Electro-optic HAN device utilizing carbon nanotube arrays and 2D hexagonal boron nitride nanosheet as alignment substrates*

Hybrid-aligned nematic (HAN) liquid crystal (LC) devices have both fundamental and technological importance for their applications in LC adaptive lenses, low voltage LC displays, smart windows, and many more.  We report the fabrication and characterization of a novel nanostructure-based HAN device employing vertically aligned carbon nanotube (VA-CNT) arrays as the homeotropic alignment agent on one side and 2D hexagonal boron nitride (h-BN) as the planar alignment agent on the other side of the LC cell.  The LC achieves the HAN configuration in the cell, i.e., homeotropic alignment at the VA-CNT side due to the π-π stacking interaction between the LC and CNTs, and planar alignment at the h-BN side due to the π-π stacking interaction between the LC and h-BN.   When an applied electric field is ramped up across this VA-CNT/h-BN HAN cell, the LC (positive anisotropic) obtains a homeotropic state, requiring no threshold voltage to start the reorientation process—this effect is similar to that of a traditional polyimide (PI)-based HAN device.  This VA-CNT/h-BN HAN cell successfully demonstrates the optical, electro-optical operations and the electric field-induced dynamic response.  This study reveals that two inorganic nanostructured surfaces, i.e., VA-CNT arrays and 2D h-BN, can efficiently replace the organic PI alignment agents when needed and retain the HAN device's necessary electro-optical performances.  These results substantially expand the fundamental understanding and the scope of utilizing various nanostructured surfaces for LC alignment mechanisms.