  Since the advent of graphene, two-dimensional (2D) materials become very
attractive and there is growing interest to explore new 2D beyond graphene.
Here, through density functional theory (DFT) calculations, we predict 2D
wide-band-gap II-V semiconductor materials of M$_3$X$_2$ (M=Zn, Cd and X=N, P,
As) with a dilated graphene-like honeycomb structure. The structure features
that the group-V X atoms form two X-atomic planes symmetrically astride the
centering group-IIB M atomic plane. The 2D Zn$_3$N$_2$, Zn$_3$P$_2$, and
Zn$_3$As$_2$ are shown to have direct band gaps of 2.87, 3.81, and 3.55 eV,
respectively, and the 2D Cd$_3$N$_2$, Cd$_3$P$_2$, and Cd$_3$As$_2$ exhibit
indirect band gaps of 2.74, 3.51, and 3.29 eV, respectively. Each of the six 2D
materials is shown to have effective carrier (either hole or electron) masses
down to $0.03\sim 0.05$ $m_0$. The structural stability and feasibility of
experimental realization of these 2D materials has been shown in terms of DFT
phonon spectra and total energy comparison with related existing bulk
materials. On the experimental side, there already are many similar
two-coordinate structures of Zn and other transition metals in various organic
materials, which can be considered to support our DFT prediction. Therefore,
these 2D semiconductors can enrich the family of 2D electronic materials and
may have promising potential for achieving novel transistors and optoelectronic
devices.
