Moiré, Interlayer, and Proximity Effects in 2D van der Waals Heterostructures

The discovery of graphene in 2004 has sparked immense interest in atomically thin two-dimensional (2D) materials as well as the heterostructures made by vertically assembling different 2D layers, called van der Waals heterostructures. These van der Waals heterostructures facilitate an incredible level of control in an experimental setting by allowing the atomic species of each layer to be selected down to individual atoms. Additionally, new tuning parameters in van der Waals heterostructures, such as the relative twist angle, facilitate the development of long-range interference patterns known as moiré patterns. These moiré patterns can give rise to long range interactions and complicated electronic superstructures. For example, correlated states and superconductivity have been discovered in bilayers of graphene when the twist angle is tuned to specific “magic angles” such that flat bands emerge in the moiré pattern Brillouin zone. In addition to moiré patterns, interlayer interactions and proximity effects between the layers of van der Waals heterostructure provide a useful platform for manifesting novel physical phenomena and exciting application potential. In this work, I explore numerous aspects of van der Waal heterostructures, primarily through scanning tunneling microscopy and spectroscopy, and develop a novel sample fabrication technique that facilitates the surface probe studies of air-sensitive 2D van der Waals heterostructures.

In this work, I first explore moiré patterns in semiconducting heterobilayers. While flat bands have been demonstrated to emerge in homobilayer systems when the twist angle is finely tuned, I show that flat bands can emerge in heterobilayers that are rotationally aligned. I proceed to analyze this system in the context of a smoothly varying potential generated by the moiré pattern and show that the model extends well to systems with non-zero misorientation. By incorporating mechanical deformation effects, a complete model is developed to understand the heterobilayer system and provide predictive capabilities for arbitrary semiconducting bilayer systems. Next, I explore the role of topology in heterostructures with WTe2. I show that the quantum spin Hall edge state persist in twisted homobilayers as long as the interlayer coupling is weak. Additionally, I demonstrate proximity-induced superconductivity in the quantum spin Hall edge state of monolayer 1T’-WTe2, which is a significant step towards realizing fault-tolerant topological superconductivity in a van der Waals material platform. Finally, I conclude with a study of graphene heterostructures where I show that band engineering holds promise for future device applications. Taken together, the results in this work demonstrate progress towards realizing application potential and insight into the next generation of electronic devices.