Grand arm 300

Cross-sectional TEM specimens were prepared by focused ion beam (FIB, Helios G4UX, FEI, America). In order to protect the thin film, a ~50 nm thick C layer was coated on the surface of the film before the FIB milling. HRTEM observations were carried out using a field emission microscope (JEM-2100F, JEOL, Japan) at an acceleration voltage of 200 kV. Further HAADF imaging, EDS mapping, and EELS analyses were performed with an Cs-corrected STEM operating at 300 kV (Grand ARM-300, JEOL, Japan) equipped with an X-ray energy dispersive spectrometer (JED-2300T) and Gatan image filter (GIF) system. In EELS analyses, an entrance aperture of 2.5 mm was used and the energy resolution of 1.0 eV was determined by measuring the full-width half-maximum (FWHM) of the zero-loss peak with the energy dispersion of 0.25 eV/channel. The EELS spectra were recorded with spectrum image mode to increase the signal/noise ratio, and the acquisition time was about 0.2 s per pixel (total acquisition time is about 3 min).

TEM samples were prepared on a dual-beam FIB system (Helios G4 UX, FEI, USA).
The atomic-scale in-situ biasing TEM analyses were performed on a Cs-TEM (JEOL Grand ARM300, Japan) with a PicoFemto double tile biasing TEM holder (ZEOTools Technology Company) at HRTEM mode. A tungsten tip was used as the mobile electrode, which was precisely controlled by a piezoelectric system. The in-situ videos were obtained using Gatan OneView camera with 4 k resolution by a speed of 25 frames/s. The in-situ video was drift-corrected and processed by using GM3 in-situ data processing. The HRTEM snapshots corresponding to different external voltages were extracted from the in-situ video and filtered to remove the high-frequency noise (Supplementary Movies 1 and 2).
Atomic-resolution imaging measurements before and after biasing were performed on the Cs-TEM at the AC-HAADF-STEM mode. Atomic-scale polarization switching under e-beam illumination was observed with the spot size of 8 C with a probe current of 23 pA or 6 C with 30 pA.
The simulated HAADF-STEM images were performed using MacTampas software. The simulated SAED patterns were obtained using CrystalKitX software.
Detailed atomic arrangement analyses for HAADF-STEM images combined with atomic quantification were performed using the Calatom software based on custom MATLAB scripts.

Electron microscopy analyses were performed in a coldFEG JEOL Grand ARM 300 electron microscope. The microscope was equipped with a double corrector from JEOL assuring a spatial resolution of 0.7 Å when operated at 300 kV. Before observation, the samples were deeply crushed using a mortar and pestle and dispersed in ethanol. A few drops of the suspension were deposited on a holey carbon copper microgrids. For imaging, both Annular Dark Field and Annular Bright Field detectors were used simultaneously, acquiring data in both detectors.