Fig. 4: Microstructural changes of GeSe at different FE states.

a–c HAADF images of GeSe flake at different FE states (a for State I, b for pristine, c for State II). Insets are schematic atomic models of GeSe at different poled states. d–f Zoom-in HAADF images of areas marked as rectangles in (a–c). The dotted white lines are used to display the angles 1–4. The Ge-Se-Ge-Se bonding angle changes are displayed in the left panels of the corresponding HAADF images. g–i Calculated atomic structures of bilayer GeSe under different external electric-fields. The dotted horizontal dashed line in each sub-panel is used to recognize the relative position of the charge center along the z direction (accurately marked by the absolute values). Insets show the charge center structures. The bonds in a–f are marked by tilted white arrows, and the net polarization of the adjacent layers in a–i are indicated by green and blue bold arrows.