Reduced ionic effect and accelerated electro-optic response in a 2D hexagonal boron nitride planar-alignment agent based liquid crystal device

Abstract

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.

Dielectric and Electro-Optic Effects in a Nematic Liquid Crystal Doped with h-BN Flakes

“…For example, the epitaxial interactions between the nematic LC and the hexagonal lattice of graphene, molybdenum diselenide, and tungsten diselenide deposited [1][2][3][4][5][6][7][8][9][10] demonstrate that the LC can achieve a planar-aligned state on these 2D hexagonal nanostructures. Our group has recently shown that the 2D hexagonal boron nitride (h-BN) film can be successfully employed as a planar-alignment agent as well as an ion-capturing agent in electro-optic LC devices [11][12][13]. Here we report the effects of ultrafine 2D h-BN flakes as a colloidal dopant in a nematic LC.…”

“…We have recently shown that when an h-BN nanosheet is utilized as a planar-alignment agent in an LC the cell, the h-BN nanosheet captures a significant amount of free ion impurities from the LC by ion-trapping process [11]. It has been also shown that the reduction of free ions in the LC results in a reduced γ 1 [5,9,[27][28][29].…”

“…Since W θ and γ 1 influence the dynamic electro-optic response in an LC device mainly, the electro-optic switching times for both the pure LC and LC+h-BN samples in 20 µm cells have been investigated. These response times in both the cells were measured by employing the standard electro-optical setup [6,[11][12][13], where a 5-mW He-Ne laser beam of wavelength 633 nm was sent through two crossed-polarizers and into a photodetector. The planar LC test cell was placed in-between the crossed polarizers wheren in the cell was aligned at 45 • with respect to the polarizer.…”

A generic approach for aligning liquid crystals using solution-processed 2D materials on ITO-free surfaces

“…The absence of such a peak in the h-BN/AZO case indicates a lower ion concentration in the medium, possibly due to the better ion trapping capability of the h-BN layer. 33,53 The relationship between N and the integral of transient/ ionic current is given by:…”

“…27 Another emphatic aspect that should be borne in mind is that the traditional methods employed to have planar LC orientation utilize the interaction of the terminal chain of the LC with the aligning layer. The latest development in this area is to use layers of twodimensional (2D) materials like graphene, [28][29][30] hexagonal boron nitride (h-BN), [31][32][33][34][35][36] molybdenum diselenide (MoSe 2 ), 37 tungsten diselenide (WSe 2 ), 37,38 highly oriented pyrolytic graphite (HOPG), 39 molybdenum disulfide (MoS 2 ), 40 etc., for this purpose. The epitaxial interaction between the ubiquitous phenyl ring of the liquid crystal and the hexagon of the 2D material results in planar alignment of the LC molecules.…”