  2D materials have intriguing quantum phenomena that are distinctively
different from their bulk counterparts. Recently, epitaxially synthesized
wafer-scale 2D metals, composed of elemental atoms, are attracting attention
not only for their potential applications but also for exotic quantum effects
such as superconductivity. By mapping momentum-resolved electronic states using
time-resolved and angle-resolved photoemission spectroscopy (ARPES), we reveal
that monolayer Ag confined between bilayer graphene and SiC is a large gap (> 1
eV) 2D semiconductor, consistent with GW-corrected density functional theory.
The measured valence band dispersion matches the DFT-GW quasiparticle band.
However, the conduction band dispersion shows an anomalously large effective
mass of 2.4 m0. Possible mechanisms for this large enhancement in the apparent
mass are discussed.
