Interfacial Structure in Thermotropic Liquid Crystals as Studied by Vibrational Sum Frequency Generation Spectroscopy

Abstract

Liquid crystals (LCs) are widely used in many optoelectronic devices. The study of LC interfaces is critical for understanding the molecular interactions within the LC devices and improving the device performance. Air-LC, liquid-LC, and polymer-LC interfaces have been extensively studied in the literature, however, the molecular orientation and ordering of the thermotropic LCs and solid interfaces remain underexplored. Emerging LC alignment techniques, such as nanopatterning, oblique evaporation, and photo-induced alignment, show the need for in-depth studies of solid-LC interfaces. In this work vibrational sum frequency generation spectroscopy (VSFG), a second-order nonlinear optical technique, was used to study the orientation of the interfacial LC molecules on solid substrates. Chapter 1 introduces the basics of LCs and LC devices, discussing the importance of solid-LC interfaces. Chapter 2 covers the theoretical background and experimental setup of VSFG. Chapter 3 discusses the design of sample cells and the considerations necessary for conducting temperature-controlled and voltage-controlled VSFG experiments. Chapter 4 presents a detailed study of the solid-LC interfaces of thermotropic 8CB LCs using VSFG and X-ray reflectivity (XRR), comparing the bulk and interfacial LC behaviors across different phases. Chapter 5 explores the interfacial molecular behavior of nematic LCs 5CB under an applied voltage in a working LC sample cell. Nickel ultra-thin metallic film (UTMF) was used as transparent electrode for the first time in a VSFG experiment.