Peculiar electronic, strong in-plane and out-of-plane second harmonic generation and piezoelectric properties of atom-thick α-M2X3 (M = Ga, In; X = S, Se): role of spontaneous electric dipole orientations

Abstract

Second harmonic generation (SHG) and piezoelectric properties of two-dimensional (2D) materials are sparking great interest. However, out-of-plane SHG in 2D materials has been rarely reported; the theoretical out-of-plane piezoelectric response in atom-thick 2D materials is very limited at the present stage. α-M₂X₃ monolayers exhibit out-of-plane spontaneous polarizations, promising out-of-plane SHG and piezoelectricity. Here, we perform first-principles calculations of the electronic, SHG and piezoelectric properties of single- and few-layer α-M₂X₃. Results indicate the bandgaps of α-M₂X₃ monolayers are in the visible range, and become much narrower as the layer number goes up. Furthermore, the narrower bandgaps are broadened by more than 1.00 eV by switching the electric dipole orientation in few-layer α-M₂X₃. α-M₂X₃ monolayers exhibit superior in-plane and out-of-plane SHG properties; in particular, their out-of-plane SHG coefficients are comparable with those of GaAs crystals. Furthermore, the out-of-plane SHG coefficients can be effectively tuned by switching the electric dipole orientation in α-M₂X₃ few-layers. α-M₂X₃ monolayers exhibit superior in-plane and considerable out-of-plane piezoelectricity, and the latter is significantly enhanced in bilayer α-M₂X₃ because of the built-in electric field originating from the parallel electric dipoles. Our work will stimulate research on the ultrathin 2D photo detection, SHG and piezoelectric devices.