Microscopic Theory of Charge Complexes in Atomically-Thin Materials

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

Microscopic Modeling of Exciton Propagation and Dissociation in Two-Dimensional Materials

Atomically thin materials have been in the spotlight of research during the last decade due to their exceptional properties, providing a platform for the study of novel physical phenomena. In particular, transition metal dichalcogenides (TMDs) have emerged as promising atomically thin materials for future optoelectronic applications owing to their strong light-matter interaction and their high tunability. Furthermore, the strong Coulomb interaction in TMDs leads to the formation of tightly-bound electron-hole pairs -- excitons -- that dominate optics, dynamics and transport properties. Therefore, an accurate microscopic description of excitons in TMDs is essential for their technological application.The aim of this thesis is to microscopically investigate the underlying many-particle mechanisms behind the main processes in optoelectronic devices, such as optical generation and relaxation of excitons as well as their propagation and dissociation into unbound electron-hole pairs. Based on the density matrix formalism, we develop equations of motion describing the dynamics in a system of interacting electrons, phonons, and photons. We investigate the density-dependence of the optical absorption and the thermalization of excitons into so-called dark states. We shed light on exciton propagation, revealing the microscopic mechanisms behind the appearance of spatial rings (halos) in the photoluminescence at strong excitation. Moreover, we tackle the problem of exciton dissociation, providing insights on the prominent role of dark excitons, and examine the tunability and optimal conditions for the efficient operation of TMD-based optoelectronic devices. Finally, we provide microscopic insights on charge separation in WS2-graphene heterostructures.Our theoretical work, together with experimental support, contributes to the understanding of the many-particle mechanisms that govern the performance of TMD-based optoelectronic devices

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

Optical properties of transition metal dichalcogenide (TMD) monolayers are dominated by excitonic effects. These are significantly altered at high carrier densities, leading to energy renormalization, absorption bleaching, and even optical gain. Such effects are experimentally accessible in ultra-fast pump-probe measurements. Herein, the semiconductor Bloch equations are combined with the generalized Wannier equation to investigate the effect that excited carriers have on the excitonic properties of TMD monolayers. In particular, the dynamics of carrier occupation, energy renormalization, and absorption bleaching as well as population inversion and optical gain are investigated

Phonon-assisted exciton dissociation in transition metal dichalcogenides

Monolayers of transition metal dichalcogenides (TMDs) have been established in the last years as promising materials for novel optoelectronic devices. However, the performance of such devices is often limited by the dissociation of tightly bound excitons into free electrons and holes. While previous studies have investigated tunneling at large electric fields, we focus in this work on phonon-assisted exciton dissociation that is expected to be the dominant mechanism at small fields. We present a microscopic model based on the density matrix formalism providing access to time- and momentum-resolved exciton dynamics including phonon-assisted dissociation. We track the pathway of excitons from optical excitation via thermalization to dissociation, identifying the main transitions and dissociation channels. Furthermore, we find intrinsic limits for the quantum efficiency and response time of a TMD-based photodetector and investigate their tunability with externally accessible knobs, such as excitation energy, substrate screening, temperature and strain. Our work provides microscopic insights in fundamental mechanisms behind exciton dissociation and can serve as a guide for the optimization of TMD-based optoelectronic devices

Phonon-assisted Exciton Dissociation in Transition Metal Dichalcogenides

Monolayers of transition metal dichalcogenides (TMDs) have been established in the last years as promising materials for novel optoelectronic devices. However, the performance of such devices is often limited by the dissociation of tightly bound excitons into free electrons and holes. While previous studies have investigated tunneling at large electric fields, we focus in this work on phonon-assisted exciton dissociation that is expected to be the dominant mechanism at small fields. We present a microscopic model based on the density matrix formalism providing access to time- and momentum-resolved exciton dynamics including phonon-assisted dissociation. We track the pathway of excitons from optical excitation via thermalization to dissociation, identifying the main transitions and dissociation channels. Furthermore, we find intrinsic limits for the quantum efficiency and response time of a TMD-based photodetector and investigate their tunability with externally accessible knobs, such as excitation energy, substrate screening, temperature and strain. Our work provides microscopic insights in fundamental mechanisms behind exciton dissociation and can serve as a guide for the optimization of TMD-based optoelectronic devices

Negative effective excitonic diffusion in monolayer transition metal dichalcogenides

While exciton relaxation in monolayers of transition metal dichalcogenides (TMDs) has been intensively studied, spatial exciton diffusion has received only a little attention-in spite of being a key process for optoelectronics and having already shown interesting unconventional behaviours (e.g. spatial halos). Here, we study the spatiotemporal dynamics in TMD monolayers and track optically excited excitons in time, momentum, and space. In particular, we investigate the temperature-dependent exciton diffusion including the remarkable exciton landscape constituted by bright and dark states. Based on a fully quantum mechanical approach, we show at low temperatures an unexpected negative effective diffusion characterized by a shrinking of the spatial exciton distributions. This phenomenon can be traced back to the existence of dark exciton states in TMD monolayers and is a result of an interplay between spatial exciton diffusion and intervalley exciton-phonon scattering

Microscopic origin of anomalous interlayer exciton transport in van der Waals heterostructures

Van der Waals heterostructures constitute a platform for investigating intriguing many-body quantum phenomena. In particular, transition-metal dichalcogenide (TMD) heterobilayers host long-lived interlayer excitons which exhibit permanent out-of-plane dipole moments. Here, we develop a microscopic theory for interlayer exciton-exciton interactions including both the dipolar nature of interlayer excitons as well as their fermionic substructure, which gives rise to an attractive fermionic exchange. We find that these interactions contribute to a drift force resulting in highly nonlinear exciton propagation at elevated densities in the MoSe2 - WSe2 heterostructure. We show that the propagation can be tuned by changing the number of hBN spacers between the TMD layers or by adjusting the dielectric environment. In particular, although counterintuitive, we reveal that interlayer excitons in freestanding samples propagate slower than excitons in hBN-encapsulated TMDs-due to an enhancement of the net Coulomb drift with stronger environmental screening. Overall, our work contributes to a better microscopic understanding of the interlayer exciton transport in technologically promising atomically thin semiconductors

The Art of Constructing Black Phosphorus Nanosheet Based Heterostructures: From 2D to 3D

Assembling different kinds of 2D nanosheets into heterostructures presents a promising way of designing novel artificial materials with new and improved functionalities by combining the unique properties of each component. In the past few years, black phosphorus nanosheets (BPNSs) have been recognized as a highly feasible 2D material with outstanding electronic properties, a tunable bandgap, and strong in-plane anisotropy, highlighting their suitability as a material for constructing heterostructures. In this study, recent progress in the construction of BPNS-based heterostructures ranging from 2D hybrid structures to 3D networks is discussed, emphasizing the different types of interactions (covalent or noncovalent) between individual layers. The preparation methods, optical and electronic properties, and various applications of these heterostructures—including electronic and optoelectronic devices, energy storage devices, photocatalysis and electrocatalysis, and biological applications—are discussed. Finally, critical challenges and prospective research aspects in BPNS-based heterostructures are also highlighted

Spatio-temporal dynamics in graphene

Temporally and spectrally resolved dynamics of optically excited carriers in graphene has been intensively studied theoretically and experimentally, whereas carrier diffusion in space has attracted much less attention. Understanding the spatio-temporal carrier dynamics is of key importance for optoelectronic applications, where carrier transport phenomena play an important role. In this work, we provide a microscopic access to the time-, momentum-, and space-resolved dynamics of carriers in graphene. We determine the diffusion coefficient to be D ≈ 360 cm 2 s -1 and reveal the impact of carrier-phonon and carrier-carrier scattering on the diffusion process. In particular, we show that phonon-induced scattering across the Dirac cone gives rise to back-diffusion counteracting the spatial broadening of the carrier distribution

Electrically tunable dipolar interactions between layer-hybridized excitons

Transition-metal dichalcogenide bilayers exhibit a rich exciton landscape including layer-hybridized excitons, i.e. excitons which are of partly intra- and interlayer nature. In this work, we study hybrid exciton-exciton interactions in naturally stacked WSe$_2$ homobilayers. In these materials, the exciton landscape is electrically tunable such that the low-energy states can be rendered more or less interlayer-like depending on the strength of the external electric field. Based on a microscopic and material-specific many-particle theory, we reveal two intriguing interaction regimes: a low-dipole regime at small electric fields and a high-dipole regime at larger fields, involving interactions between hybrid excitons with a substantially different intra- and interlayer composition in the two regimes. While the low-dipole regime is characterized by weak inter-excitonic interactions between intralayer-like excitons, the high-dipole regime involves mostly interlayer-like excitons which display a strong dipole-dipole repulsion and give rise to large spectral blue-shifts and a highly anomalous diffusion. Overall, our microscopic study sheds light on the remarkable electrical tunability of hybrid exciton-exciton interactions in atomically thin semiconductors and can guide future experimental studies in this growing field of research.Comment: 8+8 pages, 4+2 figure