  The elemental two-dimensional (2D) materials such as graphene, silicene,
germanene, and black phosphorus have attracted considerable attention due to
their fascinating physical properties. Structurally they possess the honeycomb
or distorted honeycomb lattices, which are composed of six-atom rings. Here we
find a new structure of 2D allotropes of group V elements composed of
eight-atom rings, which we name as the octagonal tiling (OT) structure.
First-principles calculations indicate that these allotropes are dynamically
stable and are also thermally stable at temperatures up to 600 K. These
allotropes are semiconductors with band gaps ranging from 0.3 to 2.0 eV, thus
they are potentially useful in near- and mid-infrared optoelectronic devices.
OT-Bi is also a 2D topological insulator (TI) with a band gap of 0.33 eV, which
is the largest among the reported elemental 2D TIs, and this gap can be
increased further by applying compressive strains.
