A first-principles study of the ferroelectricity of two-dimensional IV-VI group compounds

Two-dimensional (2D) ferroelectric materials with atomic thickness have attracted much research interest due to their potential industrial applications. The recent successful synthesis of 2D SnTe, which exhibits a spontaneous polarization along the in-plane directions, motivates further studies of the related 2D IV-VI group compounds. In this work, we focus on the theoretical calculations of the stability and ferroelectricity of 2D IV-VI compounds based on density functional theory (DFT) and the Berry phase method. Firstly, the energies of the competing 2D crystal structures of the IV-VI compounds have been computed from DFT; it is found that different structures are adopted by the 2D IV-VI compounds. To facilitate the engineering of the crystal structure, the alloying effects on the phase stability and the corresponding electronic origins have been further studied. Secondly, the spontaneous polarization is calculated using the Berry phase approach, and it is found that some of the 2D IV-VI compounds have more robust ferroelectricity than others. Moreover, the manipulation of spontaneous polarization via external strain has also been discussed.