Ferroelectric-like hysteresis loops induced by chemical reaction and flexoelectricity in electrochemical strain microscopy measurements

Abstract

Electrochemical strain microscopy (ESM) can provide useful information on electrochemical reactivity in solids at the nanometer scale. In ESM, a time variant electric field applied to the sharp tip induces the ionic flow and electrochemical reaction. This triggers strains as a result of Vegard effect and flexoelectric effect, and the corresponding surface displacement can be detected by the tip. As such, the process involves mechanical-electrical-chemical coupling. A series of analytical descriptions was established to analyze the image formation and spectroscopic mechanism of ESM. However, most of the existing models are limited to the partial coupling or ignore the flexoelectric effect. In this paper, based on a fully coupled theory for thermal-electrical-chemical-mechanical processes, a model which accounts for the bulk defect electrochemical reaction, direct and inverse flexoelectric effect, and steric effect is developed for the mixed ionic-electronic conductor with an ionically blocking electrode. Here, the bulk defect electrochemical reactions are especially taken into account, which are ubiquitous in some electrochemical systems but usually omitted in the previous works. As an application of this work, the dynamic response of ESM measurement is solved numerically under the excitation of sinusoidal voltage. Numerical results reveal that there is an unambiguous ferroelectric-like hysteresis of the displacement-voltage loops, and the response is dependent on the frequency of applied excitation. Besides, the bulk defect electrochemical reaction has a salient influence on the particle distributions and the contribution of flexoelectric effect to the local surface displacement is marked. This work may help us to explain the image formation of ESM and explore the electrochemical process in solids.