Atomic imaging of 2D transition metal dihalides

Transition metal di-iodides 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 di-iodides 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.

💡 Research Summary

This paper presents a polymer‑free, on‑grid transfer technique that enables the preparation of hermetically encapsulated, suspended monolayers and few‑layer crystals of air‑sensitive transition‑metal di‑iodides (FeI₂, NiI₂, CoI₂). By fabricating silicon nitride (SiNₓ) membranes with micron‑scale holes, coating them with thin metal layers for conductivity, and using graphene “pick‑up” and “drop‑off” steps inside an argon glovebox, the authors achieve rapid, high‑yield assembly without any organic residues. The resulting graphene‑encapsulated heterostructures remain stable for weeks, can be transferred between microscopes, and are directly compatible with scanning transmission electron microscopy (STEM).

Using high‑angle annular dark‑field (HAADF) STEM at atomic resolution, the authors image the crystal structure of each material from the monolayer up to bulk thickness. In bulk, FeI₂ adopts the 1T (AA) stacking, while NiI₂ and CoI₂ crystallise in the 3R (ABC) polytype. Strikingly, when the thickness is reduced to two or three layers, FeI₂ and CoI₂ frequently switch to the 3R stacking, whereas NiI₂ retains its 3R order. A folded trilayer region of FeI₂ shows coexistence of both polytypes, evidencing a high degree of polytype plasticity in the thin‑film limit.

First‑principles density‑functional theory (DFT) calculations reveal that the energy difference between the 1T and 3R configurations is exceptionally small: ~4.4 meV per unit cell for a bilayer FeI₂ and ~14 meV for 3–4 layers. This barrier is an order of magnitude lower than stacking‑fault energies reported for other van‑der‑Waals systems (e.g., NbSe₂). Consequently, modest external perturbations—such as strain induced during mechanical exfoliation or pressure from the graphene encapsulation—can readily drive a stacking transition. The calculations also confirm that the stripe antiferromagnetic order is the ground state for both FeI₂ and NiI₂, and that magnetic ordering does not significantly affect the relative stability of the polytypes at room temperature.

Defect analysis focuses on iodine vacancies (V_I), which dominate the point‑defect population in both FeI₂ and NiI₂ monolayers. By comparing experimental HAADF contrast with simulated images, the authors quantify vacancy densities separately for the upper and lower iodine sub‑layers, finding roughly four times higher vacancy concentration in one sub‑layer. Under electron‑beam irradiation, individual V_I sites migrate and occasionally recombine, leading to a net decrease in defect density over time—a sign that vacancy annealing outpaces beam‑induced creation. Metal vacancies (V_M) are rare. Importantly, no clustering of vacancies into pores or line defects is observed, even after prolonged imaging, indicating an intrinsic resistance to defect aggregation in these di‑iodides. Ab‑initio defect‑formation energies corroborate the experimental trend: iodine vacancies are energetically more favourable than metal vacancies in NiI₂, while the opposite holds for FeI₂.

Edge structures are also examined. Both zigzag and armchair terminations are imaged, and DFT calculations of edge formation energies suggest that different edge orientations can host distinct electronic and magnetic states. Because the graphene caps fully seal the edges, the atomic configurations remain unchanged during the experiments, providing a reliable platform for future studies of edge‑related phenomena.

Overall, the work delivers four major advances: (1) a clean, polymer‑free, on‑grid transfer method that preserves the integrity of highly air‑sensitive 2D crystals; (2) the first atomic‑resolution STEM images of FeI₂, NiI₂, and CoI₂ down to the monolayer limit; (3) quantitative determination of an ultra‑low stacking‑polytype energy barrier, establishing the feasibility of extrinsic stacking control in thin films; and (4) a comprehensive picture of point‑defect dynamics and edge stability in these materials. These findings open new avenues for engineering 2D magnetic heterostructures, exploiting stacking‑induced property tuning, and integrating air‑sensitive vdW crystals into spintronic, optoelectronic, and terahertz devices.