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

Two-dimensional (2D) iron disulfide (FeS 2 ), in its 1 T polymorph, is a promising candidate for high-Curie-temperature ferromagnetic applications. Unlike typical van der Waals materials, FeS 2 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 FeS 2 . 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 1 T polymorph. This 1 T -FeS 2 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 1 T structure, attributed to the strain induced by the substrate to achieve a perfectly commensurate 5 × 5 moiré pattern. The 1 T -FeS 2 and moiré atomic structures are fully determined with high accuracy on atomic coordinates. Finally, through Cs intercalation, we demonstrate complete decoupling of the FeS 2 layer from the substrate and the release of heteroepitaxial strains.