Excitons in Epitaxially Grown WS2 on Graphene: A Nanometer-Resolved Electron Energy Loss Spectroscopy and Density
Max Bergmann1,2, Jürgen Belz1,2, Oliver Maßmeyer1,2
1mar.quest | Marburg Center for Quantum Materials and Sustainable Technologies, Philipps-Universitat Marburg, 35032 Marburg, Germany.
ACS Nano
|December 11, 2025
Summary
We studied how the number of layers in tungsten disulfide (WS2) affects its excitonic properties. A subtle lattice mismatch, not dielectric screening, causes shifts in exciton energies in these 2D materials.
Area of Science:
- Materials Science
- Condensed Matter Physics
- Nanotechnology
Background:
- Tungsten disulfide (WS2) heterostructures are promising for optoelectronics.
- Understanding layer-dependent properties is crucial for device optimization.
- Epitaxial growth offers scalable fabrication of 2D material heterostructures.
Purpose of the Study:
- To investigate the excitonic properties of WS2 monolayers, bilayers, and multilayers grown on graphene.
- To determine the factors influencing layer-dependent excitonic features in WS2.
- To correlate nanoscale structural variations with excitonic response.
Main Methods:
- Monochromatic electron energy loss spectroscopy (EELS) with nanometer-scale resolution.
- Scanning transmission electron microscopy (STEM).
- Ab initio simulations using density functional theory (DFT) and the Bethe-Salpeter equation (BSE).
Main Results:
- Observed a systematic redshift in A and B excitons (at K-valley) with increasing WS2 layer number.
- Calculations revealed lattice mismatch, not dielectric screening, as the primary cause of exciton redshift.
- Identified heteroepitaxial alignment to the graphene substrate as the origin of lattice mismatch.
Conclusions:
- Nanoscale structural distortions significantly impact excitonic properties in epitaxial 2D materials.
- Microscopic interface effects are critical for designing and fabricating optoelectronic devices.
- Combined experimental spectroscopy and theoretical modeling provide insights into realistic heterostructures.
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