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

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 band gap 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 x 2 relative to graphene) with free-electron-like dispersion, suppressing plasmaron quasiparticles in graphene via enhanced metallization 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.

Automated summary

Recently, the graphene-SiC interface has been used to epitaxially grow stable, monoelemental, two-dimensional (2D) layers via intercalation. These 2D layers, in combination with graphene, form a new class of quantum materials with different properties compared to their bulk crystals. This study demonstrates the successful modification of band alignment between graphene and silver through surface charge-transfer doping, leading to enhanced metallization of the heterostructure.