Title: Thin Film Mechanics
Description: This doctoral thesis details the methods of determining mechanical properties of two classes of novel thin films suspended two-dimensional crystals and electron beam irradiated microfilms of polydimethylsiloxane (PDMS). Thin films are used in a variety of surface coatings to alter the opto-electronic properties or increase the wear or corrosion resistance and are ideal for micro- and nanoelectromechanical system fabrication. One of the challenges in fabricating thin films is the introduction of strains which can arise due to application techniques, geometrical conformation, or other spurious conditions. Currently, inadequate models exist to model strain within thin films, making it difficult to produce structurally robust thin films and to prevent premature failure of a coating ordevice. It is thus imperative to understand and quantify thin film behavior under strain, both to aid in the development of new materials and processing techniques, as well as to enable the implementation of thin films into new designs.

Chapters 2-4 focus on two dimensional materials. This is the intrinsic limit of thin films-being constrained to one atomic or molecular unit of thickness. These materials have mechanical, electrical, and optical properties ideal for micro- and nanoelectromechanical systems with truly novel device functionality. As such, the breadth of applications that can benefit from a treatise on two dimensional film mechanics is reason enough for exploration. This study explores the anomylously high strength of two dimensional materials. Furthermore, this work also aims to bridge four main gaps in the understanding of material science: bridging the gap between ab initio calculations and finite element analysis, bridging the gap between ab initio calculations and experimental results, nano scale to microscale, and microscale to mesoscale. A nonlinear elasticity model is used to determine the necessary elastic constants to define the strain-energy density function for finite strain. Then, ab initio calculations-density functional theory-is used to calculate the nonlinear elastic response.

Chapter 2 focuses on validating this methodology with atomic force microscope nanoindentation on molybdenum disulfide. Chapter 3 explores the convergence criteria of three density functional theory solvers to further verify the numerical calculations. Chapter 4 then uses this model to investigate the role of grain boundaries on the strength of chemical vapor deposited graphene. The results from these studies suggest that two dimensional films have remarkably high strength-reaching the intrinsic limit of molecular bonds. Chapter 5 explores the viscoelastic properties of heterogeneous polydimethylsiloxane (PDMS) microfilms through dynamic nanoindentation. PDMS microfilms are irradiated with an electron beam creating a 3 m-thick film with an increased cross-link density. The change in mechanical properties of PDMS due to thermal history and accelerator have been explored by a variety of tests, but the effect of electron beam irradiation is still unknown. The resulting structure is a stiff microfilm embedded in a soft rubber with some transformational strain induced by the cross-linking volume changes.

Chapter 5 employs a combination of dynamic nanoindentation and finite element analysis to determine the change in stiffness as a function of electron beam irradiation. The experimental results are compared to the literature. The results of these experimental and numerical techniques provide exciting opportunities in future research. Two dimensional materials and flexible thin films are exciting materials for novel applications with new form factors, such as flexible electronics and microfluidic devices. The results herein indicate that you can accurately model the strength of two dimsensional materials and that these materials are robust against nano-scale defects. The results also reveal local variation of mechanical properties in PDMS microfilms. This allows one to design substrates that flex with varying amounts of strain on the surface. Combining the mechanics of two dimensional materials with that of a locally irradiated PDMS film could achieve a new class of flexible microelectromechanical systems.

Large-scale growth of two dimensional materials will be structurally robust-even in the presence of nanostructural defects-and PDMS microfilms can be irradiated to vary strain of the electromechanical systems. These systems could be designed to investigate electromechanical coupling in two dimensional films or for a substitute to traditional silicon microdevices.