Atomic force microscope system

A 2-N/m platinum-coated tip (SCM-PIT-V2 Bruker) was used on a Bruker atomic force microscope system (Billerica, MA). During PFM measurements, an AC signal ranging from 20 up to 10 V was applied at 750 kHz. This frequency was chosen to avoid resonance. The single-frequency PFM measurements are taken at a rate of 0.1 Hz, averaged over 10 measurements each, and taken with a 20-ms delay between voltage steps. The response was measured quasi-statically on a well-defined gold top electrode. The response did not change over multiple sweeps taken over multiple days. Further quasi-static measurements were taken on the bare surface of the material, which showed a similar PFM response. Static scans remove the possibility of changes in topography, leading to a false phase signal. The use of small-signal measurements reduces the amount of injected charge and the possibility of large electrostrictive contributions, while using large electrodes averages over a wide range of material so the contributions of surface defects are reduced. All PFM and electrical measurements were taken with no illumination to reduce contributions from photogenerated carriers. For large signal measurements, the domain reorientation by poling was duplicated in an external LCR measurement system.