Ferroelectricity in ultrathin crystal has been long believed to exhibit rich phase competition physics with quantum confinement and enhanced quasiparticle interactions1. Its realization is also critical to scale down memory and develop versatile ferroelectric devices with large electrical and mechanical tunablity2. However, its own depolarizing electrostatic field prevents the existence of out-of-plane ferroelectricity at two-dimensional (2D) limit. Here we report the discovery of out-of-plane 2D ferroelectricity in atomically thin In2Se3 crystal, which was theoretically predicted recently3. We experimentally found that in-plane lattice asymmetry and out-of-plane polarization is strictly locked, a new mechanism to stabilize the polar order. Such unique locking enables robust 2D ferroelectricity at ambient conditions and results in a very high transition temperature (~700 K). In addition, it also enables electrical manipulation of atomic lattice anisotropy, which is the key to 2D spintronics and valleytronics. This discovery is potentially important to the atomically thin sensors and actuators, and ultrahigh density nonvolatile memory devices.
1. Dawber, M., Rabe, K. M. & Scott, J. F. Physics of thin-film ferroelectric oxides. Rev. Mod. Phys. 77, 1083–1130 (2005).
2. Martin, L. W. & Rappe, A. M. Thin-film ferroelectric materials and their applications. Nat. Rev. Mater. 2, 16087 (2016).
3. Ding, W. et al. Prediction of intrinsic two-dimensional ferroelectrics in In2Se3 and other III2-VI3 van der Waals materials. Nat. Commun. 8, 14956 (2017).