Ferroelectricity – Ambiguity Clarified, and Resolved

The Science

Newswise — From storing information to storing energy, a special class of materials with the right properties could get the job done. These materials, called ferroelectric materials, can be difficult to identify. This detailed assessment called into question the previous identification of ferroelectric materials based solely on scanning probe microscopy (SPM). The traditional approach is to observe a “residual” effect in the material’s response (hysteresis loop) using a SPM, for example, the material retains residual electric polarization after removal of an external electric field. Scientists showed that this approach was insufficient to determine ferroelectricity in a range of materials. After conducting a detailed assessment of ferroelectricity, they developed new experimental protocols that distinguish ferroelectricity from competing interactions.

The Impact

The new approach allows reliable study of new ferroelectric materials or ferroelectricity induced by external forces. It also provides the foundation for using a scanning probe to evaluate other properties of interest in electronic applications ranging from sensors to information technologies.

Summary

Ferroelectric materials are used in a range of applications from sensors and actuators to information technology devices. Scientists have imaged and manipulated ferroelectric properties using a particular type of scanning probe microscopy called piezo-response force microscopy (PFM). PFM measures the dynamic, electromechanical response when a voltage is applied to a scanning probe microscope (SPM) tip in mechanical contact with the sample’s surface. The bias-induced hysteresis loop that is observed in the electromechanical response has often been interpreted, and sometimes misinterpreted, as evidence of ferroelectricity. Researchers, led by Oak Ridge National Laboratory, clearly showed that both ferroelectric and non-ferroelectric materials could generate similar hysteresis loops in their electromechanical responses. They identified other mechanisms that can produce hysteretic responses such as strong electrostatic interactions (similar to the force between a proton and electron). Therefore, hysteresis in the electromechanical response measured in PFM cannot unambiguously identify ferroelectricity. This finding calls into question previously identified ferroelectric materials based only on their hysteretic responses. The team further defined experimental protocols to unambiguously identify ferroelectricity, distinguishing ferroelectric properties from competing signal-generating mechanisms. This research opens new pathways for probing other electromechanical phenomena on the nanoscale that have not been studied before, such as charge trapping and properties of dielectric materials.

Funding

U.S. Department of Energy, Office of Science, Basic Energy Sciences including support of the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility; National Basic Research Program of China; National Natural Science Foundation of China; and National Science Foundation.

Publications

N. Balke, P. Maksymovych, S. Jesse, A. Herklotz, A. Tselev, C. B. Eom, I. I. Kravchenko, P. Yu, and S. V. Kalinin, “Differentiating ferroelectric and nonferroelectric electromechanical effects with scanning probe microscopy.” ACS Nano 9, 6484 (2015). [DOI: 10.1021/acsnano.5b02227]