Flexoelectricity in periodically poled lithium niobate by PFM

Piezoresponse force microscopy (PFM) has become an indispensable tool for investigating the electromechanical coupling properties of materials at the nanoscale, from bulk ferroelectric/piezoelectric materials to thin films. However, it has been gradually accepted that PFM signals can originate from various sources, including the piezoelectric effect, electrostatic interaction, and flexoelectric effect. Herein, we utilized the contact resonance-enhanced modes of PFM and non-contact Kelvin probe force microscopy to qualitatively and quantitatively determine the electromechanical coupling properties of periodically poled lithium niobite. We used the dc bias feedback from the kelvin-probe force microscopy to compensate for the interaction from the electrostatic force between the surface potential, and we demonstrate that the amplitude of the PFM signal on adjacent domains is asymmetric owing to the flexoelectricity. Based on theoretical and experimental data, we estimated the flexoelectric coefficient of lithium niobate to be on the order of 10-8 C m-1. Because the flexoelectric effect is inversely proportional to the critical dimension of the materials, our results suggest that flexoelectricity may play an important role in electromechanical coupling in periodically poled lithium niobate (PPLN) thin films. This work provides a new insight into the electromechanical coupling in PPLN; and might be helpful in designing lithium niobate-based integrated nanoelectromechanical devices, such as photonic and waveguide devices.