Mechanisms of electromechanical coupling in strain based scanning probe microscopy

Abstract

Electromechanical coupling is ubiquitous in nature and underpins the functionality of materials and systems as diverse as ferroelectric and multiferroic materials, electrochemical devices, and biological systems, and strain-based scanning probe microscopy (s-SPM) techniques have emerged as a powerful tool in characterizing and manipulating electromechanical coupling at the nanoscale. Uncovering underlying mechanisms of electromechanical coupling in these diverse materials and systems, however, is a difficult outstanding problem, and questions and confusions arise from recent experiment observations of electromechanical coupling and its apparent polarity switching in some unexpected materials. We propose a series of s-SPM experiments to identify different microscopic mechanisms underpinning electromechanical coupling, and demonstrate their feasibility using three representative materials. By employing a combination of spectroscopic studies and different modes of s-SPM, we show that it is possible to distinguish electromechanical coupling arising from spontaneous polarization, induced dipole moment, and ionic Vegard strain, and this offer a clear guidance on using s-SPM to study a wide variety of functional materials and systems.

Ferroelectric or non-ferroelectric: Why so many materials exhibit “ferroelectricity” on the nanoscale

“…Another method to determine whether a material is piezoelectric is to determine the frequency dispersion, as shown by Seol et al 336 Reproduced from Seol et al 336 . A similar example is indicated by Chen et al 118 (e-h) Comparison of soda-lime glass (e,f) and PZT sample (g,h) hysteresis loops as a function of voltage window and frequency of the applied DC switching waveform. The variance is much larger for the glass than the PZT sample, and the change in amplitude is especially pronounced, while for the PZT sample the amplitude change is minimal with respect to the time taken for the DC sweep.…”

“…258 Subsequently, the model was extended to include both migration transport [259][260][261] and electrostrictive effects. 118,262,263 Finally, it was shown that ESM mechanism can be present even in the absence of a surface reaction. In this case, the redistribution of ionic species within the material creates an electrochemical dipole, the coupling of which to electrostriction gives rise to electromechanical coupling.…”

“…Recently, combined multimodal techniques have also been suggested, for example by Li et al 118 who utilized higher harmonic excitations and dependence of hysteresis on voltage range and frequency of the applied DC waveform, to separate electromechanical response from spontaneous polarization from those stemming from induced dipoles. One can write the electromechanical response as proportional to 2 + 2 + 2 2 .…”

“…[109][110][111][112][113][114][115] Similar approaches were pursued by other groups. [116][117][118][119][120] These analyses were extended to quantitatively describe a ferroelectric domain wall profile in vertical and lateral PFM 115,121,122 , orientational dependence of the PFM signal, 112,123 and to quantify piezoresponse spectroscopy 124 , as will be discussed below. Perhaps even more importantly, the PFM signal (more rigorously, electromechanical surface displacement) was shown to be independent of contact area, ensuring that PFM, unlike force based SPMs, can be an intrinsically quantitative technique.…”

Li transport in fresh and aged LiMn2O4 cathodes via electrochemical strain microscopy

“…Ferroelectric and dipole non-Vegard contributions considered above are polar and should have the characteristic butterfly-like shape of the amplitude hysteresis loop and 180 phase switching, while the ESM amplitude loop has different shapes and its phase does not switch at 180 due to non-polar nature of the Vegard strain response, 32 which depends only on Li concentration (see Eq. (2)).…”