thin layers*

Room-temperature stable ferroelectricity is the key for establishing non-volatile high-density memories. However, reaching the thin film limit is a long-term challenge in conventional ferroelectrics due to the critical size effect. With stable layered structure and weak interlayer couplings, van der Waals material is one of the promising candidates for developing ultra-thin 2D ferroelectrics. Here, we report room-temperature intrinsic ferroelectricity in layered α-In₂Se₃ with film thickness as thin as 5 nm. The out-of-plane ferroelectricity is evidenced by the observation of reversible spontaneous electric polarization. The observed domain size is around tens μm², indicating enough robustness for device potential. The electric hysteresis loop measurement suggests a relative low coercive electric field at ~2 X 10⁵ V/cm. We further demonstrate a practical ferroelectric diode in the form of graphene/α-In₂Se₃ heterojunction. The Schottky barrier in the device can be controlled by switching the electric polarization, exhibiting a tunable diode effect. Our results offer a new way for developing novel electronic devices based on 2D ferroelectrics.