  Two-dimensional (2D) group V elemental materials have attracted widespread
attention due to their nonzero band gap while displaying high electron
mobility. Using first-principles calculations, we propose a series of new
elemental bilayers with group V elements (Bi, Sb, As). Our study reveals the
dynamical stability of 4, 6, and 8-atom ring structures, demonstrating their
possible coexistence in such bilayer systems. The proposed structures for Sb
and As are large-gap semiconductors that are potentially interesting for
applications in future nanodevices. The Bi structures have nontrivial
topological properties with a large and direct nontrivial band gap. The
nontrivial gap is shown to arise from a band inversion at the Brillouin zone
center due to the strong intrinsic spin-orbit coupling (SOC) in Bi atoms.
Moreover, we demonstrate the possibility to tune the properties of these
materials by enhancing the ratio of 6-atom rings to 4 and 8-atom rings, which
results in wider nontrivial band gaps and lower formation energies.
