Optical Sensing with Defects in Two-Dimensional Materials

Two-dimensional (2D) materials have emerged as a versatile platform at the intersection of fundamental physics and applied science. Their atomically thin nature gives rise to distinctive electronic and optical properties, while simultaneously enabling them to function as highly sensitive, readily integrable probes of their local environment. Advanced optical techniques, such as super-resolution microscopy, have opened new opportunities to interrogate these materials at the nanoscale, providing optical access to individual defect behaviors, exciton diffusion and recombination dynamics, and charge transport pathways. Such approaches not only deepen our understanding of intrinsic material behavior but also position 2D systems as powerful sensors capable of resolving local dielectric variations, electric and magnetic fields, and other environmental perturbations. Central to this work is hexagonal boron nitride (hBN), a transparent, wide bandgap semiconductor that serves as a host for optically active defects. We investigate a previously reported class of emitters that arise from interactions between native hBN and organic solvents. These emitters are believed to originate from defect sites that bind transiently to solvent molecules. To study their behavior, we develop a platform for imaging the dynamics of these transient emitters while varying the electrochemical potential and applying electric fields with controlled orientations. By tracking the spectra of individual emitters, we enable multiplexed measurements that allow spatially resolved electrochemical imaging. Through systematic analysis, we rule out modulation mechanisms based on direct electric field effects or charge transfer. Instead, we identify a mechanism driven by changes in proton concentration, which is modulated during the oxidation of trace water present in the solvent. This finding opens the possibility of using this platform for sensitive detection of protons and trace water in methanol fuel cells, where such species critically influence operational efficiency. In the final part of the thesis, we focus on a well-characterized spin defect in hBN, the negatively charged boron vacancy, and explore strategies to enhance its photoluminescence (PL) through heterostructure engineering that facilitates energy and exciton transfer. We demonstrate the coupled structure's improved utility in optical magnetometry compared to the defect by itself and discuss how improvements in PL could advance the development of wide-field optically detected magnetic resonance (ODMR) imaging using this spin defect. Using a defect in hBN would leverage 2D material's exceptional sensing capabilities and planar integrability. Overall, this thesis aims to highlight the strengths of integrating 2D materials with optical sensing and to develop transferable techniques for introducing controlled stimuli, such as electrochemical potentials, electric fields, or electromagnetic waves, with high fidelity and minimal art