Nematic Liquid Crystal Composites Research Articles

Review of Theoretical Advances of Nanomaterials and Nematic Liquid Crystal Composites

Review of Theoretical Advances of Nanomaterials and Nematic Liquid Crystal Composites

The Role of Rubrene Concentration on Dielectric Parameters of Nematic Liquid Crystal

In this study, the dielectric parameters of E7 coded nematic liquid crystal (NLC) composites containing the different amounts of rubrene fluorescent dye were investigated. E7, E7+0.5 wt.% Rubrene, and E7+1.0 wt.% Rubrene samples were prepared. Frequency dependent dielectric constants (ɛ′ and ɛ′′), dielectric anisotropy (Δε′), and ac conductivity (σac) graphs of rubrene doped E7 NLC composites were obtained by dielectric spectroscopy method and compared with pure E7 NLC. By using these graphs, relaxation frequency (fR), relaxation time (τR), dielectric strength (δɛ′), and crossover frequency (fc) parameters of the E7 NLC and its rubrene doped composites were determined. An increase in fR from 3.045 MHz to 3.697 MHz for 0 V and from 627 kHz to 686 kHz for 40 V was observed with increasing rubrene concentration. On the other hand, a decrease in τR from 0.052 μs to 0.043 μs for 0 V and from 0.254 μs to 0.232 μs for 40 V was seen with increasing rubrene concentration. Furthermore, an increase in fc from 1.145 MHz to 1.298 MHz was obtained with increasing rubrene concentration. The results show that the dielectric parameters change with the concentration of rubrene and it is thought that this study will provide a basis for investigating the dielectric properties of rubrene doped NLC composites. Moreover, it is concluded that the produced composites are a suitable base material for electro-optical device applications such as smart displays, photonics and electrical circuit elements.

Enhanced Electro-optical Properties of Low Viscous Nematic Liquid Crystal Doped with Mixed Phase Anatase/Rutile TiO<sub>2</sub> Nanoparticles for Display Applications

Organic–inorganic composite based on liquid crystalline and TiO2 nanoparticles were obtained and investigated taking into account the crystallographic form of TiO2 i.e., anatase/rutile mixed phase. TiO2 is an important class of material having various dielectric and electro-optical properties. The existent research presents the electro-optical properties of nematic liquid crystal (NLC) E204 and TiO2 nanocomposites over an extensive range of frequencies. Various important display parameters such as response time, threshold voltage, pretilt angle and activation energy of pristine as well as TiO2 doped composites systems were measured and analyzed. In comparison with the pure, TiO2-doped composite systems has approximately 50% faster response time, owing to its remarkable decline in the relaxation time and activation energy of the LCs. The alteration in the optical intensity of the NLC composites as a function of the concentration of TiO2 nanoparticles (NPs) was also examined. It was established that the optical intensity in nano-nematic composites was decreasing with the concentration of TiO2 NPs. Also, it was observed that an escalation in the TiO2 NPs concentration in NLC composites indicates to an escalation in the birefringence. Probable mechanisms of the interactivity between NLC molecules and TiO2 NPs have been discussed. The current work shows that the TiO2 NPs doping has an encouraging application in the display devices, including other electro-optical as well as photonic applications.

Optical Properties of Nematic Liquid Crystal Composites with Semiconducting Quantum Dots

The synthetic route to obtain stable composites of liquid crystal ZhK-777 with semiconductor quantum dots CdSe/CdS and CdSe/CdS/ZnS has been proposed. The dependents of fluorescence intensity of the composites on size and concentration of used quantum dots has been established. The shift of the luminescence band and decrease of its intensity has been observed both for liquid crystal and quantum dots. This effect indicates that quantum dots interaction with liquid crystals caused by a nonradiative excitation energy transfer from liquid crystal molecules to quantum dots. The dielectric spectroscopy of ZhK-777 and composites revealed that doping with quantum dots results in change of molecular relaxation frequencies in composites.

The scientific duo of TiO2 nanoparticles and nematic liquid crystal E204: Increased absorbance, photoluminescence quenching and improving response time for electro-optical devices

The present investigation on the dielectric and electro-optical properties of nematic liquid crystal (NLC) E204 and TiO2 nanocomposites has been carried out with a wide range of frequencies. The low-frequency spectra of these composites with a concentration of 0.1 wt/wt%, 0.5 wt/wt%, and 1.0 wt/wt% are recorded in the frequency range of 50–1000 Hz, and the content of impurity ions in these samples has been analyzed. The ion trapping by adsorbing sites of TiO2 and that effect on DC conductivity and diffusion constant have also been investigated. TiO2-doped LC cell results in a two-fold faster response time of the pure LC cell, as a consequence of a remarkable reduction in the rotational viscosity of the LCs. The change in the intensity of the photoluminescence (PL) spectra of NLC composites as a function of the concentration of TiO2 nanoparticles (NPs) has also been investigated. PL spectra reveal that the PL-quenching increases in nano-nematic composites with increasing the concentration of TiO2 NPs following to the Stern-Volmer quenching equation. The pure NLC exhibits a strong absorption band nearly at 314 nm, attributing to π-π* transitions in the phenyl rings which has been slightly blue-shifted after the addition of TiO2 NPs. This blue shift was to be attributed to the n → π⁎ electronic transitions in the systems. The doping of TiO2 NPs has a promising application to the field of LC devices and displays.

Electro-optical, dielectric and optical properties of graphene oxide dispersed nematic liquid crystal composites

ABSTRACT Graphene oxide (GO) in various concentrations (0.04 wt.%, 0.1 wt.% and 0.3 wt.%) was dispersed into nematic liquid crystal (LC) 5CB and studied for electro-optical, dielectric and optical responses. The textural and phase transition studies were investigated by optical polarising microscopy in the nematic phase. The effect of GO concentration on the micro-texture, phase transition temperatures, birefringence, threshold voltage, contrast ratio, dielectric permittivity, dielectric loss and conductivity was also investigated. The nematic–isotropic transition temperature slight increases (~3%) in the 0.04 wt.% and 0.1 wt.% GO dispersed 5CB composites as compared to 5CB. Threshold voltage decreases in the GO–5CB composites with the increase of GO concentration. Additionally, the effect of GO concentration on the optical absorbance of UV–visible light and energy bandgap is also considered.

Influences of nickel plated multi-walled carbon-nanotube on the electro-optical properties of nematic liquid crystal

Some enhanced performances can be obtained by doping multi-walled carbon-nanotube (MWCNT) into self-organized nematic liquid crystal (NLC). However, the dispersion of MWCNT in NLC is very few, thus the enhancement is restricted. In this work, a nickel plated MWCNT (MWCNT@Ni) is synthesized to obtain a relatively high dispersion. The morphology, element and chemical bond differences between MWCNT and MWCNT@Ni are characterized. For MWCNT@Ni, there is a layer of coaxial nickel coated on the surface of MWCNT, which weakens the interaction energy between the adjacent MWCNTs and further results in a relatively high dispersion. Moreover, MWCNT@Ni has a more orderly arrangement in NLC compared with MWCNT. The results suggest that the dielectric anisotropy of MWCNT@Ni/NLC with mass fraction of 0.01 wt% is increased by ∼3.6%, and the saturation voltage is reduced by ∼7.3%. Besides, the rise time is decreased by ∼9.5% at 5 V and 1 kHz. These performances have been improved compared with MWCNT/NLC under the same mass fraction. The effect of mass fraction of MWCNT@Ni on rise time is further investigated. As a result, the rise time is decreased by ∼16.7% as MWCNT@Ni with mass fraction of 0.10 wt% is added into NLC. In general, the method to increase dispersion of dopant in NLC is proposed, which can serve as a reference to improve the performances of NLC composites.

Specific features of luminescence quenching in a nematic liquid crystal doped with nanoparticles

The change in the intensity of the photoluminescence (PL) spectra of nematic liquid crystal (NLC) composites as a function of the concentration of CdSe/ZnS semiconductor quantum dots (QDs) and TiO2 and ZrO2 nanoparticles ~5 nm in diameter has been investigated. It is shown that the PL-quenching intensity in composites with CdSe/ZnS QDs exceeds that in composites with TiO2 and ZrO2 nanoparticles. The lowfrequency spectra of these composites with a concentration of 0.1 wt %, recorded in the range of 102–103 Hz, and the content of mobile ions in them have been investigated. It is found that the dielectric loss in the composite with CdSe/ZnS QDs is much higher and the content of mobile ions is larger by a factor of 3 than in the composites with TiO2 and ZrO2 nanoparticles. It is shown that an increase in the CdSe/ZnS QD concentration in NLC composites leads to an increase in the dielectric loss and a decrease in the PL intensity. Possible mechanisms of the interaction between NLC molecules and CdSe/ZnS QDs are discussed.

Effect of iron phthalocyanine (FePc) concentration on electrical and dielectric properties of the nematic liquid crystal composites

Electrical and dielectric properties of iron phthalocyanine (FePc) dispersed E63 nematic LC were investigated using admittance measurements in the frequency range of 100Hz - 20MHz and current measurements in the bias range of 0–20V. In order to observe the effect of FePc concentration, E63 was dispersed with 1, 3 and 5% wt/wt FePc. Basic dielectric parameters such as dielectric anisotropy (Δε), crossover frequency (fc), real and imaginary parts of dielectric permittivity (ε' and ε") and relaxation time (τ) of the samples were obtained from the experimental data, along with threshold voltage (Vth), splay elastic constant (K11) and conductivity parameters (σac and σdc). Dispersal of FePc and its concentration caused Vth, K11, σdc and fc of the LC composites increase whereas τ is lowered. LC composites were found to exhibit relaxation in low and high frequency regions such that the former was masked due to high conductivity in the samples and hence electric modulus formalism was applied to determine low frequency relaxation. Prominent effect of FePc dispersal was found in σdc values in a way that σdc of the LC composite of E63 with 5% wt/wt FePc is about 53 times larger than that of neat E63.

Ionic impurities in nematic liquid crystal doped with quantum dots CdSe/ZnS

ABSTRACTWe investigated the dielectric losses and the ionic currents in the nematic liquid crystal (NLC) doped with semiconductor quantum dots (QDs) of CdSe/ZnS core – shell type and covered with trioctylphosphine oxide (TOPO) molecules. The dielectric loss tangent of the NLC composites increased with increasing the QDs concentration from 0.1 to 0.3 wt%. The density of mobile ions in the composites increased linearly and the average values of ions mobility in the composites decreased with increasing the QDs concentration. The fast ions with the mobility of about 10–10 m2/V·s and the slow ions with the mobility of about 10–11 m2/V·s were detected in the NLC composites. The growth of the content of slow ions took place with increasing the QDs concentrations. Increasing the dielectric loss tangent was observed with increasing the duration of sonication time of the NLC composites to prepare homogeneous suspensions. The fragmentation of the CdS/ZnS shell as a result of the sonication may lead to the appearance of the slow ions in the NLC composites.

Phase equilibrium and structure formation in gold nanoparticles—nematic liquid crystal composites: experiments and theory

A theoretical and experimental study of mixtures of a calamitic nematic liquid crystal and gold nanoparticles capped with mixed monolayers (alkylic + mesogenic ligands) is presented. The effect of the ligand monolayer composition and nanoparticle concentration on the solubility in the isotropic phase and on the isotropic–nematic phase transition is studied. Mixed monolayers show the highest miscibility and lead to the formation of well-defined cellular networks. This behaviour is explained in terms of a mean-field thermodynamic model, in combination with a phenomenological expression for the isotropic interaction parameter that accounts for entropic and enthalpic effects of the mixed ligand monolayer. The final structure of the material is shown to be determined essentially by the phase equilibrium behaviour.

Magnetite nanoparticles/chiral nematic liquid crystal composites with magnetically addressable and magnetically erasable characteristics

A magnetite (Fe3O4) nanoparticle/chiral nematic liquid crystal (N*-LC) composite was prepared and filled into a planar treated cell. The Fe3O4 nanoparticles had been modified by oleic acid so that they could be better dispersed in the composite. When a magnetic field was scanned on the outer surface of the cell locally, Fe3O4 nanoparticles moved towards the inner surface of the cell correspondingly, and the black expected information was displayed. When the magnet was applied to the opposite outer surface, the information was erased. After polymer network walls were prepared in the composite, the resolution of the information displayed increased. Then, through the formation of hydrogen bonds between the nanoparticles and chiral pyridine compound (CPC) doped in the composite, the pitch length of the N*-LC could be adjusted by altering the intensity of the applied magnetic field. The composite doped with CPC could potentially be used as a material for a type of reflective colour paper with magnetically controllable characteristics.

Nonlinear optical transmission of lead phthalocyanine-doped nematic liquid crystal composites for multiscale nonlinear switching from nanosecond to continuous wave

We have formulated composites of lead (II) tetrakis (4-cumylphenoxy) phthalocyanine (PbTCPc) doped into nematic liquid crystal (LC), 4(')-pentyl-4-biphenylcarbonitrile (5-CB), that has received a 90 degree twisted alignment and investigated the nonlinear transmission properties using both pulsed (Nd:YLF 524 nm, 5 ns) and cw (532 nm) lasers. In the nanosecond regime, this compound is a reverse saturable absorber performing similarly to low-concentration solutions of PbTCPc. Under cw conditions, we observe optically self-activated polarization switching with low threshold input energy. Our results suggest the potential for an all-optical switch working from the nanosecond time scale to cw.

Characterization of carbon nanotube–thermotropic nematic liquid crystal composites

Dispersions of carbon nanotubes (CNTs) in liquid crystals (LCs) have attracted attention due to their unique properties and possible applications in photonics and electronics. However, these are hard to stabilize, and the loading level in the equilibrium state in LC hosts is small. A practical way to monitor the quality and CNT incorporation in such equilibrium dispersions is required. Here, we compare different methods for characterising equilibrium CNT–LC composite materials.

Photorefractivity in Nematic Liquid Crystals Using a Donor−Acceptor Dyad with a Low-Lying Excited Singlet State for Charge Generation

The photorefractive effect was observed in room-temperature nematic liquid crystal composites consisting of a eutectic mixture of 65% 4‘-(n-pentyl)-4-cyanobiphenyl and 35% 4‘-(n-octyloxy)-4-cyanobiphenyl doped with a donor−acceptor dyad in which 9-(N-pyrrolidinyl)perylene-3,4-dicarboximide is directly linked to 1,8:4,5-naphthalenetetracarboxydiimide. This dyad has a broad, intense absorption band that extends from 500 to 750 nm. Measurements of two-beam asymmetric energy exchange (beam-coupling) and the diffraction efficiency of the refractive index grating produced in these liquid crystal thin films were carried out. A comparison is made between the covalent donor−acceptor dyad and the corresponding noncovalent donor−acceptor pair. In each case the covalent dyad outperforms the noncovalent donor−acceptor pair. The results show that aminoaryleneimides are promising new charge donating chromophores that are capable of extending photorefractive performance of nematic liquid crystal composites into the near-infrared region of the spectrum.

The molecular structure of polyacrylonitrile fibers

In spite of considerable effort, the understanding of the molecular structure of polyacry-lonitrile-based fibers is still incomplete. This paper shows that molecular modeling based on molecular mechanics and dynamics can be used to study the molecular structure of polyacrylonitrile-based fibers with appropriate strategies. Additional information has thus been obtained which is impossible to obtain by using other instrumental techniques. It is concluded that the chain conformations of polyacrylonitrile-based fibers are similar to those of nematic liquid crystal composites in which the chains are formed by the irregular combination of two or more species of unit groups. Hence the polyacrylonitrile molecules should be viewed as ‘aperiodic chain molecules’. © 1996 John Wiley & Sons, Inc.