Carbon Nanotubes In Liquid Crystals Research Articles

Enhancement of dielectric and electro-optical characteristics of liquid crystalline material 4'-octyl-4-cyano-biphenyl with dispersed functionalized and nonfunctionalized multiwalled carbon nanotubes.

For recent applications, liquid crystal-carbon nanotube based nanocomposite systems have been proven to be highly attractive. In this paper, we give a thorough analysis of a nanocomposite system made of both functionalized and nonfunctionalized multiwalled carbon nanotubes that are disseminated in a 4'-octyl-4-cyano-biphenyl liquid crystal medium. Thermodynamic study reveals a decrease in the nanocomposites' transition temperatures. In contrast to nonfunctionalized multiwalled carbon nanotube dispersed systems, the enthalpy of functionalized multiwalled carbon nanotube dispersed systems has increased. In comparison to the pure sample, the dispersed nanocomposites have a smaller optical band gap. A rise in the longitudinal component of permittivity and, consequently, the dielectric anisotropy of the dispersed nanocomposites has been observed by dielectric studies. When compared to the pure sample, the conductivity of both dispersed nanocomposite materials has increased by two orders of magnitude. For the system with dispersed functionalized multiwalled carbon nanotubes, the threshold voltage, splay elastic constant, and rotational viscosity all decreased. For the dispersed nanocomposite of nonfunctionalized multiwalled carbon nanotubes, the value of the threshold voltage is somewhat decreased but the rotational viscosity and splay elastic constant both are enhanced. These findings show the applicability of the liquid crystal nanocomposites for display and electro-optical systems with appropriate tuning of the parameters.

Computer and experimental study of field-induced conductivity modulation in liquid crystal–carbon nanotubes system

In this study, the possibility of percolation system conductivity control by the applied electric field has been researched based on computer simulation and experimental measurements of electrical characteristics of liquid crystal–carbon nanotubes nanocomposite. It was found that electrical resistance of obtained nanocomposites decreases with increasing nanotube concentration. It is shown that the applied voltage reduces the resistance of the nanocomposite, which is likely associated with the change in the spatial orientation of nanotubes. Nanocomposites with 2–4% nanotube content show the maximum field-controlled relative change in resistance. The effect of concentration and orientation of the nanotubes on the percolation processes in the carbon nanotube system has been studied by computer simulations. The change in the percolation probability was revealed when limiting nanotube orientation angles that are controlled by the electrical field.

Boost in the thermal stability, ionic conductivity and director relaxation frequency in the composite of liquid crystal and functionalised multi-walled carbon nanotubes

ABSTRACT In the present work, we have prepared a liquid crystal-carbon nanotube (LC-CNT) composite by dispersing a low concentration of carboxylic group (-COOH) functionalised multi-walled CNTs (MWCNTs) in a liquid-crystal 6CHBT and studied in detail the alterations in thermal and electrical parameters. Thermodynamic studies reveal that transition processes are becoming fast due to the dispersion of MWCNTs. The change in the behaviour of transitions is credited to the reduction in ionic impurities owing to the ion trapping characteristic of CNTs. The isotropic liquid to nematic transition temperature of the composite system is increasing for the exothermic cycle as compared to that of the pure sample. The average transition enthalpies and entropies for various transitions of the composite system are increasing as compared to those of the virgin sample. Longitudinal component of the permittivity and dielectric anisotropy of the composite system is slightly reduced as compared to those of the virgin sample. Relaxation frequencies of an observed relaxation mechanism corresponding to the rotation of molecules about their short axes of the composite system are enhanced as compared to the undispersed sample. Total ionic conductivity of the composite system is boosted as compared to those of the pure sample.

Heterosynaptic Plasticity Emulated by Liquid Crystal-Carbon Nanotube Composites with Modulatory Interneurons.

The aim of the neuromorphic computing is to emulate energy-efficient and smart data-processing ability of the biological brain, which is achieved by massively interconnected neurons and synapses. The strength of a connection between two neurons is modified by homosynaptic and heterosynaptic plasticity. As current research in the neuromorphic device is mainly focused on emulating homosynaptic plasticity, complex biological functions are not easy to mimic because they require both homosynaptic and heterosynaptic plasticity. We demonstrate the use of a liquid crystal-carbon nanotube (LC-CNT) composite as a resistive switching material that can emulate both the homosynaptic and heterosynaptic functions of biological neurons. The LC-CNT composite undergoes resistance change by CNT alignment and aggregated wire formation subjected to an applied electric field. A two-terminal device that exploits this mechanism achieves analog switching and homosynaptic potentiation. In a multiterminal device structure, the modulatory interneuron could tune the synaptic properties to perform heterosynaptic functions such as heterosynaptic potentiation, heterosynaptic facilitation, and synaptic weight normalization to emulate complex biological functions of a brain. Artificial synapses that exploit this multifunctionality of the LC-CNT composite have uses in next-generation neuromorphic devices.

Theoretical study on the effect of electric field for carbon nanotubes dispersed in nematic liquid crystal

Nematic liquid crystal (NLC) plays an important role in the controlled alignment of carbon nanotubes (CNTs). When nematogens are strongly anchored over the surfaces of CNTs, the Liquid Crystal (LC) order parameter is greatly influenced. Moreover, the electro-optic properties of the medium are significantly improved. Application of electric field further modifies these novel properties. The effect of electric field not only changes the phase behavior but also influences the ordering of both LC molecules and CNTs. Utilizing Doi and Landau–de Gennes type free energy mechanism, we have investigated in detail the changes incurred by the electric field. Order parameters of both LC and CNTs are calculated and modified phase diagrams are drawn. Finally, we have employed Landau–Khalatnikov equation to calculate rotational viscosity of the medium. It is found that rotational viscosity decreases with increasing volume fraction of CNTs in accordance with experimental results.

Magnetic segregation effect in liquid crystals doped with carbon nanotubes.

We study the orientational transitions in a suspension of carbon nanotubes in a nematic liquid crystal induced by an external magnetic field. The case of a finite orientational anchoring of liquid crystal molecules at the surface of doped carbon nanotubes is considered. It is shown that in a magnetic field the initial homogeneous planar texture of the liquid crystal–carbon nanotubes mixture is disturbed in a threshold manner (Fréedericksz transition). The orientational and concentration distributions of the suspension are studied for different values of the magnetic field strength and segregation intensity of the impurity subsystem. The optical phase lag between ordinary and extraordinary rays of light transmitted through a layer of a liquid crystal composite is calculated. The possibility of changing the nature of the Fréedericksz transition from second order to first order is shown. This tricritical behavior is related to the redistribution of the carbon nanotubes (segregation effect) inside the layer.

The peculiar electrical response of liquid crystal-carbon nanotube systems as seen by impedance spectroscopy

Conductive nanoparticles, especially elongated ones such as carbon nanotubes, dramatically modify the electrical behavior of liquid crystal cells. These nanoparticles are known to reorient with liquid crystals in electric fields, causing significant variations of conductivity at minute concentrations of tens or hundreds ppm. The above notwithstanding, impedance spectroscopy of doped cells in the frequency range customarily employed by liquid crystal devices, 100 Hz–10 kHz, shows a relatively simple resistor/capacitor response where the components of the cell can be univocally assigned to single components of the electrical equivalent circuit. However, widening the frequency range up to 1 MHz or beyond reveals a complex behavior that cannot be explained with the same simple EEC. Moreover, the system impedance varies with the application of electric fields, their effect remaining after removing the field. Carbon nanotubes are reoriented together with liquid crystal reorientation when applying voltage, but barely reoriented back upon liquid crystal relaxation once the voltage is removed. Results demonstrate a remarkable variation in the impedance of the dielectric blend formed by liquid crystal and carbon nanotubes, the irreversible orientation of the carbon nanotubes and possible permanent contacts between electrodes.

Great Effect of Laponite Nanoplatelets on Structure and Optical Properties of Liquid Crystal–Carbon Nanotube Composites

By using several optical methods, dramatic improvement of dispersion of carbon nano-tubes (CNTs) in nematic liquid crystal (LC) E7 is found by adding a small amount (0.1 wt.%) of Laponite particles modified with an ionic surfactant CTAB to the LC-CNT composite. This effect is explained by surrounding the nanotubes by Laponite particles that promoted their disaggregation. The enhanced dispersion of CNTs was accompanied with essential changes in electro-optical properties of the colloids, namely, in greatly improved optical transmittance and electro-optic contrast, as well as in reduction of Fredericks threshold.

Mixtures composed of liquid crystals and carbon nanotubes.

The phenomenological model to describe the liquid crystal-carbon nanotubes mixture presented in a previous paper [P. van der Schoot, V. Popa-Nita, and S. Kralj, J. Phys. Chem. B 112, 4512 (2008)] has been extended to include the isotropic carbon nanotubes-nematic thermotropic liquid crystal interaction. It is assumed that the carbon nanotubes in the isotropic phase act as an external random field on liquid crystal component. The influence of the randomly orientational disorder on the phase diagram of the mixture and orientational order parameters profiles of both components is theoretically analyzed for different values of temperature, volume fraction of carbon nanotubes, nematic carbon nanotubes-nematic liquid crystal coupling strength and the random field strength.

Dispersion and magnetic field-induced alignment of functionalized carbon nanotubes in liquid crystals

Liquid crystal (LC)-functionalized multi-walled carbon nanotubes (MWNTs) were synthesized by substituting decyloxy and phenylcyclohexyl (PCH) mesogenic moieties into MWNT surface through covalent linkage. The LC-functionalized MWNTs (LC-MWNTs) showed good dispersion stability in ethanol as well as good miscibility in a host thermotropic nematic LC (N-LC). A polarizing optical microscope image demonstrated that the LC-MWNTs could be well dispersed in the host N-LC within 5wt% content of LC-MWNTs without aggregation, which is ascribed to the substantial affinity between the PCH moieties and the host N-LC molecules. It was found that the LC-MWNTs in the host LC matrix could be well aligned parallel to the direction of the applied magnetic field of 5T. Then the high order parameter of 0.46 for LC-MWNTs was obtained from the measurement of polarized Raman spectroscopy.

Cholesteric liquid crystal–carbon nanotube composites with photo-settable reversible and memory electro-optic modes

The photoresponsive electro-optical composites based on cholesteric liquid crystal (CLC) with optically controlled chirality and a minute amount of carbon nanotubes (CNTs) are studied. In cells with homeotropic anchoring, these composites exhibit a transition from fingerprint texture to homeotropic nematic texture in the course of photoinduced unwinding of the cholesteric helix. Compared with the CLC counterpart, this transition is much delayed, because of the stabilization of cholesteric filamentary domains by CNTs. The CLC-CNT composites demonstrate dual-mode operation with optical switching between reversible and memory mode. It is found that the memory response is associated with the elastic network of filamentary cholesteric domains that stabilizes the planar CLC texture reached in an electric field. In turn, the reversible mode corresponds to the unwound cholesteric state. Potential applications of this effect are discussed.

Effects of carbon nanotubes on the physical properties of a nematic liquid crystal N-(4′-methoxybenzylidene)-4-butylaniline

In this work, the nematic liquid crystal (LC) N-(4′-methoxybenzylidene)-4-n-butylaniline (MBBA) was prepared and doped with different concentrations (0.025, 0.05, 0.06, 0.07, 0.08, and 0.1wt.%) of multi-walled carbon nanotubes (CNTs) at room temperature to study the electric properties of (LC–CNTs) cell. The experimental results showed that capacitance of (LC–CNTs) cell became higher than that of pure LC cell. The dielectric permittivity is determined as a function of applied frequency (100Hz to 100kHz) at voltage (5V); it is found that increasing concentration of CNTs (0.1wt.%) led to increase in the real part dielectric constant and decrease in imaginary part for (LC–CNTs) cell compared with the pure liquid crystal. Also conductivity of (LC–CNTs) cell was increased with increasing concentration of CNTs more than 0.05wt.%. Theoretical study was carried out by using PM3 method for stable geometries of a nematic liquid crystal molecule of MBBA assembled parallel on a molecule of single-walled carbon nanotube (CNT). The result showed that the interaction caused by π,π-stacking between MBBA molecule and the wall of CNT and that may lead to formation of the local short range orientation order by LC molecules on the surface of the CNT. The binding energy of the LC molecule on the CNT wall was within the typical van der Waals interaction.

Cholesteric liquid crystal-carbon nanotube hybrid architectures for gas detection

The ability of a hybrid material that is based on cholesteric liquid crystal and carbon nanotube to detect acetone vapor is investigated. We find that the phase transition in this cholesteric liquid crystal-carbon nanotube hybrid will enable carbon nanotube to form conducting networks under the higher vapor concentration. This cholesteric liquid crystal-carbon nanotube hybrid exhibits an obvious change in reflected color and electrical resistance in the early and later stages of gas diffusion, respectively. This hybrid architecture has potential application as a gas sensor with a high dynamic range.

Dispersions of multi-walled carbon nanotubes in liquid crystals: A physical picture of aggregation

Aggregation of carbon nanotubes dispersed in nematic liquid crystalline medium is discussed. A model assuming that the aggregates consist of a “skeleton” formed by stochastically arranged nanotubes and a “shell” (“coat”) of incorporated and adjacent nematic molecules is proposed. The aggregates of this type can be considered as large quasi-macroscopic particles in the nematic matrix. The resulting composite system is a representative of a new type of complex molecular liquids with self-organization of particles in anisotropic medium. Many essential features and implications of the theoretical model (e.g., effect of concentration, aspect ratio and orientational order parameter of the nanotubes on the size and fractal dimensionality of the aggregates formed, as well as on the rate of aggregation) are in good agreement with experimental data obtained by various physical methods.

Magnetic sensitivity of a dispersion of aggregated ferromagnetic carbon nanotubes in liquid crystals

Using carbon nanotubes filled with α-Fe, we have shown that aggregated ferronematic colloids demonstrate reliable and very effective response to a weak (<5 mT) magnetic field. The magnetic field realigns the aggregates of the particles which results in a non-threshold reorientation of the LC nearby, leading to the optically observed director distortions. The distortion regions expand with the increase of the magnetic field and achieve maximum size of several micrometres, comparable with the size of the agglomerates. In the non-distorted regions the reorientation of the director begins at the magnetic field reaching the Freedericksz transition value. Taking into account the extreme sensitivity of aggregated ferronematics to magnetic field, the following experimental and theoretical studies of the individual response of the aggregated nanoparticles to magnetic field may became the topical task of the physics and applications of ferronematics.

Liquid Crystal-Carbon Nanotubes Composites with the Induced Chirality: The Way Towards Enhancement of Electro-Optic Memory

It is found that the effect of electro-optic memory earlier revealed in the liquid crystal (LC) – carbon nanotube (CNT) suspensions can be substantially enhanced by doping the LC with small amount of chiral agent. The induced chirality weakens homeotropic anchoring set by the aligning layers helping CNT network to maintain state of planar LC alignment realized in the electric field.

Towards Efficient Dispersion of Carbon Nanotubes in Thermotropic Liquid Crystals

AbstractMotivated by numerous recent reports indicating attractive properties of composite materials of carbon nanotubes (CNTs) and liquid crystals (LCs) and a lack of research aimed at optimizing such composites, the process of dispersing CNTs in thermotropic LCs is systematically studied. LC hosts can perform comparably or even better than the best known organic solvents for CNTs such as N‐methyl pyrrolidone (NMP), provided that the dispersion process and choice of LC material are optimized. The chemical structure of the molecules in the LC is very important; variations in core as well as in terminal alkyl chain influence the result. Several observations moreover indicate that the anisotropic nematic phase, aligning the nanotubes in the matrix, per se stabilizes the dispersion compared to a host that is isotropic and thus yields random tube orientation. The chemical and physical phenomena governing the preparation of the dispersion and its stability are identified, taking into account enthalpic, entropic, as well as kinetic factors. This allows a guideline on how to best design and prepare CNT–LC composites to be sketched, following which tailored development of new LCs may take the advanced functional material that CNT–LC composites comprise to the stage of commercial application.

Tailor-designed polyphilic promotors for stabilizing dispersions of carbon nanotubes in liquid crystals

We present a potent multifunctional molecular design concept for promoting the dispersion of carbon nanotubes (CNTs) in thermotropic liquid crystals (LCs), making CNT-in-LC dispersions of unprecedented stability possible and broadening the scope of potential applications.

Effect of electrooptical memory in suspensions of carbon nanotubes in liquid crystals

Electrooptical response and microstructure of dispersions of multiwall carbon nanotubes in N-(4-ethoxybenzilidene)-4-n-butyl aniline nematic liquid crystal (LC) are studied. Irreversible response on the applied electric field (electrooptical memory) was revealed in oriented layers of such suspensions. The essence of this effect consists in the fact that, after the switch-on and subsequent switch-off of the field, the optical transmittance of suspension layer placed between two crossed polarizers substantially increases compared to the initial value which is typical for homeotropic orientation of LC. The efficiency of electrooptical memory nonmonotonically depends on the concentration of nanotubes in suspension, cCNT, reaching its maximum at cCNT = 0.02–0.05 wt %. It is shown that the memory of suspensions is caused by the incomplete relaxation of LC molecules from planar to initial homeotropic state after the electric field switch-off. The model is proposed and substantiated, according to which the planar state of LC is stabilized by the network of nanotubes formed upon the disintegration of aggregates under the action of electrohydrodynamic flows. The disclosed memory effect is rather common; it is brought about in the suspensions of carbon nanotubes based on other LCs in which electrohydrodynamic instabilities are developed.

Understanding the observation of large electrical conductivity in liquid crystal-carbon nanotube composites

We report the electrical conductivity and dielectric constant measurements in composites of single-walled carbon nanotubes (CNTs) with nematic liquid crystals, employing electrical and magnetic fields as reorienting fields. Our studies demonstrate that the large magnitude of enhancement in the electrical conductivity reported in voltage-driven reorientation measurements is not due to the inherent property of CNTs, and that dielectric breakdown and local heating effects play an important role. The calculated magnitude of the local heating effect is in agreement with the experimental observation including the nematic-isotropic transition caused by such Joule heating.