Emerging oxidized and defective phases in low-dimensional CrCl₃

Two-dimensional (2D) magnets such as chromium trihalides CrX₃ (X = I, Br, Cl) represent a frontier for spintronics applications and, in particular, CrCl₃ has attracted research interest due its relative stability under ambient conditions without rapid degradation, as opposed to CrI₃. Herein, mechanically exfoliated CrCl₃ flakes are characterized at the atomic scale and the electronic structures of pristine, oxidized, and defective monolayer CrCl₃ phases are investigated employing density functional theory (DFT) calculations, scanning tunneling spectroscopy (STS), core level X-ray photoemission spectroscopy (XPS), and valence band XPS and ultraviolet photoemission spectroscopy (UPS). As revealed by atomically resolved transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis, the CrCl₃ flakes show spontaneous surface oxidation upon air exposure with an extrinsic long-range ordered oxidized O-CrCl₃ structure and amorphous chromium oxide formation on the edges of the flakes. XPS proves that CrCl₃ is thermally stable up to 200 °C having intrinsically Cl vacancy-defects whose concentration is tunable via thermal annealing up to 400 °C. DFT calculations, supported by experimental valence band analysis, indicate that pure monolayer (ML) CrCl₃ is an insulator with a band gap of 2.6 eV, while the electronic structures of oxidized and Cl defective phases of ML CrCl₃, extrinsically emerging in exfoliated CrCl₃ flakes, show in-gap spin-polarized states and relevant modifications of the electronic band structures.