Characterization of exciton properties in two-dimensional materials

Abstract

Electrons and holes easily form a bound state under Coulomb attraction, recognized as a quasiparticle of “exciton”. In two-dimensional (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 a perfect platform for explorations on many-body behaviors and exciton physics in 2D system. This thesis reports our experimental study on the exciton properties in monolayer molybdenum diselenides and multilayer Gallium selenide with semiconductor optics technique. I also discuss the analytical solution of exciton dissociation rate with semi-classical approximations. We study the excitons and their excited state in monolayer MoSe2 under electrically tuned carrier density. The exciton-polaron coupling at the band edge A1s state, its excited state A2s and the 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. The carrier dependent polaron splitting of ground and excited state gives a strong evidence of the Fermi-polaron picture. We examine the thickness dependent photoluminescence in multilayer Ga2Se2. With the help of direction-resolved photoluminescence spectroscopy, we observed that the band edge excitons have in-plane and out-of-plane components. The intensity ratio of in-plane vs. out-of-plane increases with the increased sample thickness, 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 ~10^7 V/m. The energy shifts of 1s and 2s states under in-plane electric field are discussed with perturbation theory.