Composition‐Controllable Syntheses and Property Modulations from 2D Ferromagnetic Fe₅Se₈ to Metallic Fe₃Se₄ Nanosheets

Abstract Exploring new‐type 2D magnetic materials with high magnetic transition temperature and robust air stability has attracted wide attention for developing innovative spintronic devices. Recently, intercalation of native metal atoms into the van der Waals gaps of 2D layered transition metal dichalcogenides (TMDs) has been developed to form 2D non‐layered magnetic TMDs, while only succeeded in limited systems (e.g., Cr₂S₃, Cr₅Te₈). Herein, composition‐controllable syntheses of 2D non‐layered iron selenide nanosheets (25% Fe‐intercalated triclinic Fe₅Se₈ and 50% Fe‐intercalated monoclinic Fe₃Se₄) are firstly reported, via a robust chemical vapor deposition strategy. Specifically, the 2D Fe₅Se₈ exhibits intrinsic room‐temperature ferromagnetic property, which is explained by the change of electron spin states from layered 1T'‐FeSe₂ to non‐layered Fe‐intercalated Fe₅Se₈ based on density functional theory calculations. In contrast, the ultrathin Fe₃Se₄ presents novel metallic features comparable with that of metallic TMDs. This work hereby sheds light on the composition‐controllable synthesis and fundamental property exploration of 2D self‐intercalation induced novel TMDs compounds, by propelling their application explorations in nanoelectronics and spintronics‐related fields.