Abstract
Electrons and holes easily form a bound state under Coulomb attraction, recognized as a quasiparticle of “exciton”. In 2D materials, the spatial confinement results in weak Coulomb screening, manifesting a giant exciton binding energy, robust enough against thermal perturbation. The newly discovered 2D atomic crystals provide an exciting platform for explorations on many-body behaviors and exciton physics in 2D system. This thesis reports our experimentally study on the exciton properties in monolayer molybdenum diselenides and mutilayer Gallium selenide with semiconductor optics technique. We also discuss the analytical solution of exciton dissociation rate with semi-classical approximation.
We studied the excitons and its excited state in monolayer MoSe2 under electrically tuned carrier density. The exciton-polaron coupling at band edge A1s state, its excited state A2s and spin-split higher band B1s are observed. The energy splitting of A2s state between the repulsive and attractive polaron branch is much larger than that in A1s state.
We examine thickness dependent photoluminescence in multilayer Ga2Se2. Direction-resolved photoluminescence spectroscopy was used to determine the orientation of excitons and the ratio of in-plane vs. out-of-plane excitons. We observe that in-plane excitons increase as sample thickness decreases, which can be well explained by a naïve phenomenological model.
Finally, we construct an analytical solution for all s-state exciton dissociation rate with semi-classical approximation. The result shows that both 1s state and 2s state excitons in TMDs (transition metal dichalcogenides) have a large exciton dissociation rate under ~107 V/m. The energy shifts of 1s and 2s states under in-plane electric field are discussed with perturbation theory.
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