Structure and Properties of Moiré Interfaces in Two Dimensional Materials

Abstract

The ability to rationally design materials with specific properties is a major motivator in materials science and condensed matter physics. Heterostructures of two dimensional (2D) materials, weakly bonded out of plane via van der Waals forces, offer a versatile platform for creating materials by stacking old materials in new ways. In this thesis, we focus on a facet of that endeavor that has been growing considerably over the past decade: the effect of moiré patterns that form when two lattices are nearly, but not quite, aligned. The van der Waals forces are weak enough that materials are free to sit on each other with incommensurate alignment, due to twist, lattice constant mismatch, or strain. However, the van der Waals forces do serve to create local commensuration, leading to the formation of domains and dislocations at the interface. We use dark field transmission electron microscopy (TEM) to study the resulting structures, and connect to their unique electronic properties.

We investigate the atomic and electronic structure of twisted bilayer graphene near the small and large angle limits of 0° and 30°. We then move beyond twist (seeing how strain produces moiré patterns of its own), beyond graphene (to sublattice-asymmetric materials that can host ferroelectric domains), and finally, beyond bilayer (to twisted trilayer and quadrilayer), getting a view of how extensive the space of moiré-based materials is.