Fig. 3: Ferroelectric properties.

a Electrostatic potential curve of the relaxed monolayers along the z-direction. The electrostatic potential difference (ΔΦ) is defined as the vacuum level difference between the top and bottom surfaces. b Top and side views of charge density differences between FE-WZ′ and fcc Tl₂NO₂ (left), and between FE-ZB′ and fcc In₂NO₂ (right). Green, yellow, and violet colors denote depleted charge, accumulated charge, and electric polarization, respectively. The isovalue is 1 electron per ų. c The minimum energy pathway in the polarization reversal process for FE-WZ′ Tl₂NO₂ monolayer. d Minimum energy pathway as a function of the in-plane biaxial tensile strain (from 0% to 5%) in the structural transition between two states with opposite electric polarization (P and −P) for the FE-ZB′ In₂NO₂ monolayer. The transformation proceeds through a centrosymmetric structure in which the polarization is zero.