Layered transition metal dichalcogenides (TMDCs) have attracted great interests in recent years due to their physical properties manifested in different polytypes: Hexagonal(H)-TMDC,which is semiconducting, exhibits strong Coulomb interaction and intriguing valleytronic properties; distorted octahedral(T’)-TMDC,which is semi-metallic, is predicted to exhibit rich nontrivial topological physics. In this dissertation,we employ the polarization-resolved micron-Raman/PL spectroscopy to investigate the optical properties of the atomic layer of several polytypes of TMDC.
In the first part for polarization-resolved Raman spectroscopy, we study the lattice vibration of both H and T’-TMDC, providing a thorough understanding of the polymorphism of TMDCs. We demonstrate that Raman spectroscopy is a versatile tool to probe the symmetry as well as the quality of crystals. This becomes quite important for atomic layers of TMDCs which are sensitive to the environment and substrate. In the second part of the dissertation, we focus on fabricating high-quality monolayer tungsten diselenide samples and study their excitonic bound states by photoluminescence, reflection, resonant-Raman spectroscopy, magneto-optical measurements, and time-resolved spectroscopy. We first demonstrate the many-body correlation of the multi-particle bound excitonic states at low temperatures. The PL measurements in magnetic fields demonstrate for the first time the abnormal valleytronic properties of the biexciton and five-particle bound states, exciton-trion in 1L-TMDCs. The time-resolved PL measurement reveals the ultralong lifetime of the several bound states at even lower energy range, establishing a potential platform for further investigation on exciton condensation. In addition to the lower energy bound states, we also explore its excited Rydberg states in high magnetic fields. Surprisingly, we observe for the first time the PL of up to 4s excitons, opening doors to the investigation the physics of Rydberg exciton in a 2D system. For example, we investigate the role of electron-hole exchange-interactions in 2s exciton. Contrary to 1s exciton, the exchange interaction in 2s exciton is strongly suppressed due to its larger size. Consequently, the 2s exciton display superior valley polarization and valley coherence, paving the way to valleytronic applications
