Imaging and Control of Mesoscale Structures in Two-Dimensional Materials

Boundaries, interfaces, and material heterogeneity are ubiquitous across chemistry and physics, giving rise to emergent phenomena such as localized states and modified dynamics and enabling material applications ranging from digital memory to heterogeneous catalysis. As demand for more compact and efficient devices increases, research efforts have been extended toward realizing these structures within individual nano- and micro-scale components. Intrinsic structures such as ferroic domains and artificial junctions where discontinuities in crystalline phase or composition are engineered through synthetic and lithographic methods have both been explored. Practical implementation of these types of structures in device applications requires innovation in the methodology used for their study and manipulation. Multimodal approaches that can simultaneously capture spatial, energetic, and temporal information to characterize functional interfaces and investigate their emergent properties are particularly desirable. This dissertation addresses each of these goals through the investigation of two-dimensional materials with photoemission electron microscopy (PEEM) and other spectroscopic and microscopy techniques. Chapter 2 details the working principles of PEEM with extra emphasis on polarization dependent experiments. Chapter 3 discusses the ultrahigh vacuum and laser experimental apparatus used in conducting PEEM measurements. Chapter 4 extends these methods to the study of the antiferroelectric domains in β′-In2Se3. Excitation energy dependent measurements and geometric analysis of domain arrangements connect the observed optical responses to the atomic displacements in the material. Chapter 5 presents a laser-induced wrinkle-mediated phase transition pathway for conversion between β′ and α-In2Se3. With thermal annealing steps, this transition enables repeated cycling between phases and strain accumulated throughout the process results in the formation of multiphase heterostructures and the rearrangement of domains. In Chapter 6 the behavior of Cu–Cl Ruddlesden-Popper perovskites under ultraviolet illumination is investigated. These materials undergo a modification where ordered grooves with well defined orientations are etched on their surfaces. The symmetry of the formed pattern is controlled by the organic cation spacer through electron-phonon coupling. Finally, Chapters 7 and 8 explore the impact that these material modifications and intrinsic anisotropy have on coherent light-matter interactions and dynamics in two-dimensional materials.