Theoretical Prediction of Monolayer BeP₂O₄H₄ with Excellent Nonlinear-Optical Properties in Deep-Ultraviolet Range

Most 2D nonlinear optical (NLO) materials do not have an ultrawide bandgap, therefore, they are unsuitable for working in the deep-ultraviolet spectral range (< 200 nm). Herein, the theoretical prediction of an excellent monolayer BeP₂O₄H₄ (ML-BPOH) is reported. DFT analyses suggest a low cleavage energy (≈45 meV per atom) from a naturally existed bulk-BPOH material, indicating feasible exfoliation. This novel 2D material exhibits excellent properties including an ultrawide bandgap (E_g) of 7.84 eV, and a strong second-order nonlinear susceptibility ( $$ = 0.43 pm V^-1), which is comparable to that of benchmark bulk-KBBF crystal (d₁₆ = 0.45 pm V^-1). The wide bandgap and large SHG effect of ML-BPOH are mainly derived from the (PO₂H₂)^- tetrahedron. Notably, ML-BPOH exhibits an outstanding 50% variation in d_sheet under minor stress stimuli (±3%) due to rotation of structurally rigid (PO₂H₂)^- tetrahedron. This indicates significant potential for application in material deformation monitoring.