Study of structure and alignment of liquid crystals on self-assembled monolayers

Liquid crystals (LCs) are partially fluid anisotropic materials that possess long-range orientational order. The conformations are extremely sensitive to external perturbations such as the limiting surfaces, fields, and impurities. The surfaces not only serve the purpose of confining the fluid liquid crystal, but also interact with it to determine the molecular orientational structure.; Self-Assembled Monolayers of functional molecules chemically bonded to a surface, (SAMs), have been receiving increasing attention as a convenient and versatile way of modifying surfaces. A variety of surface properties can be produced by modifying the end functional group of the SAM molecules, and SAMs can be easily patterned by photolithography or micro-contact printing.; Two research themes probing the interaction of LCs with SAMs were pursued in this thesis--the study of LC structure when deposited on SAMs and the use of SAMs for aligning LCs.; To probe LC structure, thin freely suspended films of 8SI, a tilted hexatic LC at room temperature, were transferred onto OctadecylTriEthoxySilane (OTE) SAMs, expected to act as an atomically smooth and very inert surface. Atomic Force Microscopy (AFM) revealed the smectic lamellar organization of 8SI and in high-resolution scans, the in-plane molecular organization of the smectic layers. Buckling of the smectic film due to compressional stress from transfer was also observed.; To use SAMs as LC alignment layers, the OTE SAMs on glass were stripe-patterned by UV-lithography and studied the resulting alignment of nematic LCs. On the surface of alternating homeotropic (long molecules perpendicular to the surface) and planar (parallel to the surface) orientations, the in-plane component of the director aligns parallel to the stripes because of the elastic anisotropy (molecular twist is more readily adopted than splay or bend). The alignment is especially strongly enforced near the patterned surface where the elastic distortion is largest. The director modulation decays exponentially away from the surface, to become uniform with a pretilt determined from the ratio of the widths of the planar stripes to the homeotropic ones. Disclinations of strength 1 separate domains with the two pretilt directions. They interact with one another exhibiting one-dimensionality. Using two patterned surfaces, we have fabricated multi-state LC cells with the twisted nematic geometry. Adding chiral dopant molecules deforms the disclinations to expand the regions of preferred handedness of the molecular twist. When the chiral half-pitch is comparable to the pattern periodicity, a uniform helix in the direction of the confining plates is achieved without field. These experiments demonstrate that LC ordering on a surface composed of fundamentally isotropic regions of distinct character can be achieved by appropriate geometrical patterning of the different regions.