Title: The Characterization of Mechanical Behaviors of Two Dimensional Nanomaterials with Grains and Grain Boundaries
Description: Graphene, two dimensional lattice of covalent bonds of carbon atoms, has been studied as a prospective new material for the next generation. Pristine graphene, mechanically exfoliated graphene from graphite, has gained much attention due to its outstanding properties: conductivity, permeability, transparency, and mechanical stability. While pristine graphene has shown great promise as an innovative new material, the limitations from the randomness of sizes and domains are challenging for uniform mass production. In this dissertation, we present graphene produced by chemical vapor deposition (CVD) synthesis for producing designated sizes and domains. In order to prospect the utilization, the mechanical stability of CVD graphene should be determined. We first present mechanical properties of CVD graphene. Introducing transfer method, we present how to minimize damages on graphene during the fabrication. For the measurement of mechanical properties of CVD graphene, we introduce nanoindentation test with AFM and nanoindenter.

Experimental results are demonstrated by the results of FEA analysis on the basis of nonlinear elastic behaviors. Through the experiment and simulation, we verify the ultra-high mechanical strength of CVD graphene. We also present defect-engineered graphene for the utilization. To determine the change of the status of defects on pristine graphene, we employed plasma etching to induce defects gradually. Through the observation of change of defects from sp3 type to sp2 type on pristine graphene, we understand how the phase changes depending on defects. Using nanoindentation, the mechanical strength of defective graphene is determined and we discuss its utilization based on the mechanical stability. We next exploit grains and grain boundaries of polycrystalline graphene. Transmission electron microscope (TEM) is used for precise observation of suspended membrane with grains and grain boundaries. Applying the same nanoindentation test, we compare the values of grain boundaries to pristine lattice in order to determine how grains and grain boundaries affect the ultra-high mechanical properties of graphene as defects.

We finally present angular dependence of the mechanical properties of grains and grain boundaries. Although previous research reported the angular dependence of graphene regarding its mechanical strength, it was questionable that tilt angles among grains could not affect mechanical strength based on our previous experimental data. Therefore, here we reveal that how tilt angles among grains affect the mechanical properties. Furthermore, we investigate the crack propagation at rupture of graphene in both nanoindentation and e-beam exposure. Hence, we conclude the dissertation by a discussion of directions for future work, proposing well-stitched condition of graphene, and HR TEM for the verification of real structure of grain boundaries to apply into simulation. Therefore, this thesis is an arrangement of the outstanding mechanical properties of graphene from pristine graphene to CVD graphene in both small grain and large grain type, and from macroscopic region of interests over suspended membrane to microscopic observation such as the mechanical behaviors of grains and grain boundaries.