  In the purpose of expanding the family of two-dimensional materials, we
predict the existence of two-dimensional octa-structure of nitrogen group
elements that are composed of squares and octagons in first-principle method
based on density functional theory (DFT). From our calculations, electronic
structures of all monolayers show that they are semiconductors with indirect
(N, P, Bi) and direct (As, Sb) band gaps (0.57-2.61eV). Nano-ribbons of three
different unpassivated edges and their band structures are also investigated.
Because of the reconstruction on the edges and dangling bonds, there exist
ferromagnetic edge states in P, As, Sb nano-ribbons with different edges, and a
Dirac point near {\pi} is found in the band structure of one specific N
nano-ribbon. These structures may be useful in future applications, such as
semiconductor devices, spintronics, hydrogen storage and quantum computation.
