Microscopic Theory of Externally Tunable Exciton Signatures of Two-Dimensional Materials

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.\ua0We 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.\ua0Overall, 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

Impact of strain on the optical fingerprint of monolayer transition metal dichalcogenides

Strain presents a straightforward tool to tune electronic properties of atomically thin nanomaterials that are highly sensitive to lattice deformations. While the influence of strain on the electronic band structure has been intensively studied, there are only few works on its impact on optical properties of monolayer transition metal dichalcogenides (TMDs). Combining microscopic theory based on Wannier and Bloch equations with nearestneighbor tight-binding approximation, we present an analytical view on how uni- and biaxial strain influences the optical fingerprint of TMDs including their excitonic binding energy, oscillator strength, optical selection rules, and the radiative broadening of excitonic resonances. We show that the impact of strain can be reduced to changes in the lattice structure (geometric effect) and in the orbital functions (overlap effect). In particular, we demonstrate that the valley-selective optical selection rule is softened in the case of uniaxial strain due to the introduced asymmetry in the lattice structure. Furthermore, we reveal a considerable increase of the radiative dephasing due to strain-induced changes in the optical matrix element and the excitonic wave functions

Out of the Dark and into the Light - Microscopic Analysis of Bright, Dark and Trapped Excitons

Atomically thin transition metal dichalcogenides (TMDs) have been in the focus of current research due to their efficient light-matter interaction, as well as the remarkably strong Coulomb interaction that leads to tightly bound excitons. Due to their unique band structure, TMDs show a variety of optically accessible bright and inaccessible dark excitons. Moreover, due to their optimal surface-to-volume ratio, these materials are very sensitive to changes in their surroundings, which opens up the possibility of externally tailoring their optical properties.The aim of this thesis is to present different strategies to control the optical fingerprint of TMD monolayers via molecules, strain and impurities. Based on a fully quantum-mechanical approach, we show that the coupling of excitons to high-dipole molecules can activate dark excitonic states, resulting in an additional and well-pronounced peak in the optical spectra.\ua0Moreover, we find that these dark excitonic states are very sensitive to strain, leading to crucial energy shifts and intensity changes of the dark exciton signature. Our findings reveal the potential for optical sensing of strain through activation of dark excitons.\ua0Finally, we investigate the possibility of local impurities to trap excitons resulting in localized states.We study the formation, excitonic binding energies and wave functions of localized excitonic states, all of which depend on the trapping potential. With this, we are able to calculate the photoluminescence signal and investigate the possibility of single-photon emission

Optical fingerprint of bright and dark localized excitonic states in atomically thin 2D materials

Point defects, local strain or impurities can crucially impact the optical response of atomically thin two-dimensional materials as they offer trapping potentials for excitons. These trapped excitons appear in photoluminescence spectra as new resonances below the bright exciton that can even be exploited for single photon emission. While large progress has been made in deterministically introducing defects, only little is known about their impact on the optical fingerprint of 2D materials. Here, based on a microscopic approach we reveal direct signatures of localized bright excitonic states as well as indirect phonon-assisted side bands of localized momentum-dark excitons. The visibility of localized excitons strongly depends on temperature and disorder potential width. This results in different regimes, where either the bright or dark localized states are dominant in optical spectra. We trace back this behavior to an interplay between disorder-induced exciton capture and intervalley exciton-phonon scattering processes

Impact of strain on the excitonic linewidth in transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) are known to be highly sensitive to externally applied tensile or compressive strain. In particular, strain can be exploited as a tool to control the optical response of TMDs. However, the role of excitonic effects under strain has not been fully understood yet. Utilizing the strain-induced modification of electron and phonon dispersion obtained by first principle calculations, we present in this work microscopic insights into the strain-dependent optical response of various TMD materials. In particular, we explain recent experiments on the change of excitonic linewidths in strained TMDs and predict their behavior for tensile and compressive strain at low temperatures.Comment: 7 pages, 7 figure

Molecule signatures in photoluminescence spectra of transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) show an optimal surface-to-volume ratio and are thus promising candidates for novel molecule sensor devices. It was recently predicted that a certain class of molecules exhibiting a large dipole moment can be detected through the activation of optically inaccessible (dark) excitonic states in absorption spectra of tungsten-based TMDs. In this work, we investigate the molecule signatures in photoluminescence spectra in dependence of a number of different experimentally accessible quantities, such as excitation density, temperature as well as molecular characteristics including the dipole moment and its orientation, molecule-TMD distance, molecular coverage and distribution. We show that under certain optimal conditions, even room temperature detection of molecules can be achieved

Brightening of spin- and momentum-dark excitons in transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) have been in focus of current research, among others due to their remarkable exciton landscape consisting of bright and dark excitonic states. Although dark excitons are not directly visible in optical spectra, they have a large impact on exciton dynamics and hence their understanding is crucial for potential TMD-based applications. Here, we study brightening mechanisms of dark excitons via interaction with phonons and in-plane magnetic fields. We show clear signatures of momentum- and spin-dark excitons in WS$_2$, WSe$_2$ and MoS$_2$, while the photoluminescence of MoSe$_2$ is only determined by the bright exciton. In particular, we reveal the mechanism behind the brightening of states that are both spin- \textit{and} momentum-dark in MoS$_2$. Our results are in good agreement with recent experiments and contribute to a better microscopic understanding of the exciton landscape in TMDs.Comment: 7 pages, 4 figure

Dark excitons in transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong Coulomb interaction that manifests in tightly bound excitons. Due to the complex electronic band structure exhibiting several spin-split valleys in the conduction and valence band, dark excitonic states can be formed. They are inaccessibly by light due to the required spin-flip and/or momentum transfer. The relative position of these dark states with respect to the optically accessible bright excitons has a crucial impact on the emission efficiency of these materials and thus on their technological potential. Based on the solution of the Wannier equation, we present the excitonic landscape of the most studied TMD materials including the spectral position of momentum- and spin-forbidden excitonic states. We show that the knowledge of the electronic dispersion does not allow to conclude about the nature of the material's band gap, since excitonic effects can give rise to significant changes. Furthermore, we reveal that an exponentially reduced photoluminescence yield does not necessarily reflect a transition from a direct to a non-direct gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states

Dark exciton based strain sensing in tungsten-based transition metal dichalcogenides

The recent emergence of atomically thin two-dimensional (2D) materials has opened up possibilities for the design of ultrathin and flexible nanoelectronic devices. As truly 2D materials, they exhibit an optimal surface-to-volume ratio, which results in an extremely high sensitivity to external changes which can not be achieved by conventional semiconductors. This makes these materials optimal candidates for sensing applications. Here, we propose a dark exciton based concept for ultrasensitive strain sensors. By investigating both dark and bright excitons in tungsten-based monolayer transition metal dichalcogenides, we demonstrate that the dark-bright-exciton separation can be controlled by strain, which has a crucial impact on the activation of dark excitonic states. The predicted opposite strain-induced shifts for dark and bright excitons result in a pronounced change in photoluminescence stemming from dark excitons even at very small strain values. The predicted high optical gauge factors of up to 8000 are promising for the design of optical strain sensors

Disorder-induced broadening of excitonic resonances in transition metal dichalcogenides

The presence of impurities and disorder has an important impact on the optical response of monolayer transition metal dichalcogenides (TMDs). Here, we investigate elastic exciton-impurity scattering and its influence on the linewidth of excitonic resonances in different TMD materials. We derive an analytic expression for the linewidth broadening within the density matrix formalism. We find that the exciton linewidth increases for states up to the 3s exciton due to the scattering with impurities. For higher states, the impurity contribution decreases, reflecting the reduced scattering cross section. Furthermore, we reveal that the scattering efficiency is the largest for transitions between s and p exciton states. Finally, different TMDs show generally a similar behavior. The quantitatively smaller broadening in tungsten-based TMDs can be ascribed to their smaller effective masses resulting in a less efficient scattering