  Two-dimensional thermoelectric materials with a figure of merit $ZT$, which
is greater than 2.0 at room temperature, would be highly desirable in energy
conversion since the efficiency is competitive to conventional energy
conversion techniques. Here, we propose that the indium triphosphide (InP$_3$)
monolayer offers an extraordinary $ZT$ of 2.2 at 300 K by using quantum
calculations within the ballistic thermal transport region. A remarkably low
and isotropic phononic thermal conductivity is founded, which is due to flat
lattice vibration modes. This low thermal conductivity takes a major
responsibility to the impressively high $ZT$. Moreover, a large $ZT$ that is
greater than 1.5 can be maintained, even if a 1% mechanic extension is applied
on the lattice. These results suggest that the InP$_3$ monolayer is a promising
candidate for low dimensional thermoelectric applications.
