Shape and Microstructure Control in Carbon MEMS via Origami-based Fabrication and Electrospinning
- Author(s): George, Derosh
- Advisor(s): Madou, Marc J.;
- Peraza Hernandez, Edwin A.
- et al.
Carbon possesses a distinctive ability to form chemical bonds with other carbon atoms and create unique architectures. In the last few decades, we have witnessed the discovery of different carbon systems, including fullerene, nanotubes, and graphene. The versatile and diverse nature of the element can be fully exploited only if different shapes and microstructures can be manufactured, especially at the small length scales, given the potential of carbon in microengineering applications. Two promising miniaturization approaches for small-scale manufacturing of carbon microelectromechanical systems are photolithography and electrospinning. However, these fabrication methods need to be further evolved to have the ability to fabricate complex three-dimensional shapes and to enable tailoring of the carbon microstructure.
The first part of this work integrates photolithography and origami design, where an assisted folding approach and a self-folding approach are devised and demonstrated as three-dimensional carbon microfabrication methods. In the assisted folding method, fabrication of three-dimensional polymer shapes is achieved by adjusting the material properties of photopolymer films along their planform and utilizing the enhanced surface tension effect at small length scales. Complex surface geometries with patterned facets are fabricated by implementing capillary folding on a photo patternable material. For the self-folding method, the photolithography process is tailored such that the developer solution is non-uniformly absorbed throughout the thickness of the fold regions. The solvent is diffused into the foldable regions of low cross-linking density during the development step of the photolithography process. The solvent concentration is non-uniform across the thickness of the folds and causes a strain gradient at these regions when the solvent is removed by heating the films, enabling self-folding. Experiments are performed to calibrate a model that relates the dimensions of the folds and their exhibited fold angle. The model is incorporated into a computational implementation of the unfolding polyhedra method, a versatile approach for origami design that considers smoothly bent folds. This method, enhanced with the experimentally calibrated model, enables the design of planar films programmed to reliably self-fold into target three-dimensional shapes when heated. Polyhedral shapes are fabricated to demonstrate the developed method for origami-based fabrication.
In the second part of this work, the design of carbon microstructure using mechanical and chemical treatments is presented. By applying stress during carbonization, we demonstrate how to retain the alignment of Polyacrylonitrile (PAN) molecular chains achieved through electrospinning to produce a more uniformly graphitized carbon. The resulting carbon exhibits an oriented but fragmented lattice structure and is innately rich in nitrogen heteroatoms. Besides this mechanically induced graphitization, we also report a chemically induced graphitization method on patterned nanowires using a combination of low voltage electromechanical spinning and electrodeposition of nickel. These nickel-coated carbon wire structures are used as templates to deposit multi-layer graphene selectively. This patterning technique offers high throughput for nano writing, which outperforms other existing nanopatterning techniques, making it a potential candidate for large-scale carbon nanomanufacturing.