  The electronic structure of two-dimensional (2D) semiconductors can be
significantly altered by screening effects, either from free charge carriers in
the material itself, or by environmental screening from the surrounding medium.
The physical properties of 2D semiconductors placed in a heterostructure with
other 2D materials are therefore governed by a complex interplay of both intra-
and inter-layer interactions. Here, using time- and angle-resolved
photoemission, we are able to isolate both the layer-resolved band structure
and, more importantly, the transient band structure evolution of a model 2D
heterostructure formed of a single layer of MoS$_2$ on graphene. Our results
reveal a pronounced renormalization of the quasiparticle gap of the MoS$_2$
layer. Following optical excitation, the band gap is reduced by up to
$\sim\!$400 meV on femtosecond timescales due to a persistence of strong
electronic interactions despite the environmental screening by the $n$-doped
graphene. This points to a large degree of tuneability of both the electronic
structure and electron dynamics for 2D semiconductors embedded in a van der
Waals-bonded heterostructure.
