Thesis advisor: Brian B. Zhou / This thesis presents work in the context of multimodal magnetometry for two-dimensional (2d) materials. Research on van der Waals materials has been rapidly emerging and several imaging techniques have been developed in the past decades. Among the modern techniques, solid-state spins feature outstanding sensitivity and nano-scale spatial resolution. Yet their full capacity in sensing still has room for improvement, as the quantum nature of their properties haven't been fully utilized. My research involves developing state-of-the-art sensing techniques to add new `function modules' to the nitrogen-vacancy (NV) centers, with the goal of uncovering dynamical magnetic and electrical phenomena of 2d materials. In the first chapter I will briefly discuss the basic opto-spin properties of the NV center. One shall see why NV is preferred as a quantum sensing probe: the opto-spin property comes handy as one simply counts photons to manipulate and read out quantum states, and the stability and long quantum coherence time makes NV adaptive with various environments and engineering. In the second chapter I will discuss the experimental setup with the focus on the home-built confocal microscope, which equips our sensing technique with the pump-probe scanning ability of sub-um 2d resolution. In the third chapter I will discuss the developments of the sensing protocols, including the ac susceptometry and the opto-magnetization mapping, based on the lock-in method using the quantum dynamical decoupling sequences. In the fourth chapter I will describe the ac susceptibility measurements on thin CrBr3 flakes. The magnetization behaviors under kHz to MHz excitations reveal the domain morphology and domain wall mobility, providing insights to the exchange interaction of the chromium trihalides in the 2d limit. In the fifth chapter I will describe the pump-probe measurements on few-layer CrCl3 flakes. The mapping result demonstrates a photo-generated enhancement of the in-plane magnetization. Along with the time-resolved photoluminescence measurement, the results are indicative of a defect-assisted Auger recombination process of excitons. To conclude, the multimodal sensing techniques with NV developed in this thesis allow for more versatile experiments with sensitivity for low-dimensional systems. The developments bring up new perspectives on fundamental physics in atomically thin materials, providing new ideas for future technological applications such as spintronics and quantum memory. / Thesis (PhD) — Boston College, 2024. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.