Layered Two-Dimensional Heterostructures and Their Tunneling Characteristics

This thesis demonstrates that layered heterostructures of two-dimensional crystals graphene,hexagonal boron nitride, and transition metal dichalcogenides provide new and interestinginterlayer transport phenomena. Low-energy electron microscopy is employedto study the surface of atomically thin WSe2 prepared by metal-organic chemicalvapor deposition on epitaxial graphene substrates, and a method for unambiguouslymeasuring the number of atomic layers is presented. Using very low-energy electrons to probe the surface of similar heterostructures, a relationship between extractedwork function differences from the layers and the nature of the electrical contact betweenthem is revealed. An extension of this analysis is applied to surface studiesof MoSe2 prepared by molecular beam epitaxy on epitaxial graphene. A large workfunction difference is measured between the MoSe2 and graphene, and a model isprovided which suggests that this observation results from an exceptional defect densityin the MoSe2 film. The thesis expounds a theory for computing tunneling currents betweentwo-dimensional crystals separated by a thin insulating barrier; a few situationsresulting in resonant tunneling and negative differential resistance are illustrated bycomputed examples, as well as observed characteristics, for monolayer and bilayergraphene tunneling junctions and transistors.