Emergence of a non-bulk hexagonal Fe$_2$S$_2$ single layer via phase transformation

TL;DR

This study reports the synthesis of a novel hexagonal Fe₂S₂ single layer with a β-CuI structure, achieved through phase transformation of mackinawite on graphene/Ir(111), revealing new 2D material polymorphism.

Contribution

The paper demonstrates the formation of a previously unknown 2D hexagonal Fe₂S₂ structure via phase transformation, expanding the structural diversity of Fe-S materials.

Findings

Reproducible formation of hexagonal Fe₂S₂ upon annealing.

First-principles calculations confirm the β-CuI structure as experimentally consistent.

Reduced dimensionality stabilizes new crystal structures not found in bulk.

Abstract

Two-dimensional materials can stabilize crystal structures that are absent from their bulk counterparts, offering opportunities for materials design. Here, we report the synthesis of a previously unknown hexagonal Fe$_2$S$_2$ single layer with $\beta$-CuI structure, a buckled layer of two vertically stacked FeS honeycomb lattices, realized by thermally induced transformation of single layer mackinawite grown on graphene/Ir(111). In situ scanning tunneling microscopy and low-energy electron diffraction reveal a transition from a tetragonal to a hexagonal lattice accompanied by distinct morphological and electronic signatures. The hexagonal Fe$_2$S$_2$ forms reproducibly upon annealing and represents a new structural motif within the Fe-S material family. First-principles calculations identify the $\beta$-CuI structure as most consistent with experiment. The calculations suggest that on-site Coulomb interactions and magnetic order are relevant to understanding the stability of the new 2D Fe-S compound. The preferred nucleation of single-layer mackinawite, despite being energetically disfavored, is speculated to result from its low edge energy, analogous to the 3D case. Our results establish Fe$_2$S$_2$ as a platform for exploring structural polymorphism in two dimensions and demonstrate that reduced dimensionality can stabilize crystal structures not accessible in bulk materials.