  We directly measure the electronic structure of twisted graphene/MoS2 van der
Waals heterostructures, in which both graphene and MoS2 are monolayers. We use
cathode lens microscopy and microprobe angle-resolved photoemission
spectroscopy measurements to image the surface, determine twist angle, and map
the electronic structure of these artificial heterostructures. For monolayer
graphene on monolayer MoS2, the resulting band structure reveals the absence of
hybridization between the graphene and MoS2 electronic states. Further, the
graphene-derived electronic structure in the heterostructures remains intact,
irrespective of the twist angle between the two materials. In contrast,
however, the electronic structure associated with the MoS2 layer is found to be
twist-angle dependent; in particular, the relative difference in the energy of
the valence band maximum at {\Gamma} and K of the MoS2 layer varies from
approximately 0 to 0.2 eV. Our results suggest that monolayer MoS2 within the
heterostructure becomes predominantly an indirect bandgap system for all twist
angles except in the proximity of 30 degrees. This result enables potential
bandgap engineering in van der Waals heterostructures comprised of monolayer
structures.
