Micro- and nanostructures on deformable substrates : fabrication and application

Abstract

With the increasing demand for flexible and stretchable electronic devices, the fabrication of micro- and nanostructures on deformable substrates has attracted much interest in both academic research and industry in recent years. The research work presented in this thesis focuses on cost-effective and large-area fabrication methods of micro- and nanostructures on deformable substrates, as well as their novel applications in flexible and stretchable devices with desirable performance. The first section reports a facile method that can modulate the periodicity of ordered nanostructures with arbitrary spatially varying profiles. It is achieved by introducing a non-uniform strain field on a shape-tailored elastomeric mold that carries originally uniform periodic patterns and then transferring the modulated patterns onto rigid substrates. As a demonstration, nanograting structures with various chirp factors are obtained on stretched elastomeric molds and on rigid substrates. This work provides a low-cost and high-resolution fabrication technique for large-area periodic structures with spatially varying periodicity. The second section presents a fabrication strategy for high-performance stretchable transparent electrodes with solution-processed metal meshes. Fabricated stretchable electrodes show excellent electrical conductivity and optical transparency when stretched to 55% strain. The fracture and fatigue mechanisms of metal-mesh electrodes with various mesh patterns are investigated under different stretching conditions. Under the one-time straining of up to 55%, the serpentine electrode shows the lowest sheet resistance of less than 2 Ω/sq. While under the cyclic stretching of 500 repeated cycles at 20% strain, the hexagon electrode exhibits the best stability with sheet resistance increased to approximately 10 Ω/sq. Moreover, a stretchable electroluminescent light-emitting film is demonstrated with the stretchable metal-mesh electrodes on both top and bottom sides. The light-emitting film exhibits desirable foldability and works without noticeable failure under up to a 60% strain. The third section introduces a template-based fabrication strategy for flexible metallic nanofiber network with sub-micron line width. Transparent electrodes based on metallic nanofibers can maintain the sheet resistance < 2 Ω/sq under the bending radius of 3 mm with the transmittance > 85%. The template-based strategy provides a number of advantages, including low manufacturing cost, high throughput, and consistent performance of fabricated electrodes. It can also achieve direct patterning of transparent electrodes with arbitrary patterns, which is essential in many optoelectronic devices. With the patterned transparent electrodes, a flexible patterned electroluminescent display is demonstrated and works well under mechanical bending. The fourth part reports the scaling behaviors of optical, electrical and mechanical performances of metal-mesh transparent electrodes with their geometrical dimensions and grain sizes. Experiments, theoretical analysis, and numerical simulations are applied in this study. It is found when the line width of metal meshes is reduced to smaller than 300 nm, the optical transmittance of the metal-mesh transparent electrodes with fixed metal geometric filling ratio will greatly decrease with the increase of mesh thickness. Larger thickness and larger grain size of the metal meshes will contribute to lower sheet resistance of metal-mesh electrodes. Meanwhile, the metal meshes with larger thickness will undergo smaller stress and strain under bending.