On the atomic structure of two-dimensional materials with Janus structures.

The discrepancy between the bright theoretical projections for two-dimensional (2D) Janus structures and the lack of experimental realisation of these structures motivated us to study the effect of structural disorder on the stability of MoSSe, SnSSe, PtSSe, In 2 SSe and GaInSe 2 . The calculation results demonstrate that the difference between metal-sulfur and metal-selenium bonds makes Janus structures frustrated and less energetically favourable than less ordered allotropes of the same compounds. This result explains the difficulties encountered in the experimental fabrication of these materials. In the bulk, there is an additional contribution to the total energy from dipole-dipole interactions between layers with a Janus structure that can overcome the energetic cost of structural frustration in layers for compounds with sufficiently large dipole moments. However, the entropic contribution to the free energy decreases the favourability of the ordered Janus structure. The calculation results are used to make recommendations to enable the discovery and synthesis of 2D materials with Janus structures.