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Posted: Mar 08, 2017

Merging nanotechnology and liquid crystals for better displays

(Nanowerk Spotlight) Liquid crystals displays (LCDs) are widely used for laptops, smartphones, tablets etc. Typically, an electric field applied across a thin layer of liquid crystals changes their physical properties thus enabling the use of liquid crystalline materials in displays.
Small traces of ions inherently present in liquid crystal materials can compromise the overall performance of liquid crystal devices leading to various side effects such as image sticking, image flickering, or slow response.
Ideally, the amount of ions in liquid crystals designed for display applications should be as small as possible. However, even highly purified liquid crystals can get contaminated in uncontrollable ways during device manufacturing or during its daily use. This uncontrollable contamination is a serious challenge that needs to be overcome.
One promising solution can be found by merging liquid crystals and nanotechnology. In short, nanoparticles dispersed in liquid crystals can trap mobile ions thus reducing their concentration and providing a permanent purification of liquid crystals. This simple concept was tested by many independent research groups (more details can be found in a recent review: Crystals, "Nano-Objects and Ions in Liquid Crystals: Ion Trapping Effect and Related Phenomena").
We have highlighted the necessity to consider the ionic purity of nanoparticles for the correct interpretation of experimental results and achieving the purification and/or contamination of liquid crystals in two previous Nanowerk Spotlights: "Ionic purity of nanoparticles is key to switching between purification and contamination regimes in liquid crystal devices" and "A nanotechnology approach to purifying liquid crystals".
"The majority of the reported experimental results do not discuss the effect of substrates on the purification / contamination regimes achieved in liquid crystals doped with nanoparticles," Yuriy Garbovskiy, PhD, a researcher at the UCCS BioFrontiers Center & Department of Physics, University of Colorado, tells Nanowerk. "However, liquid crystals sandwiched between two substrates constitute a major component of practically any electro-optical device utilizing these materials. Therefore, the consideration of the interactions of ions with substrates of the liquid crystal cell is very important."
An analysis of the combined effect of nanoparticles and substrates on the concentration of mobile ions in liquid crystals is the focus of Garbovskiy's recent paper in Applied Physics Letters ("Ion capturing/ion releasing films and nanoparticles in liquid crystal devices"). His analysis considers both 100% pure and contaminated with ions substrates (alignment layers) and nanoparticles.
These results could be very useful for R&D engineers trying to apply nanotechnology to liquid crystal devices. Specifically, the control of mobile ions in liquid crystals by means of nanoparticles and substrates of the cell tailored for specific applications – liquid crystal displays, light shutters, switches, modulators, etc.
The dependence of the concentration of mobile ions in the cell filled with pristine liquid crystals on the thickness of the cell
Figure 1. The dependence of the concentration of mobile ions in the cell filled with pristine liquid crystals on the thickness of the cell. The ionic purity of substrates is quantified by means of the dimensionless contamination factor vS. (Image: Dr. Yuriy Garbovskiy, University of Colorado, Colorado Springs)
"In general, we should consider interactions of ions in liquid crystals with both substrates and nanoparticles," says Garbovskiy. "To understand how substrates of the cell can affect the concentration of mobile ions in liquid crystals consider pristine (no nanoparticles are added) liquid crystals. Mobile ions in liquid crystals can stick to the surface of the substrates (the adsorption process)."
As a result, this process leads to the reduction in the concentration of mobile ions as shown in Figure 1 (solid curve). As can be seen from this, the magnitude of this effect depends on the cell thickness. It is very pronounced if relatively thin cells are used and becomes negligibly small in the case of relatively thick cells. The reduction in the concentration of mobile ions can always be observed if 100% substrates are used.
However, the use of contaminated substrates can result in three different regimes, namely, the ion capturing regime (Figure 1, solid curve), the ion releasing regime (Figure 1, dashed curve), and no change in the concentration of mobile ions (Figure 1, dotted curve).
"If nanoparticles are added to liquid crystals, we should consider the combined effect of substrates and nanoparticles," notes Garbovskiy.
Some of these effects are shown in Figure 2.
The dependence of the concentration of mobile ions in the cell filled with pristine liquid crystals on the thickness of the cell
Figure 2. The dependence of the concentration of mobile ions in the cell filled with liquid crystal nanocolloids (liquid crystals doped with nanoparticles) on the weight concentration of nanoparticles calculated at several values of the cell thickness, d. Nanoparticles are contaminated with ions (the contamination factor vNP=0.0002) and the substrates are 100% pure (the contamination factor vS=0). (Image: Dr. Yuriy Garbovskiy, University of Colorado, Colorado Springs)
As can be seen from Figure 2, depending on the cell thickness, different types of the behavior can be observed. Garbovskiy's recent paper in APL provides a detailed discussion of scenarios which can be achieved if the combined effect of substrates and nanoparticles is taking into account.
All these effects are very sensitive to the ionic purity of both nanoparticles and substrates of the liquid crystal cell.
These results have important practical implications. Figures 1 and 2 indicate that the electrical properties of liquid crystals and liquid crystal nanocolloids depend on the thickness of the cell. Moreover, this dependence is a strong function of the ionic purity of both nanoparticles and substrates of the cell.
As Garbovskiy points out: "We should revisit standard procedures used to characterize electrical properties of liquid crystals and include electrical measurements taken at different values of the cell thickness to the existing experimental protocols. The type of substrates used in experiments should be specified. Moreover, the manufacturers of liquid crystals should add all mentioned information to the materials datasheets."
The next step in his research will be to study various types of ion-capturing/ion-releasing substrates to compile a database and identify the most promising candidates.
"In my recent paper I assumed fully ionized symmetrical species of a single type; in other words: positive and negative ions are characterized by the same values of their adsorption/desorption rate constants," Garbovskiy says. "The consideration of ionic species of several types is also very interesting and important."
"Research on ions and nano-objects in liquid crystals is very diverse; it can go different ways by exploring various combinations of substrates, nanoparticles and liquid crystals," he concludes. "The design and characterization of ion-capturing/ion-releasing films and nanoparticles for their use in liquid crystal devices is a very interesting and promising direction."
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