Composition-Controllable Syntheses and Property Modulations from 2D Ferromagnetic Fe 5 Se 8 to Metallic Fe 3 Se 4 Nanosheets.

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., Cr 2 S 3 , Cr 5 Te 8 ). Herein, composition-controllable syntheses of 2D non-layered iron selenide nanosheets (25% Fe-intercalated triclinic Fe 5 Se 8 and 50% Fe-intercalated monoclinic Fe 3 Se 4 ) are firstly reported, via a robust chemical vapor deposition strategy. Specifically, the 2D Fe 5 Se 8 exhibits intrinsic room-temperature ferromagnetic property, which is explained by the change of electron spin states from layered 1T'-FeSe 2 to non-layered Fe-intercalated Fe 5 Se 8 based on density functional theory calculations. In contrast, the ultrathin Fe 3 Se 4 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.