Interface Piezoelectric Effect and Its Role in the Flexoelectric Effect

Inversion symmetry breaking has been an essential ingredient in electromechanical coupling phenomena that convert mechanical energy to electricity, including the piezoelectric and flexoelectric effects. The piezoelectric effect originates from the intrinsic lack of centrosymmetry in crystallographic structures, while the flexoelectric effect arises due to inversion asymmetry induced by strain gradients. Here, we introduce an emerging electromechanical effect that arises from inherent inversion asymmetry at interfaces, termed the interface piezoelectric effect. We will elucidate the underlying mechanism and describe the methods employed for its characterization. A distinctive aspect of this interface-induced effect is its applicability to materials of any symmetry, including centrosymmetric semiconductors. This contrasts with traditional piezoelectric effects, which are confined to non-centrosymmetric insulators. Consequently, our findings allow for the exploration of piezoelectric effects in a broader spectrum of established semiconductors, enhancing their potential applications across various technological domains. Furthermore, through deliberate engineering of the interface polar symmetry, we have discovered an exotic electromechanical coupling phenomenon. This phenomenon mirrors the electrical equivalent of a negative Poisson’s ratio. Dubbed the auxetic piezoelectric effect, it exhibits the same sign for the longitudinal (d33) and transverse (d31, d32) piezoelectric coefficients, resulting in simultaneous contraction or expansion in all dimensions when subjected to an external electrical stimulus. Finally, we will discuss the possible role of the interface piezoelectric effect in the flexoelectric effect.