Microscopic Theory of Charge Complexes in Atomically-Thin Materials
Doktorsavhandling, 2023

Atomically-thin materials have emerged as the most promising two-dimensional platform for future optoelectronic applications and for the study of quantum many-body physics. In particular, transition metal dichalcogenides (TMDs) exhibit strong Coulomb interaction, resulting in the formation of tightly-bound electron-hole complexes that dominate optics, dynamics, and transport. In the neutral regime, excitons -- bound electron-hole pairs -- constitute the dominating many-particle species from low to moderate photoexcitation densities. In the presence of doping, however, excitons can bind to additional charges and form trions. In order to achieve an efficient and controllable implementation of TMDs in novel devices, understanding the fundamental properties of excitons and trions in these materials is crucial.

The aim of this thesis is to provide a microscopic understanding of the underlying many-particle mechanisms in TMD optoelectronic devices. Based on the density-matrix formalism, we describe the dynamics in a system of interacting electrons, holes, phonons, and photons. We model the excitonic features of optical absorption spectra and reveal how they are influenced by the excitation density. We unveil the formation dynamics of dark excitons after photoexcitation and resolve the main pathways of phonon-assisted dissociation. Furthermore, we tackle exciton diffusion, tracing the emergence of photoluminescence halos back to the large heating and thermal drift of excitons at strong excitation. Finally, we consider doped TMDs and investigate the trion dynamics, including diffusion and photoluminescence. In particular, we predict so far unobserved luminescence signatures that could shed light on the internal structure of trions.

Overall, this work provides microscopic insights into many-particle processes governing the optics, dynamics, and transport in atomically thin semiconductors.




propa- gation


many-body physics



2D materials

PJ-salen and Zoom
Opponent: Prof. Atac Imamoglu, ETH Zurich, Switzerland


Raul Perea Causin

Chalmers, Fysik, Kondenserad materie- och materialteori

Exciton Propagation and Halo Formation in Two-Dimensional Materials

Nano Letters,; Vol. 19(2019)p. 7317-7323

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Microscopic Modeling of Pump-Probe Spectroscopy and Population Inversion in Transition Metal Dichalcogenides

Physica Status Solidi (B): Basic Research,; Vol. 257(2020)

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Momentum-Resolved Observation of Exciton Formation Dynamics in Monolayer WS<inf>2</inf>

Nano Letters,; Vol. 21(2021)p. 5867-5873

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Phonon-assisted exciton dissociation in transition metal dichalcogenides

Nanoscale,; Vol. 13(2021)p. 1884-1892

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Trion-phonon interaction in atomically thin semiconductors

Physical Review B,; Vol. 106(2022)

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R. Perea-Causin, S. Brem, O. Schmidt, E. Malic, Trion photoluminescence and trion stability in atomically thin semiconductors

Unraveling the properties of atomically-thin materials

The miniaturization trend in electronics has motivated the pursuit of nanomaterials with dimensions in the atomic scale. Atomically-thin materials offer technological promise in this direction and constitute a platform for the study of novel quantum phenomena. When a material is hit by light, electrons are excited into states with higher energy, leaving behind "holes” with positive charge. In atomic layers of transition metal dichalcogenides (TMDs), electrons and holes attract each other strongly and form tightly bound pairs called excitons. The fundamental understanding of excitons and other charge complexes is essential for the efficient utilization of TMDs in novel devices.

In this work, we develop quantum-mechanical models to investigate the impact of electron-hole complexes on the optical and electronic properties of TMDs. We reveal the mechanisms that lead to the stabilization of excitons into so-called dark states and their dissociation into separate electrons and holes. In collaboration with experiments, we study the spatial motion of excitons and unravel the intricate details governing their evolution into ring-like profiles that generate luminescence halos. Finally, we investigate the spatial motion of trions (exciton-electron complexes) and the mechanisms governing their decay into electrons and light. In particular, we predict luminescence signatures that, if observed, could shed light on the internal structure of trions.

This work represents a step forward in the understanding of the physics of charge complexes and contributes to determining the optimal operation conditions, limitations and tunability of TMD-based devices.


Nanovetenskap och nanoteknik



Den kondenserade materiens fysik


C3SE (Chalmers Centre for Computational Science and Engineering)



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5343



PJ-salen and Zoom


Opponent: Prof. Atac Imamoglu, ETH Zurich, Switzerland

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