PFM measurements were conducted under nonresonant mode on a commercial atomic force microscope (AFM) (Asylum Research MFP-3D) using a stiff Pt/Ir-coated silicon tip with a spring constant of 40 N/m. An ac voltage of 1 V with a frequency of 10 kHz was applied to the conductive tip, which drives the film surface to vibrate because of converse piezoelectric effect. The amplitude and phase of the vibration were then picked up by a lock-in amplifier. Here, PFM images were plotted using the signal of amplitude*sin(phase) (−90° ≤ phase ≤ 90°), of which the absolute value and sign reflect the polarization magnitude and direction, respectively.