Ferroelectric or non-ferroelectric: why so many materials exhibit\n ferroelectricity on the nanoscale

Ferroelectric materials have remained one of the foci of condensed matter\nphysics and materials science for over 50 years. In the last 20 years, the\ndevelopment of voltage-modulated scanning probe microscopy techniques,\nexemplified by Piezoresponse force microscopy (PFM) and associated time and\nvoltage spectroscopies, opened a pathway to explore these materials on a\nsingle-digit nanometer level. Consequently, domain structures, walls and\npolarization dynamics can now be imaged in real space. More generally, PFM has\nallowed studying electromechanical coupling in a broad variety of materials\nranging from ionics to biological systems. It can also be anticipated that the\nrecent Nobel prize in molecular electromechanical machines will result in rapid\ngrowth in interest in PFM as a method to probe their behavior on single device\nand device assembly levels. However, the broad introduction of PFM also\nresulted in a growing number of reports on nearly ubiquitous presence of\nferroelectric-like phenomena including remnant polar states and\nelectromechanical hysteresis loops in materials which are non-ferroelectric in\nthe bulk, or in cases where size effects are expected to suppress\nferroelectricity. While in certain cases plausible physical mechanisms can be\nsuggested, there is remarkable similarity in observed behaviors, irrespective\nof the materials system. In this review, we summarize the basic principles of\nPFM, briefly discuss the features of ferroelectric surfaces salient to PFM\nimaging and spectroscopy, and summarize existing reports on ferroelectric like\nresponses in non-classical ferroelectric materials. We further discuss possible\nmechanisms behind observed behaviors, and possible experimental strategies for\ntheir identification.\n