  The realization of air-stable 2D metals epitaxial to SiC and capped by
graphene creates a potentially immense chemical space of 2D metals and alloys
that could expand the variety of solid-state excitations unique to 2D metals
beyond what is known for graphene and niobium/tantalum chalcogenides. We
perform a high-throughput computational survey from first-principles predicting
the structures and stabilities of all metals in the periodic table when they
intercalate graphene/SiC. Our results not only agrees with all experimentally
known metal/SiC structures explored so far, but also reveals conspicuous trends
related to metal cohesive energies and metal-silicon bonding. For special
groups of metals, a small bandgap opens, relying on appropriate electron
filling and substrate-induced symmetry breaking. From this gapping
stabilization, we derive alloying rules unique to 2D metals.
