Optical study of excitonic complexes in transition metal dichalcogenides monolayers

Abstract

Transition metal dichalcogenides (TMDs) monolayers such as MX2 (M = Mo, W; X = S, Se) feature strong exciton properties and spin-valley physics. As a result of strong quantum- and dielectric confinement in single atomic layer of TMD materials, the photoexcited electrons and holes are tightly bound via Coulomb interaction to form excitonic complexes such as charged excitons (trions), biexcitons that offer great opportunities to study many-body physics. However, the distinct electronic band structure and possible many-particle components lead to complicated excitonic features especially in the photoluminescence spectra of W-based monolayers. Their possible origins and compositions need to be further explored. In this thesis, we investigate the excitonic structures of TMDs monolayers via optical spectroscopy. With near-resonance photoluminescence excitation spectroscopy, multiple scattering processes between excitons and phonons carrying non-zero wavevector are revealed, indicating the presence of indirect excitons whose constituent electrons and holes locate at different valleys. The momentum-indirect transition in WS2 monolayers matches the energies of lower energy features in the PL spectrum, suggesting their possible origin from the recombination of electrons and holes in different valleys. Such transitions require additional electrons in order to conserve momentum, which could be provided by free electrons via the formation of charge states. The encapsulation of WS2 monolayer with hexagonal boron nitride (hBN) greatly reduces the PL linewidth and enables the observation of fine structures of the lower-energy features. By electrostatic doping to the hBN-encapsulated WS2 monolayer, we observe a Fermi edge singularity-like transition in the reflectance and PL spectra, reflecting distinct interactions between excitons and Fermi electrons under different electron densities. The Fermi edge singularity may indicate a transition from a fermion-like charged state to a boson-like particle (exciton-plasmon).Due to the inversion symmetry breaking, there are two energy-degenerate but inequivalent valleys (K and K' ). This degeneracy can be broken by applying a magnetic field, leading to a valley Zeeman splitting. The different energy splittings of the excitonic features in a magnetic field arise from several contributions, including spin magnetic moment, valley magnetic moment and atomic orbital magnetic moment. By examining the valley Zeeman splitting of different excitonic features of the hBN-encapsulated WS2 and MoS2 monolayers, information regarding the band configuration and valley magnetic moment of the electrons and holes that form the excitonic states is revealed. Our findings presented in this thesis gain critical insights into the excitonic structures and many-body physics, leading towards a complete understanding of optical and electronic properties of TMDs monolayers.