Two new phases of monolayer group-IV monochalcogenides and their piezoelectric properties.

Two stable structural phases of the monolayer group-IV monochalcogenides GeS, GeSe, SnS and SnSe have been found using first-principles calculations, and will be called as A-MX and H-MX. It is found that these two new phases are energetically nearly degenerate with the pristine phase Z-MX and are semiconductors for all four group-IV monochalcogenides. The band gaps of A-MX range from 1.62 eV to 1.86 eV, while H-MX exhibits wider band gaps in the range from 2.21 eV to 2.47 eV. Moreover, both of them exhibit significant piezoelectric properties. The piezoelectric coefficient of A-MX is about 1-2 orders of magnitude higher than those of other frequently used piezoelectric materials, showing obvious anisotropic characters, while that of H-MX is isotropic and comparable with those of other piezoelectric materials.