Room-temperature ferroelectricity and memory effects in a new van der Waals crystal

Van der Waals (vdW) ferroelectric material has emerged as an essential part of modern high-density electric-field-controlled information storage device due to its non-volatile polarization accompanied by its weak van de Waals interlayer coupling which enables the separation of atomic thick few-layer flakes from its bulk crystal. However, the number of vdW ferroelectric materials which show both room-temperature 2D ferroelectricity and few-layer air stability is still very small, mainly due to the increasing depolarization field and chemical activity in ultrathin flakes. Here, we report a new vdW ferroelectric crystal, which persists its ferroelectricity down to monolayer at room temperature.  Ferroelectric multi-domain phase is achieved by applying a tip-induced electric field on the flakes with a thickness down to 4 nm. Strikingly, the ferroelectric-paraelectric phase transition was observed at around 200°C via a combination of temperature-dependent piezoresponse force microscope (PFM) measurement and dielectric constant measurement, indicating that its polarization is able to persist at a higher temperature compared with most existing vdW ferroelectric materials. First-principle calculation was adopted to explain the origin of the ferroelectric dipoles. Moreover, a vertical ferroelectric diode was built up, showing its 2D ferroelectricity in an approach of electric transport measurement, and also acts as a prototype of the non-volatile memory device. This finding reveals this new vdW ferroelectric crystal has the capability to be integrated with other existing 2D materials into functional heterostructure and also designates it as a promising candidate for next-generation information storage nanodevices.