Emergence of a non-bulk hexagonal Fe$_2$S$_2$ single layer via phase transformation
Affan Safeer, Wejdan Beida, Felix Oberbauer, Nicolae Atodiresei, Gustav Bihlmayer + 6 more
TLDR
Researchers synthesized a new hexagonal Fe₂S₂ single layer, demonstrating how 2D materials can stabilize unique crystal structures not found in bulk.
Key contributions
- Synthesized a novel hexagonal Fe₂S₂ single layer with a β-CuI structure.
- Achieved via thermally induced phase transformation of single layer mackinawite.
- In situ STM and LEED confirmed the transition and distinct electronic signatures.
- First-principles calculations identified the structure and factors for stability.
Why it matters
This paper introduces a new 2D Fe-S compound, expanding the family of iron sulfides. It highlights how reduced dimensionality enables the stabilization of crystal structures inaccessible in bulk, opening new avenues for materials design and exploring structural polymorphism.
Original 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 $β$-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 $β$-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.
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