Van der Waals In₂Se₃: from phase transitions and phase-defined heterostructures to ferroelectrics

Due to its structural polymorphism, In₂Se₃ can take the form of multiple crystalline phases, a few of which (α and β phases, in particular) have layered structures. Recent studies of crystalline In₂Se₃ in the two-dimensional (2D) form have revealed interesting structural phase transitions, phase-specific physical properties, and room-temperature ferroelectricity down to the monolayer limit with uniquely interlocked in-plane and out-of-plane polarizations. In this talk, we will discuss distinct electrical and thermal properties of 2D α- and β-In₂Se₃, and how this large contrast in phase-specific properties leads to unique characteristics in phase-defined vertical and in-plane heterostructures, the latter of which can be realized by a scalable laser direct-writing technique. We will also discuss the recent discovery of 2D ferroelectricity sustained by covalent bonding in α-In₂Se₃. Particularly, the interlocked in-plane (IP) and out-of-plane (OOP) ferroelectricity allows the switching of the IP (OOP) polarization using an OOP (IP) external electric field. This unique aspect enables novel device concepts such as van der Waals ferroelectric memristors with multidirectional switching capabilities. Our recent studies have revealed new switching mechanisms in these devices, and the obtained mechanistic insights are critical to further development of ultrathin and novel memory devices for information storage and processing applications.