1. Introduction
Spintronics is an interdisciplinary field that merges materials science, condensed matter physics, and electronic engineering, aiming to simultaneously control both spin and charge degrees of freedom in nano-electronics.1-3 In the last two decades, it has dictated the road-map toward the fabrication of semiconducting magnetic materials with reduced dimensionality.1,2 Consequently, the efforts in the synthesis of new materials have moved from bulk4,5 toward two-dimensional (2D) materials, especially after 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 Cr-Trihalides (CrX3, X = I, Br, Cl) are van der Waals (vdW) materials exhibiting both insulating or semiconducting behavior and either ferro- or anti-ferromagnetism in their bulk phase.9 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 easily exfoliates into large flakes down to the monolayer limit, starting from the bulk material and remaining stable under ambient conditions without rapid degradation.17 This enables real applications and multiple experimental investigations of CrCl3 in reduced dimensions. However, real applications face possible air exposure, degradation and the presence of defects, which so far have received little attention in Cr-trihalides. Recently, Grönke et al.18 found traces of oxygen attributable to surface oxidation of exfoliated CrCl3 flakes during transport in air, but it is not established whether flakes are really prone to oxidation upon air exposure and what are the possible effects of oxygen on the electronic structure.6,7,19 On the other hand, 2D vdW materials have native structural vacancy-defects20 that can be exploited by processes such as thermal annealing.21 Some theoretical studies on Cr-trihalides highlighted the role of defects in determining the electronic structure.22-27 Specifically, when Cr vacancy-defects are considered, transition to a half-metal is observed in all Cr-trihalides,22,24,27 while Cl vacancies narrow the band gap, as found in YCl3.23
In this study, we investigate the defective CrCl3 surface and experimentally demonstrate the existence of surface ordered oxidized and Cl defective phases of CrCl3 elucidating their role in the electronic structure. Our results show that mechanically exfoliated CrCl3 flakes, having a thickness of the order of tens of nanometers, show ordered surface oxidation upon air exposure and controllable Cl deficiency, remaining thermally stable up to 200 °C. The electronic structures of oxidized and Cl defective phases of monolayer (ML) CrCl3, extrinsically emerging in exfoliated CrCl3 flakes, show in-gap spin-polarized states and relevant modifications of the electronic bands.
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