Fig. 4. The ionic and electronic properties of two representative as-synthesized 2D AMX₂ compounds.

a, d, g Schematic diagrams of the superionic conductor (a), ferroelectricity (d), and reversible self-doping (g) properties that arise from the ionic-electronic coupling effects. The blue arrows in (a) present the directed long-distance migration of the Cu⁺/Ag⁺ ions within the interlayer. The dashed triangles in (d) show the two enantiomeric tetrahedral sites of Cu⁺/Ag⁺ ions, corresponding to the up and down ferroelectric polarization states of the materials. b The impedance spectroscopic measurement for AgCrS₂ nanosheets with different thicknesses. The inset shows the equivalent circuit, where R_s, R_e, and R_i correspond to the contact resistance, electronic resistance, and ionic resistance, respectively, and C₁ and C₂ are the constant phase elements. The detailed calculation process of ionic conductivity is shown in Supplementary Fig. 39. c The comparison of ionic conductivity with other reported superionic conductors^16,40–46. e Piezoresponse force microscopy (PFM) phase and amplitude hysteresis loop of CuScS₂ nanosheet, the inset is the ferroelectric domains (white dashed square area) after forward and reverse DC bias polarization. The out-of-plane arrow symbols represent the P_up and P_down states of the nanosheet after polarization. f Temperature-dependent optical second harmonic generation (SHG) measurement of a CuScS₂ nanosheet, demonstrating a ferroelectric Curie temperature (T_c) of ~ 370 K. h, i Memristor behavior (h) and switchable photovoltaic behavior under illumination (λ = 532 nm, 256.6 mW/cm²) (i) after positive/negative pulse polarization of the CuScS₂-based device.