Reactivity of Ultra-Thin Kagome Metal FeSn Towards 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 novel topological phases and high-temperature magnetic ordering. Robust synthesis methods for ultra-thin FeSn films, as well as an understanding of their air stability is crucial for its development and long-term operation in future devices. In this work, we realize large area, sub-10 nm 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 atmosphere the FeSn films are shown to be highly reactive, with a stable  3 nm thick oxide layer forming at the surface within 10 minutes. Notably the surface Fe remains largely unoxidized when compared to Sn, which undergoes near-complete oxidation. This is further confirmed with controlled in-situ dosing of O2 and H2O where only the Sn2 (stanene) inter-layers 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 Fe-O bonds to the Fe3Sn layer are energetically unfavorable, and furthermore, 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 may provide new avenues in how to engineer, handle and prepare future kagome metal devices.