**4. Conclusions**

In summary, we clearly observed the domain structure of CIPS with a thickness of 20–40 nm in PFM measurements and found that the hysteresis loop of CIPS showed the same negative longitudinal piezoelectric effect as PVDF, indicating the complexity of the piezoelectric response of CIPS. We used electrochemical strain microscopy (ESM) to further study Cu2+ segregation and discovered different relaxation times between the Cu2+ and an InP2S6 <sup>2</sup><sup>−</sup> ion groups, with both ions showing significant differences in their DC voltage response. These observations imply that in the regions where the ferroelectric inversion loop cannot be observed, Cu2+ segregation occurs, and InP2S6 <sup>2</sup><sup>−</sup> paraelectric phase is formed. In addition, the Cu2+ relaxation is related to the change in the ferroelectric polarization after the application of voltage, and the longer relaxation time is relevant to the distinctive ferroelectric polarization saturation in CIPS2. Finally, we constructed a Pt/CIPS/Au FTJ. We observed resistance switches in the positive and negative voltage ranges, demonstrating that FTJ has superior continuous current modulation and selfrectification functions. Combined with its piezoelectric characteristics, layered ferroelectric CIPS is suitable not only for memory, but also for sensors, actuators, and even spin-orbit devices based on vdW heterostructures.

**Author Contributions:** Conceptualization, T.J., Q.G. and Z.C.; methodology, T.J., Y.C. (Yanrong Chen); validation, Y.C. (Yali Cai), W.D., C.Z., L.Y. and W.Y.; writing—original draft preparation, T.J. and Y.C. (Yanrong Chen); writing—review and editing, T.J., Y.Y., Q.G. and Z.C.; visualization, H.K.; formal analysis, Z.C.; investigation, H.K.; resources, T.J. and S.Y.; supervision, S.Y., T.J. and Y.C. (Yanrong Chen) contribute equally. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (51702351, 51777209), the Basic and Applied Basic Research Foundation of Guangdong Province (2020B1515120019), Guangdong Provincial Key Laboratory (2014B030301014), and the Shenzhen Science and Technology Innovation Committee (JCYJ20170413152832151, KQTD20170810160424889). And The APC was funded by JCYJ20170413152832151.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
