Frequency-dependent PFM signal induced by surface adsorbates

Piezoresponse force microscopy (PFM) is an indispensable tool for investigating local piezoelectric and ferroelectric properties by detecting the electromechanical strain response to an applied voltage on a surface. The PFM technique is highly sensitive to the surface state of a sample, such as surface adsorption and surface potential, which could lead to the misinterpretation of the quantitative value of PFM signal as well as the presence of ferroelectricity. In this study, we observed a frequency-dependent PFM signal generated by surface adsorbates. However, the surface adsorbates can be removed by sample cleaning albeit the cleaning induced a significant change in surface potential. Thus, correct evaluation of the effective piezoelectric coefficient was realized by cleaning the sample and ruling out the electrostatic contribution. This work demonstrates that one should be careful to consider the effect of adsorbates and the electrostatic effect to correctly evaluate the piezoelectric coefficient.