Analysis of morphological and electro-optical properties of silica nanoparticles induced vertically aligned liquid crystal - effect of doping and coating techniques

Abstract

Conventionally, polyimide (PI) coating materials are used on the indium tin oxide (ITO) glass substrates to induce the vertical alignment (VA) of nematic liquid crystals (LCs). However, this coating technique requires a couple of steps such as spin coating, high-temperature baking and mechanical rubbing which limits their applicability to flexible LC display device applications. In the present work, VA of nematic LCs in a confined cell is induced using two different approaches: one via doping of silica nanoparticles (SNPs) as guest particles in LCs and the other with the deposition of SNPs layer on ITO glass substrates. Further, morphological and electro-optical (E-O) studies have been studied for the SNPs doped and SNPs coated vertically aligned liquid crystals (VALCs). The morphological study shows the identical degree of dark (black) and bright (white) textures of SNPs doped and coated VALC cells at OFF and ON states, respectively under the crossed polarizers. Also, the VA of nematic LC molecules in confined cells has been confirmed from the conoscopic behaviour. Consequently, morphological and E-O results revealed that the threshold (V th ) and operating (V o ) voltages required to switch the LCs from their initial VA (black state) towards minimum transition planar state and complete planar (bright) state, respectively, are reduced with better contrast ratio (CR) in SNPs coated VALC cell as compared with SNPs doped VALC cell.