Direct and converse flexoelectricity: the effect of strain and electric field gradients on nanoscale electromechanical responses

Surface electromechanics at the nanoscale are typically studied by Piezoresponse Force Microscopy (PFM), based on the inverse piezoelectric effect. As a first approach, generally only homogeneous responses are taken into account, but it has been realized that the effect of gradients in electro-mechanical phenomena at the nanoscale can become dominating: the generation of electrical signals after the application of mechanical strain gradients with an AFM tip has been proved, and it has been shown that it is possible to write ferroelectric domains [1] or to move oxygen vacancies and charges.

In this talk, I will review how gradient-based electro-mechanical effects couples and affects the quantification of PFM measurements. I will start by demonstrating the asymmetry in mechanical properties induced by the coupling of flexoelectricity to ferroelectricity leading to ferroelectrics as smart mechanical materials [2], and opening new opportunities to mechanically read ferroelectric polarization states in both, thin films and single crystals, on the base of Contact Resonance Frequency AFM mode. Then, I will put the light in another new aspect: I will demonstrate how converse flexoelectric effect [3] due to the presence of strong local electric field gradients at the tip end can induce a mechanical strain of the sample in dielectric centrosymmetric materials with magnitudes comparable to piezoelectric d₃₃ coefficient.
References
[1] H. Lu, C.-W. Bark, D. Esque de los Ojos, J. Alcala, C. B. Eom, G. Catalan, A. Gruverman, Mechanical Writing of Ferroelectric polarization, Science 336 (2012) 59.
[2] K. Cordero-Edwards, N. Domingo, A. Abdollahi, J. Sort, G. Catalan, Ferroelectrics as Smart Mechanical Materials, Advanced Materials,29 (2017) 1702210.
[3] Abdollahi, A.; Domingo, N.; Arias, I.; Catalan, G., Converse flexoelectricity yields large piezoresponse force microscopy signals in non-piezoelectric materials. Nature Communications 10 (2019) 1266