2026 Theses Doctoral

Chemical Control of Optoelectronic Properties in van der Waals Materials

Cox, Jordan E.

Low-dimensional quantum materials offer a compelling platform for controlling collective electronic, magnetic, and optical phenomena through targeted manipulation of structure and composition. In layered van der Waals systems, relatively subtle chemical or electronic perturbations, such as halide substitution, intercalation, or interfacial heterostructures, can alter lattice symmetry, redistribute electronic states, and stabilize emergent phases. This dissertation examines how chemically driven structural tuning and materials processing enable control over symmetry, magnetism, and nonlinear optical response in atomically thin transition metal oxyhalides and van der Waals magnets. Across chemically tunable semiconductors, cluster-based magnetic materials, and air-stable two-dimensional magnets, these studies demonstrate that modest modifications to composition or dimensionality can produce pronounced changes in collective behavior, establishing versatile routes toward functional quantum materials.

Chapter 1 establishes the layered tungsten oxyhalide family, WO2X2 (X = I, Br, Cl), as a chemically tunable platform for symmetry engineering in van der Waals semiconductors. Systematic halogen substitution drives a structural evolution from centrosymmetric WO2I2 to noncentrosymmetric WO2Br2 and WO2Cl2, enabling direct control over second harmonic generation activity and nonlinear optical response. In mixed-halide compositions WO2I2−yBry, both the polar lattice distortion and optical band gap are continuously tuned across the visible range, revealing a composition-driven symmetry boundary at which SHG is suppressed. To probe the coupling between electronic structure and symmetry beyond static chemical control, the chapter discusses the impact of chemical and photodoping in WO2Br2 on the nonlinear response. Together, these results demonstrate that subtle chemical and electronic perturbations drive transitions between centrosymmetric and polar phases, establishing layered mixed anion oxyhalides as a platform for tunable nonlinear optical materials. This chapter further presents the experimental observation of polarization waves and their fundamental quanta, ferrons, which carry electric polarization, in WO2Br2 and the related oxyhalide NbOI2.

Chapter 2 extends this chemically driven approach to a cluster-based van der Waals magnetic semiconductor, V4S9Br4. The chapter explores chemical intercalation as a route to modifying its electronic and magnetic structure and explore mechanochemical cluster excision and liquid exfoliation as strategies to access reduced-dimensional building blocks. These studies outline preliminary routes toward chemically controlling magnetism and electronic structure in cluster-based layered materials.

Chapter 3 establishes the air-stable van der Waals magnet CrSBr as a robust platform for exploring low-dimensional magnetism and interfacial phenomena. This work focuses on the development of reliable sample preparation and exfoliation protocols to access high-quality mono and few-layer CrSBr, enabling systematic studies at the two-dimensional limit. By producing clean, well-characterized flakes suitable for nanoscale magnetic imaging and heterostructure assembly, this work provided the foundation for subsequent investigations of intrinsic magnetic phases, spin-flip transitions, and interfacial coupling effects. In particular, bilayer CrSBr serves as a model antiferromagnetic system for Néel-vector manipulation via lateral exchange bias, while graphene/CrSBr heterostructures enable studies of anisotropic plasmon propagation driven by proximity-induced electronic anisotropy. These results position CrSBr as a versatile materials platform for engineering magnetic and polaritonic excitations in atomically thin systems.