Understanding ionic phenomena in liquid crystals doped with nanoparticles
(Nanowerk Spotlight) Liquid crystal materials are ubiquitous in everyday life. The wide range of commercial applications of liquid crystals includes inexpensive and reliable displays for all kinds of electronic devices, tunable optical components for medical equipment, biological and chemical sensors, reconfigurable elements of wireless communication systems, just to name a few.
Typically, such applications utilize so-called thermotropic liquid crystals (i.e. if the order of their components is determined or changed by temperature). They combine the properties of crystalline solids and isotropic liquids.
Thermotropic liquid crystals are made of aligned anisometric (rod-, disk-, and bowl-like) molecules. The average orientation of liquid crystal molecules is very sensitive to external factors including electric and magnetic fields, temperature, and pressure.
Liquid crystal molecules respond to changing external stimuli via changes in their average orientation. As a result, their basic physical properties change, and those changes can be visualized and quantified (for instance, we can observe light of different color and varying intensity as it passes through a thin layer of liquid crystals).
This simple principle enabled numerous applications of liquid crystals. Commercial liquid crystal devices use electric fields to control the average orientation of liquid crystals – this average orientation of liquid crystal molecules is called a 'liquid crystal director'.
The reorientation of a liquid crystal director under the action of applied electric fields can be altered by ions, which are always present in liquid crystals in minute quantities. The applied electric field separates positive and negative ions in liquid crystals. The separated ions create their own electric field acting against the external electric field and weakening it (this process is called a screening effect), and compromising the overall performance of liquid crystal devices.
This is the reason researchers have studied ions in liquid crystals since the early 1960s. The electrical measurements have become a standard part of a general material characterization of newly synthesized liquid crystals.
Recently, a new way to create advanced liquid crystal materials by merging nanotechnology and liquid crystals was discovered: by adding nanoscale objects to liquid crystals, new materials with superior physical properties can be created. However, this raises an important question: How do nanoparticles affect ionic processes in liquid crystals?
"Nanomaterials can be used to control ions in liquid crystals and numerous research papers indicate that nanomaterials in liquid crystals can affect ions in different ways," Garbovskiy tells Nanowerk. "An important observation is that nanomaterials in liquid crystals can behave either as ion-capturing agents or as ion-generating objects. This experimental fact should be considered if liquid crystals doped with nanomaterials are used in the design of commercial products."
Ion-capturing nanoparticles can be used for permanent ionic purification of liquid crystals thus benefitting their display and photonic applications. Ion-generating nanoparticles can be utilized in the design of smart windows and light shutters.
"From an academic point of view, it is very important to understand physical and chemical mechanisms of nanoparticle-induced ionic effects in liquid crystals," Garbovskiy points out.
The accumulated knowledge generated by the collective efforts of numerous research teams around the globe offers a better understanding of basic ionic effects in liquid crystals doped with nanomaterials. However, given the complexity of these systems, there are still many unknowns.
"Ions in thermotropic liquid crystals can affect nearly all existing and emerging applications including the design and performance of tunable metamaterials, multifunctional and plasmonic materials producing structured light, liquid crystal sensors, and numerous photonic devices," Garbovskiy explains. "Many complex physical phenomena in liquid crystals such as self-assembly, the formation of topological defects and solitons can be affected by ions."
Nanomaterials and ions in liquid crystals. Science and applications of liquid crystals doped with nanomaterials (top), and future research directions (bottom). Adapted from Y. Garbovskiy 2021 Nano Ex. 2 012004 doi:10.1088/2632-959X/abe652 under the terms of the Creative Commons Attribution 4.0 licence) (click on image to enlarge)
A thorough understanding of ionic phenomena in liquid crystals/nanomaterials composites is critical for both applications and fundamental science of liquid crystals.
Garbovskiy hopes that his topical review will inspire young scientists to study ionic phenomena in liquid crystals doped with nanomaterials.
Future research directions are very broad and research opportunities in this area are nearly unlimited (see Figure above). They include the development of improved experimental techniques for measuring ions in liquid crystals doped with nanomaterials; research aimed at uncovering sources of ionic contamination of nanomaterials; studies of ion trapping and ion generating effects as a function of external factors (temperature, strong electric field, electromagnetic radiation, sound, etc.); charging of nanomaterials in liquid crystals; and the development of improved theoretical and computational models, to name a few.
"One day, full control of ions in liquid crystals by means of nanoparticles will become a reality," Garbovskiy concludes. "As a result, we can envision a much broader range of applications of liquid crystals doped with nanomaterials."