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. However, 2D ferroelectrics with out-of-plane polarizations are challenging to find. Recently, Chandrasekaran et al. proposed that the functionalized MXene Sc$_{\mathrm{2}}$CO$_{\mathrm{2}}$ has a metastable state with a large out-of-plane polarization. [1] In this work, we used density functional theory (DFT) calculations to investigate additional M$_{\mathrm{2}}$CX$_{\mathrm{2}}$ materials (M $=$ Sc, Y, La; X $=$ O, F). We found that (1) substitution of Sc with Y and/or O with F can stabilize the ferroelectric phase as the ground state (the dynamic stability was verified with phonon calculations); (2) substitution can also be used to tune the polarization and band gap; and (3) these monolayers display large piezoelectric coefficients. We also show that these properties can be continuously tuned by the application of external strain or by alloying to create Sc$_{\mathrm{2x}}$Y$_{\mathrm{2(1-x)}}$CO$_{\mathrm{2}}$ monolayers. These findings demonstrate that chemical substitution and external stimuli are powerful ways to stabilize and control the properties of low dimensional ferroelectric materials. [1] A. Chandrasekaran, A. Mishra, A. K. Singh, Nano Letters 17 3290 (2017)