Towards Angle-Controlled Van Der Waals Heterostructures

Abstract

Two-dimensional (2D) materials such as graphene exhibit a combination of unique electronic, mechanical, and optical properties that have drawn significant attention over the past decade. While there has been extensive investigation into individual 2D materials, the burgeoning field of 2D heterostructures offers an even richer array of desirable properties. This led to increasing efforts to controllably manipulate these materials and to tailor them toward potential applications. An important step toward the realization of functional 2D heterostructures is the fabrication and characterization of high quality bilayers with a uniform rotation angle ([theta]) between the constituent layers. The rotation angle represents a new degree of freedom capable of tuning both optical and electrical properties and is therefore a critical component in designing heterostructures for specific applications. In this thesis, we discuss the fabrication of bilayer graphene with a controlled rotation angle. To accomplish this, we first develop an application of transmission electron microscopy (TEM) capable of imaging the structure and atomic registry between graphene layers in bilayer and trilayer structures. We then introduce a new method for creating graphene and hexagonal boron nitride (h-BN) single layers with crystallographic alignment over large scales, and we characterize the structural and electronic uniformity of these films using a variety of techniques, including TEM, low-energy electron diffraction (LEED), and angle-resolved photoemission spectroscopy. We conclude by showing that these large-scale aligned films can be used as building blocks for 2D layered structures with a controllable rotation angle and uniform optical properties. These findings provide a framework for imaging and fabricating angle-controlled heterostructures that is extensible beyond graphene and hBN to a variety of 2D materials, thereby opening the door to a virtually limitless combination of 2D heterostructures with uniquely tailored properties.