Natural oxygen defects play a vital role in the integrity, functional properties, and performance of well-known two-dimensional (2D) materials. The recently discovered chromium triiodide (CrI3) monolayer is the first real 2D magnet. However, its interaction with oxygen remains an open fundamental question, an understanding of which is essential for further exploration of its application potentials. Employing the quantum first-principles calculation method, we investigated the influence of oxygen defects on the structural, electronic, and magnetic properties of the CrI3 monolayer at the atomic level. We considered two oxygen-defective CrI3 monolayers with either a single O-attached or single O-doped structure, comparing them with an un-defective pristine monolayer. The two different oxygen defects significantly affect the original architecture of the CrI3 monolayer, being energetically favorable and increasing the stability of the CrI3 monolayer. Moreover, these point defects introduce either deep band lines or middle gap states in the band structure. As a result, the bandgap of oxygen-defective monolayers is reduced by up to 58%, compared with the pristine sheet. Moreover, the magnetic property of the CrI3 monolayer is drastically induced by oxygen defects. Importantly, O-defective CrI3 monolayers possess robust exchange coupling parameters, suggesting relatively higher Curie temperature compared with the un-defective sheet. Our findings reveal that the natural oxygen defects in the CrI3 monolayer enrich its structural, electronic, and magnetic properties. Thus, the controlled oxidation can be an effective way to tune properties and functionalities of the CrI3 monolayer and other ultrathin magnetic materials.
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