Revealing fast Cu-ion motion in CuInP2S6 - In4/3 P2S6 flakes through the ferroelectric to paraelectric phase transition

M. Checa, X. Jin, S. Neumayer, R. Millan-Solsona, M.A. Susner, M.A. McGuire, A. O’Hara, G. Gomila, P. Maksymovych, S.T. Pantelides, L. Collins
Oak Ridge National Laboratory,
United States

Keywords: piezoresponse force microscopy, electrochemical strain microscopy, electorstatic force microscopy, layered ferroelectrics


Van der Waals layered materials that can maintain strong ferroelectric properties at room temperature provide an ideal base for making smaller miniaturized ferroelectric devices. Among these materials, CuInP2S6 (CIPS) stands out because it shows unique, ion-mediated, switching of ferroelectric polarization. This leads to several features like multiple ferroelectric states, ion-based electrical conduction, and intricate chemical interactions at surfaces. Additionally, by carefully managing the composition and the synthesis process, it's possible to create stable, self-assembled layers combining ferroelectric (CIPS) and non-ferroelectric (i.e., lacking Cu) In4/3P2S6 (IPS) phases. However, the characteristics of the CIPS/IPS interfaces, which naturally form through in-plane crystal growth, haven't been extensively studied yet. We show how we use an advanced scanning probe microscopy approach to explore in detail the nanoscale variability of the AC ion-transport dependent functional properties (electromechanical, dielectric, and conductive) in CIPS-IPS flakes during the ferrielectric-paraelectric transition. We find evidence of a kHz ionically mediated electromechanical response (i.e., Vegard’s strain) of CIPS in the paraelectric phase (above TC). By using a method based on interferometry, we propose that this could be one of the first real quantitative examples of electrochemical strain microscopy (ESM). Furthermore, we've discovered variations in both the local dielectric constant and ionic conductivity across the CIPS-IPS heterostructure. We've been able to image the peak in dielectric behaviour that occurs during the ferroelectric phase transition at the nanoscale. Additionally, we've observed increased conductivity at the CIPS/IPS interface. These findings suggest that we might be able to engineer or modify the properties at this interface and position CIPS/IPS interfaces as new platform for tunning ferroelectric properties in van der Waals ferroelectrics. email: