Ferroelectric engineering of two-dimensional group-IV monochalcogenides: The effects of alloying and strain

In this work, we studied the stability and ferroelectricity of twodimensional (2D) group-IV monochalcogenides MX (M ¼ Ge, Sn, or Pb; X ¼ S, Se, or Te).Two competing crystal structures of these 2D compounds have been considered, including the rippled crystal structure with a space group of Pmn2 1 , which can be obtained from exfoliation, and the P3m1 hexagonal crystal structure containing the X-M-M-X building blocks.We find that the total energies of the rippled phases of PbX, SnS and SnSe are lower than those of the corresponding hexagonal phases; particularly, the rippled phases of SnS and SnSe exhibit spontaneous polarizations and ferroelectricity.On the other hand, the hexagonal phases of GeX and SnTe are energetically more stable than the corresponding rippled phases; because the hexagonal phases are centrosymmetric, these 2D compounds are non-ferroelectric.To engineer the ferroelectricity of 2D group-IV monochalcogenides, we have investigated the effects of alloying and equibiaxial strain.Based on density functional theory calculations, we find that the rippled phases of GeX and SnTe can be stabilized via Pb alloying to achieve ferroelectricity.In addition, it is also found that equibiaxial tensile strain gives rise to ferroelectricity in the rippled phases of 2D PbX compounds.