Title: Symmetry engineering via angular control of layered van der Waals heterostructures
Description: Crystal symmetry and elemental composition play a critical role in determining the physical properties of materials. In layered van der Waals (vdW) heterostructures, a two-dimensional (2D) material layer can be influenced by interactions between adjacent layers, dictating that the measured properties of the combined system will be in part derived from the geometric structure within the active layers. This thesis examines active crystal symmetry tuning in composite heterostructures of two-dimensional (2D) materials, engineered via nanomechanically assisted twist angle control, and designed by careful consideration of lowest energy stacking configurations. The material systems, devices, and experimental setups described in this thesis constitute a platform featuring highly programmable properties that are on-demand and reversible. Two prototypical systems are discussed in detail. The first is graphene encapsulated between boron nitride (BN) crystals, wherein the alignment state between the three layers is controlled.

The second is the same system, but with no graphene between the encapsulating BN layers. In both systems, a long-wavelength geometric interference pattern, also known as a moiré pattern, forms between the adjacent crystals as a consequence of lattice-constant mismatch and twist angle. The moiré pattern caries its own symmetry properties that are also demonstrated to be tunable, and can be thought of as an artificially constructed superlattice of periodic potential with wavelength much greater than the lattice constants of the constituent layers. In the BN-encapsulated graphene system we show drastic tunability of band gaps at primary and secondary Dirac points (PDP and SDPs) indicating reversible on-demand inversion symmetry breaking, as well as evidence of dual coexisting moiré superlattices and additional higher-order interference patterns that form between them. The all-BN system shows substantial enhancement and suppression of second harmonic generation (SHG) response from the vdW interface between the BN crystals when the quadrupole component of the SHG response is engineered to be minimal, by controlling for total layer number and layer number parity.

Changes in the physical properties of each composite system are measured with a combination of electronic transport measurements, and optical measurements (Raman and SHG), as well as piezo-force microscopy (PFM) measurements that give direct imaging of the moiré pattern. A number of invented and adapted fabrication and actuation techniques for controlling the twist angle of a bulk vdW crystal are discussed, and in the latter portion of this thesis these techniques are extended to include actuation of monolayer flakes of 2D crystals. In this discussion several case studies are discussed, including twist angle control for a single sample monolayer tungsten diselenide on monolayer molybdenum diselenide, as well as twist angle control for twisted bilayer graphene and graphene on BN. Additionally, a novel in-plane bending mode for graphene on BN is demonstrated using similar techniques. Further discussion of actuation via traditional electrostatic MEMS techniques is also included, illustrating complete on-chip control for on-demand nanomechanical actuation of 2D materials in vdW heterostructures.