Fabrication of Polymer Nanostructures by Conventional/Unconventional Anodization Techniques and Their Applications in Liquid Crystal Displays (LCDs)

Abstract

Anodization techniques based on self-organized behavior have been of great interest, especially for the preparation of large-area, nanometer-sized structures with high aspect ratios, which are difficult to form by a conventional lithographic process based on expensive vacuum equipment. Anodic aluminum oxide (AAO) has a nanoscopically cylindrical pore arrays with a hexagonally ordered arrangement, so AAO has became a popular nanotemplate for the fabrication of various functional nanostructures such as nanorods, nanowires, and nanotubes. As nanofabrication technologies have been developed toward more complex nanostructure with low-dimension, complicated or hierarchical nanostructures become a highly noticeable architecture. In this study, we suggest a simple and novel anodization technique (unconventional anodization) for fabricating hierarchical nanostructures with various shapes, maintaining advantages of conventional anodization method. Hierarchical polymer nanostructures were replicated from AAO nanotemplate obtained by unconventional anodization, and we analyzed their surface properties. In addition, we tried to apply nanapatterned surface to alignment layer and organic thin film transistors of liquid crystal displays (LCDs). In Chapter 1, we introduce conventional anodization technique as one of nanotechnologies for the preparation of nanostructures. Additionally, we add their versatile applications in practical fields such as electronics or optoelectronics, magnetic, and biosensors. In Chapter 2, we suggest unconventional anodization technique, which makes possible to fabricate more complex nanostructures with various shapes. Hierarchical AAO was obtained through the thinning process of oxide layer and the sequential anodization process at lower voltage. According to sequential anodization conditions such as the type of electrolyte and the applied voltage, the hierarchy was varied. Moreover, we fabricated the very tiny hierarchical nanopore arrays with pore diameter less than 100nm via unconventional anodization, which are limited to manufacture by conventional lithography techniques. In Chapter 3, hierarchical structures consisting of micro- or nano-meter scale structures have been discovered in many plant leaves and insect wings or legs, and exhibited outstanding properties on their surfaces. Thus, we investigate to mimic unique hierarchical structures by unconventional anodization in order to realize their superior properties. Firstly, the artificial lotus-leaf-inspired polymer nanostructures were fabricated by the replication AAO templates that have hierarchical nanopores via subsequent anodization on concave Al surface with the variation of the applied voltages. Then, we analyzed surface wettability of these hierarchical nanostructures. Secondly, for mimicking geckos foot surface, asymmetrically bent polymer nanopillars were prepared by metal deposition onto one lateral side of polymer nanopillars after the fabrication of polymer nanopillars replicated from AAO template, because the natural gecko setae show frictional anisotropy. Anisotropic adhesion behavior was observed due to structural peculiarity. In Chapter 4, we apprehend the structure of liquid crystal displays (TFT-LCDs) in order to apply micro/nanopatterned surfaces to TFT-LCDs. The patterned surface would be suitable as an alignment layer in TFT-LCDs because the orientation of LC molecules on the alignment layer is influenced by surface topography. We study LC alignment method and alignment mechanism. Furthermore, we introduce basic theory of organic thin film transistors to utilize the patterned surface in organic semiconductor layer. In Chapter 5, we investigate the alignment behavior of LC molecules with the variation of nanostructure size. As alignment layer, one-dimensional polymer nanostructure arrays replicated from AAO template was firstly prepared. When small LC molecules are confined by nanoscopic environment, they would suffer strong packing frustration because of the size similarity between LC molecules and nanoscopic space. We observed the transition of LC alignment from random to vertical orientation with the increase of nanorod diameter and length. In Chapter 6, we demonstrate the surface chemical compositions and nanoscopic topography effect on LC alignment. Several photocurable polymers with different surface free energies were used to examine the interaction at the LC/alignment layer interface. Diameter-controlled polymer nanorods were obtained from AAO templates. The alignment transition of LC molecules was only observed in polymer nanorod structure with moderate surface energy according to the increase of nanorod diameter. It can be interpreted that the LC alignment on nanorod arrays is determined by two competition interactions

the LC-LC interaction by spatial confinement and the LC-alignment layer interaction by surface chemical nature. In Chapter 7, we investigate a simple and novel method to make micropatterned organic semiconductors. The organic semiconductor was crystallized along the patterned surfaces of PDMS pad. Similar to a commercial rubber stamp, the crystalline organic semiconductor layer onto PDMS pad was transferred to the substrate after conformal contact. The molecular ordering and crystalline structures of transferred micropatterns was confirmed by 2D-grazing incidence x-ray diffraction (GIXRD). Electrical properties of the OTFTs with micropatterned semiconductors showed considerably enhanced device performance.