Optical Studies of Ordered Monomolecular Layers: Ab Initio Simulation and Experiment

Abstract

Due to the general interest on the alignment effect of the surface-bound ultrathin layers, this thesis presents research of the investigation of anchoring properties for liquid crystal (LC) molecules adsorbed on a solid substrate, using dual polarization interferometry (DPI). A new theoretical framework is designed based on the dual polarization interferometer for the detection of anisotropic information and the average anchoring angle of the adsorbed molecules, provided the liquid crystal bulk optogeometric parameters are well known. In the experiment, the nematic compound 4'-n-pentyl-4- cyanobiphenyl (5CB) is applied onto the silicon oxynitride surface, and a complete wetting monolayer with the average 56 ° polar angle and 16.6 Å thickness is observed in the stabilized stage. The results match quite well with the theoretical predictions in terms of the DPI phase change ratio. The thesis also extends the research into the functionalized substrate, in particular the Langmuir-Blodgett (LB) films covered substrate, which can give alignment effect to the deposited LC molecules. The LB forming molecule quinolinium tricyanoquinodimethanide (Q3CNQ) with long hydrocarbon chain (typically C18H37-Q3CNQ) is chosen. In order to justify the functionality of this material as an alignment layer, the electronic structure and the optical absorptive properties of this molecule in the LB phase are explored both in experiment and theory. Based on the very controversial history of this Q3CNQ compound, a robust computational LB model is built to examine the ground state, optimal geometry and the optical absorption features. A 530 nm absorption band is obtained to conclude the properties of the Q3CNQ LB layers, and also used to compare with our own experimental results.