  One of the enduring challenges in graphene research and applications is the
extreme sensitivity of its charge carriers to external perturbations,
especially those introduced by the substrate. The best available substrates to
date, graphite and hBN, still pose limitations: graphite being metallic does
not allow gating, while both hBN and graphite having lattice structures closely
matched to that of graphene, may cause significant band structure
reconstruction. Here we show that the atomically smooth surface of exfoliated
MoS2 provides access to the intrinsic electronic structure of graphene without
these drawbacks. Using scanning tunneling microscopy and Landau-level
spectroscopy in a device configuration which allows tuning the carrier
concentration, we find that graphene on MoS2 is ultra-flat producing long mean
free paths, while avoiding band structure reconstruction. Importantly, the
screening of the MoS2 substrate can be tuned by changing the position of the
Fermi energy with relatively low gate voltages. We show that shifting the Fermi
energy from the gap to the edge of the conduction band gives rise to enhanced
screening and to a substantial increase in the mean-free-path and quasiparticle
lifetime. MoS2 substrates thus provide unique opportunities to access the
intrinsic electronic properties of graphene and to study in situ the effects of
screening on electron-electron interactions and transport.
