Chemical and Biomolecular Engineering - Theses
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ItemControlled cracking in multilayered graphene films coated on flexible substrates and their electromechanical properties( 2020)Cracking in the brittle thin films causes tremendous trouble, but controlling cracking with care brings new opportunities to flexible electronics, particularly the highly accurate strain sensing. However, the complexity of the influencing factors on crack formation poses challenges to research, accurate control, and the full utilization of the novel cracked-thin film strain sensor. Multilayered reduced graphene oxide (MLG) films, due to their tuneable structural parameters via wet fabrication and high electric conductivity, could potentially serve as a promising material platform to study the cracking behaviours and the corresponding electromechanical properties. In particular, their unique cascading 2D nanostructures may lead to unusual cracking behaviours and advantageous electromechanical properties. Thus, this dissertation aims to explore the cracking behaviours of ultrathin MLG films coated on a stretchable substrate and the electromechanical properties of the cracked thin films. This project unfolds into three parts. In the first part, the method to coat the vacuum filtrated ultrathin MLG film on a flexible substrate is developed. By utilizing a swelling-induced interfacial effect of graphene oxide, we developed a simple method to manipulate the surface adhesion between the MLG film and the filtration membrane, allowing the MLG film with high surface quality to be readily coated on a series of substrates through a simple transfer printing process. The second part investigates the cracking behaviours of the MLG film transferred on a polydimethylsiloxane (PDMS) substrate. The cracking morphology of MLG films was found to depend on thickness, interlayer distance, and corrugation. An unexpected transformation of cracking morphology from typical “parallel” cracks to random “percolative” cracks was observed when the MLG film is very thin (< 40 layers of reduced graphene oxide). The third part further explores the electromechanical properties of the cracked MLG films. A high gauge factor (GF) of 56521 at strain = 6 % and large stretchability of strain = 60 % when GF = 5.1 were achieved for the parallel cracked and highly percolative cracked MLG films, respectively. The static and dynamic electromechanical characterization indicates that the percolative cracking could readily reduce relaxation, allowing accurate strain detection. A broad-frequency range accurate strain detection is demonstrated, suggesting their potential applications in human-machine interfacing and neuromuscular disease detection.