Ferroelectricity in a semiconducting all-inorganic halide perovskite

Abstract

Ferroelectric semiconductors are rare materials with both spontaneous polarizations and visible light absorptions that are promising for designing functional photoferroelectrics, such as optical switches and ferroelectric photovoltaics. The emerging halide perovskites with remarkable semiconducting properties also have the potential of being ferroelectric, yet the evidence of robust ferroelectricity in the typical three-dimensional hybrid halide perovskites has been elusive. Here, we report on the investigation of ferroelectricity in all-inorganic halide perovskites, CsGeX₃, with bandgaps of 1.6 to 3.3 eV. Their ferroelectricity originates from the lone pair stereochemical activity in Ge (II) that promotes the ion displacement. This gives rise to their spontaneous polarizations of ~10 to 20 μC/cm², evidenced by both ab initio calculations and key experiments including atomic-level ionic displacement vector mapping and ferroelectric hysteresis loop measurement. Furthermore, characteristic ferroelectric domain patterns on the well-defined CsGeBr₃ nanoplates are imaged with both piezo-response force microscopy and nonlinear optical microscopic method.

Fig. 1.. Structure characterizations of CsGeBr₃ nanostructured single crystals.

(A) The left panel shows a projected lattice view of CsGeBr₃ (CGB) perovskite structure, which features the Ge displacement along the <111>_pc direction in each unit cell. The subscript “pc” denotes pseudo-cubic. The right panel shows a 3D view of the unit cells to illustrate the Ge coordination environments of CGB in two different phases upon ferroelectric phase transition, where Cs atoms at all corners are not sketched for simplicity. (B) Typical topography (AFM) of the CGB nanoplates and the associated SAED pattern from one nanoplate shown in the TEM image (inset), with relevant crystallographic axes and planes labeled. “ZA” denotes the zone axis. (C) SEM image of the CGB nanowires and the associated SAED pattern from nanowire shown in the inset TEM image. (D) Synchrotron x-ray microdiffraction of an individual CVT-grown CGB crystal at RT (λ= 1.54982 Å). Standard pattern shown in the bottom was indexed using the Inorganic Crystal Structure Database. a.u., arbitrary units. (E) Raman spectroscopy of CGB nanoplates at RT (633-nm excitation). The phonon modes are related to the [GeX₃]⁻ tetrahedron with C_3v (3m) symmetry (inset).

Fig. 2.. Theoretical and experimental evidence of the ferroelectric polarizations.

(A) Berry phase calculations of the polarization versus distortion percentages for CGB, in which 0% distortion means the cubic phase and 100% distortion means the ferroelectric rhombohedral phase. (B) Atomic-resolution STEM image on a localized domain of CGB nanowire displays the atomic displacement vector map. (C) Optical microscopic imaging of the patterned Cu electrodes (area of each electrode, 30 μm by 30 μm) on the CsGeI₃ thin film (thickness, 350 nm). Inset: Sketched side view of the device structure (Cu–perovskite film–Pt) for the hysteresis measurement. The voltage is applied to the perovskite film through the two tungsten (W) probes. (D) Current versus electric field loop after subtracting the leakage contribution and the corresponding ferroelectric hysteresis loop, obtained from the measurement on the device shown in (C).

Fig. 3.. Piezoelectric response and ferroelectric domain imaging via PFM.

(A) In-plane (IP) PFM phase image of a CGB nanoplate. Scale bar, 5 μm. (B) In-plane PFM phase image of the same plate with sample rotation by 90° relative to (A). Scale bar, 5 μm. In both (A) and (B), the blue arrows refer to the cantilevers and the double-ended black arrows correspond to the in-plane polarization components that are detected, perpendicular to the cantilever axis. In-plane projections of the polarizations were shown by the white arrows. (C) Polarization vectors in the four domain variants were represented by P₁, P₂, P₃, and P₄ along [$\overline{1}$11], [111], [$\overline{1}\overline{1}$1], and [1$\overline{1}$1] pseudo-cubic (pc) directions, respectively. (D and E) AFM image (D) and in-plane PFM phase image (E) of another CGB nanoplate. Scale bars, 5 μm. (F) Zoomed-in in-plane PFM amplitude image of a region on the plate indicated in (D). Scale bar, 1 μm. The bottom shows a further zoomed-in image. (G) Corresponding line trace at red dashed line in (F). (H) Phase and amplitude switching spectroscopy loops for the CGB grown on SRO film, which formed a simple device structure (inset) with DC electric field applied.

Fig. 4.. Light polarization–resolved SHG measurement and spatial mapping.

(A) Schematic representation of the experimental setup to illustrate the sample orientation with respect to the stage geometry and light propagation direction. The fundamental light pulses (900-nm wavelength) impinge on the sample CGB {001} surface at normal incidence, with a half-wave plate controlling their polarization. SHG light (450 nm) was detected in the reflection-mode scanning microscopy. The fundamental beam propagates along the z axis in laboratory coordinate system and polarizes along any direction in the x-y plane, where the polarization angle (φ) is defined by the azimuthal angle of incident electric field direction with respect to the x axis. DM, dichroic mirror. Filter: 580-nm short-wavelength pass filter. (B) Polar plots of SHG intensity versus polarization angle (φ) for a region depicted as the white circle on the plate mapped in (C) to (F). (C to F) Spatial mapping SHG intensity of the domain variants on the CGB nanoplate at different linear polarization states with φ and the double-ended black arrows indicated in each image. The white arrows indicate the possible configuration for the in-plane polarization orientations in the four domains. Scale bars, 5 μm.