  Based on first-principles calculations, we propose that a layered electride
Mg2O can serve as an effective substrate, in place of metal substrate, to grow
honeycomb borophene (h-borophene). We first confirm the thermodynamic stability
of h-B@Mg2O heterostructure by energetics analysis and dynamic stability by
absence of imaginary frequency in phonon spectra. Then, kinetically, we
identify the atomistic pathways for a preferred 2D growth mode over 3D growth,
and reveal a growth transition from 2D compact islands to h-borophene at
~13-atom cluster size, indicating the feasibility of epitaxial growth of
h-borophene on Mg2O. The h-borophene is found to be stabilized by nearly one
electron transfer from Mg2O to h-borophene based on the Bader charge analysis.
Moreover, the intrinsic band structure of the free-standing h-borophene is only
weakly perturbed by the Mg2O substrate, a significant advantage over metal
substrate. We envision that layered electrides provide an attractive family of
substrates for epitaxial growth of a range of 2D materials that can otherwise
only be grown on undesirable metal substrates.
