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

Rajratan Basu,Lukas J Atwood

doi:10.1364/ome.9.001441

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 planar-aligning 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.

Highlights

The existence of excess free ions [1,2] in liquid crystals (LC) causes complications in electrooptical liquid crystal displays (LCDs)
Investigating the influence of the free ions on the LC’s electrical, mechanical, and electro-optical properties [13,14,15,16,17,18,19,20] is an essential area of fundamental and applied research. There has been another exciting direction of research which shows that the colloidal dispersion of nanomaterials, such as ferroelectric nanoparticles [21,22], titanium nanoparticles [23], carbon nanotubes [24,25,26], graphene [27,28,29,30,31,32], and fullerenes [23,33,34] in the LC can suppress the free-ion concentration significantly by the ion-trapping process
We show that the presence of this 2D hexagonal boron nitride (h-BN) planar-alignment nanosheet can greatly suppress the free-ion concentration in the LC

Summary

Introduction

The existence of excess free ions [1,2] in liquid crystals (LC) causes complications in electrooptical liquid crystal displays (LCDs). The ion-related problems in LCDs include slow responses, short-term flickering, and long-term image sticking effects [3,4,5,6,7,8,9] These free-ions generally come from the LC’s chemical synthesis process, the LC cell’s conductive electrodes [10,11], and the polyimide (PI) alignment layers [12]. We show that the presence of this 2D h-BN planar-alignment nanosheet can greatly suppress the free-ion concentration in the LC This h-BN/PI hybrid LC cell exhibits a faster electro-optic response, higher optical transparency, and an enhanced dielectric anisotropy than a commercial PI/PI LC cell. We present (a) the fabrication of the h-BN based planar LC cell, (b) the ion-trapping phenomenon by the h-BN alignment agent, (c) the effect of free-ion reduction on the electro-optic switching of the LC, (d) the optical transmission spectroscopy of the h-BN based cell, and (e) the effect of free-ion reduction on the anisotropy of the LC

Fabrication of the 2D h-BN based LC device

Findings

Conclusions