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Electrostatic effect on off-field ferroelectric hysteresis loop in piezoresponse force microscopy

Piezoresponse force microscopy (PFM) has been extensively utilized as a versatile and an indispensable tool to understand and analyze nanoscale ferro- /piezoelectric properties by detecting the local electromechanical response on a sample surface. However, it has been discovered that the electromechanical response not only originates from piezoelectricity but also from other factors such as the electrostatic effect. In this study, we explore the dependence of off-field PFM hysteresis loops on the surface-potential-induced electrostatic effect in a prototypical ferroelectric thin film by applying an external voltage to the bottom electrode during measurement. We simplify the situation by equating the surface potential to the direct current voltage waveform variations and predicting the contribution of the surface-potential-induced electrostatic effect to the PFM hysteresis loops. The experimental results approximately match our prediction-the coercive voltage linearly decreases with the surface potential, whereas the saturated amplitude and piezoresponse remain nearly constant owing to the relatively large piezoelectric coefficient of the ferroelectric thin film.