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

TL;DR

This study demonstrates how surface charge-transfer doping via potassium deposition can effectively modify the electronic band structure and properties of a graphene/2D silver heterostructure, revealing new ways to control quantum materials.

Contribution

It introduces a method to tune the band alignment and electronic properties of a graphene/2D-Ag heterostructure through surface charge-transfer doping, advancing control over quantum-confined 2D materials.

Findings

Potassium doping shifts the band alignment in the heterostructure.

Ordered potassium adlayer suppresses plasmaron quasiparticles.

Carrier densities achieved are below the conduction band onset.

Abstract

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.