Polymorphism and Ferroelectricity in Indium(III) Selenide.

Two-dimensional indium(III) selenide (In2Se3) is characterized by rich polymorphism and offers the prospect of overcoming thickness-related depolarization effects in conventional ferroelectrics. α-In2Se3 has attracted attention as a ferroelectric semiconductor that can retain ferroelectricity at the monolayer level; thus, it can be potentially deployed in high density memory switching modes that bypasses the traditional von Neumann architecture in device design. However, studies involving α-In2Se3 are often hindered by difficulties in phase identification owing to mixing with ß-In2Se3. ß-In2Se3 has several polymorphs, among which include the antiferroelectric and ferroelastic ß'-In2Se3. It is important to understand polymorph transitions and crystal-amorphous phase transitions in ß-In2Se3 to tap into the potential of this material for resistive memory storage. In this review, we discuss how the various polymorphs and polytypes of In2Se3 can be rigorously differentiated and further highlight recent applications of these phases in ferroelectrics and memory devices.

Atomic Imaging of Electrically Switchable Striped Domains in ß'-In2 Se3.

2D ferroelectricity in van-der-Waals-stacked materials such as indium selenide (In2 Se3 ) has attracted interests because the ferroelectricity is robust even in ultrathin layers, which is useful for the miniaturization of ferroelectric field effect transistors. To implement In2 Se3 in nanoscale ferroelectric devices, an understanding of the domain structure and switching dynamics in the 2D limit is essential. In this study, a biased scanning tunnelling microscopy (STM) tip is used to locally switch polarized domains in ß'-In2 Se3 , and the reconfiguration of these domains are directly visualized using STM. The room-temperature surface of ß'-In2 Se3 breaks into 1D nanostriped domains, which changes into a zig-zag striped domains of ß″ phase at low temperatures. These two types of domains can coexist, and by applying a tip-sample bias, they can be interchangeably switched locally, showing volatile or nonvolatile like behavior depending on the threshold voltage applied. An atomic model is proposed to explain the switching mechanism based on tip-induced flexoelectric effect and the ferroelastic switching between ß' and ß″ phases.