Atomic imaging of 2D transition metal dihalides

TL;DR

This study introduces a novel polymer-free method for assembling and imaging air-sensitive 2D transition metal dihalides at the atomic level, revealing their structural properties and potential for stacking control.

Contribution

The paper presents a new transfer technique enabling atomic-resolution imaging of air-sensitive 2D dihalides, facilitating detailed structural and defect analysis at the monolayer level.

Findings

Atomic resolution imaging of monolayer di-iodides achieved.

Small energy barriers allow control over stacking phases.

Stable iodine vacancies and edge configurations identified.

Abstract

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.