  Atomically thin semiconductors made from transition metal dichalcogenides
(TMDs) are model systems for investigations of strong light-matter interactions
and applications in nanophotonics, opto-electronics and valley-tronics.
However, the photoluminescence spectra of TMD monolayers display a large number
of features that are particularly challenging to decipher. On a practical
level, monochromatic TMD-based emitters would be beneficial for low-dimensional
devices but this challenge is yet to be resolved. Here, we show that graphene,
directly stacked onto TMD monolayers enables single and narrow-line
photoluminescence arising solely from TMD neutral excitons. This filtering
effect stems from complete neutralization of the TMD by graphene combined with
selective non-radiative transfer of long-lived excitonic species to graphene.
Our approach is applied to four tungsten and molybdenum-based TMDs and
establishes TMD/graphene heterostructures as a unique set of opto-electronic
building blocks, suitable for electroluminescent systems emitting visible and
near-infrared photons at near THz rate with linewidths approaching the lifetime
limit.
