Emerging oxidized and defective phases in low-dimensional CrCl(3)

Two-dimensional (2D) magnets such as chromium trihalides CrX(3) (X = I, Br, Cl) represent a frontier for spintronics applications and, in particular, CrCl(3) has attracted research interest due its relative stability under ambient conditions without rapid degradation, as opposed to CrI(3). Herein, mechanically exfoliated CrCl(3) flakes are characterized at the atomic scale and the electronic structures of pristine, oxidized, and defective monolayer CrCl(3) 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 CrCl(3) flakes show spontaneous surface oxidation upon air exposure with an extrinsic long-range ordered oxidized O–CrCl(3) structure and amorphous chromium oxide formation on the edges of the flakes. XPS proves that CrCl(3) 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) CrCl(3) is an insulator with a band gap of 2.6 eV, while the electronic structures of oxidized and Cl defective phases of ML CrCl(3), extrinsically emerging in exfoliated CrCl(3) flakes, show in-gap spin-polarized states and relevant modifications of the electronic band structures.