Polarization–electric field (P-E) hysteresis loops were measured using a commercial ferroelectric tester (Radiant Technologies). Dielectric data were collected using a commercial impedance LCR meter (E4980A, Agilent). S-E hysteresis curves were obtained on a commercial AFM (Asylum Research MFP-3D) in contact mode by maintaining a constant tip height (Z sensor feedback) while monitoring its deflection signal during polarization switching with P-E loops simultaneously recorded. The deflection signal (unit in volts) was converted to actual displacement (unit in meters) using the calibrated inverse optical lever sensitivity (InvOLS; unit in meter per volt). The tip-sample force was carefully chosen so that it is small enough to prevent any indentation damage to the soft samples, while large enough to prevent the tip from leaving the sample surface during thickness contraction. AFM probes with very different spring constants produced comparable results. Dynamic piezoelectric measurements were achieved using standard PFM. An ac voltage (usually Vpeak = 1 V) with a frequency of 10 kHz was applied to the conductive tip (large spring constant ~40 N/m to minimize electrostatic contributions) that was in contact with the top electrode and drove the sample surface to vibrate via inverse piezoelectric effect. The amplitude and phase of the vibration were then picked up by a lock-in amplifier. The effective d33 was calculated from the amplitude signal using the conversion factor, InvOLS. To obtain a hysteresis loop, a slow triangular waveform (<0.2 Hz) was applied to the bottom electrode to switch the polarization.

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