  Optical properties of heterostructures composed of layered 2D materials, such
as transition metal dichalcogenides (TMDs) and graphene, are broadly explored.
Of particular interest are light-induced energy transfer mechanisms in these
materials and their structural roots. Here, we use state-of-the-art
first-principles calculations to study the excitonic composition and the
absorption properties of WS$_2$-graphene heterostructures as a function of
interlayer alignment and the local strain resulting from it. We find that
Brillouin zone mismatch and the associated energy level alignment between the
graphene Dirac cone and the TMD bands dictate an interplay between interlayer
and intralayer excitons, mixing together in the many-body representation upon
the strain-induced symmetry breaking in the interacting layers. Examining the
representative cases of the 0$^\circ$ and 30$^\circ$ interlayer twist angles,
we find that this exciton mixing strongly varies as a function of the relative
alignment. We quantify the effect of these structural modifications on exciton
charge separation between the layers and the associated graphene-induced
homogeneous broadening of the absorption resonances. Our findings provide
guidelines for controllable optical excitations upon interface design and shed
light on the importance of many-body effects in the understanding of optical
phenomena in complex heterostructures.
