Direct Observation of Electron Beam Induced Cu Ion Migration and Aggregation Dynamics in van der Waals Layered CuInP 2 S 6

(CIPS) is an emerging 2D room-temperature ferroelectric material that has recently attracted much attention. Its ferroelectricity is mainly affected by the motion of copper (Cu) ions in the lattice, so understanding the properties of Cu ions therein is imperative for potential applications. Here, we report the direct observation of electron beam induced Cu ion migration and aggregation dynamics in CIPS by using a combination of in situ transmission electron microscopy and simulations. Both electron beam irradiation experiments and density functional theory (DFT) calculations confirm the instability of Cu ions, with electron beam irradiation experiments capturing high mobility structural degradation dynamics at the atomic scale. The results further suggest that the lateral electric field resulting from the positive sample charging induced by the electron-matter interactions can influence the collective migration dynamics of Cu ions. Furthermore, it is found that these migrated Cu ions tend to aggregate at physical boundaries either set by the substrate or the sample itself and eventually form Cu nanoparticles. This work thus provides a comprehensive understanding of the behavior of Cu ions in CIPS ranging from the atomic scale to the mesoscopic scale, and will help guide the design of novel CIPS-based electronic devices.