Two-dimensional (2D) materials have attracted sustained intense interest in the condensed matter physics community for many years because they exhibit a variety of unique and interesting physical properties. The growth of 2D materials on the surfaces of single crystalline metal and semiconductor substrates is an important approach to fabricate these intriguing materials for fundamental study of their structures and properties. This approach is also under intense development as a scalable method for the production of 2D materials with large area and high quality suitable for practical applications. The work described in this thesis addresses several outstanding issues in the growth and structure of 2D materials using powerful low energy electron microscopy and diffraction method...[ Read more ]

Two-dimensional (2D) materials have attracted sustained intense interest in the condensed matter physics community for many years because they exhibit a variety of unique and interesting physical properties. The growth of 2D materials on the surfaces of single crystalline metal and semiconductor substrates is an important approach to fabricate these intriguing materials for fundamental study of their structures and properties. This approach is also under intense development as a scalable method for the production of 2D materials with large area and high quality suitable for practical applications. The work described in this thesis addresses several outstanding issues in the growth and structure of 2D materials using powerful low energy electron microscopy and diffraction methods.

We first performed a comparative study of the growth and structure of iron oxide monolayers formed by oxidizing Fe on Pt(111) using molecular oxygen and, for the first time, atomic oxygen. A new (9x9) coincidence superstructure of the FeO(111) monolayer formed using atomic oxygen was identified. Oxidation with atomic oxygen also drives the formation of a new oxygen-enriched monolayer that exhibits a (√403x√403) R22.8° higher order coincidence superstructure. This novel iron oxide was observed to facilitate CO oxidation at elevated temperature. The biphase surface structure of a α-Fe₂O₃(0001)/Pt(111) thin film was also revealed to be composed of a FeO(111) monolayer on top of α-Fe₂O₃(0001) film with (19x19) coincidence superstructure. These outcomes expand knowledge of reactive enriched iron oxide monolayers and resolve the controversial nature of the biphase structure.

Our investigations also addressed the structure of the well-known and decades-long puzzling (5.55x5.55) incommensurate structure of a Cu₂Si monolayer on the Si(111) surface. This monolayer attracts new interest as a possible 2D Dirac material. The key observation in our work was the coexistence of mirror reflection symmetry and broken mirror symmetry of superstructure peak intensities in the LEED pattern. We proposed that Cu₂Si monolayer forms a rotated (11/√103 x 11/√103)R9.83° structure. This structure generates a (11x11) R60° coincidence superstructure that explain the perplexing LEED pattern symmetry properties. Our proposal is confirmed by observations of domains with oppositely rotated structure that were made with tilted dark field LEEM data. We also discovered a new “2D distillation” growth mechanism that can be exploited to scalably produce high quality hexagonal boron nitride (h-BN). 2D distillation growth transformed a R24.3° misaligned h-BN monolayer into an aligned h-BN layer with well-known (12x12) coincidence superstructure and improved quality. The novel 2D distillation mechanism is understood to be an unconventional coarsening process that is driven by counter-intuitive concentration gradients.