Synthesis, Characterization, and Device Applications of Two-Dimensional Materials

Two-dimensional (2D) materials have attracted tremendous research interest, as they offer novel physics, facile visualization by electron and scanning probe microscopy, and the potential to become next-generation electronic materials, all due to reduced dimensionality. Large-area 2D single crystals are needed for both fundamental scientific experiments and electronic device applications. New methods need to be developed to exploit state-of-the-art microscopy in the scientific investigation of 2D materials. Mechanisms behind the behavior of 2D-material based devices need to be resolved and new device concepts and applications need to be explored. This dissertation addresses these three aspects of 2D materials research.Using chemical vapor deposition growth of graphene on copper as a platform, the first part of my research in this thesis demonstrates a facile method involving a simple in-situ treatment of the copper catalytic substrate right before the growth that results in mm-sized graphene single crystals, elucidating the key factors of achieving large-area 2D single crystals.The second part of this work developed experimental methods to resolve important issues in 2D materials research by employing modern transmission electron microscopy. Here, a method has been developed to determine the edge orientation and termination without imaging the edge down to the atomic scale of monolayer hexagonal boron nitride (h-BN), enabling a direct comparison to theoretical predictions. Another important issue in 2D materials research is the determination of the layer count and its lateral spatial uniformity. In this work, a method is developed to map the layer count of a 2D material at nanometer-scale lateral resolution over extended areas, utilizing a combination of mass-thickness mapping offered by STEM and element-specific quantization afforded by electron energy loss spectrum (EELS) mapping.The last part of this thesis work unravels the multiple mechanisms behind the behavior of field effect transistors (FETs) based on PdSe₂. The change in device behavior in early reports from ambipolar to n channel was puzzling. As commonly encountered in device research, many factors, including channel material properties, defects, contaminants, and contact effects, are almost always entangled. Here, I use multiple control devices to unravel various mechanisms and provide consistent explanations for device behvior variations.