Atomic structure and electronic property of two-dimensional ferroelectric CuInP2Se6

Two-dimensional (2D) CuInP2X6 (X = S or Se) materials have emerged as promising candidates for various technological applications due to their lamellar structures and ferroelectric properties. However, 2D CuInP2Se6 material has not been well explored yet for the difficulty in acquiring ultrathin flakes. Here, we prepare, for the first time, CuInP2Se6 crystals by the chemical vapor deposition method and its 2D flakes, and demonstrate that the thinnest CuInP2Se6 flakes of 4 nm are achieved. X-ray diffraction and atomic-resolution scanning transmission electron microscopy analyses reveal their single crystal phase and the corresponding atomic structure of bulk CuInP2Se6 with good crystallization. Raman results further exhibit obvious thickness dependence of Raman peak position and intensity for the CuInP2Se6 flake. First-principles calculations validate the ferroelectric property of CuInP2Se6 monolayer. Importantly, we find that the antiferroelectric (AFE) state is slightly stable than the ferroelectric (FE) state in bulk form, yet these two states are far more stable than the paraelectric (PE) state, resulting in the coexistence of AFE and FE states in this material.