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 behavior, 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. Applied to diiodides, it enables atomic resolution characterization of thin samples down to the monolayer limit using transmission electron microscopy. 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 structural characteristics of these materials in the thin limit and more broadly demonstrate the utility of our transfer platform for creating atomically clean suspended van der Waals heterostructures.