Fig. 1. (Color online) Layered structures of In₂Se₃. (a) Three-dimensional crystal structure of layered In₂Se₃, with the In atoms in blue and Se atoms in red, and a quintuple layer (QL) is indicated by the black dashed square. (b) Top view of the system along the vertical direction. Each atomic layer in a QL contains only one elemental species, with the atoms arranged in one of the triangular lattices A, B or C as illustrated. (c–h) Side views of several representative structures of one QL In₂Se₃, among which the (c) to (e) structures are derived from the zincblende, wurtzite and fcc crystals, respectively. In (f), the interlayer spacings between the central Se layer and the two neighboring In layers are displayed. The black arrows in (f) and (g) indicate the directions of the spontaneous electric polarization (P) in the FE-ZB' and FE-WZ' structures, respectively. The FE-ZB’ and FE-WZ’ structures in (f) and (g) are identified as α phase, while the metastable structure fcc and fcc’ in (e) and (h) are identified as β phase. (Courtesy of Ref. [20])

Fig. 2. (Color online) Ferroelectricity of α-In₂Se₃ thin layers. (a) The surface topography of α-In₂Se₃ thin layers (~20 nm) on the heavily doped Si substrate. The scale bar is 2 μm. (b) The corresponding PFM phase image in the out-of-plane direction, showing clear ferroelectric domains. (c) The phase profile of different ferroelectric domains as sketched by the red dashed line in (b). A phase contrast of 180° is observed, which indicates the antiparallel directions of out-of-plane electric polarization between the adjacent domains. The arrows indicate the directions of electric polarization. (d) PFM amplitude and (e) PFM phase hysteresis loop measured from α-In₂Se₃ thin layers. (f) PFM amplitude image of domain engineering in α-In₂Se₃ with a film thickness of 12 nm. The scale bar is 370 nm. (Courtesy of Ref. [24])

Fig. 3. (Color online) Electrically switching the out-of-plane ferroelectric polarization and corresponding in-plane atomic configuration through dipole locking. (a) The hysteresis of remnant out-of-plane polarization of a 3-nm-thick In₂Se₃ crystal on conductive SrRuO₃, as a function of perpendicular poling voltage. Black, red, blue curves represent the normalized piezoresponse measured with V_ac = 0.5, 1, and 1.5 V, respectively. The collapse of the hysteresis loop is similar to the behavior of the conventional field-switchable ferroelectrics, but totally different from charging artifact of dielectrics. (b) Polarized domain patterned by electrically biased scanning probe and measured by PFM. The inner box corresponds to positive applied voltage (+6 V) with positive piezoresponse while the outer box to negative voltage (−6 V) with negative piezoresponse. (c) SHG intensity mapping on another trilayer In₂Se₃ sample before PFM reversed poling. The area enclosed by dashed line was then scanning by a negatively biased AFM tip. The color bar is in linear scale with arbitrary unit. (d) SHG mapping after the electrical reversed poling. It shows dark lines at the boundary of the patterned area resulting from destructive interference, which indicates the reversal of in-plane crystal orientation and corresponding nonlinear optical polarization after reversed electrical poling. (Courtesy of Ref. [25])

Fig. 4. (Color online) Switchable ferroelectric diode based on α-In₂Se₃ thin layers. (a) Schematic and (b) optical image of the device. (c) and (d) I–V curves of the ferroelectric diode with switchable rectifying behavior. (e) I–V curves measured under high DC bias, showing clear hysteresis characteristics. The arrows indicate the voltage sweeping sequence. (f) and (g) Schematic of energy band diagrams of the graphene/In₂Se₃ heterostructure, illustrating the evolution of the Schottky barrier in the polarized state of the ferroelectric. The positive and negative charges on the vertical grey lines stand for the polarization charges on the top and bottom sides of the α-In₂Se₃ thin layer. The screening charges are visualized in the graphene/FLG electrodes. (Courtesy of Ref. [24])

Fig. 5. (Color online) Structure and optical characterization of the 2D FeFET. (a) 3D schematic diagram of the FeFET. The FeFET is fabricated by vertically stacking graphene, hBN, and α-In₂Se₃ thin layers in sequence. The white arrows indicate the direction of electric polarization. The zoomed area shows the crystal structure of ferroelectric α-In₂Se₃. (b) The hysteretic ferroelectric loop in 2D α-In₂Se₃ based FeFET. The resistance follows a butterfly-like dependence on gate voltage. Inset shows the optical image and topography of the FeFET. The substrate is Si wafer with 300 nm fused SiO₂ on top. The graphene, hBN, and ultrathin α-In₂Se₃ are indicated by black, white, and red-dashed frames, respectively. From the AFM topography, the thinnest area of ultrathin α-In₂Se₃ in the device is 2.6 nm. (Courtesy of Ref. [29])