Atomically Resolving Polymorphs and Crystal Structures of In2Se3

The structures of the various phases endow In2Se3 unique properties as well as a broad range of potential applications. However, the controversy on the structures of In2Se3 strongly hinders the exploitation of its properties and potentially gives rise to misdirection of its applications. Here, taking advantage of state-of-the-art aberration-corrected scanning transmission electron microscopy, we demonstrate the atomic-scale structures of lab-created and purchased In2Se3 compounds. Six phases in three polymorphs at room temperature have been observed among all the samples, which include 2H and 3R α-In2Se3, 1T, 2H, and 3R β-In2Se3, and none-layered γ-In2Se3. Raman spectra are directly correlated to individual In2Se3 phases, providing fingerprints for identifying various phases of In2Se3. In addition, obvious out-of-plane ferroelectricity of 2H α-In2Se3 was also observed by piezoresponse force microscopy, enabling its potential application in ferroelectric devices.