Fabrication and Dynamic Tuning of Periodic Structures From Holographic Lithography
Mechanics of Materials
In this dissertation, I fabricated one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) periodic structures through holographic lithography (HL) and backfilling conversion with different materials. Along the line, I investigated their intrinsic structure-property relationship, harness and utilize the mechanical instability, and explored novel applications as tunable periodic structures. In order to mimic butterfly wings which show both structural color and superhydrophobicity, 3D diamond photonic crystals with controllable nano-roughness (≤ 120 nm) were fabricated from epoxy-functionalized cyclohexyl polyhedral oligomeric silsesquioxanes (epoxy-POSS). The nano-roughness was generated due to microphase separation of the polymer chain segments in nonsolvents during rinsing, which could be tuned by crosslinking density of the polymer and choice of solvents. Such structure offers opportunities to realize superhydrophobicity, enhanced dye adsorption in addition to the photon management in the 3D photonic crystal. Most of current studies on tunable periodic structures show limited tunable optical property ranges, which is attractive to be expanded. 2D shape memory polymer (SMP) membranes consisting of a hexagonal array of micron-sized holes were fabricated by converting from epoxy-POSS template. Reversible color switching from transparency to colorful state was achieved through thermal-mechanical deformation, utilizing shape memory effect and mechanical instability induced pattern transformation. Continuum mechanical analyses corroborated well with experimental observations. Potential applications as displays were demonstrated via two different approaches. It is challenging to directly fabricate high aspect-ratio (AR) 1D nano-scale structures, due to depth-of-focus (DOF) limitation, pattern collapse from capillary force and distortion during solvent swelling. With HL and supercritical drying, high AR 1D nano-scale structures were fabricated with epoxy-POSS and SU-8, which avoid DOF limitation and pattern collapse. Due to enhanced thermal and mechanical stability of epoxy-POSS, 1D nanogratings (AR up to 10) with controllable periodicity, filling fraction and surface roughness, were achieved, which could be directly converted to silica-like through calcination. By exploiting swelling-induced buckling of 1D SU-8 nanowalls with nanofibers formed in-between, long-range ordered 2D nanowaves with weaker reflecting color were achieved, where degree of lateral undulation could be controlled by tuning AR and exposure dosage. Using double-exposure through photomasks, patterns with both nanowaves and nanowalls for optical display were created.