High-Performance Second-Harmonic Generation in Hybrid Ge Perovskite Ferroelectrics Enabled by Confinement Engineering

Confinement effect, by strengthening interactions between structural components, provides an effective strategy for precisely regulating functional units, which is critical for properties associated with microscopic accumulation effects, such as second harmonic generation (SHG) and ferroelectricity. Here, we propose a molecular dipole management strategy through confinement effect to enhance the SHG and ferroelectricity properties in a new hybrid germanium perovskite system. Utilizing the cyclohexylammonium (Cy) cation, we successfully synthesize three polar phases, including one-dimensional (1D) hexagonal (Cy)GeI 3 , a step-like two-dimensional (2D) (Cy) 4 Ge 3 I 10 phase, and a 2D (CyF) 2 GeI 4 phase (CyF = 4,4-difluorocyclohexanammonium). All three phases exhibit photoresponses, characteristic of semiconductors. The metastable 1D (Cy)GeI 3 phase has the highest local dipole moment (LDM) of 14.07 Debye along the c -axis, resulting in a strong SHG response, and a high octahedral distortion parameter ( D i = 0.10). The step-like 2D (Cy) 4 Ge 3 I 10 phase exhibits ferroelectric properties, where the remanent polarization of 0.34 µC/cm 2 has been confirmed by PFM and P-E hysteresis loop measurements. To achieve both high phase stability and SHG intensity, we introduce a fluorine group into the organic cation, forming strong C─H···F─C hydrogen bonds that align polar units uniformly, increasing D i to 0.11. The 2D (CyF) 2 GeI 4 phase achieves a high SHG of 4 × KH 2 PO 4 . Furthermore, its phase transition temperature also increases by 88 K compared to the (Cy) 4 Ge 3 I 10 phase. Raman spectra and DFT calculations further reveal intensified confinement effect in the (CyF) 2 GeI 4 phases, consistent with its stronger structural rigidity. Our work provides comprehensive perspective and design strategies for tuning and optimizing hybrid perovskite-based nonlinear optical materials.