Two dimensional ferroelectrics: Candidate for controllable physical and chemical applications

(a) 2D ferroelectrics and its applications in physics and chemical reaction. Four research perspectives are presented while the reversible polarization is shown in the middle. (b) Schematic diagram of mechanism for controlled physics and chemical reaction by ferroelectricity. Upper panel indicates the polarization induced spin band shifts near the Fermi level. The below panel represents the polarization mediated electron transfer between gas molecules and ferroelectric substrate

(a) The adsorption energies of NH₃, NO and NO₂ on FE surfaces of In₂Se₃, indicating the potential adsorption/release induced by the FE polarization in In₂Se₃; (b) band alignments between the frontier molecular orbitals of gases and band edge states of In₂Se₃ with different polarization direction, all the energy levels are shifted relative to the vacuum level. ((a,b) Reprinted with permission from Ref. [102]. Copyright 2020 The Royal Society of Chemistry)

The applications of 2D ferroelectric In₂Se₃ and AgBiP₂Se₆ monolayers in photocatalytic water‐splitting. (a) Imagery parts (ε₂) of dielectric functions and optical absorption (α) of In₂X₃ monolayers based on G₀W₀‐BSE level. The white dashed vertical lines represent the optical absorption peaks of In₂X₃ monolayers. (b) Band alignments and partial charge density of VBM and CBM for In₂S₃, In₂Se₃, and In₂Te₃ monoalyers, respectively. (c) Band‐edge positions with respect to water redox potential for paraelectric (upper panel) and ferroelectric (lower panel) AgBiP₂Se₆ monolayers based on the HSE06 functional. The blue and red lines represent the thermodynamic oxidation potential (ϕ^ox) and reduction potential (ϕ^re), respectively. Blue and purplish‐red bars represent the positions of the CBM (blue) and VBM (red), respectively. The value of the isosurface is set as 0.006 e/ų. ((a,b) Reprinted with permission from Ref. [94]. Copyright 2018 Elsevier; (c) Ref. [95]. Copyright 2019 American Chemical Society)

The effect of FE polarization in 2D ferroelectrics on the bandstructures of nonpolar 2D materials. (a) The graphene/In₂Se₃ heterostructures with polarization up (P↑) and down (P↓); the bands derived from the In₂Se₃ layer and the graphene layer are highlighted in red and yellow, respectively. The green circles indicate the Dirac points of the graphene layer; (b) charge density difference; charge accumulation and depletion indicated by yellow and cyan with isosurfaces value 0.00015e/A³ of sandwiched In₂Se₃‐graphene‐In₂Se₃. (c) Band alignments of monolayer InTe, monolayer In₂Se₃ and InTe/In₂Se₃ heterostructures. The horizontal dark dashed lines are the Fermi level. The vacuum level is taken as reference. The insets are side view of the InTe/In₂Se₃ heterostructures with the direction of ferroelectric up and down. (d) Schematic diagram of the proposed p‐n junction based on graphene/In₂S₃/graphene. The Electronic band structures of MnCl₃/P↑‐CuInP₂S₆ (e) and MnCl₃/P↓‐CuInP₂S₆ (f) heterostructures with respect to Fermi level. Red and green lines represent the contributions from spin‐up (S↑) and spin‐down (S↓) channels of MnCl₃, while blue lines denote the contributions from CuInP₂S₆. The insets are MnCl₃/P‐CuInP₂S₆ heterostructures with FE polarization up and down in CuInP₂S₆. ((a) Reprinted with permission from Ref. [36]. Copyright 2017 Nature Publishing Group; (b) Ref. [78]. Copyright 2020 Elsevier; (c) Ref. [79]. Copyright 2019 IOP Publishing Ltd.; (d) Ref. [80]. Copyright 2020 The Royal Society of Chemistry; (e,f) Ref. [74]. Copyright 2020 The Royal Society of Chemistry)

The magnetism controlled or tuned by 2D ferroelectrics. (a) The directions of spin texture are reserved by the FE polarization in GeTe layer. (b) The magnetic ground states of FeI₂ monolayer are switched by FE In₂Se₃ from FM to AFM. (c) The magnetic ground states of CrI₃ undergo a transition from AFM to FM when the FE polarization is changed from downwards to upwards. (d) Sketch of atom‐thick multiferroic memory whose data writing and reading are dependent on the FE polar origination of CuInP₂S₆ induced by magnetoelectrical coupling between MnCl₃ and CuInP₂S₆. ((a) Reprinted with permission from Ref. [71] Copyright 2018 American Chemical Society; (b) Ref. [72]. Copyright 2019 The Royal Society of Chemistry; (c) Ref. [73]. Copyright 2020 American Chemical Society; (d,e) Ref. [74]. Copyright 2020 The Royal Society of Chemistry)

The structures of 2D intrinsic in‐plane (a–c) and out‐of‐plane (d–f) ferroelectrics. (a) Group IV monochalcogenides MX (M = Ge, Sn; X = S, Se) monolayers; (b) single layer γ‐SbX (X = P, As) with the black (red) balls representing the Sb (X) atoms. (c) GeS monolayers with two distorted degenerate FE states (phases B and B*) and an undistorted nonpolar structure (phase A). (d) Two distorted FE phases of AgBiP₂Se₆ monolayers and the high symmetry paraelectric phase (center image). (e): TMPCs‐CuMP₂X₆ (M = Cr, V; X = S, Se) monolayers. (f) In₂Se₃ monolayers with FE and symmetric structure (center image). ((a) Reprinted with permission from Ref. [22] Copyright 2016 American Physical Society; (b) Ref. [48] Copyright 2019 The Royal Society of Chemistry; (c) Ref. [49] Copyright 2019 American Institute of Physics; (d) Ref. [38]. Copyright 2017 The Royal Society of Chemistry; (e) Ref. [41]. Copyright 2018 American Institute of Physics; (f) Ding et al. Ref. [36]. Copyright 2017 Nature Publishing Group)