Excitons in Epitaxially Grown WS ₂ on Graphene: A Nanometer-Resolved Electron Energy Loss Spectroscopy and Density Functional Theory Study

We investigate the excitonic properties of epitaxially grown WS₂ monolayers, bilayers and multilayers on graphene using monochromatic electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope. This material system is particularly attractive for optoelectronic applications, as direct growth from the gas phase offers a scalable route to wafer-sized heterostructures. The combination of nanometer-scale spatial resolution and high spectral quality in EELS allows for a detailed analysis of layer-dependent excitonic features. To complement the experimental results, we perform <i>ab initio</i> simulations based on density functional theory and the Bethe-Salpeter equation. The experimental spectra reveal a systematic redshift of both A and B excitons at the K-valley─centered near 2.0 and 2.4 eV, respectively─as the number of WS₂ layers increases. While such redshifts are often attributed to dielectric screening, our <i>ab initio</i> calculations show that the dominant contribution arises from a subtle lattice mismatch between the lower and upper WS₂ layers. We trace this mismatch to the heteroepitaxial alignment of the first WS₂ layer to the graphene substrate during the growth process. Our results highlight how nanoscale structural distortions in epitaxial 2D materials can strongly influence key excitonic properties, even in the absence of intentional strain or alloying. By combining nanometer-scale electron spectroscopy with advanced theory, we establish a direct link between atomic structure and excitonic response in realistic, nonidealized heterostructures. These findings underscore the importance of microscopic interface effects in the design and scalable fabrication of exciton-based optoelectronic devices.