UC Santa Cruz

Metal halide nanomaterials, including molecular clusters (MCs), nanocrystals (NCs), and low-dimensional perovskites, have attracted significant interest because of their tunable optical properties, strong excitonic effects, and potential applications in optoelectronic and spintronic devices. However, the relationships among structure, excited-state dynamics, lattice vibrations, and spin relaxation remain insufficiently understood, especially in highly confined metal halide systems. This dissertation addresses how chemical composition, structural confinement, lattice softness, and dopant incorporation influence the optical, vibrational, excitonic, and spin properties of metal halide clusters and nanocrystals. A combination of synthesis, steady-state spectroscopy, ultrafast spectroscopy, structural characterization, and vibrational analysis was used. Amino metal halide molecular clusters were synthesized and stabilized in the solid state, and their structural and optical properties were investigated using optical spectroscopy and crystallographic analysis. Their excited-state relaxation processes were further studied by femtosecond transient absorption spectroscopy. Furthermore, low-frequency Raman spectroscopy, supported by structural analysis, was used to investigate lattice vibrations in two-dimensional lead-free metal halide double perovskites. Finally, Ni-doped CsPbBr3 nanocrystals were examined using spin-polarized transient absorption to evaluate the influence of dopant-induced defect passivation and scattering processes on spin lifetime. The results show that amino metal halide molecular clusters can form stable, strongly quantum-confined solid-state structures around 2 nm with distinct optical properties arising from their discrete metal halide units and ligand-stabilized coordination environments. Ultrafast measurements reveal that their excited-state dynamics are governed by rapid relaxation pathways associated with strong confinement and a shallow trap state. Low-frequency Raman studies of 2D lead-free perovskites demonstrate that soft metal halide lattices exhibit characteristic vibrational modes sensitive to local structure and large octahedra distortions. In Ni-doped 3D CsPbBr3 nanocrystals, moderate Ni incorporation improves spin lifetime, likely through defect passivation and reduced defect-mediated spin relaxation, whereas excessive doping shortens the lifetime due to increased disorder and scattering. Overall, this dissertation establishes a structure–dynamics–spin relationship in metal halide clusters and nanocrystals and provides insight into the design of confined metal halide materials with tunable optical and spin-dependent properties.