Fig. 2: Ferroelectric domain structure evolution of NaNbO₃/SrTiO₃ (001) heterostructures with varying film thickness.

a, c, e Vertical and b, d, f lateral piezoresponse force microscopy (PFM) amplitude imaging, along with corresponding schematic representations of the M_C (red) and M_B (blue) phases for NaNbO₃ films with thicknesses of a, b 25 nm, c, d 80 nm, and e, f 185 nm, respectively. Schematic representations of the polarization configurations with charge neutral domain walls for g M_B and h M_C phases, respectively, with corresponding in-plane projection schematics for the possible polydomain configurations. i Areal fraction of the M_B phase as a function of thickness, showing an increase in M_B phase and a decrease in M_c phase with increasing film thickness.