Optical Effects of Excitons in Monolayer Semiconductors

Abstract

Semiconducting transition metal dichalcogenides (TMDs) are 2D sheet-like materials with atomic scale thickness. The quantum confinement effects of TMDs enhance the interactions between charge carriers, leading to strongly Coulomb bound electron-hole pairs called excitons. The unique characteristics of the monolayer TMD band structure allow them to couple strongly with light fields in two degenerate, but inequivalent valleys. TMDs do not require barrier layers, unlike quantum wells, their quasi-2D predecessors. As such, they can be stacked directly onto substrates or other 2D materials to form heterostructures where the direct contact promotes exciton interaction effects without lattice matching restrictions. In the following dissertation, I explore three unique optical effects of TMD excitons in 2D heterostructures. The first effect shows how the band alignment of MoSe2/WSe2 heterostructures results in tightly bound interlayer excitons, where the tunable stacking order and twist angle between the lattices give rise to a spatially dependent moiré potential. The moiré potential can trap the excitons and give rise to stacking order-dependent optical properties. The behavior with temperature and power can be explained with the intervalley exchange interaction for interlayer excitons, which affects the population of bright excitons differently based on the stacking order. The next effect shows how encapsulating WSe2 in hBN, a 2D insulator, improves the optical quality of the TMD. This improvement leads to narrow emission satellites with anomalous nonlinear Zeeman shifts and linear polarization of the exciton-plasmon quasiparticle when the WSe2 is heavily electron doped. Finally, the effects of organic-inorganic heterostructures are investigated, specifically, hybridization between the WSe2 B exciton and the nearly degenerate 0-0 exciton of the organic molecular semiconductor PTCDA. The excitonic coupling creates a new resonance, resulting in an order of magnitude enhancement of the molecule's Raman emission.