  Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong
Coulomb interaction that manifests in tightly bound excitons. Due to the
complex electronic band structure exhibiting several spin-split valleys in the
conduction and valence band, dark excitonic states can be formed. They are
inaccessibly by light due to the required spin-flip and/or momentum transfer.
The relative position of these dark states with respect to the optically
accessible bright excitons has a crucial impact on the emission efficiency of
these materials and thus on their technological potential. Based on the
solution of the Wannier equation, we present the excitonic landscape of the
most studied TMD materials including the spectral position of momentum- and
spin-forbidden excitonic states. We show that the knowledge of the electronic
dispersion does not allow to conclude about the nature of the material's band
gap, since excitonic effects can give rise to significant changes. Furthermore,
we reveal that an exponentially reduced photoluminescence yield does not
necessarily reflect a transition from a direct to a non-direct gap material,
but can be ascribed in most cases to a change of the relative spectral distance
between bright and dark excitonic states.
