Nanoparticles In Nematic Liquid Crystal Research Articles

Effect of varying concentration of dichroic dye on textural, electro-optical, phase transition and band gap studies of zinc oxide nanoparticles induced vertically aligned liquid crystals

Vertically aligned liquid crystal (VALC) perpetually boosts the advancement for display devices and optoelectronics. Herein, initially, the nanoparticles (NPs) induced VALC (nVALC) cell was prepared via doping of 0.3 (wt./wt.%) of zinc oxide NPs in nematic liquid crystal (LC). After that, a series of orange azo dichroic dye doped nVALC cells were prepared with varying concentrations of dye such as 0.0625, 0.125, 0.25 and 0.5 wt.%. The effect of dye concentrations on LC textures, phase transition, electro-optical and band gap properties were investigated, as well as resulted in a significantly reduced value of the threshold and operating voltages as a function of dye concentrations. The contrast ratio was also enhanced up to 0.25 wt.% dye doped nVALC cells. Additionally, the transition temperature from nematic to isotropic phase and band gap studies were also performed. The present work provides improved properties of nVALC systems for next-generation display technologies.

Polarization Z-Scan Studies Revealing Plasmon Coupling Enhancement Due to Dimer Formation of Gold Nanoparticles in Nematic Liquid Crystals.

We investigate the plasmon coupling of gold nanoparticle (AuNP) dimers dispersed in a nematic liquid crystal matrix using the polarization z-scan technique. Our experimental setup includes the precise control of incident light polarization through polarization angles of 0°, 45°, and 90°. Two distinct cell orientations are examined: parallel and twisted nematic cells. In parallel-oriented cells, where liquid crystal molecules and AuNPs align with the rubbing direction, we observe a remarkable 2-3-fold increase in the nonlinear absorption coefficient when the polarization of the incident light is parallel to the rubbing direction. Additionally, a linear decrease in the third-order nonlinear absorption coefficient is noted as the polarization angle varies from 0° to 90°. In the case of twisted nematic cells, the NPs do not have any preferred orientation, and the enhancement remains consistent across all polarization angles. These findings conclusively establish that the observed enhancement in the nonlinear absorption coefficient is a direct consequence of plasmon coupling, shedding light on the intricate interplay between plasmonic nanostructures and liquid crystal matrices.

Effect of shape of ZnO nanoparticle on electro-optic and dielectric properties of nematic liquid crystal

Properties of liquid crystal incorporated with various shapes of a nanomaterial are expected to show multifaceted behaviour as properties of nanomaterial are extremely dependent on its size and shape. In view of this, we have investigated the effect of shape of ZnO nanoparticle on electro-optic and dielectric properties of nematic liquid crystal (NLC), PCH5. Nanocomposites of 0.01 wt% ZnO nano- sphere, rods, and wires in NLC are prepared. The addition of ZnO nanoparticles to NLC leads to increase in the order parameter and reduction in threshold voltage, response time as well as ionic conductivity as compared to the pure NLC host. Dielectric permittivity measurements indicate nanoparticle induces short range order in isotropic phase. Dielectric measurements also show increase in order parameter in nanocomposites with 1D nanoparticles whereas negligible change is observed in nanocomposites with spherical nanoparticles in NLC in deep nematic range. The ATR-FTIR measurements support the enhancement in parallel correlation of dipole moments in ZnO nanocomposites as compared to pure NLC.

Nanoparticle-Induced Property Changes in Nematic Liquid Crystals.

Doping liquid crystals with nanoparticles is a widely accepted method to enhance liquid crystal’s intrinsic properties. In this study, a quick and reliable method to characterise such colloidal suspensions using an optical multi-parameter analyser, a cross-polarised intensity measurement-based device, is presented. Suspensions characterised in this work are either plasmonic (azo-thiol gold AzoGNPs) or ferroelectric Sn2P2S6 (SPS) nanoparticles in nematic liquid crystals. The elastic constants and rotational viscosity showed nonlinear dependence on the concentration of AzoGNPs, initially increasing at lower concentrations and then decreasing at higher concentrations, indicating some degree of particle aggregation. For the SPS suspension, the elastic constant decreased with doping, while the rotational viscosity increased, in agreement with previous findings. Through viscosity measurements, the stability of SPS suspension over ten years is also highlighted.

Domain growth in ferronematics: slaved coarsening, emergent morphologies and growth laws.

Ferronematics (FNs) are suspensions of magnetic nanoparticles in nematic liquid crystals (NLCs). They have attracted much experimental attention, and are of great interest both scientifically and technologically. There are very few theoretical studies of FNs, even in equilibrium. In this paper, we study the non-equilibrium phenomenon of domain growth after a thermal quench (or coarsening) in this coupled system. Our modeling is based on coupled time-dependent Ginzburg-Landau (TDGL) equations for two order parameters: the LC tensor order parameter Q, and the magnetization M. We consider both shallow and deep quenches from a high-temperature disordered phase. The system coarsens by the collision and annihilation of topological defects. We focus on slaved coarsening, where a disordered Q (or M) field is driven to coarsen by an ordered M (or Q) field. We present detailed results for the morphologies and growth laws, which exhibit unusual features purely due to the magneto-nematic coupling. To the best of our knowledge, this is the first study of non-equilibrium phenomena in FNs.

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.

Slaved coarsening in ferronematics

We focus on understanding the influence of the two-component coupling in ferronematics, a colloidal suspension of magnetic nanoparticles in nematic liquid crystals. Using coarse-grained Landau-de Gennes free energies, we study the ordering dynamics of this complex fluid in d = 2 and present a range of analytical and numerical results. Our main observations are: i) slaved coarsening for quench temperatures T intermediate to the critical temperatures of the uncoupled components, ii) slower growth similar to the Lifshitz-Slyozov law for symmetric magneto-nematic coupling, iii) sub-domain morphologies dominated by interfacial defects for asymmetric coupling strengths. These novel results will serve to guide future experiments on this technologically important system.

Site-specific attraction dynamics of surface colloids driven by gradients of liquid crystalline distortions.

The site-specific migration dynamics of small particles at the air-nematic liquid crystal (LC) interface is investigated, using a LC film with a unique gradient of LC distortions. This gradient has been identified as the direct origin of the site-specific migration, while the elastic multipole interaction between the pre-existing distortion and that induced around particles is negligible. The results reveal a basic behavior of small particles with weak anchoring strength in LC distortions, which is often hidden in prominent elastic multipole interactions typically found with larger particles. Moreover, the present mechanism of the site-specific attraction of particles is specifically relevant to directed patterning and manipulations of nano-particles in nematic LCs with distortions.

Highly improved dielectric behaviour of ferronematic nanocomposite for display application

ABSTRACTWe report the investigation of influence of nickel zinc ferrite magnetic nanoparticles (NZFO (Ni0.5Zn0.5Fe2O4)) on phase transition, optical and dielectric properties in a nematic liquid crystal (NLC). The interaction of NZFO nanoparticles with NLC was confirmed by the formation of ferronematic droplets due to the transfer of magnetic orientational effect onto the underlying NLC matrix. The doping results in shift of nematic to isotropic transition to low-temperature region. An enhancement in the value of refractive index is observed in the nematic region after the addition of NZFO nanoparticles. The dielectric constant of NLC was remarkably enhanced by 10 times after doping, which is found to be maximum at 0.1 wt% concentration of NZFO nanoparticles. The decrease in the value of dissipation factor in low-frequency region shows that the magnetic nanoparticles are able to trap ionic impurities effectively. The obtained results suggest that the optimum amount of doping concentration is 0.1 wt% of NZFO nanoparticles in NLC due to high dielectric constant with low dissipation factor and high refractive index with high dispersive power at room temperature.

Conjugation of Nanomaterials and Nematic Liquid Crystals for Futuristic Applications and Biosensors.

The established role of nematic liquid crystals (NLCs) in the recent rapid development of displays has motivated researchers to modulate the electro-optical properties of LCs. Furthermore, adding nanomaterials into NLCs has led to enhancements of the properties of NLCs, like reduced threshold of the operating voltage, variation in pretilt angle, reduced switching time, etc. These enhanced properties, due to interfacial dynamics, are enabling wider applications of NLCs and nanomaterials. The recent literature of nanomaterial-doped NLCs is rich with various kinds of nanomaterials in a variety of NLCs. The light has been focused on the most widely used and studied gold nanoparticles in NLCs. The intrinsic inherent property of easy excitation of surface plasmons polaritons (SPP) is the mediating interaction of NLC electric dipoles and the polarization of charges in the GNP surface. The concepts and methods for the application of metal nanomaterials as dopants in NLCs are discussed for future applications, especially biosensors. The biosensing application of NLCs alone has already been proven in the literature. However, it is always desirable to further enhance the detection efficiency and selectivity, which have been achieved by the conjugation of GNPs and nickel nanoparticles with NLCs and their compatibility with biological materials. This aspect of future application of nanoparticles and NLC makes the point more selective to be included in the present manuscript.

Interactions of single nanoparticles in nematic liquid crystal

We studied interactions of single silica nanoparticles with homeotropic surface anchoring in a confined planar nematic liquid crystal cell. Nanoparticles form stable pairs down to a size of 35 nm with the pair binding energy proportional to the size of the particle. 22 nm particles do not form stable pairs, only metastable states were observed due to strong thermal motion and electrostatic repulsion. These nanoparticles strongly interact with the confining surfaces and the larger clusters, which prevents the formation of stable homogenous dispersions. In addition we have shown that nanoparticles are strongly attracted by topological defects induced by the microparticles. The origin of the attractive forces in nanoparticle dispersions is the minimization of free energy. These forces strongly depend on the strength and type of the liquid crystal anchoring at the surface of the nanoparticles.

Analysis of photoluminescence, UV absorbance, optical band gap and threshold voltage of TiO2 nanoparticles dispersed in high birefringence nematic liquid crystal towards its application in display and photovoltaic devices

In the present work anatase TiO2 nanoparticles (NPs) have been dispersed into the pure nematic liquid crystal (NLC) 2020 which is consisted of fluorinated 4'-alkylphenyl-4-isothiocyanatotolanes, in different concentration. The dispersion of TiO2 NPs in NLC induces photoluminescence what is an important finding of this study. It has also been found that the dispersion of TiO2 NPs reduces the UV absorbance of pure NLC. Optical band gap has been calculated and found to be decreased for the dispersed system as compared to pure NLC. The presence of TiO2 NPs affects the molecular dynamics of the NLC molecules, which has been proved by the Fourier transformed infrared spectroscopy (FTIR). Relative electric permittivity and dielectric anisotropy have been found to be decreased for the dispersed system as compared to the pure NLC. The trapping capability of TiO2 NPs is also confirmed by the independent measurement of threshold voltage. The value of threshold voltage has decreased after dispersion of NPs in the pure NLC. This decrement is due to the property of TiO2 NPs to reduce the ionic contribution, thereby, increasing the net field of the system. The induction of photoluminescence in a non photoluminescent material with the help of TiO2 nanoparticle along with reduced band gap are the main findings of the present work. It is suggested that the output of the present work may be very useful in various display and photonic applications as well as in photovoltaic devices.

The effects of magnetic nanoparticles on oscillating behavior of a nematic liquid crystal cell

ABSTRACTMagnetic suspensions in anisotropic host fluids such as liquid crystals show interesting behavior that have attracted vast interest in many areas of science, technology, and medicine. Recently, suspensions of monodomain magnetic nanoparticles in nematic liquid crystals, called ferronematics, are extensively used in different technological applications. Dynamical ordering behavior in these magnetic materials is a complex subject due to several dynamic cross-couplings. In this work, the effects of magnetic nanoparticle dopants on the dynamical behavior of a nematic liquid crystal cell are discovered through studying the motion of a torsional oscillator in which the cell is contained. Dynamical features of the cell are investigated within the framework of continuum theory. The hydrodynamic equations of the ferronematic liquid crystal are calculated in the presence of an applied magnetic field. The velocity components and the displacement dependence of the velocity field in the cell are obtained. Then, the toque exerted on the cell contained in torsional oscillator is determined. This study is based on Leslie–Ericksen theory. The analysis is also used to study the resonant frequency as a function of the applied field. In this way, the key roles of the entropy of ferroparticle solution and anchoring induced ferronematic interactions become evident.

Molecular simulations of entangled defect structures around nanoparticles in nematic liquid crystals

ABSTRACTWe investigate the defect structures forming around two nanoparticles in a Gay–Berne nematic liquid crystal using molecular simulations. For small separations, disclinations entangle both particles forming the figure of eight, the figure of omega and the figure of theta. These defect structures are similar in shape and occur with a comparable frequency to micron-sized particles studied in experiments. The simulations reveal fast transitions from one defect structure to another suggesting that particles of nanometre size cannot be bound together effectively. We identify the ‘three-ring’ structure observed in previous molecular simulations as a superposition of the different entangled and non-entangled states over time and conclude that it is not itself a stable defect structure.

Investigation of electrical and nonlinear optical properties of colloidal composite nematic liquid crystal

The synthesized TiO2 nanoparticles (NPs) doped to nematic liquid crystals (NLCs) with 1% wt. for both alignments (homeotropic and homogeneous). Dielectric permittivity of samples was measured by LCR meter at different temperatures. TiO2 NPs doped into NLCs show higher permittivity than pure NLCs. According to the obtained results, the permittivity of ε⊥ increases more than ε׀׀ in samples. The magnitude and temperature dependence of the refractive indices of TiO2 NPs doped into NLCs are reported. Investigated electrical behavior of the sample was studied over a wide range of frequency using a complex impedance spectroscopy (CIS) technique. The third-order nonlinear optical properties of TiO2 NPs doped into NLCs which tested by a Z-scan method with continuous wave (CW) He: Ne laser beam, are reported. The Z-scan results revealed that the TiO2 NPs in NLCs exhibit self-focusing nonlinearity and saturable absorption effect for both alignments. The results showed that NLCs doped with a small amount of TiO2 NPs can have significantly improved electro-optic properties.

Applicability of TiO2 nanoparticle towards suppression of screening effect in nematic liquid crystal

We report the preliminary results observed for TiO2 nanoparticles dispersed nematic liquid crystal. We observed that dispersion of TiO2 nanoparticles (NPs) in nematic liquid crystal (NLC) has suppressed the undesired ionic effect. However unpurified nanoparticles lead to increase in the ionic conductivity, due to insertion of some more impurities. The pristine and dispersed NPs in NLC have been examined by means of optical texture, optical transmittance and dielectric spectroscopy. The diminution of ionic effects has been attributed to the strong adsorption of ionic impurities on the surface of TiO2 nanoparticles. Almost all of the ions have been adsorbed on the surface of NPs at a loading of 1wt.% in NLC. Also we didn't find any considerable change in the transition temperature of NLC, except for 2wt.% loading. The optical transmittance curve reveals that relaxation processes are faster than the host materials, after the field is turned off. The faster relaxation in dispersed system is ascribed to the decrease in the rotational viscosity and suppressed screening effect. The present study is helpful in understanding the phenomenon of suppressing the screening effect, caused by the ionic movements under applied field of LC based devices, without affecting the transition temperature of liquid crystalline material.

Effect of electric and magnetic fields on the orientation structure of a ferronematic liquid crystal

We analyze uniform orientation phases in soft ferronematics (suspensions of magnetic nanoparticles in nematic liquid crystals) induced by electric and magnetic fields. It is shown that the competition between the electric and magnetic fields can lead to various sequences of orientation transitions in a ferronematic depending on the energy of coupling between the director and magnetization. We obtain and analyze phase diagrams of these transitions. A sequence of re-entrant transitions in the orientation structure (angular phase-homeotropic phase-angular phase-planar phase) is predicted for a certain range of the coupling energies and electric field strengths.

Weak coupling effects and re-entrant transitions in ferronematic liquid crystals

In the framework of continuum theory we study the orientational and magnetooptical properties of ferronematics (i.e., colloidal suspensions of magnetic nanoparticles in nematic liquid crystals) with soft planar coupling of a disperse phase with a liquid crystal matrix in a magnetic field. It is shown that there is a threshold energy of coupling between the dispersed phase and the liquid crystal matrix, below which (weak coupling) ferronematic has a complex magnetooptical response to an applied magnetic field. We show that at weak coupling initially a uniform ferronematic structure under the field action undergoes the sequence of re-entrant transitions “uniform phase–non-uniform phase–uniform phase–non-uniform phase”, which can be of the first or second order depending on a segregation parameter value. The tricritical value of the segregation parameter is found analytically as a function of the material parameters of a suspension. We study magnetic field action on the optical phase lag and the capacity of a ferronematic cell. Comparison of results of numerical calculations with experimental data is carried out.

Influence of Surface Treatment of Ferromagnetic Nanoparticles on Properties of Thermotropic Nematic Liquid Crystals

We observe a strong effect of surfactant structure on the aggregation of ferromagnetic nanoparticles in nematic liquid crystal (LC) hosts, and the colloidal stability and ordering of the resulting nematic matrix. We find that ferromagnetic nanoparticles coated with surfactants that possess terminal mesogenic groups induce minimal aggregation of the nanoparticles, resulting in a 1.5 times reduction in the value of the Frederikscz transition field in comparison with the parent LC host.

Electro-Optical Characteristics of ZrO2 Nanoparticle Doped Liquid Crystal on Ion-Beam Irradiated Polyimide Layer

It is well known that doping liquid crystals (LCs) with nanoparticles can readily change the physical and electro-optical properties of LC mixture. In this paper, we report on how the electro-optical properties and thermal stability of an LC system were enhanced by dispersing zirconia (ZrO2) nanoparticles in nematic LCs on ion-beam irradiated polyimide layers. Homogeneous LC alignment was achieved and ZrO2/LC mixture was applied in twisted-nematic (TN) mode. The addition of ZrO2 nanoparticles contributed to improvement of electro-optical properties in the TN LC cell by lowering voltage operation and decreasing response time. The TN LC cells with a ZrO2 nanoparticle concentration of 2.0 wt% showed the lowest threshold voltage of 2.0 V and the fastest response time of 15.3 ms. This enhanced electro-optical performance was likely due to van-der waals interactions and the screening effect of the ZrO2 nanoparticles in the LC medium. The thermal stability of the ZrO2/LC mixture was also improved compared to a pristine LC system.