Department of Materials Science and EngineeringVertically stacked heterostructures based on the different types of two-dimensional (2D) materials via van der Waals (vdW) interaction have been extensively researched due to their novel properties beyond the limitations of individual 2D materials. The development of chemical vapor deposition (CVD) method enables the fabrication of various vdW heterostructures with clean interface and mass production, compared to the conventional multiple transfer method. Given the 2D nature of these materials, the interface intrinsically plays an important role in modulating or modifying their properties. For example, graphene placed on hexagonal boron nitride shows high charge carrier mobility, but a non-negligible interaction leads to the observation of ???Hofstadter???s butterfly???. In addition, in the case of transition metal dichalcogenides (TMDs), the transition from the direct band gap to the indirect band gap is apparent when the thickness increases due to the interface effect. Therefore, a systematic understanding of the impact of the interface on the intrinsic characteristics and performances of the vdW heterostructures is required in order to design desirable properties and expand the scope of applications of the vdW heterostructure.
It is well known that the structural features of the underlying substrate significantly affect the growth behavior and even the unique properties of the heterostructures. Therefore, in this dissertation, I studied novel defects in TMDs induced by an underlying graphene template with various structural features. For this research, I prepared 3 types of graphene templates: 1) Pristine, 2) wrinkle-rich, and 3) nanocrystalline graphene (ncG). Pristine graphene is a good substrate for synthesizing TMDs without dangling bonds and without friction. In addition, when TMDs grow on pristine graphene, the anti-phase boundaries (APBs) of the TMDs are generated more predominantly than the tilted grain boundaries (GBs) due to vdW epitaxial growth. Using this heterostructure, we discovered the anisotropic features of the APBs according to transition-metal-facing (saw-toothed) or chalcogen-facing (straight) APBs, and both types of APBs show metallic properties despite different in-plane charge mobility. Wrinkles in graphene cause significant friction due to out-of-plane deformation and result in AB/AC stacking boundaries (SBs) in epi-TMD layer driven by Shockley partial dislocations. AB/AC SB has a buckled structure for releasing in-plane strain and results in monolayer-like behavior by reducing interlayer coupling. Finally, ncG has lots of dangling bond based active sites for multilayer growth. Due to the diffusion limited growth regime on the ncG template, the synthesized WSe2 domain shows a fractal morphology with many Se-terminated edge states. The WSe2/ncG heterostructure shows a downshifted work function similar to the valence band maximum of WSe2, resulting in a small Schottky barrier height at the metal-semiconductor-junction.
Interface-driven novel defects and their corresponding properties are mainly observed using multi-mode of transmission electron microscopy (TEM) and other surface analysis tools (e.g. Raman, x-ray spectroscopy, atomic force microscopy). Theoretical density functional theory (DFT) calculations and TEM image simulations were supported to identify thermodynamically stable defect configurations and to confirm the exact atomic structures of novel defects. These studies could provide a systematic understanding of defect engineering, especially interface-driven defect formation mechanisms, atomic configurations, and their corresponding properties. It could pave the way for achieving and expanding the manipulation and commercialization of 2D material-based devices via defect engineering.clos
