Group-IV monochalcogenide monolayers: Two-dimensional ferroelectrics with weak intralayer bonds and a phosphorenelike monolayer dissociation energy

Abstract

We performed density functional theory calculations with self-consistent van der Waals corrected exchange-correlation (XC) functionals to capture the structure of black phosphorus and twelve monochalcogenide monolayers and find the following results: (a) The in-plane unit cell changes its area in going from the bulk to a monolayer. The change of in-plane distances implies that bonds weaker than covalent or ionic ones are at work within the monolayers themselves. This observation is relevant for the prediction of the critical temperature Tc. (b) There is a hierarchy of independent parameters that uniquely define a ground state ferroelectric unit cell (and square and rectangular paraelectric unit cells as well): only 5 optimizable parameters are needed to establish the unit cell vectors and the four basis vectors of the ferroelectric ground state unit cell, while square and rectangular paraelectric structures are defined by only 3 or 2 independent optimizable variables, respectively. (c) The reduced number of independent structural variables correlates with larger elastic energy barriers on a rectangular paraelectric unit cell when compared to the elastic energy barrier of a square paraelectric structure. This implies that Tc obtained on a structure that keeps the lattice parameters fixed (for example, using an NVT ensemble) should be larger than the transition temperature on a structure that is allowed to change in-plane lattice vectors (for example, using the NPT ensemble). (d) The dissociation energy (bulk cleavage energy) of these materials is similar to the energy required to exfoliate graphite and MoS2. (e) There exists a linear relation among the square paraelectric unit cell lattice parameter and the lattice parameters of the rectangular ferroelectric ground state unit cell. These results highlight the subtle atomistic structure of these novel 2D ferroelectrics.

Microscopic Manipulation of Ferroelectric Domains in SnSe Monolayers at Room Temperature

“…The larger value of T c obtained for SnSe MLs on graphene is consistent with a larger energy required to turn the ferroelectric phase into the paraelectric phase as the chalcogen atom (Se versus Te) becomes lighter. 23 , 42 …”

Metastable piezoelectric group IV monochalcogenide monolayers with a buckled honeycomb structure

“…A transformation of the Pnm2 1 unit cell into a four-fold symmetric (square, s) Pmmn structure with a net P = 0 takes place within the dark-blue section of the energy versus ∆z plots for nine compounds (SiS, SiSe, SiTe, GeS, GeSe, GeTe, SnS, SnSe, and SnTe) 40,42,56,58 for which the Pmmn structure has a larger energy, as verified by the increase in energy in Fig. 4.…”

“…) the atomic number of the group-IVA (VIA) atom] is listed in subsequent Figures and Tables, to emphasize structural trends in this family of compounds that depend on that variable, 40,56 such as the increase of a h , |∆z h |, a p , and P h,3 with Z in Table I.…”

Changes of Structure and Bonding with Thickness in Chalcogenide Thin Films

“…Evidence for peculiar changes in atomic and electronic structure in thin chalcogenides has been reported. [ 5,12,20–24 ] However, unambiguous evidence for changes in bonding has not been obtained so far. In this work, we investigate the properties of thin‐film models of GeTe, SnTe, GeSe, and SnSe—which are representatives of the two families of group IV chalcogenides—as a function of slab thickness.…”

“…These results are consistent with the relaxed structural parameters for single bilayers discussed in refs. [ 22–24 ] .…”