Quasi-Freestanding Bilayer Borophene on Ag(111)

The
lattice structure of monolayer borophene depends sensitively
on the substrate yet is metallic independent of the environment. Here,
we show that bilayer borophene on Ag(111) shares the same ground state
as its freestanding counterpart that becomes semiconducting with an
indirect bandgap of 1.13 eV, as evidenced by an extensive structural
search based on first-principles calculations. The bilayer structure
is composed of two covalently bonded v1/12 boron monolayers that are stacked in an AB mode. The interlayer
bonds not only localize electronic states that are otherwise metallic
in monolayer borophene but also in part decouple the whole bilayer
from the substrate, resulting in a quasi-freestanding system. More
relevant is that the predicted bilayer model of a global minimum agrees
well with recently synthesized bilayer borophene on Ag(111) in terms
of lattice constant, topography, and moiré pattern.