Flexoelectric control of physical properties by atomic force microscopy

The past decade has witnessed the tremendous scientific and technological potential of nanoscale flexoelectricity in solids. The flexoelectric effect describes the universal generation of electric polarization in response to strain gradients and could be inversely enhanced at reduced nanoscale dimensions. Based on this unique scaling effect, nanoscale flexoelectricity has shown exciting physical phenomena, promising novel electronic, electromechanical, and photovoltaic applications. One of the most powerful ways to harness nanoscale flexoelectricity is to press the surface of a material through an atomic force microscope (AFM) tip to generate large strain gradients. This so-called AFM tip pressing allows us to locally break the inversion symmetry in any materials and study all the fascinating physical phenomena associated with inversion asymmetry. Although this technique has recently facilitated many important studies on nanoscale flexoelectricity, its effective use still requires a more solid foundation. In this review, we provide a comprehensive guideline to exploring nanoscale flexoelectricity via AFM tip pressing. We also discuss recent progress and the future research direction of AFM tip pressing-driven nanoscale flexoelectricity.