The planar-alignment agent in an electro-optic liquid crystal (LC) device plays an essential role for the LC's electro-optical characteristics. Rubbed polyimide (PI) layers are conventionally used as the planar-alignment agent in traditional liquid crystal displays (LCDs). Here we experimentally demonstrate that the 2D hexagonal boron nitride (h-BN) nanosheet can serve as the planar-alignment agent in an LC cell. This h-BN has higher chemical stability and more optical transparency than the PI layer. Two h-BN-covered indium tin oxide (ITO) glass slides (without any conventional PI layers) are placed together to fabricate an LC cell. A nematic LC inside this h-BN-based cell exhibits uniform planaralignment-which is probed by a crossed polarized optical microscope. This planaralignment at the molecular scale is achieved due to the coherent overlay of the benzene rings of the LC molecules on the hexagonal BN lattice. This h-BN-based LC cell shows the typical electro-optical effect when an electric field is applied via ITO electrodes. The dielectric measurement across this h-BN-based electro-optic cell shows a standard Fréedericksz transition of the LC, confirming that the 2D h-BN, as the planar-alignment agent, supplies adequate anchoring energy-which can be overcome by the Fréedericksz threshold voltage. Finally, we show that the h-BN-based LC cell exhibits more optical transparency than a regular PI alignment layer-based LC cell.

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Two-dimensional (2D) tungsten diselenide (WSe2) nanosheets were transferred onto indium tin oxide (ITO) coated glass slides. Two such 2D WSe2-covered ITO glass slides were placed together to fabricate an electro-optic liquid crystal (LC) cell. A nematic LC inside this WSe2-based device obtains uniform planar-alignment. The coherent overlay of the LC molecules along the armchair direction on the WSe2 lattice leads to this planar-alignment at the molecular scale. This WSe2-based LC device exhibits the typical electro-optical effect on the application of an electric field. A sharp Fréedericksz transition of the nematic director from this electro-optical effect confirms that the 2D WSe2 provides sufficient planar anchoring energy in the device. Finally, the WSe2/ITO slide exhibits more optical transparency than a standard polyimide alignment layer/ITO slide.

The presence of excess free-ion impurities in liquid crystals (LCs) gives rise to a number of problems in the electro-optical liquid crystal displays (LCDs), e.g., slow electro-optical responses and image sticking effects. Here we experimentally present that the two-dimensional (2D) hexagonal boron nitride (h-BN) nanosheet can serve as a planar-alignment agent and as an ion-capturing agent at the same time in an electro-optic LC device. The 2D h-BN nanosheet is employed as a planar-alignment agent on one side of an LC cell, where the standard planaraligning polyimide (PI) layer is used on the other side of the cell. The LC exhibits uniform planar-alignment in this h-BN/PI hybrid device. It is found that the free-ion impurities in the LC are significantly suppressed in this h-BN/PI hybrid cell compared to that in a standard PI/PI LC cell. The free-ion density is reduced in the hybrid cell due to the 2D h-BN nanosheet's ion-capturing process. The reduction of ionic impurities results in an accelerated electro-optic response of the LC in the h-BN based hybrid cell-which may have potential application for faster electro-optic devices.

A small quantity of hexagonal boron nitride (h-BN) flakes is doped into a nematic liquid crystal (LC). The epitaxial interaction between the LC molecules and the h-BN flakes rising from the π−π electron stacking between the LC’s benzene rings and the h-BN’s honeycomb structure stabilizes pseudo-nematic domains surrounding the h-BN flakes. Electric field-dependent dielectric studies reveal that the LC-jacketed h-BN flakes follow the nematic director reorientation upon increasing the applied electric field. These anisotropic pseudo-nematic domains exist in the isotropic phase of the LC+h-BN system as well, and interact with the external electric field, giving rise to a nonzero dielectric anisotropy in the isotropic phase. Further investigations reveal that the presence of the h-BN flakes at a low concentration in the nematic LC enhances the elastic constants, reduces the rotation viscosity, and lowers the pre-tilt angle of the LC. However, the Fréedericksz threshold voltage stays mostly unaffected in the presence of the h-BN flakes. Additional studies show that the presence of the h-BN flakes enhances the effective polar anchoring strength in the cell. The enhanced polar anchoring strength and the reduced rotational viscosity result in faster electro-optic switching in the h-BN-doped LC cell.