A research team led by Rajratan Basu, a professor of physics at the US Naval Academy, has shown that the two-dimensional (2D) hexagonal boron nitride (h-BN) can be used as an alignment layer in liquid crystal (LC) cells. These results, published recently in Optics Express, demonstrate the potential for optimizing liquid crystal devices using 2D materials.

Liquid crystal cells are used to manipulate light by applying a voltage. In these cells, “changing the orientation of the liquid crystal molecules by applying an external field changes the polarization of light passing through the cell,” says Basu. This property, known as the electrically controlled birefringence effect, can be combined with two crossed polarizers to control the intensity of light that passes through the liquid crystal cell. When incorporated into television displays (LCD TVs), images are created by delivering specific voltages into different liquid crystal cells.

In addition to the liquid crystal material, each cell contains two transparent electrodes to supply the voltage and two alignment layers to provide an initial direction for the liquid crystal molecules in the absence of a field. In commercial devices, these alignment layers can be >100 nm thick, leading to losses in light transmitted through the cell.

Basu’s research group hopes to improve liquid crystal cell performance by incorporating atomically thin 2D materials, such as graphene or h-BN, into these device architectures. However, finding materials with high performance while remaining stable under applied voltage can be challenging.

In this work, a monolayer of h-BN was presented as a novel alignment layer inside the liquid crystal cell. The group found that the hexagonal lattice structure of the h-BN layer could be matched with that of a liquid crystal molecule containing multiple benzene rings. “The nano-architectural symmetry causes the benzenes of the liquid crystals to align onto the hexagonal lattice of h-BN,” Basu says.

The h-BN formed a template for the liquid crystal molecules, which aligned, on average, in a single direction. Because of this average alignment, light passing through the cell was polarized in a single direction. When combined with two crossed polarizers, the liquid crystal cell could be switched from light to dark by applying a voltage. Notably, the total transmission of light through the h-BN LC cell increased by 20% over an LC cell that used a conventional polyimide (PI) alignment layer. “This is likely due to the significantly reduced thickness of the [monolayer] h-BN layer in comparison to the PI layer, which is one of the key benefits of 2D materials,” says Anna Baldycheva, a professor of electrical engineering and leader of STEMM Lab at the University of Exeter. Baldycheva was not affiliated with the work published in Optics Express.

From these results, Baldycheva says that liquid crystal cells using h-BN alignment layers could be further optimized by slightly distorting the 2D material’s honeycomb lattice structure. The structure of h-BN lattice has three-fold degeneracy; therefore, the LC molecules can align on the 2D material in three possible orientations. Two of these orientations are mirror images of each other and have no effect on the average alignment. However, Baldycheva says that breaking the symmetry of the h-BN could “ensure a single alignment direction [of the LC molecules] on the h-BN surface.”

In his own laboratory, Basu hopes to replace other components in the LC cell with 2D materials. Basu envisions a “graphene, h-BN heterostructure LC device where graphene is employed as the electrode and h-BN as the alignment agent.” Using two types of 2D materials could provide another drastic improvement in liquid cell transmission.

Both researchers see this work as an indication that 2D materials can significantly improve the performance of liquid crystal cells, even for commercial applications. “These results could form the basis for the development of new commercially viable LC cells by integrating 2D materials as the alignment layers in order to improve the performance,” Baldycheva says.

Read the article in Optics Express.