Fig. 1.

Unit cells for (A) black and (B) blue phosphorene monolayers that are formed by two sublayers ($S_1$ and $S_2$) separated by a distance ${\tau }_0$. Sublayers in black phosphorene are irregular––distances among nearest atoms belonging to the same sublayer are not identical––as emphasized by the dashed lines joining atoms belonging to sublayer $S_1$ on the rightmost structure in A.