Microscopic Theory of Externally Tunable Exciton Signatures of Two-Dimensional Materials
Doktorsavhandling, 2021

Atomically thin transition metal dichalcogenides (TMDs) are in the focus of current research due to their efficient light-matter interaction and the remarkably strong Coulomb interaction that leads to tightly bound excitons. Due to their unique band structure, TMDs show a variety of bright and optically inaccessible dark excitonic states. Moreover, the optimal surface-to-volume ratio makes these materials very sensitive to changes in their surroundings, which opens up the possibility of tailoring their optical properties via adsorption of molecules, application of strain, and deposition of defects.

The aim of this thesis is to use a microscopic many-particle theory to predict different strategies to externally control the optical fingerprint of TMDs. 

We show that specific molecules can activate dark excitons leading to new pronounced resonances in optical spectra. We also find that these dark states are very sensitive to strain, leading to significant energy shifts and intensity changes. This renders 2D materials suitable for optical sensing of molecules and strain. Moreover, we investigate how local defects due to single molecules or local strain can trap excitons. We show direct signatures of localized bright excitonic states as well as indirect phonon-assisted side bands of localized momentum-dark excitons. We find that the visibility of these localized states is determined by the interplay between defect-induced exciton capture and intervalley exciton–phonon scattering. Finally, we investigate the formation dynamics and optical signatures of spatially separated interlayer excitons at interfaces of acene-based molecular crystals and 2D materials, which play a crucial role for conversion of light to electricity in photodetecting devices. 

Overall, the work contributes to a better microscopic understanding of exciton optics and its control via strain, molecules, magnetic fields and impurities in atomically thin semiconductors.

transition metal dichalcogenides

dark excitons

Bloch equations

magnetic field

density matrix formalism

organic/inorganic heterostructures

localized states

strain

PJ salen, Kemigården 1, Chalmers
Opponent: Dr. Andres Castellanos-Gomez, Materials Science Institute of Madrid, Spain

Författare

Maja Feierabend

Chalmers, Fysik, Kondenserad materie- och materialteori

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2D Materials,; Vol. 8(2021)

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M. Feierabend, E. Malic . Dark interlayer excitons in WS2/tetracene heterostructures

2D Materials as Candidates for Optical Sensors

Following graphene's promising path, atomically thin materials have gained enormous attention. This can be traced back to their unique properties: they are ultrathin, strong, flexible, transparent and
conductive at the same time. These properties simply appear by changing the dimension of the material from 3D to 2D.

Transition metal dichalcogenides (TMDs)
belong to this class of atomically thin materials, and they are especially interesting since they show a strong signal when illuminated with light. They are also very sensitive to changes in their surroundings, which makes them perfect candidates for sensor applications.

In this thesis, we have developed a theoretical model
to describe and externally tune the optical fingerprint of TMDs. In particular, we show that optically inaccessible, so-called dark, states play a crucial role. We find that dark states can be activated by molecules attached to the material's surface, resulting in a significantly altered optical fingerprint. This presents a huge potential for sensing of molecules.

Furthermore, we show that the optical fingerprint of dark states undergoes large changes
when the material is mechanically deformed, offering potential for pressure sensing devices. Finally, following the vision of "atomic lego", we show that in combination with an organic layer, the optical response of TMDs can be further tuned to be used in a wide range of
optoelectronic applications.

Styrkeområden

Nanovetenskap och nanoteknik (SO 2010-2017, EI 2018-)

Infrastruktur

C3SE (Chalmers Centre for Computational Science and Engineering)

Ämneskategorier

Den kondenserade materiens fysik

ISBN

978-91-7905-471-7

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4938

Utgivare

Chalmers tekniska högskola

PJ salen, Kemigården 1, Chalmers

Online

Opponent: Dr. Andres Castellanos-Gomez, Materials Science Institute of Madrid, Spain

Mer information

Senast uppdaterat

2021-04-01