  In this study, we investigate excitonic properties of epitaxially grown WS2,
which is of particular interest for various applications due to its potential
for upscaling to wafer sized structures. Understanding the effect of the
dielectric environment due to changing layer numbers and multi-material
heterostructures on the optical properties is crucial for tailoring device
properties. Monochromated electron energy loss spectroscopy in a scanning
transmission electron microscope is employed to characterize the excitonic
spectrum of WS2 on graphene grown by metal organic chemical vapor deposition.
This technique provides the required spatial resolution at the nanometer scale
in combination with high quality spectra. To complement the experimental
results, theoretical investigations using density functional theory and
applying the Bethe-Salpeter equations are conducted. We find that by
transitioning from mono- to bi- to multilayers of WS2 the spectra show
redshifts for both, the K-valley excitons at about 2.0 and 2.4 eV as well as
excitonic features of higher energies. The latter features originate from so
called band nesting of transitions between the Gamma- and K-point. In summary,
this study provides valuable insights into the excitonic properties of WS2 in
different layer configurations and environments, which are realistically needed
for future device fabrication and property tuning. Finally, we can show that
nanometer scale electron spectroscopy supported by careful theoretical
modelling can successfully link atomic structure and optical properties, such
as exciton shifts, in non-idealized complex material systems like multilayer 2D
heterostructures.
