  Owing to the fascinating physical characteristics of two-dimensional (2D)
materials and their heterostructure, much effort has been devoted to exploring
their basic physical properties as well as discovering other novel 2D
materials. Herein, based on first-principles calculations, we propose novel 2D
material groups with the form A$_2$B$_2$, composed of group IV (A = C, Si, Ge,
or Sn) and V (B = N, P, As, Sb, or Bi) elements; the group forms two stable
phases with the $P\bar{6}m2$ ($\mathcal{M}$ phase) and $P\bar{3}m1$
($\mathcal{I}$ phase) crystal symmetries. We found that a total of 40 different
freestanding A$_2$B$_2$ compounds were dynamically stable and displayed
versatile physical properties, such as insulating, semiconducting, or metallic
properties, depending on their elemental compositions. Our calculation results
further revealed that the newly proposed 2D materials expressed high electrical
and thermal transport properties. Additionally, some of the compounds that
contained heavy elements exhibited non-trivial topological properties due to
strong spin-orbit interactions.
