Structural asymmetry in few-layer blue phosphorene

Blue phosphorus, composed of the same layers, is believed to be of the same thickness in each layer. Here we find that in few-layer blue phosphorene systems there always exists structural asymmetry in all layers, which is absent in few-layer graphene or other blue phosphorene systems. Critical switching in the profiles of layer thicknesses or interlayer distances is observed between layer numbers 12 and 13. Furthermore, we find the structural asymmetry can induce the metal-to-semimetal transition in bilayer blue phosphorene, and can change the optical absorption spectrums in entire energy scope. The symmetric semimetal band structure can be changed into the asymmetric one by external in-plane strains. Moreover, the original in-plane isotropy of the absorption spectrum for free bilayer blue phosphorene can be easily broken by applying x or y-direction compressive and tensile strains, while the 3-dimensional anisotropy will be strengthened by the in-plane strains. The structural asymmetry suggests an inevitable factor in accurately understanding the properties of blue phosphorene.