Book/Dissertation / PhD Thesis

FZJ-2018-03776

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png The electronic structure of transition metal dichalcogenides investigated by angle-resolved photoemission spectroscopy

Gehlmann, M. (Corresponding author)FZJ*

2018
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-324-2

Please use a persistent id in citations: http://hdl.handle.net/2128/19179

Abstract: Van der Waals (vdW) materials offer a perspective to revolutionize basically every facet of nowadays technology with a new generation of atomically thin devices. Transition metal dichalcogenides (TMDCs) are a family of vdW crystals, that includes several semiconducting materials with band gaps within the optical range. This makes them ideal for numerous applications such as transistors, optical sensors, solar cells, and LEDs. In this study we focuses on two members of the TMDC family: molybdenum disufide (MoS$_{2}$) and rhenium disulfide (ReS$_{2}$). Using a combination of angle-resolved photoemission spectroscopy (APRES) with density functional theory (DFT), we provide a thorough analysis of the electronic band structure of these two exceptional materials. In monolayers of MoS$_{2}$ the combination of broken inversion symmetry with the heavy element molybdenum leads to a large spin-splitting of distinct valleys within its electronic structure. Therefore, MoS$_{2}$ combines the essential ingredients for socalled $\textit{spintronics}$ and $\textit{valleytronics}$. It was generally believed that these fascinating features are forbidden in MoS$_{2}$ bulk crystals due to their centrosymmetric space group. This study demonstrates that the strong confinement of the valleys within the vdW layers leads to a recently discovered type of $\textit{hidden spin-polarization}$, which results in quasi two-dimensional, highly spin-polarized states in this centrosymmetricbulk crystal. Furthermore, we present the first ARPES study of ReS$_{2}$ bulk, monolayer, and bilayer crystals. Recent literature reported indications for a total confinement of the bulk electronic structure within the plains of the vdW layers. Our study comes to the opposite conclusion. Based on the observation of a considerable out-of-plane dispersion in the ARPES experiments, as well as in the band structure calculations, we show that valence electrons are significantly delocalized across the vdW gap. In addition, we identify the valence band maximum of bulk, monolayer, and bilayer ReS$_{2}$ experimentally. The combination of ARPES and band structure calculations shows that ReS$_{2}$ undergoes a transition from a direct band gap in the bulk and bilayer to an indirect gap in the monolayer.

Note: Universität Duisburg, Diss., 2018

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Contributing Institute(s):

1. Elektronische Eigenschaften (PGI-6)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

Appears in the scientific report 2018

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