Light-matter interactions of two-dimensional materials and the coupled nanostructures

Abstract

Two-dimensional (2D) materials have gained increasing attention due to their unique and extraordinary electrical and optoelectronic properties. These properties can be largely attributed to the fundamental light-matter interactions. This thesis project uses spectroscopy techniques and focuses on the study of the light-matter interaction in 2D materials, as well as their coupling with other nanostructures, which are essential in achieving useful optoeletronic applications with 2D materials. First, the fundamental properties of 2D materials were investigated using spectroscopy. Photoluminescence (PL) spectroscopies of MoS₂ and its related structures were studied, showing that the interaction between MoS2 layers and other dielectrics can strongly affect their PL emissions, exciton and trion properties. Moreover, combining Raman spectroscopy and X-ray photoelectron spectroscopy, the effects of substrates and defects for MoS₂ have been revealed. Next, interlayer vibrational properties of 2D materials are studied utilizing low-frequency Raman spectroscopy. Twisted bilayer MoS2 and few-layer black phosphorus were chosen to demonstrate the interlayer coupling from the perspective of interlayer breathing and shear Raman modes. These exemplary studies offer a great tool to investigate the interlayer coupling, thickness, and stacking configurations of 2D materials using low-frequency Raman spectroscopy. The anisotropic light-matter interactions of 2D materials were also examined. Using polarization dependent Raman and optical absorption spectroscopies, together with first-principles density functional theory analysis and group theory, the anisotropy of electron-photon and electron-phonon interactions can be revealed. This method can experimentally exhibit the anisotropy of electron-phonon interactions in 2D materials, and can be generalized to other layered materials with in-plane anisotropy. The interactions of 2D materials with other materials systems were also investigated using optical spectroscopies. The interactions of 2D materials and selected organic molecules were revealed using graphene-enhanced Raman spectroscopy. The interaction between 2D materials and plasmonic nanocavities were found to exhibit an interesting enhancement phenomenon for the optical response of 2D materials. Overall, the studies presented in this thesis work show broad opportunities for using spectroscopic tools to study light-matter interactions of 2D materials, as well as the combined system of 2D materials and other nanostructures. This work is significant fundamentally, and also offers useful guidelines for practical applications of 2D materials in electronics and optoelectronics.