Abstract
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.
Highlights
IntroductionSpintronics is an interdisciplinary eld that merges materials science, condensed matter physics, and electronic engineering, aiming to simultaneously control both spin and charge degrees of freedom in nano-electronics.[1,2,3] In the last two decades, it has dictated the road-map toward the fabrication of semiconducting magnetic materials with reduced dimensionality.[1,2]the efforts in the synthesis of new materials have moved from bulk[4,5] toward two-dimensional (2D) materials, especially a er the recent breakthrough discoveries of 2D magnetic systems.[6,7] Among layered materials exhibiting magnetism in the bulk form and down to the monolayer limit, the class of chromium trihalides plays a central role.[8]
Research interest has primarily focused on few-layer CrI3,10 but its rapid degradation upon air exposure,[11] which limits device fabrication, moved the attention towards CrCl3.8,12–16 Interestingly, CrCl3 can be experimentally studied even in reduced dimensions, as it exfoliates into large akes down to the monolayer limit, starting from the bulk material and remaining stable under ambient conditions without rapid degradation.[17]
In our previous study,[17] identically synthesized crystals have been studied and characterized by X-ray diffraction (XRD), optical microscopy, atomic force microscopy, and micro-Raman spectroscopy and as-grown crystals turned out to be singlecrystalline with a monoclinic C2/m space group.[35]
Summary
Spintronics is an interdisciplinary eld that merges materials science, condensed matter physics, and electronic engineering, aiming to simultaneously control both spin and charge degrees of freedom in nano-electronics.[1,2,3] In the last two decades, it has dictated the road-map toward the fabrication of semiconducting magnetic materials with reduced dimensionality.[1,2]the efforts in the synthesis of new materials have moved from bulk[4,5] toward two-dimensional (2D) materials, especially a er the recent breakthrough discoveries of 2D magnetic systems.[6,7] Among layered materials exhibiting magnetism in the bulk form and down to the monolayer limit, the class of chromium trihalides plays a central role.[8]. Research interest has primarily focused on few-layer CrI3,10 but its rapid degradation upon air exposure,[11] which limits device fabrication, moved the attention towards CrCl3.8,12–16 Interestingly, CrCl3 can be experimentally studied even in reduced dimensions, as it exfoliates into large akes down to the monolayer limit, starting from the bulk material and remaining stable under ambient conditions without rapid degradation.[17]
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