Microscopic Theory of Exciton Dynamics in Two-Dimensional Materials
Doktorsavhandling, 2020

Transition Metal Dichalcogenides (TMDs) present a giant leap forward towards the realization of nanometer-sized quantum devices. As a direct consequence of their truly two-dimensional character, TMDs exhibit a strong Coulomb-interaction, leading to the formation of stable electron-hole pairs, so-called excitons. These quasi-particles have a large impact on optical properties as well as charge-transport characteristics in TMDs. Therefore, a microscopic understanding of excitonic degrees of freedom and their interactions with other particles becomes crucial for a technological application of TMDs in a new class of optoelectronic devices. The aim of this thesis is to investigate the many-particle processes governing the ultrafast dynamics of excitons in TMD mono- and bilayers. Based on the density matrix formalism we develop equations describing an interacting system of electrons, phonons and photons, and numerically simulate the dynamics of excitons in TMDs.

First, we provide a detailed picture of exciton-light and exciton-phonon interactions with special focus on the impact of momentum-dark exciton states. In particular, we develop and apply quantitative models for the i) broadening of excitonic resonances in linear absorption spectra, ii) formation of side peaks in photoluminescence spectra resulting from phonon-assisted recombination of momentum-dark excitons and iii) dynamical simulations of the formation of bound excitons out of a free electron-hole gas. Then, we investigate how the exciton-light interaction is modified when two TMD monolayers are vertically stacked into homo- and hetero-bilayers. Here we focus on the modification of optical spectra in bilayer systems by controlling the stacking angle. In particular, we iv) show how the interlayer hybridization of momentum-dark excitons can be controlled through the stacking angle and v) investigate how the localization phase of moir\'e excitons can be tuned. Our theoretical models have allowed us to predict experimentally accessible excitonic characteristics, which have been demonstrated in several joint experiment-theory collaborations including linear absorption, photoluminescence and ultrafast pump-probe experiments.

2D materials

exciton-phonon interaction

relaxation dynamics


van der Waals materials

density matrix formalism

Opponent: Professor Wang Yao, University of Hong Kong


Samuel Brem

Chalmers, Fysik, Kondenserad materie- och materialteori

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Ultrafast Dynamics in Nanomaterials

When a material is illuminated with light, electrons jump from their ground state to an excited level and leave a “hole” behind. This hole is positively charged and attracts the negative electron. In atomically-thin films of transition metal dichalcogenides (TMDs), a new class of quantum materials, the forces between electrons and holes are so strong that they start to orbit each other and form extremely stable pairs, so-called excitons. The concept of excitons was already introduced by Yakov Frenkel in 1931 but just the recent development of atomically-thin TMDs has brought excitons into the focus of material research. These 2D materials have the potential to revolutionize the electronics industry by enabling new nanoscale device concepts. However, the key to technological applications of TMDs is a fundamental understanding of excitons.

In this work we have developed a theoretical model of excitons in TMDs. We provide microscopic insights into the motion of excitons and their interaction with light or crystal vibrations. In particular, we identify the impact of hidden so-called dark states. We also show how exciton properties change when two different TMDs are stacked. Here, we discuss how the resulting super periodic crystal structure, the moire pattern, can trap excitons into an array of light emitters. We applied our theoretical models to explain a large variety of experiments and gained microscopic insights into the dynamics of excitons in 2D quantum materials.

Excitondynamik i atomiskt tunna material

Vetenskapsrådet (VR), 2019-01-01 -- 2024-12-31.



Den kondenserade materiens fysik



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4858


Chalmers tekniska högskola


Opponent: Professor Wang Yao, University of Hong Kong

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