Formation of 2D GaXY (X = S, Se; Y = F, Cl, Br, I) with enhanced piezoelectricity via decomposition of Ga-monochalcogenide by halogenation

Motivated by the excellent electronic and optoelectronic properties of two-dimensional (2D) Ga-monochalcogenides, we systematically investigated the effect of halogenation on physical properties of monolayer GaX (X = S, Se) by the first-principles calculations. Crystal lattices of GaX were decomposed with the halogen atom being absorbed on top of the Ga atom. Unexpectedly, a series of monolayer GaXY (X = S, Se; Y = F, Cl, Br, I) was formed. Except for GaXF, other monolayer GaXY has good structural stability at room temperature. With the broken mirror symmetry, monolayer GaXY has superior flexibility, a direct bandgap in the range of 0.78-4.43 eV, the intrinsic vertical polarization, and larger in-plane as well as out-of-plane piezoelectric coefficients than Ga-monochalcogenides. Moreover, by comparing the structural stability and side-dependent electronic band alignment, we concluded that monolayer GaXY (X = S, Se; Y = Cl, Br) is suitable for photocatalytic water-splitting applications.