Surface Vacancy-Induced Switchable Electric Polarization and Enhanced Ferromagnetism in Monolayer Metal Trihalides.

Monolayer chromium triiodide (CrI ₃ ), as the thinnest ferromagnetic material demonstrated in experiment [ Huang et al. Nature 2017 , 546 , 270 ], opens up new opportunities for the application of two-dimensional (2D) materials in spintronic nanodevices. Atom-thick 2D materials with switchable electric polarization are now urgently needed for their rarity and important roles in nanoelectronics. Herein, we unveil that surface I vacancies not only enhance the intrinsic ferromagnetism of monolayer CrI ₃ but also induce switchable electric polarization. I vacancies bring about an out-of-plane polarization without breaking the nonmetallic nature of CrI ₃ . Meanwhile, the induced polarization can be reversed in a moderate energy barrier, arising from the unique porosity of CrI ₃ that contributes to the switch of I vacancies between top and bottom surfaces. Engineering 2D switchable polarization through surface vacancies is also applicable to many other metal trihalides, which opens up a new and general way toward pursuing low-dimensional multifunctional nanodevices.