Ferroelectric materials exhibit spontaneous electric polarization that can be reversed by an external electric field, making them critical for applications in memory devices, sensors, and actuators. Understanding ferroelectric domains at atomic scales requires advanced techniques capable of resolving polarization vectors, domain walls, and defects with high precision. Two key methods for such investigations are Piezoresponse Force Microscopy (PFM) and Transmission Electron Microscopy (TEM). These techniques provide complementary insights into domain structures, polarization dynamics, and defect interactions.

Piezoresponse Force Microscopy (PFM) is a scanning probe technique that measures the electromechanical response of ferroelectric materials to an applied AC voltage. By detecting local piezoelectric deformation, PFM maps polarization orientation and domain distribution with nanoscale resolution. The method operates in contact mode, where a conductive atomic force microscopy (AFM) tip applies a voltage bias, inducing strain in the material proportional to the piezoelectric coefficient. The resulting tip displacement is measured via a lock-in amplifier, allowing phase and amplitude signals to distinguish antiparallel polarization states.

High-resolution PFM enables visualization of domain walls, including 180° and 90° configurations, with sub-10 nm spatial resolution. Advanced variants such as Dual AC Resonance Tracking (DART)-PFM and Band Excitation (BE)-PFM improve sensitivity and reduce artifacts by decoupling electrostatic contributions from purely piezoelectric responses. These techniques are particularly effective for studying polarization switching dynamics, where hysteresis loops can be acquired at individual domains to quantify coercive voltages and switching times.

Defect analysis in ferroelectrics benefits from PFM’s ability to correlate domain pinning with microstructural features. For example, oxygen vacancies in perovskite ferroelectrics like Pb(Zr,Ti)O₃ (PZT) or BaTiO₃ can locally suppress polarization switching, manifesting as immobile domains in PFM phase images. By combining PFM with conductive AFM, researchers can simultaneously map polarization and leakage currents, identifying defect-rich regions that degrade ferroelectric performance.

Transmission Electron Microscopy (TEM) offers atomic-scale imaging and diffraction for direct observation of ferroelectric domains and their atomic arrangements. High-angle annular dark-field (HAADF) scanning TEM (STEM) provides Z-contrast imaging, revealing cation displacements responsible for polarization. Coupled with electron energy-loss spectroscopy (EELS), STEM can detect oxygen vacancy distributions by analyzing changes in the O-K edge fine structure, linking defects to domain wall mobility.

Polarization mapping in TEM employs aberration-corrected imaging to measure atomic column shifts, such as the relative displacement of Ti or Zr cations in PZT. Quantitative analysis of these displacements yields polarization vectors at unit-cell resolution. Phase-contrast techniques, including off-axis electron holography, further enable electric field mapping around domain walls by detecting phase shifts in the electron wave passing through the sample.

Domain walls in ferroelectrics are critical functional elements, and TEM reveals their atomic structure with exceptional clarity. For instance, in rhombohedral BiFeO₃, TEM studies have shown that 71° domain walls exhibit mixed Ising- and Néel-type character, influencing conductivity and magnetoelectric coupling. Similarly, in tetragonal ferroelectrics, TEM has resolved head-to-head and tail-to-tail domain walls, where polarization discontinuity leads to charge accumulation and localized electric fields.

Defect-domain interactions are directly observable in TEM through dislocation imaging and strain analysis. Misfit dislocations at epitaxial interfaces can pin domain walls, while point defects like vacancies cluster near wall regions, altering local polarization. In situ TEM experiments, where electric fields or mechanical stresses are applied, provide dynamic observations of domain nucleation and growth at defects, offering insights into fatigue and retention mechanisms.

Combining PFM and TEM provides a comprehensive understanding of ferroelectric behavior. PFM’s mesoscale mapping of domain patterns and switching dynamics complements TEM’s atomic-resolution structural and chemical analysis. For example, PFM can identify regions of unusual switching behavior, which TEM then examines to reveal underlying defects or interfacial strain. This correlative approach is particularly powerful for complex systems like ferroelectric superlattices or polycrystalline films, where heterogeneity dominates performance.

Recent advances in both techniques continue to push the limits of ferroelectric characterization. Sub-Å resolution STEM now allows direct imaging of light elements like oxygen, critical for understanding polarization in materials like HfO₂-based ferroelectrics. Meanwhile, ultrafast PFM techniques capture domain switching at microsecond timescales, bridging the gap between static imaging and device-relevant dynamics.

In summary, PFM and TEM are indispensable tools for probing ferroelectric domains at atomic and nanoscales. PFM excels in mapping polarization distributions and dynamic responses, while TEM provides definitive structural and chemical information at the atomic level. Together, they enable a detailed understanding of domain physics, defect interactions, and performance-limiting mechanisms in ferroelectric materials. These insights drive the development of next-generation ferroelectric devices with enhanced reliability and functionality.