Environmental instability and effects of degradation in α -In ₂ Se ₃

Abstract Among the ever-growing family of two-dimensional (2D) materials, in recent years alpha phase indium selenide ( α -In {}₂ Se {}₃ ), has attracted interest for its 2D ferroelectricity down to the monolayer limit, high photoresponsivity and thickness-dependent band-gap. However, the environmental instability of α -In {}₂ Se {}₃ presents a significant obstacle to harnessing these properties in devices. Understanding the underlying mechanism and practical limits this degradation imposes is necessary. In this work, we employ various characterization techniques in order to probe how the structure, surface morphology, composition and ferroelectric properties of α -In {}₂ Se {}₃ change over a period of up to 14 days. We resolve some of the previous contradictions in the interpretation of Raman spectra for these and similar air-sensitive 2D materials, particularly accounting for the potentially confounding role of beam damage in elemental Se spectral features. We find that the bulk structural properties of α -In {}₂ Se {}₃ remain largely unchanged owing to the formation of a uniform surface oxide layer preventing further oxidation below the surface. In addition, piezoresponse force microscopy studies show that ferroelectric polarization in α -In {}₂ Se {}₃ remains stable for up to 2 h but decays rapidly within 22–24 h, providing a quantitative time frame for the handling of α -In {}₂ Se {}₃ in air before its ferroelectric properties are compromised. As well as these findings aiding in the implementation of α -In {}₂ Se {}₃ in devices, they can also be used to inform the characterization of air-degradation in other related 2D transition metal chalcogenides.