  The effect of external static charging on borophenes - 2D boron crystals - is
investigated by using first principles calculations. The influence of the
excess negative charge on the stability of the 2D structures is examined using
a very simple analysis of decomposition of the binding energy of a given boron
layer into contributions coming from boron atoms that have different
coordination numbers. This analysis is important to understand how the local
neighbourhood of an atom influences the overall stability of the monolayer
structure. The decomposition is done for the $\alpha$-sheet and its related
family of structures. From this analysis, we have found a preference for 2D
boron crystals with very small or very high charges per atom. The structures
with intermediate charges are energetically not favourable. We have also found
a clear preference in terms of binding energy for the experimentally seen
$\gamma$-sheet and $\delta$-sheet structures that is almost independent on the
considered excess of negative charge of the structures. On the other hand, we
have shown that a model based solely on nearest-neighbour interactions,
although instructive, is too simple to predict binding energies accurately.
