  Searching for novel two-dimensional (2D) materials is highly desired in the
field of nanoelectronics. We here propose a new 2D crystal barium tri-arsenide
($\mathrm{BaAs_3}$) with a series of encouraging functionalities. Being
kinetically and thermally stable, the monolayer and bilayer forms of
$\mathrm{BaAs_3}$ possess narrow indirect band gaps of 0.87 eV and 0.40 eV,
respectively, with high hole mobilities on the order of ~$\mathrm{10^3\
cm^{2}\,V^{-1}\,s^{-1}}$. The electronic properties of 2D $\mathrm{BaAs_3}$ can
be manipulated by controlling the layer thickness. The favorable cleavage
energy reveals that layered $\mathrm{BaAs_3}$ can be produced as a freestanding
2D material. Furthermore, by introducing vacancy defects monolayer
$\mathrm{BaAs_3}$ can be transformed from a semiconductor to a metal. 2D
$\mathrm{BaAs_3}$ may find promising applications in nanoelectronic devices.
