Surface charge-transfer doping a quantum-confined silver monolayer beneath epitaxial graphene

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