Reactive Molecular Beam Epitaxy Growth of a 1T-FeS2 Single-Layer-Atomic Structure, Moiré, and Decoupling via Intercalation.

Two-dimensional (2D) iron disulfide (FeS₂), in its 1T polymorph, is a promising candidate for high-Curie-temperature ferromagnetic applications. Unlike typical van der Waals materials, FeS₂ lacks a naturally lamellar bulk structure and thus cannot be prepared by exfoliation. Consequently, it exists solely as a synthetic 2D magnet, primarily produced via chemical vapor deposition. Here, we propose an alternative synthesis method: reactive molecular beam epitaxy, where an iron layer predeposited on a Au(111) substrate is sulfurized to form FeS₂. Structural and compositional analyses of the resulting 2D layer─employing scanning tunneling microscopy, electron diffraction, Auger electron spectroscopy, and synchrotron surface X-ray diffraction─confirm a nominal Fe ratio of 1:2, essential for achieving a high Curie temperature. Modeling and fitting the three-dimensional X-ray diffraction data further reveals that the layer crystallizes in the desired 1T polymorph. This 1T-FeS₂ grown on Au(111) exhibits exceptional crystalline quality, largely surpassing that of other 2D transition metal dichalcogenides epitaxially grown on substrates. In addition, it shows pronounced atomic distortions from an ideal 1T structure, attributed to the strain induced by the substrate to achieve a perfectly commensurate 5 × 5 moiré pattern. The 1T-FeS₂ and moiré atomic structures are fully determined with high accuracy on atomic coordinates. Finally, through Cs intercalation, we demonstrate complete decoupling of the FeS₂ layer from the substrate and the release of heteroepitaxial strains.