Artificial two-dimensional (2D) materials, such as graphene or single-layer transition metal dichalcogenides, have electronic properties that are drastically different from those of their parent compounds. Moreover, these properties do not only depend on the 2D materials as such but also to their environment, for example on the substrate they are placed on. For instance, merely changing the dielectric properties of the substrate can strongly modify the band gap size of a 2D semiconductor.
Here we explore novel 2D materials such as single layers of MoS2, WS2 and TaS2 by scanning tunnelling microscopy and (time-resolved) angle-resolved photoemission spectroscopy. The layers are grown epitaxially on Au(111), Ag(111) and graphene. For the semiconducting materials (MoS2, WS2), the Coulomb interaction in the layer can be controlled by the presence of the substrate or by excited carriers in the material as such, leading to a strong band gap renormalization. It is even possible achieve a semiconductor to metal transition in the materials, merely by the right choice of substrate.