Transient charging current in nematic liquid crystals
01 Feb 1993-Journal of Applied Physics (American Institute of Physics)-Vol. 73, Iss: 3, pp 1119-1125
TL;DR: In this paper, a single and a double-voltage pulse technique was used to investigate the role of impurity ions in the materials in the transient charging processes of nematic liquid crystals.
Abstract: Transient charging currents in nematic liquid crystals have been measured with a single‐ and a double‐voltage pulse technique to elucidate the role of impurity ions incorporated in the materials in the transient charging processes. In the single pulse experiment, a peak in the current transient is observed in the nematic phase. It is shown that occurrence of the peak is due to the alignment of the director of the liquid crystal molecules in the direction of applied electric field. In the double pulse experiment, a voltage pulse (prepulse) is applied to the liquid crystal before the measurement of the transient current. It is observed that the application of the prepulse significantly alters the shape of the current. The changes in the current can be explained by considering the impurity ion distribution, modified by the prepulse, in the nematic liquid crystals. It is concluded that the double pulse experiment is a powerful tool for the examination of the influence of impurity ions in liquid crystals on the current transients.
Citations
More filters
TL;DR: In this paper, the carrier transport of a calamitic liquid crystal, i.e., 2-(4′-octylphenyl)6-dodecyloxynaphthalene (8-PNP-O12), was investigated by the time-of-flight technique.
Abstract: The carrier transport of a new calamitic liquid crystal, i.e., 2-(4′-octylphenyl)6-dodecyloxynaphthalene (8-PNP-O12) was investigated by the time-of-flight technique. The fast ambipolar electronic conduction was observed in two smectic phases of 8-PNP-O12. The carrier mobilities were determined to be 1.6×10−3 cm2/V⋅s in the lower temperature smectic phase (Sm1) between 79 and 100 °C and to be 2.5×10−4 cm2/V⋅s in the higher temperature phase (Sm2) between 100 and 121 °C, regardless of carrier signs, i.e., electron and hole. The slower ambipolar transport was observed in isotropic phase as well, whose mobility was 8×10−5 cm2/V⋅s. These mobilities were independent of applied electric fields and temperature.
138 citations
02 Nov 2015
TL;DR: In this article, the effects of nanoscale dopants on the electrical properties of liquid crystals were investigated and the most promising inorganic and organic nanomaterials suitable to capture ions in liquid crystals was identified.
Abstract: The presence of ions in liquid crystals is one of the grand challenges that hinder the application of liquid crystals in various devices, which include advanced 3-D and flexible displays, tunable lenses, etc. Not only do they compromise the overall performance of liquid crystal devices, ions are also responsible for slow response, image sticking, and image flickering, as well as many other negative effects. Even highly purified liquid crystal materials can get contaminated during the manufacturing process. Moreover, liquid crystals can degrade over time and generate ions. All of these factors raise the bar for their quality control, and increase the manufacturing cost of liquid crystal products. A decade of dedicated research has paved the way to the solution of the issues mentioned above through merging liquid crystals and nanotechnology. Nano-objects (guests) that are embedded in the liquid crystals (hosts) can trap ions, which decreases the ion concentration and electrical conductivity, and improves the electro-optical response of the host. In this paper, we (i) review recently published works reporting the effects of nanoscale dopants on the electrical properties of liquid crystals; and (ii) identify the most promising inorganic and organic nanomaterials suitable to capture ions in liquid crystals.
119 citations
TL;DR: In this paper, the frequency dependence of the complex dielectric constant of liquid crystal materials doped with tetra-n-butylammonium iodide (TBAI) is investigated in the low-frequency region, and the experimental results are analyzed in terms of space charge polarization.
Abstract: The frequency dependence of the complex dielectric constant of liquid crystal materials doped with tetra-n-butylammonium iodide (TBAI) is investigated in the low-frequency region, and the experimental results are analyzed in terms of space charge polarization. The contribution from an electric double layer is also taken into consideration in the analysis. By means of curve fitting utilizing theoretical expressions of the space charge polarization, five sets of diffusion coefficient and density values are obtained for mobile ions. It is confirmed by experiments on the temperature dependence that five kinds of ions follow Walden's rule, and verified from the viewpoint of ion radii that two of the five kinds of ions are TBA+ and I-. The frequency-dependent dielectric properties, which are characteristic of the behaviors of ions, can be well explained by this study and the analytical method introduced here is considered to be powerful for the evaluation of the attributes of mobile ions.
79 citations
TL;DR: In this paper, a small quantity of BaTiO3 ferroelectric nanoparticles (FNPs) of 50nm diameter was doped in a nematic liquid crystal (LC), and the free ion concentration was found to significantly reduced in the LC+FNP hybrid compared to that of the pure LC.
Abstract: A small quantity of BaTiO3 ferroelectric nanoparticles (FNPs) of 50 nm diameter was doped in a nematic liquid crystal (LC), and the free ion concentration was found to be significantly reduced in the LC + FNP hybrid compared to that of the pure LC. The strong electric fields, due to the permanent dipole moment of the FNPs, trapped some mobile ions, reducing the free ion concentration in the LC media. The reduction of free ions was found to have coherent impacts on the LC's conductivity, rotational viscosity, and electric field-induced nematic switching.
66 citations
Journal Article•
TL;DR: In this article, a small quantity of graphene, containing both monolayer and multilayer flakes, was doped in a nematic liquid crystal (LC), and the nematic electro-optic switching was found to be significantly faster in the LC + graphene hybrid than that of the pure LC.
Abstract: A small quantity of graphene, containing both monolayer and multilayer flakes, was doped in a nematic liquid crystal (LC), and the nematic electro-optic switching was found to be significantly faster in the LC + graphene hybrid than that of the pure LC. Additional studies revealed that the presence of graphene reduced the free ion concentration in the nematic media by ion-trapping process. The reduction of mobile ions in the LC was found to have subsequent impacts on the LC's conductivity and rotational viscosity, allowing the nematic director to respond quicker on switching the electric field on and off.
56 citations
References
More filters
Book•
01 Feb 1974
TL;DR: In this paper, the authors define an order parameter statistical theories of the nematic order phenomonological description of the nematic-isotopic mixtures and describe the properties of these mixtures.
Abstract: Part 1 Liquid crystals - main types and properties: introduction - what is a liquid crystal? the building blocks nematics and cholesterics smectics columnar phases more on long-, quasi-long and short-range order remarkable features of liquid crystals. Part 2 Long- and short-range order in nematics: definition of an order parameter statistical theories of the nematic order phenomonological description of the nematic-isotopic mixtures. Part 3 Static distortion in a nematic single crystal: principles of the continuum theory magnetic field effects electric field effects in an insulating nematic fluctuations in the alignment hydrostatics of nematics. Part 4 Defects and textures in nematics: observations disclination lines point disclinations walls under magnetic fields umbilics surface disclinations. Part 5 Dynamical properties of nematics: the equations of "nematodynamics" experiments measuring the Leslie co-efficients convective instabilities under electric fields molecular motions. Part 6 Cholesterics: optical properties of an ideal helix agents influencing the pitch dynamical properties textures and defects in cholesterics. Part 7 Smectics: symmetry of the main smectic phases continuum description of smectics A and C remarks on phase and precritical phenomena.
9,683 citations
TL;DR: A simple model based on the time variation of an applied voltage and the internal electric field induced by the adsorbed electric charges on the cell substrate has been proposed and the results from the model agree reasonably well with the experimental observations.
Abstract: Transient currents induced by the polarity reversal of a dc voltage applied to a liquid-crystal cell have been experimentally studied. For voltages less than a characteristic value, the time occurrence of the peak current decreases with decreasing applied voltage, which is contrary to behavior expected for a normal carrier-transport process. To explain the experiments, a simple model based on the time variation of an applied voltage and the internal electric field induced by the adsorbed electric charges on the cell substrate has been proposed. The results from the model agree reasonably well with the experimental observations.
74 citations
TL;DR: In this article, an anomalous peak in discharging current transients in pentyl cyanobiphenyl liquid crystals has been observed in both nematic and isotropic phase.
Abstract: Measurements of discharging current transients in pentyl cyanobiphenyl liquid crystals have been made at various applied voltages and temperatures. A well-characterized peak in the discharging transient is observed in both nematic and isotropic phase. A computer simulation based upon a random-walk method has been also carried out in order to explore the origin of such an anomalous peak. It is concluded that the occurrence of the anomalous peak is ascribed to a relaxation of an applied-electric-field-induced nonuniform distribution of space charges due to impurity ions incorporated in the liquid crystals.
57 citations
53 citations
TL;DR: A peak current was observed in the transient electric current in an n-type nematic liquid crystal, p-methoxybenzylidene-p-n-butylaniline (MBBA) by the application of a rectangular pulse voltage as mentioned in this paper.
Abstract: A peak current was observed in the transient electric current in an n-type nematic liquid crystal, p-methoxybenzylidene-p-n-butylaniline (MBBA) by the application of a rectangular pulse voltage. It was conjectured that the peak current was obtained due to the carrier transit from one electrode to the other. Experiments on the dependences of the applied voltage and sample thick ness on the transient current were carried out. The carrier mobility was calculated ca. 1.6×10-5 cm2/Vsec at 30°C from these experiments. The activation energy was derived as ca. 0.20 eV from the temperature dependence of the current. Modified Walden's rule seems applicable to the transient electric current in MBBA.
23 citations