Elastic properties and Ion-mediated domain switching of self-assembled heterostructures CuInP₂ S₆-In_4/3P₂S₆

Van der Waals (vdW) ferroelectric CuInP₂S₆ (CIPS) has attracted intense research interest due to its unique ferroelectric properties that make it promising for potential applications in flexible electronic devices. A mechanical mean, or so-called strain gradient engineering, has been proven as an effective method to modulate its ferroelectric properties, but the key parameter elastic constants Cij has not been accurately measured. Here, we utilized nanoindentation and contact resonance atomic force microscopy (CR-AFM) techniques to measure the elastic modulus on the (001) plane of nanoscale phase separated CuInP₂S₆-In_4/3 P₂ S₆ (CIPS-IPS). The Young’s modulus of the CIPS was slightly less than that of the IPS. Density Functional Theory was introduced to obtain the accurate full elastic constant Cij of CIPS and IPS, and we deduced their respective Young’s moduli, all of which are in good agreement with our experimental values. We further discovered the asymmetrical domain switching and proposed an ion-mediated domain switching model. The results provide a reliable experimental reference for strain gradient engineering in the phase field simulation in CIPS-IPS.