Optical and Electronic Properties of Atomically Thin Transition-Metal Dichalcogenides

Abstract

Two-dimensional semiconducting monolayers of transition metal dichalcogenides (TMDs) are of pivotal interest due to their fascinating optical and electronic properties. High optical yield, the direct band gap, sensitivity to the surrounding material or strain, and the ability to stack various heterostructures by exfoliation techniques opens up new possibilities for device concepts. The promising properties originate from the exceptionally strong Coulomb interaction between charge carriers due to the two-dimensional geometry and weak dielectric screening. It is necessary to analyse the interplay of many-body Coulomb interaction effects in order to provide a microscopic understanding of the underlying physics. High densities of excited charge carriers populating the band structure valleys cause strong renormalizations that are investigated in this thesis for the typical monolayer TMDs MoS2, MoSe2, WS2 and WSe2. The semiconductor Bloch equations are evaluated, including many-body Coulomb interaction. Excitation-induced band structure renormalizations cause a transition from direct to indirect band gaps, which drains carriers from the bright optical transition to dark states. Thus the advantageous properties of a direct gap semiconductor vanish with increasing carrier density, which has strong implications for optical applications such as TMD nanolasers. Monolayer TMDs are often studied in experiments that involve photoexcitation of charge carriers, which requires the knowledge of the charge carrier density in order to interpret the results. Estimating the density from the linear absorption coefficient is a common yet misleading concept that does not reflect optical non-linearities emerging at elevated pump power. Here, the evaluation of the population dynamics for a non-equilibrium state provides insight into the fluence dependence of the photoexcited density, which originates from the balance between Pauli-blocking and band structure renormalizations as well as scattering processes that dominate in a different regime. The stacking of TMD layers to build van der Waals heterostructures has opened a growing field of research. TMD heterobilayers of type-II band-alignment exhibit interlayer excitons (ILX) that are characterized by spatial separation of electron and hole and long lifetimes. The twist angle between the two layers provides further possibilities to tailor the bilayer properties. The dependence of the ILX lifetime on the twist angle and the temperature is analysed in detail, revealing the physics behind indirect Moiré excitons.