  High quality epitaxial graphene films can be applied as templates for
tailoring graphene-substrate interfaces that allow for precise control of the
charge carrier behavior in graphene through doping and many-body effects. By
combining scanning tunneling microscopy, angle-resolved photoemission
spectroscopy and density functional theory we demonstrate that oxygen
intercalated epitaxial graphene on Ir(111) has high structural quality, is
quasi free-standing, and shows signatures of many-body interactions. Using this
system as a template, we show that tunable pn-junctions can be patterned by
adsorption and intercalation of rubidium, and that the n-doped graphene regions
exhibit a reduced Coulomb screening via enhanced electron-plasmon coupling.
These findings are central for understanding and tailoring the properties of
graphene-metal contacts e.g. for realizing quantum tunneling devices.
