Visualization of ferroelectric domains via friction asymmetry

Abstract

Ferroelectric materials have been studied actively in sensor, actuator, energy harvester, and the key materials for next generation semiconductor device due to its switchable polarization state and electromechanical properties. Since the most of application of ferroelectric material is derived from domain distribution, in-depth study about the ferroelectric domains is essential for advanced application of ferroelectric materials. Piezoresponse force microscopy (PFM) is one of the representative methods for visualization of ferroelectric domains. PFM uses the converse piezoelectric effect, through which electric field induces the mechanical deformation of materials. In this method, the AC field is applied through conductive probe and the mechanical deformation is induced. The amplitude and phase values extracted from this induced deformation using a lock-in amplifier. However, speed of PFM imaging is limited by lock-in amplifier, and quality of PFM imaging is affected by electrostatic artifact that can be generated due to the electric signal. We suggest new method to visualize ferroelectric domain to overcome this limitation. New method based on the mechanical properties of ferroelectric domain according to polarization direction reveals the ferroelectric domains direction by friction asymmetry. As it is purely mechanical observation, the imaging speed can drastically increase without compromising the quality of the image. In this study, we visualized the ferroelectric domains via friction asymmetry, and confirmed that friction asymmetry depends on ferroelectric vertical domain direction only. Also, although friction asymmetry shows inversed trend in particular environment, it is always dominated by mechanical properties at high force regime. Using this method, we imaged the ferroelectric domain at high scan rates beyond 625 Hz, i.e., ~4.4 images per second in 128 ×128 pixels. Also, in-situ observation of ferroelectric domain switching was conducted. By observing the friction images, we could visualize the ferroelectric domain switching behavior.