Confined monolayer Ag as a large gap 2D semiconductor and its quasiparticle interactions with high-density Dirac electrons*

2D materials have been spotlighted due to their intriguing quantum phenomena distinctive from their bulk counterparts as shown in graphene, transition metal dichalcogenides (TMDs), or other 2D families. On top of that, epitaxially synthesized wafer-scale 2D metals, composed of elemental atoms, are recently attracting attention not only for their future applications but also for exotic quantum effects such as superconductivity [1].  It was theoretically predicted that, unlike its conductive bulk form, the monolayer In and Ag have a gapped electronic structure.  Here, we utilize time- and angle-resolved photoemission spectroscopy (trARPES) to reveal the unoccupied electronic structure of monolayer Ag and verify its gapped feature. The gap size was measured to be ~ 1eV and, along with valence band dispersion, it showed a good agreement with GW corrected DFT calculations. In addition, interestingly, we observed a strong mass enhancement of the Γ point conduction band, a factor of 2.6 larger than the theoretical prediction. We attribute this massive renormalization of the conduction band dispersion to the high-density Dirac electrons (~ 4 × 10¹³/cm²) of the capping bilayer graphene.

[1] Briggs et al. Atomically thin half-van der Waals metals enabled by confinement heteroepitaxy. Nat. Mater. 19, 637–643 (2020)