Atomic imaging of 2D transition metal diiodides

Transition metal diiodides such as FeI₂, NiI₂ and CoI₂ are an emerging class of 2D magnets exhibiting rich and diverse magnetic behaviour, but their study at the monolayer limit has been severely hindered by fabrication challenges due to their air-sensitivity. Here, we introduce a polymer-free method for clean, rapid, and high-yield assembly of hermetically encapsulated suspended samples of air-sensitive monolayers. Applying it to diiodides enables atomic resolution characterisation of thin samples – down to the monolayer limit – for the first time. Our imaging, combined with complementary first-principles calculations, reveals an unusually small energy barrier between alternate stable stacking polytypes in few-layer films, enabling extrinsic control of the stacking phase. We also observe stable isolated iodine vacancies that do not aggregate to form extended structures, and identify and verify the stability of the various edge configurations of thin samples. These results establish the unique structural characteristics of these materials in the thin limit, and more broadly demonstrate the utility of our transfer platform for creating atomically clean suspended vdW heterostructures.