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Designing Hierarchical Nanostructures from Conformable and Deformable Thin Materials Won-Kyu Lee†,§ and Teri W. Odom*,†,‡ Department of Materials Science and Engineering and ‡Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
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ABSTRACT: This Perspective focuses on the design of hierarchical structures in deformable thin materials by patterning mechanical instabilities. Fabrication of threedimensional (3D) structures with multiple length scales starting at the nanoscalecan result in on-demand surface functionalities from the modification of the mechanical, chemical, and optical properties of materials. Conventional top-down lithography, however, cannot achieve 3D patterns over large areas (>cm2). In contrast, a bottomup approach based on controlling strain in layered nanomaterials conformally coated on polymeric substrates can produce multiscale structures in parallel. In-plane and out-of-plane structural hierarchies formed by conformal buckling show unique structure−function relationships. Programmable hierarchical surfaces offer prospects to tune global- and local-level characteristics of nanomaterials that will positively impact applications in nanomechanics, nanoelectronics, and nanophotonics.
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such as anisotropic water spreading and directional adhesion.2,4,22 Strain-induced buckling of stiff, thin materials commonly referred to as skin layerson soft substrates is a promising method to achieve spontaneously ordered and disordered patterns.23 Although buckle periodicity is easily tuned at the microscale, nanoscale control over the 3D structure is challenging because of materials limitations of the skin layer. Recently, softer skin layers deposited by plasma-mediated polymerization on prestrained thermoplastic substrates have resulted in nanowrinkles and nanoridges with continuously tunable feature sizes following strain relief.17,24 Control over local and global disorder of the hierarchical nanostructures is possible by patterning strain in the soft skin layer.25,26 Previous work on 3D structure formation driven by mechanical instabilities has focused on the mechanics of wrinkling and associated surface morphologies.27−30 Structural transformations in nonlinear strain regimes (e.g., microscale wrinkle-to-fold transitions) have been theoretically and experimentally summarized.31 Also, several reviews have reported the crumpling of graphene and other two-dimensional (2D) nanomaterials with tunable mechanical and electrical properties and applications.32−34 In this Perspective, we focus on designing hierarchical structured nanomaterials from thin materials using conformal buckling. First, we review principles to form out-of-plane hierarchical nanostructures via a memorybased, sequential nanowrinkling process. Second, we discuss fabrication of in-plane hierarchical structures consisting of
atural and man-made materials derive many of their properties from structural characteristics at multiple scales.1 In hierarchical materials, the smallest structural elements along with the global architecture synergistically determine physical and chemical properties of the system.2 Patterning topography in three dimensions (3D) has been used to create nature-inspired functional surfaces,2−5 such as mimicking reversible adhesive properties of gecko feet,6,7 reduced drag of shark skin,8,9 and self-cleaning lotus leaves.10,11 Rational design of out-of-plane patterns over multiple length scales is important because microscale and nanoscale features support different functions. For example, microstructures provide mechanical strength, stability, and flexibility; nanostructures offer additional functions such as superhydrophobicity/philicity, structural color, and selective filtration.2,12−14 Most systems created for a desired response consist of surface patterns and underlying substrates made from different materials.2,15,16 Because discontinuous and delaminated interfaces hinder robustness, tailoring the adhesion between the textured surface and the supporting substrate is required. Monolithic 3D features offer a way to preserve structural functionalities under dynamic conditions, including substrate bending and stretching.15,17 Complex top-down processes such as multistep photolithography and imprinting can produce hierarchical patterns with length scales spanning several orders of magnitude (nanometers to micrometers).2,18,19 These tools, however, cannot tune pattern periodicity and nanostructure orientation over large areas (>cm2).2,15,20,21 Controlling the orientations of multiscale features is critical to realize unique physical properties © XXXX American Chemical Society
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DOI: 10.1021/acsnano.9b03862 ACS Nano XXXX, XXX, XXX−XXX
Perspective
Cite This: ACS Nano XXXX, XXX, XXX−XXX
ACS Nano
Perspective
(G) spanning 5 orders of magnitude in wavelength (Figure 1a). After strain relief, the first generation (G1) of wrinkles acts as an effective skin to form the second G (G2) of wrinkles with a larger λ; additional G, such as G3 and G4, can be formed until the substrate is fully compressed.35 Sequential strain relief of skin layers on biaxially stretched PDMS can also control structural hierarchies (Figure 1b). By changing combinations of skin thickness (h) and directional strain in the elastomeric substrates, microstructures with tunable wrinkle orientations with different wavelengths can be realized.36 Nonlinear strain relief in viscoelastic substrates such as polyurethane can also result in hierarchical folding and networking (Figure 1c).37 Neither one-step wrinkling under high strain for self-similar wrinkles nor sequential wrinkling, however, can result in out-ofplane hierarchical architectures with independent control over λ and orientation.4 Either all generations of self-similar wrinkles form spontaneously under a single and continuous strain-relief step or previous-generation features are deformed under sequential wrinkling. Hybrid approaches that combine nanolithography with microscale buckling have overcome this issue.2,38 For example, orientations of microscale wrinkles and imprinted nanostructures can be tuned independently by changing the relative angle of directional strain (to form the wrinkles) with respect to the orientation of the nanopatterns.39,40 Although design flexibility of this hybrid method has improved length-scale control, the resulting surfaces lack mechanical robustness because the interfaces between the patterned nanostructures and supporting substrates typically consist of more than two different materials. Delamination and
multiscale wrinkles and crumples, with an emphasis on 2D atomically thin materials as skin layers.
In this Perspective, we focus on designing hierarchical structured nanomaterials from thin materials using conformal buckling. Designing Out-of-Plane Hierarchical Nanostructures. Conventional Wrinkling Approaches for Nanostructures. Wrinkling a stiffer skin layer on a softer, prestrained substrate by allowing the substrate to relax and compress the skin layer can generate 3D textures spontaneously.23 The resulting wrinkles are characterized by wavelength λ ∼ 2πh(ES/EB)1/3, where h is the thickness of the skin layer, ES is Young’s modulus of the skin, and EB is Young’s modulus of the substrate. The wrinkle amplitude (A) is proportional to λ and the applied strain (ε) from the substrate. Because wrinkling is a general phenomenon, λ can span several orders of magnitude, from 104 to 10−8 m. Recently, the wrinkling of skin layers has emerged as a simple method to fabricate surface textures at the nanoscale by reducing h and the modulus ratio ES/EB.24 λ and A of the nanowrinkles can also be engineered by changing h and ε, respectively.25 Wrinkling at high strain (ε) can transform a flat surface into a 3D hierarchical structure without requiring complex lithographic methods.2 Oxidation of stretched polydimethylsiloxane (PDMS) substrates at ε > 0.3 followed by strain relief can achieve self-similar structures with multiple wrinkle generations
Figure 1. Hierarchical micro- and nanostructures by (a) self-similar wrinkling at high strain regimes; (b) sequential strain relief; and (c) wrinkle-to-fold transition and networking. Panel a adapted with permission from ref 35. Copyright 2005 Nature Publishing. Panel b adapted with permission from ref 36. Copyright 2008 RSC Publishing. Panel c adapted with permission from ref 37. Copyright 2011 Nature Publishing. B
DOI: 10.1021/acsnano.9b03862 ACS Nano XXXX, XXX, XXX−XXX
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monolithic 3D patterns was realized because of two key characteristics of CHF3 plasma-treated PS (or PO): (1) strain can be manipulated by heating above the glass transition temperature (Tg) of the substrate materials, and (2) plasmamediated skin layers can conformably coat any surface. Unique to the memory-based process in thermoplastics, both λ and the orientation of previous-generation wrinkles are preserved as the next G of wrinkles is formed.44 By tuning the skin thickness h for each cycle with varying plasma treatment time, the wrinkle wavelength of each G can be controlled independently (Figure 2b). Hierarchical nanowrinkles with different orientation combinations cannot be realized in one-step wrinkling because the strain direction for each cycle changes the orientation of each G feature.35,36 Notably, memory-based sequential wrinkling offers exquisite control over the orientation for each G by relieving strain in 2D (biaxial direction) or one dimension (1D; uniaxial direction). As a result, unconventional 3D patterns such as three-generational (G1−G2−G3) hierarchical wrinkles can be created with a nearly unlimited combination of λ and orientation (Figure 2b). In addition, sequential nanowrinkling can create hierarchical nanostructures in a range of low-dimensional nanomaterials45 where sacrificial skin layers are used to produce the larger wrinkle wavelengths (higher G) (Figure 3). First, G1 wrinkles (i.e., the smallest wavelength wrinkles) are formed by strain relief of PS with a skin layer of monolayer 1T-MoS2. Next, larger G2 wrinkles are created by casting a solution-processable polymer, poly(vinylpyrrolidone) (PVP), with thickness h2 on the nanostructured surface, followed by a second strain-relief step. The sacrificial PVP skin can then be rinsed to reveal G1+G2 features in MoS2. Importantly, the 3D features of the nanomaterial are not deformed during dissolution of the skin. By increasing the number of cycles (n), higher-order wrinkles up to G5 features were demonstrated. Multigenerational MoS2 wrinkles enhanced the hydrogen evolution reaction by lowering the overpotential for catalytic performance. The overpotential
cracks typically occur at the heterogeneous interface when the substrate is bent or stretched.15,17,22
Sequential cycles of skin formation followed by strain relief can form threedimensional out-of-plane hierarchical nanostructures with distinct generational features. Out-of-Plane Hierarchical Structures by Memory-Based, Sequential Nanowrinkling. To achieve nanoscale wrinkles, the Young’s modulus of the skin layer must be close to and larger than the substrate. In general, there are two approaches to form skin layers: (1) deposition of a thin metal film on a prestrained substrate23,41 and (2) modification of the top layer of the substrate with an ion beam42 or ultraviolet light.37 Although nanowrinkles can be produced from thin metal and polymer skin layers on prestrained polystyrene (PS) and PDMS substrates, continuously tunable λ down to sub-100 nm based on systematic control over skin thickness h was not yet possible.2,41,43 Recently, we reported a chemical patterning methodCHF3 plasma-mediated polymerization using reactive ion etching (RIE)to produce fluoropolymer thin films on PS.24 Because h can be controlled with ±1 nm accuracy by changing RIE time, λ can be varied from several micrometers to as small as 30 nm after relieving strain in the system. Applying the same texturing process to prestrained polyolefin (PO) films resulted in nanoscale ridges instead of wrinkles.17 Ridges have aspect ratios (>4) higher than those of the wrinkles (
