Evaporation induced self-assembly of silica nanoparticles on ITO substrates in a confined cell for vertical alignment of liquid crystals and performance analysis

Abstract

The work reported herein advances the approach to achieve the vertical alignment (VA) of liquid crystals (LCs) by directly forming the multilayer self-assembly of nanoparticles (NPs) on the indium tin oxide (ITO) substrates in a confined cell. In this method, for confined evaporative self-assembly of NPs, the colloidal solution of uniformly dispersed 0.3 wt% silica NPs (SNPs) in deionized (DI) water was injected in between the two ITO coated glass substrates by capillary method. The ITO cell filled with colloidal solution was kept at a fixed temperature of 60°C for uniform thermal evaporation of water to deposit the multilayer of NPs on ITO substrates. The multilayered self-assembly of SNPs on ITO substrates was confirmed through field emission scanning electron microscopy (FESEM) and fourier transform infrared (FTIR) spectroscopy studies. The surface roughness profile of the deposited SNPs-multilayer was analysed by atomic force microscopy (AFM). Further, the contact angle (CA) and surface energy values of SNPs-multilayer deposited ITO substrates were also measured and analyzed comparatively with the values of polyimide (PI) coated ITO substrates. After complete evaporation of water, a negative dielectric anisotropy nematic LC was filled in to the cell using capillary method again and the versatility of prepared substrates was assessed for the VA of the liquid crystal (LC). Inquisitively, confirmation of VA of LC in the present approach was found similar with the PI coated VA (conventional approach) using polarized optical microscopy (POM) and conoscopy studies. In addition, effects of external applied electric field were observed well in order with the orientation of LCs to switch vertically aligned LC (VALC) cells from opaque OFF state to transparent ON state, under crossed polarizers. Also, the obtained results in terms of transmission, threshold voltage, operating voltage, and contrast ratio (CR) of VALC cell prepared with this easy and new approach were found comparable to the conventional approach.