Underlying data and analysis code from '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. Applied to diiodides, it enables atomic resolution characterisation 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.

过渡金属二碘化物（transition metal diiodides），如碘化亚铁（FeI₂）、碘化镍（NiI₂）与碘化钴（CoI₂），是一类新兴的二维磁体（2D magnets），展现出丰富多样的磁学行为。但由于其空气敏感性带来的制备难题，针对单层极限下这类材料的研究长期受到严重阻碍。本研究提出了一种无聚合物（polymer-free）的制备方法，可洁净、快速且高收率地组装得到空气敏感性单层材料的密封封装悬浮样品。将该方法应用于二碘化物体系后，我们可借助透射电子显微镜（transmission electron microscopy）实现低至单层极限的薄层样品原子级分辨率表征。结合配套的第一性原理计算（first-principles calculations），我们的成像研究揭示出少层薄膜中不同稳定堆叠多型间存在异常小的能垒，使得对堆叠相的外源性调控成为可能。我们还观测到了稳定存在的孤立碘空位（iodine vacancies），这类空位不会聚集形成扩展结构；同时我们鉴定并验证了薄层样品各类边缘构型的稳定性。上述结果明确了这类材料在超薄极限下的结构特征，更从更广维度证明了我们的转移平台在制备原子级洁净的悬浮范德华异质结（van der Waals heterostructures）方面的实用价值。