Spectroscopic investigations of complex van der Waals systems

Complex chalcogenides are superb platforms for revealing the interplay between structure, charge, and magnetism as well as the unusual states of matter that develop under external stimuli. These van der Waals solids can also be exfoliated into few- and single-layer sheets that sport a number of unexpected properties, including quantum confinement, magnetic excitations and size-induced magnetic states, and symmetry breaking. A commonality amongst sheet studies is the role of Raman scattering to assure sample quality, probe even-symmetry vibrational modes, and uncover single-layer properties. Odd-symmetry modes were, however, completely underexplored in few-layer materials. Infrared spectroscopy is well suited for examining the fundamental excitations of the lattice, and because the technique probes oddsymmetry vibrations, it is useful for revealing ferroelectric, vibronic, and spin-lattice coupling mechanisms. Although it is highly desirable to extend toward few- and single-layer systems, traditional infrared spectroscopy cannot overcome the diffraction limit for small-sized (exfoliated) flakes. Synchrotron-based near-field infrared nanospectroscopy offers an important path forward. A majority of my dissertation is focused on the layered MPS3 (M = Mn, Fe, and Ni) family. In exfoliated MnPS3 I discovered a symmetry crossover at n = 11 from C2/m to a higher order space group, P¯31m, due to the restoration of a C3 axis of rotation. Similar effects were found in the Fe analog but over a crossover range. NiPS3 is different in that this system resists distortion and sports P¯31m symmetry iv at all thicknesses. The closely related CrPS4 behaves similarly. This system has C2 symmetry, not only the bulk form, but also in the ultrathin limit. This implies that ferroelectricity and chirality exist through the monolayer. Finally, we explore the dynamics of HfS2 in order to examine the effects of spin orbit coupling on chemical bonding. We extract Born effective charge, polarizability, and ionicity all of which are in good agreement with our theoretical calculations - thus addressing long standing controversies in the literature.