Phase instability in van der Waals In₂ Se₃ determined by surface coordination

Abstract

van der Waals In₂Se₃ has attracted significant attention for its room-temperature 2D ferroelectricity/antiferroelectricity down to monolayer thickness. However, instability and potential degradation pathway in 2D In₂Se₃ 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′-In₂Se₃ originating from the relatively unstable octahedral coordination. Together with the broken bonds at the edge steps, it leads to moisture-facilitated oxidation of In₂Se₃ in air to form amorphous In₂Se₃−₃ₓO₃ₓ layers and Se hemisphere particles. Both O₂ and H₂O are required for such surface oxidation, which can be further promoted by light illumination. In addition, the self-passivation effect from the In₂Se₃−₃ₓO₃ₓ layer can effectively limit such oxidation to only a few nanometer thickness. The achieved insight paves way for better understanding and optimizing 2D In₂Se₃ performance for device applications.