Effect of Nanoscopic Curvature from Anodic Aluminum Oxide on Physical Structuring of Organic and Inorganic Materials and Its Applications

Abstract

Nanostructures have been received considerable interests, since they have exhibited unique properties that are not the same as that in bulk or in molecular level. Many investigations have proved that their physiochemical properties greatly depend on the size, hardness of confinement, surface curvature, and anisotropy/asymmetry of nanostructures. This interesting behavior has suggested creation of more advanced materials which properties are tailored based on the structural parameters. In this thesis, we introduced another new structural parameter and investigated its effect on materials properties and it applicability as well. Utilizing self-assembled anodic aluminum oxide, we obtained hexagonally packed mesoscopic concave patterns. A replication technique enabled us to produce both concave and convex patterns accompanied with packing transition. Hence, our system incorporated pattern curvature and packing structure simultaneously. . On the basis of the interesting structural parameter, we exhibited the importance of our finding to control the physical structuring of organic and inorganic materials. Firstly, coating polymer thin films on concavely and convexly patterned Al substrates, we studied how the structure influenced the thin film stabilities. Even though the polymer films are inherently unstable on the substrate material, the polymer thin films on the concave patterns were found to present wetting behavior whereas those on the convex patterns exhibited extensive rupturing. We ascribed the behavior to the opposition in topological gradient in each pattern. Thus, it was recognized that pattern curvature can be an important parameter to enhance film stability. Secondly, we investigated thin film stabilities on concavely patterned and convexly patterned PI substrate whereby the coated polymer films are originally stable on the substrate. In spite of its inherent stability, polymer thin films were found to dewet on the convex patterns by hole nucleation at the peaks of the patterns. Although dewetting was initiated by the local process, dewetting on the convex patterns presented long-range-order rupturing morphologies. We speculate that the long range correlation came from pattern-directed dewetting on the regularly packed patterns and its subsequent amplification likewise amplification of capillary wave in spinodal dewetting. Thirdly, we exploited the inverse packing characteristics of the two patterns and investigated its effect on pore growth in anodization. Anodizization of the concavely patterned Al produced typical circular pores in hexagonal packing. Anodization of the inversely patterned Al formed triangular pores in inverse-hexagonal packing. Thus, we discovered that both circular and triangular pore shapes are achievable from the self-assembled structure of AAO itself without any assistance of expensive lithographic techniques. The opposition in pit packing structures and the topographies around the pits were discussed to regulate the pore shape. Lastly, by incorporating pattern-directed dewetting phenomena, we controlled the topographies of the peaks in the patterns and investigated its alteration on pore formation in anodization. Reducing the curvature and depth of the convex patterns, triangular pores were found to become more isotropic, accompanying formation of subsidiary pores in the regions other than pits. In addition, the regulation in pattern topographies allowed us to obtain pore arrays with the same ordering structure even from the oppositely packed patterns. Therefore, we found that pattern topography as well as pit order structure is an important parameter to control the pore shape and its ordering. In conclusion, we find importance of the structure incorporating curvature and packing to control physical structuring of materials. As a new finding of structural parameter in nanomaterials has created advanced materials of better tailored properties, our finding of a new structural parameter and investigation through its utilization will suggest a new strategy to control nanomaterials.