstrated in the current investigations. Improvement in spontaneous polarization and response time in nanocomposites has been attributed to magneto-electric (ME) coupling resulting from the strong interaction among the ferromagnetic nanoparticle's exchange field (due to unpaired e−) and the field of liquid crystal molecular director. Electron paramagnetic resonance spectrum of FLC material gives a broad resonance signal with superimposed components indicating the presence of a source of spin. This paramagnetic behavior of host FLC material had been a major factor in strengthening the guest host interaction by giving an additional possibility of (a) spin-spin interaction and (b) interactions between magnetic-dipole and electric-dipole moments (ME effects) in the composite materials. Furthermore, the phenomenon of dielectric and static memory effect in these composites are also observed which yet again confirms the coupling of magnetic NP's field with FLC's director orientation. We therefore believe that such advanced soft materials holding the optical and electrical properties of conventional LCs with the magnetic and electronic properties of ferromagnetic nanoparticles are going to play a key role in the development of futuristic multifunctional optical devices.