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New insights into the electrical properties of liquid crystals doped with nanoparticles

(Nanowerk Spotlight) Many of the electronic devices we use in our daily life – laptops, smartphones, smart watches, tablets, car navigation, etc. – rely on liquid crystal display (LCD) technologies.
LCDs get their name from the special liquid crystal solution that is contained between two thin glass plates inside the display. An electric field applied across the liquid crystal layer changes optical properties of the liquid crystals thus enabling their use in displays.
"Typically, high resistivity liquid crystals are required for display applications," Yuriy Garbovskiy, PhD, a researcher at the UCCS BioFrontiers Center & Department of Physics, University of Colorado, tells Nanowerk. "As a result, the concentration of mobile ions in liquid crystals should be as small as possible. However, small traces of ions are always present in liquid crystal materials leading to many undesirable side effects including image sticking, image flickering, and slow time response of the device."
That is why the concentration of mobile ions in liquid crystals is a very important parameter which determines the suitability of particular liquid crystals for display applications.
The penetration of nanotechnology into liquid crystal science resulted in the development of new ways to control the concentration of mobile ions in liquid crystals by means of nanoparticles. Recently we reported some of these findings in several Nanowerk Spotlights: "Ionic purity of nanoparticles is key to switching between purification and contamination regimes in liquid crystal devices"; "A nanotechnology approach to purifying liquid crystals"; and "Merging nanotechnology and liquid crystals for better displays".
In many practical cases, the consideration of fully ionized species-contaminants of a single type is sufficient to describe ions in liquid crystals. A new paper by Garbovskiy, just published in Chemical Physics Letters("Ions and size effects in nanoparticle/liquid crystal colloids sandwiched between two substrates. The case of two types of fully ionized species"), considers two types of fully ionized species-contaminants in liquid crystals doped with nanoparticles and sandwiched between two substrates.
"The need to discuss two types of fully ionized species can be clearly seen by considering liquid crystals characterized by one type of fully ionized species-contaminants, and substrates or nanoparticles contaminated with the other type of fully ionized species," Garbovskiy points out.
This new paper reports several interesting size effects including monotonous and non-monotonous dependence of the total concentration of mobile ions in liquid crystals on the thickness of the cell and/or on the concentration of nanoparticles.
Interestingly, this dependence can be tuned by varying the concentration of nanoparticles, their size and ionic purity (the ionic purity of substrates and nanoparticles can be quantified by means of the dimensionless contamination factor ω (0 ≤ ω ≤ 1).
The total concentration of mobile ions in liquid crystals doped with nanoparticles as a function of their weight concentration calculated at several values of the cell thickness
Figure 1. The total concentration of mobile ions in liquid crystals doped with nanoparticles as a function of their weight concentration calculated at several values of the cell thickness.
An example of non-monotonous dependence of the concentration of mobile ions in liquid crystals on the weight concentration of nanoparticles is shown in Figure 1 above. Nanoparticles were assumed contaminated with ions prior to dispersing them in liquid crystals and were characterized by the contamination factor ωNP = 0.0001.
These results indicate that the type of the observed behavior (monotonous or non-monotonous) depends on the cell thickness.
Another interesting example of both monotonous and non-monotonous behavior is shown in Figure 2. In this case, the dependence of the concentration of mobile ions on the cell thickness can be altered by the presence of nanoparticles.
The total concentration of mobile ions in liquid crystals doped with nanoparticles as a function of the cell thickness calculated at several values of the weight concentration of nanoparticles
Figure 2. The total concentration of mobile ions in liquid crystals doped with nanoparticles as a function of the cell thickness calculated at several values of the weight concentration of nanoparticles.
In both cases shown in Figures 1-2, the presence of fully ionized species of two types along with the adsorption of these ions onto the surface of substrates and nanoparticles are major physical reasons leading to the observed behavior.
"Our new findings have important practical implications," notes Garbovskiy. "Ionic contamination of both substrates and nanoparticles should be checked by experimentalists to adequately interpret experimental data."
"In addition" he continues, "electrical measurements of newly synthesized liquid crystal materials or nanocomposites made of liquid crystals and nanoparticles should be taken at several values of the cell thickness. More generally, existing protocols to characterize liquid crystals and liquid crystal nanocolloids should be modified to account for tricky behavior illustrated by Figures 1-2."
Given the enormous variety of liquid crystals and nanomaterials, future studies of the complex behavior of ions in liquid crystal nanocolloids can go different ways by exploring various types of materials and effects of external factor such as temperature, strong electric field, irradiation with light, etc.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk
 

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