Strain-induced ferroelectric phase transition and second-harmonic generation enhancement in NbOCl2 monolayer

Two-dimensional (2D) materials exhibiting both second-harmonic generation (SHG) and ferroelectric properties are promising candidates for high-performance optoelectronic devices. However, the relationship between SHG response and ferroelectric polarization and bandgap in such materials has not been revealed clearly. Therefore, by using first-principles calculations based on many-body theory, we comprehensively study the structural, electronic, linear and nonlinear (SHG) properties of ferroelectric NbOCl2 monolayer under strain regulation to clarify the influence of ferroelectric polarization (Ps) and bandgap on the SHG coefficient. Our results show a ferroelectric to nonpolar phase transition under compressive strain larger than 5% along the polar direction. Moreover, under strain, the electronic bandgap, optical gap Eo, and SHG coefficient can be modified to a large extent and show different variation tendencies before and after phase transition. Importantly, a volcano relationship between SHG coefficient and a combined parameter (β1 = Ps*Eo) is established. Our findings unambiguously demonstrates the correlation between structural distortion, spontaneous polarization, and SHG property and have important implications for developing ferroelectric and nonlinear optical devices.