Room-temperature out-of-plane and in-plane ferroelectricity of two-dimensional β-InSe nanoflakes

Two-dimensional (2D) layered semiconductors have shown great application potential in next generation nanoelectronic devices. The ferroelectric and piezoelectric properties of 2D semiconductors are also highly desirable in many applications, such as ferroelectric nonvolatile memory and switch. In the present work, we experimentally demonstrate the simultaneous out-of-plane and in-plane ferroelectricity of β-indium selenide (β-InSe) nanoflakes at room temperature. The polarization switching in the as-prepared β-InSe with the P63/mmc symmetry is studied by piezoresponse force microscopy. Out-of-plane polarization hysteresis loops are observed in a 7-nm-thick sample, and the in-plane and out-of-plane ferroelectric switching under the forward and reverse direct current bias are obtained in a 10-nm-thick sample at room temperature. These results indicate that β-InSe is a promising intrinsic 2D van der Waals ferroelectric material. Our work has connected the 2D materials with ferroelectric materials and inspired their applications in electronic devices.