Reactivity of Ultrathin Kagome Metal FeSn toward Oxygen and Water

The kagome metal FeSn consists of alternating layers of kagome-lattice Fe₃Sn and honeycomb Sn₂ and exhibits great potential for applications in future low-energy electronics and spintronics because of an ideal combination of topological phases and high-temperature magnetic ordering. Robust synthesis methods for ultrathin FeSn films, as well as an understanding of their air stability, are crucial for its development and long-term operation in future devices. In this work, we realize large-area, <10 nm thick, epitaxial FeSn thin films and explore the oxidation process via synchrotron-based photoelectron spectroscopy using in situ oxygen and water dosing, as well as ex situ air exposure. Upon exposure to the atmosphere, the FeSn films are shown to be highly reactive, with a stable ∼3 nm thick oxide layer forming at the surface within 10 min. Notably, the surface Fe remains largely unoxidized when compared with Sn, which undergoes near-complete oxidation. Additionally, the band structure remains metallic under oxygen exposure. These are further confirmed with controlled in situ dosing of O₂ and H₂O, where only the Sn₂ (stanene) interlayers within the FeSn lattice oxidize, suggesting the Fe₃Sn kagome layers remain almost pristine. These results are in excellent agreement with first-principles calculations, which show that Fe-O bonds to the Fe₃Sn layer are energetically unfavorable and a large formation energy preference of 1.37 eV for Sn-O bonds in the stanene Sn₂ layer over Sn-O bonds in the kagome Fe₃Sn layer. The demonstration that oxidation only occurs within the stanene layers and the preservation of the Dirac bands may provide additional avenues in how to engineer, handle, and prepare future kagome metal devices.