Suppressed ionic effect and low-frequency texture transitions in a cholesteric liquid crystal doped with graphene nanoplatelets.

Po-Chang Wu,Longin N Lisetski,Wei Lee

doi:10.1364/oe.23.011195

Abstract

We focus on investigating the dielectric behaviors and the low-frequency texture transitions in a cholesteric liquid crystal (CLC) doped with graphene nanoplatelets (GNPs) by means of dielectric spectroscopy and measurements of electro-optical responses. The experimental results indicate that incorporating GNPs at a content of 0.5 wt% into the CLC leads to significant suppression of ionic behaviors, as manifested by the reduction in ionic density, diffusivity, and relaxation frequency. In addition, the electro-optical properties of the GNP-doped CLC cell show the lowered operation voltage for the switching from the planar to focal conic state and the absence of the low-frequency focal-conic-to-uniform-lying-helix texture transition. Such results are attributable to the effects of GNPs as nuclei in the CLC medium, giving rise to the repression of the ionic and electrohydrodynamic effects.

Highlights

Liquid crystals (LCs), known for their uniquely anisotropic material properties, have found a wide spectrum of applications in electro-optical and photonic devices, especially the flatpanel displays
The differences between the doped cholesteric liquid crystal (CLC) and the pure one within a designated temperature regime are rather small (Fig. 1(b)). This suggests that graphene nanoplatelets (GNPs) as the dopant dispersed in the CLC do not show a significant effect on the helical structure and the optical profile of the Bragg reflection band of the CLC
The shift of the curve towards higher frequencies with increasing temperature for both the undoped and doped CLC cells can be attributable to the promotion of the ion transport at higher temperatures or to an increase in the number of charge carriers contributing to electric conduction in the cells

Summary

Introduction

Liquid crystals (LCs), known for their uniquely anisotropic material properties, have found a wide spectrum of applications in electro-optical and photonic devices, especially the flatpanel displays. When applying voltages to the CLC, two typically stable states—the planar (P) and focal conic states (FC)—can be achieved Based on such unique features, CLCs have widely been proposed for applications in memory-enabling reflective LC displays [2], lasers [3] and some other photonic devices such as optical switches, modulators as well as diodes [4,5,6]. Other studies of graphene-doped LC systems have shown the controlled alignment in electroclinic LCs [25], controlled optical properties in nematic LCs [26], stabilization of the LC state and improved operating voltage in blue phases [27,28], and enhanced electro-optical switching in ferroelectric LCs [29]. Since the space-charge energy plays a crucial role in the induction of the low-frequency electrohydrodynamic (EHD) effect in the CLC [30,31], the electro-optical responses of both the GNP-doped and pure CLC cells are compared to clarify the operation voltage and field-induced texture transition behaviors, especially for cells driven by low-frequency voltages

Experiment

Results and discussion

Conclusions