The physical and chemical properties in 2D crystals can be tuned not only by the native structure and chemistry of the crystal, but also by the impact of defects, dopants and interfaces.  In 2D crystals, defects exist on the surface of the crystal and can exhibit unique structures and migration dynamics.   In addition, the ability to isolate and manipulate two dimensional (2D) crystals has enabled us to create both in-plane and vertically stacked van der Waals heterostructures.  When creating a heterostructure via stacking dissimilar 2D crystals, new physical, chemical and electronic properties can emerge due to, but distinct from, the native two dimensional properties of the component materials.  In this study, ultra-high resolution aberration-corrected electron microscopy imaging is used to probe the atomic and chemical structure of the defects, i.e. dislocations, dopants, and grain boundaries, and their local relaxations and their migration dynamics in transition metal dichalcogenides (TMDs)  monolayers, WS2, WSe2 and MoS2, and their heterostructures with graphene .  This talk also presents a unique approach in formation of atomically thin freestanding van der Waals graphene/TMD heterostructures, where one is able to investigate the role of defects on the nucleation of a second layer in vertically stacked 2D heterostructures.

The physical and chemical properties in 2D crystals can be tuned not only by the native structure and chemistry of the crystal, but also by the impact of defects, dopants and interfaces.  In 2D crystals, defects exist on the surface of the crystal and can exhibit unique structures and migration dynamics.   In addition, the ability to isolate and manipulate two dimensional (2D) crystals has enabled us to create both in-plane and vertically stacked van der Waals heterostructures.  When creating a heterostructure via stacking dissimilar 2D crystals, new physical, chemical and electronic properties can emerge due to, but distinct from, the native two dimensional properties of the component materials.  In this study, ultra-high resolution aberration-corrected electron microscopy imaging is used to probe the atomic and chemical structure of the defects, i.e. dislocations, dopants, and grain boundaries, and their local relaxations and their migration dynamics in transition metal dichalcogenides (TMDs)  monolayers, WS2, WSe2 and MoS2, and their heterostructures with graphene .  This talk also presents a unique approach in formation of atomically thin freestanding van der Waals graphene/TMD heterostructures, where one is able to investigate the role of defects on the nucleation of a second layer in vertically stacked 2D heterostructures.