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

Abstract

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)GeI₃, a step-like two-dimensional (2D) (Cy)₄Ge₃I₁₀ phase, and a 2D (CyF)₂GeI₄ phase (CyF = 4,4-difluorocyclohexanammonium). All three phases exhibit photoresponses, characteristic of semiconductors. The metastable 1D (Cy)GeI₃ 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)₄Ge₃I₁₀ phase exhibits ferroelectric properties, where the remanent polarization of 0.34 µC/cm² 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)₂GeI₄ phase achieves a high SHG of 4 × KH₂PO₄. Furthermore, its phase transition temperature also increases by 88 K compared to the (Cy)₄Ge₃I₁₀ phase. Raman spectra and DFT calculations further reveal intensified confinement effect in the (CyF)₂GeI₄ 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.

Keywords

confinement effect / dipole moment / ferroelectric / SHG

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