Ferroelectricity and Nonlinear Optical Responses in Two-Dimensional Distorted MX₂Y (M = Cu, Ag, Au; X = Chalcogens; Y = Halogen) Monolayers

Two-dimensional (2D) materials with spontaneous polarization can exhibit large second-order nonlinear optical (NLO) effects. Here, we present a series of stable distorted MX₂Y monolayers by using first-principles calculations and lattice vibration analysis. The structural distortion leads to a lower polar symmetry, giving rise to intrinsic ferroelectricity with a Curie point up to room temperature. We show that the bulk photovoltaic effect (BPVE) in polar MX₂Y monolayers has an enhanced shift current, an order of magnitude larger than that in the undistorted nonpolar counterparts. Meanwhile, the second harmonic generation (SHG) susceptibility reaches up to the order of 10⁶ pm²/V, superior to that of 2D conventional materials such as MoS₂, h-BN, and GeS. Our study advances the research in 2D ferroelectric materials and would stimulate more efforts in developing optoelectronic devices based on NLO effects.