On the Atomic Structure of Two-Dimensional Materials with Janus Structures

TL;DR

This paper investigates why 2D Janus structures are difficult to synthesize experimentally, showing that structural disorder and energetic frustration hinder their stability, but certain bulk interactions may facilitate their formation.

Contribution

The study provides a detailed analysis of the energetic factors affecting the stability of 2D Janus materials and offers guidelines for their experimental realization.

Findings

Janus structures are less stable due to bond frustration.

Dipole interactions in bulk can stabilize Janus layers.

Entropy reduces the stability of ordered Janus configurations.

Abstract

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, In2SSe and GaInSe2. 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 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.