Controllable synthesis of two-dimensional ferroelectric α-In<sub>2</sub>Se<sub>3</sub> by low-temperature chemical vapor deposition

Two-dimensional ferroelectric α-In<sub>2</sub>Se<sub>3</sub> possesses many fascinating physical properties. However, chemical-vapor-deposited ferroelectric α-In<sub>2</sub>Se<sub>3</sub> typically requires high temperatures (>650 ℃). In this work, α-In<sub>2</sub>Se<sub>3</sub> is synthesized at 400 to 460 ℃ by introducing a KCl/LiCl/NH<sub>4</sub>Cl ternary catalyst, resulting in a 200 ℃ reduction in growth temperature compared with ferroelectric α-In<sub>2</sub>Se<sub>3</sub> synthesized by the traditional chemical vapor deposition (CVD) method. The surface morphology of the as-prepared material is controlled by temperature and gas flow rate. As the growth temperature increases from 400 to 460 ℃, the synthesized α-In<sub>2</sub>Se<sub>3</sub> is changed from discrete hexagonal flakes to a continuous and uniform thin film, which is confirmed by the scanning electron microscope. Raman spectroscopy shows that the characteristic peaks of In<sub>2</sub>Se<sub>3</sub> are located at 103, 180, and 195 cm<sup>–1</sup>, respectively, indicating that the CVD-grown In<sub>2</sub>Se<sub>3</sub> is α-phase. Furthermore, energy dispersive spectrometer and X-ray photoelectron spectroscopy indicate that the elemental composition is close to the ideal stoichiometric ratio, confirming the successful synthesis of the α-In<sub>2</sub>Se<sub>3</sub>. Then, the as-prepared α-In<sub>2</sub>Se<sub>3</sub> is transferred onto Si/SiO<sub>2</sub> substrate for device fabrication. Atomic force microscope indicates that the film is uniform, with an approximate thickness of 63 nm. The fabricated two-terminal memristors exhibit analogous resistive switching behaviors. And such memristors are used to achieve synaptic functions of long-term potentiation/long-term depression. For artificial neural network simulations based on the synaptic memristors, the recognition accuracy for hand-written digit image exceeds 90%. This work provides a practical method for growing two-dimensional ferroelectric α-In<sub>2</sub>Se<sub>3</sub> at low temperatures for applications in synaptic devices and neuromorphic computing.