*Article* **Ferroelectricity and Piezoelectricity in 2D Van der Waals CuInP2S6 Ferroelectric Tunnel Junctions**

**Tingting Jia 1,2,\*, Yanrong Chen 1,3, Yali Cai 1,2, Wenbin Dai 1,3, Chong Zhang 1,2, Liang Yu 1,4, Wenfeng Yue 1,4, Hideo Kimura 5, Yingbang Yao 3, Shuhui Yu 1, Quansheng Guo 2,\* and Zhenxiang Cheng 6,\***


**Abstract:** CuInP2S6 (CIPS) is a novel two-dimensional (2D) van der Waals (vdW) ferroelectric layered material with a Curie temperature of *T*C~315 K, making it promising for great potential applications in electronic and photoelectric devices. Herein, the ferroelectric and electric properties of CIPS at different thicknesses are carefully evaluated by scanning probe microscopy techniques. Some defects in some local regions due to Cu deficiency lead to a CuInP2S6–In4/3P2S6 (CIPS–IPS) paraelectric phase coexisting with the CIPS ferroelectric phase. An electrochemical strain microscopy (ESM) study reveals that the relaxation times corresponding to the Cu ions and the IPS ionospheres are not the same, with a significant difference in their response to DC voltage, related to the rectification effect of the ferroelectric tunnel junction (FTJ). The electric properties of the FTJ indicate Cu+ ion migration and propose that the current flow and device performance are dynamically controlled by an interfacial Schottky barrier. The addition of the ferroelectricity of CIPS opens up applications in memories and sensors, actuators, and even spin-orbit devices based on 2D vdW heterostructures.

**Keywords:** two-dimensional materials; ferroelectric properties; scanning probe microscope; negative piezoelectricity; phase segregation
