Vertical Alignment of Liquid Crystals Using a Nanoscale Self-Assembled Molecular Layer of Alkoxybenzoic Acids

Polyimide-free alignment is an important objective toward reducing the fabrication costs of liquid crystal displays. In this study, we applied alkoxybenzoic acids to an indium tin oxide substrate to vertically align liquid crystals in the absence of a polyimide layer. Alkoxybenzoic acids can hydrogen bond to the indium tin oxide surface, and their extended alkyl chains can then align vertically. During liquid crystal cell preparation, four alkoxybenzoic acids were mixed at low concentrations with liquid crystals. The vertical alignment of the alkoxybenzoic acids was evaluated using Fourier-transform infrared spectroscopy, water contact angle measurements, and atomic force microscopy. In the infrared analysis, we confirmed that the alkoxybenzoic acid formed hydrogen bonds with the indium tin oxide substrate by observing the shift of the carbonyl peak from a doubly to a singly hydrogenbonded state. We measured the surface contact angles to confirm the extension of the hydrophobic alkyl chain away from the electrode surface. Atomic force microscopy was used to evaluate the surface topography and layer thickness. We confirmed the presence of vertical pillars on the indium tin oxide substrate, which helped the liquid crystals to orient vertically and formed a nanoscale self-assembled molecular layer. We also evaluated the vertical alignment when an alkoxybenzoic acid/liquid crystal mixture was injected into a fabricated cell; the alignment was improved as the alkyl chain length grew. Finally, a liquid crystal cell fabricated by the proposed method showed better electro-optical properties such as voltage-transmittance curve and response times than one prepared by the conventional method. Based on these findings, we expect that the vertical alignment of liquid crystals induced by the presence of alkoxybenzoic acids can reduce the fabrication costs of liquid crystal displays.