Phase Instability in van der Waals In2Se3 Determined by Surface Coordination

van der Waals In₂ Se₃ has attracted significant attention for its room-temperature 2D ferroelectricity/antiferroelectricity down to monolayer thickness. However, instability and potential degradation pathway in 2D In₂ Se₃ have not yet been adequately addressed. Using a combination of experimental and theoretical approaches, we here unravel the phase instability in both α- and β'-In₂ Se₃ originating from the relatively unstable octahedral coordination. Together with the broken bonds at the edge steps, it leads to moisture-facilitated oxidation of In₂ Se₃ in air to form amorphous In₂ Se_3-3x O_3x layers and Se hemisphere particles. Both O₂ and H₂ O are required for such surface oxidation, which can be further promoted by light illumination. In addition, the self-passivation effect from the In₂ Se_3-3x O_3x layer can effectively limit such oxidation to only a few nanometer thickness. The achieved insight paves way for better understanding and optimizing 2D In₂ Se₃ performance for device applications.