Halide-driven polarity tuning and optimized SHG-bandgap balance in (C4H11N2)ZnX3 (X = Cl, Br, I)

Abstract

In the quest for organic–inorganic metal halides (OIMHs) that harmoniously combine large second harmonic generation (SHG) efficiency with broad bandgaps, our study introduces a series of noncentrosymmetric and polar piperazine zinc halides, (C₄H₁₁N₂)ZnX₃ (X = Cl, Br, I). The distinct influence of halide anion size on the configuration of ZnNX₃ tetrahedra is uncovered, revealing a design principle for enhancing SHG activity and manipulating bandgap characteristics. Specifically, (C₄H₁₁N₂)ZnCl₃ is the first example of deep-ultraviolet (deep-UV) nonlinear optical (NLO) OIMHs, with UV transparency lower than 190 nm and moderate SHG effect, 0.8 times KH₂PO₄ (KDP). Meanwhile, compared with (C₄H₁₁N₂)ZnI₃ (2.1 times KDP, 4.52 eV), (C₄H₁₁N₂)ZnBr₃ boasts a widened bandgap of 5.53 eV while maintaining a striking SHG response (2.5 times KDP), representing a pinnacle in the SHG-bandgap balance among OIMHs with E_g exceeding 5.0 eV. Computational analyses underscore the critical roles of halide orbitals and ZnNX₃ distortions in dictating SHG efficiency, with increasing halide polarizability correlating with heightened SHG contributions. This study paves the way for a novel approach to designing short-UV NLO crystals based on OIMHs.