Size-dependent electromechanical properties in piezoelectric superlattices due to flexoelectric effect

- A strong size dependence of electromechanical properties is predicted for the piezoelectric BaTiO3/PbTiO3 superlattices.
- For the superlattices with a specific thickness ratio of different layers, the piezoelectric response can be several times larger than that of bulk counterparts.
- The present work demonstrates a practical way to design the piezoelectric superlattices with high piezoelectric coefficient by using flexoelectric effect.

Piezoelectric superlattice is a potential component for nanoelectromechanical systems. Due to the strong nonlocal effect such as flexoelectric effect at interfaces, classical piezoelectric theory is unable to accurately describe the electromechanical response of piezoelectric superlattice at nanoscale scale. Based on the previous nonlocal thermodynamics theory with flexoelectric effect Liu et al. (2016), the size-dependent electromechanical properties of piezoelectric superlattices made of BaTiO3 (BTO) and PbTiO3 (PTO) layers are investigated systematically in the present work. Giant strain gradient is found near the interface between BTO and PTO layers, which leads to the significant enhancement of polarization in the superlattice due to the flexoelectric effect. For the piezoelectric BTO-PTO superlattices with different unit-cell sizes, the thickness of interface with nontrivial strain gradient is almost constant. The influence of strain gradient at the interface becomes significant when the size of superlattice decreases. As a result, a strong size dependence of electromechanical properties is predicted for the piezoelectric BTO-PTO superlattices. In particular, for the superlattices with a specific thickness ratio of BTO and PTO layers, the piezoelectric response can be several times larger than that of bulk structure. The present work demonstrates a practical way to design the piezoelectric superlattices with high piezoelectric coefficient by using the nonlocal effect at nanoscale.