One of the enduring challenges in graphene research and applications is theextreme sensitivity of its charge carriers to external perturbations,especially those introduced by the substrate. The best available substrates todate, graphite and hBN, still pose limitations: graphite being metallic doesnot allow gating, while both hBN and graphite having lattice structures closelymatched to that of graphene, may cause significant band structurereconstruction. Here we show that the atomically smooth surface of exfoliatedMoS2 provides access to the intrinsic electronic structure of graphene withoutthese drawbacks. Using scanning tunneling microscopy and Landau-levelspectroscopy in a device configuration which allows tuning the carrierconcentration, we find that graphene on MoS2 is ultra-flat producing long meanfree paths, while avoiding band structure reconstruction. Importantly, thescreening of the MoS2 substrate can be tuned by changing the position of theFermi energy with relatively low gate voltages. We show that shifting the Fermienergy from the gap to the edge of the conduction band gives rise to enhancedscreening and to a substantial increase in the mean-free-path and quasiparticlelifetime. MoS2 substrates thus provide unique opportunities to access theintrinsic electronic properties of graphene and to study in situ the effects ofscreening on electron-electron interactions and transport.
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