From Chaos to Control: Programmable Crack Patterning with Molecular Order in Polymer Substrates

Abstract

The cost, form, and performance of the device are limited by the conditions necessary for these processing techniques. For example, conventional photolithography typically requires patterned light to generate user-defined microscale patterns. [3][4][5] More sophisticated techniques such as electron-beam lithography or dip-pen lithography can pattern nanoscale features, but these multistep scanning processes are highcost and labor-intensive. [6,7] Thus, fabrication techniques that can produce highly organized patterns or structures at small size scales with a simple process may offer significant advantages.In synthetic systems, cracking is normally an unwelcomed, uncontrollable, and chaotic phenomenon caused by mechanical conditions that induce material failure. However, living systems, such as the skin of crocodiles and elephants, often show examples of self-organized crack patterns caused by intrinsic anisotropy. [8,9] Inspired by nature, recent advances in crack-assisted fabrication techniques, also referred to as cracklithography, have demonstrated how the humble crack can be Cracks are typically associated with the failure of materials. However, cracks can also be used to create periodic patterns on the surfaces of materials, as observed in the skin of crocodiles and elephants. In synthetic materials, surface patterns are critical to micro-and nanoscale fabrication processes. Here, a strategy is presented that enables freely programmable patterns of cracks on the surface of a polymer and then uses these cracks to pattern other materials. Cracks form during deposition of a thin film metal on a liquid crystal polymer network (LCN) and follow the spatially patterned molecular order of the polymer. These patterned sub-micrometer scale cracks have an order parameter of 0.98 ± 0.02 and form readily over centimeter-scale areas on the flexible substrates. The patterning of the LCN enables cracks that turn corners, spiral azimuthally, or radiate from a point. Conductive inks can be filled into these oriented cracks, resulting in flexible, anisotropic, and transparent conductors. This materials-based processing approach to patterning cracks enables unprecedented control of the orientation, length, width, and depth of the cracks without costly lithography methods. This approach promises new architectures of electronics, sensors, fluidics, optics, and other devices with micro-and nanoscale features.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202008434. IntroductionProcesses that allow patterning of materials at the micro-or nanoscale enable many devices, including active electronics,

Recent progress on crack pattern formation in thin films

“…Kim et al presented a strategy to achieve controllable crack patterns by depositing a thin metal film on a liquid crystal substrate. 21 The molecular orientation of the substrate induces an anisotropic residual stress in the film, which is relieved by the formation of parallel cracks along the molecular orientation. By patterning the liquid crystal substrate (including molecular orientation, domain size and shape), various complex cracks can be freely programmable, as shown in Fig.…”

“…15–18 In particular, controllable fabrication of various crack patterns in thin films provides an efficient way to fabricate crack-assisted nanoscale patterns for various functional applications. 19–22…”

“…[15][16][17][18] In particular, controllable fabrication of various crack patterns in thin films provides an efficient way to fabricate crack-assisted nanoscale patterns for various functional applications. [19][20][21][22] Crack pattern formation in thin films is significantly influenced by mechanical loadings. There are different ways to exert a force to enable a crack to nucleate or propagate: in-plane tensile or shear loading (mode I or II) and tearing loading (mode III).…”

Farming‐Inspired Continuous Fabrication of Grating Flexible Transparent Film with Anisotropic Conductivity

“…The fabricating methods for microgrooves mainly include photolithography, [31,39] direct laser writing, [40] nanoimprint lithography, [41] and the crack method. [42,43] Unfortunately, most of the aforementioned methods often suffers from costly, complex process, and small-scale production. In a word, it remains a challenge to fabricate conductive gratings on flexible transparent polymer film by a cost-effective, simple, and continuous manufacturing method.…”

Periodic Nanoporous Inorganic Patterns Directly Made by Self‐Ordering of Cracks

“…[21][22][23][24] Even though such a method allows for precise control over crack nucleation and further propagation, it typically employs lithographic techniques for prepatterning. [25] An alternative is the Solution-processed inorganic nanoporous films are key components for the vast spectrum of applications ranging from dew harvesting to solar cells. Shaping them into complex architectures required for advanced functionality often needs time-consuming or expensive fabrication.…”