  Recently the graphene/SiC interface has emerged as a versatile platform for
the epitaxy of otherwise unstable, monoelemental, two-dimensional (2D) layers
via intercalation. Intrinsically capped into a van der Waals heterostructure
with overhead graphene, they compose a new class of quantum materials with
striking properties contrasting their parent bulk crystals. Intercalated silver
presents a prototypical example where 2D quantum confinement and inversion
symmetry breaking entail a metal-to-semiconductor transition. However, little
is known about the associated unoccupied states, and control of the Fermi level
position across the bandgap would be desirable. Here, we n-type dope a
graphene/2D-Ag/SiC heterostack via in situ potassium deposition and probe its
band structure by means of synchrotron-based angle-resolved photoelectron
spectroscopy. While the induced carrier densities on the order of $10^{14}$
cm$^{-2}$ are not yet sufficient to reach the onset of the silver conduction
band, the band alignment of graphene changes relative to the rigidly shifting
Ag valence band and substrate core levels. We further demonstrate an ordered
potassium adlayer ($2\times 2$ relative to graphene) with free-electron-like
dispersion, suppressing plasmaron quasiparticles in graphene via enhanced
metalization of the heterostack. Our results establish surface charge-transfer
doping as an efficient handle to modify band alignment and electronic
properties of a van der Waals heterostructure assembled from graphene and a
novel type of monolayered quantum material.
