J. Ekstein, R. Vasudevan, M. Checa, S. Raghuraman, K.P. Kelley, S. Jesse, N. Domingo
Oak Ridge National Laboratory,
United States
Keywords: ferroelectrics, switching dynamics, piezoelectricity, domain walls
Summary:
Piezoresponse Force Microscopy (PFM) is a well-established technique for mapping the nanoscale piezoelectric response of ferroelectric materials. Its extension, Switching Spectroscopy PFM (SS-PFM), combines local imaging with the application of a DC bias to the tip, enabling the induction and mapping of ferroelectric switching properties with tens-of-nanometer spatial resolution. SS-PFM has revealed that domain nucleation and growth are influenced by intrinsic factors such as defects, dopants, and pre-existing domain structures. However, conventional PFM and SS-PFM approaches provide only static or quasi-static information—capturing the end states of domain switching—while offering limited insight into the real-time dynamics of bias-driven domain wall motion. Because the AFM tip is not positioned over the domain wall during motion, these techniques cannot directly capture transient wall displacements. Moreover, the temporal resolution of typical PFM experiments is insufficient to resolve millisecond-scale dynamics, and electrostatic artifacts can further obscure interpretation. Yet, understanding domain wall motion is crucial, as it dictates macroscopic ferroelectric performance, fatigue, and functional stability. To overcome these limitations, we developed Scanning Oscillator Piezoresponse Force Microscopy (SO-PFM), a dynamic variant of PFM designed to visualize and quantify bias-driven domain wall motion in ferroelectrics. In SO-PFM, the tip is simultaneously excited by a high-frequency AC bias (for conventional piezoresponse detection) and a low-frequency modulation bias. The scanning and bias sequences are synchronized such that each point on the sample experiences an identical excitation waveform. The resulting data form a three-dimensional dataset comprising two spatial dimensions (the scan area) and one temporal dimension, providing a real-time sequence of domain configurations with an effective temporal resolution of ~1 ms. For modulation bias below the coercive field, we can study reversible domain wall dynamics and visualize domain wall evolution under applied bias, revealing the kinetics of reversible motion at sub-coercive fields. We demonstrate this capability on epitaxial lead titanate (PbTiO₃) films with 180° domain walls. The c⁺/c⁻ antiparallel domains exhibit reversible expansion and contraction under alternating bias cycles, with reproducible behavior across multiple scans. The measured wall displacements and velocities depend strongly on local structural context, including the orientation relative to a– and c-domain lamellae and density of defects: comparative studies on samples with different defect concentrations confirm the critical role of local disorder in modulating domain wall susceptibility to motion. However, for modulation bias above the coercive field, the Scanning Oscillator mode also enables the study of domain switching dynamics, since it allows direct visualization of domain nucleation and growth processes when voltages exceeding the coercive field are applied, thus extending its applicability to dynamic polarization switching phenomena. This work was supported by the Center for Nanophase Materials Sciences (CNMS), a U.S. Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory.