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. 2019 Apr 10;10:1661. doi: 10.1038/s41467-019-09650-8

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 1 — Ferroelectric and non-ferroelectric contributions in PFM. a Owing to the linear coupling between piezoelectric activity and spontaneous polarization in the low-field range, ferroelectric domain imaging can be carried out by detecting the electrically induced surface displacement due to converse piezoelectric effect. b In addition to the mechanism mentioned in a, other mechanisms may contribute to the image formation in PFM in ambient conditions: electrochemical reactions facilitated by the water meniscus at the tip–sample junction, field-induced migration of anions/cations, surface charging, and carrier injection, as well as non-linear high-order strain effects. c Switching spectroscopy PFM on amorphous non-ferroelectric HfO₂ films yields hysteretic behavior in the PFM phase and amplitude signals. (Reprinted with permission from ref. ⁴². Copyright 2015 American Chemical Society). As both ferroelectric and non-ferroelectric materials can exhibit hysteretic behavior in switching spectroscopy PFM, the observation of such loops alone is insufficient to confirm the ferroelectric behavior. d Non-ferroelectric signals due to charge injection and electrostatic forces can mimic the ferroelectric behavior in PFM measurements of non-ferroelectric LaAlO₃ films. This image demonstrates reversible and long-living PFM phase contrast, which arises after applying a ±5 V d.c. bias to the tip. The image size is 8 × 8 µm². (Reprinted from ref. ³⁵, with the permission of AIP Publishing). e Electrically switchable and stable PFM phase contrast in Pt/LaAlO₃/SrRuO₃ capacitors is attributed to electrically induced oxygen vacancy migration. (Reprinted with permission from ref. ⁸¹). f Redistribution and accumulation of oxygen vacancies either on the surface or at the interface can cause hysteretic field-dependent behavior of the PFM signal in LaAlO₃/SrTiO₃ heterostructures. (Reprinted with permission from ref. ³⁶. Copyright 2012 American Chemical Society). g, h Effect of environment in PFM manifests itself in creating different boundary conditions for domain writing and retention. PFM images of domains generated by voltage pulses under ambient conditions (g) and low humidity (h) reveal drastically different domain sizes and shapes (g, h are both reprinted from ref. ⁴⁷, with the permission of AIP Publishing)