Fig. 1. (Color online) (a) Schematic crystal structure of In₂Se₃ (reproduced with permission from Ref. [77], © Ding, W. J.et al. 2017). (b) PFM phase and amplitude images of a thinα-In₂Se₃ flake, respectively (reproduced with permission from Ref. [45], © 2017 American Chemical Society). (c) The phase images for both OOP and IP polarization of a 6 nm thick In₂Se₃ flake (reproduced with permission from Ref. [46], © 2018 American Chemical Society). (d) Schematic crystal structure of 2Hα-In₂Se₃ in 1 to 4 layers. (e) Schematic of IP polarization rearrangement under electric field. (d) and (e) Reproduced with permission from Ref. [50], © The Royal Society of Chemistry 2021. (f) Ferroelectric hysteresis loops of 8 nm thickα-In₂Se₃ film (reproduced with permission from Ref. [48], © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020).

Fig. 2. (Color online) (a) Schematic structure ofα-In₂Se₃ FeS-FET and switching characteristics ofα-In₂Se₃ FeS-FET (reproduced with permission from Ref. [3], Copyright © 2019, Mengwei Siet al.). (b) Schematic structure ofα-In₂Se₃ FeCTs and switching characteristics ofα-In₂Se₃ FeCTs (reproduced with permission from Ref. [19], Copyright © 2021, Shuiyuan Wanget al.).

Fig. 3. (Color online) (a) Schematic crystal structure of CuInP₂S₆. (b) AFM images and BEPFM images of CuInP₂S₆ flake (reproduced with permission from Ref. [51], Copyright © 2015, American Chemical Society). (c) AFM images and PFM images of CuInP₂S₆ flake. (d) Electric characterization of the vdW CuInP₂S₆/Si diode (Inset: schematic structure of ferroelectric diode based on CuInP₂S₆ heterostructure). (a), (c) and (d) Reproduced with permission from Ref. [52], Copyright © 2016, Fucai Liuet al.

Fig. 4. (Color online) (a) Optical microscopy and AFM images of d1T-MoTe₂. (b) PFM phase hysteretic and butterfly loops of monolayer d1T-MoTe₂. (c) Schematic atomic structure of d1T-MoTe₂. (d) Electric characterization and structure of the FTJ device d1T-MoTe₂. (a-d) Reproduced with permission from Ref. [44], Copyright © 2019, Shuoguo Yuanet al.

Fig. 5. (Color online) (a) Schematic crystal structure of Bernal-stacked bilayer graphene (reproduced with permission from Ref. [91], © Springer Nature 2022). (b) Schematic of dual-gate devices N0, H2 and H4. (c) Four-probe resistance for devices N0, H2 and H4. (d) Hysteretic transport behavior for device H4. (Inset: ‘zigzag’ patterns illustrate how data are obtained.) (e) and (f) Forward and backward scan forV_BG sweep between –50 and 50 V. (g) The difference between measured in (e) and (f). (b-g) Reproduced with permission from Ref. [74], © Zheng Zet al. 2020.

Fig. 6. (Color online) (a) Schematic crystal structure of AB-stacked and BA-stacked BN. (b) PFM phase and amplitude images of twisted bilayer BN, respectively. (c) ResistanceR_xx of graphene for the device as a function of the top gate and bottom gate, respectively (Inset: dual-gate P-BBN device structure). (d) Ferroelectric switching in twisted bilayer BN. (e) Temperature dependence of polarization and graphene resistance R_xx, respectively. (f) Room-temperature operation of a ferroelectric field-effect transistor. (a-f) Reproduced with permission from Ref. [75], © 2021 American Association for the Advancement of Science.

Fig. 7. (Color online) (a) Schematic crystal structure of H-stacked and R-stacked (MX and XM) TMDs, respectively. (b) PFM phase and amplitude images of MoSe₂, respectively. (c) Schematic illustration of lateral PFM measurement on MoSe₂. (d) ResistanceR_xx of graphene for TMDs device as a function of the top gate and bottom gate, respectively (Inset: dual-gate R-stacked TMDs device structure). (e) Ferroelectric switching in small-angel twisted bilayer WSe₂ d1 device. (f) Schematic of polarization switching in WSe₂ d1 and d2 device and temperature dependence of graphene resistanceR_xx, respectively. (a-f) Reproduced with permission from Ref. [76], © 2021 Springer Nature 2022.

Table 0. Twisted 2D ferroelectricity.

Table 0. Proved 2D ferroelectric materials.