Nanoparticles can cause unusual reorientation transitions in liquid crystals
(Nanowerk Spotlight) Many of the electronic devices we use in our everyday life employ liquid crystals in their optical displays – laptops, cell phones, flat screen TVs, digital watches, and calculators. Applications of liquid crystals are numerous and not just limited to displays: There are tunable liquid crystal filters, optical waveplates, camera lenses, beam-steering devices, spatial light modulators, and reconfigurable antennas for wireless communication, to name a few.
Electric field induced orientational transitions of elongated liquid crystal molecules are at the heart of the afore-mentioned technologies. Changing the applied voltage controls the orientation of liquid crystals and their properties such as color.
"Aligned liquid crystals can also impose their long-range orientational order on various organic and inorganic dopants," Yuriy Garbovskiy, Assistant Professor in the Department of Physics & Engineering Physics at Central Connecticut State University, tells Nanowerk. "In the simplest case, we can dissolve dopants such as pleochroic dyes – they absorb light of different colors depending on their orientation relative to the polarization of light – in a liquid crystal host."
"The applied electric field reorients the liquid crystal host, and the liquid crystal host via interactions with a dopant (a 'guest') reorients the guest," he explains. "As a result, we can control the orientation of the dopant by changing the orientation of the liquid crystal host. This effect is called the guest-host effect."
As a result, orientational transitions in liquid crystals can be controlled by ferroelectric nanoparticles dispersed in such materials.
Figure 1. Inverse guest-host effect. An image on the left shows the orientation of a pure (LC) and doped (FNP+LC) nematic liquid crystals with negative dielectric anisotropy (Δε < 0) when electrical field is not applied (the cell is placed in between two crossed polarizers). An image on the right shows the same twin cell under the action of the applied electric field: Ferroelectric nanoparticles (FNP), being aligned by an external electric field, hold liquid crystal (LC) molecules in homeotropic state even when an external electric field attempts to orient a liquid crystal in an orthogonal direction. Inset shows a twin cell (each cell is separated with a polymer stripe into two identical regions). The optical view of the cells placed in between two crossed polarizers and the difference in colors in two regions reveals immediately whether the LC is influenced by the embedded FNP. (Provided by Dr. Garbovskiy who adapted it from Nanoscale, 2020, 12, 16438 with permission from The Royal Society of Chemistry) (click on image to enlarge)
The team's research findings reveal an extreme behavior of ferroelectric nanomaterials in nematic liquid crystals never before observed in such systems. Two major factors resulted in the possibility to observe this unconventional electro-optical effect experimentally.
The first important factor is an enhanced ferroelectric response of ferroelectric nanoparticles prepared by means of wet grinding. As a result, the researchers were able to use relatively small ferroelectric nanoparticles (to make sure they do not disturb the orientation of liquid crystals via the creation of defects) at relatively low concentrations (to avoid excessive aggregation and sedimentation of nanoparticles in liquid crystals) that enabled the production of true liquid crystal colloids employed in their experiments. The size of the prepared ferroelectric nanoparticles was in the range of 10–20 nm (BaTiO3), 20–60 nm (Sn2P2S6), 20–50 nm (CuInP2S6) and 40–90 nm (SbSI).
The second important factor is the use of the unconventional configuration of the prepared samples (a twin cell shown in the middle inset, Figure 1).
These two factors enabled the observation of an extreme behavior of ferroelectric nanomaterials in liquid crystals when nanodopants can fully control the orientational transitions in nematic liquid crystals. As a result, the team was able to observe an inverse guest-host effect: ferroelectric nanoparticles reorient and hold liquid crystal molecules in a direction of the orientation of nanoparticles even when an external electric field attempts to orient a liquid crystal in an orthogonal direction as shown at the right of Figure 1.
"The possibility to control orientational transitions in nematic liquid crystals by means of ferroelectric nanoparticles broadens our understanding of such systems and opens the door to a variety of applications relying on LCs," they say. "In addition, this effect can be employed to study in situ guest–host interactions in liquid crystal nanocomposites."
Since the discovery of the guest-host effect in the early 1960s, researchers have explored different types of dopants in liquid crystals. The initially used molecular dopants such as organic dyes were gradually replaced with nanomaterials in the early 2000s.
"Nanomaterials in liquid crystals is a hot research topic, highly promising for a variety of electronic and photonic applications," Garbovskiy points out. "The number of research papers reporting the properties of various types of nanomaterials – carbon nanotubes, fullerenes, metal, dielectric, semiconductor, magnetic, and ferroelectric nanoparticles – in liquid crystals is growing rapidly. In the majority of cases, researches take advantage of the guest-host effect to study the behavior of the aligned nanodopants in the liquid crystal host."
The full potential of ferroelectric nanomaterials in liquid crystals is not fully understood yet and there are many experimental and theoretical unknowns – making this an exciting research field for years to come.
Experimental studies are typically limited to relatively low concentrations of nanomaterials in liquid crystals because of their tendency to aggregate. One experimental challenge is how to reduce the aggregation of ferroelectric nanoparticles and at the same time increase their concentration.
In addition, the role of nanoparticle size and their size distribution is also important. The theoretical model described in this paper is based on a continuum theory. According to the authors, it would also be very interesting to study the limits of this model, fine-tune it, and investigate transient phenomena in their systems.