Xu, XiaodongFonseca, Jordan2024-10-162024-10-162024Fonseca_washington_0250E_27279.pdfhttps://hdl.handle.net/1773/52608Thesis (Ph.D.)--University of Washington, 2024In the past 20 years, 2D van der Waals materials have garnered broad interest across a wide range of scientific and engineering disciplines. They provide a highly tunable platform for exploring emergent quantum phenomena in solid state systems. Their low defect density, atomic thickness, and high surface-to-volume ratio make them stand out in two ways in particular: First, their charge carrier density can be tuned dramatically with only an electrostatic gate. Moiré superlattices formed by stacking 2D crystals with either a twist angle or lattice mismatch have provided a highly tunable platform for exploring remarkably rich physical phenomena. Since the moiré pattern arises from interfering lattices, its wavelength and symmetry are highly sensitive to the effects of strain, which modifies the lattice parameters of the constituent layers. The combination of a large, experimentally tunable “superlattice” constant and the ability to fine-tune the carrier density in a single device at cryogenic temperatures provide unprecedented control over the electronic environment and have made gated moiré systems one of the most promising material platforms for exploring and simulating the physics of strongly correlated 2D electron systems. Second, 2D materials can be stretched, bent, and compressed more than bulk materials before they yield. Due to their remarkable strength, one particularly enticing tuning knob is strain since the ability to break rotational symmetries and change interatomic spacing both have significant implications for nearly all material properties that are rooted in the multi-orbital makeup of band structure and the crystal symmetries that underlie the tensors governing essentially all material properties. Despite the importance and potential for strain in tuning moiré systems, there have been limited experimental investigations utilizing in-situ strain control to date, not mentioning gated device geometry. In this dissertation, I tackle the challenge of simultaneously implementing cryogenic, tunable uniaxial strain in a high-quality, gated moiré superlattice. After establishing optical techniques as a probe of crystal symmetries in Chapter 2 and exploring how localized, out-of-plane strain pulses generated by an ultrafast laser can be used to perturb and probe material properties in 2D van der Waals crystals in Chapter 3, the following three chapters build on each other sequentially. In Chapter 4, I directly image the effects of applying uniaxial stress to an exposed WS2/WSe2 moiré superlattice with piezoresponse force microscopy (PFM). I use a combination of basic FFT analysis as well as an implementation of Geometric Phase Analysis (GPA) to qualitatively and quantitatively understand how this uniaxial stress strains the moiré superlattice. In Chapter 5, I report our work to address the strain transmission problem—graphene and hBN do not transmit strain from the substrate to a target layer in a functional device, requiring that we explore alternative dielectric substrates that provide an insulating, atomically flat, uniform dielectric surface that also transmits significant strain. I show that bismuth oxy-selenite is a suitable material for this purpose that transmits ~1% uniaxial tensile strain to a gate-tunable monolayer WS2 flake. Finally, in Chapter 6 I unify the previous results by demonstrating the ability to fabricate high-quality, top-gated WS2/WSe2 moiré heterostructure devices that can be continuously, reversibly strained up to 1% at cryogenic temperatures. I report, for the first time, an interlayer exciton strain gauge factor, a splitting of the interlayer exciton into two distinct species with distinct dipole moments, optical selection rules, and gate-dependent oscillator strengths.application/pdfen-USnone2D materialsmoire materialsopticsstraintransition metal dichalcogenidesPhysicsMaterials SciencePhysicsThesis