Room-Temperature Ferroelectricity in Hexagonally Layered α-In2\nSe3\n Nanoflakes down to the Monolayer Limit

2D ferroelectric material has emerged as an attractive building block for high-density data storage nanodevices. Although monolayer van der Waals ferroelectrics have been theoretically predicted, a key experimental breakthrough for such calculations is still not realized. Here, hexagonally stacking α-InSe nanoflake, a rarely studied van der Waals polymorph, is reported to exhibit out-of-plane (OOP) and in-plane (IP) ferroelectricity at room temperature. Ferroelectric multidomain states in a hexagonal α-InSe nanoflake with uniform thickness can survive to 6 nm. Most strikingly, the electric-field-induced polarization switching and hysteresis loop are, respectively, observed down to the bilayer and monolayer (≈1.2 nm) thicknesses, which designates it as the thinnest layered ferroelectric and verifies the corresponding theoretical calculation. In addition, two types of ferroelectric nanodevices employing the OOP and IP polarizations in 2H α-InSe are developed, which are applicable for nonvolatile memories and heterostructure-based nanoelectronics/optoelectronics.