Optical Properties of TMDC Monolayers and Their Heterostructures

Recently, the 2D semiconductors represented by transition-metal dichalcogenides (TMDCs) received strong attention owing to a number of important features. These include the large exciton binding energies relevant for room-temperature applications, the valley pseudo-spin degree of freedom and polariz...

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Bibliographic Details
Main Author: Schneider, Lorenz Maximilian
Contributors: Koch, Martin (Prof. Dr.) (Thesis advisor)
Format: Dissertation
Published: Philipps-Universität Marburg 2020
Online Access:PDF Full Text
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Summary:Recently, the 2D semiconductors represented by transition-metal dichalcogenides (TMDCs) received strong attention owing to a number of important features. These include the large exciton binding energies relevant for room-temperature applications, the valley pseudo-spin degree of freedom and polarization sensitivity, and the overall strong light-matter interactions at the monolayer limit which motivate their use for optoelectronics. Furthermore, van-der-Waals stacking of different 2D crystals results in out-of-plane heterostructures, which can, in addition to the inherited properties of the individual layered constituents, even exhibit tailored properties caused by the strong influence of the environment and hybridization of atomic orbitals. Accordingly, a mixture of unique and novel properties can arise. Ultimately, in contrast to conventional semiconductor heterostructure growth, there is no direct need for lattice matching or a fixed orientation during assembly. Nonetheless, the relative orientation between different or similar 2D lattices may indeed be important for the composed stack’s features. Thus, the vast number of possible combinations of 2D-materials with each other, as well as with substrates and with conventional semiconductors or molecular materials, offer huge opportunities for engineering and tailoring the material stack properties to meet the demand of a specific application. To do that effectively and systematically, several questions need to be addressed, to the answering of which the following studies contribute with important insights focusing on the optical properties of semiconductor monolayers and van-der-Waals stacks. In this work, four central chapters discuss excitonic signatures in different TMDC 2D structures, shedding light on the role of the environment and stacking configuration, as well as on the quasi-particle energy-momentum dispersion, valley polarization as well as light-matter interactions. Firstly, the influence of the surroundings on the fundamental properties of 2D-semiconductor monolayers, such as the energetics, the exciton–phonon coupling, exciton–exciton annihilation and exciton diffusion, is addressed based on time-integrated and time-resolved photoluminescence spectroscopy. Thereby, the important role of hexagonal BN as substrate or capping layer and as encapsulant is discussed. Following that, a better understanding of high-symmetry alignments for bilayers is gained employing epitaxially grown tungsten-disulfide samples with two distinct and deterministically obtained configurations. While formerly only the symmetry of the stack itself was considered, the study presented here shows that also the symmetry of the surrounding has to be considered as it can lift the degeneracy between the layers. Thereby, the out-of-plane symmetry break renders homobilayers in fact heterojunctions. Moreover, the aspect of spin–valley and spin–layer locking has been discussed for the natural and artificial bilayer type, with eyes towards valleytronic applications. In contrast to high-symmetry stacks, investigations on arbitrarily stacked heterostructures are at the starting point for explorations on the impact of moiré patterns and interlayer hybridization. Here, a preliminary study on a tungsten-based heterostructure exhibits pronounced spectral features attributed to such interlayer effects, besides the occurrence of conventional intra- and interlayer excitons. Next, regarding linewidth improved encapsulated monolayers, a unique access to the excitonic energy–momentum dispersion is demonstrated with the help of angle-resolved spectrospopy. The analysis of Fourier-space-resolved emission and reflection spectra hereby facilitate the ongoing discussion of dispersion relations in 2D semiconductors. The so far unrivalled optical measurements show novel experimental evidence of meV strong excitonic dispersion within the light cone in support of theories discussed in the literature. Furthermore, Fourier-space spectroscopy delivers a tool to identify the radiative patterns of bright and partially dark excitonic states and provided evidence for the phonon-sidebands in agreement with the prediction in the literature. Finally, improvements of the light–matter interactions and of the emission behavior towards optoelectronic applications are crucial, taking further into account challenges in the integration of van-der-Waals materials in established silicon-, III/V semiconductors- or fiber-based technology. Therefore, a nanostructured photonic substrate landscape for lateral confinement of optical fields and vertical enhancement of coupling of light into and out of 2D-materials is investigated as a prototype structure for possible nanophotonic applications.
Physical Description:181 Pages