  We report on the observation of photoluminescence (PL) with a narrow 18 meV
peak width from molecular beam epitaxy grown MoS$_2$ on graphene/Ir(111). This
observation is explained in terms of a weak graphene-MoS$_2$ interaction that
prevents PL quenching expected for a metallic substrate. The weak interaction
of MoS$_2$ with the graphene is highlighted by angle-resolved photoemission
spectroscopy and temperature dependent Raman spectroscopy. These methods reveal
that there is no hybridization between electronic states of graphene and
MoS$_2$ and a different thermal expansion of graphene and MoS$_2$. Molecular
beam epitaxy grown MoS2 on graphene is therefore an important platform for
optoelectronics which allows for large area growth with controlled properties.
