Spectroscopy of spin-valley excitons in two-dimensional semiconductors and heterostructures

Funk, Victor (2024): Spectroscopy of spin-valley excitons in two-dimensional semiconductors and heterostructures. Dissertation, LMU München: Fakultät für Physik

Abstract

Transition metal dichalcogenides such as tungsten diselenide (WSe2) and molybdenum diselenide (MoSe2) are two-dimensional semiconductors that exhibit unique optical and electronic properties. In the monolayer limit, they exhibit a direct band gap with optical transitions in the visible to near-infrared spectrum. At the energy minima of the inequivalent K and K' valleys, light-matter interactions couple electrons of the conduction band with empty valence band states. The reduced electrostatic screening due to the two-dimensional nature of the crystal structure leads to the formation of tightly bound excitons. These are energetically degenerate and exhibit valley-index dependent optical selection rules, making them ideal for next generation optoelectronic devices. This thesis focuses on the light-matter interactions in WSe2 mono- and bilayers as well as MoSe2-WSe2 heterobilayers at cryogenic temperatures to add to the understanding of their exciton transitions, phonon interactions and valley polarisation. The first part of this study investigates the temperature-dependent photoluminescence of WSe2 mono- and bilayer under controlled electrical doping. A pronounced asymmetry in the spectral profile of phonon-assisted luminescence from momentum-indirect exciton reservoirs were observed. In contrast, excitons with direct radiative decay pathways display thermally broadened, symmetric spectral profiles. The photon dispersion relation reduces the number of allowed radiative states due to energy and momentum conservation. This restriction is removed by phonons. The results of this work add to the understanding of phonon-assisted recombination of momentum-dark excitons and, more generally, establish means to access the thermal distribution of finite-momentum excitons in atomically thin semiconductors with indirect bandgaps. The second part of the thesis explores the spin dynamics of interlayer excitons in MoSe2-WSe2 heterobilayers in spin-triplet configuration. Cryogenic, spectral and time-resolved measurements of the photoluminescence in magnetic field were performed under controlled polarisation. The measured degree of polarisation was decomposed into magnetic and optical contributions and described within a rate equation model. The model includes thermalisation between Zeeman-split reservoirs and dephasing due to long-range Coulomb interactions. Within these minimal assumptions, momentum-indirect interlayer excitons were found as a polarisation-maintaining reservoir. The resulting valley polarisation of interlayer excitons as a function of magnetic field was described for both the steady-state and time-resolved regimes. This work provides an analytical framework applicable to the whole class of interlayer-excitons in heterobilayer systems.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 13. November 2024
• 1. Bericht­erstatter:in: Högele, Alexander