Ferroelectricity usually exists in a special class of dielectric materials. The spatial inversion symmetry of these materials is broken, so that the positive and negative charge centers of the system are separate, resulting in a spontaneous polarization. The direction of this spontaneous polarization can be reversed by the external electric field, resulting in a hysteresis loop^1-2 similar to ferromagnetic materials, so this kind of property is called ferroelectric in analogy with ferromagnetism. In ferroelectric materials, the two electrical states of the electric polarization vectors before and after flipping can correspond to “0” and “1” in binary language respectively. Because the electrical state can be read and written repeatedly and preserved for a long time, ferroelectric materials are widely used in many fields, such as ferroelectric memory, ferroelectric tunnelling junction(FTJ)³, ferroelectric capacitor and ferroelectric diode. The traditional ferroelectric materials are mainly ABO₃ type perovskite materials⁴.

Under the general trend of miniaturization and integration of electronic equipment, exploring the new ferroelectric materials with small size and foldable properties has become a hot topic in the low dimensional ferroelectricity fields. In traditional ferroelectric materials (ABO₃-type perovskite), when the thickness of the material reduce to several nanometers, the surface depolarization effect is strong, so that the ferroelectric property significantly weakened or even disappeared, corresponding to the ferroelectric critical size. Therefore, how to design and prepare high performance ferroelectric devices with small size has become one of the major challenges in this field.