Topological and optoelectronic properties of stacked two-dimensional materials

When two-dimensional (2D) materials, such as graphene, hexagonal boron nitride, and transition metal dichalcogenides, are stacked on top of each other, their properties can change significantly. In this work, we explore the emergent topological and optoelectronic properties of stacked two-dimensional materials using scanning probe microscopy (SPM), including scanning tunneling microscopy (STM) and photocurrent atomic force microscopy (PC-AFM).
We describe the fabrication of stacked two-dimensional materials geared towards SPM. We explain the procedure of picking up flakes, how to stack them into (twisted) heterostructures, and how to transfer them to a substrate of choice. Finally, we explain the microsoldering method to electrically connect the samples to the SPM system.
We study a topological valley Hall network that emerges in bilayer graphene with a small twist angle. The twist creates domains of different stacking orders, which form a triagonal lattice. When an electric field is applied perpendicular to the graphene, the graphene develops a bandgap. Furthermore, the symmetry between the AB and BA stacking domains is broken, creating a topological boundary. At this boundary, electrons from different valleys counterpropagate, and they do so dissipationlessly as long as no intervalley scattering occurs. We explore the emergence of the network in a naturally occuring twisted bilayer in highly oriented pyrolithic graphite and in a stacked sample of twisted bilayer graphene on hBN.
Using a photoconductive AFM, we measure the photocurrent at sub-nanometer resolution in transition metal dichalcogenides. We show that exciton peaks can be clearly resolved, even at room temperature. We then study the excitonic properties of a stacked TMDC sample, were we show that excitons from multiple layers can be measured in the photocurrent. We then dive deeper into the effects of atomic defects and flake edges on the excitonic structure of molybdenum diselenide on hBN, were we show that the excitonic response is significantly altered as compared to the pristine material.