Controlling the Orientation-Dependent Second Harmonic Generation in Hybrid Germanium Perovskites

Manipulating the crystal orientation plays a crucial role in the conversion efficiency during second harmonic generation (SHG). Here, we provide a new strategy in controlling the surface-dependent anisotropic SHG with the precise design of (101) and (2 $$ 0) MAGeI₃ facets. Based on the SHG measurement, the (101) MAGeI₃ single crystal exhibits larger SHG (1.3×(2 $$ 0) MAGeI₃). Kelvin probe force microscopy imaging shows a smaller work function for the (101) MAGeI₃ compared with the (2 $$ 0), which indirectly demonstrates the stronger intrinsic polarization on the (101) surface. X-ray photoelectron spectroscopy confirms the band bending within the (101) facet. Temperature-dependent steady-state and time-resolved photoluminescence spectroscopy show shorter lifetime and wider emission band in the (101) MAGeI₃ single crystal, revealing the higher defect states. Additionally, powder X-ray diffraction patterns show the (101) MAGeI₃ possesses larger in-plane polar units [GeI₃]^- density, which could directly enhance the spontaneous polarization in the (101) facet. Density functional theory (DFT) calculation further demonstrates the higher intrinsic polarization in the (101) facet compared with the (2 $$ 0) facet, and the larger built-in electric field in the (101) facet facilitates surface vacancy defect accumulation. Our work provides a new angle in tuning and optimizing hybrid perovskite-based nonlinear optical materials.