Inelastic and Elastic Light-Matter Interactions in Two-Dimensional Semiconductors
Creators: name: Yang, Jiong
Publisher: name: The Australian National University
Year: 2017
Subjects: light-matter interactions
two-dimensional
semiconductors

description: Two-dimensional (2D) semiconductors have attracted significant attention over past decades. Graphene, transition metal dichalcogenides (TMDs) and phosphorene, etc. have been studied on their unique electrical, optical and optoelectronic properties. Most of previous studies are focused on inelastic light-matter interactions of 2D semiconductors. This dissertation will continue the research of inelastic light-matter interactions of novel 2D semiconductors, and will expand to their elastic interactions with light. In this dissertation, by exploiting inelastic interactions of thin-layer TMDs and phosphorene with excitation laser, Raman/PL spectroscopic analysis will be conducted to investigate their optical properties. Their layer-dependent phonon modes and optical bandgaps will be revealed. Low-temperature Raman measurements will be conducted to study their thermal properties. Low-temperature PL measurements can help understand band structure evolution of 2D TMD semiconductors with the influence of temperature. And anisotropic properties of phosphorene will be studied by polarization-resolved Raman/PL spectroscopy. When laser inelastically interacts with 2D semiconductors, exciton, trion and higher-order complexes will form, and PL spectroscopy is a useful tool to detect those quasi-particles by analysing their photon emissions during recombination. External electric, cryogenic and optical factors will all have impacts on and can tune the dynamics of those quasiparticles. In this dissertation, exciton and trion dynamics in bilayer (2L) MoS2, monolayer (1L) molybdenum ditelluride (MoTe2), and 1L phosphorene will be presented. Exciton and trion dynamics in 2L MoS2 and 1L MoTe2 will be tuned by external electric field at cryogenic temperature. And exciton and trion dynamics in 1L phosphorene will be tuned by direct optical injection. Phosphorene has been reported to be highly unstable in ambient conditions and it can be easily oxidized at the presence of light and moisture. Theoretical calculations have revealed that, if oxidized, certain types of oxygen defects in phosphorene can introduce new valence-band-like or conduction-band-like sub-bands to its electronic bandgap. In this dissertation, controllable oxidation will be applied to 1L phosphorene by interfacial defects, and it can brighten excitons and trigger efficient photon emissions at new wavelengths. Finally, we will expand to elastic light-matter interactions of 2D semiconductors. Strong elastic interactions rely on substantial changes of amplitude and phase of lights. Our phase-shifting interferometer measurement results indicate that thin-layer TMDs, taking MoS2 as an example, own large optical path length (OPL) values despite of their very thin nature. By taking advantage of such giant OPL, atomically thin optical lenses and gratings based on thin-layer MoS2 have been fabricated with focused Gallium ion beam. The measured focal length of the fabricated MoS2 micro-lens is consistent with the calculated value. And gratings fabricated from thin-layer MoS2 exhibit higher diffraction efficiency than those fabricated from silica and graphene.