  We find that several layer-phase group-III monochalcogenides, including GaS,
GaSe and InSe, are piezoelectric in the monolayer form. First-principles
calculations reveal that the piezoelectric coefficients of monolayer GaS, GaSe
and InSe are on the same order of magnitude as the earlier discovered
two-dimensional piezoelectric materials, such as BN and MoS2 monolayers. Our
study expands the family of two dimensional piezoelectric materials, suggesting
that strong piezoelectric response can occur in a wide range of two dimensional
materials with broken inversion symmetry. The co-existence of piezoelectricity
and superior photo-sensitivity in these two-dimensional semiconductors enables
the integration of electromechanical and optical sensors on the same material
platform.
