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ABSTRACT: Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention owing to their unique electrical, optical, mechanical, and thermal properties not found in their 3D counterparts. Monolayer TMDs can be obtained by mechanical, chemical, or electrochemical exfoliation. However, these strategies lack uniformity and produce defect-rich samples, making it impossible for large-scale device fabrication. The chemical vapor deposition (CVD) method emerges as the viable candidate to create atomically thin specimens at the technologically relevant scale. However, the large-scale growth of monolayer TMD films with spatial homogeneity and high electrical performance remains an unsolved challenge.
This dissertation centers on orientation and dimensionality controlling of TMDs. At first, we systematically study the growth of stereotypical molybdenum disulfide (MoS2) monolayer on a c-plane sapphire with chemical vapor deposition (CVD) to elucidate the factors controlling their orientation. We have arrived at the conclusion that the concentration of precursors- that is, the ratio between sulfur and molybdenum oxide, plays a key role in the size and orientation of seeds, subsequently controlling the orientation of MoS2 monolayers. We demonstrate a ledge-directed epitaxy (LDE) of dense arrays of continuous, self-aligned, monolayer, and single-crystalline MoS2 nanoribbons on β-gallium (iii) oxide (β-Ga2O3) (100) substrates. In the second part, we theoretically reveal and experimentally determine the origin of resonant modulation of contrast as a result of the residual 3-fold astigmatism in modern scanning transmission electron microscopy (STEM) and its unintended impact on violating the power-law dependence of contrast on coordination modes between the transition metal and chalcogenide atoms.