Monolithic Hierarchical Nanostructuring of Thin Materials

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Patterning hierarchical structures in three-dimension (3D) has created nature-inspired functional surfaces. Gecko feet structures have been mimicked for reversible adhesive properties, shark skin for reduced drag, and lotus leaves for self-cleaning surfaces. Rational design of out-of-plane patterns across multiple length scales is crucial because each micro- and nano-feature has different functions. Microstructures support mechanical strength, stability, and flexibility, while nanostructures realize main functionalities such as superhydrophobicity/-philicity, structural color, and selective filtration. Most of multiscale systems for desired applications are consisting of surface patterns and underlying substrates with different materials. Because discontinuous and delaminated interfaces hinder system robustness, tailoring the adhesion between the hierarchical structures and the substrates is an another design factor to be considered. Monolithic integration of 3D patterns into flexible and soft materials is ideal to preserve the structural functionalities under dynamic conditions such as substrate bending or stretching.', 'Most efforts to fabricate hierarchical patterns with length scales spanning several orders of magnitude (e.g., nm to μm) have relied on complex top-down processes such as multistep photolithography or imprinting. These tools, however, cannot easily manipulate characteristic features organizing structural hierarchies over large-areas (> cm2). Independent control over the feature size and order/disorder of nanostructures is crucial for modulating the mechanical, chemical, and optical properties of the surfaces. Strain-induced buckling of thin materials (or skin layers) on a soft base is an emerging alternative to realize ordered and disordered patterns in parallel. Although wrinkle wavelength is tunable at microscale, nanoscale control over wrinkle structures is challenging because of materials limitations on the skin; hence, designing 3D hierarchy via multiscale wrinkling has been limited. Recently, our group showed how skin layers can be monolithically formed on plasma-treated thermoplastics where nanowrinkles with tunable wavelength could resulted. Local and global feature disordering of nanostructures has been manipulated with this materials system by patterning strain in the skin layers.', 'In this dissertation, we focus on the structural designs of deformable thin materials using conformal buckling at the nanoscale where unusual surface functionalities occur. Chapter 2 discusses the general design principle of out-of-plane, three-dimensional (3D) hierarchical nanostructures via a memory-based, sequential nanowrinkling process. Chapter 3 covers the realization of the structural hierarchy in a fully monolithic system by high-fidelity pattern transfer. Chapter 4 focuses spatial control of in-plane hierarchical nanostructures consisting of multiscale wrinkles and crumples side-by-side, with an emphasis on graphene as a skin layer. In this chapter, we further explain both electrical and mechanical properties of textured graphene at local and global areas. Chapter 5-6 discuss post-buckling approaches for the formation of monolithic nanostructures to build-up functional systems in soft and hard thin materials. In more detail, Chapter 5 describes the surface modulation of as-fabricated monolithic nanoridges by plasma etching to realize programmably tunable wetting transitions. Chapter 6 introduces wrinkle nanolithography integrated with concurrent design to produce and optimize quasi-random photonic nanostructures for broadband light-trapping.

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  • 09/30/2019
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