Tuning Polarization and Electronic Structure via Chemical Substitution in Two Dimensional Ferroelectrics

Two dimensional materials with switchable spontaneous electric polarizations (“2D ferroelectrics”) have recently been garnering attention as components for ultrathin electronic devices. Recently, it was proposed by Chandrasekaran et al. that the functionalized MXene Sc₂CO₂ has a metastable state with a comparatively large out-of-plane ferroelectric polarization. [1] Here, we used density functional theory (DFT) calculations to investigate additional M₂CX₂ materials (M = Sc, Y, La; X = O, F). We found that (1) chemical substitution of Sc with Y and/or O with F can preferentially stabilize the ferroelectric phase as the ground state (with the dynamic stability of these phases verified with phonon calculations); (2) such substitution can be used to systematically tune the polarization and band gap; and (3) these monolayers display large piezoelectric coefficients. Finally, we demonstrate how these properties can be continuously tuned by the application of external strain or alloying to create Sr_2xY_2-2xCO₂ monolayers. These findings demonstrate that chemical substitution and external stimuli are powerful ways to stabilize and control the properties of low dimensional ferroelectrics.

[1] Chandrasekaran et al., Nano Lett. 17 3290 (2017)