Reactivity of Ultrathin\nKagome Metal FeSn toward Oxygen\nand Water

The kagome metal FeSn consists of alternating layers\nof kagome-lattice\nFe<sub>3</sub>Sn and honeycomb Sn<sub>2</sub> and exhibits great potential\nfor applications in future low-energy electronics and spintronics\nbecause of an ideal combination of topological phases and high-temperature\nmagnetic ordering. Robust synthesis methods for ultrathin FeSn films,\nas well as an understanding of their air stability, are crucial for\nits development and long-term operation in future devices. In this\nwork, we realize large-area, <10 nm thick, epitaxial FeSn thin\nfilms and explore the oxidation process <i>via</i> synchrotron-based\nphotoelectron spectroscopy using <i>in situ</i> oxygen and\nwater dosing, as well as <i>ex situ</i> air exposure. Upon\nexposure to the atmosphere, the FeSn films are shown to be highly\nreactive, with a stable ∼3 nm thick oxide layer forming at\nthe surface within 10 min. Notably, the surface Fe remains largely\nunoxidized when compared with Sn, which undergoes near-complete oxidation.\nAdditionally, the band structure remains metallic under oxygen exposure.\nThese are further confirmed with controlled <i>in situ</i> dosing of O<sub>2</sub> and H<sub>2</sub>O, where only the Sn<sub>2</sub> (stanene) interlayers within the FeSn lattice oxidize, suggesting\nthe Fe<sub>3</sub>Sn kagome layers remain almost pristine. These results\nare in excellent agreement with first-principles calculations, which\nshow that Fe–O bonds to the Fe<sub>3</sub>Sn layer are energetically\nunfavorable and a large formation energy preference of 1.37 eV for\nSn–O bonds in the stanene Sn<sub>2</sub> layer over Sn–O\nbonds in the kagome Fe<sub>3</sub>Sn layer. The demonstration that\noxidation only occurs within the stanene layers and the preservation\nof the Dirac bands may provide additional avenues in how to engineer,\nhandle, and prepare future kagome metal devices.