Atomic-scale strain created during growth turns out to be the hidden lever controlling excitons in WS2 layered on graphene.
Study: Excitons in Epitaxially Grown WS2 on Graphene: A Nanometer-Resolved Electron Energy Loss Spectroscopy and Density Functional Theory Study. Image Credit: BCKGRDS/Shutterstock.com
Researchers have shown that lattice distortions created during epitaxial growth, rather than dielectric screening alone, can directly reshape exciton energies in WS2 layered on graphene, revealing a new level of structural control over 2D optoelectronic materials.
Two-dimensional (2D) transition metal dichalcogenides (TMDs), particularly tungsten disulfide (WS2), are central to emerging optoelectronic and valleytronic technologies.
When thinned to a single layer, WS2 transitions from an indirect to a direct band gap semiconductor, dramatically enhancing its interaction with light. With this direct band gap, excitons, bound electron-hole pairs, dominate the optical response, strengthened by reduced dielectric screening and spatial confinement.
Understanding how excitons behave in realistic, device-scale materials is a challenge. Most prior studies have relied on mechanically exfoliated samples, which lack the subtle strain and structural nonuniformities introduced during large-area growth.
The new study, published in ACS Nano, shifts the focus to epitaxially grown WS2 on graphene, a technologically scalable platform where growth-induced distortions become unavoidable – and informative.
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Probing Excitons at the Nanoscale
To investigate how structure and excitons interact at the smallest scales, the researchers combined nanometer-resolved electron energy loss spectroscopy (EELS) with scanning transmission electron microscopy (STEM).
Unlike optical techniques, STEM-EELS offers both high spatial resolution and access to indirect electronic transitions through momentum transfer, enabling the detection of excitonic states beyond the reach of photon-based probes.
The WS2-graphene heterostructures were synthesized using metal organic chemical vapor deposition (MOCVD) on graphene-coated sapphire substrates. High-angle annular dark-field (HAADF) imaging allowed the team to identify monolayer, bilayer, and multilayer regions with atomic precision, while EELS spectra were recorded across these regions to track excitonic signatures.
Systematic Redshift with a Structural Origin
The measurements revealed a clear redshift of the A and B excitons located near 2.0 eV and 2.4 eV at the K-point of the Brillouin zone, as WS2 transitioned from monolayer to bilayer and multilayer configurations.
Similar trends have been reported in exfoliated samples, however, the spatial resolution of EELS made it possible to directly link these energy shifts to local structural features.
In bilayer regions, the redshifted A exciton consistently coincided with a lattice mismatch moiré (LMM) pattern. Atomic-scale analysis showed that this pattern arises from a minute lattice expansion (approximately one picometer) in the upper WS2 layer relative to the lower one.
The mismatch traces back to the heteroepitaxial alignment of the first WS2 layer to the graphene substrate, which slightly compresses its lattice during growth.
Simulations Confirm the Mechanism
To disentangle competing effects such as quantum confinement and dielectric screening, the team performed ab initio simulations using density functional theory combined with the Bethe-Salpeter equation (DFT-BSE).
Because the experimentally observed moiré spans more than 100 nm, a simplified bilayer model with uniform strain was used to capture the essential physics.
The calculations showed that dielectric screening alone would produce a slight blueshift in bilayer WS2. However, introducing even a 0.6 pm lattice mismatch reverses this trend, producing a redshift that matches the experimental observations.
The simulations, therefore, identify strain-induced band-gap renormalization as the dominant driver of the exciton energy shift.
The effect is even stronger for the C exciton, observed between 2.7 and 2.9 eV, which originates from electronic states between the Γ and K points. Its larger redshift reflects a heightened sensitivity to stacking order and interlayer coupling.
Photoluminescence measurements independently confirmed the presence and energy position of the A exciton across the sample, demonstrating that the observed shifts are an intrinsic property of the WS2-graphene heterostructure rather than an artefact of the electron-based probe.
Future 2D Devices
The study establishes a direct, nanoscale link between growth-induced structural distortions and excitonic response in a realistic, scalable 2D material system. Rather than treating strain as an unwanted byproduct of fabrication, the findings suggest it can be harnessed as a design parameter for tuning excitonic properties.
As epitaxial growth methods continue to mature, such insights could prove critical for the development of next-generation optoelectronic, photonic, and quantum devices based on layered materials.
Journal Reference
Bergmann M., et al. (2025). Excitons in Epitaxially Grown WS2 on Graphene: A Nanometer-Resolved Electron Energy Loss Spectroscopy and Density Functional Theory Study. ACS Nano. DOI: 10.1021/acsnano.5c11994