Stable out-of-plane piezoelectric group-IV monochalcogenide monolayers with a buckled honeycomb structure*

We used DFT to study 12 group-IV monochalcogenide monolayers MX (M=Si, Ge, Sn, Pb and X =S, Se, Te) with a 2-fold degenerate honeycomb structure. The exchange of M and X atoms yields a two-fold degenerate structure and the bucking of atoms in the unit cell provides a thickness Δz and an out-of-plane electric polarization. A 2D structural phase transformation is discussed from the perspective of elastic energy barriers J [1,2,3,4]; J is the smallest energy required to change the buckled unit cell (Δz>0) onto a planar unit cell (Δz=0). These materials will not undergo a 2D phase transformation. For comparison, silicene has a similar structure and a lower J and undergoes the 2D structural transformation [5]. This study increases the understanding of relationships among the structure, ferroelectric behavior, and structural phase transformations in 2D materials with structural degeneracies.

References:
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2. T. Bishop et al., PRL 122, 015703 (2019)
3. S. Barraza-Lopez et al., PRB 97, 024110 (2018)
4. S. P. Poudel et al., under review
5. G. G. Naumis et al., Rep. Prog. Phys. 80, 096501 (2017)