Air stability and composition evolution in van der Waals Fe₃GeTe₂

Layered two-dimensional ferromagnetic materials have emerged as a promising platform for spintronic applications, owing to their extraordinary physical properties. Remarkably, the representative two-dimensional ferromagnet Fe3GeTe2 has been extensively investigated due to its high Curie temperature and strong perpendicular magnetic anisotropy, which are beneficial for high-density storage at room temperature. However, there are few reports on its stability in the air and the surface oxidation products, which may prohibit its future application. Here, we report the natural oxidation process of Fe3GeTe2 films grown by molecular beam epitaxy evolved in the atmosphere, which was studied by x-ray photoelectron spectroscopy measurements and transmission electron microscopy. Our research shows that the surface of the Fe3GeTe2 film is oxidized quickly when exposed to air and shows two obviously evolving stages in the whole oxidation process. In the first stage, metallic Ge atoms are almost completely oxidized to form Ge-O bonds in GeO2, while partial metallic Fe and Te atoms are oxidized into Fe2O3 and TeO2, respectively. The second stage of oxidation is dominated by changes in the valence state of the Fe element, where Fe2O3 is reduced to FeCO3 through the participation of carbon adsorbed on the surface with the final oxidation product of FeCO3 GeO2 TeO2. Our findings provide insight into the subsequent growth and protection of Fe3GeTe2 thin films, which is of great significance for in-depth study and further application of spintronic devices in two-dimensional limits.