Effect of net charge on the relative stability of different 2D boron allotropes*

We study the effect of electron doping on the bonding character and
stability of 2D structures of elemental boron, called borophene,
which is known to form many stable allotropes. Our ab
initio calculations reveal a previously unknown stable 2D
ε-B structure for the neutral system. We find that the
chemical bonding characteristic in this and other boron structures
is strongly affected by extra charge. Beyond a critical degree of
electron doping, the most stable allotrope changes from ε-B
to a buckled honeycomb structure. Additional electron doping,
mimicking a transformation of boron to carbon, causes a gradual
decrease in the degree of buckling of the honeycomb lattice that
can be interpreted as piezoelectric response. Net electron doping
can easily be achieved by placing borophene in direct contact with
the layered Ca₂N electride. In particular, electron
doping can be doubled by changing from a bilayer to a sandwich
geometry.