Band
gap tuning in 2D monolayers is one of the most attractive
approaches in design and production of new atomically thin semiconductors.
Following our recent computational design of stable two-dimensional
octahedral boranes, we present comprehensive computational study of
electronic structure, stability and properties of members belonging
to family B4X2 (X = B, N, P, As, Sb). We select
15 group atoms (pnictogens) as apical substituents to stabilize quasi-octahedral
units by delocalized bonding and create interlayer “pressure”
caused by vertically aligned lone pairs. We first substitute apical
B atoms by N and then go down the 15 group to capture electronic structure
trends. B→N substitution opens band gap, while further substitution
consistently narrows band gap. We revealed elegant band gap trend
which is inversely proportional to the size of octahedral units. Thus,
old as world isoelectronic substitution could be used for band gap
engineering in superoctahedral 2D boranes and other monolayers. We
also discovered superatomic bonding in B4As2 and B4P2 octahedra which agrees with their
exceptional stability (up to 800 K) and magnetic response properties.
Remarkably, both B4As2 and B4P2 units have 14 valence electrons, making them exceptional
examples of stable nonconventional electron count of spherical aromaticity.
Alongside with a considerable negative formation energy and phonon
stability, B4As2 has extremely low exfoliation
energy of 0.02 eV/atom implying probability of synthetic route from
a putative bulk material. It gives a strong basis to believe in possibility
of experimental fabrication of B4As2 monolayer
which can serve as alternative to MoS2 and silicon.