  Light emission in atomically thin heterostructures is known to depend on the
type of materials, number and stacking sequence of the constituent layers. Here
we show that the thickness of a two-dimensional substrate can be crucial in
modulating the light emission. We study the layer-dependent charge transfer in
vertical heterostructures built from monolayer tungsten disulphide (WS2) on
one- and two-layer epitaxial graphene, unravelling the effect that the
interlayer electronic coupling has on the excitonic properties of such
heterostructures. We bring evidence that the excitonic properties of WS2 can be
effectively tuned by the number of supporting graphene layers. Integrating WS2
monolayers with two-layer graphene leads to a significant enhancement of the
photoluminescence response, up to one order of magnitude higher compared to WS2
supported on one-layer graphene. Our findings highlight the importance of
substrate engineering when constructing atomically thin layered
heterostructures.
