Prediction of two-dimensional ferroelectric metal Mxenes

Abstract

Metals with spontaneous electric polarization have exhibited fascinating characteristics that could improve the performance of nanoelectronics. As of yet, only a few metals with electrical polarization (polar metals) have been discovered. Most of those polar metals are bulk materials, thus, the polarization direction cannot be switched using an external electric field. Here, using first principles calculations, we demonstrate that 2D Nb₂NF₂, which belongs to the Mxene family, has good reversible out-of-plane electrical polarization and the characteristics of a metal. The ground state structure of ferroelectric 2D Nb₂NF₂ is unique and a 2D material with such a ground state structure has not been reported elsewhere. Furthermore, 72 2D surface functionalized Mxene compositions (M₂XS₂:M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and Mn; X = C and N; S = O, OH and F) were investigated and only 2D Nb₂NF₂ showed dynamic and thermal stability in the novel ground state. However, there is a large variety of Mxenes to be explored, therefore, the identified ferroelectric ground state will pave the way to discover novel 2D ferroelectric Mxenes with various attributes. Showing metallicity and ferroelectricity simultaneously, 2D Nb₂NF₂ can play a major role in designing next generation materials with extraordinary properties.