Regensburg 2025 – scientific programme
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HL: Fachverband Halbleiterphysik
HL 49: 2D Materials: Electronic Structure and Exitations III (joint session O/HL/TT)
HL 49.6: Talk
Thursday, March 20, 2025, 11:45–12:00, H11
Probing Excitonic Properties and Structural Effects in WS2-Graphene Heterostructures Using EELS and DFT-BSE Modeling — •Max Bergmann, Jürgen Belz, Oliver Maßmeyer, Robin Günkel, Badrosadat Ojaghi Dogahe, Andreas Beyer, Stefan Wippermann, and Kerstin Volz — Department of Physics, Philipps-Universität Marburg, Germany
This study investigates the excitonic properties of WS2 epitaxially grown on graphene by metal-organic chemical vapor deposition. We focus on understanding the effects of structural changes, such as variations in the number of WS2 layers. Using monochromatic electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM), we observe in the monolayer region of WS2 an excitonic spectrum with excitonic peaks at 2.0 eV and 2.4 eV, as well as additional spectral features at higher energies. Measurements in the bilayer region show a small redshift of these features due to the additional layer. Complementary density functional theory and Bethe-Salpeter calculations show that this redshift in the K-valley excitons is due to both a change in quantum confinement and a change in the WS2 lattice constant, with the latter being the dominant effect. Using STEM, this lattice distortion can be attributed to the heteroepitaxial alignment of the lower WS2 layer to the graphene substrate, while the upper layer is relaxed. This study provides valuable insights into the relationship between atomic structure and optical properties in complex material systems, providing essential knowledge for the design and optimization of 2D heterostructures for advanced device applications.
Keywords: WS2; Excitons; Graphene; Redshift; Epitaxial alignment